Content uploaded by Christine S Siddoway
Author content
All content in this area was uploaded by Christine S Siddoway on May 06, 2021
Content may be subject to copyright.
uncorrected proofs
3. The Geology of West
Antarctica
Christine Siddoway
3.1. Overview
The physiographic province of West Antarctica (Fig. 3-1) borders the Pacic sector of
the Southern Ocean and supports the West Antarctic Ice Sheet. Unlike East Antarctica,
it has no cratonic elements and consists largely of thinned continental crust of the West
Antarctic rift. In geographical terms, West Antarctica is that portion of the Antarctic
continent that resides primarily in the western hemisphere, seaward of the 030° W –
170° E meridian. Bounded by the Transantarctic Mountains along its interior margin,
West Antarctica encompasses Marie Byrd Land, Thurston Island, and the Ellsworth
Mountains block, with the Ross Embayment and Weddell Sea, which altogether compri-
se the West Antarctic rift province and its seaward margins (Fig. 3-2). The West Antarctic
ice sheet, with its dynamic fast-owing glacier ice streams, lies within the bounds of the
rift. The Paleozoic-Mesozoic bedrock of this glaciated region crops out in coastal expo-
sures and forms extensive subglacial bedrock, encountered in offshore dredge and drill
Fig. 3-1. Map of West Antarctica, showing selected place names, tectonic terranes, and physiographic
features mentioned in the text. Gridded basemap is BEDMAP2 digital elevation model (Fretwell et al.
2013). Cartography by Brad Herried, Polar Geospatial Center, USA.
uncorrected proofs
2 3. The Geology of West Antarctica
core samples. West Antarctica hosts a Neogene volcanic province that includes eruptive
centers as young as Pleistocene and Holocene. Alkalic polygenetic volcanoes crown the
central elevated region of Marie Byrd Land, where volcano summits reach elevations up
to 4180 meters.
Formed along the Paleozoic – Mesozoic active margin of Gondwanaland, the lithos-
phere of West Antarctica consists of disparate Paleozoic-Mesozoic crustal blocks − tec-
tonic terranes (Fig. 3-3) − that record the stabilization and growth of continental crust
along a convergent margin, followed by intracontinental extension and wrench tectonics.
The prevalent Cenozoic record is of sparse, widespread alkalic volcanism; narrow-mode
Fig. 3-2. Crustal thickness map for West Antarctica, from Chaput et al. (2014). Uses continent-scale
surface wave constrained estimates of Sun et al. Thickness refers to the distance from the base of the
ice sheet to the Moho, estimated using POLENET-ANET data and other seismic station constraints. Grey
boxes indicate seismographic stations. Inset shows the demarcation of terranes in West Antarctica (Sto-
rey et al. 1988a, Randall & MacNiocaill 2004), with the oldest terranes of cratonic afnity shown in grey.
uncorrected proofs
3.1. Overview 3
Fig. 3-3. Tectonic reconstructions showing tectonic terranes of West Antarctica within the Gondwana su-
percontinent and their comparative positions at a) 250 Ma and b) 180 Ma, from Torsvik et al. (2008). Terrane
abbreviations are: AP: Antarctic Peninsula, EWM: Ellsworth-Whitmore, TI: Thurston Island-Eights Coast, and
MBL: Marie Byrd Land. Other abbreviations are: PM: Pensacola Mountains, CL: Coats Land, FI: Falkland Is-
lands, FB: Filchner Block, BH: Bouvet hotspot, NZ: Zealandia, P: Patagonia. The black thick line in (a) marks
the location and trend of the Permian-Early Mesozoic Gondwanide orogen (Dalziel & Grunow 1992, Curtis
2001, Randall & MacNiocaill 2004), a tectonic element that links the EWM block to the Pensacola Mountains,
Falkland Islands, and South Africa. The EWM block formed within the Natal Embayment in (a) and under-
went at least 90° of counterclockwise rotation during supercontinent breakup. The dark grey pattern in (b)
shows the extent of the Ferrar magmatism in Antarctica and Karroo magmatism in South Africa, expressions
of plume activity centered upon the Bouvet hotspot at the time of breakup. Absolute plate motion vectors are
denoted with white arrows and mean plate velocities for the “East Antarctic Craton” are given.
uncorrected proofs
4 3. The Geology of West Antarctica
rifting from Miocene to present; and landscape rejuvenation in response to post-Pliocene
focused incision by ice streams and outlet glaciers.
The terranes of West Antarctica are distinguished on tectonostratigraphic grounds and
on the basis of isotopic signatures. The single known exposure of Precambrian rock, at
Haag Nunataks (Fig. 3-3), is >1050 Ma. Isotopic characteristics of ultramac xenoliths
obtained from Miocene to Pleistocene volcanoes suggest a considerable extent of con-
cealed old lithosphere. The major geological-tectonic provinces that are distinguished in
West Antarctica are: the Ellsworth-Whitmore Mountains, the Ross and Amundsen pro-
vinces in Marie Byrd Land, Eights Coast-Thurston Island, the West Antarctic rift system;
and the Antarctic Peninsula. Within this volume, the geology of the Antarctic Peninsula
(chapter 2: Smellie, this vol.) is presented separately from that of West Antarctica. New
geophysical evidence reveals the lithospheric-scale structures that form the boundaries
between these crustal blocks and provinces: they correspond to deep, linear, subglacial
troughs. The data also show that the Ellsworth-Whitmore Mountains block has a deep
crustal root and that Marie Byrd Land is underlain by anomalous mantle characterized
by slow seismic velocities, indicative of a major thermal anomaly. The margins of the
Thurston Island block are delineated by gravity and magnetic anomalies indicative of
recently active or active rifts.
This chapter begins with a review of the stratigraphy, geological evolution, and struc-
tural architecture of the three West Antarctica terranes (Fig. 3-2, inset) (Dalziel & Elliot
1982, Storey et al. 1988a, Storey et al. 1991) that border the West Antarctic rift provin-
ce. Next it examines the geotectonic characteristics of the West Antarctic rift system,
from the perspective of West Antarctica, itself, and explores the question of the genetic
relationship between the West Antarctic rift system and the Transantarctic Mountains
that form the rift province boundary with the East Antarctic craton. Understanding of
the bedrock geology beneath the Ross Sea, the vast expanse of the West Antarctic ice
sheet, and the continental shelf of the Amundsen Sea is provided by the results of air-
borne and marine geophysical explorations. The chapter concludes with an examination
of the Marie Byrd Land volcanic province and other geological associations of special
contemporary signicance or scientic potential in respect to the dynamic lithosphere-
cryosphere-ocean system of West Antarctica.
3.2. Ellsworth-Whitmore Mountains Terrane
Extensive rock exposures in the Sentinel and Heritage Ranges, that together comprise
the Ellsworth Mountains, form the backbone of the geology of the Ellsworth-Whitmore
Mountains terrane. The crustal thickness in this region (Fig. 3-2) is 32 to 37 km (Chaput
et al. 2014), in dramatic contrast to adjacent crust within the West Antarctic rift province
that is between ~26 and 19 km thick (Jordan et al. 2010). Recent aerogeophysical in-
vestigation of this region (Jordan et al. 2013, Ross et al. 2014) broadens the known sub-
glacial extent of the Ellsworth-Whitmore Mountains terrane to the south and southeast,
toward the TAM. Deep, narrow basins separating the tectonic blocks are interpreted to
be lithospheric-scale faults having Mesozoic tectonic inheritance. The Bentley subgla-
cial trough is the most sharply dened of these basins (Lloyd et al. 2015). The greatest
topographic relief in West Antarctica occurs upon one such structure that controls the
Ellsworth Mountains block margin and the position of the Rutford Ice Stream (Fig. 3-4).
A vertical exchange of 7 km is achieved over a lateral distance of 40 km, from the sum-
mit of Mt. Vinson at 4892 m, to the bed of Rutford Ice Stream at –2200 m. This structural
uncorrected proofs
3.2. Ellsworth-Whitmore Mountains Terrane 5
zone separates the Ellsworth range from a tectonic microblock that hosts Haag Nunataks
( ). The Whitmore Mountains are an isolated group of four summits of crystalline be-
drock at ~3000m elevation, located SW of the Ellsworth Mountains. The small expo-
sures of Proterozoic gneiss forming Haag Nunataks stand above Fowler Ice Rise, east
of the Heritage Range. The Haag Nunataks are distinguished as the only exposures of
Precambrian rock in West Antarctica (Millar & Pankhurst 1987, Flowerdew et al. 2007).
The Ellsworth Mountains consist of a 13-km-thick succession of Paleozoic volcanic
and sedimentary rocks (Fig. 3-5) deposited in active rift to passive margin settings along
the margin of West Gondwana (Curtis 2001, Elliot et al. 2015 and references therein).
Fig. 3-4. (a) Satellite image map of the Ellsworth Mountains (GoogleEarth, DigitalGlobe, ©U.S. Geologi-
cal Survey 2015a). The box outlines the area detailed in (b). (b) Structural map of the southern Ellsworth
Mountains (Heritage Range), and (c) selected cross sections to illustrate the geometries of upright to steeply
inclined folds, and bedding-cleavage-fault relationships in the Springer Peak Formation. From Curtis (2001).
The structural cross sections are along proles d and f, indicated on the map. The Heritage Group consists of
siliciclastic sediments deposited as synrift ll in latest Early Cambrian to Late Cambrian time. The overlying
Crashsite Group (Fig. 3-4) represents the transition to passive margin sedimentation. The Cambrian through
Permian strata were together deformed during the Permian-Triassic Gondwanian orogeny (Fig. 3-3a).
uncorrected proofs
6 3. The Geology of West Antarctica
Early Paleozoic strata are grouped into the Early to Late Cambrian Heritage Group and
Late Cambrian to Devonian Crashsite Group (Fig. 3-5). The Heritage Group (Curtis et
al. 1999) is dominated by siliciclastic sediments that include lahar and ash-ow tuff de-
posits, uvial to shallow marine deltaic deposits and black shales, and lesser carbonates.
Basaltic volcanic and subvolcanic rocks occur at intervals, wherein there are growth
faults and intraformational unconformities. Conglomerates, derived in part from contem-
poraneous volcanic rocks, reect higher energy uvial transport arising from develop-
ment of topographic relief. Taken together, the characteristics indicate rapid deposition
in a continental rift basin. The contact between the Heritage and Crashsite Groups marks
the transition from rifted to passive margin, comparable and correlative to that recorded
in Middle to Late Cambrian strata in southern Africa (Curtis 2001, Flowerdew et al.
2007), creating a sound basis for tectonostratigraphic correlation between the Ellsworth
Mountains and Cape Fold Belt, Africa, in early Paleozoic time (Curtis & Storey 1996,
Curtis 2001, Randall & MacNiocaill 2004, Flowerdew et al. 2007).
Fig. 3-5. Tectonic-stratigraphic column for the Ellsworth Mountains, from Curtis (2001) and Flowerdew
et al. (2007). The depositional age of 512 Ma for Union Glacier formation is indicated. Asterisk symbols
upon the column show the position of age determinations from clasts (e.g. Rees & Duebendorfer 1997),
used to constrain formations’ ages and reveal tectonic discontinuities. The solid lines with shear arrows
correspond to major tectonic breaks with direction of tectonic transport. Wavy lines mark sequence boun-
daries and dashed lines are for inferred stratigraphical contacts.
uncorrected proofs
3.2. Ellsworth-Whitmore Mountains Terrane 7
The Crashsite Group (Spörli 1992) consists of 3000m of varicolored orthoquartzites
and argillites accumulated in a shallow marine setting during Late Cambrian through
Devonian time (Fig. 3-5). Uppermost in the Paleozoic succession are the Carboniferous
to Permian Whiteout Conglomerate and Permian Polarstar Formation. The Whiteout
Conglomerate (Matsch & Ojakangas 1992) is massive glacial diamictite up to 1000m in
thickness, deposited during Carboniferous-Permian Gondwanide glaciation. Silicic and
carbonate clasts, generated and transported by ice sheets, have East Antarctica provenan-
ce. Characteristics of some deposits indicate direct emplacement by glacier ice whereas
others suggest deposition from oating ice and currents in proximity to the glacial groun-
ding line (Matsch & Ojakangas 1992). The Polarstar Formation (Collinson et al. 1992,
Elliot et al. 2015) consists of argillite in the lower part, coarsening-upward argillite to
sandstone cycles with lenticular to aser bedding in the middle, and ning-upward cross-
bedded channel sandstones in the upper portion. Capping the sequence are carbonaceous
strata and siltstones containing Glossopteris plant fossils. The formation contains abun-
dant volcanogenic detritus, attributed to sources on the Gondwana convergent margin.
New U-Pb geochronology data for igneous zircon from the ashbeds and youngest detrital
grains from sandstone and argillite provide evidence that the Polarstar Formation is ent-
irely Permian, accumulated in the interval 270 to 258 Ma (Elliot et al. 2015).
The Whitmore Mountains (Figs. 3-1, 3-3) consist of Early Jurassic Mount Seelig gra-
nite (Webers et al. 1992a) that intruded arkosic turbidite rocks; the feldspathic sedimen-
tary rocks form a metamorphic aureole and roof pendants within the granite (Storey &
Macdonald 1987). High grade assemblages occur at one locality, Mt. Woolard, where mig-
matitic garnet-biotite-plagioclase paragneiss and two-pyroxene amphibolite gneiss are
found (Storey & Dalziel 1987). Recent investigation of U-Pb age and Hf isotope charac-
teristics of detrital zircon from the paragneiss point to derivation from proximal sources,
and a correlation to rift sediments of the Heritage Group in the Ellsworth Mountains
(Flowerdew et al. 2007). High uppermost mantle velocities beneath the Whitmore region,
indicative of cool, viscous, older sublithospheric mantle (Chaput et al. 2014), however,
show that the Whitmore Mountains merits designation as a tectonic subprovince.
The oldest gneisses in West Antarctica crop out at Haag Nunataks, northeast of the
Ellsworth Mountains. Rb-Sr whole-rock isochron ages are 1176±76 Ma for the protolith,
and a minimum age is provided by a crosscutting microgranite of 1058±53 Ma (Millar
& Pankhurst 1987). This occurrence is the single known exposure of Precambrian rocks
in West Antarctica. Recent aerogeophysical surveys of this region (Jordan et al. 2013)
indicate the presence of Haag-type gneisses within the Ellsworth Mountain block, and a
greater extent of Proterozoic crystalline basement rocks than had been previously known.
Deformation during the Permian-Triassic Gondwanian orogeny imparted folds and
cleavage upon the Cambrian to Permian strata of the Ellsworth Mountains (Fig. 3-4)
(Curtis 2001, Curtis et al. 2004), and fabrics in Whitmore Mountains gneisses. The rst
phase of deformation, D1, is marked by N-S to NE-SW trending thrust faults and meso-
scale folds with axial planar cleavage. D2 deformation commenced upon bedding-parallel
thrust faults, accompanied by intraformational imbrication and duplexing, and myloniti-
zaton upon faults. As deformation progressed, intense folding and a penetrative regional
cleavage developed that overprinted or transposed D1 and early D2 structures in discrete
zones, accompanied by a change from dip slip reverse to dextral transpression. Dominant
NW-SE trending folds are a product of this phase (Fig. 3-4). As emphasized, Cambrian
through Permian strata of the Ellsworth-Whitmore Mountains terrane are affected, making
it plain that the deformation events are Permian or younger. There is a paucity of evidence
of the effects of the end-Cambrian Ross Orogeny (see chapter 4: Goodge, this volume).
uncorrected proofs
8 3. The Geology of West Antarctica
This lack of evidence of deformation arising from the Ross Orogeny has been much de-
liberated (Curtis 1998, Duebendorfer & Rees 1998, Curtis 2001 and references therein),
because Middle to Late Cambrian contractional to transpressional deformation did affect
other sites in this broad region (see Chapter 4: Goodge, this volume), such as the Stewart
Hills, Thiel Mountains, and Pensacola Mountains, that contain potentially correlative stra-
ta (Rees & Duebendorfer 1997, Duebendorfer & Rees 1998, Curtis et al. 2004). Recent
detrital zircon investigations (Flowerdew et al. 2007), however, show clear distinctions in
respect to U-Pb age and Hf isotopes for the Heritage Group of the Ellsworth Mountains,
compared to the other locations that share an afnity. The detrital zircon evidence, toge-
ther with the correlation to the contemporaneous Middle to Late Cambrian continental
rift evolution for the Cape fold belt (Curtis 2001) resolve the dilemma. The continental
rifting and mac volcanism in the Ellsworth Mountains and South African sectors of the
Gondwana margin likely occurred in a back-arc or inboard setting with respect to the sub-
duction margin (Fig. 3-3), which was undergoing extension at the same time that conver-
gent processes along the active margin affected the rest of the Transantarctic Mountains
(Curtis & Storey 1996, Rees & Duebendorfer 1997, Curtis 2001).
A later phase of intracontinental deformation during the Permo-Triassic Gondwanan
orogeny (Curtis 1998) and initial stages of breakup between East and West Gondwana
led to plate reorganization and the development of the Ellsworth-Whitmore Mountains
as a discrete terrane. The Jurassic plutonism in the Whitmore Mountains and Pirrit Hills
(Storey et al. 1988b, Lee et al. 2012), and inferred faults corresponding to well dened
magnetic lineaments (Jordan et al. 2013), are expressions of the early phase of breakup
in the Weddell Sea sector. Disparities in the geometry of fold-thrust structures and of pa-
leomagnetic pole positions indicate that the Ellsworth block moved independently from
South Africa and East Antarctica, and underwent a ca 90° anticlockwise rotation with
respect to its original position along the Gondwana margin (Fig. 3-3), and contempora-
neous units in the Transantarctic and Pensacola Mountains (Dalziel et al. 1987, Grunow et
al. 1987, Randall & MacNiocaill 2004). Some amount of lateral translation for the block
is called upon to explain the absence, in the Ellsworth Mountains, of the effects of de-
formation and magmatism that occurred in the Transantarctic and Pensacola Mountains,
to which the Ellsworth Mountains restore (Curtis 2001; chapter 4: Goodge, this volume).
The kinematics and amount of lateral translation is debated, however (Grunow et al.
1987, Curtis et al. 1999, Randall & MacNiocaill 2004, Jordan et al. 2013). It is possible
that the newly recognized wrench systems bounding the Ellsworth-Whitmore Mountains
terrane (Jordan et al. 2013) accommodated this displacement, however this interpreta-
tion is difcult to test directly due to the cover of the Antarctic ice sheet. According to
results from apatite ssion-track analysis of samples from a 4.2-kilometer vertical trans-
ect upon the west side of the Vinson Massif (Fitzgerald & Stump 1991), bedrock uplift
in the Ellsworth Mountains commenced after the initial separation of East and West
Gondwana and coincided with the opening of the Weddell Sea in the Early Cretaceous
Period (Fitzgerald & Stump 1991, Fitzgerald & Stump 1992). Mountain relief has been
sustained in the Ellsworth Mountains since that time.
3.3. Marie Byrd Land
Marie Byrd Land is a product of the Paleozoic and Mesozoic convergent accretionary
margin of East Gondwana (Figs. 3-3, 3-6) (i.e. Collins 2002). The terrain contains the
most complete record of the growth and stabilization of the new continental crust of West
uncorrected proofs
3.3. Marie Byrd Land 9
Antarctica (Antarctic Peninsula, excluded: see Chapter 2 of this volume for an overview
of the development of the Antarctic Peninsula terrane) provided by four principal rock
associations. The geological associations are: 1) Cambrian–Ordovician turbiditic sedi-
mentary rocks, several kilometres in thickness (Fig. 3-7a) (Bradshaw et al. 1983, Adams
1986), 2) Paleozoic calc-alkaline metaluminous to peraluminous plutonic rocks (Fig.
3-7b) (Weaver et al. 1991, Pankhurst et al. 1993, Pankhurst et al. 1998, Mukasa & Dalziel
2000, Siddoway & Fanning 2009, Yakymchuk et al. 2015), 3) migmatite-granite com-
plexes that represent exhumed middle crust (Fig. 3-7c) (Richard et al. 1994, Smith 1996,
Mukasa & Dalziel 2000, Siddoway et al. 2004b, McFadden et al. 2010a, McFadden et
al. 2010b, Korhonen et al. 2010a, Korhonen et al. 2010b); and 4) Mesozoic-dominantly
Cretaceous-alkalic to calc-alkaline granites and mac dikes (Fig. 3-7d) (Weaver et al.
Fig. 3-6. (a) Geographic map of the Ford Ranges in western Marie Byrd Land, with locations of features
mentioned in the text. The inferred boundary between the Ross Province and Amundsen provinces is as
identied by Pankhurst et al. (1998). (b) Tectonic reconstruction of the Cretaceous convergent margin
East Gondwana, from Yakymchuk et al. (2015); drawing upon elements rendered in Eagles et al. (2004),
Mortimer et al. (2006), and Veevers (2012). Abbreviations are RP: Ross Province, AP: Amundsen Provin-
ce, and TI: Thurston Island terrane.
uncorrected proofs
10 3. The Geology of West Antarctica
1991, Weaver et al. 1994, Adams et al. 1995, Mukasa & Dalziel 2000, Siddoway et al.
2005, Saito et al. 2012, Yakymchuk et al. 2013). The most extensive rock exposures of
these units are in the Ford Ranges of western Marie Byrd Land (Fig. 3-1).
As a consequence of distinctions in bedrock units, timing of plutonism, Nd model ages,
and paleomagnetic characteristics, Marie Byrd Land is divided into two tectonic provin-
ces (Fig. 3-6). The Ross Province consists of voluminous Cambrian turbidites and ysch
of the Swanson Formation, intruded by 375–345 Ma calc-alkaline granitoids of the Ford
Plutonic suite that have Nd model ages of 1.5–1.3 Ga (Pankhurst et al. 1998). Detrital
zircon U-Pb age distributions from Swanson Formation and metamorphosed equivalents
contain a signicant age component of ca. 1.2–1.0 Ga (Yakymchuk 2014, p. 174). The
Amundsen Province consists of plutonic rocks of Ordovician–Silurian (450–420 Ma)
and Permian-Triassic age, that yield younger Nd model ages of 1.3–1.1 Ga (Pankhurst
et al. 1993, Pankhurst et al. 1998); sedimentary rocks are lacking. Both provinces host
Cretaceous calcalkaline to alkaline plutonic rocks (Weaver et al. 1991, McFadden et al.
2010a, Yakymchuk et al. 2013, Brown et al. 2016) and Miocene to Present mac alkalic
volcanoes (LeMasurier & Rocchi 2005, LeMasurier et al. 2011). Based on limited evidence
Fig. 3-7. Photographs illustrating the principal rock formations of Marie Byrd Land. (a) Dark-colored tur-
bidite sedimentary rocks of the Swanson Formation, along-strike view along the northeast-dipping limb of
a steep, upright fold. Strata are metamorphosed to lower greenschist grade, associated with development
of axial planar cleavage and quartz veins (Photo: C. Yakymchuk). Inset is an equal area stereographic
plot of planes of Swanson Formation bedding (solid lines) and average planes of cleavage (dashed lines)
corresponding to NW-SE oriented symmetrical tight upright folds. (b) Phaneritic, unfoliated main phase of
the Ford Granodiorite suite, containing dark roof pendants of Swanson Formation, southern Ford Ranges.
(c) Granite-migmatite association of the Fosdick Mountains gneiss dome, view to east along the north
ank of the Fosdick range. (d) Byrd Coast Granite, Bowman Peak, Edward VII Peninsula.
uncorrected proofs
3.3. Marie Byrd Land 11
from Re–Os isotopic investigation of lherzolite and harzburgite xenoliths from young
volcanoes, central Marie Byrd Land appears to be underlain by Amundsen Province-type
crust: Re–Os model ages for lithosphere stabilization are ca. 1.1 to >1.3 Ga (Handler et
al. 2003), consistent with Proterozoic Nd model ages obtained for Marie Byrd Land gra-
nites and orthogneisses (Pankhurst et al. 1998). Paleomagnetic observations support the
presence of a tectonic boundary between the two distinct provinces (DiVenere et al. 1996,
Luyendyk et al. 1996), in the vicinity of Land Glacier (Fig. 3-1), however the orientation
is unknown due to the extensive ice cover and the lack of geophysical data in that area.
3.3.1. Ross Province
The oldest exposed rocks in the Ross Province are the Swanson Formation, voluminous
Cambrian–Ordovician turbidites and ysch (Fig. 3-7a) of the type that dominates the
sedimentary record of Gondwana’s proto-Pacic margin (Figs. 3-3, 3-6). The source of
the voluminous sediments of East Gondwana was the Ross-Delamerian orogen (Ireland
et al. 1994, Adams et al. 2005, Adams et al. 2014; chapter 4: Goodge, this volume). The
Swanson Formation is intruded by 375–345 Ma calc-alkaline, I-type plutons and asso-
ciated granites that comprise the Ford Plutonic Suite (Fig. 3-7b) (Pankhurst et al. 1998,
Siddoway & Fanning 2009, Yakymchuk et al. 2015) and by Cretaceous alkali granites of
circa 115 to 97 Ma (Fig. 3-7c) (zircon U-Pb ages: Siddoway et al. 2004a, McFadden et
al. 2010a, Yakymchuk et al. 2013). These occur as plutons intruding Swanson Formation
and Ford intrusives, and as stacked, thick sills and discordant dikes in the Fosdick
Mountains. The Ross Province appears to extend at least as far south as Roosevelt Island
and Siple Dome, based on the presence of 370–330 Ma (Ford suite provenance) and
110–100 Ma (derived from Cretaceous arc or A-type magmatic rocks) U-Pb age popula-
tions in detrital zircon sampled from sub-ice stream sediment-age groups that are absent
from subglacial tills of East Antarctic ice streams (Licht et al. 2014).
Detrital zircon provenance is central to the understanding of sediment transport
and tectonic relationships for the Gondwana margin in early Paleozoic time. Swanson
Formation and its correlatives in Australia (Lachlan Group), northern Victoria Land
(Robertson Bay Group), and New Zealand (Greenland Group) represent sedimentary de-
tritus eroded from and accumulated outboard of the Cambrian Delamerian–Ross Orogen
(Flöttmann & Kleinschmidt 1993, Ireland 1992, Ireland et al. 1994, Ireland et al. 1998,
Adams 2004, Veevers & Saeed 2011; chapter 4: Goodge, this volume). The regionally
extensive greywackes contain some common detrital zircon U-Pb age populations of ca.
650–490 Ma and ca. 1.2–1.0 Ga, and a scattering of small Archean age groups (e.g. Adams
et al. 2014, Hawkesworth & Kemp 2006). New Hf and O isotope analyses for detrital
zircon from Swanson Formation and equivalent metasedimentary rocks allow the inter-
pretation of provenance to be rened (Fig. 3-8, Yakymchuk et al. 2015). Three principal
age populations in western Marie Byrd Land include Neoproterozoic–Cambrian zircons
with evolved Hf isotope values that indicate derivation from reworked Mesoproterozoic
crust, interpreted to be igneous and metamorphic rocks of the Ross–Delamerian Orogen;
Mesoproterozoic detrital zircons with juvenile Hf isotope values that are consistent with
derivation from crust resembling the gneiss of Haag Nunataks (Fig. 3-8; see section
3.2., above) or a sedimentary derivative Haag-type crust that is concealed beneath the
Antarctic ice sheet; and Paleoproterozoic zircons sourced from crystalline basement of
the sort that is exposed in the central Transantarctic Mountains (Yakymchuk et al. 2015).
All are of continental derivation (e.g. Glen 2005, Gelen 2013).
uncorrected proofs
12 3. The Geology of West Antarctica
Fig. 3-8. Histograms and
normalized probability dis-
tribution curves (inset) of
detrital zircon U–Pb ages
obtained from Swanson
Formation (A–C) and me-
tasedimentary rocks (D–G)
of western Marie Byrd
Land. Diagram is from Ya-
kymchuk et al. (2015).
uncorrected proofs
3.3. Marie Byrd Land 13
A current model for formation of the thick turbidites of the Australian Tasmanides, the
Lachlan Group, involves deposition as submarine fan complexes in a back-arc or margi-
nal oceanic basin, followed by tectonic imbrication, thickening, and low grade metamor-
phism due to accretion to the continental margin (Foster et al. 2009, Glen 2013). As for
the Lachlan, so was the succession of events for the immature sedimentary strata in other
sectors of Gondwana’s active margin, albeit during different intervals of early Paleozoic
time. In the Ross Province, folding and low greenschist grade metamorphism affected
the Swanson Formation in the Late Ordovician, between 450–421 Ma (Adams 1986,
Adams et al. 1995, Kleinschmidt & Petschick 2003). Kilometres-scale upright folds and
cleavage in the Swanson Formation trend NW-SE, and folds plunge gently to the NW
(Fig. 3-7a) (Wade & Couch 1982, Siddoway & Fanning 2009). If the Tasmanides model
applies broadly to the Gondwana margin and Ross Province, then the cleavage develop-
ment and folding of Swanson Formation reects accretion and telescoping of the Ross
Province of Marie Byrd Land during Paleozoic plate convergence along the Gondwana
margin (Boger 2011).
Calc-alkaline, primarily I-type plutonism became established in the Ross Province
from 375–345 Ma, in the latest Devonian to Carboniferous (Pankhurst et al. 1998,
Siddoway & Fanning 2009, Yakymchuk et al. 2015). The intrusives form the Ford
Granodiorite suite (Fig. 3-7b) and are a product of convergent margin arc magmatism
(Weaver et al. 1991, Pankhurst et al. 1998, Yakymchuk et al. 2015) that was widespread
in East Gondwana (Tulloch et al. 2009, Borg et al. 1986, Borg et al. 1987, Fioretti et al.
1997a, Fioretti et al. 1997b, Chappell et al. 1988). Hf and O isotope compositions of
zircons from the Ford Granodiorite record the mixing of a juvenile magma with melts
derived from Swanson Formation, and an increase in the Swanson-derived component in
younger members of the Ford Suite (Yakymchuk et al. 2015). Somewhat elevated δ18O
values of zircons from Ford Granodiorite are attributed to the Swanson metasedimentary
components that were subjected to surface processes such as weathering and sediment
transport (e.g. Hawkesworth & Kemp 2006).
Consistent with the geochemical and isotopic evidence for crustal melting of the
Swanson Formation, mid-crustal exposures of in the Fosdick Mountains (Fig. 3-7c) con-
sist of migmatitic paragneiss derived from Swanson Formation (Siddoway et al. 2004b,
Yakymchuk 2014). Peak conditions at ca. 360 Ma entailed temperatures in excess of 800
°C, and pressure P >750 MPa (7.5 kbar) (Fig. 3-9a) (Korhonen et al. 2010b, Yakymchuk
2014), corresponding to crustal depths of 25 km or greater. Metasedimentary rock pro-
tolith compositions yielded melt volumes up to 25%. Under sustained high temperature
conditions within the Fosdick complex, anatexis of the Devonian granodiorites in the
mid-crust (early Ford Granodiorite suite) also occurred, producing 2 to 10 volume per
cent melt from the breakdown of hornblende and plagioclase at temperatures above bio-
tite stability (Korhonen et al. 2010a, Korhonen et al. 2010b). Material derived from both
sources surpassed the melt connectivity transition of ~7 vol.%, leading to development
of melt extraction pathways and melt loss from the midcrustal source region (Fig. 3-9a).
Peraluminous granites of ca. 330 Ma in the Ford Ranges are possible derivatives of the
crustal melting (Pankhurst et al. 1998, Korhonen et al. 2010b, Brown et al. 2016).
The temporal trend in source compositions in Paleozoic plutonic rocks in the Ford
Ranges indicates an advancing or neutral accretion mode along the Gondwana con-
vergent margin in Marie Byrd Land (Yakymchuk et al. 2015) that contrasts with the
rapid extension mode documented in same-aged granitoids in Western Province of New
Zealand (Tulloch et al. 2009).
uncorrected proofs
14 3. The Geology of West Antarctica
Fig. 3-9. Pressure-temperature (P-T) conditions
for regional metamorphism in Marie Byrd Land,
based on mid-crustal exposures of the Fosdick
Mountains migmatite-granite complex, exhumed
during Cretaceous detachment faulting and gneiss
dome formation (McFadden et al. 2010a). P-T
pseudosections are from Korhonen et al. (2011).
The thin dashed lines are melt isopleths in mol.%
(~vol.%); the thick dashed lines represent the soli-
di. Over the course of the model run for melt loss,
the solid shift to higher temperatures because
compositions become more residual due to melt
loss. (a) Pseudosection used to characterize M1,
regional metamorphism affecting western Marie
Byrd Land during Devonian–Carboniferous plate
convergence and orogenic I-type magmatism. Bulk
composition for the sedimentary protolith was esta-
blished from whole rock analyses of representative
Swanson Formation samples. The prograde path
(dotted line) within the sillimanite eld is based
on petrological evidence (Korhonen et al. 2010b).
Melt-loss events ML1 and ML2 (black circles) cor-
respond to points of modeled melt production of 7
mol.% (~vol.%). (b) Pseudosection calculated for
Cretaceous high temperature metamorphism (M2),
modeled on the basis of representative Swanson
Formation composition modied by Devonian–
Carboniferous melt loss, M1. The bold line delimits
the peak eld for M2 metamorphism, and the box
denotes the pseudosection detail in Fig. 3-9c. (c)
Expanded area from (b), contoured for mol.% melt,
garnet and cordierite. The interpreted Cretaceous
P–T path is shown by the heavy solid line.
uncorrected proofs
3.3. Marie Byrd Land 15
Subsequent to the Paleozoic convergent margin, there is a gap in the geological record
in Marie Byrd Land between 320 Ma (Mukasa & Dalziel 2000, Pankhurst et al. 1998)
and 143 Ma, when calc-alkaline magmatism recommenced (Adams 1987, Weaver et
al. 1991, Weaver et al. 1992). In the neighboring Thurston Island/Eights Coast terrane,
there is a fuller record of Mesozoic magmatism (Riley et al. 2017b), that is summarized
below. Contemporaneous I-type magmatism, with emplacement of leucogranite to mon-
zogranite occurred across both tectonic provinces of Marie Byrd Land (Pankhurst et al.
1993, Mukasa & Dalziel 2000), linked to subduction of the Phoenix oceanic plate and
possibly other oceanic microplates (Sutherland & Hollis 2001, Seton et al. 2012, Wobbe
et al. 2014). Geochemical characteristics, including pronounced negative Nb anomal-
ies and large-ion lithophile element abundances, support the association to subduction
(Pankhurst et al. 1993, Weaver et al. 1994, Mukasa & Dalziel 2000).
In the Ross Province (Fig. 3-6) in middle Cretaceous time, regional A-type plutonism
succeeded arc-related I-type activity, leading to emplacement of the Byrd Coast granite
suite in western Marie Byrd Land (Adams 1987, Weaver et al. 1991, Weaver et al. 1995,
Brown et al. 2016). A-type granites (Whalen et al. 1987) have distinctive geochemistry
in respect to major elements (for example, high SiO2 and Na2O+K2O; low CaO and Sr),
discriminants such as Y, Nb, and Ce, and ratios of Ga/Al and Fe/Mg. The isotopic and
geochemical characteristics reect the derivation of A-type granites from partial melting
of an enriched granulitic lower crust that is a residual of orogenic I-type magmatism.
A-type granites commonly occur within the tectonic context of intracontinental exten-
sion, and such is the case for the Byrd Coast Granite (Fig. 3-7d) (Siddoway et al. 2004a,
Siddoway 2008, McFadden et al. 2015).
The main phase of Byrd Coast Granite plutonism occurred from 115 to 95 Ma, deter-
mined from U-Pb zircon geochronology (Siddoway et al. 2004a, McFadden et al. 2010b,
Yakymchuk et al. 2013, Brown et al. 2016) and whole rock Rb-Sr isochron ages (Adams
1987), for sites in the Ford Ranges and Edward VII Peninsula. On the basis of aeroma-
gnetic evidence Byrd Coast Granite plutons are interpreted to exist in the subglacial en-
vironment (Ferriaccioli et al. 2002, Luyendyk et al. 2003). Active tectonism is recorded
along the entirety of the margin of the proto-Pacic Ocean, during this interval of the
Cretaceous Period, in Antarctica (Siddoway 2008, Vaughan et al. 2012), in Zealandia
(Weaver et al. 1992, Muir et al. 1995, Waight et al. 1998), in Patagonia (Hervé et al.
2007), and along the North American Cordillera (Paterson & Ducea 2015). Regionally,
emplacement of alkalic mac dykes overlapped in time with Byrd Coast Granite plu-
tonism (Siddoway et al. 2005, Saito et al. 2012), as did mid-crustal anatexis leading to
gneiss dome emplacement in the footwall of the Fosdick Mountains detachment zone
(see below).
In the case of mid-Cretaceous A-type plutonism in Ross province, there is strong
direct evidence for renewed melting of lower middle crust, at granulite facies conditi-
ons (Korhonen et al. 2010a, Korhonen et al. 2010b, Korhonen et al. 2011, Brown et al.
2016). K-feldspar rich anatectic granites within the Fosdick Mountains have Sr, Nd, Hf
and O isotope values consistent with derivation from Ford Granodiorite suite and meta-
Swanson Formation represented by residual migmatite gneisses in the Fosdick complex
(Yakymchuk et al. 2013, Brown et al. 2016) that experienced T > 800 °C at P ~700
MPa (7.0 kbar) (Figs. 3-9b, 3-9c). Cretaceous anatectic granites within the gneiss dome
display cumulate structures, and have variable major oxide and trace element concentra-
tions, with low SREE contents and common positive Eu anomalies (Brown et al. 2016).
They are interpreted as evidence of fractional crystallization and the accumulation of
early-crystallized feldspar and quartz within intrusions within the source and transfer
uncorrected proofs
16 3. The Geology of West Antarctica
zone, that are genetically linked to Byrd Coast intrusions at higher crustal levels in the
Ford Ranges (Brown et al. 2016). Dolerite dikes of the Ross province give a wider a
range of ages from 142 to 96 Ma (Siddoway et al. 2005, Saito et al. 2012), with oldest
dike ages matched by an early phase of Byrd Coast alkalic granite that is ca. 142 Ma in
the Allegheny Mountains and ca. 131 Ma at Mt. Corey (Fig. 3-6a) (Adams 1987). Byrd
Coast Granite elsewhere in the Ford Ranges gives ages of ca. 105–103 Ma and ca. 99
Ma (Richard et al. 1994, Yakymchuk 2014), and in Edward VII Peninsula is 103–98 Ma
in age (Mukasa & Dalziel 2000, Siddoway et al. 2004a). The plutonism coincided with
transtension to extension during a transition from oblique convergence to oblique exten-
sion occurred along the Cretaceous East Gondwana plate margin (Siddoway et al. 2004a,
Siddoway et al. 2005, McFadden et al. 2010a, McFadden et al. 2010b, McFadden et al.
2015), possibly induced in part by oceanic ridge-trench interaction (Bradshaw 1989,
Weaver et al. 1994, Luyendyk 1995).
This generation of alkalic dikes and syeno-granites was emplaced over a wide region
of the Amundsen Province, also (Mukasa & Dalziel 2000, Weaver et al. 1994, Wandres
& Bradshaw 2005). Timing is established from a dolerite dike swarm emplaced at 107±5
Ma, followed closely by 102–95 Ma syenite and alkalic granite (Storey et al. 1999), mar-
king the rapid change to transtension- and rift-related alkalic magmatism.
3.3.2. Amundsen Province
The Amundsen Province of central and eastern Marie Byrd Land (Pankhurst et al. 1998)
contains a record of Ordovician-Silurian (450–420 Ma) and Permo-Triassic plutonism,
comparable in timing to that of Thurston Island (see below) and the Antarctic Peninsula
(chapter 2, Smellie: this volume). Sedimentary precursors are found in the form of pa-
ragneiss. The gneisses of central versus easternmost Marie Byrd Land yield distincti-
ve populations of detrital zircons, with respect to U-Pb age range and abundance. The
central region yields ages of 270 to 400 Ma (Patton Bluff and Mt. Petras), whereas the
basement rocks underlying the late Miocene Mt. Murphy stratovolcano (Fig. 3-1) range
from circa 480 to 1000 Ma (Fig. 3-10; Pankhurst et al. 1998). The biotite-muscovite pa-
ragneiss and granodiorite orthogneiss at Mt. Murphy are the oldest known constituents
of the Amundsen Province. The U-Pb zircon emplacement age for the plutonic precursor
of the orthogneiss is 505± 5 Ma, and Nd isotope model ages of 1.3 to 1.1 Ga indicate the
presence of a Precambrian lithosphere at depth (Pankhurst et al. 1998). The isotope ages
and geological history of the Amundsen Province contrast with Thurston Island, to the
east (see below). Deep, narrow subglacial troughs, occupied by ice streams, separate the
terranes (Jordan et al. 2010).
The single macrofossil occurrence in Marie Byrd Land comes from the Amundsen
Province. Considered to be late Devonian in age, the fossils are plant impressions and
carbonized plant remains, found within glacial erratics of meta-argillite that were depo-
sited upon a granite nunatak, Milan Rock, along a headward segment of Land Glacier
(Grindley et al. 1980). The erratics consist of dark carbonaceous slaty argillite, ascri-
bed to a freshwater deltaic environment, that was tentatively assigned to the Swanson
Formation (Grindley et al. 1980). The recognition of fossil material from the lycopsid
Haskinsia colophylla, of Middle Devonian age (Xu & Berry 2008), places the correla-
tion of the meta-argillite in question, because pre-late Swanson Formatoin is pre-late
Ordovician (Adams 1986, Kleinschmidt & Petschick 2003).
uncorrected proofs
3.3. Marie Byrd Land 17
Permian-aged arc intrusives (276±2 Ma) underlie the Kohler Range, providing evi-
dence of longstanding convergence along the Gondwana margin (Pankhurst et al. 1998).
Their geochemistry is adakitic, suggesting derivation by melting of the subducted basal-
tic oceanic crust (Castillo 2006). The magmatic system is a potential source of volca-
nic detritus now found within the Beacon Supergroup of the Transantarctic Mountains
(chapter 4: Goodge, this volume) and Crashsite Quartzite of the Sentinel Range (see
above). If true, the Marie Byrd Land terrane was in proximity to East Antarctica by
Permian time. A province boundary between eastern and western Marie Byrd Land (Ross
and Amundsen provinces; see Fig. 3-6) is inferred on paleomagnetic grounds between
the Ross and Amundsen provinces (DiVenere et al. 1996), but the region is nearly com-
pletely glaciated and there is an absence of airborne or marine geophysical data that
could reveal the position of the province boundary. The boundary is expected to have
a clear expression in the gravity and/or magnetic anomalies, judging from the situation
for comparable features imaged in the Ford Ranges (Luyendyk et al. 2003). Restoration
of the dextral component motion along the Cretaceous plate boundary (e.g. Sutherland
& Hollis 2001) potentially would place the Mesozoic magmatic arc of the Amundsen
province and Thurston Island outboard of the Ford Ranges (Mortimer et al. 2006, Kipf
et al. 2012).
The Amundsen province and Thurston Island terranes both record plate convergen-
ce and calc-alkaline plutonism from 124–110 Ma, arising from subduction of oceanic
crust (Storey et al. 1991, Pankhurst et al. 1998, Mukasa & Dalziel 2000, Riley et al.
2017b). Rock types span a range of compositions from hornblende-biotite granodiorite
to monzogranite. Plutons in central to eastern MBL are calcalkaline through adakitic arc
rocks, based on trace element criteria (Pankhurst et al. 1998). Anatectic granites from
a migmatite-granite association in the Demas Range yielded conventional U–Pb zircon
lower intercept ages of ca. 127–128 Ma and 118 to 113 Ma. Onset of A-type pluto-
nism in the Amundsen Province is believed to be diachronous, arising at 102 Ma in the
Fig. 3-10. Summary diagram showing detrital zircon age histograms of SHRIMP 238U-206Pb data for
paragneisses of the central and eastern Amundsen Province, from Pankhurst et al. (1998). The localities
sampled in the central Amundsen Province are Mt. Petras (solid white) and Patton Bluff (solid grey). The
two sites display a great deal of overlap in age; an expanded scale is therefore shown in the inset for those
two samples. Mt. Murphy (Fig. 3-1) yields detrital zircon age results (solid black pattern) that are utterly
dissimilar from the other Amundsen province samples, but similar to the U-Pb ages and abundance of
detrital zircons in the Swanson Formation (Fig. 3-8).
uncorrected proofs
18 3. The Geology of West Antarctica
west, near Land Glacier (Yakymchuk et al. 2015), and commencing at 96–94 Ma in the
east (Mukasa & Dalziel 2000). Further east in the Thurston Island terrane, calc-alkali-
ne magmatism continued during the same interval of ca. 96 Ma to 94 Ma, then ceased
(Mukasa & Dalziel 2000, Riley et al. 2017b). Intermediate to mac plutonic rocks are
the prevalent constituents of the rock exposed at Bear Peninsula in easternmost MBL,
but andesitic breccia also occurs (Pankhurst 1990, Pankhurst et al. 1998). Bear Peninsula
is separated from the next terrane to the east, the Thurston Island terrane, by the deep
narrow lineaments in the subglacial topography beneath the Pine Island and Thwaites ice
streams (Holt et al. 2006, Vaughan et al. 2006).
On the basis of granitoid petrogenesis and ages, the Amundsen Province displays an
afnity to the Takaka terrane and Median Tectonic Zone of Zealandia (Bradshaw et al.
1997, Pankhurst et al. 1998, Tulloch et al. 2009). Adakitic granites, which may be gene-
rated by melting of young subducted lithosphere, are found not only in the Amundsen
Province but in New Zealand in Cretaceous time (Muir et al. 1995, Wandres et al. 1998,
Allibone & Tulloch 2004) and lend support to the interpretation that the Phoen ix-Pacic
spreading ridge (Bradshaw 1989, Luyendyk 1995) and/or Hikurangi Plateau igneous
province (Storey et al. 1999, Mortimer et al. 2006, Kipf et al. 2014) encroached on the
Cretaceous subduction margin of Gondwana in this sector leading to plate reorganization.
3.3.3. Fosdick Mountains migmatite-granite complex and gneiss
dome
Singular access to areally extensive exposures of migmatite and granite, representative of
the lower middle and middle crust of the Ross Province, exists in the Fosdick Mountains
of the northern Ford Ranges (Figs. 3-1, 3-6a). The Fosdick range is a migmatite-granite
complex forming a gneiss dome, as was recognized by Wilbanks originally (1972). The
elongate, 80 x 15 km structure is delimited on the south by a S-dipping, dextral-oblique
detachment zone (McFadden et al. 2010b) and by an inferred steep, dextral strike-slip
zone on the north (Siddoway et al. 2005, McFadden et al. 2010a) (Fig. 3-11). Field- and
laboratory-based research of the migmatite-granite complex in the Fosdick Mountains
has led to advances in understanding of: a) the differentiation and stabilization of the con-
tinental crust of Marie Byrd Land (Siddoway & Fanning 2009, Korhonen et al. 2010a,
Korhonen et al. 2010b, Korhonen et al. 2011, Brown et al. 2011, Yakymchuk et al. 2013,
Yakymchuk et al. 2015, Brown et al. 2016), and b) the kinematics, timing, and dura-
tion of tectonic exhumation of the migmatite-granite association within a Cretaceous
detachment system (Richard et al. 1994, Siddoway et al. 2004a, Siddoway et al. 2005,
Siddoway 2008, McFadden et al. 2010a, McFadden et al. 2010b, McFadden et al. 2015).
Magmatic arcs within greywacke-dominated accretionary orogenic settings are effec-
tive granite production factories (Vielzeuf et al. 1990, Brown 1994), particularly along
advancing to transpression boundaries wherein crustal thickening occurs and deforma-
tion can enhance the physical and chemical segregation, migration, and coalescence of
melts (e.g. Hollister & Crawford 1986, Brown 1994). Within the Fosdick Mountains,
correlatives of the Ford Granodiorite suite and Swanson Formation were twice subjected
to granulite facies conditions that produced voluminous anatectic granite from bioti-
te-breakdown melting (Korhonen et al. 2011). The initial episode, broadly contempo-
raneous with Paleozoic arc magmatism, entailed conditions of T = 820–870 °C, P =
750 to 1150 MPa (7.5–11.5 kbar) (Fig. 3-9a), as determined from mineral equilibrium
modeling (Korhonen et al. 2010a, Korhonen et al. 2011). In paragneisses, a prevalent
uncorrected proofs
3.3. Marie Byrd Land 19
mineral assemblage is sillimanite-garnet-biotite±cordierite, associated with cordierite-
bearing anatectic granites (Siddoway et al. 2004b). This widespread residual assemblage
is not retrogressed, an indication of the extraction of anatectic melt and migration of
melt and uids out of the source region. Geochemical and isotope evidence from Ford
Granodiorite suite and isotope evidence from zircon extracted from diverse plutonic
phases in the Fosdick Mountains and Ford Ranges reect the processes of crustal melting
and differentiation that occurred in the Ross Province during middle Paleozoic oroge-
nesis (Pankhurst et al. 1998, Korhonen et al. 2010a, Korhonen et al. 2010b, Yakymchuk
et al. 2015).
Fig. 3-11. Three-dimensional block diagrams that illustrate the structures developed during melt-present
conditions in the middle crust in Marie Byrd Land during Cretaceous oblique convergence on the Gondwa-
na margin (McFadden et al. 2010a). Exposures of migmatite and granite in the Fosdick Mountains gneiss
dome reveal: a) an initial, strike-slip-dominated phase of deformation at 117 to 114 Ma, associated with
steep fabrics and vertical pathways for melt migration, and b) a subsequent stage of oblique divergence
from at 109 to 102 Ma, deformation and shows the Fosdick dome as a magma accumulation zone.
uncorrected proofs
20 3. The Geology of West Antarctica
The Cretaceous Gondwana margin (Fig. 3-6b) experienced oblique convergence bet-
ween the Phoenix oceanic plate and Gondwana (Sutherland & Hollis 2001, Larter et
al. 2002, Seton et al. 2012). Stretching axes oriented azimuth 074 – 254, oblique to the
convergent boundary, are determined from orientations of fold axes, mineral stretching
lineations, and the geometries of mac and felsic dike arrays in the Ford Ranges and wit-
hin the Fosdick Complex, corresponding to dextral transcurrent strain on land in the Ford
Ranges (Siddoway et al. 2004b, Siddoway et al. 2005). High angle faults are delineated
by steep geophysical gradients (Luyendyk et al. 2003) and contrasting mineral cooling
ages between ranges (Richard et al. 1994, Siddoway et al. 2004b, McFadden et al. 2015).
Transcurrent deformation coincided with a second stage of granulite facies metamor-
phism and anatexis/biotite breakdown in the Fosdick migmatite-granite complex, with T
= 830–870 °C and P = 600 to 750 MPa (6–7.5 kbar) (Korhonen et al. 2010a, Korhonen et
al. 2011). The steep foliation imparted by early wrench deformation provided a pathway
for granite migration to higher crustal levels at circa 117 to 114 Ma. Anatectic granites
within vertical dikes that crystallized at 117 to 114 Ma (U-Pb zircon) underwent vertical
shortening deformation. A change to oblique extension oriented 055°–235° led to the
development of dilatant zones into which anatectic granites migrated. Thick subhori-
zontal sheets of cumulate granite were emplaced and crystallized from 109 to 101 Ma,
coincident with onset of the normal-oblique South Fosdick detachment system. Stacked,
sheeted granite intrusions immediately underlie the South Fosdick detachment zone
(McFadden et al. 2010a, McFadden et al. 2010b). Within the ~2 km thick detachment
zone (Fig. 3-11), solid state mylonitic fabrics are successively overprinted by kinemati-
cally compatible discrete brittle faults. Together the sequential structures provide a re-
cord of unroong of the Fosdick dome (McFadden et al. 2010a, McFadden et al. 2010b);
this is corroborated by mineral cooling ages (McFadden et al. 2015).
The past decade of research has produced U-Pb zircon geochronology and 40Ar/39Ar
hornblende and biotite thermochronology at high spatial resolution for the Fosdick
Mountains (McFadden et al. 2015, and references therein) and the Ford Ranges (Richard
et al. 1994, Lisker & Olesch 1998, Contreras et al. 2012). In the Fosdick Mountains, U-Pb
zircon crystallization ages of 102 Ma for youngest anatectic granites coincide spatially
and temporally with 40Ar/39Ar cooling ages for hornblende (103 to 101 Ma) and biotite
(101 to 100 Ma) in granitoids. The data indicate cooling rates >100 °C/m.y. that are a
consequence of near-isothermal decompression and detachment-induced exhumation of
hot, deep migmatites and granites to shallow crustal levels where conductive heat loss
led to rapid cooling. A similar evolution of granite crystallization, exhumation, and rapid
cooling is documented for Edward VII Peninsula (Fig. 3-1) at ca. 100 Ma (Siddoway et
al. 2004a, Yakymchuk et al. 2015). Apatite ssion track (Richard et al. 1994, Lisker &
Olesch 1998) and (U-Th)/He zircon (Contreras et al. 2012) thermochronology research
yields a dominance of ages between 96 to 87 Ma for samples from upper elevations, and
scattered results down to 67 Ma from lower sites, reecting a broad regional extent of
Cretaceous tectonism in western Marie Byrd Land. The apparent absence of Cenozoic
cooling ages likely is a consequence of tectonic stability and stabilization of lithosphere
in western MBL since the time of breakup of MBL-Zealandia (Fig. 3-6b), an interpreta-
tion that is supported by the presence of thicker and faster lithosphere beneath this sector
of West Antarctica (Heeszel et al. 2016).
Pleistocene alkali basalt lavas occur in small centers and as subvertical dikes across
the Fosdick Mountains (Gaffney & Siddoway 2007). The lavas are of limited extent
but high importance because they are mineralogically and chemically diverse and con-
tain ultramac xenoliths. The lavas have incompatible element enrichment patterns that
uncorrected proofs
3.4. Thurston Island/Eights Coast 21
resemble those of the central and east Marie Byrd Land volcanoes (see below), but they
have differing Sr-Nd-Pb isotope compositions. The xenoliths (Chatzaras et al. 2013,
Chatzaras et al. 2016) represent a range of depths, and so may illuminate mantle conditi-
ons in zones of seismic anisotropy. The xenoliths display a great deal of variation in rock
fabric, deformation intensity and mechanisms of deformation, as well as a wide range of
equilibration temperatures of 780 to 1065 °C, based on pyroxene geothermometers and
olivine-spinel Fe-Mg exchange geothermometry (calculated at P = 1500 MPa [15 kbar]).
The wide range of temperatures of equilibration at approximately 50 km depth provides
an indication of heterogeneities in the mantle beneath MBL, consistent with the variation
in seismological characteristics that is being discovered for Marie Byrd Land (Accardo
et al. 2014, Lloyd et al. 2015, Heeszel et al. 2016). The subcontinental lithospheric mant-
le of once-neighboring Zealandia is similarly heterogeneous (Scott et al. 2014).
3.4. Thurston Island/Eights Coast
Sparse rock exposures in the Thurston Island–Eights Coast terrane (Figs. 3-1, 3-3) yield
a surprisingly detailed record of Late Carboniferous through Mesozoic calc-alkaline plu-
tonism (Grunow et al. 1991, Storey et al. 1991, Leat et al. 1993, Pankhurst et al. 1993,
Kipf et al. 2012, Riley et al. 2017b). Nd and Sr model ages indicate a maximum age of
1150–1400 Ma for underlying crust (Pankhurst et al. 1993), resembling the result from
granitic orthogneiss of Haag Nunataks (Millar & Pankhurst 1987). The earliest U-Pb
zircon isotopic age of circa 349 Ma for metaplutonic ‘basement’ rocks is very close in
age to the Carboniferous granites of the Ross Province, suggesting a genetic association
(Riley et al. 2017b). Subsequently, gabbro and diorites formed during Permian-Triassic
mac magmatism, at circa 239 Ma based upon U-Pb zircon geochronology (Riley et al.
2017b), potentially as a part of the broader convergent margin magmatism that is better
recorded in the Antarctic Peninsula. Those rock formations had previously been cons-
trained by 87Sr/86Sr determinations of 309±5 Ma and mineral cooling ages of 240–220
Ma (Leat et al. 1993, Pankhurst et al. 1993). Jones Mountain granite, emplaced at circa
200 Ma, ties the region to more widespread magmatism that occurred in the Antarctic
Peninsula and Ellsworth-Whitmore Mountains. The majority of Mesozoic I-type mag-
matism occurred in pulses, with one at ca. 182 Ma involving silicic volcanism, follo-
wed by activity at 157 to 145 Ma (Riley et al. 2017b) and 108–90 Ma. This Thurston
Island evidence contributes to the evidence of the broad regional magmatism in middle
Cretaceous time. Subduction of oceanic crust is indicated by initial 87Sr/86Sr ratios of
0.705–0.706 and εNdt values of +2 to –4, revealing the presence of juvenile lower crust
or slightly enriched mantle along the Andean-type subduction margin of Gondwana. The
Late Jurassic evolved magmas that dominate the record in Thurston Island resemble but
are temporally distinct from the middle Jurassic granitoids in other sectors of the con-
vergent margin such as the Antarctic Peninsula and Patagonia (Pankhurst et al. 1993).
A small contribution from crustal melts in Early Jurassic time is reected by musco-
vite-bearing granite with an age of 198±2 Ma, initial 87Sr/86Sr ratio of 0.710, and εNdt
values of –5 to –7. Silicic volcanism from 100 to 90 Ma in the Jones Mountains produced
lavas and tuffs that have 87Sr/86Sr initial ratios of 0.706–0.709 and εNdt values of –3 to –6
(Leat et al. 1993), suggesting crustal anatexis and a link to the intracontinental extension
within the West Antarctica rift system (cf. Mukasa & Dalziel 2000, Yakymchuk et al.
2015) rather than to the long-standing subduction margin. Mac dikes of Late Cretaceous
age (ca. 90 Ma; Leat et al. 1993), oriented parallel to the coast of Thurston Island, cut the
uncorrected proofs
22 3. The Geology of West Antarctica
older rocks. Granodiorite in the easternmost exposure of the terrane, at Lepley Nunatak,
has an initial 87Sr/86Sr ratio of 0.7066 and a low δNd value, suggestive of crustal contri-
butions. New U-Pb zircon data provide an age of ca. 108 Ma (Riley et al. 2017b), with a
constraint on cooling and thermal evolution provided by a 40Ar/39Ar biotite age of ca. 87
Ma (Pankhurst et al. 1993). The Jones Mountains also host pillowed alkalic basalts and
palagonitized volcaniclastic rocks (Hole & LeMasurier 1994) of Miocene age (~10 Ma)
that reach a thickness of 700 m. The primitive lavas have unradiogenic Sr-isotope ratios
of 0.703, radiogenic Nd, and low large ion lithophile element concentrations, indicative
of an aesthenospheric source (Hole & LeMasurier 1994).
Contemporary tectonism at the margins of the Thurston Island terrane is indicated by
Holocene volcanism in the Hudson Mountains (ca. 2 ka, Corr & Vaughan 2008), narrow
subglacial rifts (Bingham et al. 2012), geophysical and heat ow anomalies (Jordan
et al. 2010), and new thermochronology data that reveal differential cooling histories
across physiographic lineaments (Lindow 2014, Spiegel et al. 2016). The exhumation of
a gabbro intrusion from 3 km depth at ca 29 to 27 Ma (Rocchi et al. 2006) and apatite
thermochronometry data indicative of rapid bedrock cooling at 29–25 Ma (Lindow 2014,
Spiegel et al. 2016) are potential signs of fault reactivation or rift propagation along the
boundary between the Thurston Island and Antarctic Peninsula terranes.
The Thurston Island and Eights Coast blocks are separated by and bounded on the
west by tectonic basins. On the west, airborne gravity data (Cochran et al. 2015) reve-
al a well-dened sedimentary basin within the continental shelf of the Amundsen Sea
Embayment. The basin accommodated 80–100 km of extension (δ = 1.5 to 1.7) and
contains a thickness of approximately 6 km of sediment. The basin margin abuts the do-
minant granitic rocks of the Thurston Island and Eights Coast blocks, and cuts across a
system of east-west trending, stepped half-grabens that separate the two blocks. The gra-
bens correspond to an elongate depression formed by distributed, minor extension, that
now is occupied by seawater supporting the Abbott Ice Shelf (Bingham et al. 2012). The
grabens’ orientation parallel to the Cretaceous mac dyke array on Thurston Island (Leat
et al. 1993), and their gravity characteristics, suggest that they formed during the onset of
rifting between West Antarctica and Zealandia (Cochran et al. 2015). Breakup ultimately
localized along the north margin of Thurston Island and eastern Marie Byrd Land.
3.5. West Antarctic Rift Province
Covering an area of ~1.2 x 106 km2, the vast West Antarctic rift system (WARS) is
comparable to other active rift provinces on Earth from the standpoint of aerial extent
and range of geotectonic characteristics (LeMasurier 2008). The system’s interior mar-
gin exhibits some of the most dramatic topographic relief found on Earth’s continents
(Fig. 3-1), with a vertical exchange of 7700 meters between the deepest subglacial low
and highest neighboring summits. Within the WARS there are profound, narrow deeps,
including the Bentley subglacial trench and Byrd Subglacial Basin, that likely are struc-
turally controlled (Winberry & Anandakrishnan 2004, Jordan et al. 2010, Chaput et al.
2014). The deep troughs have been attributed to Oligocene transcurrent faults reactivated
under extension (Granot et al. 2013) or grabens formed during Cretaceous extension
(Trey et al. 1999). The Bentley subglacial trench coincides with the north boundary of
the Ellsworth-Whitmore Mountains terrane (Lloyd et al. 2015).
The region of the WARS is characterized by thin crust, high heat ow, active volca-
nism, and presence of low velocity, low viscosity, warm mantle (Behrendt et al. 1991a,
uncorrected proofs
3.5. West Antarctic Rift Province 23
Behrendt et al. 1994, Behrendt et al. 1996, Behrendt 1999, Ritzwoller et al. 2001, Danesi
& Morelli 2001, Pollard et al. 2005). It hosts active seismicity of moderate magnitude: in
2012, there were four tectonic earthquakes of M.4.4 to M5.5 in the southern Ford Ranges
of Marie Byrd Land (USGS Earthquake Hazards Program, accessed May 2015) – some
of the rst to be recorded within the continental crust of West Antarctica (cf. Reading
2006). Elevated heat ow of 83 to 126 mW/m2 occurs across the WARS (Blackman et
al. 1987, Berg et al. 1989), with ux as high as 285±80 mW/m2 at one locality (Fisher et
al. 2015). The WARS is the only active continental rift province on earth to host a polar
ice sheet during a time of rapid glaciological change, making it a feature of considerable
contemporary scientic interest.
West Antarctica’s subglacial topography now is known with improved resolution, sin-
ce the release of BEDMAP2 gridded surface elevation, ice-thickness, bathymetry and
subglacial bed elevation data (Fretwell et al. 2013). Surface topography is becoming
available for the rst time as a result of new methods of digital elevation model (DEM)
extraction from high-resolution stereoscopic Worldview 1 & 2 imagery (e.g. Porter et al.
2011, Noh & Howat 2015). Advances in characterization of the geophysical properties of
the lower lithosphere and aesthenospheric mantle are being achieved via the A-NET and
POLENET GPS and seismic networks upon Antarctica (http://polenet.org).
The international research efforts that are bringing West Antarctica into focus provide a
foundation for contemporary research in West Antarctica, banishing long-standing enig-
mas that exist for the dominantly ice covered region. These include: the character of West
Antarctica crust and lithosphere (e.g. Behrendt et al. 1996, Bell et al. 1998, Luyendyk
et al. 2003, Handler et al. 2003, Chaput et al. 2014); the tectonic context for Neogene
to present alkaline magmatism (Finn et al. 2005, Rocchi et al. 2006, Sutherland et al.
2010); the causes for Cenozoic tectonic reactivation (e.g. Rossetti et al. 2003b, Salvini
et al. 1997, Paulsen & Wilson 2010) and seismicity (Winberry & Anandakrishnan 2004,
Lough et al. 2013), and the inuence of inherited structures upon ice-ocean-bedrock
interactions of the dynamic West Antarctic ice sheet (Lowe & Anderson 2002, Vaughan
et al. 2006, Holt et al. 2006, Sorlien et al. 2007, Gohl 2011) during contemporary climate
change (Jordan et al. 2010, Bingham et al. 2012).
Knowledge of the Mesozoic tectonic evolution of the WARS within Gondwana also
is critical to understanding of geological complexity and regional variations arise from
structural inheritance (e.g. Lowe & Anderson 2002, Siddoway 2008, Vaughan et al. 2006,
Holt et al. 2006, Sorlien et al. 2007, McFadden et al. 2010b) and the Cretaceous to pre-
sent landscape evolution (LeMasurier & Landis 1996, LeMasurier 2006, Bialas et al.
2007, Huerta 2007, Wilson & Luyendyk 2006, Wilson et al. 2012). Mesozoic structures
reactivated within the late Miocene to present tectonic framework commonly coincide
with sites of dynamic ice-bedrock interactions for the West Antarctic ice sheet (Paulsen
& Wilson 2009, Granot et al. 2010, Granot et al. 2013, Gohl 2011, Kipf et al. 2014).
3.5.1. Character of crust and lithosphere
Crustal thickness <28 km for the rift province (Figs. 3-2, 3-12) is determined from seis-
mic refraction, seismic reection, and gravity data (Trey et al. 1999, Luyendyk et al.
2003, Wobbe et al. 2012), broadband seismic data (Winberry & Anandakrishnan 2004),
gridded effective topography (Müller et al. 2007; see also references therein) and seismic
receiver functions of P- to S-wave conversions (Chaput et al. 2014). These differing and
complementary methods all determine 21 to 28 km thicknesses for the broad arcuate
uncorrected proofs
24 3. The Geology of West Antarctica
Fig. 3-12. Schematic prole across Mary Byrd Land and the Ross Embayment (Tinto et al. 2018), view toward South, with Marie Byrd Land on left and Transan-
tarctic Mountains part sector of East Antarctica on right. Vertical exaggeration: 5x. Grey colors on left denote distal, marine, deeper water sediments derived from
Ross Orogen, and underlying lithosphere. White and pale grey colors denote terrigenous to shallow marine, proximal sediments derived from Ross Orogen, and
underlying basement of 600 to 170 Ma age, upon faulted lithosphere. Other crustal elements are as labeled on diagram. Two contrasting Moho proles are provi-
ded: black line, seismic discontinuity and velocity contrast of Trey et al. (1999); pink line, P- to S-wave conversion of Chaput et al. (2014).
uncorrected proofs
3.5. West Antarctic Rift Province 25
region that extends from the Ross Sea and Ice Shelf to the Thurston Island block, with
somewhat thicker crust, up to 32 km thick (Luyendyk et al. 2003, Chaput et al. 2014),
underlying the region of highest topography in central Marie Byrd Land.
Continental crust of the continent–ocean transition zone (Fig. 3-13) north of Marie
Byrd Land, that has only sparse volcanism, is 24 km thick, based on new marine gravity
and seismic reection data (Wobbe et al. 2012). Bordering the Ross Sea and western
Marie Byrd Land, the margin is steep and abrupt (Lawver & Gahagan 1994, Cooper et
al. 1991b), and the width of the zone is 145 km, quite narrow when compared to the 670
km wide transition zone in the Bellingshausen Sea, where there is abundant magmatism.
The age of the rst denitive oceanic crust is 84 Ma, based on the identication of ma-
gnetic anomaly c34y (McAdoo & Laxon 1997, Wobbe et al. 2012). Magnetic anomalies
mapped in the Amundsen Sea embayment, that are interpreted on the basis of gravity
characteristics to be mac intrusions, are attributed to continental rifting-related magma-
tism between 100–85 Ma, preceding breakup between Zealandia and Marie Byrd Land.
Extension factors are ß = 1.3 to 1.8 based on Müller et al. (2007) crustal thinning grid
derived from effective topography (sedimentary cover, removed) relationships. Thin
crust (~25 km) along the south ank of Marie Byrd Land corresponds to high topogra-
phy, with bedrock elevation 1000 m greater than is predicted by Airy isostasy (Winberry
& Anandakrishnan 2004, cf. LeMasurier & Landis 1996). The lack of isostatic compen-
sation corroborates the evidence from seismic velocity anomalies for low-density mantle
beneath southern MBL that contrasts with average-density mantle beneath the interior of
the rift (Winberry & Anandakrishnan 2004).
Fig. 3-13. Map of sediment thicknesses within marine shelf/basins bordering West Antarctica, from
Wobbe et al. (2014). Contours correspond to water depth, in meters, and isopach thicknesses are ac-
cording to the barscale. Sediment catchment areas are delineated by dashed lines; coverage area and
estimates of compacted sediment volume are given by histogram in upper left. DSDP sites are annotated
with black diamond symbol. Abbreviations: RS = Ross Sea, AS = Amundsen Sea, BS = Bellingshausen
Sea, AT = Adare Trough, DGS = De Gerlache Seamounts, MBS = Marie Byrd Seamounts.
uncorrected proofs
26 3. The Geology of West Antarctica
The most profound depths and thinnest crust correspond to the Byrd Subglacial Basin,
Pine Island Rift and Ferrigno Rift (Figs. 3-1, 3-2), where the depth to Moho is determi-
ned to be 19±1 km (Jordan et al. 2010, Bingham et al. 2012, Chaput et al. 2014). The
narrow dimension and linearity of the features are suggestive of steep structural geome-
tries associated with transform faults, an interpretation that is supported by kinematic
models derived from Cenozoic plate circuits (Müller et al. 2007, Granot et al. 2013). The
deep trenches and troughs therefore may have originated as releasing bends (Müller et al.
2007), or occupy strike slip faults that have been reactivated by extensional deformation
(Jordan et al. 2010, Bingham et al. 2012).
3.5.2. Bedrock structures’ inuence upon ice-ocean dynamics
A focus of much contemporary research is the deep, narrow bathymetric features that
underlie the West Antarctic ice streams in eastern Marie Byrd Land and Thurston Island,
because they coincide with sites of dynamic thinning of the West Antarctic ice sheet
(Jordan et al. 2010, Bingham et al. 2012). The narrow channels allow inltration/circu-
lation of warm ocean water beneath the ice sheets. A zone of elevated geothermal heat
ow is known to underlie one of these narrow rift areas so far (Schroeder et al. 2014).
Thermal anomalies are likely to correspond with reactivated faults within subglacial be-
drock and sites of Pleistocene-Holocene active volcanism (Wilch et al. 1999, Gaffney &
Siddoway 2007, Corr & Vaughan 2008, Narcisi et al. 2006, Lough et al. 2013). The ques-
tion of basal geothermal ux arising from warm mantle beneath thinned crust (Maule et
al. 2005, Shapiro & Ritzwoller 2004, Lawrence et al. 2006a, Lawrence et al. 2006b) is
of obvious consequence for ice sheet dynamics. There is pressing need to determine the
inuence of underlying crustal structures on heat ow, volcanism, basal melting, and
inux of marine waters, that have immediate effect on the stability of glaciological and
glacial-marine systems that are subject to atmospheric and ocean warming (e.g. Jordan
et al. 2010, Bingham et al. 2012, Feldman & Levermann 2015).
3.6. Tectonic Evolution and Timing of the WARS
3.6.1. Relationship between the Transantarctic Mountains and West
Antarctic rift system
The spatial correspondence of the WARS’ western and southern limits of thinned crust
with the dramatic mountain relief of the Transantarctic Mountains (Figs. 3-1, 3-12) has
naturally led to investigation the following questions: 1) did the high topography and
curvilinear front of the Transantarctic range originate at the same time and within the
same tectonic framework as broad extension across the WARS, during narrow extension
in the Terror Rift, or a combination of both? and 2) How thick and how high was the crust
of West Antarctic prior to extension?
There is ongoing debate and an array of different standpoints on these two questions.
The rst question has been investigated through apatite ssion track thermochronology
and multi-method thermochronometry (e.g. Glorie et al. 2012), and geodynamics, and
the second has been explored via kinematic and thermal-mechanical models and pa-
leotopographic reconstructions that track eroded sediment volumes. The sparse on-land
sedimentary deposits and small number of deep drill cores prevent the use of traditional
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 27
stratigraphic methods that would allow reconstruction of the uplift and exhumation his-
tory (e.g. Lisker & Läufer 2013). Thermochronology data (multiple 40Ar/39Ar mineral
systems, and low temperature thermochronology data from zircon (U-Th)/He, apatite
ssion track, and apatite (U–Th–Sm)/He systems), therefore, provide the current basis of
understanding of the stages of landscape evolution of the TAM-WARS.
Apatite ssion track ages for the TAM have been interpreted in terms of both diffe-
rential or possibly episodic denudation between crustal blocks separated by crustal-scale
structures, in the Cretaceous to Paleogene (Fitzgerald 1994, Fitzgerald 2002, and refe-
rences therein), or as a record of subdued topography and sedimentary burial throughout
the Mesozoic, followed by Cenozoic extension (Lisker & Läufer 2013).
Cretaceous episodes of cooling are recorded in south Victoria Land (Fitzgerald 2002,
and references therein), north of David Glacier (Balestrieri et al. 1997), and across a
transect of the Pacic margin of north Victoria Land (Lisker 2002), overlapping in time
with the formation of the Ross Sea epicratonic basins (Figs. 3-12, 3-14). Three to 4 km
of sediments accumulated in the generally north-south-oriented elongate basins, mac
magmatism occurred, corresponding to regions of high seismic velocities and gravity
anomalies at 6 to 11 km in the upper crust (Trey et al. 1999). Regional denudation of 2 to
3 km is interpreted for the Mesozoic, with a modest volume of eroded detritus provided
to Ross Sea basins and depocenters at the continent-ocean boundary (Karner et al. 2005,
Decesari et al. 2007, Wilson et al. 2012). A fairly wide range of ages span the broad
extension phase in the WARS, and overlap with the time of breakup between Australia-
East Antarctica and West Antarctica-Zealandia. Correspondingly, there must have been
Fig. 3-14. (a) Map of sedimentary basins (shaded grey) of the Ross Sea, from Decesari et al. (2007).
Basins are delimited by a depth to basement greater than 2.5 km (ANTOSTRAT 1995). Abbreviations are
VLB: Victoria Land Basin, NB: Northern Basin, CT: Central Trough, and EB: Eastern Basin. The location
of Deep Sea Drilling Program sites 270 and 272 are indicated with numerals. The bold line denotes the
location of seismic proles collected orthogonal to the basin structure (ANTOSTRAT 1995, Trey et al.
1999, Busetti et al. 1999). (b) Bathymetric prole with geo-tectonic interpretation of a modelled prole
that closely parallels the seismic transect indicated with a bold line in (a), from Busetti et al. (1999). The
tectonic basins contain Late Cretaceous (?) early rift sediments through Palaeogene basin ll/The basin
deposits and intervening basement highs are overlain unconformably by Neogene glacial and glacio-ma-
rine sediments. The Moho, designated with a solid line, is interpreted on the basis of refraction and OBS
data, and 2-D gravity models.
uncorrected proofs
28 3. The Geology of West Antarctica
a development of mountain relief in the TAM at circa 115 to 100 Ma, in agreement with
a new geodynamic model and plate tectonic reconstruction of East Gondwana by Jacob
& Dyment (2014). Using rst order conjugate features and pseudo-isochron/gravity
anomalies between Australia, East Antarctica and West Antarctica, Jacob & Dyment’s
(2014) reconstruction predicts lithospheric thickening and the development of topogra-
phic relief in a linear belt corresponding to the TAM. If correct, the reconstruction pro-
vides a plausible framework for the development of an orogenic plateau that has been
proposed by some (Bialas et al. 2007, Huerta & Harry 2007).
Apatite ssion track data indicate a rapid cooling interval in the Eocene Epoch, also,
attributed to rapid bedrock uplift at rates of 100 to 200 meters per million years (Sugden et
al. 1995, Busetti et al. 1999, Fitzgerald 2002) and denudation on the order of 4 to 10 km.
Tectonism related in part to seaoor spreading in the Adare Trough, where there was 160 km
of spreading between 40 and 26 Ma (Granot et al. 2013). The focused extension in the Adare
Trough within the Antarctic Plate was accompanied by distributed extension in the Ross
Sea basins (Karner et al. 2005), with 95 km of extension in the Victoria Land Basin (Davey
et al. 2006) and stretching of 40 km in both the Central and Eastern Basins (Decesari et al.
2007). Supporting evidence for this comes from thermal subsidence models of the eastern
Ross Sea’s broad marine platforms, regionally extensive surfaces that have been depressed
to depths of 100 m to 350 m below sea level (Wilson & Luyendyk 2006). Investigators of
the landscape evolution on the margins of the RIS (Lisker & Läufer 2013, Foley et al. 2013)
have recently challenged the established view that the topographic relief between TAM and
the West Antarctic rift province is a product of episodic uplift since 55 Ma, however.
A nal phase of tectonism that is recorded by thermochronology and basin sedimen-
tation is Oligocene-Miocene, from 34–23 Ma. The period is marked by formation of
prominent incised troughs separated by bedrock highs, interpreted as a product of intense
localized erosion after onset of continental glaciation at ca. 34 Ma (e.g. Sorlien et al.
2007, Lisker & Läufer 2013) or even later, at ca. 14 Ma when there was a shift toward
warm-based glaciation (e.g. Stern et al. 2005, Sugden et al. 2005). Seismic stratigraphic
records for the Ross Sea provide information about the volume of sedimentary detritus
transferred from ‘continental’ Marie Byrd Land to depocenters in the Ross Sea and on the
continental shelf (e.g. Wilson et al. 2012, Wobbe et al. 2012). The least-known Central
and Eastern basins contain records that reect large scale regional systems, whereas the
Victoria Land and Northern basins contain sediments derived from proximal sources in
the Transantarctic Mountains (De Santis et al. 1999, Wobbe et al. 2012).
Although there is a lack of concensus about the time of formation, origins of the
dramatic relief of the Transantarctic Mountains, and relationship to the West Antarctic
rift system, there nevertheless is an impetus to obtain a resolution to the issue due to the
need for tighter constraints on the timing and kinematics of relative motions within West
Antarctica for development of a robust tectonic plate circuit for the southwest Pacic
(Matthews et al. 2015, Granot et al. 2013, and references therein). A synthesis here fol-
lows of the geological and geochronological factors that bear on the extent and timing of
the West Antarctic rift system.
3.6.2. Mesozoic development
The origins of the West Antarctic rift system (WARS) date to the time of Pangaea brea-
kup and Jurassic crustal extension between East and West Gondwana. The emplacement
of Ferrar Dolerite (Fleming et al. 1997, Elliot & Fleming 2004, Fleming 2008, Leat
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 29
2008) and opening of the Weddell Sea (Grunow et al. 1987, Jokat et al. 2003, Veevers
2012) mark this initial stage that formed an early part of what became the WARS. The
Ferrar tholeiitic sills and associated volcanic rocks span a phenomenal 3500 km length
of the margin of East Antarctica, yet were emplaced in a span of just 1 million years at
circa 183 Ma (Fleming et al. 1997, Elliot & Fleming 2004, Leat 2008). The Weddell
Sea opened in two stages, as evidenced by marine magnetic anomalies identied from
marine geophysical surveys (König & Jokat 2006, Jokat 2007) and potential elds data
(Jordan et al. 2013). The rst deep continental rift basin to develop at circa 167 Ma Ma
was the Somali-Mozambique basin between Queen Maud Land, India-Sri Lanka, and
southern Africa (Mozambique). The connection from Africa (Natal Embayment) was
unbroken (Fig. 3-3). At ca. 147 Ma, slow separation between the Antarctic Peninsula,
southern South America and Africa commenced, with creation of true ocean oor in the
western Weddell Sea. Restricted sedimentation began, but communication between the
Weddell Sea and Somali-Mozambique basins was not yet established.
During the Cretaceous, a dramatic episode of intracontinental extension affected
central West Antarctica and Zealandia, that at the time were continguous within East
Gondwana. The term Zealandia refers to the entirety of the continental landmass of
New Zealand, including the submerged portions of the Campbell Plateau, Lord Howe
and Chatham Rises (Fig. 3-6b). In less than 20 million years, central West Antarctica
underwent 600 kilometres of extension, and across the Ross Sea through western Marie
Byrd Land,>1000 kilometres of stretching occurred (DiVenere et al. 1996, Luyendyk et
al. 1996, Storey et al. 1999). The broad basin beneath the Ross Sea and Ice Shelf for-
med, and the Bentley/Byrd troughs (Fig. 3-1) were created (Cooper et al. 1991b, Lawver
& Gahagan 1994, Davey & Brancolini 1995). In New Zealand, the Campbell Plateau/
Great South Basin developed (Fig. 3-6b) (Sutherland 1999, Grobys et al. 2009). Plate
separation between West Antarctic and Zealandia occurred along a trend orthogonal to
the rift basins (Siddoway 2008; see Fig. 3-6b). Tectonic restoration of the abrupt, steep,
rifted margins of the two land masses produces a tight t of the already-extended crust,
providing clear evidence that major extension across West Antarctica occurred prior to
breakup at 83 Ma (Lawver & Gahagan 1994, McAdoo & Laxon 1997). The degree of
extension and timing of events in Antarctica are corroborated by structural/thermochro-
nological data from extensional detachment faults at three sites spaced across 1000 km
from the central Ross Sea to its eastern margin (Fig. 3-12) (Fitzgerald & Baldwin 1997,
Siddoway et al. 2004a, McFadden et al. 2010b). Middle to lower middle crustal rocks
were tectonically exhumed and cooled extremely rapidly between 110 to 96 Ma, well in
advance of onset of seaoor spreading at 84–83 Ma (McAdoo & Laxon 1997, Eagles
et al. 2004).
The ca. 100 Ma crustal thinning across MBL and the eastern Ross Sea-Ross Ice Shelf
occurred within the context of distributed transtension, according to records from well-
studied bedrock exposures within extended crust on the eastern margin of the Ross Sea
(Siddoway et al. 2004b, McFadden et al. 2010a). The wrench component of deforma-
tion arose from the rapid oblique subduction of young, hydrated oceanic lithosphere
of the Phoenix plate (Sutherland & Hollis 2001, Finn et al. 2005). Widespread rapid
cooling (Siddoway 2008) led to landscape stabilization and formation of regionally ex-
tensive erosion surfaces at two or more elevations (LeMasurier & Landis 1996, Wilson
& Luyendyk 2006). The lower, better-characterized erosion surface formed as a wavecut
marine platform, based on the areal extent of the low relief surface that is unaffected
by variations in rock type (Wilson & Luyendyk 2006). The erosion surface(s) provide
a valuable datum for subsequent differential movements due to deformation, erosion,
uncorrected proofs
30 3. The Geology of West Antarctica
isostatic adjustment in response to glacial loading/unloading, and/or thermal subsidence.
There is indirect evidence for signicant Eocene-Oligocene extension in the southern
and eastern Ross Embayment (e.g. Cande et al. 2000, Granot et al. 2013), where there is
evidence of broad thermal subsidence that cannot be accounted for by the lithospheric
response to ice sheet loading/unloading, and must be post-middle Eocene (Wilson &
Luyendyk 2006).
Beneath the Ross Sea, normal-fault-bounded basement highs and basins have been
mapped from marine multichannel seismic surveys, ocean bottom seismograph, and gra-
vity surveys (Cooper & Davey 1985, Cooper et al. 1987, Cooper et al. 1991a, Davey
& Brancolini 1995, Trey et al. 1999). Interpreted as early-rift structures, the elongate,
north-south-oriented basins display positive gravity anomalies and high seismic velo-
cities in the lower crust, suggesting the presence of dense mac igneous rock. Crustal
thicknesses beneath the basins may be as low as 14 km, increasing to 21 to 24 km be-
neath the Coulman and Central Highs (Trey et al. 1999, Busetti et al. 1999, Chaput et
al. 2014). The basement highs divide the Ross Sea sector of the rift into four basins:
the Northern Basin, Victoria Land Basin, Central Trough, and Eastern Basin (Cooper et
al. 1995). Prevalent normal faults bound the basins (Cooper et al. 1991a, Cooper et al.
1991b, Tessensohn & Wörner 1991), and tectonic lineaments marking transfer systems
both partition the basins and segment the Transantarctic Mountains into blocks with dif-
fering levels of crustal exposure (Rossetti et al. 2003b). The basins contain a ll of 1 km
to 4 km of inferred Mesozoic sedimentary ll, overlain by thin Paleogene deposits, and a
thick accumulation of glacial sediments that expands beyond the bounds of the structural
basins (Anderson et al. 2001, Karner et al. 2005). Bounded by faults that accommodated
6 km or more of relative motion (Hamilton et al. 2001), the Victoria Land Basin in the
western Ross Sea contains a thickness of sediments up to 14 km (Cooper et al. 1987,
Brancolini et al. 1995, Busetti et al. 1999).
For the region of the Ross Ice Shelf, where bathymetric and geophysical data co-
verage is sparse, it is nevertheless clear that accumulated sediments are thin or absent.
BEDMAP2 bathymetry is dominated by elongate, narrow bedrock highs (Fretwell et al.
2013). There is no sediment upon the bedrock highs and only thin (200–600 m) sedi-
ments are found in the glacial troughs (Fig. 3-15) (Chaput et al. 2014, their Table 1).
Gravity characteristics also differ from those of the Ross Sea region, in that the anomal-
ies associated with the elongate bedrock highs indicate the presence of multiple types of
bedrock, including both basement and sedimentary deposits (Wilson & Luyendyk 2006).
An inherited structure, the Byrd Glacier fault zone, that has ~1 km of south-side-up re-
lative motion since 40 Ma (Foley et al. 2013), appears to form the boundary between the
Ross Sea and Ice Shelf regions.
In summary, considered in light of the temporal/denudational history of major topo-
graphic/bathymetric features, thermochronological data indicate that the present con-
guration of the WARS resulted from Mesozoic broad extension followed by Cenozoic
narrow extension focused in the western Ross Sea. Diverse modeling approaches have
been employed using elastic, kinematic and thermal-mechanical methods, to explore the
two-stage extension. These include Stern & ten Brink (1989), Fitzgerald et al. (1986),
Lawrence et al. (2006b), Bialas et al. (2007), Huerta & Harry (2007), and van Wijk et
al. (2008).
Models for the West Antarctic rift system that emphasize the role of lithospheric-
scale faults bounding the TAM include the crustal detachment model of Fitzgerald et al.
(1986) and the exural origin models of Stern & ten Brink (1989) and ten Brink et al.
(1997). The moderately dipping crustal detachment acts as a transfer or relay linking
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 31
the region of extensional strain in the upper crust, beneath the WARS, to the realm of
subcrustal lithosphere extensional strain beneath the TAM (Fitzgerald et al. 1986). The
lithospheric exure models, by contrast, envision a steep normal fault separating exten-
ded West Antarctica from intact East Antarctica lithosphere, causing exural uplift of the
TAM due to lateral variations in temperature, density, and amount of erosion. Ten Brink
Fig. 3-15. Interpretive block diagram with schematic representation of bedrock units and bathymetric
features beneath the Ross Ice Shelf, a region that appears to lack glaciouvial sediments that elsewhere
obscure the bedrock structure (e.g. Ross Sea). View is to southeast. Labels are: 1) Thick package of
immature sedimentary rocks, low-grade metamorphosed (metagreywacke), intruded by 2) intermediate to
felsic plutons (e.g. Yakymchuk et al. 2013). 3) Mac igneous rocks, related to WARS development (e.g.
Davey & Brancolini 1995). 4) Tertiary strata deposited after breakup between West Antarctica and New
Zealand (e.g. Wilson et al. 2012). 5) Low relief surfaces at 100 to 350 meters below sea level, interpreted
as remnants of a pre-glacial extensive wavecut platform (Wilson & Luyendyk 2006). 6) Inferred high angle
normal faults of Mesozoic-Cenozoic origin (Davey & Brancolini 1995, Luyendyk et al. 2001). 7) Linear
troughs formed by localized intense glacial erosion (e.g. Stern et al. 2005, Sorlien et al. 2007), ca. 700
mbsl, with moderate to small sediment thickness (cf. Chaput et al. 2014) suggestive of effective removal
of detritus or young age of incision (mid-Miocene or younger).
uncorrected proofs
32 3. The Geology of West Antarctica
et al. (1997) assign the change in the effective elastic thickness across the TAM boun-
ding fault to the Eocene. A role for anomalously warm asthenosphere created by rifting,
crustal thinning and magma injections into the middle and lower crust of the rift zone, is
a component of the thermally driven uplift models of Smith & Drewry (1984) and Berg
et al. (1989). Recent work by Lawrence et al. (2006b) calls upon the dual effects of a
buoyant thermal load and exural uplift, at the juncture between warm, thin lithosphere
underlying the WARS and the cold lithosphere of East Antarctica.
Other numerical models of viscoelastic-plastic, non-Newtonian behavior explore the
consequences of extension across a speculative thick crust of an orogenic plateau for-
med during Mesozoic plate convergence and subjected to a range of thermal conditions
(Bialas et al. 2007, Huerta & Harry 2007). In addition to the abrupt, faulted margin at
the TAM, geological evidence of a “West Antarctic plateau” comes from the sedimentary
detritus in the Triassic Beacon Supergroup in the Transantarctic Mountains, that is attri-
buted to East Gondwana margin sources (Elliot & Fanning 2008), in particular, detrital
zircon, interpreted to have been transported from the Mesozoic marginal arc across a
forearc basin to the site of deposition in the TAM, or alternatively transported subaerially
across a broad highland (e.g. Collinson et al. 1994). Another source of indirect evidence
for elevated terrain in West Antarctica, albeit in Paleocene-Eocene time, is the thermal
subsidence history of the extensive submerged bedrock platforms in the southern and
eastern Ross Embayment (Wilson & Luyendyk 2006).
Still under consideration and debate, the numerical models for the formation of the
TAM in the context of the WARS encompass mechanically driven uplift, thermal sup-
port, extensional collapse of a West Antarctic plateau, and combination models. Ongoing
evaluation and renement will need to take into account geological factors from rock
records and rened knowledge of the character of lithosphere and mantle of West
Antarctica.
3.6.3. The Cenozoic: tectonic sedimentation, structural reactiva-
tion, and seismicity
Intracontinental basin sediments
The record of the distribution and thickness of Cretaceous through Cenozoic sediments
and sedimentary rocks in interior West Antarctica has been obtained principally through
marine geophysical surveys in the Ross Sea and airborne geophysical surveys over in-
terior West Antarctica and the Ford Ranges. Surveys of the past quarter-century are re-
ported by Behrendt (1999), Behrendt et al. (1991a, 1991b), Cooper et al. (1995), Davey
& Brancolini (1995), De Santis et al. (1995, 1999), Bell et al. (1998, 2006), Busetti et
al. (1999), Luyendyk et al. (2001, 2003), Hamilton et al. (2001), Wilson & Luyendyk
(2006), Sorlien et al. (2007), Fielding et al. (2008), and by works cited within those
reports.
Young glacial and glacial marine sediments ll and conceal the fault-bounded be-
drock highs and structural basins of the Ross Sea. The deposits form ridges of unconsoli-
dated glacial detritus that extend seaward into sedimentary wedges with seaward dipping
reectors (Anderson et al. 2001). In between are elongate troughs Exquisite mega-scale
glacial lineations, drumlins, and gullies record the seaward transport of voluminous sedi-
ments and dynamic modications by glacial meltwater.
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 33
Drilling recovery of sediment cores has been achieved at a few locations in the Ross
Sea, by the Deep Sea Drilling Program (e.g. Hayes et al. 1975), Cape Roberts Project
(Bohaty et al. 2000, Naish et al. 2001), CIROS (Barrett 1989) and ANDRILL projects
(Antarctic Drilling Program 2015). Sampling of unconsolidated subglacial sediments
has been achieved through use of hot water drilling to reach the base of the central West
Antarctica ice streams and ice sheet (e.g. Tulaczyk et al. 1998, Licht et al. 2014).
The major Ross Sea basins are the Victoria Land Basin in the west, Central Trough,
Northern Basin, and Eastern Basin (Figs. 3-12, 3-14). The Victoria Land Basin contains
wedges of coarse-grained sediments deposited in fans and deltas within half-grabens,
and in grabens deposited during an early rift phase (Fielding et al. 2008). These are
bounded or truncated by active faults. The coarse sediment wedges are overlain by sym-
metrical, inward thickening lenses of cylic shallow marine to glaciomarine sediments.
Material of this type was sampled from Deep Sea Drilling Program Site 270, Leg 28, co-
red to 412 meters. The site was situated on the margin of a basement high, where drilling
penetrated through the sedimentary deposits into 30 meters of regolith/sedimentary brec-
cia, then an underlying carbonaceous metasedimentary complex (Ford & Barrett 1975,
Mortimer et al. 2011). The regolith/breccia was formed subaerially, and it is overlain by
thin (<5 m) margin-facies clastic rocks, specically, carbonaceous quartz sand followed
by glauconitic greensand. K/Ar dating of the glauconite yielded an age of circa 26 Ma.
Above the sandstones are glaciomarine sediments of late Oligocene-early Miocene age,
including >300 m of silty claystones with suspended granules and pebbles, capped by an
angular unconformity. Plio-Pleistocene sediments above the unconformity are feldspat-
hic sandstones containing granules (Ford & Barrett 1975).
Sites 271, 272, and 273 of the Deep Sea Drilling Program (DSDP) Leg 28 drilled in
to sediments of the Central and Northern Basins of the Ross Sea (Hayes et al. 1975).
Sites 271, 272, and 273 were cored to sub-bottom depths of 265, 443 and 346 meters,
respectively. There was poor to no recovery of strata from the sites, and none contain
pre-Miocene rocks. All four sites are dominated by lithied, unstratied, silty mudstone
containing marine microfossils and sparse lithic clasts; the mudstones are of predomi-
nantly Miocene age. The cores provide ‘ground truth’ for the seismic stratigraphic units
mapped across the Ross Sea (Brancolini et al. 1995, De Santis et al. 1995, Luyendyk et
al. 2001, Sorlien et al. 2007), named the Ross Sea seismic (RSS) sequence and Victoria
Land Basin seismic (V) sequence (Cooper et al. 1987). The RSS contains eight units
separated by unconformities. The deepest sedimentary package is identied as syntec-
tonic extensional deposits marked by inclined seismic reectors that correspond to rot-
ated bedding. The package is cut by faults, as evidenced by discontinuity of layers and
tectonic breccia that was penetrated by the drill core recovery at DSDP site 270 (Ford
& Barrett 1975, Ford 1991). The eastern Ross Sea basin displays a landward-deepening
bathymetric prole (De Santis et al. 1999) that is typical many areas around the Antarctic
continental margin (Cooper et al. 1991a). In the Ross Sea, the prole is attributable to
the effects of many cycles of ice sheet grounding line and retreat (ten Brink et al. 1997).
An unconformity named RSU-6 overlaps and cuts across the faults and seismic-sedi-
mentary layering of the lower RSS-1 (Brancolini et al. 1995). The unconformity bevels
the lower stratal package and is overlain by laterally continuous, at lying, undistur-
bed reectors considered to be of late Oligocene-early Miocene age (RSS2, Luyendyk
et al. 2001, Sorlien et al. 2007). Age control of ca. 25 Ma is provided by microfossil
identication from the upper RSS2 at DSDP site 270. Undeformed, laterally continuous
glaciomarine deposits, including ice front features such as deltas and moraines occur
in the RSS2 (De Santis et al. 1999, Sorlien et al. 2007). Unconformities dip northward
uncorrected proofs
34 3. The Geology of West Antarctica
and the seismic sequences downlap to the north. The Central Trough and Eastern Basin
contain several kilometers of Oligocene sediments, interpreted as deposits accumulated
during extension and subsidence that mark an end to the sediment-poor conditions of
the Paleogene (Decesari et al. 2007). Approximately 40 km of extension was distributed
across the Central and Eastern basins, compared to 95 km of extension accommodated
by the Northern and Victoria Land Basins in the western Ross Sea (Cande et al. 2000,
Granot et al. 2013).
Counter to expectations, broad positive Bouger gravity anomalies (wavelengths of
100–200 km) in the Ross Sea and interior West Antarctica exist over the sedimentary ba-
sins and negative anomalies occur over the basement highs (Karner et al. 2005, Bell et al.
2006). Shallow magnetic anomalies over the shallow basement highs alternate with deep
magnetic anomalies at >3800 m depth that are associated with the broad basins (Bell et
al. 2006), and likely are attributable to mac igneous rocks or thinned crust (Tréhu et al.
1993). Small circular shallow magnetic anomalies perturb the broad basin anomalies;
these are attributed to small volcanic centers. This perplexing inverse density relation-
ship for depocenters potentially arose as a result of exural rigidity imparted during a
period of slow/low sedimentation during lithospheric cooling following rifting (Karner
et al. 2005) between the Eocene to Miocene (Wilson et al. 1998, Cape Roberts Science
Team 2000, Hamilton et al. 2001, Luyendyk et al. 2001).
Within the WARS, active Cenozoic extension and alkali volcanism is ongoing in the
Terror rift (Fig. 3-14), within the Victoria Land Basin at the western limit of the Ross Sea,
bordering the Transantarctic Mountains (Fielding et al. 2008). The narrow zone of active
extension and magmatism is kinematically linked to Eocene-Oligocene oceanic sprea-
ding across the Adare Trough to the north, within the oceanic portion of the Antarctic
plate (Cande et al. 2000, Davey et al. 2006). Extension of ~150 km has occurred across
the Terror rift (Stock & Cande 2002, Davey et al. 2006, Granot et al. 2010, Granot et
al. 2013). Rift-related alkaline magmatism began at ca. 50 Ma along the west boundary
of the West Antarctic rift province (Rocchi et al. 2002), and at ca. 28 Ma in Marie Byrd
Land (LeMasurier et al. 2011), and magmatism continues today. In central Marie Byrd
Land, polygenetic volcanism began at ca. 14 Ma, and a chronology of volcanism from
the Miocene to Recent is well known (e.g. Wilch et al. 1999, Wilch & McIntosh 2002,
Rocchi et al. 2006). Volcanoes of 19 to 6 Ma age form north-south chains, whereas vol-
canoes younger than ca. 6 Ma are distributed along east-west trends.
Oligocene to Present kinematic and geodynamic models predict active tectonism in
central and eastern West Antarctica, also (Müller et al. 2007, Croon et al. 2008, Granot
et al. 2013), particularly through fault reactivation (Müller et al. 2007, Paulsen & Wilson
2010). Resolution of structures is low due to the presence of the West Antarctic Ice Sheet.
Despite this, there is considerable evidence of the dynamic state from bed topographic
and geophysical datasets, including: narrow, deep troughs inferred to be controlled by
faults (e.g. Jordan et al. 2010, Bingham et al. 2012); Cenozoic magmatism (Behrendt et
al. 1996) and young volcanism (Wilch et al. 1999, Gaffney & Siddoway 2007, Corr &
Vaughan 2008, Lough et al. 2013); rapid exhumation of Cenozoic intrusive rocks to the
surface (Rocchi et al. 2006); and a high geothermal ux high ux of 115 mW/m2, locally
reaching 200 mW/m2 (Maule et al. 2005, Schroeder et al. 2014, Dziadek et al. 2017,
Wiens et al. 2013).
The Miocene and younger polygenetic volcanoes form elongate vents and volcanic
edices have a shape anisotropy (alignment, elongate shape, and vent distributions) that
is interpretable in terms of a maximum horizontal strain orientation, which in turn re-
veals the Neogene state of stress across West Antarctica (Paulsen & Wilson 2010). The
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 35
orientation of the maximum horizontal stress direction based on the elongate orientation
of post-6 Ma volcanic centers is oriented east–west (Paulsen & Wilson 2010), which
is parallel to the absolute motion of the Antarctic plate (Müller et al. 2007, Croon et
al. 2008). This geodynamic framework makes the preexisting tectonic boundaries bet-
ween the Marie Byrd Land, Thurston Island/Eights Coast, and the Ellsworth-Whitmore
Mountains terranes susceptible to strike-slip reactivation (Dalziel 2006, Müller et al.
2007). Therefore, recent plate dynamic models attribute the profound depths of the Byrd
Subglacial Basin and Bentley Subglacial Trench (Fig. 3-1) to tectonic activity upon trans-
tensional structures or strike-slip releasing bends along a branch of the Cenozoic West
Antarctic rift (Müller et al. 2007, Gohl et al. 2013a, b). Fault reactivation and potentially
related processes such as geothermal activity or volcanism are of consequence because
they may inuence the dynamic behavior of the ice streams that have localized upon the
terrane boundaries (e.g. Holt et al. 2006, Gohl et al. 2013a, Schroeder et al. 2014).
Continental shelf sediments
The rifted margin of West Antarctica is conjugate to the Chatham Rise (Fig. 3-6), formed
as a result of separation between Zealandia and West Antarctica, circa 87 Ma ago. This
contrasts with the active margin in the Antarctic Peninsula, where subduction continued
until the Antarctic–Phoenix spreading ridge migrated into the trench (Larter et al. 2002;
Eagles et al. 2004). Sediment thicknesses on the shelf have been calculated using multi-
channel seismic reection, swath bathymetry, borehole, and gravity modeling data. The
sediment volume along the West Antarctic margin exceeds 3 million km3 (Wilson et al.
2012, ANTOSTRAT 1995), and may be as great as 10 million km3 (Wobbe et al. 2014,
and references therein). Wobbe et al. (2014) estimate that 2.78 million km3 of sediment
reside in the Ross Sea basin, with the remainder positioned in the Amundsen Sea and
Bellingshausen Sea basins along the continent-ocean boundary (Fig. 3-13). Much of
the sediment is of glaciouvial origins. The rst big Antarctic ice sheets initiated at the
Eocene–Oligocene boundary, at ca. 34 Ma, when there was a transition from greenhouse
to icehouse climate conditions in Antarctica (Zachos & Kump 2005, and references the-
rein). Continental glaciation initiated, according to numerical models, in elevated areas
of the West Antarctica terranes, followed by outward growth to form ice sheets (e.g.
DeConto & Pollard 2003, Pollard & DeConto 2009).
The shelf and slope sequences of the outer shelf and upper slope, revealed by mul-
ti-channel seismic reection and swath bathymetry (Nitsche et al. 2000, Nitsche et al.
2007, Lowe & Anderson 2002), are a product of glacial transport and sedimentation
since the Oligocene (cf. Rocchi et al. 2006, Sorlien et al. 2007) or after. Lower aggra-
dational sequences formed in a glacial marine environment when the West Antarctic ice
sheet was of reduced extent on the inner shelf. Upper progradational sequences indicate
the presence of grounded ice sheets of maximum extent that reached the shelf edge
(Nitsche et al. 2000).
Wilson et al. (2012) use seismic stratographic records from ANTOSTRAT (1995) for
Ross Sea sediment volume, and Nitsche et al. (2000, 2007), Scheuer et al. (2006), and
Gohl et al. (2013b) for coastal eastern MBL, to calculate minimum and maximum bounds
on the amount of crustal material from continental West Antarctica that must have been
eroded to form the Oligocene and younger marine sediments that accumulated in the
Ross Sea and on the Amundsen-Bellingshausen continental margins. There is an obser-
ved volume of 3.1 to 5.6 million cubic kilometres of sediment in these depocenters, that
uncorrected proofs
36 3. The Geology of West Antarctica
corresponds to a source volume of 2.05 to 4.94 million km3 in ‘onshore’ West Antarctica.
Wilson et al. (2012) used the estimated volumes of material eroded from the interior source
regions to reconstruct a plausible Eocene-Oligocene paleogeography for West Antarctica.
The restoration of the large volume of eroded sediments (that now reside on the shelf) to
their prior position on land yields a reconstructed paleogeography with elevations ran-
ging from present-day sea level up to a few hundred meters in elevation. The presence of
upland areas during a time of cooling of global climate potentially inuenced the growth
of the rst continental ice sheets (Wilson et al. 2012, cf. Pollard & DeConto 2009).
3.6.4. Characteristics and origin of the Marie Byrd Land volcanic
province
Cenozoic volcanism in Marie Byrd Land occurs along a distance of nearly 1000 km
along the northern edge of the West Antarctic rift province (Fig. 3-16). Alkalic volca-
nism in this province initiated at circa 30 Ma and continued through Quaternary time
(Hart et al. 1995, Hart et al. 1997, Hole & LeMasurier 1994, Wilch et al. 1999, Wörner
Fig. 3-16. Map of of the West Antarctic Rift province from Storey et al. (2013), with location of Cenozoic al-
kaline volcanic centers indicated by star symbols. The outline of the extent Middle Jurassic Ferrar magmatism
is shown with the dashed line symbol, and large Jurassic magmatic centers are shown in black. The region
affected by mid Cretaceous alkaline magmatism in Marie Byrd Land is delimited with a dashed line in grey.
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 37
1999, LeMasurier & Rocchi 2005, Rocchi et al. 2006, Storey et al. 2013). Eighteen large
alkaline volcanoes that attain circa 3000m elevation consist of dominant felsic alkaline
lavas, phonolite, trachyte, and intermediate differentiates. Upon these volcanoes, parasi-
tic vents produced scoria and lava ows made up of alkali basalt, hawaiite, and basanite.
There is high interest in the geochemistry, history and time scales of volcanism due to re-
cognition of Late Pleistocene to Holocene explosive tephra eruptions from four >3000m
volcanoes spaced along the length of the volcanic province (Wilch et al. 1999). There
have been recent discoveries of basaltic tephra from a large eruption at circa 2.2 ka
that blanketed an area of 23,000 km2 (Corr & Vaughan 2008), and of small-magnitude,
long-period earthquake swarms indicative of deep magma movement that may presage
an eruption (Lough et al. 2013; see below). A large subglacial eruption could have enor-
mous consequences for the West Antarctic ice sheet and its subglacial hydrological sys-
tem, as a result of meltwater production, bed lubrication, and ice discharge to the ocean
(Vogel & Tulaczyk 2006, Fricker et al. 2007, Nitsche et al. 2013).
Neogene to present alkalic volcanism
Many of the alkali volcanic centers in Marie Byrd Land erupt lavas that have Sr-Nd-
Pb isotope and trace element compositions characteristic of the HIMU mantle reser-
voir of ocean island basalts (Panter et al. 1997, Panter et al. 2000, Handler et al. 2003,
LeMasurier & Rocchi 2005, Gaffney & Siddoway 2007, LeMasurier et al. 2011). The
abbreviation HIMU (Hanyu et al. 2011) refers to lavas with high μ, where μ = 238U/204Pb.
Isotopic characteristics include low ɛNd (≤+4), low ɛHf (≤+3), and radiogenic Pb iso-
topes (206Pb/204Pb ≥ 21.5). The radiogenic Pb isotopes are a product of processes that
fractionate U/Pb and Th/Pb signicantly over a long period, as for example within sub-
ducted, altered, partially melted oceanic crust that is surrounded by and hybridizes with
surrounding deep mantle (Hanyu et al. 2011) over a long time period (109 years), or via
stepwise development may be generated from a shallower source with a high U/Pb ratio
in a shorter period of time, ca. 108 years (Rocchi et al. 2002).
The majority of magmatism in Marie Byrd Land occurred from 19 Ma to present, and
therefore volcanism occurred in the presence of the West Antarctic ice sheet (LeMasurier
& Rocchi 2005). Only a single volcano, Mt. Petras, is known to have originated earlier,
from 29–25 Ma. The easternmost basalt/trachyte volcano, Mt. Murphy (LeMasurier et
al. 1994), is a magnicent shield edice that is underlain by hydrovolcanic deposits, ab-
out 400 m thick, that record lava-ice-water interactions. Exposures of basement gneisses
exist beneath the lavas, a rare occurrence in Marie Byrd Land (see Fig. 3-10). The hya-
loclastites and pillow lavas, with interbedded tillite indicative of the subglacial environ-
ment, are succeeded by subaerial basalt ows reaching a thickness of 1000 m, followed
by felsic ows up to 550 m thick that form the summit of Mt. Murphy.
The westernmost well-exposed basaltic lavas occur in the Fosdick Mountains, where
basalt and basanite ows are circa 1.4 Ma (Gaffney & Siddoway 2007). The Fosdick
Mountains lavas have overall incompatible element abundances consistent with the
HIMU-type signature (Hanyu et al. 2011, cf. Finn et al. 2005) of ocean island basalts.
Their Pb isotopic compositions, however, are not radiogenic enough to be considered a
direct representation of the HIMU mantle source in the strict sense. Each of the three
centers in fact have distinct Sr-Nd-Pb isotopic characteristics, suggesting that there are
at least three source components to the west MBL lavas. The interpretation is borne out
by the lack of homogeneity among spinel peridotite xenoliths entrained within the ows:
uncorrected proofs
38 3. The Geology of West Antarctica
the xenoliths exhibit wide variation in mineral assemblages, rock fabric, deformation
intensity, and deformation mechanisms (Chatzaras et al. 2013, Chatzaras et al. 2016),
despite the spatial proximity of the deep-sourced but comparatively low volume ows
(Gaffney & Siddoway 2007).
The central MBL volcanic province (Fig. 3-16) is made up of isolated volcanic moun-
tains and volcanoes along chains that are controlled by bedrock structures (Fig. 3-17)
(e.g. LeMasurier & Rocchi 2005), presumed to be inherited faults. A majority are po-
lygenetic shield volcanoes constructed from subaerial ows of nepheline-normative
Fig. 3-17. (a) Satellite image of the Ames and Flood Ranges, sites of Miocene through Pliocene mac
alkali volcanism (GoogleEarth, DigitalGlobe, ©U.S. Geological Survey 2015b). (b) Sketch geologic map
of the felsic and intermediate volcanic rocks at the crest of the Ames and Flood Ranges, from LeMasurier
et al. (2011). Labeled sample localities show isotopic age information.
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 39
basanite and hawaiite, hypersthene-olivine transitional basalts and trachytes, capped off
by parasitic cinder cones and tephras of basanite containing nodules of spinel lherzolite,
dunite, websterite, and other ultramac xenoliths. An overall trend toward more felsic
compositions from base to summit reects magmatic differentiation in the centers over
time, and is borne out by the late appearance of intermediate alkalic lavas (phonolite,
which is an alkali feldspar – nepheline rock) and iron-rich, silica-poor rhyolite (pan-
tellerite) in some centers (LeMasurier et al. 2011). The contemporaneity of the young
magmas that differ strongly in composition potentially is a consequence of fractional
crystallization over a long time period (LeMasurier et al. 2011).
Mount Sidley is a large polygenetic stratovolcano at the south end of the Executive
Committee Range (Fig. 3-1). It consists of three major vent complexes that erup-
ted phonolitic and trachytic lavas (Panter et al. 1997), with lesser pyroclastic rocks
(Smellie et al. 1990). Phonolites were erupted between 5.7 and 4.2 Ma, giving way to
trachytes at 4.6–4.5 Ma and small volumes of silica undersaturated lavas (benmoreite-
mugearite series) at 4.4–4.3 Ma, along a southward compositional trend. Within Mt.
Sidley, and along the Executive Committee Range as a whole, an apparent directionali-
ty to the petrological trends suggests south-directed pulses of volcanic activity attribu-
table to tectonically driven or magma assisted fracture propagation (Panter et al. 1997).
Sequential release of magmas due to progressive reactivation of an inherited structure
may explain a perceived trend toward younger ages (Panter et al. 1994, LeMasurier &
Rex 1989). New age and isotopic data do not support the radial age pattern (Paulsen &
Wilson 2009).
New evidence for south-migrating magmatic activity along the trend of the Executive
Committee range comes from two swarms of deep earthquakes in 2010 and 2011, re-
corded by the POLENET/ANET broadband seismic network (Lough et al. 2013). The
swarms have unusual frequencies attributed to long-period seismicity, of a type that el-
sewhere in the world arises as a result of deep magma movements. The swarms are loca-
ted beneath a subglacial topographic high, a positive magnetic anomaly, and an englacial
mac tephra layer. Taken together, the features point to the existence of a sub-ice vol-
cano situated >55 km south of the southernmost exposure of volcanic rock in the area.
An ancillary effect of magma migration in this sector could be elevated background heat
ow beneath one of the West Antarctic ice streams. A possible signicant consequence
could be a subglacial volcanic eruption causing an outburst of glacial melt water in the
ice streams’ catchment (Lough et al. 2013).
Youngest eruptions across Marie Byrd Land (Wilch et al. 1999, Gaffney & Siddoway
2007) are dominated by peralkaline trachytes, with minor phonolites. These emanated
from Mounts Takahe (Fig. 3-18), Berlin (2.7 Ma), Moulton, Siple, and small centers in the
Fosdick Mountains in the Plio-Pleistocene. Volcanic products include agglutinated pyroc-
lastic fall deposits containing pumice fragments, welded trachyte ignimbrite, welded auto-
clastic breccias, and lithic- and ash-rich explosion breccias containing volcanic bombs.
Mt. Siple, an isolated symmetrical volcano that rises from sea level along the Hobbs
Coast (Fig. 3-1), has extensive ice cover and offers few exposures for petrological and
geochronological investigation. Satellite cones around the base are basanite of <100 ka
age. The caldera rim consists of pyroclastic fall deposits that are moderately to densely
welded and yield an age of 227 ka. A subsidiary cone below the summit crater is 168
ka (Wilch et al. 1999). Mt. Takahe volcano to the east is, like Mt. Siple, a graceful, ice-
shrouded, symmetrical volcano that is virtually undissected by erosion. Very limited
exposures exist of lava alternating with densely welded pyroclastic rocks occur near the
rim of the volcano’s broad, low summit crater that is 8 km across (Wilch et al. 1999).
uncorrected proofs
40 3. The Geology of West Antarctica
40Ar/39Ar dating of anorthoclase yields ages of 192 and 93 ka. The rim is composed of
hydrovolcanic tuffs, obsidian-bearing bomb-and-block layers, welded to nonwelded py-
roclastic lapilli deposits, and lavas that are just 8200 years old (Fig. 3-18).
The hydrovolcanic deposits are produced by lava – ice interactions on the Marie Byrd
Land volcanoes. The preservation of these easily eroded deposits, on the upper slopes
of >3000 m volcanoes, is due to the greater extent and height of the West Antarctic ice
sheet during the Miocene to Holocene (Ackert et al. 1999, Wilch & McIntosh 2002,
LeMasurier & Rocchi 2005). A superb example of lithofacies analysis of ice-contact
versus subaerial eruption products in the Crary Mountains (Wilch & McIntosh 2002),
documents a record of uctuations between pillow lava and hyaloclastite breccia –
indicative of wet, ice-contact conditions – and effusive lavas with welded breccia de-
posits, reecting ‘dry’ subaerial environments. Sequences of this type are documented
also at Mt. Murphy, 150 km away, where they are interrupted by erosion surfaces
and intravolcanic tillites. The ages for the volcanic rocks are 9 to 6 Ma, and hydro-
volcanic facies predominate between 9 and 8 Ma, based on 40Ar/39Ar geochronology,
suggesting the a maximum extent of the ice sheet in the late Miocene (9.3–8.2 Ma BP)
in eastern Marie Byrd Land (Mt. Murphy–Crary Mountains area; Wilch & McIntosh
2002). Hyaloclastic breccias of ca. 590± 15 Ma near the summit of Mt Murphy, along
the coast, indicate that the most recent ‘high stand’ of the WAIS was attained at 550 m
above current levels.
Fig. 3-18. (a) Satellite image of Mt. Takahe trachyte volcano, Pleistocene in age (GoogleEarth, Digital-
Globe, ©U.S. Geological Survey 2015c). Diameter of crater is 8 km and base is ~30 km, with vertical relief
~2000 m. Trachytic compositions are characteristic of most exposures at Mt. Takahe, with subordinate
mugearite and basanite/hawaiite. (b) Geologic sketch map of Mt. Takahe, from LeMasurier (2013), sho-
wing trachyte and subordinate mugearite and basanite/hawaiite. Labeled sample localities show isotopic
age information.
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 41
Tectonic context for magmatism and relationship to West Antarctic
Rift System
The origin of diffuse alkaline magmatism across West Antarctica, thermal and seismic
anomalies in the mantle beneath the region, high topography in areas of thin crust, and
relationship of magmatism to the rift province is much debated, and multiple viable
hypotheses are in play (LeMasurier & Rex 1989, LeMasurier & Rex 1991, LeMasurier
& Landis 1996, Finn et al. 2005, Sutherland et al. 2010, LeMasurier et al. 2011, Storey
et al. 2013). A hypothesis for the presence of a mantle plume (LeMasurier & Rex 1989,
LeMasurier & Rex 1991, Hole & LeMasurier 1994, LeMasurier & Landis 1996) or fos-
sil mantle plume (Hart et al. 1997, Panter et al. 2000) beneath Marie Byrd Land has
gained much traction, insofar as it potentially explains the trace elements and Sr-Nd-Pb
isotopes pertaining to the HIMU-ocean island basalt characteristics, indicative of a pri-
mitive mantle source; the increase in elevation toward the interior of Marie Byrd Land;
and a perceived radial, outward-younging age pattern for the trachytes. Advocates of a
Cenozoic/active plume place the time of onset at ca. 30 Ma (e.g. Hole & LeMasurier
1994, LeMasurier & Landis 1996).
Relevant for the mantle plume hypothesis, an historic conception has been that the
Antarctic tectonic plate had been stationary in respect to global plate tectonic circuit
since 80 Ma (e.g. LeMasurier & Rex 1989, Le Masurier & Landis 1996), due to the
dominance of divergent boundaries surrounding the plate. However that situation is now
known to be false (Croon et al. 2008): contemporary models indicate the effects of in-
tegrated ‘ridge-push’ forces upon West Antarctica, that arise from the ridge-transform
zones (Zoback 1992). Furthermore, the perceived outward-younging, radial age pattern
for felsic volcanism in MBL (LeMasurier & Rex 1989) has been disproven by more
comprehensive sampling and in-depth eld study (Rocchi et al. 2002, Paulsen & Wilson
2010, Kipf et al. 2014).
One recent, alternative hypothesis attributes the alkali volcanism, mantle anomalies,
and topographic effects (Fig. 3-16) to decompression melting of the mantle due to crustal
thinning achieved by rifting (e.g. Wörner 1999) or wrench deformation (e.g. Rocchi et
al. 2002). In this instance, the HIMU characteristics may be a relic of a Mesozoic plume
that underplated the lithosphere (Weaver et al. 1994) prior to/during mid-Cretaceous
rifting in the WARS and breakup of east Gondwana, rather than evidence of an active
plume (e.g. Rocholl et al. 1995, Hart et al. 1997, Panter et al. 2000). A third hypothesis
is for upward ow of enriched, lower density mantle above a zone of sinking of oceanic
slabs introduced into the sublithospheric mantle during Paleozoic and Mesozoic subduc-
tion (Finn et al. 2005), with the possibility of mantle upwelling induced in response to
cessation of Gondwana subduction (Sutherland et al. 2010). The compositional range
of Marie Byrd Land lavas appears to reect mixing between melts derived both from
dehydrated oceanic crust and from subcontinental lithosphere metasomatized by uids
released from the slabs. Moreover, the residual, potassic, hydrous phases indicate more
moderate temperatures in the mantle domains, that could produce “hot-spot-like” basalts
without the need for a deep mantle plume. Such a scenario is consistent with the tectonic
setting of MBL magmatism and the depth of the mantle anomaly, that is attributable
to subduction of altered (hydrated) oceanic lithosphere beneath East Gondwana (West
Antarctica) occurred over a long period of Paleozoic and Mesozoic time, leading to the
release of uids from the subducted slab(s) (Finn et al. 2005, Rocchi et al. 2002, Rocchi
et al. 2006, Sutherland et al. 2010).
uncorrected proofs
42 3. The Geology of West Antarctica
Finally, the elevated region is not circular, as typically is the case for mantle plu-
mes, and centers of magmatism are distributed along linear rather than radial trends.
Furthermore, the plume hypothesis does not explain the onset of magmatism in the
middle Eocene. Valuable information about the intraplate stress state and the locus of
volcanism comes from the orientation and alignment of Miocene to Holocene volcanic
cones and vents in Marie Byrd Land provide (Paulsen & Wilson 2010). Elongation direc-
tions of volcanic vents, craters, and edices reveal a maximum horizontal strain directi-
on oriented north-south in early Miocene time. Younger volcanoes,<6 Ma in age, display
a shape anisotropy that is orthogonal to that of Miocene volcanic centers, revealing that
there has been a change in the maximum horizontal strain direction. Neogene plate mo-
tions produced north-south-oriented maximum horizontal strain in West Antarctica from
the Late Miocene through Pliocene (~20 to 12 Ma;), when the Flood Range and Toney
Mountain developed (LeMasurier & Rocchi 2005, Paulsen & Wilson 2010). From 6 Ma
to present, the maximum horizontal strain has been oriented east-west during the time of
construction of most of the Executive Committee Range, culminating with the eruption
of Mt Waesche. The current standpoint is that the magmatism in the province is strong-
ly localized along preexisting zones of weakness imparted during Paleozoic-Mesozoic
convergence along the margin of East Gondwana and as a result of Cretaceous intracon-
tinental extension (Finn et al. 2005, Rocchi et al. 2002, Rocchi et al. 2005, Paulsen &
Wilson 2010, Kipf et al. 2014).
3.6.5. Paleotopography and landscape rejuvenation
The highest non-volcanic peaks in MBL are spaced over a wide region of outcrop
in the Ford Ranges and Edward VII Peninsula (Fig. 3-6a), where summit elevations
are between 700 and 1200 m (Stone et al. 2003). The remaining coastal nunataks and
summits range in elevation from 150 to 600m, and emerged from beneath the West
Antarctic Ice Sheet since Last Glacial Maximum (Stone et al. 2003, Sugden et al. 2005;
Lisker & Olesch 1998, Lindow 2014, Spiegel et al. 2016). The low-lying topography
and comparatively small range of bedrock elevations in coastal and western Marie Byrd
Land precludes the use of thermochronology ‘elevation proles,’ of the type that illumi-
nate the Cenozoic landscape evolution of the Ellsworth and Transantarctic Mountains
(Gleadow & Fitzgerald 1987, Fitzgerald & Stump 1991, Lisker 2002, Balestrieri &
Bigazzi 2001).
As an alternative in MBL, valuable insights about age and exhumation history come
from thermochronology data obtained over limited range of elevation and a broad coas-
tal area (Fig. 3-12). Diverse isotopic systems including U-Pb zircon and Ti-in-zircon for
igneous crystallization ages, U-Pb ages for growth of metamorphic monazite, 40Ar/39Ar
for closure ages for multiple mineral systems, and apatite ssion track and (U-Th)/He
zircon cooling ages for migmatite and granite complexes (Richard et al. 1994, Mukasa
& Dalziel 2000, Lisker & Olesch 1998, Siddoway et al. 2004b, Contreras et al. 2012,
Lindow 2014, McFadden et al. 2015, Spiegel et al. 2016) all produce a consistent narrow
range of ages from 102 to 88 Ma. Unimodal distribution of apatite ssion track lengths
of >13.4 µm indicate rapid translation through the apatite partial annealing zone (PAZ)
(Lisker & Olesch 1998, Contreras et al. 2012).
The (U-Th)/He zircon and apatite ssion track ages for fteen non-volcanic sum-
mit locations all yield of 98 Ma to 88 Ma (Richard et al. 1994, Lisker & Olesch 1998,
Contreras et al. 2012, Lindow 2014, Spiegel et al. 2016) that overlap with both U-Pb
uncorrected proofs
3.6. Tectonic Evolution and Timing of the WARS 43
igneous crystallization and 40Ar/39Ar mineral cooling ages of granite or migmatite from
the same sites. The close correspondence between Cretaceous igneous crystallization
ages and low temperature thermochronometers, and the narrow range of ages from the
low temperature methods, provide evidence of very rapid cooling across the broad re-
gion due to tectonic exhumation during intracontinental extension (McFadden et al.
2015). Evidently, mid-crustal rocks that originated at >700 °C at depths of 20 to 24 km at
ca. 100 Ma passed through the 180–200 °C zone for partial retention of helium in zircon
(Reiners 2005) and the 120–60 °C apatite partial annealing zone at depths <2 km (Kohn
et al. 2005) by ca. 88 Ma. This likely was achieved by the tectonic translation of high
temperature rocks to shallow depths upon regional-scale detachment faults (Fig. 3-12),
including those that are well-documented at three locations in Marie Byrd Land and the
Ross Sea (Fitzgerald & Baldwin 1997, Siddoway et al. 2004a, McFadden et al. 2010b).
The tightly clustered older ages from summit elevations (>800m) potentially reects the
preservation of a ‘fossil’ apatite partial annealing zone (e.g. Toraman et al. 2014) as a
result of slow cooling and very limited denudation after 88 Ma. The presence of a “fossil
PAZ” is compatible with the preservation of a Cretaceous erosion surface in Marie Byrd
Land (LeMasurier & Landis 1996) and the apparent lack of Moho relief beneath the Ford
Ranges (Luyendyk et al. 2003).
Lower-elevation outcrops,<600 m above sea level, yield more scattered apatite s-
sion-track and (U-Th)/He zircon ages, between 83–60 Ma (Richard et al. 1994, Lisker
& Olesch 2003, Contreras et al. 2012, Spiegel et al. 2016). The low-elevation sites are
coastal, or at the border of fast-owing outlet glaciers, which are subject to change due to
glacial loading/unloading, accommodated in part by small differential fault movements.
The scattered younger ages and lack of interpretable pattern at elevations <700m suggest
that incision into the inferred “fossil PAZ” commenced somewhat recently and in an un-
even manner (e.g. Toraman et al. 2014). Possible evidence for the onset of active glacial
erosion into the regionally extensive crust that contains the imprint of anatexis and rapid
cooling in the Cenomanian-Turonian Stage is the presence of detrital zircons of 110–100
Ma age within contemporary glacial sediments of Bindschadler and Kamb ice streams
(Licht et al. 2014).
Evidence for the time of onset of erosional incision into a speculative “fossil PAZ” in
West Antarctica in the Oligocene possibly is provided by widely separated sites at Mt.
Murphy in eastern MBL and Roosevelt Island in western MBL. Mt. Murphy was disco-
vered to be the site of rapid exhumation of an Oligocene gabbro from depths of 3 km or
more (Rocchi et al. 2006), a nding that is corroborated by apatite ssion track ages of
31 to 28 Ma from three sites (Spiegel et al. 2016). The exhumation or incision may mark
the onset of growth of the elevated topography of the interior of Marie Byrd Land (i.e.
“Marie Byrd Land dome”) since 30 Ma (Spiegel et al. 2016). In broadly contempora-
neous strata on the opposite, western margin of Marie Byrd Land, deep, linear troughs
are seismically imaged to incise pre-25 Ma sediments of the Roosevelt subbasin, bord-
ering Edward VII Peninsula (Sorlien et al. 2007). The troughs clearly are sites of glacial
incision and constitute the earliest evidence for landscape reactivation due to erosional
incision in Marie Byrd Land Peninsula (Sorlien et al. 2007).
A broader distribution and denser sampling for thermochronology across MBL will
be needed to verify the presence of a vestigial PAZ, and rene our understanding of the
spatial-temporal evolution of topography in MBL. If achieved, the endeavor will help to
clarify whether Miocene and younger bedrock incision occurs in response to dynamic
support due to presence of warm mantle beneath MBL (LeMasurier & Landis 1996,
Heeszel et al. 2016), onset of wet-based glaciation (Flower & Kennett 1994, Sugden et
uncorrected proofs
44 3. The Geology of West Antarctica
al. 2005, Lindow 2014, Spiegel et al. 2016), or differential movement upon inherited
faults due to change in the tectonic stress state (Rocchi et al. 2002, Rocchi et al. 2003a,
Croon et al. 2008, Paulsen & Wilson 2010) and/or glacial unloading.
Pine Island Bay – Thwaites Glacier region
The tectonic boundary between MBL and Thurston Island/Eights Coast corresponds to
the location of a north-south-oriented, deep, broad bedrock trough that underlies and
extends inland from the Amundsen Sea Embayment. The Thwaites Glacier ice stream
occupies this bedrock low (Fig. 3-1). A very narrow, deep, east-west oriented trough
enters the Amundson Sea Embayment on the southeast, and is identied as an active rift
(Jordan et al. 2010). This Pine Island rift forms the southern boundary of the Thurston
Island block, and is occupied by Pine Island Glacier ice stream. The two ice streams to-
gether drain 4% of the outow from the entire Antarctic Ice Sheet (Vaughan et al. 2006).
The behavior of the ice streams is dynamically inuenced by the geology and geomor-
phology of the subglacial bedrock, and is highly sensitive to warming of the ocean and
atmosphere. The crustal thickness in this section is 19±1 km based on the estimated
Moho depth.
Bedrock geology and presence/absence of subglacial sediments are known to inu-
ence ice sheet behavior, in respect to the distribution of slow-moving sheet ice versus
fast-moving ice streams. Across most of West Antarctica, the motion of the ice sheet is
slow (Neumann et al. 2008) or stationary, where the ice sheet is xed to the underlying
bedrock (Bell et al. 1998). Crystalline bedrock (granitic rocks or migmatite gneiss; Kipf
et al. 2012) or competent greenschist-metamorphosed siliciclastic rocks underlie such
regions. Some evidence of the geology of the bedrock comes from the age distributions
of detrital zircons obtained from subglacial sediments from the central to eastern ice
streams; there is an age correspondence to bedrock sources in Marie Byrd Land, and no
known sources in East Antarctica (e.g. Licht et al. 2014).
Elsewhere the ice sheet overlies sedimentary basin deposits or water-saturated till,
materials that are porous and permeable, allowing the migration of water along and be-
neath the base of the ice sheet (Bell et al. 1998, Bell et al. 2006, Studinger et al. 2001,
Fricker et al. 2007). Airborne geophysical surveys and ice drilling have shown that the
fast-moving ice streams of West Antarctica, that ow several hundred meters per year,
exploit zones where there are basal lubricants such as water or deformable till (Tulaczyk
et al. 1998), that in some instances correspond with zones of higher heat ow. The mar-
gins of the fast-owing ice correspond to high gradient anomalies that are interpreted as
bedrock faults, an indication of bedrock structural control upon the locations for strea-
ming ice (Studinger et al. 2001, Holt et al. 2006, Bingham et al. 2012).
The future outlook for the ice streams and West Antarctic ice sheet hinges on ice-bed
dynamics. The ice streams that ow north toward the Amundsen Sea have beds below
sea level that deepen toward the interior, a conguration that allows inltration of seawa-
ter beneath the ice streams, allowing the seawater to inltrate along the base, promoting
streaming behavour or otation of the glacier ice, leading to grounding line retreat (Holt
et al. 2006, Rignot et al. 2014).
A third geological element of the subglacial environment, identied on the ba-
sis of shallow-sourced magnetic anomalies that correlate with bed topography, is late
Cenozoic volcanic rocks erupted from subaerial or subglacial centers (Behrendt et al.
2004, Behrendt 2013). Lavas erupted beneath and into the ice sheet would consist of
uncorrected proofs
3.7. Conclusion 45
hyaloclastite, pillow breccia and other volcanic debris, that would be easily eroded by
moving ice, whereas subaerial volcanoes, erupted during interglacial conditions, consist
of more competent bedrock that forms higher-standing features. The subglacial volca-
noes, together with crustal-scale structures that may inuence the location of the centers
of volcanism, are expected or documented (Blankenship et al. 1993, Blankenship et al.
2001, Schroeder et al. 2014) to be sites of high heat ow. Direct evidence of contrasts in
the basal thermal regime and geological variations in the glacial substrate in Marie Byrd
Land comes from landforms observed at upper versus lower elevations in recently degla-
ciated coastal regions (Sugden et al. 2005). Periglacial landforms at summit elevations
(tors, well-developed granite grus) that form beneath cold-based ice are found in close
proximity to glacially incised, scoured and sculpted bedrock that are produced beneath
wet-based outlet glaciers at lower elevations along the coast.
3.7. Conclusion
The denition of the tectonic terranes of West Antarctica, achieved on the basis of direct
eld and laboratory investigations in geology, isotope geochemistry, and paleomagne-
tism, is being rened and expanded in the 21st century through the acquisition of net-
work and grid geophysical and seismological/geodetic survey data (e.g. Wilson et al.
2011, Tinto et al. 2018). Variations in glacial bed topography, thickness of crust and who-
le-lithosphere, and mantle characteristics have been/are being modeled and mapped (e.g.
An et al. 2016), to obtain a dynamic understanding of the lithospheric evolution of West
Antarctica. The framework introduces new interdisciplinary questions about cryosphere-
lithosphere interaction during a time of global change. Much new research focuses upon
development of new continent-scale digital geological map resources with online access
that encourages cross-discipline use (Cox et al. 2016), evaluation of the lithospheric
scale structures that bound the terranes (e.g. Müller et al. 2007) and control the patterns
of Neogene volcanism and heat ow (Paulsen & Wilson 2009, Fisher et al. 2015), and
the discovery of the origin and consequences of anomalous mantle that underlies West
Antarctica (e.g. Hansen et al. 2014).
