The Pangea conundrum
J. Brendan Murphy1, R. Damian Nance 2
1
Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada
2
Department of Geological Sciences, Ohio University, Athens, Ohio 45701, USA
ABSTRACT
Geodynamic models for supercontinent assembly, whereby the dispersing continental fragments of a supercontinent break up and migrate from geoid highs to reassemble at
geoid lows, fail to account for the amalgamation of Pangea. Such models would predict that
the oceans created by continental breakup in the early Paleozoic (e.g., Iapetus, Rheic) would
have continued to expand as the continents migrated toward sites of mantle downwelling in
the paleo-Pacific, reassembling an extroverted supercontinent as this ocean closed. Instead,
Pangea assembled as a result of the closure of the younger Iapetus and Rheic Oceans. Geodynamic linkages between these three oceans preserved in the rock record suggest that the
reversal in continental motion may have coincided with the Ordovician emergence of a superplume that produced a geoid high in the paleo-Pacific. If so, the top-down geodynamics used
to account for the breakup and dispersal of a supercontinent at ca. 600–540 Ma may have been
overpowered by bottom-up geodynamics during the amalgamation of Pangea.
Keywords: Pangea, geodynamics, supercontinents, Appalachian orogen, Terra Australis orogen.
INTRODUCTION
Although the existence of Pangea is one of the cornerstones of geology, the mechanisms responsible for its amalgamation are poorly understood. To a first order, we know where and when it formed (e.g., Scotese,
1997; Cocks and Torsvik, 2002; Stampfli and Borel, 2002), but not why.
The formation of Pangea is primarily recorded in the origin and evolution of the Paleozoic oceans (e.g., Iapetus, Rheic, Prototethys) between
Laurentia (ancestral North America), Baltica (northwestern Europe), and
northern Gondwana (South America–West Africa), the closure of which
resulted in its amalgamation (e.g., van Staal et al., 1998). Although well
documented, this record runs counter to many widely accepted geodynamic models for the forces that drive the supercontinent cycle. There
is a broad consensus that sinking of cold lithosphere (slab pull) provides
90% of the force needed to drive plate tectonics (e.g., Anderson, 2001) and
is indirectly responsible for upwelling beneath mid-ocean ridges (Stern,
2004). This scenario is compatible with geodynamic arguments (Gurnis,
1988), which propose that supercontinents break up and migrate from
sites of mantle upwelling (geoid highs) toward subduction zones, and they
reassemble at sites of mantle downwelling (geoid lows).
The Mesozoic breakup and dispersal of Pangea, for example,
occurred above sites of mantle upwelling, and its fragments are currently
being dispersed toward sites of mantle downwelling as the oceans between
them (interior oceans; Murphy and Nance, 2003) widen, and the exterior
ocean (Panthalassa-Pacific) that surrounded Pangea contracts. If current
plate motions are projected into the future, then the next supercontinent
(Amasia; Hoffman, 1999) should assemble as the exterior ocean closes, a
process termed extroversion (Fig. 1; Murphy and Nance, 2003).
However, if these geodynamic arguments are applied to the early
Paleozoic, they fail to predict the formation of Pangea in the correct configuration. In the Late Neoproterozoic and Ediacaran, most reconstructions
show Laurentia joined to West Gondwana, while the eastern Gondwanan
blocks converged and ultimately collided with West Gondwana. This produced a hemisphere dominated by the assembly of continental fragments
surrounded by the hemispheric paleo–Pacific Ocean. By the early Paleozoic, Laurentia, Baltica, and Gondwana had separated (Fig. 1A), resulting
in the opening of the Iapetus and Rheic Oceans and convergence in the
paleo-Pacific. However, for Pangea to have assembled at a site of mantle
downwelling, the dispersing continents should have migrated toward the
subduction zones in the exterior paleo–Pacific Ocean, and it is this ocean
that should have closed. Instead, a wealth of geologic evidence indicates that subduction commenced in the relatively newly formed interior
(Iapetus and Rheic) oceans located between Laurentia, Baltica, and Gondwana, and that Pangea assembled as the interior oceans closed, a process
termed introversion (Fig. 2; Murphy and Nance, 2003).
To gain insight into the processes leading to the formation of Pangea,
we investigated the geodynamic linkages between the Paleozoic evolution
of the interior (Iapetus and Rheic) oceans, as recorded in the orogens produced by their closure (Ouachita, Appalachian, Caledonia, Variscan), and
the penecontemporaneous evolution of the exterior (paleo-Pacific) ocean.
Oceanic domains at the leading (exterior) edges of the dispersing continents are recorded in the 18,000 km Terra Australis orogen, which preserves a record of subduction until at least 230 Ma (e.g., Cawood, 2005).
GEOLOGICAL FRAMEWORK
Initial rifting of the Iapetus Ocean occurred in stages from ca. 600 Ma
to 550 Ma between Laurentia, Baltica, and Amazonia (Fig. 1A; Cawood
et al., 2001). The onset of subduction in the Iapetus realm is recorded
by the development of ca. 500–480 Ma arc-related ophiolitic complexes, which were obducted soon after onto the Laurentian margin, an
event assigned to the Taconic orogeny (Appalachians) and the Grampian
orogeny (Caledonides) (van Staal et al., 1998). At about the same time
as these orogenic events, the Rheic Ocean formed as a result of the Late
Cambrian–Early Ordovician drift of peri-Gondwanan terranes (including
Ganderia-Avalonia and Carolinia) away from the northern Gondwanan
margin, which preserves a ca. 500 Ma passive-margin assemblage and
coeval voluminous bimodal magmatism in southern Mexico (Acatlán
Complex; Nance et al., 2006), Britain, Iberia, and the Bohemian Massif
(e.g., Sánchez-García et al., 2003; Kryza et al., 2003). An Early Ordovician breakup unconformity and widespread rift-related subsidence are
indicated by the distribution of the Armorican Quartzite across much of
mainland Europe (Quesada et al., 1991; Linnemann et al., 2004).
By the Middle Ordovician, the peri-Gondwanan terranes were
positioned ~2000 km north of the Gondwanan margin and separated the
Iapetus Ocean to the north from the Rheic Ocean to the south (Fig. 1).
The Iapetus Ocean was closed in stages between the Late Ordovician and
Middle Silurian, in the north by the collision of Laurentia and Baltica to
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GEOLOGY,
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Geology,
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v. 36; no. 9; p. 703–706; doi: 10.1130/G24966A.1; 2 figures.
703
A
High angle between plate boundary and ridge
Laurentia
A
B
Supercontinent
C
Breakup
Drift
460 Ma
540 Ma
Laurentia
A-C
Iapetus
D
E
Introversion
F
Extroversion
Combination
Gondwana
A-C
South Pole
Laurussia
Passive
margin
Variscan
orogen
AlleghanianOuachita
orogen
A-C
Rheic Ocean
280 Ma
370 Ma
B
North America
280 Ma
A
C
Ch-Oax-Y
South
America
?
FI
?
NW Africa
BM
A
?
ARM
NW-I
OM W Europe
Rheic Suture
Figure 1. Paleozoic reconstructions (modified from Scotese, 1997;
Cocks and Torsvik, 2002; Stampfli and Borel, 2002; Murphy et al.,
2006; Cawood and Buchan, 2007). A: By 540 Ma, Iapetus Ocean had
formed between Laurentia and Gondwana. By 460 Ma, AvaloniaCarolinia (A–C) had separated from Gondwana, creating the Rheic
Ocean. By 370 Ma, Laurentia, Baltica, and A–C had collided to form
Laurussia, and Rheic Ocean began to contract, closing by 280 Ma,
to form Pangea. Terra Australis orogen at 280 Ma was located
along periphery of Pangea, representing vestige of tectonic activity
within paleo–Pacific Ocean. B: Location of Rheic Ocean suture within
Alleghanian-Ouachita and Variscan orogens and early Mesozoic
position of peri-Gondwanan terranes mentioned in text (see Keppie
et al., 2003). Ch-Oax-Y—Chortis, Oaxaquia-Yucatan; F—Florida;
C—Carolina; A—Avalonia; O-M—Ossa Morena; NW-I—Northwest
Iberia, ARM—Armorica; BM—Bohemian massif.
form Laurussia, and in the south by the accretion of Ganderia, Avalonia,
and Carolina to Baltica and then Laurentia (van Staal et al., 1998; Hibbard
et al., 2002). Northerly directed subduction of the Rheic Ocean beneath
Laurussia commenced in the Silurian and continued until its closure and
the amalgamation of Pangea in the Carboniferous.
In the early Paleozoic, Laurentia and Gondwana were surrounded
by the paleo–Pacific Ocean, vestiges of which are preserved in the Terra
Australis orogen, which extends from eastern Australia and Tasmania into
New Zealand, the Transantarctic Mountains, southern Africa, and South
704
Ocean ridge
Subduction zone
Transform fault
Collisional orogeny
Mafic terranes
Interior arc
Exterior arc
Figure 2. A–C: Schematic diagrams (modified from Murphy and
Nance, 2003) showing stages in breakup of a supercontinent, fate
of relatively old oceanic lithosphere that surrounds supercontinent
before breakup (exterior ocean, dark shade), and creation of relatively new oceanic lithosphere between dispersing blocks (interior
ocean, pale shade). In the case of Pangea, paleo-Pacific represents
exterior ocean, and Iapetus and Rheic Oceans are examples of
interior oceans. In B and C, note subduction commences along the
boundaries between interior and exterior oceans. D: Configuration
of a supercontinent formed by introversion (i.e., preferential subduction of interior oceanic lithosphere). E: Configuration of a supercontinent formed by extroversion (i.e., preferential subduction of exterior
oceanic lithosphere). F: An intermediate case in which one ocean is
closed by introversion and the other is closed by extroversion.
America (Fig. 1; Cawood, 2005). Subduction along this margin began at
ca. 580 Ma (Cawood and Buchan, 2007) and continued until the end of
the Paleozoic, by which time the orogen was distributed along the periphery of Pangea. The orogen is interpreted to have formed by accretionary
processes without the involvement of major continental collisions. Its
evolution is best documented in the Adelaide, New England, and Lachlan
fold belts of eastern Australia (Collins, 2002; Gray and Foster, 2004). The
most complete record of continental-margin evolution is preserved in the
Adelaide fold belt, where Neoproterozoic to Cambrian passive-margin
sedimentary successions occur in a series of rift-sag basins. Abundant
early Paleozoic volcanic complexes and intra-oceanic sequences are preserved in the New England fold belt, and the 700-km-wide Lachlan fold
belt is dominated by 520–480 Ma oceanic volcanic rocks overlain by a
blanket of sediments analogous to the modern Bengal fan. Between 440
and 400 Ma, intra-oceanic arc and basin development recorded in these
belts by sedimentary and mafic volcanic rocks is interpreted to reflect the
retreat of the subducted slab during episodes of upper-plate extension and
basaltic magmatism, and transient flat subduction attributed to the arrival
of buoyant oceanic plateaus (Collins, 2002).
Intra-oceanic subduction in the paleo–Pacific Ocean was clearly
well established ~80 m.y. before subduction commenced in the Iapetus
Ocean (Cawood and Buchan, 2007) and before the Rheic Ocean opened.
According to conventional geodynamic models, slab-pull forces associated with this subduction should have resulted in the migration of the
dispersing continents toward the paleo-Pacific subduction zones, as was
the case when the Iapetus Ocean opened. However, this motion reversed
itself when subduction commenced in the relatively newly formed Iapetus
and Rheic Oceans between Laurentia, Baltica, and Gondwana. During the
GEOLOGY, September 2008
assembly of Pangea, therefore, subduction was not only initiated within
the new interior oceans, but the rates of subduction of this relatively young
Paleozoic lithosphere must also have overpowered those of the already
well established subduction zones within the exterior (paleo-Pacific)
ocean. Thus, rather than forming by extroversion, as most geodynamic
models would predict, the formation of Pangea was actually achieved by
introversion (see Fig. 2).
GEODYNAMIC SCENARIO
A key problem that needs to be addressed in resolving this conundrum
is the geodynamic scenario that facilitated the initiation of subduction in
the Iapetus and Rheic Oceans and generated sufficient slab pull to drive
the dispersing continental fragments toward the Pangea configuration.
Subduction initiation is controversial, although it is generally accepted
that initiation at passive margins is mechanically difficult because of
the flexural rigidity of the oceanic lithosphere (Cloetingh et al., 1989).
No simple modern analogues for such a process have been documented.
On the other hand, Stern and Bloomer (1992) and Stern (2004) showed
that intra-oceanic subduction initiation along the Isu-Bonin-Mariana arc
system occurred along a transform system where younger, less dense
oceanic lithosphere was in contact with older, denser lithosphere. This
was followed by rapid trench retreat and voluminous mafic (including
boninitic) magmatism as the older lithosphere subducted beneath the
younger, and it is consistent with geodynamic models that show subduction initiation followed by rapid back-arc extension (Gurnis et al., 2004).
A modern analogue for this process may be occurring along the Atlantic
margin beneath Gibraltar, where subduction initiation of the Atlantic has
been documented at the eastern end of the Azores-Gibraltar transform
fault (Gutscher et al., 2002).
Within the Iapetan realm, such a scenario would have existed in the
early Paleozoic along boundaries been the young Iapetan and old paleoPacific oceanic lithosphere (see Figs. 2B and 2C). Such boundaries are
a requirement of supercontinent breakup. The high angle between these
boundaries and the Iapetan ocean ridge suggests that they had a strong
strike-slip component and, hence, were likely sites for transform faults
to nucleate. The age of the paleo-Pacific oceanic lithosphere adjacent
to these transform faults may never be known, but it is almost certain
to have been older than the adjoining interior (Iapetus) oceanic lithosphere. The age and density contrast between them, and therefore the
most favorable conditions for subduction initiation, would have been
most pronounced when the Iapetus Ocean was young. We view this to be
the most favorable geodynamic scenario for the origin of the arc-related
oceanic complexes that were respectively obducted onto the margins of
Laurentia and Baltica during the ca. 500–480 Ma Taconic and Grampian
orogenies (see van Staal et al., 1998). The implication of this interpretation is that these mafic arc-related complexes were formed from the
subduction of paleo-Pacific, rather than Iapetan, lithosphere in a manner
analogous to the Scotia arc and the Mesozoic-Cenozoic “capture” of the
subduction zones around the Caribbean plate within the Atlantic realm
(e.g., Pindell et al., 2006). The evolution of the Iapetan–paleo-Pacific
plate boundaries would have depended on a number of factors, including the spreading rate at the Iapetan ridge, the drift of the dispersing
continents, and the rate of rollback of the subduction zone. However,
the presence of boninites and mafic sheeted dikes in Iapetan ophiolitic
complexes (e.g., Bédard et al., 1998) indicates that forearc extension,
and hence slab rollback, occurred. The strike of an arc produced in this
fashion would have been oriented at a high angle to the Iapetan ocean
ridge, and this geometry would have facilitated its subsequent obduction
(see Suhr and Cawood, 1993). However, as pointed out by Gurnis et al.
(2004), subduction zones initiated in this manner cannot pull the large
overriding plates toward them, and so cannot be considered a mechanism that began the assembly of Pangea.
GEOLOGY, September 2008
Global reconstructions (e.g., Scotese, 1997; Stampfli and Borel,
2002) imply that assembly of Pangea began after the closure of the Iapetus
Ocean. They also imply that the rate of subduction of Rheic Ocean lithosphere exceeded subduction rates in the paleo–Pacific Ocean despite
the fact that subducted Rheic oceanic lithosphere was a maximum of
60 m.y. old, and that subduction zones were already well established in the
paleo–Pacific Ocean.
Although the causes are hotly debated, a geodynamic linkage
between tectonic events in the Rheic and paleo–Pacific Oceans is suggested by the dramatic change in tectonic environment of paleo-Pacific
subduction zones between 440 Ma and 420 Ma, best documented in
the Lachlan fold belt of eastern Australia (e.g., Gray and Foster, 2004), the
time at which the Iapetus Ocean was closing and subduction within
the Rheic Ocean was commencing.
Proxy records for oceanic lithosphere evolution are also consistent
with this linkage. Rapid sea-level rise in the Cambrian and Early Ordovician (e.g., Hallam, 1992), generally attributed to the development of relatively young, elevated Iapetan oceanic lithosphere, gave way to a drop in
sea level from the Late Ordovician to the Carboniferous, implying that
the average age of the oceanic lithosphere increased during the middle to
late Paleozoic. This trend is compatible with preferential subduction of
relatively young Rheic Ocean lithosphere over older paleo-Pacific oceanic
lithosphere. Similarly, compared to the early Paleozoic, the lower initial
87
Sr/86Sr value deduced for ocean waters in the late Paleozoic (e.g., Veizer
et al., 1999) implies an increase in the rate of seafloor spreading. Taken
together, the sea-level and Sr isotope data suggest that any increased
seafloor spreading in the paleo–Pacific Ocean during Pangea assembly
was compensated, not by subduction of old paleo-Pacific lithosphere, but
by subduction of newer Rheic Ocean lithosphere.
Top-down geodynamic models adequately account for the early
Paleozoic dispersal of continents, but they cannot account for the middle
to late Paleozoic introversion that resulted in the amalgamation of Pangea. This conundrum suggests that top-down geodynamic forces can be
overridden by other forces. Assuming that continents migrate from geoid
highs to geoid lows, the reversal of their movement in the middle Paleozoic implies that the geoid lows became geoid highs. The mechanisms for
this are unclear. However, one possibility is that the descent of subducted
slabs in the paleo–Pacific Ocean to the core-mantle boundary thermally
insulated that boundary, leading to the ponding of hot mantle material that
built up and rose toward the surface as a superplume (Zhong and Gurnis,
1997; Condie, 1998; Tan et al., 2002). Such an event would result in a
geoid high and enhanced seafloor spreading in the paleo–Pacific Ocean
that could have been responsible for the reversal in the direction of motion
of the continents. In this scenario, “top-down” tectonics of the early Paleozoic gave way to “bottom-up” tectonics in the late Paleozoic. An Ordovician superplume has been proposed by Condie (2004) to account for the
period’s high sea levels, high paleotemperatures, abundant black shales,
and strong biological activity.
CONCLUSIONS
The amalgamation of Pangea cannot be explained by top-down
geodynamic models. If these principles are applied to an Early Ordovician world, they would not generate Pangea in the correct configuration. Instead, the geologic record implies that the rates of subduction in
the relatively new interior oceans (e.g., the Iapetus and Rheic Oceans)
exceeded those in the exterior paleo–Pacific Ocean. Geodynamic linkages
between subduction in the interior and exterior oceans are implied by dramatic changes in the style of subduction in the paleo-Pacific that coincide
with the onset of subduction in the Rheic Ocean. Assuming continents
migrate from geoid highs to geoid lows, then the reversal in continental motions that led to the formation of Pangea may have coincided with
the emergence of a geoid high in the paleo-Pacific during the Ordovician.
705
We tentatively suggest that this may reflect the formation of a superplume,
for which there is independent, supporting evidence in the rock record.
If so, the preferential destruction of interior oceans, required to produce
introverted supercontinents such as Pangea, may occur when top-down
tectonics give way to bottom-up tectonics.
ACKNOWLEDGMENTS
Murphy acknowledges the continuing support of the Natural Sciences and
Engineering Research Council, Canada, through Discovery and Research Capacity
grants. Nance is supported by National Science Foundation grant EAR-0308105.
We are indebted to Peter Cawood, Michael Gurnis, and Joe Meert for their insightful reviews. This paper is a contribution to the International Geological Correlation
Programme (IGCP) Projects 453 and 497.
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Manuscript received 18 March 2008
Revised manuscript received 16 May 2008
Manuscript accepted 20 May 2008
Printed in USA
GEOLOGY, September 2008