Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?

Abstract

The Rheic Ocean is widely believed to have formed in the Late Cambrian-Early Ordovician as a result of the drift of peri-Gondwanan terranes, such as Avalonia and Carolina, from the northern margin of Gondwana, and to have been consumed in the Devonian Carboniferous by continent-continent collision during the formation of Pangea. Other peri-Gondwanan terranes (e.g., Armorica, Ossa-Morena, northwest Iberia, Saxo-Thuringia, Moldanubia) remained along the Gondwanan margin at the time of Rheic Ocean formation. Differences in the Neoproterozoic histories of these peri-Gondwanan terranes suggest the location of the Rheic Ocean rift may have been inherited from Neoproterozoic lithospheric structures formed by the accretion and dispersal of peri-Gondwanan terranes along the northern Gondwanan margin prior to Rheic Ocean opening. Avalonia and Carolina have Sm-Nd isotopic characteristics indicative of recycling of a juvenile ca. 1 Ga source, and they were accreted to the northern Gondwanan margin prior to voluminous late Neoproterozoic arc magmatism. In contrast, Sm-Nd isotopic characteristics of most other peri-Gondwanan terranes closely match those of Eburnian basement, suggesting they reflect recycling of ancient (2 Ga) West African crust. The basements of terranes initially rifted from Gondwana to form the Rheic Ocean were those that had previously accreted during Neoproterozoic orogenesis, suggesting the rift was located near the suture between the accreted terranes and cratonic northern Gondwana. Opening of the Rheic Ocean coincided with the onset of subduction beneath the Laurentian margin in its predecessor, the Iapetus Ocean, suggesting geodynamic linkages between the destruction of the Iapetus Ocean and the creation of the Rheic Ocean.

Full text

ON THE COVER:
The strata in the synclinal structure in the Narcea River valley of Asturias, northern Spain,
record the origin and evolution of the Rheic Ocean. The Early Ordovician strata record the
rift-drift transition during the development of the Rheic Ocean. The deformation of the
strata is Carboniferous (Variscan) in age and is associated with closure of the Rheic Ocean
and the formation of Pangea. See Origin of the Rheic Ocean: Rifting along a
Neoproterozoic suture? by Murphy et al., p. 325-328. Photo by: Gabriel Gutierrez-Alonso
Cover design by: Margo Y. Sajban

q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology; May 2006; v. 34; no. 5; p. 325–328; doi: 10.1130/G22068.1; 1 figure. 325
Origin of the Rheic Ocean: Rifting along a Neoproterozoic suture?
J. Brendan Murphy Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada
Gabriel Gutierrez-Alonso Departamento de Geologı´a, Universidad de Salamanca, 33708 Salamanca, Spain
R. Damian Nance Department of Geological Sciences, Ohio University, Athens, Ohio 45701, USA
Javier Fernandez-Suarez Departamento de Petrologı´a y Geoquı´mica, Universidad Complutense, 28040 Madrid, Spain
J. Duncan Keppie Institute de Geologia, Universidad Nacional Autonoma de Mexico, 04510 Mexico D.F., Mexico
Cecilio Quesada IGME, Direccio´n de Geologı´a y Geofı´sica, c/La Calera, 1 28760 Tres Cantos, Madrid, Spain
Rob A. Strachan School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth PO1 3QL, UK
Jarda Dostal Department of Geology, St. Marys University, Halifax, Nova Scotia B3H 3C3, Canada
ABSTRACT
The Rheic Ocean is widely believed to have formed in the Late
Cambrian–Early Ordovician as a result of the drift of peri-
Gondwanan terranes, such as Avalonia and Carolina, from the
northern margin of Gondwana, and to have been consumed in the
Devonian Carboniferous by continent-continent collision during
the formation of Pangea. Other peri-Gondwanan terranes (e.g., Ar-
morica, Ossa-Morena, northwest Iberia, Saxo-Thuringia, Moldan-
ubia) remained along the Gondwanan margin at the time of Rheic
Ocean formation. Differences in the Neoproterozoic histories of
these peri-Gondwanan terranes suggest the location of the Rheic
Ocean rift may have been inherited from Neoproterozoic litho-
spheric structures formed by the accretion and dispersal of peri-
Gondwanan terranes along the northern Gondwanan margin prior
to Rheic Ocean opening.
Avalonia and Carolina have Sm-Nd isotopic characteristics in-
dicative of recycling of a juvenile ca. 1 Ga source, and they were
accreted to the northern Gondwanan margin prior to voluminous
late Neoproterozoic arc magmatism. In contrast, Sm-Nd isotopic
characteristics of most other peri-Gondwanan terranes closely
match those of Eburnian basement, suggesting they reflect recy-
cling of ancient (2 Ga) West African crust. The basements of ter-
ranes initially rifted from Gondwana to form the Rheic Ocean were
those that had previously accreted during Neoproterozoic orogen-
esis, suggesting the rift was located near the suture between the
accreted terranes and cratonic northern Gondwana. Opening of
the Rheic Ocean coincided with the onset of subduction beneath
the Laurentian margin in its predecessor, the Iapetus Ocean, sug-
gesting geodynamic linkages between the destruction of the Iapetus
Ocean and the creation of the Rheic Ocean.
Keywords: Pangea, Rheic Ocean, Neoproterozoic suture, Sm-Nd data,
terranes.
INTRODUCTION
The tectonic evolution of the Paleozoic Era is dominated by early
Paleozoic continental dispersal and ocean development, followed, in
the middle to late Paleozoic, by convergence culminating in collisional
tectonics that led to the amalgamation of Pangea (Fig. 1). During this
time, terranes were transferred from Laurentia to Gondwana (Thomas
and Astini, 1996) and from Gondwana to Laurentia (van Staal et al.,
1998; Keppie et al., 2003). Two key oceans, the Iapetus and Rheic,
developed during the early Paleozoic by fundamentally different rifting
mechanisms. It is generally accepted that the Iapetus Ocean developed
in the Early Cambrian with the separation of two major continents
(Laurentia and Gondwana, e.g., Cawood et al., 2001), but that the
Rheic Ocean initiated in the Late Cambrian–Early Ordovician with the
rifting of peri-Gondwanan terranes (e.g., Avalonia and Carolina) from
the northern (Amazonia West African) margin of Gondwana (Fig. 1;
van Staal et al., 1998; Cocks and Torsvik, 2002).
On separation, these terranes defined the boundary between of the
expanding Rheic Ocean to the south and the contracting Iapetus Ocean
to the north during the middle to late Paleozoic (Fig. 1; Cocks and
Torsvik, 2002; Stampfli and Borel, 2002). The implications of such
differences in rift mechanisms are profound, yet they have received
little attention in the literature.
Iapetus was largely closed by either the mid-Silurian (e.g., Hib-
bard et al., 2002) or the Devonian (Hatcher, 1989) through Laurentia-
Baltica collision and peri-Gondwanan terrane accretion. It was the clo-
sure of the Rheic Ocean, some 100 million years later, that led to the
assembly of Pangea through the collision of Laurentia-Baltica with
Gondwana. In this paper, we focus on the processes that determined
the site of initial rifting and development of the Rheic Ocean. We show
that the peri-Gondwanan terranes that separated from the northern
Gondwanan margin to form the Rheic Ocean had previously been ac-
creted to this margin in the Neoproterozoic, whereas those that re-
mained on this margin had formed part of cratonic Gondwana. This
implies that rifting was focused on Neoproterozoic sutures between the
accreted terranes and cratonic Gondwana and, more generally, that (1)
ocean development along continental margins shows a remarkable de-
gree of structural inheritance, (2) accreted terranes are susceptible to
subsequent rifting and may therefore experience repeated episodes of
accretion, and (3) the contrasting thermal conditions of intracratonic
and continental margin breakup produce quite different styles of rifting.
EVOLUTION OF THE IAPETUS AND RHEIC OCEANS
The Iapetus Ocean (Fig. 1A) opened in stages between ca. 600
Ma (Laurentia-Baltica) and 550 Ma (Cawood et al., 2001). The sub-
sequent onset of convergence is recognized by the development of arc-
related mafic complexes in the Late Cambrian–Early Ordovician (van
Staal et al., 1998). Iapetus was closed to the north in the early to mid-
Silurian by the collision between Laurentia and Baltica to form Lau-
russia and, to the south, by the accretion of Gondwana-derivedterranes,
such as Avalonia and Carolina, to Laurentia and Baltica, either by the
Late Ordovician to mid-Silurian (Hibbard et al., 2002) or by the De-
vonian (Hatcher, 1989).
Although a small ocean existed between Avalonia/Carolina and
the Amazonian–West African margin of Gondwana in the Cambrian
(Keppie et al., 2003), the main Rheic Ocean opened in the Late
Cambrian–Early Ordovician (Fig. 1A) (e.g., Cocks and Torsvik, 2002).
By the end of the Paleozoic, the Rheic Ocean had been closed by the
collision between Gondwana and Laurussia, which produced the
Appalachian-Variscan orogen, one of the key events in the formation
of Pangea (van Staal et al., 1998).
NEOPROTEROZOIC EVOLUTION OF PERI-GONDWANAN
TERRANES
A wealth of paleontological, paleomagnetic, and isotopic data in-
dicates that a number of terranes in the Appalachian and Variscan or-
ogens were positioned along the northern margin of West Gondwana
in the Neoproterozoic and early Paleozoic at considerable latitudinal
distance from Laurentia (e.g., Cocks and Torsvik, 2002; Landing,

326 GEOLOGY, May 2006
Figure 1. A: Paleozoic reconstructions (modified from Stampfli and Borel, 2002) showing outboard position of Avalonian-type terranes
relative to Cadomian-type terranes, their translation along Gondwanan margin in early Paleozoic, their separation from Gondwana with
opening of Rheic Ocean, and their accretion to Laurentia with closure of Iapetus Ocean. Note that opening of Rheic Ocean coincides with
onset of northwesterly directed subduction and subduction of Iapetan Ocean ridge along Laurentian margin (see Stampfli and Borel, 2002).
B: Early Mesozoic position of peri-Gondwanan terranes mentioned in text (modified from Keppie et al., 2003).Ch—Chortis,Oax—Oaxaquia,
Y—Yucatan, Fl—Florida, C—Carolina, A—Avalonia, OM—Ossa Morena, CAD—Cadomia, NW-I—northwest Iberia, Arm—Armorica, BM—
Bohemian Massif. C: Sm-Nd isotopic data for Avalonia (A) (from Nance and Murphy, 1996; Murphy et al., 2000) and Carolina (e.g., Samson
et al., 1995), which have relatively juvenile signatures. D: Sm-Nd isotopic data for Armorica and Icart Gneiss (Samson et al., 2003) and
Saxo-Thuringia (Bohemian Massif, Linnemann et al., 2004), which have more ancient crustal components. Shaded area in C shows field
for peri-Rodinian oceanic lithosphere defined by time of amalgamation (a) and breakup (b) of Rodinia, calculated assuming a typicalSm/Nd
oceanic crustal ratio of 0.18. CHUR 5 chondrite uniform reservoir.
2005). The tectonothermal histories of these peri-Gondwanan terranes
show that they continuously faced an open ocean for this time, thereby
providing fundamental constraints to paleocontinental reconstructions
(e.g., Murphy et al., 2000; Nance et al., 2002; Keppie et al., 2003).
Peri-Gondwanan terranes are characterized by late Neoproterozoic
arc-related tectonothermal histories beginning at ca. 760 Ma and peak-
ing at ca. 635–570 Ma with the generation of voluminous arc-related
igneous rocks. High-grade metamorphism between ca. 670 and 650 Ma
is thought to represent accretion of early Avalonian arcs to the Gond-
wanan margin (Murphy et al., 2000). Coeval sedimentary successions
were deposited in a variety of intra-arc and interarc basins, which were
bounded by active faults (e.g., Keppie et al., 2003). Diachronous ter-
mination of arc magmatism between ca. 610 and ca. 530 Ma is attri-
buted to the progressive development of a transform system in the
latest Neoproterozoic to Early Cambrian (Fig. 1A) resulting from ridge-
trench collision, during which some terranes were transferred laterally
along the Gondwanan margin (e.g., NW Iberia; Gutierrez-Alonso et al.,
2005). This scenario is analogous to the Cenozoic development of the
San Andreas transform fault system of western North America (Mur-
phy et al., 2000; Nance et al., 2002; Keppie et al., 2003).
Despite their broadly similar tectonostratigraphy, peri-Gondwanan
terranes (Fig. 1B) can be subdivided into two groups by the contrasting
age and composition of their basements: (1) Cadomian-type terranes
(northern Armorica, Ossa-Morena, Saxo-Thuringia, Moldanubia), and
(2) Avalonian-type terranes (e.g., West Avalonia, East Avalonia, Car-
olina, Moravia-Silesia, NW Iberia, and possibly Armorica south of the
North Armorican shear zone). Only in Armorica is undisputed base-
ment (2.06 Ga Icart Gneiss; Samson and D’Lemos, 1998) exposed in
any of the peri-Gondwanan arc terranes. However, the contrasting base-
ment composition of Avalonian-type and Cadomian-type terranes is
clear from the very different U-Pb detrital zircon populations in their
Neoproterozoic clastic successions and the contrasting Sm-Nd isotopic
characteristics of their Neoproterozoic and early Paleozoic crustally
derived igneous rocks (Figs. 1C and 1D; e.g., Nance and Murphy,
1996; Samson et al., 2003). The Sm-Nd isotopic composition of
Cadomian-type felsic rocks is characterized by predominantly negative

GEOLOGY, May 2006 327
«
Nd
values (typically between 11.6 to 29.9 for t 5 610 Ma) and
depleted mantle model ages (T
DM
) of 1.0 to 2.0 Ga (e.g., Samson and
D’Lemos, 1998). In contrast, felsic rocks in Avalonian-type terranes
are characterized by «
Nd
values (t 5 610 Ma) that range between 21.0
and 15.0, and T
DM
model ages of 0.75–1.1 Ga (e.g., Murphy et al.,
2000), and show little overlap with those of Cadomian-type terranes.
Available U-Pb detrital zircon data for Neoproterozoic clastic succes-
sions are similarly distinct. Detrital zircons in Cadomian-type turbidites
reveal clusters of ages at 0.60–0.65 Ga, 2.0–2.2 Ga, 2.4 Ga, and 2.6
Ga (Fernandez-Suarez et al., 2002; Gutierrez-Alonso et al., 2005; Sam-
son et al., 2003), whereas similar-aged clastic successions in Avalonia,
NW Iberia, and the Bohemian Massif typically contain populations
with ages of 0.6 Ga, 1.0–1.2 Ga, 1.5 Ga, 1.8–2.0 Ga, and 2.6 Ga
(Keppie et al., 1998; Linnemann et al., 2004).
The combined Sm-Nd and U-Pb database suggests that the Ca-
domian arc developed upon 2.0–2.1 Ga crystalline basement (Icart
Gneiss), which can be correlated with similar-aged (Icartian) rocks in
the West African craton (Samson and D’Lemos, 1998). The range in
«
Nd
values is most simply attributed to the mixing of magmas derived
from this Icartian–West African basement with juvenile mantle com-
ponents of Cadomian (ca. 600 Ma) age (Nance and Murphy, 1996;
Samson and D’Lemos, 1998).
For Avalonian-type terranes, the 0.75–1.1 Ga T
DM
model ages
suggest derivation of the arc magmas from relatively juvenile (mafic)
basement (proto-Avalonia), which was itself extracted from a depleted
mantle source during an interval that coincides with the ca. 1.1–0.75
Ga life span of the supercontinent Rodinia. Thus ‘‘proto-Avalonia’’ is
inferred to have been formed above one or more subduction zones
within the oceanic lithosphere that surrounded Rodinia (Murphy et al.,
2000). If so, the basements to the Avalonian-type terranes must have
accreted to the northern Gondwanan margin prior to the main 635–570
Ma arc event, at which time the juvenile signature was inherited by
arc-related magmas generated by renewed subduction beneath that mar-
gin. Evidence of such accretion includes the ca. 650 Ma high-grade
metamorphic events recorded in the Avalonian rocks in Britain, south-
ern Newfoundland, Maine, and possibly the Roanoke Rapids terrane
in Carolina (e.g., Murphy et al., 2000; Hibbard et al., 2002). Recent
studies of terrane accretion in the Canadian Cordillera suggest that such
processes can be thick-skinned, and involve up to 100–150 km of
lithosphere, which has isotopic characteristics inherited by younger ig-
neous bodies (e.g., MacKenzie et al., 2005).
The contrasting early histories of the Avalonian- and Cadomian-
type terranes imply that a fundamental suture of Neoproterozoic age
(ca. 650 Ma) must have developed along the northern Gondwanan mar-
gin between the accreted Avalonian-type and the cratonic Cadomian-
type terranes, which is analogous to that between accreted terranes and
continental North America in the Cordillera. Additional sutures may
have also existed between the various Avalonian-type terranes.
PALEOZOIC HISTORY
Peri-Gondwanan terranes are characterized by a widespread Cam-
brian platformal sequence (e.g., Landing, 2005). Faunal and paleomag-
netic data indicate a peri-Gondwanan location for the Avalonian-type
terranes until the Early Ordovician (e.g., Cocks and Torsvik, 2002),
although faunal endemism indicates the temporary presence of a nar-
row seaway between Avalonia and Gondwana in the Early Cambrian
(Landing, 2005).
Recent U-Pb (zircon) and Ar-Ar (muscovite) data from clastic
strata in Iberia (Gutierrez-Alonso et al., 2005) suggest that strike-slip
displacement of terranes along the Gondwanan margin continued from
the late Neoproterozoic into the Cambrian. For example, terranes with
Avalonian-type and Cadomian-type basement signatures were juxta-
posed prior to the Early Ordovician (ca. 485 Ma) deposition of the
regionally extensive Armorican quartzite (e.g., NW Iberia, SW Iberia;
Gutierrez-Alonso et al., 2005), which is thought to reflect the Rheic
rift-drift transition. Dextral motion of Avalonian-type terranes relative
to the Gondwanan margin is consistent with their outboard position
and suggests reactivation of the Neoproterozoic sutures between pre-
viously accreted outboard terranes and Gondwana. Such motion might
have been accompanied by microplate capture of Avalonia in a manner
analogous to the Cenozoic capture of Baja California by the Pacific
plate (Keppie et al., 2003), leading to the development of the narrow
seaways documented by Landing (2005).
Separation from Gondwana by ca. 485 Ma is supported by the
accelerated subsidence recorded in Early Ordovician Avalonian and
Gondwanan passive margin sequences in Britain (e.g., Prigmore et al.,
1997), and by voluminous bimodal rift-related igneous rocks and thick
(;10 km) passive margin strata in various locations along the Gond-
wanan margin (e.g., Ossa-Morena zone in Iberia, Sanchez-Garcia et al.,
2003; the Acatlan and Oaxacan complexes of Mexico, Keppie et al.,
2006). By 460 Ma, Avalonia lay at ;408S (Hamilton and Murphy,
2004), about 1700–2000 km south of Laurentia, whereas Gondwana
remained at 608S (e.g., Cocks and Torsvik, 2002). This implies a north-
erly component of drift for Avalonia of ;8 cm/yr. We view this drift
as recording the opening of the Rheic Ocean.
Paleomagnetic studies indicate minimal latitudinal separation be-
tween the Carolina terrane and Laurentia by ca. 455 Ma, implying that
Avalonia was ;2000 km south of Carolina at this time. However, as
Hibbard et al. (2002) pointed out, the current distance between the
sampled sites is ;1900 km, so that Carolina and Avalonia could rep-
resent the leading and trailing edges, respectively, of the same
microplate.
ORIGIN OF THE RHEIC OCEAN
Avalonia and Carolina were geodynamically linked to the north-
ern Gondwanan margin from ca. 650 Ma to ca. 490 Ma, but had drifted
;2000 km north of this margin by ca. 460 Ma. Opening of the Rheic
Ocean began with the ca. 500–490 Ma separation of Avalonia and
Carolina from Gondwana, while the Cadomian-type terranes remained
along the Gondwanan margin (e.g., Cocks and Torsvik, 2002). Hence,
the inception of the Rheic Ocean involved the rift and drift of terranes
that were outboard of the Neoproterozoic suture zone between the ac-
creted Avalonia-type terranes and cratonic northwestern Gondwana.
NW Iberia has Avalonian-type basement yet remained along the Gond-
wanan margin during initial rifting. We suggest that its Neoproterozoic
transfer from a peri-Amazonian realm to a peri–West African realm
removed it from its more outboard position (Fig. 1).
The suture zone was reactivated, first in the late Neoproterozoic
during oblique subduction associated with the main arc phase, and
second during the progressive generation of a transform margin in the
Ediacaran–Early Cambrian, at which time, the movement of Avalonia
relative to Gondwana may have been accompanied by microplate cap-
ture (Keppie et al., 2003). This diachronous generation of a transform
margin may have also produced a slab window (Dickinson and Snyder,
1979), resulting in crustal thinning and asthenospheric upwelling that
could have caused the voluminous Late Cambrian–Early Ordovician
rift-related magmatism and provided the thermally weakened rheology
to facilitate the rift-drift transition of Avalonia-Carolina. Such a mech-
anism would account for the apparent longevity of magmatism after
Avalonia-Carolina had separated.
Taken together, the tectonothermal evolution of the Avalonian-
and Cadomian-type terranes suggests that both the site of initial rifting
and the subsequent development of the Rheic Ocean were profoundly
influenced by the reactivation of suture zones generated at ca. 650 Ma
by the accretion to the Gondwanan margin of the juvenile Avalonian-
type basement derived from the peri-Rodinian ocean. If so, the thermal

328 GEOLOGY, May 2006
structure and style of magmatism and passive margin development dur-
ing the formation of the Rheic Ocean were profoundly influenced by
earlier events.
The actual mechanism of rift-to-drift is uncertain. Several models
(e.g., van Staal et al., 1998) imply that the Rheic Ocean initiated as a
backarc basin, but evidence for arc-related rocks coeval with rifting
along the northern Gondwanan margin is equivocal. Alternatively,
since the opening of the Rheic Ocean is coeval with a polarity flip
along the northern Iapetus margin and the onset of northwesterly di-
rected subduction and ridge-trench collision (Fig. 1A; e.g., Stampfli
and Borel, 2002; van Staal et al., 1998), the portion of the Avalonian-
Carolinian microplate captured from Gondwana during the Early Cam-
brian may have been pulled away from Gondwana by slab pull in a
manner analogous to the opening of the Neotethys in the Cenozoic
(Stampfli and Borel, 2002). This would require the absence of a spread-
ing ridge between Avalonia-Carolina and the Laurentian margin and,
given the moderately rapid northerly component of motion of Avalonia
between 480 and 460 Ma (8 cm/yr; Hamilton and Murphy, 2004), the
presence of a spreading ridge in the Rheic Ocean that was approxi-
mately E-W in orientation.
ACKNOWLEDGMENTS
We acknowledge the support of the Natural Sciences and Engineering
Council, Canada (Murphy and Dostal), Ministerio de Educacion y Ciencia Re-
search Projects BTE2003-05128 (Gutierrez-Alonso), CGL2004-0463-CO2/BTE
(Fernandez-Suarez), and 1FD2003-2324 (Quesada), National Science Founda-
tion grant EAR-0308105 (Nance), and Papiit grant IN103003 (Keppie). We are
grateful to Ulf Linnemann, Victor Ramos, Steve Whitmeyer, Nigel Woodcock,
and an anonymous reviewer for constructive reviews. This is a contribution to
International Geological Correlation Project 497.
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Manuscript received 15 July 2005
Revised manuscript received 25 October 2005
Manuscript accepted 29 October 2005
Printed in USA

ca. 540 Ma
s
s
s
s
C
NW-I
OM-CAD
Ch-Oax-Y-Fl
A
I
a
p
e
t
u
s
O
c
e
a
n
B
A
C
D
ca. 500 Ma
s
s
s
s
s
s
s
I
a
p
e
t
u
s
O
c
e
a
n
ca. 480 Ma
B
a
l
t
i
c
a
s
s
s
s
s
s
s
I
a
p
e
t
u
s
O
c
e
a
n
ca. 470 Ma
B
a
l
t
i
c
a
I
a
p
e
t
u
s
O
c
e
a
n
s
s
s
s
s
s
s
ca. 450 Ma
B
a
l
t
i
c
a
I
a
p
e
t
u
s
O
c
e
a
n
s
s
s
s
s
s
s
s
s
s
s
s
ca. 430 Ma
B
a
l
t
i
c
a
R
h
e
i
c
O
c
e
a
n
Subduction Zones
Sutures and paleo-sutures
Mid Oceanic Ridges
Tornquist Transform Fault
s
s
s
s
s
s
R
h
e
i
c
O
c
e
a
n
R
h
e
i
c
O
c
e
a
n
Arrested subduction
into transform fault
Avalonia Seaway
Cadomian Basement
Cratons and pan-African Belts
Iapetus and Rheic oceans
Avalonian Basement
Taconic Volcanic Arc
Continental Shelf
CHUR
0 0.4 0.8 1.2 1.6
-6
-4
0
-2
+2
+4
+6
+8
DEPLETED MANTLE
Carolina
Avalonia
Avalonian Type
ε
Nd
a
b
Rodinia
a-amalgamation
b-breakup
ε
Nd
CHUR
0 0.4 0.8 1.2 1.6
DEPLETED MANTLE
2.0
2.4
0
-6
-10
-2
+2
+6
+10
-14
-18
-22
Armorica
Icart Gneiss
Saxo Thuringia
Cadomian Type
Oaxaquia
Age (Gy)
Age (Gy)
NW AFRICA
NORTH AMERICA
SOUTH
AMERICA
?
?
NW-I
OM
ARM
Fl
C
A
A
Ch-Oax-Y
W EUROPE
BM
Neoproterozoic Suture
?
Rheic Suture
ARM
OM