GSA Bulletin: Neoproterozoic-Paleozoic geography …...the Appalachian or ogen in the Canadian Mar itimes, and of the Caledonian orogen in the Br itish Isles, contin ues to be emphasiz

ABSTRACT

The ever-changing distribution of continents and ocean basins onEarth is fundamental to the environment of the planet. Recent ideas re-garding pre-Pangea geography and tectonics offer fresh opportunitiesto examine possible causative relations between tectonics and environ-mental and biologic changes during the Neoproterozoic and Paleozoiceras. The starting point is an appreciation that Laurentia, the rift-bounded Precambrian core of North America, could have been juxta-posed with the cratonic cores of some present-day southern continents.This has led to reconstructions of Rodinia and Pannotia, superconti-nents that may have existed in early and latest Neoproterozoic time, re-spectively, before and after the opening of the Pacific Ocean basin.

Recognition that the Precordillera of northwest Argentina consti-tutes a terrane derived from Laurentia may provide critical longitudi-nal control on the relations of that craton to Gondwana during the Pre-cambrian-Cambrian boundary transition, and in the early Paleozoic.The Precordillera was most likely derived from the general area of theOuachita embayment, and may have been part of a hypotheticalpromontory of Laurentia, the “Texas plateau,” which was detachedfrom the Cape of Good Hope embayment within Gondwana betweenthe present-day Falkland-Malvinas Plateau and Transantarctic Moun-tains margins. Thus the American continents may represent geometric“twins” detached from the Pannotian and Pangean supercontinents inEarly Cambrian and Early Cretaceous time, respectively—the newmid-ocean ridge crests of those times initiating the two environmentalsupercycles of Phanerozoic history 400 m.y. apart. In this scenario, theextremity of the Texas plateau was detached from Laurentia during theCaradocian Epoch, in a rift event ca. 455 Ma that followed Middle Or-dovician collision with the proto-Andean margin of Gondwana as partof the complex Indonesian-style Taconic-Famatinian orogeny, whichinvolved several island arc-continent collisions between the two majorcontinental entities. Laurentia then continued its clockwise relative mo-tion around the proto-Andean margin, colliding with other arc ter-ranes,Avalonia, and Baltica en route to the Ouachita-Alleghanian-Her-cynian-Uralian collision that completed the amalgamation of Pangea.

The important change in single-celled organisms at the Mesopro-terozoic-Neoproterozoic boundary (1000 Ma) accompanied assemblyof Rodinia along Grenvillian sutures. Possible divergence of metazoanphyla, the appearance and disappearance of the Ediacaran fauna (ca.650–545 Ma), and the Cambrian “explosion” of skeletalized metazoans(ca. 545–500 Ma) also appear to have taken place within the frameworkof tectonic change of truly global proportions. These are the opening of

the Pacific Ocean basin; uplift and erosion of orogens within the newlyassembled Gondwana portion of Pannotia, including a collisionalmountain range extending ≈7500 km from Arabia to the Pacific mar-gin of Antarctica; the development of a Pannotia-splitting oceanicspreading ridge system nearly 10 000 km long as Laurentia broke awayfrom Gondwana, Baltica, and Siberia; and initiation of subductionzones along thousands of kilometres of the South American andAntarctic-Australian continental margins. The Middle Ordovician sea-level changes and biologic radiation broadly coincided with initiationof the Appalachian-Andean mountain system along >7000 km of theTaconic and Famatinian belts. These correlations, based on testable pa-leogeographic reconstructions, invite further speculation about possi-ble causative relations between the internally driven long-term tectonicevolution of the planet, its surface environment, and life.

INTRODUCTION

“Apparently, when evolution becomes sluggish, it is not so much for want of an ap-propriate chance mutation as for the lack of a worthy environmental challenge.” (deDuve, 1995, p. 213)

The most striking change in the history of the Earth recorded in the geo-logic record marks the boundary between rocks formed during its≈4.0-b.y.-long Archean and Proterozoic history and strata of the overlyingCambrian System. The Cambrian strata contain the fossil evidence of theapparently rapid development of skeletalized metazoans that commencedbetween 545 and 540 Ma, the dawn of Phanerozoic time (Brasier, 1991,1992a; Conway Morris, 1993; Knoll and Walter, 1992; Bowring et al.,1993; Grotzinger et al., 1995; see Fig. 1 for time scale). The cause of thisbiologic “explosion” that peaked at the Tommotian-Atdabanian boundary,ca. 528 Ma, is fundamental to the understanding of the development of lifeon the planet, even though the new line of evidence provided by molecularbiology indicates that divergence among metazoan phyla may have oc-curred in the “Deep Precambrian” (Wray et al., 1996), near the beginningof Neoproterozoic time.

The Neoproterozoic Era, from 1.0 Ga to the Precambrian-Cambrianboundary, was an interval of dramatic changes in global environmentalconditions. There were several glaciations, possibly extending to tropicalor even equatorial latitudes, and carbonates were apparently depositedfrom the equator to very high latitudes (Hambrey and Harland, 1981;Young, 1995). Isotopic data from Neoproterozoic sedimentary rocks indi-cate major swings in the chemistry of ocean waters (Asmerom et al., 1991;Brasier, 1992b; Kaufman et al., 1993; Ripperdan, 1994; Nicholas, 1996).Global sea level rose during Cambrian time, marking the start of the first-order eustatic cycle that lasted throughout the Paleozoic Era, the earlier of

16

OVERVIEWNeoproterozoic-Paleozoic geography and tectonics: Review, hypothesis,environmental speculation

Ian W. D. Dalziel* Institute for Geophysics and Department of Geological Sciences, University of Texas,8701 North Mopac Expressway, Austin, Texas 78759-8397

GSA Bulletin; January 1997; v. 109; no. 1; p. 16–42; 17 figures; 2 tables.

*E-mail address: [email protected]

OVERVIEW: NEOPROTEROZOIC-PALEOZOIC GEOGRAPHY AND TECTONICS

Geological Society of America Bulletin, January 1997 17

Figure 1. Time-scale; right-hand col-umn, indicates hypothetical superconti-nental entities and ocean basins referredto in text. Sources are: Ordovician—In-ternational Union of Geological Sci-ences, Subcommission on OrdovicianStratigraphy, Williams, S. H., 1995, per-sonal commun.; Cambrian—Grotz-inger et al., 1995; Proterozoic—Interna-tional Union of Geological Sciences,Plumb, 1991.

the two Phanerozoic environmental supercycles (Fischer, 1984). Importantsecond-order falls in sea level (Hallam, 1992), one of the most pronouncedradiations of marine organisms in Earth history, and a glaciation in Gond-wana (Beuf et al., 1971) marked the Ordovician Period.

The ever-changing distribution of continents and ocean basins is a criti-cal element in understanding Earth’s environment (Crowley and North,1991). Tectonic change, such as sea-floor spreading, formation of oceanicplateaus, and the uplift of mountain ranges and major plateaus, is believedto have played a major role in Mesozoic and Cenozoic environmentalchange (Raymo et al., 1988; Hallam, 1992). In this article I review the theproblems of global geography and tectonics in Neoproterozoic and earlyPaleozoic time, expanding on recent ideas that Laurentia, the ancestral cra-ton of North America, might have been juxtaposed to, and interacted tec-tonically with, elements of the southern supercontinent of Gondwana dur-ing that time interval. This may explain the otherwise unmatched, if poorlyunderstood, late Precambrian Pacific margins of Laurentia, South Amer-ica, Antarctica, and Australia. I suggest a new hypothesis, in which a re-cently established geologic relationship between the Andean Precordilleraof northwestern Argentina and the southern United States may, by provid-ing constraint on the relative longitude of Laurentia and Gondwana at twocritical times, prove to be a key element in understanding the geography

I. W. D. DALZIEL

18 Geological Society of America Bulletin, January 1997

and tectonics of the planet as multicellular life became firmly established.I speculate briefly and in general terms on possible causative relations overthat time interval between tectonics, the environment, and biologic evolu-tion. The geographic and tectonic scenario that I propose presents ample“challenges” to spur evolution if Christian de Duve’s observation, quotedat the beginning of this introduction, is on target.

REVIEW: “ARCHETYPAL” AND “ALTERNATIVE”PALEOGEOGRAPHIC SCENARIOS

Every student of stratigraphy learns of the distinction between the olenel-lid benthic trilobite realm in northwest Scotland and western Newfoundlandand the paradoxidid realm of southern Britain and the Avalon platform, the

Figure 2. (a) Distribution of “Atlantic” (horizontal lines) and “Pacific” (vertical lines) benthic trilobite faunas in the North Atlantic– borderingcontinents ( from Wilson, 1966). (b) Interpretation of the proto-Atlantic ocean of early Paleozoic time according to Wilson (1966; the Iapetus oceanof Harland and Gayer, 1972, see text); note separation of faunal provinces, and the matching conjugate rifted margins (see text). Stipple indicatesOrdovician island arcs.

Figure 3. (a) Paleomagnetically controlledcontinental reconstruction for Early Ordoviciantime (Torsvik and Trench, 1991); note that aclockwise change in the paleolongitude of Lau-rentia with respect to Gondwana, which is per-missible given the axial symmetry of Earth’smagnetic field, would place the proto-Appala-chian and proto-Andean margins in close prox-imity (see Fig. 4). Equal area projection. (b) In-terpretation by McKerrow et al. (1991) of thegeologic relationship of present-day NorthAtlantic–bordering continents in Early Ordovi-cian time using the paleogeography of Figure 3a.Brick ornament shows extent of carbonate plat-forms; coarse stipple represents thick clastic de-posits; fine dots are shelf clastics. Black trianglesrepresent active island-arc systems.

a



classic “Pacific” and “Atlantic” provinces of Walcott (1889; Fig. 2a). Evenbefore continental drift was widely accepted, the issue of whether theseearly Paleozoic benthic faunas were separated by a deep and wide oceanbasin or by a “geanticlinal” ridge was discussed in my late 1950s stratigra-phy class at the University of Edinburgh (Holmes, 1965). J. Tuzo Wilsonbrought it to the forefront of geologic thought in the 1960s: he extended thedeveloping plate tectonic paradigm back to early Paleozoic time when,seeking to explain the two-sided geologic as well as faunal nature of theAppalachian orogen (Williams, 1964), he asked the question, “Did the At-lantic Ocean open, close, and then re-open?” (Wilson, 1966). He introducedboth the term “proto-Atlantic” for the early Paleozoic ocean separating Lau-rentia from Baltica and Gondwana (Fig. 2b), and the now orthodox conceptof the Wilson cycle of ocean basin opening and closure to generate an oro-genic belt. The paradigm of plate tectonics and the concept of the Wilsoncycle were first applied to the detailed analysis of an orogen, the sameAppalachian-Caledonian belt (Dewey, 1969; Bird and Dewey, 1970).

The faunal and environmental distinction between the two forelands ofthe Appalachian orogen in the Canadian Maritimes, and of the Caledonianorogen in the British Isles, continues to be emphasized by modern workers(Cocks and Fortey, 1982; Williams et al., 1996; Fig. 2b). The term Iapetusocean was substituted for proto-Atlantic ocean by Harland and Gayer(1972), the wide acceptance of this change to the name of the mythologicfather of Atlas reflecting appreciation that the early Paleozoic ocean basinoff proto-Appalachian-Caledonian Laurentia was a separate entity, ratherthan a direct precursor of the present Atlantic Ocean basin.

What has not changed is the widespread belief that through most of earlyPaleozoic time, the Iapetus ocean basin off the proto-Appalachian-Caledonian rifted margin of Laurentia separated that craton from Baltica,northwest Africa, and northwestern South America (Morel and Irving,1978; Fortey and Cocks, 1992; McKerrow and Scotese, 1990; McKerrow etal., 1991, 1992; Jurdy et al., 1995; Pickering and Smith, 1995; Torsvik et al.,1995, 1996a, 1996b; Van der Pluijm et al., 1995; Mac Niocaill et al., 1996,1997). In the absence of any real evidence of late Precambrian juxtapositionof the Laurentian and West African cratons, this appears to have sprungfrom Tuzo Wilson’s original question, illustration (Fig. 2b), and implicationof a type of accordion tectonics with ocean basins opening and closing in“approximately but not precisely” the same configuration of the borderingcratons (Wilson, 1966, p. 677).

Traditionally, therefore, the Appalachian margin of Laurentia has tendedto be juxtaposed with northwest Africa in reconstructions for times prior tolate Precambrian rifting (Bird and Dewey, 1970; Hatcher, 1978, 1987,1989), or else a “North Atlantic craton,” joining Laurentia and Baltica, isportrayed in isolation (e.g., Winchester, 1987; Gower et al., 1990). Like-wise, paleomagnetically controlled reconstructions of early Paleozoic ge-ography mostly show the proto-Appalachian and northwest African mar-gins opposed to each other, even when separated by 4500 km of Iapetusocean basin (e.g., Torsvik and Trench, 1991; Fig. 3a). Does this traditionreflect, at least in part, the fact that by far the largest concentration of geol-ogists working on problems of early Paleozoic geography happens to becentered on the margins of the present-day North Atlantic Ocean basin? It

Figure 3. (Continued).

b

I. W. D. DALZIEL


leads to a comprehensive and elegant analysis of early Paleozoic New Eng-land, the Canadian Maritimes, the British Isles, and Scandinavia, but itdoes nothing to explain the origin and development of the rifted Neopro-terozoic-Cambrian southeastern and southern continental margins of Lau-rentia (Fig. 3b). Nor, on a global scale, does it explain the origin of the Pa-cific-facing late Precambrian margins of Laurentia, proto-Andean SouthAmerica, Antarctica, and Australia. The proto-Andean margin is the site ofa long-recognized enigmatic lower Paleozoic carbonate platform, contain-

ing the “Pacific” benthic olenellid trilobite fauna characteristic of the Lau-rentian craton, in the Andean Precordillera of northwest Argentina (Keidel,1921; Borrello, 1965, 1971; Palmer, 1972).

A challenge to this “archetypal” view has emerged in recent years with thesuggestion of an “alternative paleogeographic reconstruction,” to use thephrases of Torsvik et al. (1995). The alternative started with the introductionof the “SWEAT” hypothesis of Moores (1991). This revived an idea of Cana-dian geologists to explain the late Precambrian Cordilleran margin of Lau-

Figure 4. Hypothetical relative motion of Laurentia (North America—NAM) around the proto-Andean margin of Gondwana (AFR—Africa;AUS—Australia; EANT—East Antarctica; IND—India; SAM—South America) during the Paleozoic Era (Dalziel, 1991). Numbered Laurent-ian outlines show paleomagnetically permissible positions of that continent relative to Gondwana at successive time (in millions of years beforepresent) during the Neoproterozoic and Paleozoic. Map is an elliptical mercator projection with Africa in its present-day position, and Gondwanaassembled using Mesozoic-Cenozoic sea-floor spreading data. EWM—Ellsworth-Whitmore block of Antarctica, and FI—Falkland-Malvinas Is-lands platform (Lafonian microplate) of South America. Both are shown restored to their pre-Jurassic positions (for explanation of paleomag-netic poles used, see Dalziel and Grunow, 1992; Grunow et al., 1987).



rentia by juxtaposition of the southwest United States and East Antarctica(SWEAT) prior to the opening of the Pacific Ocean basin in Neoproterozoictime. The determination of longitude was the major problem of early navi-gators, and the determination of relative paleolongitude is the most difficultchallenge in pre-Pangea geography. Hence, in refining and amplifying thishypothesis, I pointed out that it is paleomagnetically acceptable, given theabsence of longitudinal control, for Laurentia to have been located between

East and West Gondwana at the end of Precambrian time, and to have movedrelatively clockwise around the proto-Andean margin of South America dur-ing the Paleozoic Era en route to its final Ouachita-Alleghanian collisionwith northwest Africa to complete the assembly of Pangea (Dalziel, 1991;Fig. 4). The location of Laurentia between East Antarctica–Australia andSouth America at the end of Precambrian time was also advocated by Hoff-man (1991). We both suggested that opening of the Pacific ocean basin might

Figure 5. Reconstruction by DallaSalda et al. (1992b) of the early Fama-tinian (Ocloyic)-Taconic orogen as acontinuous Middle Ordovician Lauren-tia-Gondwana collisional belt. Note thesuggestion that the carbonate platformof the Precordilleran terrane of north-west Argentina, shown as part of thelarger Occidentalia terrane of DallaSalda et al. (1992a), was detached fromthe Ouachita embayment. Laurentia:AM—Avalon-Meguma terrane; C—Carolina terrane; MU—Marathon up-lift; OM—Ouachita fold belt; OT—Ouachita trough; RR—Reelfoot rift;SOA—Southern Oklahoma aulacogen.South America: NP—North Patagoniamassif; P—Puna; PC—Precordilleranbelt; PR—Pampean Ranges.

I. W. D. DALZIEL


have been linked to the amalgamation of Gondwana by closure of PanAfrican and Brazilide ocean basins. This was the first hypothesis attemptingto explain the Neoproterozoic-Cambrian rifted margins of East Antarc-tica–Australia and South America, together with the proto-Cordilleran andproto-Appalachian margins of the same general age in North America. To-gether these constitute a very high percentage of the “global budget” of Neo-proterozoic-Cambrian rifted margins (Bond et al., 1984).

Dalla Salda et al. (1992a) suggested that, if Laurentia had rotated clock-wise around Gondwana (Fig. 4), its Appalachian margin might have col-lided with South America, initiating the Paleozoic Famatinian orogen ex-posed as the basement of the Mesozoic-Cenozoic Andes and broadly coevalwith the Taconian orogeny of North America. My Argentine colleagues andI then proposed (Dalla Salda et al., 1992b; Dalziel et al., 1994) that the earlyPaleozoic Taconic and Ocloyic (early Famatinian) orogens of North and

Figure 6. Stratigraphic analysis and interpretation of Thomas (1991) of the evolution of the Ouachita embayment, showing (a) his interpreta-tion that the embayment was formed by an Early Cambrian ridge-crest jump from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform (from Thomas and Astini, 1996); and (b) his reconstructed section of the Southern Oklahoma fault system (from Thomas,1991), where synrift rocks are overlapped by middle Upper Cambrian strata (the section runs north-northeast, right, to south-southwest, left).



South America, respectively, had once been continuous. The Cambrian-Ordovician carbonate bank that forms the Precordillera terrane of northwestArgentina (Ramos et al., 1986), known for many years as the San Juan orMendoza faunal province and hereafter the Precordillera, is the only local-ity outside the Laurentian craton (inclusive of northwest Scotland) withstrata containing the olenellid benthic trilobite fauna (Borello, 1965, 1971).The Cambrian-Ordovician sedimentary strata forming the Precordillera arenow recognized as part of a larger terrane including Precambrian basementexposed to the east, the Cuyania terrane of Ramos (1995).

The Precordillera lies on the Pacific side of the Famatinian orogen,which is also therefore a two-sided orogen like the Appalachians and theCaledonides—a Laurentian benthic fauna occurs in the Precordilleran plat-form carbonates to the west, and a Gondwanan fauna in the craton cover tothe east. Noting the debate over a possible missing fragment of the Lau-rentian platform from the Ouachita embayment, the ancestral Gulf of Mex-ico and most prominent embayment in the early Paleozoic margin of Lau-rentia (Thomas, 1976, 1989, 1991; Lowe, 1985, 1989; Arbenz, 1989; Vieleand Thomas, 1989), we proposed that the Precordillera was detached fromLaurentia in a Late Ordovician rifting event following Middle Ordoviciancollision with Gondwana (Fig. 5; for a general review of these hypothesessee Dalziel, 1995). There is no evidence for such a rifting event in Lauren-tia, but there is in the Precordillera, as discussed in the following.

Astini et al. (1995) and Thomas and Astini (1996) compared the earlyPaleozoic stratigraphy and fauna of the Precordillera and the southeasternLaurentian platform in detail, and have greatly strengthened the DallaSalda et al. (1992b) suggestion that the Precordillera was derived from thegeneral area of the Ouachita embayment, a conclusion supported by evi-dence of a Grenvillian basement beneath the cover of the Precordillera andexposed to the east (Abruzzi et al., 1993; Kay et al., 1996). Consistent withThomas’s analysis of the rifting and subsidence history of the embayment(Thomas, 1991; Fig. 6), and to explain the affinities of the Cambrian-Ordovician fauna in the strata of the Precordillera (Benedetto et al., 1995;Fig. 7), however, Astini and his colleagues and Thomas proposed that itwas detached as a separate microcontinent during the Cambrian Period,and drifted across the Iapetus ocean basin independently to collide withSouth America in Middle Ordovician time (Astini et al., 1995; Thomas andAstini, 1996; Fig. 8). Thomas (1991) interpreted the overstep of synrift fa-cies in the Birmingham rift, the Mississippi graben, and the Southern Ok-lahoma aulacogen by Upper Cambrian strata to indicate completion of rift-drift transition within the Ouachita embayment by middle Late Cambriantime (Fig. 6). A cosmopolitan pelagic agnostid fauna invaded the Pre-cordillera during the Cambrian, and a warm-water Gondwanan benthictrilobite fauna reached the Precordilleran platform during the late Areni-gian. It was not until Middle Ordovician time, however, that lichid, ptery-gometopid, and calyminid trilobites, found in Cambrian and Lower Or-dovician strata of Gondwana, spread to Laurentia (Droser et al., 1996). Acomprehensive review of the development of the Precordillera was pro-vided by Astini et al. (1996b).

The microcontinental interpretion of Astini et al. (1995), and now ofThomas and Astini (1996; Fig. 8) was shared by most of the participants inthe recent Penrose Conference “The Argentine Precordillera: A Laurentianterrane?” cosponsored by the Geological Society of America and the Aso-ciación Geológica Argentina. This was mainly because the distinction be-tween the faunas of the Laurentian and Gondwana cratons was largelymaintained through the Middle Ordovician docking of the Precordillera,and because Thomas’s interpretation of the rifting history of the Ouachitaembayment is consistent with much of the geology of the Precordillera(Dalziel et al., 1996). Likewise, authors have noted that while the Lauren-tia-Gondwana collision advocated by Dalla Salda et al. (1992a, 1992b) ispaleomagnetically permissible, they preferred for various reasons to main-tain an “archetypal” paleogeography for early Paleozoic time, with the

proto-Appalachian margin opposed to northwest Africa (Torsvik et al.,1995, 1996a; van der Pluijm et al., 1995; Pickering and Smith, 1995).

The difference between the two models is not only a question of the causeof the Ocloyic orogeny: it is also crucial for global Paleozoic geography. Ifthe Astini et al. and Thomas and Astini microcontinental, or Laurentian ben-thic fauna “funeral ship,” model is correct, debate about the relative posi-tions of Laurentia and Gondwana will continue. Arguments about likelyrates and directions of motion for the Precordilleran microcontinent acrossthe Iapetus ocean basin, the floor of which has been recycled into the man-tle, will be added to those involving traditional paleomagnetic, faunal, andpaleoenvironmental controls (Dalziel and Dalla Salda, 1996a, 1996b; Astiniet al., 1996a; Torsvik et al., 1996b). For example, I compute the reconstruc-tions of Mac Niocaill et al. (1997) to require a convergence rate of the Pre-cordilleran terrane with respect to South America of just over 11 cm/yr(≈5600 km in ≈50 m.y.); is this reasonable or not? If, on the other hand, thePrecordilleran terrane were detached after a continent-continent collision,then it is a tectonic tracer (Dalziel, 1993) that records the relative positionsof Laurentia and Gondwana at the time of the collision during the Ordovi-cian Period. In this “calling card” model, the separation of the Laurentianand Gondwana faunas is explained by a Taconic-Ocloyic physiographicbarrier analogous to the European Alps or the Indonesian archipelago be-tween Australia and Asia, “Wallace’s line” (Dalziel and Dalla Salda, 1996a).

The process in the “calling card” model would have been similar to themuch more recent detachment from Laurentia, during the opening of theNorth Atlantic and after the Caledonian-Scandian Laurentia-Baltica colli-sion of the Scottish fragment of its craton and overlying Cambrian–LowerOrdovician platform cover with its “Pacific” trilobite fauna (Salter, 1859;Peach et al., 1907). In the case of the Precordillera, if attached to Laurentiaat the time of Ordovician docking in Gondwana, it is a paleogeographickey, defining the relative paleolongitudes of those two major continentalentities at certain times prior to the amalgamation of Pangea and thegrowth of the present ocean basins.

HYPOTHESIS: A MISSING TEXAS PLATEAU?

I propose the new hypothesis that the Precordillera was formerly part ofa Laurentian plateau analogous to, and originally adjacent to, the present-day Falkland-Malvinas Plateau in the South Atlantic Ocean basin. I makethis proposal because I perceive major difficulties with the microcontinen-

Figure 7. Changing affinities of the early Paleozoic benthic fauna ofthe Precordilleran carbonate platform in terms of genera percentage(from Benedetto et al., 1995). Time intervals: Camb—Middle to LateCambrian; Arenig—middle to late Arenig; Llanv—early Llanvirn;Carad—early Caradoc; Ashg—Hirnantian (see Fig. 1).

I. W. D. DALZIEL


tal model of Astini et al. (1995) and Thomas and Astini (1996). In addition,the “missing” plateau could explain not only the geology and faunas of thePrecordillera and the Ouachita embayment, including the long-enigmaticoffshore “Llanoria” landmass of Dumble (1920), Miser (1921), and King(1937), but also a long-standing enigma of the Pacific margin of Gond-wana. In this model of the evolution of the Appalachian-Andean mountainsystem, a refinement of the one proposed by Dalla Salda et al. (1992b) andamplified by Dalziel et al. (1994), the southward-tapering extremities ofthe North and South American cratons, the “southern cones,” are virtuallyidentical continental “twins” born more than 400 m.y. apart in the EarlyCambrian and Early Cretaceous opening of the Iapetus and South Atlanticocean basins, respectively—the start of the Paleozoic and Mesozoic eusta-tic and environmental supercycles.

That several major continents of the present day, notably South Amer-ica, Africa, and India, have pointed southern terminations has been notedfor many years (e.g., Holmes, 1944). Although the geometry of these threecontinents is, to a first approximation, explained by the pattern of Meso-zoic sea-floor spreading controlled by regional stress, plume-related stress,and lines of old lithospheric weakness such as Pan African and Brazilidebelts (Porada, 1989; Dalziel and Lawver, 1995), the origin of the “southerncone” of Laurentia from the southern Appalachians to Baja California hasremained obscure. As previously pointed out, the origin of Laurentia’ssouthern margins is not addressed by traditional or “archetypal” views ofthe Iapetus ocean basin (e.g., Fig. 3b). Explanation of the Precordillera asa microcontinent detached from the Ouachita embayment could help pro-vide an explanation, but does not involve consideration of possible conju-

Figure 8. Laurentian benthicfauna “funeral ship” model of thePrecordilleran terrane of Argen-tina as the “Guillermo Thomasia”microcontinent (PR) that was de-tached from the Ouachita embay-ment during the Cambrian (a andb), and drifted independentlyacross the Iapetus ocean basin as itsLaurentian fauna was replaced byendemic and Gondwanan forms(c and d), to collide with the proto-Andean margin in Middle Ordovi-cian time (e). Modified by me fromThomas and Astini (1996) by theaddition of arrows to show ridge-crest jumps required by the model(Early Cambrian, RJ1, see Fig. 6a)and by the geology of the Pre-cordillera (Late Ordovician, RJ2,see text). Brick ornament—Cam-brian and Lower Ordovician plat-forms; diagonal hachures—riftedmargin prisms; crosses—oceaniccrust.



gate margins to those of southern Laurentia. The problem may involvemore than two continents.

Thomas (1991), in his description and analysis of the origin of the late Pre-cambrian to early Paleozoic Appalachian-Ouachita rifted margin, empha-sized the influence of northeast-trending rifts and northwest-trending trans-form faults. The timing of rift-drift transition off the Appalachian Blue Ridge,as along the rest of the ≈4500-km-long orogen (e.g.,Williams and Hiscott,1987; Bond et al., 1984), is clearly marked by latest Precambrian cessation ofsynrift magmatism and the onlap of Lower Cambrian sandstone at the baseof a retrogradational passive margin succession that oversteps uppermostPrecambrian rift-fill sequences (see also Thomas and Astini, 1996). Inboardof this margin, however, it is Upper Cambrian strata that overstep uppermostPrecambrian and Cambrian igneous and sedimentary rocks in the Missis-sippi Valley–Rough Creek–Rome graben system, the Birmingham basementfault system, and the southern Oklahoma fault system (Fig. 6b).

It should be noted that a recent review of the paleomagnetic data for thelatest Proterozoic and early Paleozoic concludes that a 5000-km-wide oceanexisted between Laurentia and Gondwana prior to the Precambrian-Cam-brian boundary (Torsvik et al., 1996a). This is in direct contradiction to theinterpretations of the geologic evidence cited above. These place the rift-drift transition along the proto-Appalachian margin from Newfoundland toGeorgia close to the Precambrian-Cambrian boundary (545–540 Ma,Table 1; Williams and Hiscott, 1987; Thomas and Astini, 1996). The sametime is inferred from geologic evidence for the separation of Laurentia andSiberia (Pelechaty, 1996), although Baltica may have separated earlier.Other interpretations of the paleomagnetic data do permit considerablygreater proximity, if not actual juxtaposition of Laurentia and Gondwana atthe time (Grunow et al., 1996; Powell et al., 1995). Here, however, relianceis placed on the geologic interpretation, the age of rift-related igneous rocksand Lower Cambrian overstep. Initiation of a major spreading system at the


Figure 9. Satellite altimetry-derived gravity map (free-air) of the South Atlantic Ocean basin (Sandwell and Smith, 1992) showing initial (I) andpresent-day (P) offsets of the spreading ridge at the Falkland-Malvinas-Agulhas fracture zone. Abandoned spreading center (C) and relict topo-graphic expressions of ridge-crest jumps (W, X, Y, Z) were discussed by Barker (1979); all are located south of the fracture zone. F/MP—Falk-land-Malvinas Plateau; M—Maurice Ewing Bank (massif); NV—Natal Valley.

I. W. D. DALZIEL


Proterozoic-Cambrian boundary would explain the start of the global ma-rine transgression of the Paleozoic supercycle.

Thomas (1989, 1991) and Thomas and Astini (1996) interpreted the dif-ference in age of rift-related rocks between the Appalachian margin and theOuachita embayment as the expression of an Early Cambrian ridge crest“jump” of ≈800 km within the Iapetus ocean basin at the beginning of theCambrian Period from the Blue Ridge rift along the Appalachian margin tothe Ouachita rift in the cratonic embayment southwest of the Alabama-Oklahoma transform fault. This led to their interpretation that a branch ofthe mid-Iapetus ridge, named the Ouachita mid-ocean ridge, generated≈1500 km of oceanic lithosphere before the end of the Cambrian Period,separating a microcontinent of ≈800 km2 covered by Laurentian shelf fa-cies from the main continent (Thomas, 1991; Figs. 6 and 8, a and b). It isthis microcontinent that Dalla Salda et al. (1992a, 1992b), Dalziel et al.(1994), Astini et al. (1995), the 1995 San Juan Penrose Conference Partic-ipants (Dalziel et al., 1996), and Thomas and Astini (1996) believed to bethe Precordillera of the present-day Andes. In the “calling card” model ofDalla Salda et al. (1992b) and Dalziel et al. (1994), the Precordillera wasdetached from Laurentia in Caradocian time following continent-continent

collision. In the microcontinental “funeral ship” model of Astini et al.(1995) and of Thomas and Astini (1996), it departed Laurentia in the LateCambrian and drifted across the Iapetus ocean basin as the discrete micro-continent dubbed “Bill Thomas land” or “Guillermo Thomasia” at the Pen-rose Conference.

With the exception of a few questions with regard to distances of sepa-ration at certain times, both from Laurentia and Gondwana, the adaptationof the Thomas (1991) model for the origin of the Ouachita embayment toexplain the origin of the Precordillera along the lines of Astini et al. (1995)and Thomas and Astini (1996) at first appears straightforward, as in the re-constructions of Mac Niocaill et al. (1997, in press). The continuity of theshallow-marine environment to allow the Laurentian olenellid benthictrilobite fauna to survive into Early Ordovician time (Benedetto et al.,1995; Fig. 7), for example, may have required a lesser separation of the mi-crocontinent from Laurentia than that suggested in the models of Thomas(1991), Astini et al. (1995), and Thomas and Astini (1996) (Figs. 6a and8c). However, a fracture zone ridge, common along Mesozoic-Cenozoictransform faults, could have been present. Some degree of oceanic isola-tion from the parent continent would have allowed the cosmopolitan ag-

Figure 10. (a) Mesozoic extensional basins and Magallanes (Andean) foredeep of the southern cone of South America, including the Falkland-Malvinas Plateau (modified from Urien et al., 1995). BB—Burdwood Bank (North Scotia Ridge); CPB—Central plateau basin; DA—Dungenessarch; DM—Deseado massif; F/MI—Falkland-Malvinas Islands platform (Lafonian microplate); MEM—Maurice Ewing massif; NPM—NorthPatagonian massif; RB—Rawson basin; SJgB—San Jorge basin; SJnB—San Julian basin; SMB—South Malvinas basin; SSI—South ShetlandIslands trench. (b) Cross section of the South Malvinas basin (after Yrigoyen, 1989). For location see Figure 10a.

a



nostid pelagic fauna to invade the Precordilleran microcontinent before theend of the Cambrian. A source of siliciclastic sediment, either an island ora pathway from the craton proper, had to have been maintained into LateCambrian time. Gondwana faunas became established as the microconti-nent converged on the proto-Andean margin in Arenigian time. Accomo-dation has to be made to explain the warmer water “Pacific” graptolitefauna in the Llandeilian strata of the Precordillera (Stanley Finney, 1995,personal commun.), but perhaps this could be explained by circulation pat-terns within the Iapetus ocean.

Comparison with the record of opening of the present-day oceans, how-ever, reveals major difficulties with this Thomas and Astini et al. micro-continental “funeral ship” scenario (Fig. 8). A critical aspect is the timingof sea-floor spreading between Laurentia and the Precordillera (Dalziel etal., 1996). The Early Cambrian “inboard” ridge crest jump, originally pro-posed by Thomas (1989, 1991) and advocated in this Precordilleran model,from the Blue Ridge margin to the Ouachita embayment, would havelengthened the separation of mid-Iapetus spreading centers across theAlabama-Oklahoma transform during the Cambrian by ≈800 km (ridgejump 1 in Fig. 8a). A jump of this type is not represented in the Mesozoic-Cenozoic along any of the rifted passive margins formed during the frag-mentation of Pangea and birth of the Atlantic, Indian, and Southern oceanbasins. On the contrary, ridge crest jumps at transform faults in theseoceans shorten, rather than lengthen the separation of actively spreadingridges. A striking example can be demonstrated in the South AtlanticOcean basin using shipborne magnetic and satellite altimeter-derived grav-ity data. Three major ridge crest jumps have occurred since the oceanicfloor of the basin was initiated ca. 130 Ma, shortening the separation of thespreading centers north and south of the Falkland-Agulhas fracture zonefrom its original ≈1400 km to its present 200 km (Barker, 1979; Fig. 9).

There are several modern examples of cratonic fragments that are nowcomparatively widely separated geographically from their parent continentby stretching of continental lithosphere and rift-basin subsidence. Thesemicrocontinents have not, however, been set free from that parent by actualsea-floor spreading, but rather form horsts within plateaus extending fromtheir parent continents. The most striking example is the submerged Mau-rice Ewing Bank (or massif) at the eastern end of the Falkland-Malvinas

Plateau (Figs. 9 and 10a). Others are the British Isles, Sri Lanka, and Tas-mania (Sandwell and Smith, 1992). The closest example to the “funeralship” type of scenario along a passive margin, is the separation of Mada-gascar from Africa. Even in this case, however, the initial spreading tookplace in the Somali basin on the African side of the microcontinent. Sub-sequently the spreading center jumped outboard to the southwest Indianridge (Lawver et al., 1991). I therefore conclude that the “funeral ship”model for the origin of the Precordillera put forward by Astini et al. (1995)and Thomas and Astini (1996) is not actualistic, and that another modelneeds to be sought for the development of the Ouachita embayment andseparation of the Precordillera from that region of Laurentia. In searchingfor an alternative explanation, it becomes apparent that the microcontinen-tal model may not be a unique explanation for the stratigraphic relation-ships and geologic evolution of the Ouachita embayment.

Striking similarities exist between the early Paleozoic rifting and subsi-dence within the Ouachita embayment as described by Thomas (1989, 1991)and the early Mesozoic rifted South American margin of the South AtlanticOcean basin. The Falkland-Malvinas Plateau extends ≈1500 km east of theSouth American continental shelf edge into the South Atlantic Ocean basin(Figs. 9 and 10a). It tapers eastward from 600 km to 300 km wide, and isbounded on the north by the Falkland-Agulhas fracture zone and on the southby the Falkland Trough, a continuation of the Andean foredeep separating theplateau from the North Scotia Ridge. The Falkland-Malvinas Plateau con-sists of three geologic provinces (Fig. 10a): from east to west, these are theMaurice Ewing Bank, a Cenozoic carbonate bank built upon a horst of Pre-cambrian crystalline basement, i.e., the Maurice Ewing massif of Urien et al.(1995), that was exposed during the Late Jurassic initiation of rifting (Barkeret al., 1977); the late Mesozoic central Falkland-Malvinas Plateau rift basin,which is floored by highly extended continental crust, and possibly to a lim-ited extent by oceanic crust of a failed spreading center (Lorenzo and Mutter,1988); and the Falkland-Malvinas Islands platform, or Lafonian microplate,on which the Falkland Islands with their Grenvillian basement and Paleozoicto lower Mesozoic sedimentary cover are located (Greenway, 1972; Cin-golani and Varela, 1976). West of the platform, there is another late Mesozoicrift basin, the Malvinas, or South Malvinas, basin (Turic et al., 1980;Yrigoyen, 1989; Urien et al., 1995; Fig. 10b).

MAGALLANES DUNGENESS ARCH SOUTH MALVINASBASIN BASIN


b

I. W. D. DALZIEL


Discussion of the complex evolution of the Falkland-Malvinas Plateauduring the Mesozoic and Cenozoic fragmentation of western Gondwana isoutside the scope of this paper. For a recent analysis see Marshall (1994).Critical points here are as follows. (1) The entire 1500 km × ≈350 km sub-merged plateau is unquestionably part of the South American continent,the possibility of a failed spreading center in the central basin notwith-standing. (2) The Falkland-Malvinas Plateau is partly floored by Precam-brian crystalline basement comparable to that in southern Africa, where itoriginated in the Natal Valley (Fig. 9), and in adjacent Patagonia. (3) Thecentral plateau basin formed as an extensional feature in Early to MiddleJurassic time, prior to the Early Cretaceous development of the earliestocean lithosphere in the South Atlantic Ocean basin. (4) Geologic and pa-leomagnetic data demonstrate that extensional separation of the Falkland-Malvinas Plateau from southern Africa involved >100° of rotation of theFalkland-Malvinas platform (or Lafonian microplate) relative to the SouthAmerican craton. (5) Early motion along the Falkland-Agulhas fracturezone continued westward into South America along the Gastré shear zone.(6) The present length of the Falkland-Malvinas Plateau is its length afterseveral hundred kilometres of extension, i.e., at the time of onset of driftbetween South America and Africa. (7)Although detailed reconstruction issubject to interpretation, the plateau can be accurately restored to the posi-tion in the Natal Valley that it occupied immediately prior to the Early Cre-taceous opening of the South Atlantic Ocean basin.

These facts about the Falkland-Malvinas Plateau lead me to concludethat the Precordillera could have been part of a similar plateau attached toLaurentia, outboard of the Ouachita embayment, during the Cambrian andEarly Ordovician. In particular, the fact that the South American Falkland-Malvinas Plateau includes rift basins several kilometres deep oversteppedby sedimentary strata younger than the initial floor of the surroundingSouth Atlantic Ocean basin (Turic et al., 1980; Yrigoyen, 1989) leads meto further question Thomas’s model of a ridge crest jump within the open-ing Iapetus ocean basin, and hence his conclusion that there was a≈1500-km-wide Ouachita ocean by Late Cambrian time (Thomas, 1991;Thomas and Astini, 1996). As noted by Dalla Salda et al. (1992b) andDalziel et al. (1994), another interpretation, based on sedimentary facies,provenance, and paleocurrent analyses, suggests that the Ouachita troughwas a narrow basin during the early and middle Paleozoic, with an out-board sediment source similar to the rifted proto-Appalachian margin(Rosenfeld, 1981, in Denison, 1995; Lowe, 1985, 1989; Arbenz, 1989).This interpretation is now supported by geochemical data that indicate re-stricted circulation in the basin during early Paleozoic time (Reid et al.,1996), as well as by additional sedimentologic data (Dix et al., 1996). It isalso in keeping with the long-standing evidence from the Marathon marginof trans-Pecos Texas that formed the basis for an offshore Llanoria land-mass in the early Paleozoic, as discussed by Keller and Dickerson (1996).

The presence of slope facies on the present Pacific side of the Pre-cordillera is interpreted by Astini et al. (1995) and Thomas and Astini(1996) to indicate the presence of an ocean basin by Late Cambrian time,but could equally well indicate merely a rift basin as seen on the Falkland-Malvinas Plateau today (Figs. 9 and 10). A continental plateau model iscompatible with the evidence of faunal and environmental change noted inthe Precordillera during Cambrian and Early to Middle Ordovician time, in-cluding the encroachment of the cosmopolitan agnostoid trilobites in theCambrian, and the persistence of “Pacific” pelagic graptolites into the Llan-deilian. Deep open ocean water flowed over the Falkland-Malvinas Plateauduring late Mesozoic and Cenozoic time after separation from Africa, de-positing an abyssal, cold water benthic foraminiferal fauna on continentalbasement (Barker et al., 1977). A continental plateau that was part of Lau-rentia until after an Ordovician (Ocloyic) collision with South America isalso compatible with the fact that the oldest known rocks of unquestionedoceanic affinities on the Pacific margin of the Precordillera are mafic pillow

lavas of Llandeilian-Caradocian rather than Cambrian age (Ramos et al.,1986; Dalziel et al., 1996). The Astini et al. (1995) and Thomas and Astini(1996) microcontinental interpretation therefore requires a second enig-matic ridge crest jump to explain the presence of these rocks. In their model,the ridge crest would have had to shift more than 1000 km to the proto-An-dean margin from its mid-Iapetus position (see ridge jump 2, Fig. 8e).

I believe that the microcontinental model mistakenly assumes that thepresence around the Ouachita embayment and in the Precordillera of riftand slope facies, open ocean sediments, and pelagic faunas demands theformation of oceanic lithosphere between the Precordillera and Laurentiaduring the Cambrian and Early Ordovician. On the contrary, exactly thesame sedimentary environments and faunal types can exist within an ex-tending continental plateau like the Falkland-Malvinas Plateau, without theonset of full sea-floor spreading. Extension of continental crust permitshundreds of kilometers of separation between two points on the surface ofthe Earth, but only sea-floor spreading can result in separations of thou-sands of kilometers. Hence, the Laurentian affinities of the Precordillera,its olenellid benthic trilobite fauna, and Grenvillian basement, plus the“switch over” to endemic and Gondwana faunas prior to docking in SouthAmerica, could be explained by its representing a feature like the MauriceEwing massif as part of an early Paleozoic Laurentian plateau outboard ofthe Ouachita embayment within the Iapetus ocean basin, analogous to theFalkland-Malvinas Plateau of South America within the present-day SouthAtlantic Ocean basin. Is this merely an ad hoc hypothesis? Let us considerwhere this hypothetical feature could have originated within the Neoprot-erozoic–early Paleozoic Earth.

NEOPROTEROZOIC GEOGRAPHY

Following publication of the SWEAT hypothesis (Moores, 1991), PaulHoffman and I both suggested that Laurentia could have “broken out” ofthe Neoproterozoic supercontinent, which formed at the time of globalGrenvillian orogenesis and is most commonly referred to as Rodinia(McMenamin and McMenamin, 1990), from between East and WestGondwana (Dalziel, 1991; Hoffman, 1991; Fig. 11). We suggested thatopening of the Pacific Ocean basin was compensated, on a globe of con-stant radius, by the closing of Pan African and Brazilide ocean basinswithin the amalgamating cratons of Gondwana during Neoproterozoic to,at latest, Cambrian time. Subsequently, I made the specific suggestion thatthe Greenland-Scotland-Labrador promontory of Laurentia may have beenrifted from the ancestral Arica embayment of South America followingopening of the Pacific Ocean basin, the amalgamation of Gondwana, andhence the ephemeral existence of a Laurentia-Gondwana supercontinent inlatest Neoproterozoic time (Dalziel, 1992b, 1994; Fig. 12). I suggestedspecific tests of this hypothesis, and am actively pursuing some of them.

Of interest here is one particular aspect of the quantitative reconstructionof Rodinia, and of the hypothetical latest Precambrian supercontinent thatmay have existed briefly following the opening of the Pacific Ocean basin(Dalziel, 1992b, 1994). The name Pannotia, the “all-southern” superconti-nent, has been suggested for this latter entity (Powell, 1995; Powell et al.,1995). It mainly resulted from the amalgamation of East Gondwana withWest Gondwana and Laurentia along the East African orogen to close theMozambique ocean (Fig. 12). Amalgamation of West Gondwana itself in-volved closure of several Pan African–Brazilide ocean basins (Trompette,1994; Unrug, 1996). Also involved was “Turkic-type” (Íengör and Na-tal’in, 1996) accretion of arcs and oceanic plateaus, together with the clo-sure of small ocean basins (Stern, 1994; Stein and Goldstein, 1996). Bear-ing in mind that both Laurentia and Gondwana are portrayed withgeometric accuracy in the Rodinia and Pannotia reconstructions (see cap-tions for Figs. 11 and 12), it is notable that with the relative positions ofLaurentia and Gondwana controlled by the suggested Scotland-Arica con-



nection, the Ouachita embayment of Laurentia is located opposite a majorembayment in the Precambrian craton of Gondwana between South Amer-ica and East Antarctica, and immediately adjacent to the Falkland-Mal-vinas Plateau discussed above.

The embayment off the present-day Cape of Good Hope is geologicallyunique along the ≈12 000 km of the Pacific margin of Gondwana, and hasbeen recognized for several years (Dalziel, 1982). This part of southern-most Africa and Antarctica is the site of a Cambrian to Permian sedimen-tary succession that was the central part of the “Samfrau geosyncline” ofDuToit (1937), and was deformed in the early Mesozoic to form the centralpart of the Pacific margin Gondwanide fold belt.

The margin of Gondwana immediately opposed to the Ouachita embay-ment margin of Laurentia in my hypothetical reconstructions of Rodiniaand Pannotia is the Ellsworth-Whitmore mountains crustal block ofAntarctica (Figs. 11 and 12), restored to what I believe to be its original po-sition between Africa and Antarctica using geologic and paleomagneticdata (Schopf, 1969; Watts and Bramall, 1981; Dalziel and Elliott, 1982;Dalziel and Grunow, 1992). Both the Falkland-Malvinas Plateau and theEllsworth-Whitmore block have some Precambrian basement of Grenvil-lian age, (Cingolani and Varela, 1976; Millar and Pankhurst, 1987). EarlyCambrian bimodal magmatism, associated with rapid synrift sedimenta-tion in the Ellsworth-Whitmore block (Fig. 13a) is generally interpreted asbeing related to continental extension (Vennum et al., 1992). A daciticstock has been dated using the U-Pb zircon method at 535 ± 3 Ma (Rees et

al., 1995). Initiation of subsidence of the adjacent southern African marginthat eventually accommodated the Cape Supergroup also dates from latestPrecambrian or earliest Phanerozoic deposition of the Klipheuwel Forma-tion that underlies the Ordovician Table Mountain Sandstone (Winter,1984; Hiller, 1992; Fig. 13b). The Cape Supergroup did not undergo com-pressive deformation until the early Mesozoic Gondwanide orogeny (Du-Toit, 1937; Hälbich, 1992). The dominant deformation of the succession inthe Ellsworth Mountains was also Gondwanide (Webers et al., 1992a), al-though some evidence of early Paleozoic deformation and a pre-Early De-vonian unconformity has been revealed by recent mapping (M. N. Rees,1996, personal commun.). In contrast, by Cambrian time the Gondwanamargin was affected by subduction-related magmatism and orogenic de-formation from the immediately adjacent Pensacola Mountains of Antarc-tica to Australia, and along the proto-Andean margin of South America(Stump, 1995; Dalla Salda et al., 1990).

TABLE 1. PALEOMAGNETIC POLES IN RECONSTRUCTIONS

Age (Ma) Latitude Longitude α95 References(N) (E)

Early to Mid-NeoproterozoicA. 1110 Ma (late Mesoproterozoic)Coats Land 22.9 80.3 6.8 Gose et al., 1997, in pressLaurentia 54.7 –142.2 * Caldwell A pole of Constanzo-

Alvarezet al. 1993, in Gose et al., 1997, in press

B. ca. 725 Ma (mid-Neoproterozoic)Laurentia –5.9 –17.6 * Fahrig et al. pole, in Powell et al.,

1993India –78.4 –140 Powell et al., 1993

545 Ma (End Neoproterozoic)Laurentia 13.0 –15.0 15.0 Meert & Van der Voo, 1997,

in pressAustralia –45 –14 13.0 mean calculated from Li & Powell,

1993

515 Ma (Mid-Cambrian)Laurentia 4.0 –3.0 12.0 Meert et al., 1994bAfrica 25.0 –4.0 25 Grunow, 1995Baltica 38.3 4.9 * Torsvik et al., 1996

475 Ma (Early-Ordovician) (all in van der Pluijm et al., 1995)Laurentia –10.8 –3.5 * Mac Niocaill & Smethurst, 1994†

Africa 34.0 7.0 * Van der Voo, 1988Baltica 30.0 54.0 * Torsvik et al., 1992Siberia 42.0 –50.0 * Torsvik et al., 1994

465 Ma (Mid-Ordovician) (all in van der Pluijm et al., 1995)Laurentia -12.6 –9.2 * Mac Niocaill & Smethurst, 1994§

Africa 34.0 7.0 * Van der Voo, 1988Baltica 19.0 49.0 * Torsvik et al., 1992Siberia 27.0 –46.0 * Torsvik et al., 1994East Avalonia 17.0 18.00 * Torsvik & Trench, 1991

430-420 Ma (Early Silurian)Laurentia 18.0 127.0 * Van der Voo, 1993 (Table 5.1)Africa 43.0 –171.0 * Van der Voo 1993, (Table 5.2A)Baltica –15 –15 * Torsvik et al., 1996a (Table 1)Siberia –4.0 121.0 19.0 Van der Voo, 1993 (Table 5.4)

*No α95 supplied; arbitrary 10° shown†Northern Scotland pole of 15°S, 17°E rotated to Laurentian coordinates§Northern Scotland pole of 16°S, 11°E rotated to Laurentian coordinates

TABLE 2. EULER (FINITE) POLES OF ROTATION FOR THE MAJOR PLATES

Age Latitude Longitude Angle References

Laurentia to absolute framework725 22.0 –129.0 –92.53 This study, using Powell et al. 1993545 16.5 –127.0 –109.47 This study, using Meert et al. 1994515 26.3 –119.0 –109.40 Meert et al. (1994b)475 29.4 –121.3 –94.09 This study, using Mac Niocaill and

Smethurst 1994*465 27.8 –124.2 –89.94 This study, using Mac Niocaill and

Smethurst 1994*422 12.4 –152.2 –74.00 This study, using Van der Voo 1993

Baltica to absolute framework725 38.2 –107.2 –108.65 based on Dalziel 1994545 29.4 –105.5 –120.50 based on Dalziel 1994515 17.6 –41.2 –138.56 Torsvik et al. (1992)475 13.4 –60.4 –125.82 This study465 15.9 –64.4 –115.38 This study422 31.4 –133.0 –90.18 This study

Siberia to absolute framework725 33.5 –137.8 –115.85 This study545 29.2 –131.9 –131.10 This study515 4.5 45.2 126.93 Van der Voo (1993)475 5.9 53.5 133.39 This study465 –3.0 –131.1 –116.74 This study422 14.6 47.4 98.32 This study

East Antarctica to Laurentia725 12.8 119.9 134.85 based on Dalziel, 1991

East Antarctica to South Africa725 81.6 –124.5 110.16 This study545–422 7.1 144.8 –56.35 based on Lawver et al. 1992

East Gondwana fit poles Australia to East Antarctica725–422 –2.0 38.9 –31.50 Royer and Sandwell 1989India to East Antarctica725–422 –4.4 16.7 –92.77 Lawver and Scotese 1987Northern South America to Laurentia725 2.3 –23.6 –99.34 Dalziel et al. 1994545 2.3 –23.6 –99.34 Dalziel et al. 1994515 5.0 155.7 74.73 This study475 4.3 143.5 77.64 This study465 2.5 –38.8 –80.24 This study422 40.1 159.5 52.87 This study

West Gondwana fit polesSouthern Africa to northern South America725–422 50.0 –32.5 –55.08 Nürnberg and Müller 1991Northwest Africa to northern South America725–422 50.8 –34.2 –53.69 Nürnberg and Müller 1991Coats Land–Maudheim-Grunehogna block to South Africa725–422 7.1 144.8 –56.35 based on Lawver et al. 1992Ellsworth Mountains to South Africa725–422 50.0 –80.0 83.28 based on Dalziel and Grunow 1992

*Northern Scotland pole transferred to Laurentia using Lawver et al.’s (1990)Eurasia–North America fit pole (69.1 156.7 –23.64).

I. W. D. DALZIEL


I suggest that when proto-Appalachian Laurentia rifted from Gondwanaclose to the Precambrian-Cambrian boundary (Bond et al., 1984; Williamsand Hiscott, 1987), it had a continental plateau attached that filled the gapbetween the Cape and Ouachita embayments. This missing “Texas plateau,”to distinguish it from the smaller Texas promontory on the margin of theOuachita embayment (Thomas, 1991, Fig. 6), would have had almost ex-actly the length of the adjacent Falkland-Malvinas Plateau, 1500 km, butcould have been as much as two to three times wider, 800–1000 km, an ap-proximate maximum figure for the present along-strike extent of both thecombined Ouachita-Marathon margin (Fig. 6a) and for the Precordillera ofnorthwest Argentina, the latter measured as the full extent of the largerCuyania terrane (Ramos et al., 1986, 1995; Thomas, 1991; Thomas and As-tini, 1996). It is important to recall, however, that the present length of theFalkland-Malvinas Plateau is its length immediately prior to sea-floorspreading. Hence the dimensions of both the Falkland-Malvinas Plateauand the hypothetical Texas plateau are reconstructed here after significantrifting and extension. The opening of the Laurentia-Gondwana portion ofthe Iapetus ocean basin would, in this scenario, have taken place as an exactanalog to the Mesozoic opening of the South Atlantic Ocean basin betweenSouth America and Africa (Figs. 14 and 15).

The Texas plateau would have been bounded by transform faults nearlyparallel, and directly analogous, to the Falkland-Agulhas and North ScotiaRidge transforms of today. It seems reasonable to assume that the early Pa-leozoic faults would have had a similar development to those of Mesozoic

and Cenozoic age. The mid-Iapetus ridge would have stepped outboard ofthe Texas plateau just as the mid-Atlantic ridge stepped outboard of the Falk-land-Malvinas Plateau (Fig. 9). A ridge crest jump (or jumps) would havetaken place to shorten the lengthening northerly transform, as it did in thecase of the mid-Atlantic ridge around the Falkland-Malvinas Plateau(Barker, 1979). The Alabama-Oklahoma transform of Thomas (1976, 1989,1991) would have been continuous with the original fault bounding theoceanic (now Pacific) side of the Falkland-Malvinas Plateau margin ofGondwana, and closely analogous to the Mesozoic Agulhas-Falkland-Malv-inas transform fault and its continuation into South America, the Gastré faultzone (Rapela and Pankhurst, 1992; Figs. 9 and 10a). I refer to it as the Malv-inas-Alabama-Oklahoma transform (Figs. 12, 14, and 15a). The otherbounding fault of the Texas plateau marks the present-day southern marginof the Laurentian craton. It was perhaps the precursor of the hypotheticalMojave-Sonora megashear (Silver and Anderson, 1974; Anderson andSchmidt, 1983), if that does indeed mark the present southern limit of cra-tonic Laurentia. Like the Shackleton fracture zone between the present-dayScotia and Antarctic plates (Barker et al., 1991, Fig. 10a), this would havebeen a fault separating areas of plate growth and plate convergence. I refer toit as the Ellsworth-Sonora-Mojave transform (Figs. 12, 14, and 15a).

Paleomagnetic data permit the juxtaposition of Laurentia and WestGondwana controlled by the correlation of the Greenland-Scotland-Labrador promontory and the Arica embayment, as in reconstructions ofboth Rodinia and Pannotia (Figs. 11 and 12). The work of German and

Figure 11. Geologically constrained reconstruction of the hypothetical supercontinent Rodinia formed by amalgamation of older cratons in theglobal Grenvillian orogeny (ca. 1000 Ma; green belts), shown in a paleomagnetically constrained latitude and orientation ca. 725 Ma, prior to sep-aration of the Cordilleran margin of Laurentia from East Antarctic-Australia and the opening of the Pacific Ocean basin (see Fig. 1). The Mozam-bique ocean basin between East and West Gondwana, and other Panafrican-Brazilide (or southern “Pannotian”; Stump, 1987) ocean basins be-tween the cratons of West Gondwana are of unknown relative width. The outlines of present-day South America and Africa are shown only forreference. All reconstructions use digitized continental outlines, and are shown in orthographic projection. See Table 1 for paleomagnetic poles—crosses with 95% confidence circles; note that arbitrary confidence circles of 10° are drawn around those for which no errors are supplied; l1 andc—Laurentia and Coats Land (part of CMG, see below) ca. 1110 Ma; l2—Laurentia ca. 725 Ma, coordinates are based on this Laurentian pole; i—India ca. 720 Ma. See Table 2 for rotation parameters. Laurentia–East Gondwana (SWEAT) reconstruction modified from Moores (1991) andDalziel (1991, 1992a, 1992b) by exclusion of younger terranes in eastern Australia and relocation of Weddell sea margin (CMG) block as discussedin text; Laurentia-Amazonia–Rio de la Plata reconstruction as in Dalziel (1992b, 1994); Laurentia-Kalahari as in Gose et al. (1997); Kalahari-Congo based on data suggesting that the Zambesi belt was intracratonic (Wilson et al., 1993; Munyanyiwa et al., 1996), but note that widths of in-tra-Gondwana oceans are unknown, and this may be incorrect (see Meert et al., 1994a; Meert and Van der Voo, 1996a, 1996b; Trompette, 1994; Un-rug, 1996a); Siberia is positioned with respect to Laurentia approximately as suggested by Hoffman (1991) and refined by Pelechaty (1996);Laurentia-Baltica as in Torsvik et al. (1996a), but note that they showed a 180° rotation between 650 and 580 Ma. The hemisphere not shown wasvirtually entirely oceanic, the Mirovia ocean of McMenamin and McMenamin (1990). A—Arequipa massif; AM—Amazonian craton ; AV—Aval-onia; B—Baltica (Russian craton); C—Congo craton; CMG—Coats Land–Maudheim-Grunehogna province of East Antarctica (Gose et al.,1997); E—Ellsworth-Whitmore mountains block (in Pangea position); F—Florida (in pre-Pangea position within Gondwana and including theCarolina terrane, but see text); F/MP—Falkland-Malvinas Plateau ; K—Kalahari craton; MBL—Marie Byrd Land; NG—New Guinea; R—Rockall Plateau with adjacent northwest Scotland and northwest Ireland; RP—Rio de la Plata craton; S—Siberia (Angara craton); SF—São Fran-cisco craton; SV—Svalbard block (Barentia); WA—West African craton; TxP—hypothetical Texas plateau (see text). Omitted are the South andNorth China blocks and the Tarim block; for discussion of their possible positions within Rodinia, see Li et al. (1995) and Pelechaty (1996). Shortred lines in East Gondwana and Laurentia are dikes suggested by Park et al. (1995) to be part of the one “giant radiating dike swarm” ca. 780 Ma.

Figure 12. Geologically constrained reconstruction of the Pannotia supercontinent that may have existed ephemerally near the Precambrian-Cambrian boundary (Dalziel, 1992b; Powell et al., 1995; see Fig. 1). See Table 1 for paleomagnetic poles; l—Laurentia; g—Gondwana. PanAfrican-Brazilide (Pannotian; Stump, 1987) basins are shown closed, although some of them may have persisted into the Cambrian (see Grunowet al., 1996). Note that Torsvik et al. (1996a) interpreted the paleomagnetic data to require separation of Laurentia and Gondwana by >5000 kmat the Precambrian-Cambrian boundary; the interpretation shown here is based primarily on geologic arguments as the paleomagnetic polesshown do not support such close juxtaposition of Laurentia and Gondwana(see text). See Table 2 for rotation parameters. Barbs—upper platesof subduction zones (Cadomian) and incipient subduction zones (Delamerian-Ross arc, D-R-A); thick lines—incipient mid-Iapetus ridges; diag-onal lines—East African collisional orogen involving East and West Gondwana. For projection, coordinates, confidence circles, and abbrevia-tions, see Figure 11. Additions: EA—east Avalonia (southern British Isles); ESMT—hypothetical Ellsworth-Sonora-Mojave transform (see text);MAOT—hypothetical Malvinas-Alabama-Oklahoma transform (see text); WA—west Avalonia (Nova Scotia, Avalon platform, etc.).


Figure 11.

Figure 12.

I. W. D. DALZIEL


South African geologists in Dronning Maud Land indicates that this part ofthe present-day East Antarctica, immediately south of Africa (CoatsLand–Maudheim–Grunehogna block, CMG in Fig. 11), was sutured to theKalahari craton during Grenvillian (Kibaran) orogenesis ca.1000 (Groen-wald et al., 1995). This has led me to juxtapose the Kalahari and EastAntarctic cratons in my earlier attempts to reconstruct early Neoprotero-zoic Rodinia (e.g., Dalziel, 1992a, 1992b, 1995). The Kalahari craton isalso shown attached to East Antarctica in a new map of Gondwana assem-bly (Unrug, 1996). New paleomagnetic and geochronologic data, however,indicate that this interpretation is flawed. A well-dated pole for 1110 Ma

(concordant U-Pb zircon and titanite dates) obtained by Gose et al. (1997)from another part of Weddell Sea margin (Coats Land), together with ear-lier results from Dronning Maud Land and the Kalahari craton, demon-strates that the Coats Land–Maudheim–Grunehogna block was part of theKalahari craton and of West Gondwana, rather than part of East Gond-wana. It could, moreover, have been located off the proto-Appalachianmargin of Laurentia (Fig. 11; note comparison of the new Coats Land polewith a Laurentian pole of the same age). The suture between East and WestGondwana, the East African orogen, must therefore extend to the Pacificmargin of the East Antarctic craton at the Shackleton Range (Fig. 12),

Figure 13. (a) Stratigraphy of thePaleozoic Heritage Group of theEllsworth Mountains, Antarctica(Webers et al., 1992a; but note thatthere may be an unconformity be-low the Crashsite Group accordingto M. N. Reese, 1996, personal com-mun.). Note the Lower(?) to MiddleCambrian synrift Union GlacierFormation at the base of the section.(b) Stratigraphy of southern Africa(Hiller, 1992). Note the uppermostPrecambrian to Cambrian syn-riftKlipheuwel Formation at the base ofthe section.

a



where there is evidence of Pan African orogenesis (Marsh, 1983), as sug-gested by Grunow et al. (1996) and by Tessensohn (1995). Although un-certainty in the width and age of closure of the intra-Gondwana Pan Afri-can and Brazilide basins needs to be emphasized (see captions, Figs. 11and 12; Unrug, 1996), the collisional suture formed during latest Precam-brian to Early Cambrian closure of the Mozambique ocean between Eastand West Gondwana is ≈7500 km long.

Recognition that the Grenvillian age rocks of Coats Land and DronningMaud Land were part of West Gondwana rather than East Gondwana as farback as 1100 Ma negates one of the main lines of evidence of the SWEAThypothesis for the early Neoproterozoic juxtaposition of the Pacific mar-gins of Laurentia and East Antarctica–Australia (Moores, 1991; Dalziel,1991). Such a fit is still broadly supported, however, by the length of themargins (Dalziel, 1991), limited paleomagnetic data (Powell et al., 1993),and the global “budget” of probable late Precambrian rifted margins cou-pled with the fact that a Grenvillian age orogen is absent from both of thesemargins, but present along the proto-Appalachian and proto-Andean mar-gins (Fig. 11). Future tests include the dating and paleomagnetic poles ofthe Australian portion of the proposed ca. 780 Ma giant radiating dikeswarm of Park et al. (1995, Fig. 11).

To summarize, my suggestion based on geologic and faunal data that thePrecordillera might have originated as a bathymetric high on a Falkland-Malvinas Plateau-like feature outboard of the Ouachita embayment of Lau-rentia appears to be independently supported by paleogeographic analysis.The presence of the hypothetical Texas plateau adjoining the Falkland-Malvinas Plateau within Rodinia and Pannotia during the Neoproterozoic isnot only geometrically in keeping with quantitative reconstructions of thosesupercontinents with the Greenland-Scotland-Labrador promontory match-ing the Arica embayment, but explains the geometry and geology of thelong-enigmatic Cape of Good Hope embayment and Gondwanide fold belt,which are unique along the 12 000 km Pacific margin of Gondwana.

Comparison of the structure, rift history, and subsidence history of theearly Paleozoic Ouachita embayment and of the Mesozoic Falkland-Mal-vinas plateau is illuminating (Fig. 15b). The rifting and subsidence in the

Ouachita embayment analyzed by Thomas (1991) may reflect the forma-tion of a basin, the Ouachita rift, directly analogous to the South Malvinasbasin (Turic et al., 1980; Urien et al. 1995; Figs. 6 and 10), rather than abasin floored by true oceanic lithosphere. The South Malvinas basin wasformed early in continental separation and overstepped by sediment in thelater stages of rifting, just as in the case of the Birmingham graben(Thomas, 1991). The San Julian basin of Argentina is in a position adjoin-ing the Agulhas-Falkland-Gastré transform fault that makes it a direct ana-log of the South Oklahoma fault system (aulacogen) with respect to theAlabama-Oklahoma transform of early Paleozoic time. The Rawson basinsnorth of the Agulhas-Falkland-Gastré transform are analogous to the aban-doned Mississippi graben and Birmingham fault systems.

As previously mentioned, there is a significant body of sedimentologicevidence to indicate that the Ouachita rift, like the Marathon rift (Kellerand Dickerson, 1996), was a narrow two-sided basin throughout the earlyand middle Paleozoic, with a block to the southeast supplying sedimentfrom a source like the proto-Appalachian passive margin (Lowe, 1985,1989; Denison, 1995; Dix et al., 1996). In the light of the above discussion,I suggest that the Precordillera, with its Cambrian-Lower Ordovicianolenellid trilobite fauna, its southeastern Laurentian stratigraphic affinities,and its Grenvillian basement, was not derived from within the Ouachitaembayment. Rather, it originated as a major horst block closely analogousto the Falkland-Malvinas platform (Lafonian microplate) or the MauriceEwing massif within an extended continental plateau ouboard of the Texasmargin of the Ouachita embayment, the Texas plateau (Fig. 15b). If cor-rect, this means that it was still attached to Laurentia at the time it dockedalong the proto-Andean margin of Gondwana (Fig. 16; discussed below),thereby defining the relative longitudes and positions of Laurentia andGondwana in the Middle Ordovician and representing a vital key to the his-tory of the Earth during the Paleozoic Era.

PALEOZOIC TECTONICS

Following rifting from Gondwana near the Precambrian-Cambrian bound-ary, rapid motion of Laurentia across lines of latitude to an equatorial position


b

I. W. D. DALZIEL


during the Cambrian is recognized to require a relatively fast sea-floor-spread-ing rate. Even without longitudinal motion, it approached the rate of motion ofIndia during the Mesozoic Era (Meert et al., 1993). It is interesting to note thatLaurentia did not develop a “bow wave” continental margin orogen like theMesozoic and Cenozoic North American and South American cordilleras.Those formed along the leading edge of the continents as they moved forwardrelative to the mantle over eastward-dipping subduction zones, causing com-pression along the leading edge of the upper plate (Dalziel, 1986; Coney andEvenchick, 1994; Russo and Silver, 1996). The absence of such an orogenalong the Pacific margin of early Paleozoic Laurentia suggests that the conti-nent lay at that time within a larger plate, and that convergence during theopening of Iapetus between Laurentia and South America ocurred elsewherewithin or beyond the Pacific Ocean basin. Rapid Mesozoic motion of the In-dian plate appears to be due to its being driven by both ridge push to the southand slab-pull to the north. The same situation may have affected CambrianLaurentia, and may have induced rifting and subsidence along its Pacific mar-gin, leading to the “double rift event” paradox and the initiation of the lowerPaleozoic miogeocline (Stewart, 1972; Bond et al., 1984; Link et al., 1993).The most obvious location for the subduction zone is along its East Gond-wana margin, the Ross-Delamerian arc (Fig. 12).

Separation of Laurentia and Gondwana had to be sufficiently great by lateEarly Cambrian time (ca. 520 Ma) for the Laurentian benthic fauna to de-velop in isolation. However, a previously unexplained mingling of NorthAmerican and Australo-Sinian biofacies is represented by the Aphelaspiszone trilobites and by mollusca in the Ellsworth Mountains of Antarctica(Webers and Yochelson, 1989; Shergold and Webers, 1992; Webers et al.,1992b). Hence the proximity suggested here for the southern cone of Lau-rentia and the Cape embayment of Gondwana during the Cambrian (Figs. 12and 14) has some paleobiologic support. If, as suggested by the molecular bi-ology studies of Wray et al. (1996), metazoan divergences between phyladate back well into the Precambrian, isolation of the Laurentian trilobitefauna might be required even earlier than late Early Cambrian time.

A major difference between the Paleozoic Iapetus ocean and the Meso-zoic South Atlantic Ocean basin lies in the rapidity with which the SouthAmerican margin of Iapetus became an active consuming margin. Thishappened relatively early in Cambrian time, ca. 530 Ma; subduction-re-lated magmatism apparently continued until the appearance of what are in-terpreted as collisional granitoids in Middle Ordovician time (e.g., DallaSalda et al., 1990; Pankhurst et al., 1996; Sato et al., 1996; Toselli et al.,1996). A possible explanation of this phenomenon may lie in the nature ofthe present-day Shackleton fracture zone (Fig. 10a). At its northern end, theShackleton fracture zone has started to subduct the Antarctic plate beneaththe Scotia plate as South America moves west with respect to the mantle,Africa, and Antarctica. Gondwana moved over the South Pole during latestProterozoic to Cambrian time (Figs. 12 and 14), and the hypothetical an-cestor of the continuation of the Mojave-Sonora megashear along the mar-gin of the Falkland-Malvinas Plateau, the Ellsworth-Mojave-Sonora trans-form (Fig. 12), may have changed to a subduction zone in like fashion,propagating northward (in present coordinates) along the proto-Andean Ia-petus margin during Cambrian time. As Laurentia moved to the equatorduring the Cambrian, the Gondwana supercontinent moved across theSouth Pole. In Vendian time western Amazonia (present coordinates) waslocated over the pole; during the Cambrian it was the West African craton(Figs. 12 and 14), and by the Early Ordovician the pole lay near the thenorthern coast of Africa (Fig. 15). Thus if, following Dalla Salda (1992a,1992b) and Dalziel et al. (1994) and as suggested here, the proto-Appala-chian and proto-Andean margins were at the same longitude, the interven-ing Iapetus oceanic lithosphere would have contracted between the twoduring the Cambrian and Early to Middle Ordovician epochs by subduc-tion beneath the Pampean magmatic arc.

According to the scenario suggested here, during the early Paleozoic

subduction of Iapetus oceanic lithosphere beneath Gondwana, Laurentiamust have been moving in a net dextral sense, so that the Texas plateau thatwas detached from the Cape embayment eventually collided with theproto-Andean margin of South America in the position of the present-dayPrecordillera in the early Middle Ordovician, ca. 465 Ma (Figs. 14–16). Pa-leomagnetic data permit a maximum separation of Laurentia and the SouthAmerican part of the Gondwana craton margins of ≈2000 km in MiddleOrdovician time if they were at the same longitude (Fig. 16). This isslightly greater than the extended length allowable for the Texas plateau ifit originated between the Texas margin of the Ouachita embayment and theEllsworth-Whitmore Mountains block of the Antarctic margin (Fig. 12).Although precise dating remains to be done, studies of major shear zonesin Precambrian basement of Argentina to the east of the Precordillera indi-cate top to the west thrusting with no significant component of strike-slipmotion (Simpson et al., 1995). This suggests that docking of the Pre-cordillera may have involved dominantly down-dip subduction.

Two main objections have been raised to the Laurentia-Gondwana col-lision proposed by Dalla Salda et al. (1992a, 1992b) and amplified byDalziel et al. (1994): (1) it contradicts the faunal and paleoenvironmentaldifferences between the tropical to equatorial Laurentian craton and the bo-real Gondwana craton, and (2) it barely permits room for the island-arcsystems known to have collided with Laurentia in the New England–Cana-dian Maritimes region, and probably farther south, during Ordovician time(see Astini et al., 1995; Torsvik et al. 1995; van der Pluijm et al., 1995;Mac Niocaill et al., 1996, 1997; Dalziel and Dalla Salda, 1996a, 1996b;Astini et al., 1996a; Torsvik et al., 1996b ). The Texas plateau hypothesis,I submit, overcomes these problems and at the same time addresses “. . . amajor problem in Appalachian geology . . . to quantify a realistic causativemechanism for the Taconic deformation in the central and southern Ap-palachians” (Drake et al., 1989, p. 162).

Like the microcontinental model, the Texas plateau hypothesis is com-patible with the following paleontologic facts: (1) a Laurentian benthic trilo-bite fauna in the Precordillera through Cambrian, Tremadocian, and earlyArenigian time: shallow marginal ridges are characteristic of transform con-tinental margins like the Falkland-Malvinas Plateau and West Africa(Lorenzo and Mutter, 1988; see Figs. 9 and 10) and would permit a Lau-rentian fauna to exist in suitable habitats along the length of the plateau;(2) Middle to Late Cambrian cosmopolitan pelagic agnostid trilobites in thePrecordillera: as previously mentioned, the analogous Falkland-MalvinasPlateau was inundated with deep water from the Southern Ocean during theLate Cretaceous during subsidence after its separation from Africa (Barkeret al., 1977); (3) incoming tropical Gondwanan fauna to the Precordillera bythe Arenigian Epoch: the Texas plateau approached the subducting Pam-pean margin at that time (Fig. 15, a and b), and modern arthropod larvae canmigrate for up to ≈1000 km (see discussion in Dalziel et al., 1996; Thomasand Astini, 1996); (4) pelagic “Pacific” graptolites in the Precordillera aslate as Llandeilian time: even after collision of the tip of the Texas plateauwith the Pampean subduction zone, its length would have been open to low-latitude circum-Laurentian ocean waters (Figs. 15 and 16); (5) benthicGondwana lichid, pterygometopid, and calyminid trilobites first spread toLaurentia in the Middle Ordovician (Droser et al., 1996): that is the time ofsuggested convergence of the two continental masses; (6) cool water Hir-nantian faunas and evidence of glaciation in the Precordillera by Carado-cian-Ashgillian time: the Precordillera was isolated within high-latitudeGondwana after collision of the Texas plateau in Middle Ordovician time,and rifting in the Caradocian (Figs. 16, 17, and following discussion).

Recent criticism notwithstanding, the Texas plateau hypothesis for col-lision of an asperity of Laurenia with the proto-Andean margin of Gond-wana appears to accommodate the known South American and Laurentianarc terranes within the Ordovician Iapetus ocean basin, in accordance withthe faunal and paleomagnetic data of Van der Voo (1993), van der Pluijm



et al. (1995), Harper et al. (1996), Mac Niocaill et al. (1996, 1997), Torsviket al. (1996a), and Williams et al. (1996). This is demonstrated in Fig-ures 15–17. The change in facies and fauna of the Precordilleran strata tocooler water peri-Gondwana conditions (Astini et al., 1995, 1996b;Benedetto et al., 1995; Thomas and Astini, 1996) prior to Middle Ordovi-cian docking appears to pose a problem with both microcontinental andTexas plateau models, and incompatability of the paleomagnetic and geo-logic evidence. The situation is not, however, vastly different from thepresent-day relative proximity of the southern limit of the Great BarrierReef and the cold-temperate waters around Tasmania, especially given thefact that the first Gondwana faunas in the Precordilleran platform werewarm water forms.

The Taconic orogeny is generally regarded as one involving diachronoussubduction, ophiolite obduction, and arc collision along the almost 4000 kmlength of the Appalachian mountains without continent to continent colli-sion (Hatcher, 1989). The nearly contemporaneous Ocloyic phase of theFamatinian orogen along the proto-Andean margin of South America alsoinvolves subduction, obduction of ophiolites, and arc collision. In this re-finement of the proposal by Dalla Salda et al. (1992a, 1992b), that the twoorogens were once continuous and resulted from Laurentia-Gondwana col-lision, I suggest that actual craton to craton contact may have been limitedto the docking of the extremity of the Texas plateau, the Precordillera,within Gondwana, and possibly the docking of the Carolina terrane, whichhas strong Gondwanan affinities (Secor et al., 1983), within Laurentia. Pa-leomagnetic data place the latter at ≈23° south latitude during the Middle toLate Ordovician (Vick et al., 1987), which means that it could have dockedas part of the Gondwana margin in the vicinity of the Famatinian-Puna arcor arcs (Conti et al., 1996) during the Middle Ordovician collision of theTexas plateau (Fig. 16). A major Taconic foredeep is developed to the westof the Carolina terrane (Shanmugam and Lash, 1982).

I regard the proposed Ordovician interaction between the Laurentianand Gondwana plates as analogous to the present-day convergence be-tween the Indo-Australian and Eurasian plates north of Australia. The In-donesian orogen, with which the Taconic orogen is frequently comparedby others (e.g., Shanmugam and Lash, 1982) is a complex orogenic collageinvolving subduction and arc magmatism, ophiolite obduction, and the col-lision of the New Guinea asperity of the Australian continent (Fig. 11) withsoutheastern Asia (Hamilton, 1979). If the Precordillera was still attachedto Laurentia as part of the Texas plateau at the time of Middle Ordoviciandocking with Gondwana, the combined Taconic-Famatinian (Ocloyic) oro-gen would have been every bit as complex, a major orogen (Fig. 16) withthe southern Appalachians continuing into the Famatinian belt (the pierc-ing points of Dalla Salda et al., 1992b), although perhaps offset by a majortransfer zone along the Malvinas-Alabama-Oklahoma fault that I suggestbounded the Texas plateau. I refer to the ephemeral Laurentia-Gondwanasupercontinent of the Middle Ordovician (Fig. 1) that would have resultedfrom the docking of the Texas plateau as “Artejia,” thereby reflecting thegeologic connection that appears to suggest its former existence, as well asthe hispanic background of both Argentina and Texas.

What of the Late Ordovician separation of the Precordillera from Lau-rentia, and what became of the part of the hypothetical ≈1500-km-longTexas plateau that is not represented in the Precordillera? The only hard ev-idence of rifting involving the formation of oceanic lithosphere between thePrecordillera and Laurentia is, as mentioned previously, the presence alongits Pacific margin of mafic pillow lavas with mid-ocean ridge basalt chem-istry interlayered stratigraphically with graptolite-bearing Llandeilian-Caradocian sedimentary strata (Ramos et al., 1986). By general comparisonwith the present-day physiography of the Falkland-Malvinas Plateau, and inkeeping with the horst and graben interpretation of Keller (1995) and Kellerand Dickerson (1996) for the Precordillera during the Cambrian and EarlyOrdovician, I therefore suggest that the Texas plateau may have been di-

vided into at least three parts (Figs. 15b and 16): (1) an easternmost (rela-tive to present-day Texas) passive margin that rifted from the Cape embay-ment and is now subducted beneath cratonic South America; (2) a horst withcarbonate capping that became the Precordillera after limited subduction ofits passive margin; and (3) the Ouachita-Marathon rift basin.

The passive margin and the Precordilleran horst would have been short-ened in the Ocloyic collision and further shortened during the post-Ocloyicpre-Andean deformation along the proto-Andean margin, as well as in theMesozoic formation of the Andean Cordillera (Astini et al., 1996b; Ramoset al., 1996). The eastern shoulder of the Ouachita basin may have under-gone some Ocloyic shortening prior to Landeilian-Caradocian rifting thatseparated the Precordillera. Such shortening could explain the reportedoutboard early to middle Paleozoic sediment sources in the Ouachita-Marathon embayment (Lowe, 1985, 1989; Denison, 1995; Keller andDickerson, 1996), including the influx of sand represented by the MiddleOrdovician “Mountain Fork” clastic wedge of the Ouachita allochthon,Oklahoma (Dix et al., 1996). The Laurentian remnants of the Texas plateauwere shortened during the Ouachita-Alleghanian collision of Laurentia andnorthwestern Africa, and then extended during Pangea breakup when theysubsided again. It is posssible that some remnants of the Texas plateauwere also incorporated into other pre-Andean terranes such as Chilenia(Ramos et al., 1986).

Following the separation of the Precordillera, Laurentia continued itsclockwise relative motion around the Pacific margin of Gondwana, as dis-cussed by Dalziel et al. (1994), colliding with the various exotic terranes ofthe Canadian Maritimes and southern British Isles (Fig. 17). Collision withBaltica (the Russian craton) at the end of the Silurian formed Laurussia,which amalgamated with Gondwana and Siberia (the Angara craton) toform the Pangean supercontinent that can be reconstructed using the mag-netic anomaly and structural fabrics of the present-day ocean basins. Theapparent rapid relative motion of Laurentia and Gondwana between theLate Ordovician and Silurian time (Figs. 16 and 17) is somewhat puzzling,and may reflect poor resolution in the Gondwana paleomagnetic record forthe interval, as discussed elsewhere (McKerrow and Scotese, 1990; Van derVoo, 1993; Dalziel et al., 1994).

An important line of evidence bearing on Paleozoic geography lies in thedistribution K-bentonites. These altered volcanic-ash deposits are commonin Ordovician strata of eastern and midwestern Laurentia, East Avalonia(England and Wales), Baltica, and the Precordillera. It has even been sug-gested that particularly well developed Caradocian bentonite layers of Lau-rentia and Baltica that correlate very closely in time might have resultedfrom the same “ultraPlinian” explosive eruption. This is difficult to envis-age in the geographic scenario presented here, as pointed out in a review byHuff et al. (1995), unless there is an undiscovered equivalent layer in north-ern South America. A search for K-bentonites in the Precordillera has re-vealed several layers in the lower Llanvirnian part of the section and one inthe Caradocian (Huff et al., 1995; Bergstrom et al., 1996). They are thusolder than almost all of those in Laurentia. The possible discovery of K-ben-tonites in Ordovician strata of the Marathon region of southwestern Texas,first in the entire state, was reported by Patricia Dickerson at the GeologicalSociety of America South-Central Section Meeting in Austin, Texas (seeKeller and Dickerson, 1996), and has considerable potential significance asthere is no obvious Laurentian source. Preliminary study of clay minerals,however, has cast some doubt on the presence of K-bentonites in west Texas(W. D. Huff, 1996, personal commun.). The full paleogeographic meaningof these important volcanic layers is unlikely to emerge without a substan-tial amount of further study on several continents, as emphasized by Huff etal. (1995), and by Samson (1996), in an article pointing out geochemicalreasons for a possible South American source for Laurentian K-bentonites.A search for K-bentonites needs to be undertaken in the cratonic areas ofnorthern South America to fill in this emerging picture.

I. W. D. DALZIEL


SPECULATION: ENVIRONMENTAL AND BIOLOGICALIMPLICATIONS

“Natural selection, the cornerstone of the Neodarwinian paradigm, con-cerns the interaction between heritable variation and the environment.” Af-ter making this opening statement in his discussion of the environmentalcontext of evolutionary change, Knoll (1991, p. 77) emphasized that theperiod of major biological innovation at the end of the Proterozoic Eon,was also a time of “profound tectonic, climatic, and biogeochemicalchange.” Intense multidisciplinary studies, undertaken all around the globeover the past decade particularly as a result of the activities of InternationalGeologic Correlation Project No. 320 on “Neoproterozoic Events and Re-sources,” have generated a remarkable faunal, climatic, and geochemicalrecord. The marked geochemical changes, in particular (Asmerom et al.,1991; Brasier, 1992b; Kaufman et al., 1993; Montañez et al., 1996;Nicholas, 1996), but also major climatic change such as the growth of ex-tensive ice sheets (Young, 1995), are often ascribed to tectonic causes suchas rifting, ocean basin formation, and orogenesis. This link to tectonics,however, has remained vague. As pointed out by Brasier (1996), there ap-pears to be an important underlying dichotomy of opinion: do the litho-logic changes of the times reflect biologic change, a sort of “Gaian” con-trol, or are all the changes fundamentally driven by tectonics, by theplanet’s internal thermal energy?

Full discussion of possible tectonic causes of the environmental changesthat accompanied the emergence of soft-bodied metazoans during the Neo-proterozoic following nearly 3.0 b.y. of single-celled life, the possible di-vergence of chordates from invertebrates early in Neoproterozoic time, theCambrian “explosion” of skeletalized animals, and the Middle Ordovicianradiation of marine biota, is far beyond the scope of this review. The pale-ogeographic reconstructions presented here for Neoproterozoic and earlyPaleozoic time do, however, permit evaluation of the possible magnitudeof the major tectonic events that occurred during that critical time interval.This may eventually lead to a better understanding of the factors responsi-ble for the environmental changes, and hence perhaps for the unique bio-logic revolution. It is only by taking a global view of the paleogeographicchanges that we can start to consider the long-term evolution of the wholeEarth system. Is it a coincidence, for example, that the global Grenvillianorogeny that resulted in the amalgamation of Rodinia at the end of theMesoproterozoic, however that supercontinent was configured, marks amajor change in Earth’s biosphere; i.e., an increase in the abundance anddiversity of eukaryotic organisms (Knoll, 1994; Knoll and Sergeev, 1995),and the conjectured time of divergence of major metazoan phyla (Wray etal., 1996)?

A relationship between environmental changes and Gondwana-wideNeoproterozoic to earliest Paleozoic Pan African and Brazilide (Pannotian)orogenesis has been suggested by many. This appears even more likely

Figure 17. Latest Ordovician–Early Silurian paleogeography constrained by paelomagnetic, paleoenvironmental, and faunal data. Note in-cipient Taconic and Caledonian collisions of arc terranes, west and east Avalonia, and Baltica with Laurentia. See Table 1 for paleomagnetic poles;l—Laurentia; g—Gondwana; b—Baltica; s—Siberia; paleolatitude for Avalonia after van der Pluijm et al. (1995). See Table 2 for rotation para-meters. For projection, coordinates, confidence circles, and abbreviations, see Figures 11, 12, 14, 15, and 16. Additions: APC—Argentine Pre-cordillera; OE—Ouachita embayment.

Figure 14. Middle Cambrian paleogeography constrained by geologic arguments, and by paleomagnetic, faunal, and paleoenvironmental data(see text; and for Baltica and Siberia, see Torsvik et al., 1996a). Note that the reconstruction is for a time after the initiation of subduction alongthe proto-Andean margin and some dextral motion of Laurentia with respect to Gondwana. See Table 1 for paleomagnetic poles; l—Laurentia;g—Gondwana. See Table 2 for rotation parameters. For projection, coordinates, confidence circles, and abbreviations, see Figures 11 and 12. Ad-ditions: ET—Antarctic termination of hypothetical Ellsworth-Sonora-Mojave transform (see Fig. 12); PA—Pampean arc; RA—Ross segment ofDelamerian-Ross arc (see Fig. 12); crosses—hypothetical marginal fracture ridge.

Figure 15. (a) Early Ordovician (Arenig) paleogeography constrained by geologic arguments, and by paleomagnetic, faunal, and paleoenvi-ronmental data (see text). See Table 1 for paleomagnetic poles; l—Laurentia; g—Gondwana; b—Baltica; s—Siberia. See Table 2 for rotation pa-rameters. For projection, coordinates, confidence circles, and abbreviations, see Figures 11, 12, and 14. Intraoceanic arcs: Exa—Exploits arc,Pava—Peri-Avalonian arc, Pla—peri-Laurentian arc, all positioned as in Mac Niocaill et al., 1997; Fpa + Famatinian/Puna arc positioned as inConti et al. (1996). (b) Comparison of Early Ordovician Iapetus ocean basin (as interpreted here; Fig. 15a) and the mid-Cretaceous South At-lantic Ocean basin (from sea-floor spreading data); both are shown with South America in present-day coordinates. Faunal influxes to the pro-posed Texas plateau (CF—cosmopolitan pelagic fauna; GF—Gondwanan fauna), and the Falkland-Malvinas Platea (CF—pelagic fauna) areshown with arrows. Compare the structure of the early Paleozoic Ouachita embayment (Thomas, 1991; Fig. 6a) and hypothetical Texas plateauwith that of the Mesozoic southern cone of South America and the Falkland-Malvinas Plateau (Urien et al., 1995; Fig. 10a). Basins and passivemargins of the Falkland-Malvinas Plateau (FM/P) and Texas plateau (TxP) are shaded. Laurentia: ?APC—hypothetical location of future Ar-gentine Precordilleran cover (see Fig. 16); B—Birmingham rift; M—Mississippi rift; MR—Marathon rift; OR—Ouachita rift; S—Southern Ok-lahoma fault system (aulacogen). South America: FI—Falkland-Malvinas Islands platform; M—Maurice Ewing Bank (massif); R—Rawsonbasins; J—San Julian basin; S—South Malvinas basin.

Figure 16. Geologically constrained reconstruction showing initial stage of hypothetical Middle Ordovician collision of Texas plateau with theproto-Andean margin and extent of the Taconic-Famatinian (Ocloyic) orogen. See Table 1 for paleomagnetic poles; a—Avalonia; l—Laurentia; b—Baltica; g—Gondwana; s—Siberia; paleolatitude of Avalonia from van der Pluijm et al. (1995). Cross-hatched area of the Texas plateau representsthe horst of Figure 15b that hypothetically became the Precordilleran terrane of northwest Argentina. See Table 2 for rotation parameters. For pro-jection, coordinates, confidence circles, and abbreviations, see Figures 11, 12, 14 and 15. Additions:APC—location of present Argentine Precordillera,dashed yellow line (digitized as the larger Cuyania terrane of Ramos, 1995; see text); SATs—Southern Appalachian terranes (e.g., Carolina terrane),see text; x—southern Appalachian foredeep (Sevier basin). Intraoceanic arcs positioned as in Conti et al. (1996) for Fpa, and Mac Niocaill et al. (1997)for Exa and Pava.



a

b

with the recognition that the collisional orogen between East and WestGondwana extended for ≈7500 km from Arabia to the Pacific margin ofEast Antarctica (Fig. 12). This was a collisional belt far exceeding the≈3000 km length of the Himalayas, nearly as long as the entire Tethysidesfrom the Mediterranean Sea to the Pacific Ocean. Given the influence theuplift and erosion of the Himalayas is believed to have had on Cenozoicseawater composition and global climate (Raymo et al., 1988; Richter etal., 1992), it it not unreasonable to speculate that the extended Mozam-bique belt or East African orogen may have played a similar role inchanges at the end of Neoproterozoic time, including the marked increasein the 87Sr/86Sr ratio in seawater (Nicholas, 1996) that extended into theLate Cambrian (Montañez et al., 1996) and the burial of organic carbonthat is believed to have triggered a critical rise in the availability of atmos-

Figure 14. Figure 15.

Figure 16.

Figure 17.

I. W. D. DALZIEL


pheric oxygen (Knoll and Walter, 1992). The timing of discrete eventsalong the East African orogen, as well as the other Pannotian belts, needsto be refined before their environmental effects can be more thoroughlyevaluated.

There have been two major cycles of eustatic rise and fall in sea level,the first-order or supercycles, during the Phanerozoic Eon (Fischer, 1984;Hallam, 1992). The more recent cycle started with the fragmentation ofPangea in the Jurassic and extends to the present. The cause of the eustaticrise during the Cretaceous Period is most likely to have been the rapid vol-ume increase of oceanic spreading ridges and volcanic plateaus (Kominz,1984; Larson, 1991). The fall corresponds in time to the late CenozoicAlpine-Himalayan continent-continent collision and the growth of theAntarctic ice sheet, the former resulting in an increase in the volume of theocean basins, and the latter, possibly causatively linked, extracting waterfrom the oceans. The Paleozoic cycle extends from the time of separationof Laurentia from the amalgamating Gondwana, i.e., the breakup of the hy-pothetical Pannotia (Fig. 12), to the amalgamation of Pangea. The conti-nent-continent collisions along the Caledonian-Scandian and Ouachita-Al-leghanian-Hercynian-Uralian sutures may be causatively linked to thedevelopment and growth of the late Paleozoic Gondwana ice sheet (Pow-ell and Veevers, 1987; Raymo, 1991).

By analogy with the Cretaceous, the global rise in sea level that startedduring the Early Cambrian is most likely to have been the result of thegrowth of new, buoyant oceanic spreading centers. The reconstruction ofPannotia suggested here yields a hypothetical length of mid-Iapetusspreading ridges of nearly 10 000 km (Laurentia–South America, Lauren-tia-Baltica, and Laurentia-Siberia). The new lithospheric surface was bal-anced by the loss of surface area along the Ross-Delamerian and Pampeansubduction zones, but the volume increase at a new trench is small com-pared with the increase along an equal length of new spreading ridge(Southam and Hay, 1977). Hence, the major Cambrian transgression ofocean water across the continents was probably driven by the tectonicprocesses responsible for the fragmentation of Pannotia. A marked de-crease in the generally rising 87Sr/86Sr ratio of global seawater at the Ne-makit-Daldynian–Tommotian boundary (530–535 Ma, Nicholas, 1996;Fig. 1) could reflect hydrothermal activity along the newly formed mid-Ia-petus spreading ridge.

If supercontinental amalgamation and fragmentation, the latter being ac-companied by decrease in ocean volume and transgression, have occurredseveral times in the history of the planet, why should an explosive burst ofmulticellular hard-skeletal life have accompanied latest Neoproterozoicgrowth of new ocean basins? Why did this not occur 250 m.y. earlier dur-ing the breakup of Rodinia and the opening of the vast Pacific Oceanbasin? The explanation must surely lie in the interaction between the tec-tonic development of the solid earth, the chemical development of the at-mosphere, and the stage of biologic evolution. At the time of the fragmen-tation of Rodinia, eukaryotes were at an early stage of development (Knolland Sergeev, 1995) and the atmosphere appears to have contained too littleoxygen to support multicellular life (Knoll and Walter, 1992). WhenPangea broke up, most of the metazoan ecological niches had been filled,despite the biologic crisis at the end of the Paleozoic Era. However, the Ia-petus ocean basin formed within the Pannotian supercontinental assembly≈200 m.y. later than the breakup of Rodinia, after the initial developmentof soft-bodied metazoans, notably the Ediacaran fauna (Grotzinger et al.,1995), and possibly after the divergence of metazoan phyla (Wray et al.,1996). The oxygen content of the atmosphere had passed a critical thresh-old (Knoll and Walter, 1992), and the advancing Cambrian seas appear tohave opened up a myriad of unoccupied habitats, permitting the unique“explosion” of skeletalized metzaoans of phyla already established.

During the early to middle Paleozoic transgressive phase of the Panno-tia-Pangea eustatic sea-level cycle, there were several important interrup-

tions in the rise of sea level. Some of these could perhaps be explained bythe hypothetical Laurentia-Gondwana interaction discussed earlier. Thesuggestion of a relationship between orogenesis and regression goes backto the views of Stille in the 1920s (for a historical review, see Hallam,1992). Many workers have studied the possibility of a causative relation-ship between the Taconic orogeny, epeirogenic movements in the Laurent-ian interior, and sea-level change during the Ordovician (e.g., Quinlan andBeaumont, 1984; Howell and van der Pluijm, 1990; Coakley and Gurnis,1995). The two main problems are timing and the mechanism. Taconicorogenesis is diachronous, and arc-continent collision may not generate alarge enough drop in sea level to explain observations around the world.

The possibility that the Taconic orogen in North America was part of amuch longer mountain belt that involved at least local continent-continentcollision suggests to me that these problems should be reevaluated. Mycolleagues and I have pointed out the coincidence in time of the sequenceboundary in Gondwana and Laurentia (Dalziel et al., 1994). We drew at-tention to the fact that continent-continent collision transmits compressivestress far into cratonic interiors (Craddock and van der Pluijm, 1989). Per-haps the sequence boundaries record the time of direct continent-continentinteraction, while the complexity of events in the orogens themselves blurthe time signal. For example, the limited expression of the Sauk-Tippeca-noe unconformity near the Ouachita embayment, and its absence in thePrecordillera, may reflect local flexure related to collision of the Texasplateau with Gondwana. Convergence of two major continental masses in-volving not only arc and terrane collision, but also collision of asperitieslike the hypothetical Texas plateau, will have a greater likelihood of in-creasing the volume of the ocean basins and hence of lowering sea level,than will arc-continent collision alone. By comparison with the Cretaceoushistory of the Andes, the deformation in the cratonic interior of NorthAmerica and volume of siliciclastic sediments spreading westward fromthe Appalachians in Middle to Late Ordovician time seems difficult to rec-oncile with a mere arc-continent collision.

The possibility of a causal link between Middle Ordovician orogenesisand the diversification of marine biota through the development of newhabitats has been raised by Miller and Mao (1995). If the orogenic eventwas on a larger scale than previously envisioned and involved Laurentia-Gondwana collision to at least a limited extent, any such effect would havebeen more pronounced. Moreover, an orogenic event of this magnitudecould also have a major effect on world climate. The Late Ordovicianglacial event has long been somewhat enigmatic, coming as it did during a“greenhouse” stage associated with the early to middle Paleozoic eustaticsea-level rise (Crowley and Baum, 1991). Nonetheless it has been linked tothe Taconic orogeny in North America through “drawdown” of carbon diox-ide (Raymo, 1991; Kump et al., 1996). If the Taconic event in North Amer-ica was merely part of a more extensive orogen like that between Australiaand Asia today, this would perhaps make the causative connection far morelikely and strengthen the case for a global connection between orogenic epi-sodes and glaciation through geologic time envisaged by Chamberlin(1899). As pointed out by Bickle (1996), however, carbon dioxide is bothreleased and absorbed in orogenesis, so this is a complex problem.

CONCLUSIONS

Geologic considerations have led to testable hypotheses for the config-uration of pre-Pangea supercontinents in the early to mid-Neoproterozoic(Rodinia), and the latest Neoproterozoic (Pannotia). These were separatedby the opening of the Pacific Ocean basin and the amalgamation, or nearlycomplete amalgamation, of Gondwana. The critical breakthrough in thedevelopment of the hypotheses was recognition that the ancestral NorthAmerican craton, Laurentia, may formerly have been juxtaposed withsouthern continents of the present day.



Traditional geographic reconstructions that juxtapose the proto-Appala-chian margin of Laurentia with northwestern Africa in the latest Protero-zoic, and keep the two margins at the same longitude through the early Pa-leozoic, may be incorrect. Recognition that the Cambrian-Ordoviciancarbonate platform composing the Precordillera of northwest Argentinawas part of Laurentia, and that it could have been detached from the gen-eral area of the Ouachita embayment, implies some close interaction be-tween the proto-Appalachian and proto-Andean margins throughout thatbiologically critical time interval. This would have been the global settingwithin which the well-established subduction, ophiolite obduction, andarc-continent collision of the Taconic orogenesis took place.

The hypothesis that the Precordillera originated as a microcontinent de-tached from within the Ouachita embayment, may also be incorrect.Rather, when Laurentia separated from the proto-Andean margin of Pan-notia to open Iapetus in the latest Precambrian to Early Cambrian, it mayhave done so in a manner analogous to the Mesozoic opening of the SouthAtlantic Ocean basin. An ≈1500 km × 800–1000 km continental plateaumay have extended into the Iapetus ocean basin from the present site of theOuachita embayment. The “Texas plateau” could have originated along-side the Falkland-Malvinas Plateau of the present day, its tip rifted from theEllsworth-Whitmore crustal block of Antarctica in its restored position ad-jacent to southernmost Africa. This was a unique location along the Pacificmargin of Gondwana, the site of a Cambrian-Permian passive margin insouthernmost Africa and the early Mesozoic Gondwanide fold belt. Thefaults bounding the Texas plateau can be viewed as analogs to those bound-ing today’s Falkland-Malvinas Plateau. Basins within the Mesozoic Falk-land-Malvinas Plateau also appear to be analogous in their tectonic settingto those of the early Paleozoic Ouachita embayment.

Subduction started along the proto-Andean and Antarctic-Australianmargins early in Cambrian time as Gondwana moved over the south pole.Rapid motion of Laurentia to equatorial latitudes may have been driven bya combination of ridge push from the mid-Iapetus ridge and trench pullfrom a subduction zone within or bounding the Pacific ocean basin. Theonly well-established convergence zone of the time was the Ross-Del-amerian arc in Antarctica and Australia. By Arenigian time, the Texasplateau had moved to within ≈1000 km of the proto-Andean margin, andthe invasion of Gondwana faunas had started. Ocloyic collision of theplateau with the proto-Andean margin and subduction beneath the Gond-wana craton took place in Middle Ordovician time. This collision of an as-perity of Laurentia with Gondwana was part of a highly complex Indone-sian-style convergence between two major plates that initiated theAppalachian-Andean mountain system and involved subduction arc-con-tinent collision and ophiolite obduction. Caradocian rifting separated thetwo cratons, leaving a horst of the Texas plateau within Gondwana to beuplifted as a result of subsequent Andean orogenesis, forming the present-day Precordillera.

Following separation, the Appalachian margin of Laurentia could havereached its widely accepted position for latest Ordovician and Siluriantime, opposed to the northwest South American and African margins, asthe volcanic arc and other terranes of the Canadian Maritimes, west Aval-onia, and east Avalonia (southern Britain) collided with the northern Appa-lachian and Scottish margins in the Taconic and Caledonian orogenicevents of those regions. Baltica then collided with Greenland in the Scan-dian orogeny, forming Laurussia, prior to the final Ouachita-Alleghanian-Hercynian-Uralian collision of that entity with Africa and Asia to completethe amalgamation of Pangea.

The environmental and biologic changes that marked the Proterozoic-Cambrian transition were contemporaneous with uplift and erosion of thePannotian orogens, including the collisional East African belt that extendedfor ≈7500 km from Arabia to the Pacific margin of Antarctica. The nearly10 000-km-long oceanic spreading ridge system that originated between

Laurentia, the proto-Andean margin, Baltica, and Siberia at the Precam-brian-Cambrian boundary, initiated the first order cycle of eustatic sea-level change that lasted through the Paleozoic Era. The tectonically drivenCambrian transgression, coming at a time following the rise of the oxygencontent of the atmosphere and the evolution of soft-bodied metazoans, mayhave been critical in the “explosion” of hard-shelled animals.

Eustatic sea-level drops and epeirogenic movements during Ordoviciantime may have resulted from a complex Taconic-Ocloyic collision involv-ing the Laurentian and Gondwana cratons; a significantly larger tectonicevent than the arc-continent collision that is well-documented in Lauren-tia. This collision may also have played a role in the Middle Ordovician ra-diation of marine fauna, as well as in triggering the Late Ordovician glacia-tion of Gondwana.

ACKNOWLEDGMENTS

The work was supported by National Science Foundation GrantsEAR-94 18236 and OPP-91 17996 and the Institute for Geophysics andGeology Foundation of the University of Texas at Austin. I thank Robert J.Hatcher, Jr. and A. R. (Pete) Palmer for constructive reviews that signifi-cantly improved the manuscript. I also benefitted from informal reviews,comments, and discussion with many colleagues. In particular I thankRichard Buffler, Carlos Cingolani, John Cooper, Luis Dalla Salda, Roger(Tim) Denison, Patricia Dickerson, Robert H. Dott, Jr., Stanley Finney,Anne Grunow, Mark Helper, Warren Huff, Frederick Hutson, Dennis Ko-lata, Larry Lawver, Z. X. “Lee” Li, Sharon Mosher, William Muehlberger,Samuel Mukasa, Robert Neuman, Chris Powell, Victor Ramos, MargaretRees, William Thomas, Cees van Staal, and Trond Torsvik. Anne Grunow,Conan Mac Niocaill, and their colleagues kindly furnished copies of man-uscripts in press. Responsibility for the interpretations in the article, how-ever, is mine alone. The reconstructions could not have been done withoutthe outstanding and generous help and advice of Lisa Gahagan of thePLATES project at the Institute for Geophysics, University of Texas atAustin. Drafting was carried out by Gwen Watson.

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GSA Bulletin: Neoproterozoic-Paleozoic geography …...the Appalachian or ogen in the Canadian Mar itimes, and of the Caledonian orogen in the Br itish Isles, contin ues to be emphasiz