The Acadian Orogeny in the Northern Appalachians

Abstract

The Acadian orogeny, involving deposition of clastic wedges, deformation, metamorphism, magmatism, and exhumation, is limited in time to the Devonian, and in space to the northern mainland Appalachians. Conventional interpretations attribute it to collision between Laurentia and Avalon or Meguma terranes. However, advances in paleogeography indicate that the Avalon and Meguma terranes were accreted to Laurentia in the Late Ordovician and Early Silurian, which was synchronous with closure of the Iapetus Ocean. On the other hand, the Rheic Ocean remained open to the south. In this context, we propose that the Acadian orogeny developed on an Andean-type margin, and attribute it to flattening of the subduction zone as a consequence of collision of an oceanic plateau surrounding a plume. This model explains: (1) SE to NW diachronism in the onset of deformation throughout the Devonian; (2) the development of a ~400-380 Ma magmatic arc gap in Maritime Canada that was abruptly terminated in the Meguma terrane at ~380-370 Ma by (3) intrusion of voluminous felsic magmatism and plume-related lamprophyres as the plume thermally eroded the oceanic lithosphere, causing melting of the lower crust; (4) accompanying regional high-T, low-P metamorphism related to thermal anomalies above a plume; (5) emplacement of gold deposits and associated siderophile elements, possibly derived from fluid circulation above an ascending plume; and (6) rapid Late Devonian exhumation of ~10 km attributed to dynamic uplift over the plume. As the plume head migrated northward, the anomalously intense bimodal magmatism shifted into the Cobequid Highlands (Avalon terrane) at ~360 Ma, and then to the Magdalen Islands, where ~330 Ma plume-related magmatism occurred above a high-density, lower crustal lens interpreted as plume-derived underplated mafic rocks. Late Carboniferous formation of the Maritimes basin is attributed to cooling of this decapitated plume head.

Full text

663
International Geology Review, Vol. 47, 2005, p. 663–687.
Copyright © 2005 by V. H. Winston & Son, Inc. All rights reserved.
0020-6814/05/810/663-25 $25.00
The Acadian Orogeny in the Northern Appalachians
J. BRENDAN MURPHY1
Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada
AND J. DUNCAN KEPPIE
Instituto de Geología, Universidad Nacional Autónoma de México, México D.F. 04510, México
Abstract
The Acadian orogeny, involving deposition of clastic wedges, deformation, metamorphism, mag-
matism, and exhumation, is limited in time to the Devonian, and in space to the northern mainland
Appalachians. Conventional interpretations attribute it to collision between Laurentia and Avalon or
Meguma terranes. However, advances in paleogeography indicate that the Avalon and Meguma ter-
ranes were accreted to Laurentia in the Late Ordovician and Early Silurian, which was synchronous
with closure of the Iapetus Ocean. On the other hand, the Rheic Ocean remained open to the south.
In this context, we propose that the Acadian orogeny developed on an Andean-type margin, and
attribute it to flattening of the subduction zone as a consequence of collision of an oceanic plateau
surrounding a plume. This model explains: (1) SE to NW diachronism in the onset of deformation
throughout the Devonian; (2) the development of a ~400–380 Ma magmatic arc gap in Maritime
Canada that was abruptly terminated in the Meguma terrane at ~380–370 Ma by (3) intrusion of
voluminous felsic magmatism and plume-related lamprophyres as the plume thermally eroded the
oceanic lithosphere, causing melting of the lower crust; (4) accompanying regional high-T, low-P
metamorphism related to thermal anomalies above a plume; (5) emplacement of gold deposits and
associated siderophile elements, possibly derived from fluid circulation above an ascending plume;
and (6) rapid Late Devonian exhumation of ~10 km attributed to dynamic uplift over the plume. As
the plume head migrated northward, the anomalously intense bimodal magmatism shifted into the
Cobequid Highlands (Avalon terrane) at ~360 Ma, and then to the Magdalen Islands, where ~330
Ma plume-related magmatism occurred above a high-density, lower crustal lens interpreted as
plume-derived underplated mafic rocks. Late Carboniferous formation of the Maritimes basin is
attributed to cooling of this decapitated plume head.
Introduction
THE APPALACHIAN OROGEN extends for more than
3000 km from Newfoundland to Alabama along the
eastern margin of North America, and pre-Mesozoic
reconstructions imply former continuity and genetic
linkages with the Caledonide and Variscan orogens
of western Europe (Fig. 1). It is now generally
accepted that the Appalachian-Caledonide-
Variscan orogen represents the destruction of Paleo-
zoic oceans such as the Iapetus and Rheic oceans,
in which the accretion of suspect terranes to Lauren-
tia-Baltica at various times during the Ordovician–
Devonian was followed in the Permo-Carboniferous
by terminal continental collision with Gondwana
and the formation of Pangea (e.g., Williams, 1979;
Keppie, 1985; van Staal et al., 1998 and references
therein).
The destruction of these oceans is recorded by
several episodes of orogenic activity that have
generally been related to collision of various
terranes with each other or with cratons (see Fig. 2).
Thus, the earliest orogenic event, the Late Cam-
brian–Middle Ordovician Penobscotian ororeny, has
been attributed to the amalgamation of composite
terranes within the Iapetus Ocean (Keppie, 1993;
Stewart et al., 1995). The early to mid-Ordovician
Taconic orogeny is inferred to have been the result
of collision between Laurentia and outboard oceanic
arcs, backarcs, and oceanic terranes that originated
in the Iapetus ocean and contain sparse Laurentian
fauna (e.g., van Staal et al., 1998). The Late Ordovi-
cian–Silurian Salinic orogeny has been related to
the accretion of the Gander, Avalon, Nashoba, and
Carolina terranes to Laurentia during the closure of
1Corresponding author; email:bmurphy@stfx.ca

664 MURPHY AND KEPPIE
Iapetus (e.g., Keppie, 1985, 1993; van Staal et al.,
1998; Hibbard et al., 2002). The Devonian Acadian
orogeny has been attributed to collision of either
Avalonia with Laurentia (Robinson et al., 1998) or to
the accretion of the Meguma terrane (e.g., van Staal
et al., 1998). There is general consensus that the last
orogenic event, the Late Carboniferous–Permian
Alleghenian orogeny, was due to terminal collision
between Gondwana and Laurentia-Baltica that
closed the Rheic Ocean and resulted in the forma-
tion of Pangea.
Although modern analogs record orogenic events
due to collision of terranes, they also document
widespread orogenic activity related to other mech-
anisms, such as flat-slab subduction and changes of
the convergence rates and directions. Thus in the
Andes, flat-slab subduction occurs in four geo-
graphically limited zones, two of which are related
to collision of a mid-oceanic ridge with the trench
(the Chile and Cocos ridges; Ramos and Alleman,
2001), one is inferred to result from subduction of an
oceanic plateau (the Inca plateau; Gutscher, 2002),
and one is genetically connected with subduction of
a seamount chain (San Fernandez ridge; Ramos et
al., 2002). In the Cordillera of western North Amer-
ica, the Late Cretaceous–Early Tertiary Laramide
orogeny has been attributed to overriding of the
Yellowstone hotspot (Murphy et al., 1998). These
different mechanisms produce different geological
records. Thus, ridge-trench collision is unique
in producing a gap in the magmatic arc and slab
windows that are associated with rift-related mag-
matism (Thorkelson and Taylor, 1989). Plume-
trench collision also produces a magmatic-arc gap
but is associated with a wide zone of deformation,
and may be followed by extensive plume-related
magmatism after the plume burns through the
subducting slab and into the overlying lithosphere.
Subduction of aseismic ridges and changes in the
rate of convergence, although producing a mag-
matic-arc gap and a wide zone of deformation, are
not associated with rift or plume magmatism. In the
southwestern Pacific Ocean, Collins (2002) has
related contractions (i.e., orogenic events) and
extension to alternations of flat-slab and steep sub-
duction that are restricted in both time and space.
The difference between the southwestern and
eastern Pacific may be related to different rates of
convergence across the trench. However, in all of
these cases, subduction is ongoing and its effects
are restricted to a narrow zone, whereas flat-slab
subduction is limited to short episodes with effects
that have dimensions congruent with the along-
strike width of the flat slab but a wide across-strike
extent. In such cases, narrow and wide orogenic
zones should alternate along any one active margin.
This necessitates a close examination of the extent
and duration of orogenic events in an orogen, rather
than assuming that they may be correlated along the
entire length of the orogen.
Flat-slab and convergence rate mechanisms for
orogenic activity are probably more common in the
FIG. 1. Pre-Mesozoic reconstruction of the Appalachian-Caledonide-Variscan orogen, (from McKerrow et al., 2000).
Locations of Figures 5 (Maritime Canada), 6 (New England) , 7 (Carolinas) , and 8 (Newfoundland) also are shown.

ACADIAN OROGENY 665
Appalachians than presently recognized. Murphy et
al. (1999) and Keppie and Krogh (2000) proposed
that the geological record of the Meguma and Avalon
terranes in Maritime Canada during the Acadian
orogeny is consistent with flat-slab subduction
caused by subduction of an oceanic plateau and a
related plume. This paper extends the implication
of plume-trench collision as a mechanism for the
Acadian orogeny to the northern Appalachians.
The mechanism deduced for the various orogenic
events in the Appalachians is largely conditional
upon inferred paleogeographic reconstructions. For
example, the closure of ocean basins between
terranes is generally related to collision orogenic
events. Thus the closure of the Iapetus Ocean has
been related to both the Ordovician–Silurian Salinic
orogeny (e.g. Keppie, 1985, 1993; van Staal et al.,
1998; Hibbard et al., 2002) and the Devonian
Acadian orogeny (Robinson et al., 1998). Similarly
the closure of the Rheic Ocean between the Meguma
and Avalon terranes has been related to the Acadian
orogeny (e.g., Schenk, 1997; van Staal et al., 1998),
although some authors have suggested that the
Meguma Group was deposited upon an Avalonian
FIG. 2. Early Ordovician to Late Silurian paleogeographic reconstructions of the Circum-Atlantic region (based on
Cocks and Torvik, 2002; Fortey and Cocks, 2003).

666 MURPHY AND KEPPIE
basement (Keppie and Dostal, 1991; Keppie et al.,
1997; Murphy et al., 2004b). In view of these con-
trasting views, we first address the paleogeography
just before the onset of the Acadian orogeny.
Silurian Paleogeography
The time of closure of the Iapetus Ocean between
eastern Laurentia and Avalonia has been inferred to
be either Late Ordovician–Early Silurian or Devo-
nian; however, geological and geophysical data are
accumulating in support of the former. Lithologic
(O’Brien et al., 1983, 1996) and paleomagnetic data
(Johnson and Van der Voo, 1986; Van der Voo, 1988)
as well as faunal evidence (Pickering et al., 1988;
Cocks and Fortey, 1990; Landing, 1996; Cocks and
Torsvik, 2002; Fortey and Cocks, 2003) indicate
that Avalonia was located along the periphery of
Gondwana from the late Neoproterozoic to the Early
Ordovician. Separation between Avalonia and
Gondwana gradually increased during the Ordovi-
cian (Figs. 2A and 2B). Paleontological evidence
supports faunal linkages of Avalonia with Baltica by
the Late Ordovician (e.g., Williams et al., 1995;
Fortey and Cocks, 2003). Recent geochemical and
isotopic data from the ~1900 m Late Ordovician–
Early Devonian sedimentary sequence of the
Arisaig Group show that the clastic rocks contrast
with the underlying Avalonian basement rocks, indi-
cating that they were not derived from Avalonian
basement (Murphy et al., 1996b, 2004a).
All sedimentary rocks are characterized by
strongly negative εNd (from –4.8 to –9.3) and TDM
ages >1.5 Ga, with an overall trend toward increas-
ingly negative εNd values from the base to the top of
the group (Fig. 3). U-Pb detrital zircon data from
Lower Silurian (Beechill Cove Formation), Middle
Silurian (French River Formation), and Lower Devo-
nian (Stonehouse Formation) strata of the Arisaig
Group have similar zircon populations (Fig. 4).
Some zircons are close to their respective deposi-
tional ages, suggesting that Arisaig basin formation
may have been broadly coeval with active volcanism
in the orogen. These rocks are also characterized by
minor Ordovician and Cambrian zircons, significant
populations of Neoproterozoic (550–700 Ma), Meso-
proterozoic (0.95–1.3 Ga), Paleoproterozoic (2.0–
2.2 Ga), and minor Archean zircons. A comparison
between these data and the age of tectonothermal
events in potential source areas (Fig. 4), together
with regional geologic data, suggest that Arisaig
Group strata were primarily derived from Baltica
(Murphy et al., 2004a).
Paleomagnetic data indicate a 41°S ± 8° paleo-
latitude for the Avalon terrane in Nova Scotia (Dunn
Point volcanics: Van der Voo and Johnson, 1985,
recently dated at ~460 Ma; Hamilton and Murphy,
2004). Inasmuch as Laurentia lay at a paleolatitude
of ~20°S between 460 and 440 Ma (e.g., Mac-
Niocaill and Smethurst, 1994), this implies that
Avalonia was located about 1700–2000 km south of
the Laurentian margin at 460 Ma. Paleomagnetic
data suggest that any paleolatitudinal separation
between Avalonia and Laurentia had disappeared
by the Early Silurian (Miller and Kent, 1988; Trench
and Torsvik, 1992; Potts et al., 1993; MacNiocaill et
al., 1997). This indicates a latitudinal convergence
rate between Avalonia and Laurentia between 460
and 440 Ma of about 5.5 cm/yr (i.e., the destruction
of the Iapetus Ocean). As Gondwana remained rela-
tively stationary during this period (e.g., Torsvik et
al., 1996), these data imply a northerly component
of drift of Avalonia from Gondwana of about 8 cm/yr
(development of the Rheic Ocean). Taken together,
these data define the southern limit of Iapetus and
the northern limit of the Rheic Ocean at 460 Ma.
Thus these data indicate a connection between the
death of the Iapetus Ocean and the Salinic orogeny,
rather than with the Acadian orogeny.
In the southern and central Appalachians, paleo-
magnetic studies indicate minimal latitudinal sepa-
ration between the Carolina terrane and Laurentia
by ~455 Ma (Vick et al., 1987; Noel et al., 1988).
According to Hibbard (2000), sinistral accretion of
Carolina and related terranes to Laurentia began in
the Middle to Late Ordovician and continued into
the Early Silurian, as evidenced by 40Ar/39Ar cool-
ing ages on micas that define the regional cleavage
in the Carolina zone. Although paleomagnetic data
indicate that Avalonia was at 41°S (Johnson and Van
der Voo, 1990; Hamilton and Murphy, 2004) at ~460
Ma, suggesting a separation of ~2000 km, Hibbard
et al. (in press) point out that because the current
direct separation between sampled sites is ~1900
km, Carolina and Avalonia could represent the lead-
ing and trailing edges, respectively, of the same
plate.
The Acadian orogeny has also been attributed to
the collision of the Meguma and Avalona terranes
and the closure of the Rheic Ocean. However, the
original relationship between the Meguma and
Avalon terranes is controversial (e.g., Keppie 1993;
van Staal et al., 1998). Many authors have inferred

ACADIAN OROGENY 667
FIG. 3. A. εNdt versus 147Sm/144Nd diagram (t = 430 Ma) comparing Sm-Nd isotopic data for the Arisaig Group with
typical Sm-Nd isotopic compositions of Avalonian crust (Murphy and MacDonald, 1993; Murphy et al., 1996b). The Sm-
Nd isotopic characteristics for the average upper crust are bracketed between modern global average river sediment
(147Sm/144Nd = 0.114; TDM = 1.52 Ga; Goldstein and Jacobsen, 1988) and the average age of sedimentary mass (Miller
et al., 1986). See also Thorogood (1990). Iapetan crust includes normal and depleted island arc tholeiites, and ophiolitic
complexes in Newfoundland and Norway (Pedersen and Dunning, 1997; MacLachlan and Dunning, 1998). Silurian
strata of England and Wales (SEW) from Thorogood et al. (1990), and Meguma terrane metasedimentary rocks (MMS) from
Clarke et al. (1997). B. εNdt versus time (Ga) diagram (t = 430 Ma) comparing Sm-Nd isotopic data for the Arisaig Group
with typical Sm-Nd isotopic compositions of Avalonian crust (Murphy et al., 1996b, 2000). Field for Grenville rocks after
Patchett and Ruiz (1989), Dickin and McNutt (1989), Dickin et al. (1990), Daly and McLelland (1991), McLelland et al.,
(1993), Dickin (2000). C. Variation in εNdt (t = 430 Ma) with stratigraphic height in the Arisaig Group. Beechill Cove
Formation data from Murphy et al., 1996b.

668 MURPHY AND KEPPIE
FIG. 4. Detrital zircon ages (open circles) from coeval uppermost Ordovician to Lower Devonian clastic sequences in the Avalon terrane (Arisaig Group) and Meguma terrane (Annap-
olis Valley), after Murphy et al. (2004b). In the Avalon terrane, BC-1 is from the Lower Silurian Beechill Cove Formation and SH-1 is from the Lower Devonian Stonehouse Formation. In
the Meguma terrane, WR-1 is from the Upper Ordovician–Lower Silurian Whiterock Formation and TB-1 is from the Lower Devonian Torbrook Formation. These data are compared with
detrital-zircon data from underlying uppermost Neoproterozoic–Lower Ordovician Meguma Group (Krogh and Keppie, 1990) and Neoproterozoic Avalonia (Keppie et al., 1998; Bevier et
al., 1990). Symbols: x = concordant U-Pb zircon ages; filled circles = discordant 207Pb/206Pb ages. Also shown are tectonothermal events in Baltica (Gower et al., 1990; Roberts, 2003),
eastern Laurentia (Cawood et al., 2001), Amazon craton (Sadowski and Bettencourt, 1996), northwest Africa (Rocci et al., 1991), and Gander (van Staal et al., 1996). Abbreviations: NS =
Nova Scotia; NB = New Brunswick; NE = New England.

ACADIAN OROGENY 669
that the Meguma Group represents a Cambrian–
Early Devonian passive margin bordering northwest
Africa that was transferred to Laurentia during the
Acadian orogeny (e.g., Schenk, 1997 and references
therein). This was based primarily upon: (1)
proposed stratigraphic correlations between the
Cambro-Silurian strata in the Meguma terrane and
coeval sequences in Morocco (Schenk, 1997); and
(2) the Middle Devonian age of the Acadian orogeny,
the oldest accretionary event recognized in the
Meguma terrane. According to this model, the
Avalon and Meguma terranes lay on the opposite
sides of the Rheic Ocean, and were juxtaposed
during the Acadian orogeny.
Alternatively, the Cambrian–Early Devonian
strata of the Meguma terrane have been interpreted
as a passive margin bordering the Avalon microcon-
tinent, which would imply that the Meguma Group
was deposited on Avalonian continental crust (e.g.,
Keppie et al., 2003 and references therein). This is
based upon (1) the proposed correlation of the
Upper Ordovician–Lower Devonian units in the
Meguma terrane with coeval sequences in the rest of
the Appalachians (Keppie and Krogh, 1999); and
(2) the similarity of Nd-isotopic signatures in Late
Ordovician–Early Silurian crustally derived igneous
suites in the Meguma and Avalon terranes (Keppie
et al., 1997). As the Silurian to Lower Devonian
strata predate the Acadian orogeny, and no older
deformational events are recorded, these data have
been interpreted to indicate that the Meguma Group
rested depositionally on Avalonian basement in the
Cambro-Ordovician. In this interpretation, Avalonia
and Meguma lay on the same (northern) flank of the
Rheic Ocean, and together collided with Laurentia
by the Early Silurian. Inasmuch as the Meguma is
the most outboard terrane in the Appalachians, this
latter model would imply that the eastern flank of
the Appalachians would have resembled a modern
Cordilleran or Andean margin.
U-Pb detrital zircon data from Ordovician and
Lower Devonian clastic sedimentary rocks in the
Meguma terrane (Fig. 4); (White Rock and Torbrook
formations in the Annapolis Valley, respectively) are
very similar to those from the Arisaig Group (Avalon
terrane; Murphy et al., 2004b). In addition to abun-
dant Cambrian–Late Neoproterozoic and Paleopro-
terozoic zircons, Late Ordovician–Early Devonian
samples from the Meguma terrane have important
Mesoproterozoic zircon populations (1.0–1.4 Ga)
that are typical of Avalonia, and strongly suggest
their contiguity by Late Ordovician–Early Silurian
time. Because Avalonia had accreted to Laurentia-
Baltica by the Late Ordovician, these data suggest
that the Meguma terrane, like Avalonia, resided
along the same (northern) margin of the Rheic
Ocean at that time. That these terranes were juxta-
posed throughout the Early Paleozoic and probably
into the Neoproterozoic is also indicated by the
absence of evidence for a Cambro-Ordovician
accretionary event in both the Meguma and Avalo-
nian stratigraphy, the lack of intervening suture
zone ophiolitic units, and the similarity of Avalonian
and Meguma basement Nd isotopic signatures in
Paleozoic igneous suites (Keppie et al., 1998). In
view of these recent data, the Acadian orogeny can-
not be due to accretion of the Meguma and Avalon
terranes.
Acadian Orogeny
The Acadian orogeny is named for Acadia, the
old French name for the Maritime Provinces of
Canada (Nova Scotia, New Brunswick, and Prince
Edward Island). It is the term originally defined for
the middle Paleozoic deformation event in the
Appalachian orogen of Maritime Canada (including
eastern Quebec). The term “Acadian” was intro-
duced by Schuchert (1923) to explain Late Devo-
nian deformation in the Chaleur Bay region of New
Brunswick and Gaspé, Quebec. Several workers
including Boucot et al. (1964) and Poole et al.
(1970) showed that this deformation was regionally
extensive in the northern Appalachians (Fig. 5). In
Maritime Canada, the Acadian orogeny affected the
entire width of the orogen, from the most outboard
terrane (Meguma) to the Laurentian foreland. Dono-
hoe and Pajari (1974) showed that the age of peak
Acadian deformation became progressively younger
northward from coastal Maine to Gaspé, varying
from Gedinnian to post-Eifelian in age, a trend con-
firmed in a recent synthesis by Robinson et al.
(1998).
The effects of the Acadian orogenic event are
now widely interpreted to extend along the south-
eastern Laurentian margin from Pennsylvania to
Newfoundland, and to correlative rocks in Western
Europe (Bradley, 1989; Williams, 1979, 1993;
Woodcock and Strachan, 2000) where Late Paleo-
zoic deposits overlie, with angular unconformity,
previously deformed Early Paleozoic sequences.
This regional unconformity is commonly expressed
by widespread Lower Devonian platformal carbon-
ates overlying Taconic foredeep deposits, both

670 MURPHY AND KEPPIE
FIG. 5. Map of Appalachian orogen in Atlantic Canada and northern Maine showing Silurian–Devonian igneous rocks, the migration of Silurian–Devonian Acadian foredeep and
deformation front through time (data from Robinson et al., 1998, extrapolated into New Brunswick), and Devonian–Carboniferous magmatism associated with plume activity (modified from
Murphy and Keppie, 1998). Silurian arc magmatism and Early Devonian bimodal magmatism are associated with convergence of Avalonia and Meguma to Laurentia, and possibly related
to resultant slab break-off (see van Staal et al., 1998). The proposed plume track from ca. 390 to 330 Ma is shown with a large arrow. The flat-slab model applies to the Acadian orogeny
in Maritime Canada and adjacent Maine. Motions of upper and lower plates in a hotspot reference frame dictate that swell and plume would be overridden at different locations along the
continental margin. Hence Late Devonian tectonism in New England may also be explained by this model.

ACADIAN OROGENY 671
traditionally recognized as clear demarcations
between earlier Taconic and Acadian orogenic
events. More recently, this demarcation has been
related to a switch from sinistral-to dextral-domi-
nated kinematics along major NE-trending faults
(e.g., Keppie, 1993; Nance and Dallmeyer, 1993;
Cawood et al., 1994; Anderson et al., 2001).
Meguma terrane
Stratigraphy. The Meguma terrane is exposed
only in mainland Nova Scotia (Fig. 5), where it is
separated from the Avalon terrane to the north by
the Minas fault zone. The terrane is underlain by
thick (~13 km) Cambrian–Ordovician turbidites
(Schenk, 1997) of the Meguma Group containing a
Gondwanan fauna (Pratt and Waldron, 1991) that
are disconformably to conformably overlain by a 2.3
to 4.5 km thick Upper Ordovician to Lower Devo-
nian sequence of bimodal volcanic rocks overlain by
shallow-marine to continental clastic rocks with
Rhenish-Bohemian fauna (Boucot, 1975). The
Ordovician switch from turbidites to shallow marine
deposits was accompanied by a change from a
south-southwesterly to northwesterly source. Detri-
tal zircons in the Goldenville Formation (lower unit
of the Meguma Group) yielded ca. 3.0 Ga, 2.0 Ga,
and 600 Ma ages and also indicate a Gondwanan
(West African) source prior to separation from Gond-
wana (Krogh and Keppie, 1990). Near the base of
the Upper Ordovician–Lower Devonian sequence, a
rhyolitic tuff of the White Rock Formation yielded
concordant U-Pb ages of ~440 Ma (Keppie and
Krogh, 2000; MacDonald et al., 2002).
Structure. The earliest effects of the Acadian
orogeny in the Meguma terrane began at ~415 Ma
with regional metamorphism (predominantly green-
schist-facies), polyphase deformation, and cleavage
formation (Keppie and Dallmeyer, 1995) and contin-
ued to the Late Devonian (Hicks et al., 1999). On a
regional scale, cleavage is axial planar to NE-trend-
ing shallowly plunging periclinal folds. Several
studies propose that the onset of the Acadian orog-
eny at ~415 Ma was associated with oblique dextral
convergence between the Meguma and Avalon ter-
ranes (Keppie and Dallmeyer, 1995). The current
boundary between these terranes is defined by the
E-W Minas fault zone (Fig. 5), along which several
episodes of late Paleozoic dextral motion has
occurred (e.g., Keppie, 1982; Murphy, 2003). Adja-
cent to this fault zone, regional folds are rotated
clockwise, consistent with late Paleozoic dextral
motion. Traced westward, this boundary swings into
the NE-SW Bay of Fundy (Fig. 5). Seismic profiles
in the Bay of Fundy show the major structure to be a
SE-dipping Mesozoic listric normal fault inferred
to be located along a late Paleozoic listric thrust
zone (Keen et al., 1991). Coeval late Paleozoic
(Alleghanian) deformation has been documented by
Culshaw and Liesa (1997). The down-dip extension
of the sole thrust either maintains its SE dip to the
Moho or flattens at 15 km depth between the lower
and upper crusts (Keen et al., 1991). The latter
model implies that the Meguma terrane is allochth-
onous and that the Minas fault zone may be a lateral
ramp (Keppie, 1993).
Magmatism. In the Meguma terrane, intrusion of
widespread late syntectonic ~375–370 Ma grani-
toids and minor mafic dikes (Clarke et al., 1993,
1997) were accompanied by high T-low P metamor-
phism and shear zone deformation. Although these
plutons clearly cross-cut early Acadian fabrics,
studies by Benn et al. (1997) show that they crystal-
lized during the latest stages of Acadian deformation
and that their geometry is structurally controlled.
Geophysical studies (e.g., Keen et al., 1991) show
that the plutons have a broadly laccolithic geometry,
and extend to depths of 5–10 km, with feeder zones
extending to 25 km.
Exhumation. The earliest evidence of exhuma-
tion of Meguma terrane rocks is their occurrence as
clasts in latest Devonian–Tournaisian Horton Group
rocks (Martel et al., 1993), implying an uplift of
between 6 and 10 km between ~370 and 360 Ma
(e.g., Keppie and Dallmeyer, 1995; Jennex et al.,
2000; Murphy, 2000; Murphy and Hamilton, 2000).
Avalon terrane in mainland Nova Scotia
and New Brunswick
To the north of the Minas fault zone in mainland
Nova Scotia, Avalonia (Fig. 5), the largest suspect
terrane in the northern Appalachian orogen, occu-
pies much of the southern flank of the Appala-
chians. Correlative rocks occur in Ireland, southern
Britain, and adjacent parts of continental Europe,
thereby providing a potential genetic linkage
between orogenic events in Laurentia and Western
Europe (e.g., McKerrow et al., 1991, 2000; Wood-
cock and Strachan, 2000).
Stratigraphy. Ordovician–Lower Devonian rocks
in Avalonia of mainland Nova Scotia unconformably
overlie older Neoproterozoic and Cambrian strata
that contain typical Avalonian fauna (Landing and
Murphy, 1991). Ordovician strata consist of ~460
Ma bimodal intracontinental volcanic rocks (Dunn

672 MURPHY AND KEPPIE
Point Formation) and interbedded red clastic sedi-
ments disconformably overlain by an 1800 m con-
tinuous stratigraphic sequence dominated by
shallow-marine to continental, fossiliferous silici-
clastic rocks (Arisaig Group and correlatives).
Waldron et al. (1996) interpreted the Silurian–
Early Devonian subsidence history of the Arisaig
Group as an initial phase of rapid subsidence and
extension (30% to 60%) in the Early Llandovery,
followed by thermal relaxation and slower subsid-
ence rates in the Wenlockian and Ludlow. Vastly
increased subsidence rates and accommodation
space in the Pridoli (as represented by the Stone-
house Formation) was attributed to loading of the
Avalonian margin due to interaction with a neigh-
boring terrane (Waldron et al., 1996), probably the
Meguma terrane.
The Arisaig Group is unconformably overlain by
Middle Devonian interbedded basalts and continen-
tal rocks, and this unconformity is widely inter-
preted to represent the local manifestation of the
Acadian orogeny.
Structure. In the Fredericton trough, the earliest
Acadian deformation consists of upright, isoclinal
folds with a low-grade cleavage deformed Lland-
overy-Gedinnian rocks before intrusion of granitoid
plutons at 406 ± 7 Ma.
Magmatism. In the Cobequid Highlands of main-
land Nova Scotia, Middle to Late Devonian (~360
Ma) magmatism is particularly voluminous, and is
represented by bimodal volcanic and plutonic rocks
(Doig et al., 1996; Piper et al., 1993).
In southern New Brunswick, the Kingston Com-
plex (e.g., Currie, 1987) or terrane (Barr et al.,
2002), is considered to be either part of Avalonia
(e.g., Keppie and Dostal, 1991) or Ganderia (Barr et
al., 2002). It consists of arc-related metavolcanic
and metasedimentary rocks intruded by Early Sil-
urian granite plutons and mafic dikes. The intrusive
age of the mafic dikes in uncertain, but 40Ar/39Ar
analyses on igneous hornblende are interpreted as a
~415 Ma cooling age (Nance and Dallmeyer, 1993).
Magmatism in the Kingston Complex has been
genetically related to the compositionally similar
Late Silurian coastal Maine Complex.
The geochemistry of ~430–422 Ma Avalonian
granitoids is typical of subduction, but a younger
event (~396–367 Ma) has more mafic components
and more juvenile signatures, which have been
attributed to crustal delamination by Whalen et
al. (1994). Silurian–Early Devonian arc-related
plutonic and volcanic rocks switch off at the same
time as deformation was occurring in the Central
Maine Basin.
Siluro-Devonian geology in central and southern
New Brunswick (north of the Kingston Complex) is
dominated by relatively low grade volcanic and
sedimentary rocks occurring in anticlinoria, which
are affected by widespread plutonism and deforma-
tion, separated by basins that preserve a relatively
continuous stratigraphic record. Deformation was
accompanied by regional dextral shear (e.g., Nance
and Dallmeyer, 1993; Schreckengost and Nance,
1996). Voluminous Siluro-Devonian granitoid rocks
(Whalen et al., 1996) have metaluminous to slightly
peraluminous, within-plate chemistry. Silurian
plutons have complex histories that may involve
input of a juvenile component, whereas Devonian
granitoids represent partial melting of hybridized
Avalonian crust. Whalen et al. (1996) attribute this
magmatism to collision and lithosphere delamina-
tion accompanying and following the accretion of
Avalonia to Laurentia.
Central and Northern Cape Breton Island
Important Early Silurian (~435 Ma) arc-related
magmatism, metamorphism, followed by collisional
orogeny in central and northern Cape Breton (Barr
and Raeside, 1989; Keppie et al., 1991; see Fig. 5)
may be correlative with similar-aged events in the
La Poile Group of Newfoundland (e.g., Chandler et
al., 1987; Dunning et al., 1990 ; O’Brien et al.,
1991; Barr et al., 1998), which are attributed to the
Salinic orogeny. According to Lin et al. (1994), these
events collectively represented subduction followed
by promontory-promontory collision between Lau-
rentia and Avalonia during the Silurian.
Central and northern New Brunswick
and neighboring Maine
Much of central and northern New Brunswick
and neighboring Maine (Figs. 5 and 6) is underlain
by Gondwana-derived Cambrian–Lower Ordovician
strata of the Gander terrane (van Staal et al., 1998),
which is unconformably overlain by a widespread
Middle Ordovician volcano-sedimentary sequence
interpreted as an ensialic rifted arc (van Staal,
1994) that oversteps the boundary with the Exploits
subzone to the north and Avalonia to the south (Wil-
liams and Piasecki, 1990; van Staal et al., 1996) and
contains peri-Gondwanan (Celtic) fauna (Neuman
and Harper, 1992). Complex Late Ordovician–
Silurian deformation is related to sinsistral oblique
convergence and collision of peri-Gondwanan

ACADIAN OROGENY 673
terranes against the Laurentian margin (van Staal et
al., 1990). According to van Staal and de Roo
(1995), several short-lived episodes of arc accretion,
slab roll-back and breakup, and rapid exhumation
of blueschist-facies metamorphic rocks are attrib-
uted to post-collisional extension.
A prominent feature of this region is the
Silurian–Lower Devonian Tobique-Piscataquis
magmatic belt and correlatives, which have been
variously interpreted to represent arc (Rankin,
1968; Bradley, 1983; Thirlwall, 1988) or post-colli-
sional magmatism (Arth and Ayuso, 1997; Ayuso
and Arth, 1997) related to the closure of the Iapetus
Ocean. Alternatively, they have been interpreted as
intra-plate continental tholeiites intruded in an
extensional environment (Dostal et al., 1989; Hon et
al., 1992).
Foreland clastic wedge
In the Appalachian foreland of Quebec, Sil-
urian–Lower Devonian sedimentary facies in the
Gaspé Belt south of the Québec reentrant and the
St. Lawrence promontory preserves evidence of sig-
nificant syn-depositional tectonism, attributed by
Malo et al. (1995) to along-strike Acadian structural
variations as a consequence of collision along an
irregular margin and by Bourque et al. (2000) to the
Salinic “disturbance” beginning in late Llandover-
ian time and continuing into the Early Devonian.
Llandoverian–Wenlockian regression is attributed
to post-Taconian successor basin filling that culmi-
nated with extensive carbonate platform develop-
ment. This was followed by extensional tectonics
that produced shelf faulting and block tilting, on top
of which block reefs and reef complexes developed
and extended along the Gaspé-Témiscouata shelf.
Early Devonian rapid subsidence in the Québec
reentrant area was coeval with uplift in the St.
Lawrence promontory, attributed to loading of the
Laurentian margin associated with the further
thrusting of Avalon and Meguma terranes over
Laurentia. Lower to Middle Devonian clastic wedge
FIG. 6. Geology of northern New England (modified from Robinson et al., 1998; Tucker et al., 2001). Abbreviations:
CM, GL, and SP = Coastal Maine, Graham Lake, and Sennebec Pond fault zones, respectively.

674 MURPHY AND KEPPIE
deposits of the eastern Gaspé Peninsula record a
gradual displacement of source areas from the
southeast to the southwest.
Equivalent clastic wedge deposits in western
New York state and Pennsylvania are the Middle–
Upper Devonian clastic wedge (Quinlan and
Beaumont, 1984) that thins to less than 30 m near
Alabama (Hatcher, 1989), where subsidence curves
in Middle Paleozoic strata show no effect of loading
the Appalachian margin (Thomas and Whiting,
1995). The Catskill clastic wedge is widely inter-
preted to reflect this loading of the Laurentian
margin and a westerly migrating foredeep and clas-
tic wedge ahead of an advancing Acadian orogenic
front (Bradley, 1989). Some of the source rocks for
the Catskill delta deposits may have been located in
the New York promontory in southern New England,
where the Siluro-Devonian recumbent folds and
high-grade metamorphism was followed by rapid
uplift, and removal of 20 km of crust. Stewart et al.
(1995) and Bradley (1989) attributed the Siluro-
Devonian tectonothermal activity and westerly
migration of the Catskill foredeep to the Acadian
orogeny.
Post-Acadian structures in the Gaspé Peninsula
affected rocks that are as young as Namurian, and
such transpressive deformation, more than 1000 km
away from areas of peak coeval Alleghanian meta-
morphism in the southeastern United States, is
attributed by Jutras et al. (2003) to the far-field
effects of rigid indenters during terminal continental
collision. Middle-Late Devonian bimodal igneous
rocks followed by Middle Carboniferous mafic rocks
with plume-related geochemical signatures (Bedard,
1986) are consistent with intracontinental wrench
syn-to-post Acadian tectonic regime.
In central Maine (Fig. 6), shallow-marine
Silurian strata gave way to Lower Devonian deeper-
water flysch deposits. The Central Maine Basin
preserves a record of continuous Late Ordovician–
Early Devonian deep-marine sedimentation (Berry
and Osberg, 1989), with the lower strata derived
from the west, and younger strata from the east. This
basin has been interpreted as either a vestige of
ocean between Avalonia and Laurentia (Bradley,
1989), or a post-collisional basin intracontinental
regime (e.g., Hon et al., 1992).
Southern and Central Appalachians
Effects of the Acadian orogeny in the southern
and central Appalachians (Fig. 7) are limited to: (1)
a shear zone within the Carolina zone; (2) in the
Blue Ridge province of western North Carolina, a
phase of Devonian dextral strike-slip tectonics is
partitioned into narrow shear zones where it is
superimposed on earlier high-pressure Ordovician
deformation (Willard and Adams, 1994); and (3) an
upper Paleozoic ductile shear zone that defines the
western limit of the Carolina zone may represent the
latest stage in the movement of the zone against
Laurentia. Late Devonian deformation in the central
and southern Appalachians has been genetically
linked with the Upper Devonian clastic wedge.
Newfoundland
The effects of the Acadian orogeny in Newfound-
land are limited (Fig. 8). In central Newfoundland
(Humber, Dunnage, and Gander zones), widespread
Silurian (~435–415 Na) deformation, metamor-
phism, plutonism and exhumation are generally
considered to be the climactic event following the
Early Silurian, Salinic oblique sinistral collision
of the Gander and Avalon terrane with Laurentia
(Dunning et al., 1990; O’Brien et al., 1991; Will-
iams, 1993; Cawood et al., 1994; Kerr, 1997;
Schofield and D’Lemos, 2000) that occurred before
onset of the Acadian orogeny. Devonian deformation
in the Avalon terrane is limited to SE-vergent thrust-
ing and folding adjacent to dextral faults (Keppie,
1993). An apparent hiatus in plutonism exists
between the Silurian and the mid-Devonian. Devo-
nian magmatism in Newfoundland occurs in isolated
plutons in the Dunnage and Gander zones, but is
most voluminous in the Avalon zone and adjacent to
the Avalon-Gander boundary. The compositions of
Devonian granites are also indicative of a post-colli-
sional setting. Renewed plutonism was accom-
panied by a switch from sinistral kinematics of
the Silurian to Devonian dextral kinematics (as
exemplified by the Dover fault; Holdsworth, 1994)
suggesting that the magmatism developed in
response to wrench kinematics.
Summary
The data presented above indicates that the
Acadian orogeny is limited in both time and space:
it lasted from the Late Silurian to Late Devonian and
is centered on the northern Appalachians. Outside
this region, its effects are limited to strike-slip faults
and sporadic magmatism. It appears to have spread
diachronously from south to north with clastic
wedges migrating northward and then westward,
closely followed by the deformation front. In space it
coincides with a gap in the magmatic arc that was

ACADIAN OROGENY 675
subsequently interrupted by voluminous magma-
tism, some of which has a plume signature, and
which also migrated from south to north.
Model for the Acadian Orogeny
The conclusion that the Avalon and Meguma ter-
ranes shared a common history during the Paleozoic
has profound consequences for interpretation of the
Acadian orogeny. Inasmuch as the Meguma is the
most outboard terrane, this implies that all peri-
Gondwanan terranes in the northern Appalachians
were accreted prior to the mid-Silurian. The Late Sil-
urian–Devonian Acadian orogeny, therefore, cannot
be related to collision of the Meguma terrane with
Avalonia or with the Laurentian margin. Instead, this
scenario suggests that the Acadian orogeny occurred
in a Cordilleran- or Andean-type setting.
Paleocontinental reconstructions imply contin-
ued convergence between Gondwana and Laurentia
during the Late Silurian and Devonian, and conse-
quently, subduction of the Rheic Ocean. However,
over a wide region of Maritime Canada, voluminous
magmatism ceased between ca. 395 and 380 Ma,
and a period of relative magmatic quiescence
occurred. During this period, the diachronous onset
of deformation associated with the Acadian orogeny
in the northern Appalachians migrated northwest-
ward from Ludlovian in the southeast to Frasnian in
the northwest (Fig. 3; Bradley, 1989; Keppie, 1993;
Robinson et al., 1998), to extend across the entire
width of the Appalachian orogen.
In the Meguma terrane, the period of magmatic
quiescence was abruptly terminated by a volumi-
nous ~380–370 Ma magmatic episode most dram-
atically represented by intrusion of the biggest
FIG. 7. Simplified geological map showing the general geology of the Southern Appalachians (after Hibbard et al.,
2002; Hibbard, 2004).

676 MURPHY AND KEPPIE
FIG. 8. Tectonostratigraphic subdivisions of central Newfoundland (after Williams and Piasecki, 1990; Williams, 1993). Age data for plutons compiled from Kerr (1997).

ACADIAN OROGENY 677
granitoid batholiths in the northern Appalachians
and coeval lamprophyric dikes (e.g., Clarke et
al., 1997). Coeval abundant gold and siderophile
mineralization has an isotopic signature indicating a
source within the lower crust and probably formed
as a result of dehydration by mantle-derived magma
and fluids (Kontak et al., 1990).
Magmatism in the Meguma terrane terminated
abruptly by ~370 Ma. To the north, in the Cobequid
Highlands on the Avalon terrane, voluminous ~360
Ma, bimodal magmatism commenced (Pe-Piper et
al., 1989; Doig et al., 1996; Pe-Piper and Piper,
1998), with final emplacement of plutons at a shal-
low level, controlled by coeval dextral transpression
along major E-W faults (Koukouvelas et al., 2002).
Magmatism in the Cobequid Highlands terminated
before ~355 Ma. Farther north, middle Carbonifer-
ous mafic rocks with plume-related geochemical
signatures crop out around the periphery of the
Maritimes basin and in the Magdalen Islands
(Bedard, 1986). Although the Maritimes basin is
poorly exposed, geophysical and borehole data indi-
cate that ~3 km thick deposit of mainly terrestrial
Carboniferous and Permian rocks. Beneath the
Maritimes basin, seismic velocities of 7.2 m/s and a
pronounced positive Bouguer anomaly are attrib-
uted to a 10–20 km thick Carboniferous underplat-
ing of mantle-derived magma (Marillier and
Verhoef, 1989).
Murphy et al. (1999) and Keppie and Krogh
(1999) proposed that the Silurian–Devonian
Acadian orogeny in the type area was caused by flat-
slab subduction, in a manner analogous to the
modern Andes or the late Mesozoic–Cenozoic evolu-
tion of the southwestern United States (Fig. 9). This
model provides a potential explanation for several
anomalous aspects of the geology of the northern
Appalachians that otherwise remain enigmatic.
Following ~415 Ma convergence between the
Avalon and Meguma terranes, the 395–380 Ma
period of magmatic quiescence and the northwest-
ward migration of the deformation front is attributed
to the development of a flat slab beneath the Cana-
dian Appalachians (Figs. 9A and 9B) and ensuing
Laramide-style orogenic activity in which intermit-
tent coupling between the subducted and overriding
plates resulted in deformation that migrated to at
least 600 km inboard of the continental margin. The
flat-slab model is capable of explaining both the
lack of 395–380 Ma arc magmatism associated with
the convergence (a problem implicit in several pre-
vious models), and the diachronous migration of the
deformation front and overstep sequences across the
entire 500–800 km width of the orogen.
Flat-slab subduction has been attributed to
several mechanisms: (1) far-field effects of terrane
collision (Maxson and Tikoff, 1996); (2) overriding
of an ocean plateau or dormant seamounts (e.g.,
Pilger, 1981; Gutchner et al., 2000); (3) subduction
of young oceanic lithosphere (Gutscher, 2002) or a
ridge (Yañez et al., 2002); (4) increased rates of con-
vergence or changes in the direction of convergence
(Ramos et al., 2002); or (5) overriding of a swell and
an active plume (Murphy et al., 1999). All these
mechanisms produce a migrating deformation front
and temporarily switch off arc magmatism. However,
only overriding of a plume would terminate a period
of magmatic quiescence with voluminous magma-
tism with a linear, diachronous distribution, coupled
with compositional changes from felsic- to mafic-
dominated magmatism.
The Laramide orogeny is characterized by an
almost complete absence of magmatism (the Paleo-
cene magmatic gap), widespread deformation, and
thick-skinned tectonics associated with basement
uplifts located about 1500 km inboard of the conti-
nental margin. Most authors attribute these features
to flat-slab subduction (e.g., Dickinson and Snyder,
1979; Liviccarri et al., 1981; Bird, 1988; Severin-
haus and Atwater, 1990). However, origin of the flat
slab is controversial, and not well constrained.
Recent geodynamic analysis of the modern flat-
slab subduction zones has drawn attention to their
spatial and temporal correlation with a subducting
oceanic plateau (e.g., Gutscher et al., 2000; Yañez et
al., 2002), suggesting a similar possibility for the
Laramide orogeny. The Andean margin, for exam-
ple, has several flat-slab segments, up to 500 km
wide, that are each correlated with subduction of
anomalously warm oceanic crust, represented by
oceanic plateau (Gutscher et al., 2000). Murphy et
al. (1998) speculated that the origin of flat-slab sub-
duction in the western United States is due to the
overriding by the continental margin of a mantle
plume and related swell related to the ancestral
Yellowstone hotspot. According to plate reconstruc-
tions, the ancestral Yellowstone hotspot would have
collided with the margin at about 50 Ma. The model
implies that the underlying plume would have been
positioned beneath the Kula and/or Farallon
oceanic plates prior to 55–50 Ma. Evidence for the
existence of the plume in the oceanic realm is
derived from Late Cretaceous basaltic terranes of
the Coastal Ranges and from the Yukon Territory.

678 MURPHY AND KEPPIE
FIG. 9. Plate tectonic model for Acadian orogeny in the Maritime Appalachians. Cross-sections are coincident with
the plume track in Figure 3. A. Between 415 and 395 Ma, plume resided beneath oceanic crust. Coeval magmatism and
metamorphism (see Fig. 1) in Laurentia is interpreted to reflect telescoping of Avalonia (A) and Meguma (M) to Laurentia
during which slab break-off may have occurred (van Staal et al., 1998). B. Between 395 and 380 Ma, the plume is
overridden by the Laurentian margin, resulting in subhorizontal subduction and temporary cessation of magmatism and
coeval migration of the Acadian foredeep and deformation front. C. Between 380 and 360 Ma, the plume head thermally
erodes the subducted oceanic plate, causing intracrustal melting and minor mafic magmatism. D: Between 360 and 300
Ma, breakthrough of plume-related magmatism occurs. As the plume eventually dies, underplating by plume-derived
magma results in the formation of a positive gravity anomaly and in the formation of the Maritimes basin. The region of
flat-slab subduction may not extend to the central or southern Appalachians. Note that the process in which a continent
overrides a plume and swell is considered diachronous, so that timing of events might be different in the northeastern
United States.

ACADIAN OROGENY 679
Duncan (1982) and Wells et al. (1984) proposed that
some basaltic provinces of the Coastal Ranges (such
as the 60–50 Ma Crescent terrane) were seamounts
generated by the Yellowstone hotspot that were
accreted to western North America by the Eocene.
In the Yukon Territory, the ca. 70 Ma Carmacks
basaltic volcanics have plume-type geochemistry.
Paleomagnetic data indicate that the Crescent and
Carmacks basalts were both erupted at similar
paleolatitudes to the Yellowstone hotspot (Johnston
et al., 1996) although the Crescent volcanics under-
went much less subsequent northward translation
(Babcock et al., 1992).
The 395–380 Ma period of magmatic quiescence
in Maritime Canada may be related to an incubation
period in which the plume progessively assimilated
or thermally eroded the overlying lithosphere (Fig.
9B), thereby exposing the continental lithosphere to
the hot asthenosphere (Fig. 9C). The resulting melts
led to an abrupt termination of magmatic quiescence
and intrusion of voluminous ~380–370 Ma intra-
crustal granitoid rocks and lamprophyres of the
Meguma terrane. Between 380 and 330 Ma, the site
of magmatism moved northward to reside beneath
the Maritimes basin by Visean times. The mafic
component of the ~360 Ma magmatism in the
Cobequid Highlands may represent melting of the
lithospheric mantle above the plume. Visean-West-
phalian mafic rocks around the Maritimes basin
have plume-related geochemistry (Pe-Piper and
Piper, 1998; Bedard, 1986) and are interpreted to
reflect the penetration of the plume-derived magmas
into the overlying continental crust (Fig. 9D).
Anomalously high heat flow associated with the
plume provides a viable mechanism for regional-
scale high-T, low-P metamorphism in the Meguma
terrane that is broadly coeval with the intrusion of
the plutons. In addition, current models for plumes
involve ascent from the core-mantle boundary, a
region thought to be anomalously rich in gold and
siderophile elements (Brimhall, 1987), and the
ascending plume may have transported these ele-
ments into the continental lithosphere (Rock and
Groves, 1988). The ~20 m.y. interval between the
overridding of the leading edge of the swell and its
peak thermal effect provide a time window for circu-
lating fluids to scavenge metals from the dehydrated
lower crust.
The record of uplift and erosion within the
Meguma and Avalon terranes can also be explained
by the model. Overriding of the leading edge of the
plume swell can account for the Early Devonian
transition from shallow-marine to continental sedi-
mentation recorded in both the Meguma and Avalon
terranes. This change is synchronous with the earli-
est development of slaty cleavage in the southeast-
ern Meguma terrane. The rapid exhumation of ~10
km in the Meguma terrane between 370 and 360 Ma
(Keppie and Dallmeyer, 1995) and rapid uplift of
the Cobequid Highlands at ~360 Ma (Ryan et al.,
1987) may be the topographic expression of an over-
ridden plume.
The high-density lens beneath the Maritimes
basin (Marillier and Verhoef, 1989) may represent
the beheaded plume, the cooling of which would
induce sinking relative to surrounding uplifted
areas. Such sinking could account for the geophysi-
cal anomalies and formation of the Maritimes basin
(Fig. 9D). As the plume is considered stationary
relative to the more mobile overlying lithosphere,
there is no geophysical expression of plume activity
in the Cobequid Highlands and the Meguma
terrane.
The limited effects of the Acadian orogeny out-
side the northern mainland Appalachians may be a
reflection of steepening of the subduction zone,
resulting in a narrow active zone that may have
become extensional rather than contractional, a
factor that depends on the rate of convergence.
Discussion
In the Appalachians, several orogenic events
have been recognized: Taconian, Penobscotian,
Salinian, Acadian, and Alleghanian, and these have
generally been correlated along the whole length of
the orogen. Of these, the Salinian and Alleghenian
orogenies have been interpreted as due to continent-
continent collision, Laurentia-Avalonia and Lauren-
tia-Gondwana, respectively. Thus their widespread
distribution is to be expected. The Taconian and
Penobscotian orogenies have been related to con-
vergence in island arc settings, and if these island
arcs have great length, their accretion to the margins
may also be widespread.
The Acadian orogeny was initially interpreted to
have resulted from continent-continent collision
between Laurentia and Avalonia, or between the
amalgamated Laurentia/Avalonia and Meguma/
Africa. The former can now be discounted, inas-
much as data collected within the last 30 years
indicate that Laurentia-Avalonia collision took
place in the Late Ordovician–Silurian, i.e. before
onset of the Acadian orogeny. On the other

680 MURPHY AND KEPPIE
hand, recent detrital zircon data supports the conti-
guity of the Meguma and Avalon terranes during the
Paleozoic, and imply that they accreted to Laurentia
as a single microcontinent by the mid- to Late
Silurian (~415 Ma). Hence, the collision of Avalonia
or Meguma is unlikely to be the cause of the Late
Devonian–Late Carboniferous tectonism and mag-
matism. The data support the hypothesis that the
Meguma terrane is the passive margin on the south-
ern margin of Avalonia, and that it formed along the
northern (Laurentian) margin of the Rheic Ocean
(Murphy et al., 2004b), which did not close until the
Permo-Carboniferous.
This implies that the Acadian orogeny formed
along an Andean-type margin, possibly by overrid-
ing a plume and its swell (Murphy et al., 1999;
Keppie and Krogh, 1999). Arc-related magmatism
and deformation related to crustal shortening
occurred along most of the Andean margin. How-
ever, coeval orogeny related to flat-slab subduction,
characterized by an absence of arc magmatism and
deformation well inboard of the plate boundary,
occurs in Peru and the Pampean, and appears to
have been synchronous with changes in conver-
gence rate and direction, and with thick-skinned
tectonics related to intermittent coupling between
the overridden oceanic lithosphere and the South
American plate (Ramos et al., 2002). Similar varia-
tions in the dip of the subduction zone along the
northern margin of the Rheic Ocean may account for
the variation in the style of Acadian orogenesis from
the southern Appalachians to Atlantic Canada.
Elsewhere in the circum-Pacific region, subduc-
tion-related orogenic activity appears to alternate
between steeply and shallowly dipping subduction,
a phenomenon called tectonic switching by Collins
(2002). During periods of steep subduction, the
overriding plate is generally subjected to extension,
in which sedimentary basins may form, with conver-
gent deformation limited to the trench area. An
exception occurs if the convergence rate is high;
then orogenic effects are more widespread. During
periods of flat-slab subduction the overriding plate
is subjected to convergence, causing medium- to
high-grade metamorphism, polyphase deformation,
and magmatism in the basins. According to Collins
(2002), these tectonothermal events are often
mistakenly ascribed to arc-continental collision, so
tectonic switching may offer an alternative model
that could resolve controversies about the timing
and nature of “collisional” tectonics in the central
and southern Appalachians.
Flattening of the Benioff zone is generally the
result of collision of buoyant regions in the oceans
such as seamounts, mid-oceanic ridges, young
oceanic lithosphere, oceanic plateaus, island arc
complexes, hot spots, etc. As these buoyant regions
are of limited size, it follows that the associated
orogenic events are limited in areal distribution.
Furthermore, flattening of the subduction zone is
gradual in producing diachronous orogenic effects
that start at the trench and migrate inboard. On the
other hand, if the flattening of the subduction zone is
the result of increased convergence rates, the
orogenic effects may have wider distribution.
Inasmuch as flat-slab subduction is a common
feature of modern convergent margins, it should also
be common in the geologic record. We believe that
the Acadian orogeny in the northern mainland
Appalachians may be the expression of such a pro-
cess. The relative motion between the overriding
plate and the plume implies that features such as
the magmatic gap, plume-related magmatism, defor-
mation fronts, and basin formation are separated in
space as well as in time. As a result, these features
may be difficult to recognize in ancient orogenic
belts, especially where they are overprinted by
polyphase deformation and dismembered by subse-
quent faulting. The presence of continental mafic
rocks with plume-related chemistry is an important
clue to the identification of such processes. Because
even the strongest plumes have difficulty in pene-
trating continental lithosphere, the presence of
plume-related volcanic rocks indicates that the
plume and its related swell may have had a pro-
tracted earlier history.
Acknowledgments
We acknowledge the continuing support of the
Natural Sciences and Engineering Research Coun-
cil of Canada (Murphy) and Universidad Nacional
Autónoma de México (Keppie) for facilitating
the research. We are grateful for the thoughtful,
constructive reviews by Ulf Linnemann and Rob
Strachan. Contribution to IGCP (International
Geological Correlation Programme) Projects 453
and 497.
REFERENCES
Anderson, S. D., Jamieson, R. A., Reynolds, P. H., and
Dunning, G. R, 2001, Devonian extension in New-
foundland: 40Ar/39Ar and U-Pb data from the Mings

ACADIAN OROGENY 681
Bight area, Baie Verte Peninsula: Journal of Geology, v.
109, p. 191–211.
Arth, J. G., and Ayuso, R. A., 1997, The Northeast King-
dom Batholith, Vermont: Geochronology and Nd, O,
Pb, and Sr isotopic constraints on the origin of Acadian
granitic rocks, in Sinha, A. K. et al., eds., Magmatism
in the Appalachians: Geological Society of America
Memoir, 191, p. 1–18.
Ayuso, R. A., and Arth, J. G., 1997, The Spruce Head
composite pluton: An example of mafic to silicic Salin-
ian magmatism in coastal Maine, northern Appala-
chians, in Sinha, A. K. et al., eds., Magmatism in the
Appalachians: Geological Society of America Memoir,
191, p. 19–43.
Babcock, R. S., Burmester, R. F., Engebretson, D. C., War-
nock, A., and Clark, K. P., 1992, A rifted margin origin
for the Crescent basalts and related rocks in the north-
ern coast range volcanic province, Washington and
British Columbia: Journal of Geophysical Research, v.
97(B5), p. 6799–6821.
Barr, S. M., and Raeside, R. P., 1989, Tectonostratigraphic
zonation of Cape Breton Island, Nova Scotia: Implica-
tions for the configuration of the northern Appalachian
orogen: Geology, v. 17, p. 822–825.
Barr, S. M., Raeside, R. P., and White, C. E., 1998, Geo-
logical correlations between Cape Breton Island and
Newfoundland, northern Appalachian orogen: Cana-
dian Journal of Earth Sciences, v. 35, p. 1252–1270.
Barr, S. M., White, C. E., and Miller, B. V., 2002, The
Kingston terrane, southern New Brunswick, Canada:
Evidence of an Early Silurian arc: Geological Society
of America Bulletin, v. 114, p. 964–982.
Bedard, J. H., 1986, Pre-Acadian magmatic suites of the
southeastern Peninsula: Geological Society of America
Bulletin, v. 97, p. 1177–1191.
Benn, K., Horne, R. J., Kontak, D. J., Pignotta, G. S., and
Evans, N. G., 1997, Syn-Acadian emplacement model
for the South Mountain batholith, Meguma Terrane,
Nova Scotia: Magnetic fabric and structural analyses:
Geological Society of America Bulletin, v. 109, p.
1279–1293.
Berry, H. N., IV, and Osberg, P. H., 1989, A stratigraphic
synthesis of eastern Maine and western New Brun-
swick, in Tucker, R. D., and Marvinney, R. G., eds.,
Studies in Maine geology, volume 2: Structure and
stratigraphy: Augusta, ME, Maine Geological Survey,
p. 1–32.
Bevier, M. L., Barr, S. M., and White, C. E., 1990, Late
Precambrian U-Pb ages for the Brookville Gneiss, New
Brunswick: Journal of Geology, v. 98, p. 955–968.
Bird, P., 1988, Formation of the Rocky Mountains, western
United States: A continuum computer model: Science,
v. 239, p. 1601–1507.
Boucot, A. J., 1975, Evolution and extinction rate con-
trols: Developments in paleontology and stratigraphy,
v. 1: Amsterdam, Netherlands, Elsevier Scientific Pub-
lication Co., 427 p.
Boucot, A. J., Field, N. T., Fletcher, R., Forbes, W. H.,
Naylor, R. S., and Pavlides, L., 1964, Reconnaissance
bedrock geology of the Preque Isle Quadrangel, Maine:
Augusta, ME, Maine Geological Survey Quadrangle
Map Series, no. 2, pp. 123.
Bourque, P. A., Malo, M., and Kirkwood, D., 2000, Paleo-
geography and tectono-sedimentary history at margin
of Laurentia during Silurian–Earliest Devonian time:
The Gaspé Belt, Québec: Geological Society of Amer-
ica Bulletin, v. 112, p. 4–30.
Bradley, D. C., 1983, Tectonics of the Acadian orogeny in
New England and adjacent Canada: Journal of Geol-
ogy, v. 91, p. 381–400.
Bradley, D. G., 1989, Taconic plate kinematics as revealed
by foredeep stratigraphy: Tectonics, v. 8, p. 1037–
1049.
Brimhall, G. H., Jr., 1987, Preliminary fractionation pat-
terns of ore metals through Earth history: Chemical
Geology, v. 64, p. 1–16.
Cawood, P. A., Dunning, G. R., Lux, D., and van Gool,
J. A. M., 1994, Timing of peak metamorphism and
deformation along the Appalachian margin of Lauren-
tia in Newfoundland: Silurian, not Ordovician: Geol-
ogy, v. 22, p 399–402.
Cawood, P. A., McCausland, P. J. A., and Dunning, G. R.,
2001, Opening Iapetus: Constraints from the Lauren-
tian margin of Newfoundland: Geological Society of
America Bulletin, v. 113, p. 443–453.
Chandler, F. W., Sullivan, R. W., and Currie, K. L., 1987,
Springdale Group and correlative rocks: A Llandovery
overlap sequence in the Canadian Appalachians:
Royal Society of Edinburgh: Earth Sciences v. 78, p.
41–49.
Clarke, D. B., MacDonald, M. A., Reynolds, P. H., and
Longstaffe, F. J., 1993, Leucogranites from the eastern
part of the South Mountain batholith, Nova Scotia:
Journal of Petrology v. 34, p. 653–679.
Clarke, D. B., MacDonald, M. A., and Tate, M. C., 1997,
Late Devonian mafic-felsic magmatism in the Meguma
Zone, Nova Scotia, in, Sinha, A. K., et al. eds., The
nature of magmatism in the Appalachian orogen: Geo-
logical Society of America Memoir, 191, p. 107–127.
Cocks, L. R. M., and Fortey, R. A., 1990, Biogeography of
Ordovician and Silurian faunas, in McKerrow, W. S.,
and Scotese, C. R., eds., Paleozoic Paleogeography
and Biogeography: Geological Society Memoir 12, p.
97–104.
Cocks, L. R. M. and Torsvik, T. H., 2002, Earth geography
from 500 to 400 million years ago: A faunal and
palaeomagnetic review: Journal of the Geological Soci-
ety of London, v. 159, p. 631–644.
Collins, W.J. 2002, Hot orogens, tectonic switching and
creation of continental crust: Geology v. 31, p. 535–
538.
Culshaw, N., and Liesa, M., 1997, Alleghanian reactiva-
tion of the Acadian fold belt, Meguma zone, southwest

682 MURPHY AND KEPPIE
Nova Scotia: Canadian Journal of Earth Sciences, v.
34, p. 833–847.
Currie, K. L., 1987, Relations between metamorphism and
magmatism near Cheticamp, Cape Breton Island: Nova
Scotia Geological Survey of Canada, Paper 85-23, 66
p.
Daly, J. S., and McLelland, J. M., 1991, Juvenile middle
Proterozoic crust in the Adirondack Highlands, Gren-
ville Province, northeastern North America: Geology,
v. 19, p. 119–122.
Dickin, A. P., 2000, Crustal formation in the Grenville
province: Nd isotopic evidence: Canadian Journal of
Earth Sciences, v. 37, p. 165–181.
Dickin, A. P., and McNutt, R. H., 1989, Nd model age
mapping of the southeast margin of the Archean fore-
land in the Grenville Province of Ontario: Geology, v.
17, p. 299–302.
Dickin, A. P., McNutt, R. H., and Clifford, P. M., 1990, A
neodymium isotope study of plutons near the Grenville
Front in Ontario, Canada: Chemical Geology, v. 83, p.
315–324.
Dickinson, W. R., and Snyder, W. S., 1979, Geometry of
subducted slabs related to the San Andreas transform:
Journal of Geology, v. 87, pp. 609–627.
Doig, R., Murphy, J. B., Pe-Piper, G., and Piper, D. J. W.,
1996, U-Pb geochronology of Late Paleozoic plutons,
Cobequid Highlands, Nova Scotia, Canada: Evidence
for Late Devonian emplacement adjacent to the
Meguma-Avalon terrane boundary: Geological Jour-
nal, v. 31, p. 179–188.
Donohoe, H.V., Jr., and Pajari, G., 1974, The age of Aca-
dian deformation in Maine–New Brunswick: Maritime
Sediments, v. 9, p. 78–82.
Dostal, J., Wilson, R. A., and Keppie, J. D., 1989,
Geochemistry of the Siluro-Devonian Tobique belt in
northern and central New Brunswick (Canada):
Tectonic implications: Canadian Journal of Earth
Sciences, v. 26, p. 1282–1296.
Duncan, R. A., 1982, A captured island chain in the Coast
Range of Oregon and Washington: Journal of Geophys-
ical Research, v. 87, p. 10,827–10,837.
Dunning, G. R., O’Brien, S. J., Colman-Sadd, S. P., Black-
wood, R. F., Dickson, W. L., O’Neill, P. P., and Krogh,
T. E., 1990, Silurian orogeny in the Newfoundland
Appalachians: Journal of Geology, v. 98, p. 895–913.
Fortey, R. A., and Cocks, L. R. M., 2003, Palaeontological
evidence bearing on global Ordovician–Silurian conti-
nental reconstructions: Earth Science Reviews, v. 61,
p. 245–307.
Goldstein, S. J., and Jacobsen, S. B., 1988, Nd and Sr iso-
topic systematics of river water suspended material:
Implications for crustal evolution: Earth and Planetary
Science Letters, v. 87, p. 221–236.
Gower, C. F., Ryan, A. B., and Rivers, T., 1990, Mid-Prot-
erozoic Laurentia–Baltica: An overview of its geologi-
cal evolution and summary of the contributions by this
volume, in Gower, C. F., Rivers, T., and Ryan, B., eds.,
Mid-Proterozoic Laurentia–Baltica: Geological Asso-
ciation of Canada Special Paper 38, p. 1–20.
Gutscher, M.-A., Spakman, W., Bijwaard, H., and
Engdahl, E. R., 2000, Geodynamics of flat subduction:
Seismicity and tomographic constraints from the
Andean margin: Tectonics, v. 19, p. 814–833.
Gutscher, M.-A., 2002, Andean subduction styles and
their effect on thermal structure and interplate cou-
pling: Journal of South American Earth Sciences, v.
15, p. 3–10.
Hamilton, M. A., and Murphy, J. B., 2004, Tectonic signif-
icance of a Llanvirn age for the Dunn Point volcanic
rocks, Avalon terrane, Nova Scotia, Canada: Implica-
tions for the evolution of the Iapetus and Rheic oceans:
Tectonophysics, v. 379, p. 199–209.
Hatcher, R. D., Jr., 1989, Tectonic syntheses of the U.S.
Appalachians, in Hatcher, R. D., Thomas, W. A., and
Viele, G. W., eds., The Appalachian-Ouachita orogen
in the United States: Geological Society of America,
The Geology of North America, v. F-2, p. 511–535.
Hibbard, J. P., 2000, Docking Carolina: Mid-Paleozoic
accretion in the southern Appalachians: Geology v. 28,
p. 127–130.
Hibbard, J., 2004, The Appalachian orogen, in van der
Pluijm, B.A., and Marshak, S., eds., Earth structure,
2nd ed.: New York, NY, Norton, p. 582–592.
Hibbard, J. P., Miller, B. V., Tracy, R., and Carter, B., in
press, The Appalachian peri-Gondwanan realm: A
paleogeographic perspective from the south, in
Vaughan, A. P. M., Leat, P. L., and Pankhurst, R. J.,
eds., Terrane processes at the Pacific Margin of Gond-
wana: Geological Society of London, Special Publica-
tion, in press.
Hibbard, J. P., Stoddard, E. F., Secor, D. T., and Dennis,
A. J., 2002, The Carolina Zone: Overview of Neoprot-
erozoic to early Paleozoic peri-Gondwanan terranes
along the eastern flank of the southern Appalachians:
Earth Science Reviews, v. 57, p. 299–339.
Hicks, R. J., Jamieson, R. A., and Reynolds, P. H., 1999,
Detrital and metamorphic 40Ar/39Ar ages from musco-
vite and whole-rock samples, Meguma Supergroup,
southern Nova Scotia: Canadian Journal of Earth Sci-
ences, v. 36, p 23–32.
Holdsworth, R. E., Structural evolution of the Gander-
Avalon terrane boundary: A reactivated transpres-
sional zone in the NE Newfoundland Appalachians:
Journal of the Geological Society of London, v. 151, p.
629–646.
Hon, R., Fitzgerald, J. P., Sargant, S. L., Schwartz, W. D.,
Dostal, J., and Keppie, J. D., 1992, Silurian–Early
Devonian mafic rocks of the Piscataquis volcanic belt
in northern Maine: Atlantic Geology, v. 28, p. 163–
170.
Jennex, L. C., Murphy, J. B., and Anderson, A. J., 2000,
Post-orogenic exhumation of an auriferous terrane:
The paleoplacer potential of the early Carboniferous

ACADIAN OROGENY 683
St. Marys Basin; Canadian Appalachians: Mineralium
Deposita, v. 35, p. 776–790.
Johnson, R. J. E., and Van der Voo, R., 1986, Paleomag-
netism of the Late Precambrian Fourchu Group, Cape
Breton Island, Nova Scotia: Canadian Journal of Earth
Sciences, v. 23, p. 1673–1685.
Johnson, R. J. E., and Van der Voo, R., 1990, Pre-folding
magnetization reconfirmed for the Late Ordovician–
Early Silurian Dunn Point volcanics, Nova Scotia: Tec-
tonophysics, v. 178, p. 193–205.
Johnston, S. T., Wynne, P. J., Francis, D., Hart, C. J. R.,
Enkin, R. J., and Engebretson, D. C., 1996, Yellow-
stone in Yukon: The Late Cretaceous Carmacks Group:
Geology, v. 24, p. 997–1000.
Jutras, P., Prichonnet, G., and McCutcheon, S., 2003,
Alleghanian deformation in the eastern Gaspé Penin-
sula of Quebec, Canada: Geological Society of Amer-
ica Bulletin, v. 115, 1538–1551.
Keen, C. E., Kay, W. A., Keppie, J. D., Marillier, F., Pe-
Piper, G., and Waldron, J. W. F., 1991, Deep seismic
reflection data from the Bay of Fundy and Gulf of
Maine: Tectonic implications for the northern Appala-
chians: Canadian Journal of Earth Sciences, v. 28, p.
1096–1111.
Keppie, J. D. 1982, The Minas geofracture, in St. Julien,
P., and Beland, J., eds., Major structural zones and
faults of the Northern Appalachians: Geological Asso-
ciation of Canada Special Paper 24, p. 263–280.
Keppie, J. D., 1985, The Appalachian collage, in Gee, D.
G., and Sturt, B. eds., The Caledonide orogen, Scandi-
navia, and related areas: New York, NY, John Wiley
and Sons, p. 1217–1226.
Keppie, J. D., 1993, Synthesis of Paleozoic deformational
events and terrane accretion in the Canadian Appala-
chians: Geologische Rundschau, v. 82, p. 381–431.
Keppie, J. D., and Dallmeyer, R. D., 1995, Late Paleozoic
collision, delamination, short-lived magmatism, and
rapid denudation in the Meguma terrane (Nova Scotia,
Canada): Constraints from 40Ar/39Ar isotopic data:
Canadian Journal of Earth Sciences, v. 32, p. 644–659.
Keppie, J. D., Davis, D. W., and Krogh, T. E., 1998, U-Pb
geochronological constraints on Precambrian stratified
units in the Avalon Composite Terrane of Nova Scotia,
Canada: Tectonic implications: Canadian Journal of
Earth Sciences, v. 35, p. 222–236.
Keppie, J. D., and Dostal, J., 1991, Late Proterozoic tec-
tonic model for the Avalon terrane in Maritime Can-
ada: Tectonics, v. 10, p. 842–850.
Keppie, J. D., Dostal, J., Murphy, J. B., and Cousens,
B. L., 1997, Palaeozoic within-plate volcanic rocks in
Nova Scotia (Canada) reinterpreted: Isotopic con-
straints on magmatic source and paleocontinental
reconstructions: Geological Magazine, v. 134, p. 425–
447.
Keppie, J. D., and Krogh, T. E., 1999, U-Pb geochronology
of Devonian granites in the Meguma terrane of Nova
Scotia Canada: Evidence for hotspot melting of a
Neoproterozoic source: Journal of Geology, v. 107, p.
555–568.
Keppie, J. D., and Krogh, T. E., 2000, 440 Ma igneous
activity in the Meguma terrane, Nova Scotia, Canada:
Part of the Appalachian overstep sequence?: Ameri-
can Journal of Science, v. 300, p. 528–538.
Keppie, J. D., Nance, R. D., Murphy, J. B., and Dostal, J.,
1991, Northern Appalachians: The Avalon and
Meguma terranes, in Dallmeyer, R. D., and Lecorche,
J. P., eds., The Western African orogens and Circum
Atlantic correlatives: Berlin, Germany, Springer Ver-
lag, p. 315–333.
Keppie, J. D., Nance, R. D., Murphy, J. B., and Dostal, J.,
2003, Tethyan, Mediterranean, and Pacific analogues
for the Neoproterozoic–Paleozoic birth and develop-
ment of peri-Gondwanan terranes and their transfer to
Laurentia and Laurussia: Tectonophysics, v. 365, p.
195–219.
Kerr, A., 1997. Space-time composition relationships
among Appalachian-cycle plutonic suite in Newfound-
land, in Sinha, A. K. et al., eds., Magmatism in the
Appalachians: Geological Society of America Memoir,
191, 193–220.
Kontak, D. J., Smith, P. K., Kerrich, R., and Williams,
P. F., 1990, Integrated model for Meguma Group lode
gold deposits, Nova Scotia, Canada: Geology, v. 18, p.
238–242.
Koukouvelas, I., Pe-Piper, G., and Piper, D. J. W., 2002,
The role of dextral transpressional faulting in the evo-
lution of the early Carboniferous mafic-felsic plutonic
and volcanic complex: Cobequid Highlands, Nova
Scotia, Canada: Tectonophysics, v. 348, p. 219–246.
Krogh, T. E., and Keppie, J. D., 1990, Age of detrital zir-
con and titanite in the Meguma Group, southern Nova
Scotia, Canada: Clues to the origin of the Meguma Ter-
rane: Tectonophysics, v. 177, p. 307–323.
Lambert, R. St. J., and McKerrow, W. S., 1976, The Gram-
pian orogeny: Scottish Journal of Geology, v. 12, p.
271–293.
Landing, E., 1996, Avalon: Insular continent by the latest
Precambrian, in Nance, R. D., and Thompson, M. D.,
eds., Avalonian and related peri-Gondwanan terranes
of the circum-North Atlantic: Geological Society of
America Special Paper 304, p. 29–63.
Landing, E., and Murphy, J. B., 1991, Uppermost Precam-
brian(?)–Lower Cambrian of mainland Nova Scotia:
Faunas, depositional environments, and stratigraphic
revision: Journal of Paleontology, v. 65, p. 382–396.
Lin, S., van Staal, C. R., and Dube, B., 1994, Promontory-
promontory collision in the Canadian Appalachians:
Geology, v. 22, p. 897–900.
Livaccari, R. F., Burke, K., and Sengor, A. M. C., 1981,
Was the Laramide orogeny related to subduction of an
oceanic plateau? Nature, v. 289, p. 276–279.
MacDonald, L. A., Barr, S. M., White, C. E., and Ketchum,
J. W. G., 2002, Petrology, age, and tectonic setting of
the White Rock Formation, Yarmouth area, Nova

684 MURPHY AND KEPPIE
Scotia: Canadian Journal of Earth Sciences, v. 39, p.
259–277.
MacLachlan, K., and Dunning, G., 1998, U-Pb ages and
tectonomagmatic relationships of early Ordovician
low-Ti tholeiites, boninites, and related plutonic rocks
in central Newfoundand: Contributions to Mineralogy
and Petrology, v. 133, p. 235–258.
MacNiocaill, C., and Smethurst, M. A., 1994, Palaeozoic
palaeogeography of Laurentia and its margins: A reas-
sessment of the paleomagnetic data: Geophysical Jour-
nal International, v. 116, p. 715–725.
MacNiocaill, C., van der Pluijm, B. A., and Van der Voo,
R., 1997, Ordovician paleogeography and the evolu-
tion of the Iapetus Ocean: Geology, v. 25, p. 159–162.
Malo, M., Tremblay, A., Kirkwood, D., and Cousineau, P.,
1995, Along-strike Acadian structural variations in
the Quebec Appalachians: Consequence of a collision
along an irregular margin: Tectonics, v. 14, pp. 1327–
1338.
Marillier, F., and Verhoef, J., 1989, Crustal thickness
under the Gulf of St. Lawrence, northern Appala-
chians, from gravity and deep seismic data: Canadian
Journal of Earth Sciences, v. 26, p. 1517–1532.
Martel, A. T., McGregor, D. C., and Utting, J., 1993, Strati-
graphic significance of Upper Devonian and Lower
Carboniferous miospores from the type area of the Hor-
ton Group, Nova Scotia: Canadian Journal of Earth
Sciences, v. 30, p. 1091–1098.
Maxson, J., and Tikoff, B., 1996, Hit-and-run collision
model for the Laramide orogeny, western United
States: Geology, v. 24, p. 968–972.
McLelland, J. M., Daly, J. S., and Chiarenzelli, J., 1993,
Sm-Nd and U-Pb isotopic evidence of juvenile crust in
the Adirondack lowlands and implications for the evo-
lution of the Adirondack Mountains: Journal of Geol-
ogy, v. 101, p. 97–105.
McKerrow, W. S., Dewey, J. F., and Scotese, C. R., 1991,
The Ordovician and Silurian development of the Iape-
tus Ocean: Special Papers in Paleontology, v. 44, p.
165–178.
McKerrow, W. S., MacNiocaill, C., and Dewey, J. F., 2000,
The Caledonian orogeny redefined: Journal of the Geo-
logical Society of London, v. 157, p. 1149–1154.
Miller, J. D., and Kent, D. V., Paleomagnetism of the Sil-
uro-Devonian Andreas redbeds: Evidence of a Devo-
nian supercontinent?: Geology, v. 16, p. 195–198.
Miller, R. G., O’Nions, R. K., Hamilton, P. J., and Welin,
E., 1986, Crustal residence ages of clastic sediments,
orogeny and crustal evolution: Chemical Geology, v.
57, p. 87–99.
Murphy, J. B. 2000, Tectonic influence on sedimentation
along the southern flank of the Late Paleozoic
Magdalen Basin in the Canadian Appalachians:
Geochemical and isotopic constraints on the Horton
Group in the St. Mary’s Basin, Nova Scotia: Geological
Society of America Bulletin, v. 112, p. 997–1011.
Murphy, J. B., 2003, Late Paleozoic formation and devel-
opment of the St. Marys Basin, mainland Nova Scotia,
Canada: A prolonged record of intra-continental
strike-slip deformation during the assembly of Pan-
gaea, in Storti, F., Holdsworth, R. E., and Salvini, F.,
eds., Intraplate strike-slip deformation belts: Geologi-
cal Society of London, Special Publication 210, p.
185–196.
Murphy, J. B., Fernández-Suárez, J., and Jeffries, T. E.,
2004a, Lithogeochemical, Sm-Nd, and U-Pb isotopic
data from the Silurian–Early Devonian Arisaig Group
clastic rocks, Avalon terrane, Nova Scotia: A record of
terrane accretion in the Appalachian-Caledonide oro-
gen: Geological Society of America Bulletin, v. 116, p.
1183–1201.
Murphy, J. B., Fernández-Suárez, J., Keppie, J. D., and
Jeffries,T. E., 2004b, Contiguous rather than discrete
Paleozoic histories for the Avalon and Meguma ter-
ranes based on detrital zircon data: Geology, v. 32, p.
585–588.
Murphy, J. B., and Hamilton, M. A., 2000, U-Pb detrital
zircon age constraints on evolution of the Late Paleo-
zoic St. Marys Basin, central mainland Nova Scotia:
Journal of Geology, v. 108, p. 53–72.
Murphy, J. B., and Keppie, J. D., 1998, Late Devonian
palinspastic reconstruction of the Avalon-Meguma ter-
rane boundary: Implications for terrane accretion and
basin development in the Appalachian orogen: Tec-
tonophysics, v. 284, p. 221–231.
Murphy, J. B., Keppie, J. D., Dostal, J., and Cousins, B. L.,
1996a, Repeated late Neoproterozoic–Silurian lower
crustal melting beneath the Antigonish Highlands,
Nova Scotia: Nd isotopic evidence and tectonic inter-
pretations, in Nance, R. D., and Thompson, M. D.,
eds., Avalonian and related peri-Gondwanan terranes
of the Circum-North Atlantic: Geological Society of
America Special Paper 304, p. 109–120.
Murphy, J. B., Keppie, J. D., Dostal, J., Waldron, J. W. F.,
and Cude, M. P., 1996b, Geochemical and isotopic
characteristics of Early Silurian clastic sequences in
Antigonish Highlands, Nova Scotia, Canada: con-
straints on the accretion of Avalonia in the Appala-
chian-Caledonide orogen: Canadian Journal of Earth
Sciences, v. 33, p. 379–388.
Murphy, J. B., and MacDonald, D. A., 1993, Geochemistry
of Late Proterozoic arc-related volcaniclastic turbidite
sequences, Antigonish Highlands, Nova Scotia: Cana-
dian Journal of Earth Sciences, v. 30, p. 2273–2282.
Murphy, J. B., Oppliger, G. L., Brimhall, G. H., Jr., and
Hynes, A., 1998, Plume-modified orogeny: An exam-
ple from the western United States: Geology, v. 26, p.
731–734.
Murphy, J. B., Strachan, R. A., Nance, R. D., Parker,
K. D., and Fowler, M. B., 2000, Proto-Avalonia: A 1.2–
1.0 Ga tectonothermal event and constraints for the
evolution of Rodinia: Geology, v. 28, p. 1071–1074.

ACADIAN OROGENY 685
Murphy, J. B., van Staal, C. R., and Keppie, J. D., 1999, Is
the mid to late Paleozoic Acadian orogeny a plume-
modified Laramide-style orogeny?: Geology, v. 27, p.
653–656.
Nance, R. D., and Dallmeyer, R. D., 1993, 40Ar/39Ar
amphibole ages from the Kingston complex, New
Brunswick: Evidence for Silurian–Devonian tectono-
thermal activity and implications for the accretion of
the Avalon composite terrane: Journal of Geology, v.
101, p. 375–388.
Neuman, R. B., and Harper, D. A. T., 1992, Paleogeo-
graphic Significance of Arenig-Llanvirn Toquima-
Table Head and Celtic brachiopod assemblages, in
Webby, B. D., and Laurie, J. R., eds., Global perspec-
tives on Ordovician geology: Rotterdam, Netherlands,
and Brookfield, VT, A. A. Balkema, p. 241–254.
Noel, J., Spariosu, D., and Dallmeyer, R. D., 1988, Paleo-
magnetism and 40Ar/39Ar ages from the Carolina slate
belt, Albemarle, North Carolina: Implications for ter-
rane amalgamation with North America. Geology, v.
16, p. 64–68.
O’Brien, B. H., O’Brien, S. J., and Dunning, G. R., 1991,
Silurian cover, late Precambrian–Early Ordovician
basement, and the chronology of Silurian orogenesis in
the Hermitage flexure (Newfoundland Appalachians):
American Journal of Science, v. 291, p. 760–799.
O’Brien, S. J., O’Brien, B. H., Dunning, G. R., and Tucker,
R. D., 1996, Late Neoproterozoic Avalonian and
related peri-Gondwanan rocks of the Newfoundland
Appalachians, in Nance, R. D., and Thompson, M. D.,
eds., Avalonian and related peri-Gondwanan terranes
of the circum-North Atlantic: Geological Society of
America Special Paper 304, p. 9–28.
O’Brien, S. J., Wardle, R. J., and King, A. F., 1983, The
Avalon zone: A Pan-African terrane in the Appala-
chian orogen of Canada: Geological Journal, v. 18, p.
195–222.
Patchett, P. J., and Ruiz, J., 1989, Nd isotopes and the ori-
gin of the Grenville-age rocks in Texas: Implications
for Proterozoic evolution of the United States mid-con-
tinent region: Journal of Geology, v. 97, p. 685–695.
Pedersen, R. B., and Dunning, G. R., 1997, Evolution of
arc crust and relations between contrasting sources: U-
Pb (age), Nd and Sr isotopic systematics of the ophio-
lite terrain of SW Norway: Contributions to Mineralogy
and Petrology, v. 128, p. 1–15.
Pe-Piper, G., Murphy, J. B. and Turner, D. S., 1989,
Petrology, geochemistry, and tectonic setting of some
Carboniferous plutons of the Eastern Cobequid Hills:
Atlantic Geology, v. 25, p. 37–49.
Pe-Piper, G., and Piper, D. J. W., 1998, Geochemical evo-
lution of Devonian–Carboniferous igneous rocks of the
Magdalen basin, eastern Canada: Pb- and Nd-isotope
evidence for mantle and lower crustal sources: Cana-
dian Journal of Earth Sciences, v. 35, p. 201–221.
Pickering, K., Basset, M. G., and Siveter, D. J., 1988, Late
Ordovician–Early Silurian destruction of the Iapetus
Ocean: Newfoundland, British Isles, and Scandina-
via—a discussion: Transactions of the Royal Society of
Edinburgh, v. 79, p. 361–382.
Pilger, R. H., 1981, Plate reconstruction, aseismic ridges,
and low-angle subduction beneath the Andes: Geolog-
ical Society of America Bulletin, v. 92, p. 448–456.
Piper, D. J. W., Pe-Piper, G., and Loncarevic, B. D., 1993,
Devonian–Carboniferous deformation and igneous
intrusion in the Cobequid Highlands: Atlantic Geol-
ogy, v. 29, p. 219–232.
Poole, W. H., Sandford, B. V., Williams, H., and Kelley,
D. G., 1970, Geology of southeastern Canada, in Dou-
glas, R. J. W., ed., Geology and economic minerals of
Canada: Geological Survey of Canada, Economic
Geology Report no. 1, p. 227–304.
Potts, S., Van der Pluijm, B., and Van der Voo, R., 1993,
Discordant Silurian paleolatitudes for central New-
foundland: New paleomagnetic evidence from the
Springdale Group: Earth and Planetary Science Let-
ters, v. 120, p. 1–12.
Pratt, B. R., and J. W. F. Waldron, 1991, A Middle Cam-
brian trilobite faunule from the Meguma Group of
Nova Scotia: Canadian Journal of Earth Sciences, v.
28, p. 1843–1853.
Quinlan, G. M. and Beaumont, C., 1984, Appalachian
thrusting, lithospheric flexure, and the Paleozoic
stratigraphy of the Eastern Interior of North America:
Canadian Journal of Earth Sciences, v. 21, p. 973–996.
Ramos, V. A., and Aleman, A. 2001, Tectonic evolution of
the Andes, in Cordani, U. G., Thomaz Filho, A., and
Campos, D. A., eds., Tectonic evolution of South
America: 31st International Geological Congress, Rio
de Janeiro, Brazil, p. 635–688.
Ramos, V. A., Cristallini, E. O., and Pérez, D. J., 2002,
The Pampean flat-slab of the Central Andes: Journal of
South American Earth Sciences, v. 15, p. 59–78.
Rankin, D. W., 1968, Volcanism related to tectonism in
the Piscataquis volcanic belt, an island arc of early
Devonian age in north-central Maine, in Zen, E-an,
White, W. S., Hadley, J. B., and Thompson, J. B., Stud-
ies in Appalachian geology: Northern and maritime:
New York, NY, Interscience, p. 83–94.
Roberts, D., 2003, The Scandinavian Caledonides: Event
chronology, palaeogeographic settings, and likely
modern analogues: Tectonophysics, v. 365, p. 283–
299.
Robinson, P., Tucker, R. D., Bradley, D., Berry, H. N. V.,
and Osberg, P. H., 1998, Paleozoic orogens in New
England, USA: Geologiska Föreningens Förhandlin-
gar, v. 120, p. 119–148.
Rocci, G., Bronner, G., and Deschamps, M., 1991, Crystal-
line basement of the West African craton, in Dallm-
eyer, R. D., and Lecorche, J. P., eds., The West African
orogens and circum-Atlantic correlatives: Heidelberg,
Germany, Springer-Verlag, p. 31–61.

686 MURPHY AND KEPPIE
Rock, N. M. S., and Groves, D. I., 1988, Can lamprophyres
resolve the genetic controversy over mesothermal gold
deposits?: Geology, v. 16, p. 538–541.
Ryan, R. J., Calder, J. H., Donohoe, H. V., Jr., and Naylor,
R., 1987, Late Paleozoic sedimentation and basin
development adjacent to the Cobequid Highlands
massif, eastern Canada, in Beaumont, C., and Tankard,
A. J., eds., Sedimentary basins and basin-forming
mechanisms: Canadian Society of Petroleum Geolo-
gists Memoir 12 and Atlantic Geoscience Society Spe-
cial Publication 5, p. 311–317.
Sadowski, G. R., and Bettencourt, J. S., 1996, Mesoprot-
erozoic tectonic correlations between eastern Lauren-
tia and the western border of the Amazon craton:
Precambrian Research, v. 76, p. 213–227.
Schenk, P. E., 1997, Sequence stratigraphy and prove-
nance on Gondwana’s margin: The Meguma zone
(Cambrian-Devonian) of Nova Scotia, Canada: Geolog-
ical Society of America Bulletin, v. 109, p. 395–409.
Schofield, D. I., and D’Lemos, R. S., 2000, Granite petro-
genesis in the Gander Zone, NE Newfoundland: Mix-
ing of melts from multiple sources and the role of
lithospheric delamination: Canadian Journal of Earth
Sciences, v. 37, p. 535–547.
Schreckengost, K. A., and Nance, R. D., 1996, Silurian–
Devonian dextral reactivation near the inboard margin
of the Avalon Composite Terrane: Kinematic evidence
from the Kingston complex, southern New Brunswick,
Canada, in Nance, R. D., and Thompson, M. D., eds.,
Avalonian and related peri-Gondwanan terranes of the
circum–North Atlantic: Geological Society of America
Special Paper 304, p. 165–178.
Schuchert, C., 1923, Sites and names of the North Ameri-
can geosynclines: Geological Society of America Bul-
letin, v. 34, p. 151–229.
Severinghaus, J., and Atwater, T., 1990, Cenozoic geome-
try and thermal state of the subducting slabs beneath
North America, in Wernicke, B. P., ed., Basin and
Range extensional tectonics near the latitude of Las
Vegas, Nevada: Geological Society of America Memoir
176, p. 1–22.
Stewart, D. B., Unger, J. D., and Hutchinson, D. R., 1995,
Silurian tectonic history of Penobscot Bay region,
Maine: Atlantic Geology, v. 31, p. 67–79.
Thirlwall, M. F., 1988, Wenlock to mid Devonian volcan-
ism of the Caledonian-Appalachian orogen, in Harris,
A. L., and Fettes, D. J., eds., Caledonian-Appalachian
orogen: Geological Society of London Special Publica-
tion, v. 18, p. 415–428.
Thomas, W. A., and Whiting, B. M., 1995, The Alabama
promontory: An example of the evolution of the Appa-
lachian-Ouachita thrust-belt recess at a promontory of
the rifted continental margin, in Hibbard, J., Van Staal,
C., and Cawood, P., New perspectives in the Cale-
donian-Appalachian orogen: Geological Association of
Canada Special Paper 41, p. 7–18.
Thorkelson, D. J., and Taylor, R. P., 1989, Cordilleran slab
windows: Geology, v. 17, p. 833–836.
Thorogood, E. J., 1990, Provenance of the pre-Devonian
sediments of England and Wales: Sm-Nd isotopic evi-
dence: Journal of the Geological Society of London, v.
147, p. 591–594.
Trench, A., and Torsvik, T. H., 1992, The closure of the
Iapetus Ocean and Tornquist Sea: New paleomagnetic
constraints: Journal of the Geological Society of Lon-
don, v. 149, p. 867–870.
Torsvik, T. H., Smethurst, M. A., Meert, J. G., Van der Voo,
R., McKerrow, W. S., Brasier, M. D., Sturt, B. A., and
Walderhaug, H. J., 1996, Continental break-up and
collision in the Neoproterozoic and Palaeozoic—a tale
of Baltica and Laurentia: Earth Science Reviews, v.
40, p. 229–258.
Tucker, R. D., Osberg, P. H., and Berry, H. N., IV, 2001,
The geology of a part of Acadia and the nature of the
Acadian orogeny across central and eastern Maine:
American Journal of Science, v. 301, p. 205–260.
Van der Voo, R., 1988, Palaeozoic paleogeography of
North America, Gondwana, and intervening displaced
terranes: Comparisons of palaeomagnetism within
palaeoclimatology and biogeographical patterns: Geo-
logical Society of America Bulletin, v. 100, p. 311–
324.
Van der Voo, R., and Johnson, R. J. E., 1985, Paleomag-
netism of the Dunn Point Formation (Nova Scotia):
High paleolatitudes for the Avalon terrane in the Late
Ordovician: Geophysical Research Letters, v. 12, p.
337–340.
van Staal, C. R., 1994, The Brunswick subduction com-
plex in the Canadian Appalachians: Record of the Late
Ordovician to Late Silurian collision between Lauren-
tia and the Gander margin of the Avalon: Tectonics, v.
13, p. 946–962.
van Staal, C. R., and de Roo, J. A., 1995, Mid-Palaeozoic
tectonic evolution of the Appalachian Central Mobile
Belt in northern New Brunswick, Canada: Collision,
extensional collapse, and dextral transpression, in
Hibbard, J., van Staal, C. R., and Cawood, P., eds.,
New perspectives in the Appalachian–Caledonian
Orogen: Geological Association of Canada Special
Paper 41, p. 367–389.
van Staal, C. R., Dewey, J. F., MacNiocaill, C., and McK-
errow, W. S., 1998, The Cambrian–Silurian tectonic
evolution of the Northern Appalachians and British
Caledonides: History of a complex, west and southwest
Pacific-type segment of Iapetus, in Blundell, D., and
Scott, A. C., eds., Lyell: The past is the key to the
present: Geological Society of London Special Publi-
cation 143, p. 199–242.
van Staal, C. R., Ravenhurst, Winchester, J. A., Roddick,
J. C., and Langton, J. P., 1990, Post Taconic blueschist
suture in the northern Appalachians of New Brun-
swick, Canada: Geology, v. 18, p. 1073–1077.

ACADIAN OROGENY 687
van Staal, C. R., Sullivan, R. W., and Whalen, J. B., 1996,
Provenance and tectonic history of the Gander zone in
the Caledonide/Appalachian orogen: Implications for
the origin and assembly of Avalon, in Nance, R. D.,
and Thompson, M. D., eds., Avalonian and related
peri-Gondwanan terranes of the Circum-North Atlan-
tic: Geological Society of America Special Paper 304,
p. 347–367.
Vick, H., Channell, J., and Opdyke, N., 1987, Ordovician
docking of the Carolina slate belt: Paleomagnetic data:
Tectonics, v. 6, p. 573–583.
Waldron, J. W. F., Murphy, J. B., Melchin, M., and Davis,
G., 1996, Silurian tectonics of western Avalonia:
Strain-corrected subsidence history of the Arisaig
Group, Nova Scotia: Journal of Geology, v. 104, p.
677–694.
Wells, R. E., Engebretson, D. C., Snavely, P. D., Jr., and
Coe, R. S., 1984, Cenozoic plate motions and the vol-
cano-tectonic evolution of western Oregon: Tectonics,
v. 3, p. 275–294.
Whalen, J. B., Jenner, G. A., Currie, K. L., Barr, S. M.,
Longstaffe, F. J., and Hegner, E., 1994, Geochemical
and isotopic characteristics of granitoids of the Avalon
Zone, southern New Brunswick: Possible evidence for
repeated delamination events: Journal of Geology, v.
102, p. 269–282.
Whalen, J. B., Jenner, G. A., Longstaffe, F. J., and Hegner,
E., 1996, Nature and evolution of the eastern margin of
Iapetus: Geochemical and isotopic constraints from
Siluro-Devonian granitoid plutons in the New Brun-
swick Appalachians: Canadian Journal of Earth
Sciences, v. 33, p. 140–155.
Willard, R. A., and Adams, M. G., 1994, Newly discov-
ered eclogite in the southern Appalachian orogen,
northwestern Carolina: Earth and Planetary Science
Letters, v. 123, p. 61–70.
Williams, H., 1979, Appalachian orogen in Canada: Cana-
dian Journal of Earth Sciences, v. 16, p. 792–807.
Williams, H., 1993, Acadian orogeny in Newfoundland, in
Roy, D. C., and Skehan, J. W., eds., The Acadian orog-
eny: Recent studies in New England, Maritime Can-
ada, and the autochthonous foreland: Geological
Society of America Special Paper 275, p. 123–133.
Williams, H., and Piasecki, M. A. J., 1990. The Cold
Spring mélange and a possible model for Dunnage-
Gander zone interaction in central Newfoundland:
Canadian Journal of Earth Sciences, v. 27, p. 1126–
1134.
Williams, S. H., Harper, D. A. T., Neuman, R. B., Boyce,
W. D., and MacNiocaill, C., 1995, Lower Palaeozoic
fossils from Newfoundland and their importance in
understanding the history of Iapetus Ocean: Geologi-
cal Association of Canada, Special Paper no. 41, p.
115–126.
Woodcock, N., and Strachan, R. A., 2000, Geological his-
tory of the British Isles: Oxford, UK, Blackwell
Science.
Yañez, G., Cembrano, J., Pardo, M., Ranero, C., and
Selles, D., 2002, The Challenger–Juan Fernández–
Maipo major tectonic transition of the Nazca-Andean
subduction system at 33–34°S: Geodynamic evidence
and implications: Journal of South American Earth
Sciences, v. 15, p. 23–38.