OVERVIEW - geography and tectonics: Review, hypothesis, environmental speculation

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

ABSTRACT the basin; uplift and erosion of orogens within the newly assembled portion of , including a collisional The ever-changing distribution of and ocean basins on mountain range extending ≈7500 km from Arabia to the Pacific mar- is fundamental to the environment of the planet. Recent ideas re- gin of ; the development of a Pannotia-splitting oceanic garding pre-Pangea geography and tectonics offer fresh opportunities spreading ridge system nearly 10 000 km long as broke away to examine possible causative relations between tectonics and environ- from Gondwana, , and ; and initiation of subduction mental and biologic changes during the Neoproterozoic and Paleozoic zones along thousands of kilometres of the South American and eras. The starting point is an appreciation that Laurentia, the - Antarctic-Australian continental margins. The Middle Ordovician sea- bounded core of , could have been juxta- level changes and biologic radiation broadly coincided with initiation posed with the cratonic cores of some present-day southern continents. of the Appalachian-Andean mountain system along >7000 km of the This has led to reconstructions of and Pannotia, superconti- Taconic and Famatinian belts. These correlations, based on testable pa- nents that may have existed in early and latest Neoproterozoic time, re- leogeographic reconstructions, invite further speculation about possi- spectively, before and after the opening of the Pacific Ocean basin. ble causative relations between the internally driven long-term tectonic Recognition that the Precordillera of northwest Argentina consti- of the planet, its surface environment, and life. tutes a terrane derived from Laurentia may provide critical longitudi- nal control on the relations of that to Gondwana during the Pre- INTRODUCTION -Cambrian boundary transition, and in the early Paleozoic. The Precordillera was most likely derived from the general area of the “Apparently, when evolution becomes sluggish, it is not so much for want of an ap- Ouachita embayment, and may have been part of a hypothetical propriate chance mutation as for the lack of a worthy environmental challenge.” (de promontory of Laurentia, the “Texas plateau,” which was detached Duve, 1995, p. 213) from the Cape of Good Hope embayment within Gondwana between the present-day Falkland-Malvinas Plateau and Transantarctic Moun- The most striking change in the history of the Earth recorded in the geo- tains margins. Thus the American continents may represent geometric logic record marks the boundary between rocks formed during its “twins” detached from the Pannotian and Pangean in ≈4.0-b.y.-long and history and strata of the overlying Early Cambrian and time, respectively—the new Cambrian System. The Cambrian strata contain the fossil evidence of the mid-ocean ridge crests of those times initiating the two environmental apparently rapid development of skeletalized metazoans that commenced supercycles of history 400 m.y. apart. In this scenario, the between 545 and 540 Ma, the dawn of Phanerozoic time (Brasier, 1991, extremity of the Texas plateau was detached from Laurentia during the 1992a; Conway Morris, 1993; Knoll and Walter, 1992; Bowring et al., Caradocian Epoch, in a rift event ca. 455 Ma that followed Middle Or- 1993; Grotzinger et al., 1995; see Fig. 1 for time scale). The cause of this dovician collision with the proto-Andean margin of Gondwana as part biologic “explosion” that peaked at the Tommotian-Atdabanian boundary, of the complex Indonesian-style Taconic-Famatinian , which ca. 528 Ma, is fundamental to the understanding of the development of life involved several island arc- collisions between the two major on the planet, even though the new line of evidence provided by molecular continental entities. Laurentia then continued its clockwise relative mo- biology indicates that divergence among metazoan phyla may have oc- tion around the proto-Andean margin, colliding with other arc ter- curred in the “Deep Precambrian” (Wray et al., 1996), near the beginning ranes, , and Baltica en route to the Ouachita-Alleghanian-Her- of Neoproterozoic time. cynian-Uralian collision that completed the amalgamation of Pangea. The Neoproterozoic , from 1.0 Ga to the Precambrian-Cambrian The important change in single-celled organisms at the Mesopro- boundary, was an interval of dramatic changes in global environmental terozoic-Neoproterozoic boundary (1000 Ma) accompanied assembly conditions. There were several glaciations, possibly extending to tropical of Rodinia along Grenvillian sutures. Possible divergence of metazoan or even equatorial latitudes, and carbonates were apparently deposited phyla, the appearance and disappearance of the fauna (ca. from the equator to very high latitudes (Hambrey and Harland, 1981; 650Ð545 Ma), and the Cambrian “explosion” of skeletalized metazoans Young, 1995). Isotopic data from Neoproterozoic sedimentary rocks indi- (ca. 545Ð500 Ma) also appear to have taken place within the framework cate major swings in the chemistry of ocean waters (Asmerom et al., 1991; of tectonic change of truly global proportions. These are the opening of Brasier, 1992b; Kaufman et al., 1993; Ripperdan, 1994; Nicholas, 1996). Global rose during Cambrian time, marking the start of the first- *E-mail address: [email protected] order eustatic cycle that lasted throughout the Paleozoic Era, the earlier of

GSA Bulletin; January 1997; v. 109; no. 1; p. 16Ð42; 17 figures; 2 tables.


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Figure 1. Time-scale; right-hand col- umn, indicates hypothetical superconti- nental entities and ocean basins referred to in text. Sources are: Ordovician—In- ternational Union of Geological Sci- ences, Subcommission on Ordovician Stratigraphy, 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). Important second-order falls in sea level (Hallam, 1992), one of the most pronounced radiations 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 oceanic plateaus, and the uplift of mountain ranges and major plateaus, is believed to have played a major role in Mesozoic and Cenozoic environmental change (Raymo et al., 1988; Hallam, 1992). In this article I review the the problems of global geography and tectonics in Neoproterozoic and early Paleozoic 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 of Gondwana dur- ing that time interval. This may explain the otherwise unmatched, if poorly understood, late Precambrian Pacific margins of Laurentia, South Amer- ica, Antarctica, and . I suggest a new hypothesis, in which a re- cently established geologic relationship between the Andean Precordillera of northwestern Argentina and the southern United States may, by provid- ing constraint on the relative of Laurentia and Gondwana at two critical times, prove to be a key element in understanding the geography

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

and tectonics of the planet as multicellular life became firmly established. REVIEW: “ARCHETYPAL” AND “ALTERNATIVE” I speculate briefly and in general terms on possible causative relations over PALEOGEOGRAPHIC SCENARIOS that time interval between tectonics, the environment, and biologic evolu- tion. The geographic and tectonic scenario that I propose presents ample Every student of stratigraphy learns of the distinction between the olenel- “challenges” to spur evolution if Christian de Duve’s observation, quoted lid benthic trilobite realm in northwest Scotland and western Newfoundland at the beginning of this introduction, is on target. and the paradoxidid realm of southern Britain and the Avalon platform, the

Figure 3. (a) Paleomagnetically controlled continental reconstruction for Early Ordovician time (Torsvik and Trench, 1991); note that a clockwise change in the paleolongitude of Lau- rentia with respect to Gondwana, which is per- missible given the axial symmetry of Earth’s magnetic 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 the geologic relationship of present-day North AtlanticÐbordering 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 triangles represent active island-arc systems.


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Figure 3. (Continued).


classic “Pacific” and “Atlantic” provinces of Walcott (1889; Fig. 2a). Even What has not changed is the widespread belief that through most of early before continental drift was widely accepted, the issue of whether these Paleozoic time, the Iapetus ocean basin off the proto-Appalachian- early Paleozoic benthic faunas were separated by a deep and wide ocean Caledonian rifted margin of Laurentia separated that craton from Baltica, basin or by a “geanticlinal” ridge was discussed in my late 1950s stratigra- northwest , and northwestern (Morel and Irving, phy class at the University of Edinburgh (Holmes, 1965). J. Tuzo Wilson 1978; Fortey and Cocks, 1992; McKerrow and Scotese, 1990; McKerrow et brought it to the forefront of geologic thought in the 1960s: he extended the al., 1991, 1992; Jurdy et al., 1995; Pickering and Smith, 1995; Torsvik et al., developing plate tectonic paradigm back to early Paleozoic time when, 1995, 1996a, 1996b; Van der Pluijm et al., 1995; Mac Niocaill et al., 1996, seeking to explain the two-sided geologic as well as faunal of the 1997). In the absence of any real evidence of late Precambrian juxtaposition Appalachian orogen (Williams, 1964), he asked the question, “Did the At- of the Laurentian and West African , this appears to have sprung lantic Ocean open, close, and then re-open?” (Wilson, 1966). He introduced from Tuzo Wilson’s original question, illustration (Fig. 2b), and implication both the term “proto-Atlantic” for the early Paleozoic ocean separating Lau- of a type of accordion tectonics with ocean basins opening and closing in rentia from Baltica and Gondwana (Fig. 2b), and the now orthodox concept “approximately but not precisely” the same configuration of the bordering of the of ocean basin opening and closure to generate an oro- cratons (Wilson, 1966, p. 677). genic belt. The paradigm of and the concept of the Wilson Traditionally, therefore, the Appalachian margin of Laurentia has tended cycle were first applied to the detailed analysis of an orogen, the same to be juxtaposed with northwest Africa in reconstructions for times prior to Appalachian-Caledonian belt (Dewey, 1969; and Dewey, 1970). late Precambrian rifting (Bird and Dewey, 1970; Hatcher, 1978, 1987, The faunal and environmental distinction between the two forelands of 1989), or else a “North Atlantic craton,” joining Laurentia and Baltica, is the Appalachian orogen in the Canadian Maritimes, and of the Caledonian portrayed in isolation (e.g., Winchester, 1987; Gower et al., 1990). Like- orogen in the British Isles, continues to be emphasized by modern workers wise, paleomagnetically controlled reconstructions of early Paleozoic ge- (Cocks and Fortey, 1982; Williams et al., 1996; Fig. 2b). The term Iapetus ography mostly show the proto-Appalachian and northwest African mar- ocean was substituted for proto-Atlantic ocean by Harland and Gayer gins opposed to each other, even when separated by 4500 km of Iapetus (1972), the wide acceptance of this change to the name of the mythologic ocean basin (e.g., Torsvik and Trench, 1991; Fig. 3a). Does this tradition father of Atlas reflecting appreciation that the early Paleozoic ocean basin reflect, at least in part, the fact that by far the largest concentration of geol- off proto-Appalachian-Caledonian Laurentia was a separate entity, rather ogists working on problems of early Paleozoic geography happens to be than a direct precursor of the present Atlantic Ocean basin. centered on the margins of the present-day North Atlantic Ocean basin? It

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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 before present) during the Neoproterozoic and Paleozoic. Map is an elliptical mercator projection with Africa in its present-day position, and Gondwana assembled using Mesozoic-Cenozoic sea-floor spreading data. EWM—Ellsworth-Whitmore block of Antarctica, and FI—Falkland-Malvinas Is- 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).

leads to a comprehensive and elegant analysis of early Paleozoic New Eng- ing the “Pacific” benthic olenellid trilobite fauna characteristic of the Lau- , the Canadian Maritimes, the British Isles, and Scandinavia, but it rentian craton, in the Andean Precordillera of northwest Argentina (Keidel, does nothing to explain the origin and development of the rifted Neopro- 1921; Borrello, 1965, 1971; Palmer, 1972). terozoic-Cambrian southeastern and southern continental margins of Lau- A challenge to this “archetypal” view has emerged in recent years with the rentia (Fig. 3b). Nor, on a global scale, does it explain the origin of the Pa- suggestion of an “alternative paleogeographic reconstruction,” to use the cific-facing late Precambrian margins of Laurentia, proto-Andean South phrases of Torsvik et al. (1995). The alternative started with the introduction America, Antarctica, and Australia. The proto-Andean margin is the site of of the “SWEAT” hypothesis of Moores (1991). This revived an idea of Cana- a long-recognized enigmatic lower Paleozoic carbonate platform, contain- dian geologists to explain the late Precambrian Cordilleran margin of Lau-

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

rentia by juxtaposition of the southwest United States and East Antarctica East and West Gondwana at the end of Precambrian time, and to have moved (SWEAT) prior to the opening of the Pacific Ocean basin in Neoproterozoic relatively clockwise around the proto-Andean margin of South America dur- time. The determination of longitude was the major problem of early navi- ing the Paleozoic Era en route to its final Ouachita-Alleghanian collision gators, and the determination of relative paleolongitude is the most difficult with northwest Africa to complete the assembly of Pangea (Dalziel, 1991; challenge in pre-Pangea geography. Hence, in refining and amplifying this Fig. 4). The location of Laurentia between East AntarcticaÐAustralia and hypothesis, I pointed out that it is paleomagnetically acceptable, given the South America at the end of Precambrian time was also advocated by Hoff- absence of longitudinal control, for Laurentia to have been located between man (1991). We both suggested that opening of the Pacific ocean basin might

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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).

have been linked to the amalgamation of Gondwana by closure of Pan Dalla Salda et al. (1992a) suggested that, if Laurentia had rotated clock- African and Brazilide ocean basins. This was the first hypothesis attempting wise around Gondwana (Fig. 4), its Appalachian margin might have col- to explain the Neoproterozoic-Cambrian rifted margins of East Antarc- lided with South America, initiating the Paleozoic Famatinian orogen ex- ticaÐAustralia and South America, together with the proto-Cordilleran and posed as the basement of the Mesozoic-Cenozoic Andes and broadly coeval proto-Appalachian margins of the same general age in North America. To- with the Taconian orogeny of North America. My Argentine colleagues and gether these constitute a very high percentage of the “global budget” of Neo- I then proposed (Dalla Salda et al., 1992b; Dalziel et al., 1994) that the early proterozoic-Cambrian rifted margins (Bond et al., 1984). Paleozoic Taconic and Ocloyic (early Famatinian) orogens of North and

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South America, respectively, had once been continuous. The Cambrian- Ordovician carbonate bank that forms the Precordillera terrane of northwest Argentina (Ramos et al., 1986), known for many years as the San Juan or Mendoza faunal province and hereafter the Precordillera, is the only local- ity outside the Laurentian craton (inclusive of northwest Scotland) with strata containing the olenellid benthic trilobite fauna (Borello, 1965, 1971). The Cambrian-Ordovician sedimentary strata forming the Precordillera are now recognized as part of a larger terrane including Precambrian basement exposed to the east, the 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 the Caledonides—a Laurentian benthic fauna occurs in the Precordilleran plat- form carbonates to the west, and a Gondwanan fauna in the craton cover to the 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- Figure 7. Changing affinities of the early Paleozoic benthic fauna of rentia (Thomas, 1976, 1989, 1991; Lowe, 1985, 1989; Arbenz, 1989; Viele the Precordilleran carbonate platform in terms of genera percentage and Thomas, 1989), we proposed that the Precordillera was detached from (from Benedetto et al., 1995). Time intervals: Camb—Middle to Late Laurentia in a Late Ordovician rifting event following Middle Ordovician Cambrian; Arenig—middle to late Arenig; Llanv—early Llanvirn; collision with Gondwana (Fig. 5; for a general review of these hypotheses Carad—early Caradoc; Ashg—Hirnantian (see Fig. 1). see 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 early proto-Appalachian margin opposed to northwest Africa (Torsvik et al., Paleozoic stratigraphy and fauna of the Precordillera and the southeastern 1995, 1996a; van der Pluijm et al., 1995; Pickering and Smith, 1995). Laurentian platform in detail, and have greatly strengthened the Dalla The difference between the two models is not only a question of the cause Salda et al. (1992b) suggestion that the Precordillera was derived from the of the Ocloyic orogeny: it is also crucial for global Paleozoic geography. If general area of the Ouachita embayment, a conclusion supported by evi- the Astini et al. and Thomas and Astini microcontinental, or Laurentian ben- dence of a Grenvillian basement beneath the cover of the Precordillera and thic fauna “funeral ship,” model is correct, debate about the relative posi- exposed to the east (Abruzzi et al., 1993; Kay et al., 1996). Consistent with tions of Laurentia and Gondwana will continue. Arguments about likely Thomas’s analysis of the rifting and subsidence history of the embayment rates and directions of motion for the Precordilleran microcontinent across (Thomas, 1991; Fig. 6), and to explain the affinities of the Cambrian- the Iapetus ocean basin, the floor of which has been recycled into the man- Ordovician fauna in the strata of the Precordillera (Benedetto et al., 1995; tle, will be added to those involving traditional paleomagnetic, faunal, and Fig. 7), however, Astini and his colleagues and Thomas proposed that it paleoenvironmental controls (Dalziel and Dalla Salda, 1996a, 1996b; Astini was detached as a separate microcontinent during the Cambrian Period, et al., 1996a; Torsvik et al., 1996b). For example, I compute the reconstruc- and drifted across the Iapetus ocean basin independently to collide with tions of Mac Niocaill et al. (1997) to require a convergence rate of the Pre- South America in Middle Ordovician time (Astini et al., 1995; Thomas and cordilleran terrane with respect to South America of just over 11 cm/yr Astini, 1996; Fig. 8). Thomas (1991) interpreted the overstep of synrift fa- (≈5600 km in ≈50 m.y.); is this reasonable or not? If, on the other hand, the cies in the Birmingham rift, the Mississippi graben, and the Southern Ok- Precordilleran terrane were detached after a continent-continent collision, lahoma aulacogen by Upper Cambrian strata to indicate completion of rift- then it is a tectonic tracer (Dalziel, 1993) that records the relative positions drift transition within the Ouachita embayment by middle Late Cambrian of Laurentia and Gondwana at the time of the collision during the Ordovi- time (Fig. 6). A cosmopolitan pelagic agnostid fauna invaded the Pre- cian Period. In this “calling card” model, the separation of the Laurentian cordillera during the Cambrian, and a warm-water Gondwanan benthic and Gondwana faunas is explained by a Taconic-Ocloyic physiographic trilobite fauna reached the Precordilleran platform during the late Areni- barrier analogous to the European Alps or the Indonesian archipelago be- gian. It was not until Middle Ordovician time, however, that lichid, ptery- tween Australia and , “Wallace’s line” (Dalziel and Dalla Salda, 1996a). gometopid, and calyminid , found in Cambrian and Lower Or- The process in the “calling card” model would have been similar to the dovician strata of Gondwana, spread to Laurentia (Droser et al., 1996). A much more recent detachment from Laurentia, during the opening of the comprehensive review of the development of the Precordillera was pro- North Atlantic and after the Caledonian-Scandian Laurentia-Baltica colli- vided by Astini et al. (1996b). sion of the Scottish fragment of its craton and overlying CambrianÐLower The microcontinental interpretion of Astini et al. (1995), and now of Ordovician platform cover with its “Pacific” trilobite fauna (Salter, 1859; Thomas and Astini (1996; Fig. 8) was shared by most of the participants in Peach et al., 1907). In the case of the Precordillera, if attached to Laurentia the recent Penrose Conference “The Argentine Precordillera: A Laurentian at the time of Ordovician docking in Gondwana, it is a paleogeographic terrane?” cosponsored by the Geological Society of America and the Aso- key, defining the relative paleolongitudes of those two major continental ciación Geológica Argentina. This was mainly because the distinction be- entities at certain times prior to the amalgamation of Pangea and the tween the faunas of the Laurentian and Gondwana cratons was largely growth of the present ocean basins. maintained through the Middle Ordovician docking of the Precordillera, and because Thomas’s interpretation of the rifting history of the Ouachita HYPOTHESIS: A MISSING TEXAS PLATEAU? embayment is consistent with much of the geology of the Precordillera (Dalziel et al., 1996). Likewise, authors have noted that while the Lauren- I propose the new hypothesis that the Precordillera was formerly part of tia-Gondwana collision advocated by Dalla Salda et al. (1992a, 1992b) is a Laurentian plateau analogous to, and originally adjacent to, the present- paleomagnetically permissible, they preferred for various reasons to main- day Falkland-Malvinas Plateau in the South Atlantic Ocean basin. I make tain an “archetypal” paleogeography for early Paleozoic time, with the this proposal because I perceive major difficulties with the microcontinen-

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Figure 8. Laurentian benthic fauna “funeral ship” model of the Precordilleran terrane of Argen- tina as the “Guillermo Thomasia” microcontinent (PR) that was de- tached from the Ouachita embay- ment during the Cambrian (a and b), and drifted independently across the Iapetus ocean basin as its Laurentian fauna was replaced by endemic and Gondwanan forms (c and d), to collide with the proto- Andean margin in Middle Ordovi- cian time (e). Modified by me from Thomas and Astini (1996) by the addition 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—rifted margin prisms; crosses—oceanic crust.

tal model of Astini et al. (1995) and Thomas and Astini (1996). In addition, That several major continents of the present day, notably South Amer- the “missing” plateau could explain not only the geology and faunas of the ica, Africa, and India, have pointed southern terminations has been noted Precordillera and the Ouachita embayment, including the long-enigmatic for many years (e.g., Holmes, 1944). Although the geometry of these three offshore “Llanoria” landmass of Dumble (1920), Miser (1921), and King continents is, to a first approximation, explained by the pattern of Meso- (1937), but also a long-standing enigma of the Pacific margin of Gond- zoic sea-floor spreading controlled by regional stress, plume-related stress, wana. In this model of the evolution of the Appalachian-Andean mountain and lines of old lithospheric weakness such as Pan African and Brazilide system, a refinement of the one proposed by Dalla Salda et al. (1992b) and belts (Porada, 1989; Dalziel and Lawver, 1995), the origin of the “southern amplified by Dalziel et al. (1994), the southward-tapering extremities of cone” of Laurentia from the southern Appalachians to Baja California has the North and South American cratons, the “southern cones,” are virtually remained obscure. As previously pointed out, the origin of Laurentia’s identical continental “twins” born more than 400 m.y. apart in the Early southern margins is not addressed by traditional or “archetypal” views of Cambrian and Early Cretaceous opening of the Iapetus and South Atlantic the Iapetus ocean basin (e.g., Fig. 3b). Explanation of the Precordillera as ocean basins, respectively—the start of the Paleozoic and Mesozoic eusta- a microcontinent detached from the Ouachita embayment could help pro- tic and environmental supercycles. vide an explanation, but does not involve consideration of possible conju-

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Figure 8. (Continued).

gate margins to those of southern Laurentia. The problem may involve It should be noted that a recent review of the paleomagnetic data for the more than two continents. latest Proterozoic and early Paleozoic concludes that a 5000-km-wide ocean Thomas (1991), in his description and analysis of the origin of the late Pre- existed between Laurentia and Gondwana prior to the Precambrian-Cam- cambrian to early Paleozoic Appalachian-Ouachita rifted margin, empha- brian boundary (Torsvik et al., 1996a). This is in direct contradiction to the sized the influence of northeast-trending and northwest-trending trans- interpretations of the geologic evidence cited above. These place the rift- form faults. The timing of rift-drift transition off the Appalachian Blue Ridge, drift transition along the proto-Appalachian margin from Newfoundland to as along the rest of the ≈4500-km-long orogen (e.g.,Williams and Hiscott, Georgia close to the Precambrian-Cambrian boundary (545Ð540 Ma, 1987; Bond et al., 1984), is clearly marked by latest Precambrian cessation of Table 1; Williams and Hiscott, 1987; Thomas and Astini, 1996). The same synrift magmatism and the onlap of Lower Cambrian sandstone at the base time is inferred from geologic evidence for the separation of Laurentia and of a retrogradational passive margin succession that oversteps uppermost Siberia (Pelechaty, 1996), although Baltica may have separated earlier. Precambrian rift-fill sequences (see also Thomas and Astini, 1996). Inboard Other interpretations of the paleomagnetic data do permit considerably of this margin, however, it is Upper Cambrian strata that overstep uppermost greater proximity, if not actual juxtaposition of Laurentia and Gondwana at Precambrian and Cambrian igneous and sedimentary rocks in the Missis- the time (Grunow et al., 1996; Powell et al., 1995). Here, however, reliance sippi ValleyÐRough CreekÐRome graben system, the Birmingham basement is placed on the geologic interpretation, the age of rift-related igneous rocks fault system, and the southern Oklahoma fault system (Fig. 6b). and 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) and present-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.

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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—Dungeness arch; DM—Deseado massif; F/MI—Falkland-Malvinas Islands platform (Lafonian microplate); MEM—Maurice Ewing massif; NPM—North Patagonian massif; RB—Rawson basin; SJgB—San Jorge basin; SJnB—San Julian basin; SMB—South Malvinas basin; SSI—South Shetland Islands trench. (b) Cross section of the South Malvinas basin (after Yrigoyen, 1989). For location see Figure 10a.

Proterozoic-Cambrian boundary would explain the start of the global ma- collision. In the microcontinental “funeral ship” model of Astini et al. rine transgression of the Paleozoic supercycle. (1995) and of Thomas and Astini (1996), it departed Laurentia in the Late Thomas (1989, 1991) and Thomas and Astini (1996) interpreted the dif- Cambrian and drifted across the Iapetus ocean basin as the discrete micro- ference in age of rift-related rocks between the Appalachian margin and the continent dubbed “Bill Thomas land” or “Guillermo Thomasia” at the Pen- Ouachita embayment as the expression of an Early Cambrian ridge crest rose Conference. “jump” of ≈800 km within the Iapetus ocean basin at the beginning of the With the exception of a few questions with regard to distances of sepa- Cambrian Period from the Blue Ridge rift along the Appalachian margin to ration at certain times, both from Laurentia and Gondwana, the adaptation the Ouachita rift in the cratonic embayment southwest of the Alabama- of the Thomas (1991) model for the origin of the Ouachita embayment to Oklahoma transform fault. This led to their interpretation that a branch of explain the origin of the Precordillera along the lines of Astini et al. (1995) the mid-Iapetus ridge, named the Ouachita mid-ocean ridge, generated and Thomas and Astini (1996) at first appears straightforward, as in the re- ≈1500 km of oceanic lithosphere before the end of the Cambrian Period, constructions of Mac Niocaill et al. (1997, in press). The continuity of the separating a microcontinent of ≈800 km2 covered by Laurentian shelf fa- shallow-marine environment to allow the Laurentian olenellid benthic cies from the main continent (Thomas, 1991; Figs. 6 and 8, a and b). It is trilobite fauna to survive into Early Ordovician time (Benedetto et al., this microcontinent that Dalla Salda et al. (1992a, 1992b), Dalziel et al. 1995; Fig. 7), for example, may have required a lesser separation of the mi- (1994), Astini et al. (1995), the 1995 San Juan Penrose Conference Partic- crocontinent from Laurentia than that suggested in the models of Thomas ipants (Dalziel et al., 1996), and Thomas and Astini (1996) believed to be (1991), Astini et al. (1995), and Thomas and Astini (1996) (Figs. 6a and the Precordillera of the present-day Andes. In the “calling card” model of 8c). However, a fracture zone ridge, common along Mesozoic-Cenozoic Dalla Salda et al. (1992b) and Dalziel et al. (1994), the Precordillera was transform faults, could have been present. Some degree of oceanic isola- detached from Laurentia in Caradocian time following continent-continent tion from the parent continent would have allowed the cosmopolitan ag-

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Figure 10. (Continued).

nostid pelagic fauna to invade the Precordilleran microcontinent before the Plateau (Figs. 9 and 10a). Others are the British Isles, Sri Lanka, and Tas- end of the Cambrian. A source of siliciclastic sediment, either an island or mania (Sandwell and Smith, 1992). The closest example to the “funeral a pathway from the craton proper, had to have been maintained into Late ship” type of scenario along a passive margin, is the separation of Mada- Cambrian time. Gondwana faunas became established as the microconti- gascar from Africa. Even in this case, however, the initial spreading took nent converged on the proto-Andean margin in Arenigian time. Accomo- place in the Somali basin on the African side of the microcontinent. Sub- dation has to be made to explain the warmer water “Pacific” graptolite sequently the spreading center jumped outboard to the southwest Indian fauna in the Llandeilian strata of the Precordillera (Stanley Finney, 1995, ridge (Lawver et al., 1991). I therefore conclude that the “funeral ship” personal commun.), but perhaps this could be explained by circulation pat- model for the origin of the Precordillera put forward by Astini et al. (1995) terns within the Iapetus ocean. and Thomas and Astini (1996) is not actualistic, and that another model Comparison with the record of opening of the present-day oceans, how- needs to be sought for the development of the Ouachita embayment and ever, reveals major difficulties with this Thomas and Astini et al. micro- separation of the Precordillera from that of Laurentia. In searching continental “funeral ship” scenario (Fig. 8). A critical aspect is the timing for an alternative explanation, it becomes apparent that the microcontinen- of sea-floor spreading between Laurentia and the Precordillera (Dalziel et tal model may not be a unique explanation for the stratigraphic relation- al., 1996). The Early Cambrian “inboard” ridge crest jump, originally pro- ships and geologic evolution of the Ouachita embayment. posed by Thomas (1989, 1991) and advocated in this Precordilleran model, Striking similarities exist between the early Paleozoic rifting and subsi- from the Blue Ridge margin to the Ouachita embayment, would have dence within the Ouachita embayment as described by Thomas (1989, 1991) lengthened the separation of mid-Iapetus spreading centers across the and the early Mesozoic rifted South American margin of the South Atlantic Alabama-Oklahoma transform during the Cambrian by ≈800 km (ridge Ocean basin. The Falkland-Malvinas Plateau extends ≈1500 km east of the jump 1 in Fig. 8a). A jump of this type is not represented in the Mesozoic- South American continental shelf edge into the South Atlantic Ocean basin Cenozoic along any of the rifted passive margins formed during the frag- (Figs. 9 and 10a). It tapers eastward from 600 km to 300 km wide, and is mentation of Pangea and birth of the Atlantic, Indian, and Southern ocean bounded on the north by the Falkland-Agulhas fracture zone and on the south basins. On the contrary, ridge crest jumps at transform faults in these by the Falkland Trough, a continuation of the Andean foredeep separating the oceans shorten, rather than lengthen the separation of actively spreading plateau from the North Scotia Ridge. The Falkland-Malvinas Plateau con- ridges. A striking example can be demonstrated in the South Atlantic sists of three geologic provinces (Fig. 10a): from east to west, these are the Ocean basin using shipborne magnetic and satellite altimeter-derived grav- Maurice Ewing Bank, a Cenozoic carbonate bank built upon a horst of Pre- ity data. Three major ridge crest jumps have occurred since the oceanic cambrian crystalline basement, i.e., the Maurice Ewing massif of Urien et al. floor of the basin was initiated ca. 130 Ma, shortening the separation of the (1995), that was exposed during the initiation of rifting (Barker spreading centers north and south of the Falkland-Agulhas fracture zone et al., 1977); the late Mesozoic central Falkland-Malvinas Plateau rift basin, from its original ≈1400 km to its present 200 km (Barker, 1979; Fig. 9). which is floored by highly extended continental crust, and possibly to a lim- There are several modern examples of cratonic fragments that are now ited extent by oceanic crust of a failed spreading center (Lorenzo and Mutter, comparatively widely separated geographically from their parent continent 1988); and the Falkland-Malvinas Islands platform, or Lafonian microplate, by stretching of continental lithosphere and rift-basin subsidence. These on which the Falkland Islands with their Grenvillian basement and Paleozoic microcontinents have not, however, been set free from that parent by actual to lower Mesozoic sedimentary cover are located (Greenway, 1972; Cin- sea-floor spreading, but rather form horsts within plateaus extending from golani and Varela, 1976). West of the platform, there is another late Mesozoic their parent continents. The most striking example is the submerged Mau- rift basin, the Malvinas, or South Malvinas, basin (Turic et al., 1980; rice Ewing Bank (or massif) at the eastern end of the Falkland-Malvinas Yrigoyen, 1989; Urien et al., 1995; Fig. 10b).

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Discussion of the complex evolution of the Falkland-Malvinas Plateau lavas of Llandeilian-Caradocian rather than Cambrian age (Ramos et al., during the Mesozoic and Cenozoic fragmentation of western Gondwana is 1986; Dalziel et al., 1996). The Astini et al. (1995) and Thomas and Astini outside the scope of this paper. For a recent analysis see Marshall (1994). (1996) microcontinental interpretation therefore requires a second enig- Critical points here are as follows. (1) The entire 1500 km × ≈350 km sub- matic ridge crest jump to explain the presence of these rocks. In their model, merged plateau is unquestionably part of the South American continent, the ridge crest would have had to shift more than 1000 km to the proto-An- the possibility of a failed spreading center in the central basin notwith- dean margin from its mid-Iapetus position (see ridge jump 2, Fig. 8e). standing. (2) The Falkland-Malvinas Plateau is partly floored by Precam- I believe that the microcontinental model mistakenly assumes that the brian crystalline basement comparable to that in southern Africa, where it presence around the Ouachita embayment and in the Precordillera of rift originated in the Natal Valley (Fig. 9), and in adjacent Patagonia. (3) The and slope facies, open ocean sediments, and pelagic faunas demands the central plateau basin formed as an extensional feature in Early to Middle formation of oceanic lithosphere between the Precordillera and Laurentia Jurassic time, prior to the Early Cretaceous development of the earliest during the Cambrian and Early Ordovician. On the contrary, exactly the ocean lithosphere in the South Atlantic Ocean basin. (4) Geologic and pa- same sedimentary environments and faunal types can exist within an ex- leomagnetic data demonstrate that extensional separation of the Falkland- tending continental plateau like the Falkland-Malvinas Plateau, without the Malvinas Plateau from southern Africa involved >100¡ of rotation of the onset of full sea-floor spreading. Extension of continental crust permits Falkland-Malvinas platform (or Lafonian microplate) relative to the South hundreds of kilometers of separation between two points on the surface of American craton. (5) Early motion along the Falkland-Agulhas fracture the Earth, but only sea-floor spreading can result in separations of thou- zone continued westward into South America along the Gastré shear zone. sands of kilometers. Hence, the Laurentian affinities of the Precordillera, (6) The present length of the Falkland-Malvinas Plateau is its length after its olenellid benthic trilobite fauna, and Grenvillian basement, plus the several hundred kilometres of extension, i.e., at the time of onset of drift “switch over” to endemic and Gondwana faunas prior to docking in South between South America and Africa. (7) Although detailed reconstruction is America, could be explained by its representing a feature like the Maurice subject to interpretation, the plateau can be accurately restored to the posi- Ewing massif as part of an early Paleozoic Laurentian plateau outboard of tion in the Natal Valley that it occupied immediately prior to the Early Cre- the Ouachita embayment within the Iapetus ocean basin, analogous to the taceous opening of the South Atlantic Ocean basin. Falkland-Malvinas Plateau of South America within the present-day South These facts about the Falkland-Malvinas Plateau lead me to conclude Atlantic Ocean basin. Is this merely an ad hoc hypothesis? Let us consider that the Precordillera could have been part of a similar plateau attached to where this hypothetical feature could have originated within the Neoprot- Laurentia, outboard of the Ouachita embayment, during the Cambrian and erozoicÐearly Paleozoic Earth. Early Ordovician. In particular, the fact that the South American Falkland- Malvinas Plateau includes rift basins several kilometres deep overstepped NEOPROTEROZOIC GEOGRAPHY by sedimentary strata younger than the initial floor of the surrounding South Atlantic Ocean basin (Turic et al., 1980; Yrigoyen, 1989) leads me Following publication of the SWEAT hypothesis (Moores, 1991), Paul to further question Thomas’s model of a ridge crest jump within the open- Hoffman and I both suggested that Laurentia could have “broken out” of ing Iapetus ocean basin, and hence his conclusion that there was a the Neoproterozoic supercontinent, which formed at the time of global ≈1500-km-wide Ouachita ocean by Late Cambrian time (Thomas, 1991; Grenvillian orogenesis and is most commonly referred to as Rodinia Thomas and Astini, 1996). As noted by Dalla Salda et al. (1992b) and (McMenamin and McMenamin, 1990), from between East and West Dalziel et al. (1994), another interpretation, based on sedimentary facies, Gondwana (Dalziel, 1991; Hoffman, 1991; Fig. 11). We suggested that provenance, and paleocurrent analyses, suggests that the Ouachita trough opening of the Pacific Ocean basin was compensated, on a globe of con- was a narrow basin during the early and middle Paleozoic, with an out- stant radius, by the closing of Pan African and Brazilide ocean basins board sediment source similar to the rifted proto-Appalachian margin within the amalgamating cratons of Gondwana during Neoproterozoic to, (Rosenfeld, 1981, in Denison, 1995; Lowe, 1985, 1989; Arbenz, 1989). at latest, Cambrian time. Subsequently, I made the specific suggestion that This interpretation is now supported by geochemical data that indicate re- the Greenland-Scotland-Labrador promontory of Laurentia may have been stricted circulation in the basin during early Paleozoic time (Reid et al., rifted from the ancestral Arica embayment of South America following 1996), as well as by additional sedimentologic data (Dix et al., 1996). It is opening of the Pacific Ocean basin, the amalgamation of Gondwana, and also in keeping with the long-standing evidence from the Marathon margin hence the ephemeral existence of a Laurentia-Gondwana supercontinent in of trans-Pecos Texas that formed the basis for an offshore Llanoria land- latest Neoproterozoic time (Dalziel, 1992b, 1994; Fig. 12). I suggested mass in the early Paleozoic, as discussed by Keller and Dickerson (1996). specific tests of this hypothesis, and am actively pursuing some of them. The presence of slope facies on the present Pacific side of the Pre- Of interest here is one particular aspect of the quantitative reconstruction cordillera is interpreted by Astini et al. (1995) and Thomas and Astini of Rodinia, and of the hypothetical latest Precambrian supercontinent that (1996) to indicate the presence of an ocean basin by Late Cambrian time, may have existed briefly following the opening of the Pacific Ocean basin but could equally well indicate merely a rift basin as seen on the Falkland- (Dalziel, 1992b, 1994). The name Pannotia, the “all-southern” superconti- Malvinas Plateau today (Figs. 9 and 10). A continental plateau model is nent, has been suggested for this latter entity (Powell, 1995; Powell et al., compatible with the evidence of faunal and environmental change noted in 1995). It mainly resulted from the amalgamation of East Gondwana with the Precordillera during Cambrian and Early to Middle Ordovician time, in- West Gondwana and Laurentia along the East African orogen to close the cluding the encroachment of the cosmopolitan agnostoid trilobites in the Mozambique ocean (Fig. 12). Amalgamation of West Gondwana itself in- Cambrian, and the persistence of “Pacific” pelagic graptolites into the Llan- volved closure of several Pan AfricanÐBrazilide ocean basins (Trompette, deilian. Deep open ocean water flowed over the Falkland-Malvinas Plateau 1994; Unrug, 1996). Also involved was “Turkic-type” (Íengör and Na- during late Mesozoic and Cenozoic time after separation from Africa, de- tal’in, 1996) accretion of arcs and oceanic plateaus, together with the clo- positing an abyssal, cold water benthic foraminiferal fauna on continental sure of small ocean basins (Stern, 1994; Stein and Goldstein, 1996). Bear- basement (Barker et al., 1977). A continental plateau that was part of Lau- ing in mind that both Laurentia and Gondwana are portrayed with rentia until after an Ordovician (Ocloyic) collision with South America is geometric accuracy in the Rodinia and Pannotia reconstructions (see cap- also compatible with the fact that the oldest known rocks of unquestioned tions for Figs. 11 and 12), it is notable that with the relative positions of oceanic affinities on the Pacific margin of the Precordillera are mafic pillow Laurentia and Gondwana controlled by the suggested Scotland-Arica con-

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TABLE 1. PALEOMAGNETIC POLES IN RECONSTRUCTIONS TABLE 2. EULER (FINITE) POLES OF ROTATION FOR THE MAJOR PLATES α Age (Ma) Latitude Longitude 95 References Age Latitude Longitude Angle References (N) (E) Laurentia to absolute framework Early to Mid-Neoproterozoic 725 22.0 Ð129.0 Ð92.53 This study, using Powell et al. 1993 A. 1110 Ma (late ) 545 16.5 Ð127.0 Ð109.47 This study, using Meert et al. 1994 Coats Land 22.9 80.3 6.8 Gose et al., 1997, in press 515 26.3 Ð119.0 Ð109.40 Meert et al. (1994b) Laurentia 54.7 Ð142.2 * Caldwell A pole of Constanzo- 475 29.4 Ð121.3 Ð94.09 This study, using Mac Niocaill and Alvarez Smethurst 1994* et al. 1993, in Gose et al., 1997, 465 27.8 Ð124.2 Ð89.94 This study, using Mac Niocaill and in press Smethurst 1994* B. ca. 725 Ma (mid-Neoproterozoic) 422 12.4 Ð152.2 Ð74.00 This study, using Van der Voo 1993 Laurentia Ð5.9 Ð17.6 * Fahrig et al. pole, in Powell et al., 1993 Baltica to absolute framework India Ð78.4 Ð140 Powell et al., 1993 725 38.2 Ð107.2 Ð108.65 based on Dalziel 1994 545 29.4 Ð105.5 Ð120.50 based on Dalziel 1994 545 Ma (End Neoproterozoic) 515 17.6 Ð41.2 Ð138.56 Torsvik et al. (1992) Laurentia 13.0 Ð15.0 15.0 Meert & Van der Voo, 1997, 475 13.4 Ð60.4 Ð125.82 This study in press 465 15.9 Ð64.4 Ð115.38 This study Australia Ð45 Ð14 13.0 mean calculated from Li & Powell, 422 31.4 Ð133.0 Ð90.18 This study 1993 Siberia to absolute framework 515 Ma (Mid-Cambrian) 725 33.5 Ð137.8 Ð115.85 This study Laurentia 4.0 Ð3.0 12.0 Meert et al., 1994b 545 29.2 Ð131.9 Ð131.10 This study Africa 25.0 Ð4.0 25 Grunow, 1995 515 4.5 45.2 126.93 Van der Voo (1993) Baltica 38.3 4.9 * Torsvik et al., 1996 475 5.9 53.5 133.39 This study 465 Ð3.0 Ð131.1 Ð116.74 This study 475 Ma (Early-Ordovician) (all in van der Pluijm et al., 1995) 422 14.6 47.4 98.32 This study Laurentia Ð10.8 Ð3.5 * Mac Niocaill & Smethurst, 1994† Africa 34.0 7.0 * Van der Voo, 1988 East Antarctica to Laurentia Baltica 30.0 54.0 * Torsvik et al., 1992 725 12.8 119.9 134.85 based on Dalziel, 1991 Siberia 42.0 Ð50.0 * Torsvik et al., 1994 East Antarctica to South Africa 465 Ma (Mid-Ordovician) (all in van der Pluijm et al., 1995) 725 81.6 Ð124.5 110.16 This study Laurentia -12.6 Ð9.2 * Mac Niocaill & Smethurst, 1994¤ 545Ð422 7.1 144.8 Ð56.35 based on Lawver et al. 1992 Africa 34.0 7.0 * Van der Voo, 1988 Baltica 19.0 49.0 * Torsvik et al., 1992 East Gondwana fit poles Siberia 27.0 Ð46.0 * Torsvik et al., 1994 Australia to East Antarctica East Avalonia 17.0 18.00 * Torsvik & Trench, 1991 725Ð422 Ð2.0 38.9 Ð31.50 Royer and Sandwell 1989 India to East Antarctica 430-420 Ma (Early Silurian) 725Ð422 Ð4.4 16.7 Ð92.77 Lawver and Scotese 1987 Laurentia 18.0 127.0 * Van der Voo, 1993 (Table 5.1) Northern South America to Laurentia Africa 43.0 Ð171.0 * Van der Voo 1993, (Table 5.2A) 725 2.3 Ð23.6 Ð99.34 Dalziel et al. 1994 Baltica Ð15 Ð15 * Torsvik et al., 1996a (Table 1) 545 2.3 Ð23.6 Ð99.34 Dalziel et al. 1994 Siberia Ð4.0 121.0 19.0 Van der Voo, 1993 (Table 5.4) 515 5.0 155.7 74.73 This study α 475 4.3 143.5 77.64 This study *No 95 supplied; arbitrary 10¡ shown †Northern Scotland pole of 15¡S, 17¡E rotated to Laurentian coordinates 465 2.5 Ð38.8 Ð80.24 This study ¤Northern Scotland pole of 16¡S, 11¡E rotated to Laurentian coordinates 422 40.1 159.5 52.87 This study West Gondwana fit poles Southern Africa to northern South America nection, the Ouachita embayment of Laurentia is located opposite a major 725Ð422 50.0 Ð32.5 Ð55.08 Nürnberg and Müller 1991 Northwest Africa to northern South America embayment in the Precambrian craton of Gondwana between South Amer- 725Ð422 50.8 Ð34.2 Ð53.69 Nürnberg and Müller 1991 ica and East Antarctica, and immediately adjacent to the Falkland-Mal- Coats LandÐMaudheim-Grunehogna block to South Africa 725Ð422 7.1 144.8 Ð56.35 based on Lawver et al. 1992 vinas Plateau discussed above. Ellsworth Mountains to South Africa The embayment off the present-day Cape of Good Hope is geologically 725Ð422 50.0 Ð80.0 83.28 based on Dalziel and Grunow 1992 unique along the ≈12 000 km of the Pacific margin of Gondwana, and has *Northern Scotland pole transferred to Laurentia using Lawver et al.’s (1990) been recognized for several years (Dalziel, 1982). This part of southern- EurasiaÐNorth America fit pole (69.1 156.7 Ð23.64). most Africa and Antarctica is the site of a Cambrian to Permian sedimen- tary succession that was the central part of the “Samfrau geosyncline” of DuToit (1937), and was deformed in the early Mesozoic to form the central al., 1995). Initiation of subsidence of the adjacent southern African margin part of the Pacific margin Gondwanide fold belt. that eventually accommodated the Cape Supergroup also dates from latest The margin of Gondwana immediately opposed to the Ouachita embay- Precambrian or earliest Phanerozoic deposition of the Klipheuwel Forma- ment margin of Laurentia in my hypothetical reconstructions of Rodinia tion that underlies the Ordovician Table Mountain Sandstone (Winter, and Pannotia is the Ellsworth-Whitmore mountains crustal block of 1984; Hiller, 1992; Fig. 13b). The Cape Supergroup did not undergo com- Antarctica (Figs. 11 and 12), restored to what I believe to be its original po- pressive deformation until the early Mesozoic Gondwanide orogeny (Du- sition between Africa and Antarctica using geologic and paleomagnetic Toit, 1937; Hälbich, 1992). The dominant deformation of the succession in data (Schopf, 1969; Watts and Bramall, 1981; Dalziel and Elliott, 1982; the Ellsworth Mountains was also Gondwanide (Webers et al., 1992a), al- Dalziel and Grunow, 1992). Both the Falkland-Malvinas Plateau and the though some evidence of early Paleozoic deformation and a pre-Early De- Ellsworth-Whitmore block have some Precambrian basement of Grenvil- vonian unconformity has been revealed by recent mapping (M. N. Rees, lian age, (Cingolani and Varela, 1976; Millar and Pankhurst, 1987). Early 1996, personal commun.). In contrast, by Cambrian time the Gondwana Cambrian bimodal magmatism, associated with rapid synrift sedimenta- margin was affected by subduction-related magmatism and orogenic de- tion in the Ellsworth-Whitmore block (Fig. 13a) is generally interpreted as formation from the immediately adjacent Pensacola Mountains of Antarc- being related to continental extension (Vennum et al., 1992). A dacitic tica to Australia, and along the proto-Andean margin of South America stock has been dated using the U-Pb zircon method at 535 ± 3 Ma (Rees et (Stump, 1995; Dalla Salda et al., 1990).

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Figure 11. Geologically constrained reconstruction of the hypothetical supercontinent Rodinia formed by amalgamation of older cratons in the global 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 for reference. 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 and c—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. LaurentiaÐEast Gondwana (SWEAT) reconstruction modified from Moores (1991) and Dalziel (1991, 1992a, 1992b) by exclusion of younger terranes in eastern Australia and relocation of Weddell sea margin (CMG) block as discussed in text; Laurentia-AmazoniaÐRio 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 was virtually entirely oceanic, the Mirovia ocean of McMenamin and McMenamin (1990). A—Arequipa massif; AM— ; AV—Aval- onia; B—Baltica (Russian craton); C—; CMG—Coats LandÐMaudheim-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 the Carolina terrane, but see text); F/MP—Falkland-Malvinas Plateau ; K—; 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—; TxP—hypothetical Texas plateau (see text). Omitted are the South and North China blocks and the Tarim block; for discussion of their possible positions within Rodinia, see Li et al. (1995) and Pelechaty (1996). Short red 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.

I suggest that when proto-Appalachian Laurentia rifted from Gondwana and Cenozoic age. The mid-Iapetus ridge would have stepped outboard of close to the Precambrian-Cambrian boundary (Bond et al., 1984; Williams the Texas plateau just as the mid-Atlantic ridge stepped outboard of the Falk- and Hiscott, 1987), it had a continental plateau attached that filled the gap land-Malvinas Plateau (Fig. 9). A ridge crest jump (or jumps) would have between the Cape and Ouachita embayments. This missing “Texas plateau,” taken place to shorten the lengthening northerly transform, as it did in the to distinguish it from the smaller Texas promontory on the margin of the case of the mid-Atlantic ridge around the Falkland-Malvinas Plateau Ouachita embayment (Thomas, 1991, Fig. 6), would have had almost ex- (Barker, 1979). The Alabama-Oklahoma transform of Thomas (1976, 1989, actly the length of the adjacent Falkland-Malvinas Plateau, 1500 km, but 1991) would have been continuous with the original fault bounding the could have been as much as two to three times wider, 800Ð1000 km, an ap- oceanic (now Pacific) side of the Falkland-Malvinas Plateau margin of proximate maximum figure for the present along-strike extent of both the Gondwana, and closely analogous to the Mesozoic Agulhas-Falkland-Malv- combined Ouachita-Marathon margin (Fig. 6a) and for the Precordillera of inas transform fault and its continuation into South America, the Gastré fault northwest Argentina, the latter measured as the full extent of the larger zone (Rapela and Pankhurst, 1992; Figs. 9 and 10a). I refer to it as the Malv- Cuyania terrane (Ramos et al., 1986, 1995; Thomas, 1991; Thomas and As- inas-Alabama-Oklahoma transform (Figs. 12, 14, and 15a). The other tini, 1996). It is important to recall, however, that the present length of the bounding fault of the Texas plateau marks the present-day southern margin Falkland-Malvinas Plateau is its length immediately prior to sea-floor of the Laurentian craton. It was perhaps the precursor of the hypothetical spreading. Hence the dimensions of both the Falkland-Malvinas Plateau Mojave-Sonora megashear (Silver and Anderson, 1974; Anderson and and the hypothetical Texas plateau are reconstructed here after significant Schmidt, 1983), if that does indeed mark the present southern limit of cra- rifting and extension. The opening of the Laurentia-Gondwana portion of tonic Laurentia. Like the Shackleton fracture zone between the present-day the Iapetus ocean basin would, in this scenario, have taken place as an exact Scotia and Antarctic plates (Barker et al., 1991, Fig. 10a), this would have analog to the Mesozoic opening of the South Atlantic Ocean basin between been a fault separating areas of plate growth and plate convergence. I refer to South America and Africa (Figs. 14 and 15). it as the Ellsworth-Sonora-Mojave transform (Figs. 12, 14, and 15a). The Texas plateau would have been bounded by transform faults nearly Paleomagnetic data permit the juxtaposition of Laurentia and West parallel, and directly analogous, to the Falkland-Agulhas and North Scotia Gondwana controlled by the correlation of the Greenland-Scotland- Ridge transforms of today. It seems reasonable to assume that the early Pa- Labrador promontory and the Arica embayment, as in reconstructions of leozoic faults would have had a similar development to those of Mesozoic both Rodinia and Pannotia (Figs. 11 and 12). The work of German and

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. Pan African-Brazilide (Pannotian; Stump, 1987) basins are shown closed, although some of them may have persisted into the Cambrian (see Grunow et al., 1996). Note that Torsvik et al. (1996a) interpreted the paleomagnetic data to require separation of Laurentia and Gondwana by >5000 km at the Precambrian-Cambrian boundary; the interpretation shown here is based primarily on geologic arguments as the paleomagnetic poles shown do not support such close juxtaposition of Laurentia and Gondwana(see text). See Table 2 for rotation parameters. Barbs—upper plates of 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.).

30 Geological Society of America Bulletin, January 1997

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Figure 12.

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Figure 13. (a) Stratigraphy of the Paleozoic Heritage Group of the Ellsworth Mountains, Antarctica (Webers et al., 1992a; but note that there may be an unconformity be- low the Crashsite Group according to M. N. Reese, 1996, personal com- mun.). Note the Lower(?) to Middle Cambrian synrift Union Formation at the base of the section. (b) Stratigraphy of southern Africa (Hiller, 1992). Note the uppermost Precambrian to Cambrian syn-rift Klipheuwel Formation at the base of the section.


South African geologists in Dronning Maud Land indicates that this part of (concordant U-Pb zircon and titanite dates) obtained by Gose et al. (1997) the present-day East Antarctica, immediately south of Africa (Coats from another part of Weddell Sea margin (Coats Land), together with ear- LandÐMaudheimÐGrunehogna block, CMG in Fig. 11), was sutured to the lier results from Dronning Maud Land and the Kalahari craton, demon- Kalahari craton during Grenvillian (Kibaran) orogenesis ca.1000 (Groen- strates that the Coats LandÐMaudheimÐGrunehogna block was part of the wald et al., 1995). This has led me to juxtapose the Kalahari and East Kalahari craton and of West Gondwana, rather than part of East Gond- Antarctic cratons in my earlier attempts to reconstruct early Neoprotero- wana. It could, moreover, have been located off the proto-Appalachian zoic Rodinia (e.g., Dalziel, 1992a, 1992b, 1995). The Kalahari craton is margin of Laurentia (Fig. 11; note comparison of the new Coats Land pole also shown attached to East Antarctica in a new map of Gondwana assem- with a Laurentian pole of the same age). The suture between East and West bly (Unrug, 1996). New paleomagnetic and geochronologic data, however, Gondwana, the East African orogen, must therefore extend to the Pacific indicate that this interpretation is flawed. A well-dated pole for 1110 Ma margin of the East Antarctic craton at the Shackleton Range (Fig. 12),

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Figure 13. (Continued).


where there is evidence of Pan African orogenesis (Marsh, 1983), as sug- Ouachita embayment analyzed by Thomas (1991) may reflect the forma- gested by Grunow et al. (1996) and by Tessensohn (1995). Although un- tion of a basin, the Ouachita rift, directly analogous to the South Malvinas certainty in the width and age of closure of the intra-Gondwana Pan Afri- basin (Turic et al., 1980; Urien et al. 1995; Figs. 6 and 10), rather than a can and Brazilide basins needs to be emphasized (see captions, Figs. 11 basin floored by true oceanic lithosphere. The South Malvinas basin was and 12; Unrug, 1996), the collisional suture formed during latest Precam- formed early in continental separation and overstepped by sediment in the brian to Early Cambrian closure of the Mozambique ocean between East later stages of rifting, just as in the case of the Birmingham graben and West Gondwana is ≈7500 km long. (Thomas, 1991). The San Julian basin of Argentina is in a position adjoin- Recognition that the Grenvillian age rocks of Coats Land and Dronning ing the Agulhas-Falkland-Gastré transform fault that makes it a direct ana- Maud Land were part of West Gondwana rather than East Gondwana as far log of the South Oklahoma fault system (aulacogen) with respect to the back as 1100 Ma negates one of the main lines of evidence of the SWEAT Alabama-Oklahoma transform of early Paleozoic time. The Rawson basins hypothesis for the early Neoproterozoic juxtaposition of the Pacific mar- north of the Agulhas-Falkland-Gastré transform are analogous to the aban- gins of Laurentia and East AntarcticaÐAustralia (Moores, 1991; Dalziel, doned Mississippi graben and Birmingham fault systems. 1991). Such a fit is still broadly supported, however, by the length of the As previously mentioned, there is a significant body of sedimentologic margins (Dalziel, 1991), limited paleomagnetic data (Powell et al., 1993), evidence to indicate that the Ouachita rift, like the Marathon rift (Keller and the global “budget” of probable late Precambrian rifted margins cou- and Dickerson, 1996), was a narrow two-sided basin throughout the early pled with the fact that a Grenvillian age orogen is absent from both of these and middle Paleozoic, with a block to the southeast supplying sediment margins, but present along the proto-Appalachian and proto-Andean mar- from a source like the proto-Appalachian passive margin (Lowe, 1985, gins (Fig. 11). Future tests include the dating and paleomagnetic poles of 1989; Denison, 1995; Dix et al., 1996). In the light of the above discussion, the Australian portion of the proposed ca. 780 Ma giant radiating dike I suggest that the Precordillera, with its Cambrian-Lower Ordovician swarm of Park et al. (1995, Fig. 11). olenellid trilobite fauna, its southeastern Laurentian stratigraphic affinities, To summarize, my suggestion based on geologic and faunal data that the and its Grenvillian basement, was not derived from within the Ouachita Precordillera might have originated as a bathymetric high on a Falkland- embayment. Rather, it originated as a major horst block closely analogous Malvinas Plateau-like feature outboard of the Ouachita embayment of Lau- to the Falkland-Malvinas platform (Lafonian microplate) or the Maurice rentia appears to be independently supported by paleogeographic analysis. Ewing massif within an extended continental plateau ouboard of the Texas The presence of the hypothetical Texas plateau adjoining the Falkland- margin of the Ouachita embayment, the Texas plateau (Fig. 15b). If cor- Malvinas Plateau within Rodinia and Pannotia during the Neoproterozoic is rect, this means that it was still attached to Laurentia at the time it docked not only geometrically in keeping with quantitative reconstructions of those along the proto-Andean margin of Gondwana (Fig. 16; discussed below), supercontinents with the Greenland-Scotland-Labrador promontory match- thereby defining the relative and positions of Laurentia and ing the Arica embayment, but explains the geometry and geology of the Gondwana in the Middle Ordovician and representing a vital key to the his- long-enigmatic Cape of Good Hope embayment and Gondwanide fold belt, tory of the Earth during the Paleozoic Era. which are unique along the 12 000 km Pacific margin of Gondwana. PALEOZOIC TECTONICS Comparison of the structure, rift history, and subsidence history of the early Paleozoic Ouachita embayment and of the Mesozoic Falkland-Mal- Following rifting from Gondwana near the Precambrian-Cambrian bound- vinas plateau is illuminating (Fig. 15b). The rifting and subsidence in the ary, rapid motion of Laurentia across lines of latitude to an equatorial position

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during the Cambrian is recognized to require a relatively fast sea-floor-spread- subduction of Iapetus oceanic lithosphere beneath Gondwana, Laurentia ing rate. Even without longitudinal motion, it approached the rate of motion of must have been moving in a net dextral sense, so that the Texas plateau that India during the Mesozoic Era (Meert et al., 1993). It is interesting to note that was detached from the Cape embayment eventually collided with the Laurentia did not develop a “bow wave” continental margin orogen like the proto-Andean margin of South America in the position of the present-day Mesozoic and Cenozoic North American and South American cordilleras. Precordillera in the early Middle Ordovician, ca. 465 Ma (Figs. 14Ð16). Pa- Those formed along the leading edge of the continents as they moved forward leomagnetic data permit a maximum separation of Laurentia and the South relative to the mantle over eastward-dipping subduction zones, causing com- American part of the Gondwana craton margins of ≈2000 km in Middle pression along the leading edge of the upper plate (Dalziel, 1986; Coney and Ordovician time if they were at the same longitude (Fig. 16). This is Evenchick, 1994; Russo and Silver, 1996). The absence of such an orogen slightly greater than the extended length allowable for the Texas plateau if along the Pacific margin of early Paleozoic Laurentia suggests that the conti- it originated between the Texas margin of the Ouachita embayment and the nent lay at that time within a larger plate, and that convergence during the Ellsworth-Whitmore Mountains block of the Antarctic margin (Fig. 12). opening of Iapetus between Laurentia and South America ocurred elsewhere Although precise dating remains to be done, studies of major shear zones within or beyond the Pacific Ocean basin. Rapid Mesozoic motion of the In- in Precambrian basement of Argentina to the east of the Precordillera indi- dian plate appears to be due to its being driven by both ridge push to the south cate top to the west thrusting with no significant component of strike-slip and slab-pull to the north. The same situation may have affected Cambrian motion (Simpson et al., 1995). This suggests that docking of the Pre- Laurentia, and may have induced rifting and subsidence along its Pacific mar- cordillera may have involved dominantly down-dip subduction. gin, leading to the “double rift event” paradox and the initiation of the lower Two main objections have been raised to the Laurentia-Gondwana col- Paleozoic miogeocline (Stewart, 1972; Bond et al., 1984; Link et al., 1993). lision proposed by Dalla Salda et al. (1992a, 1992b) and amplified by The most obvious location for the subduction zone is along its East Gond- Dalziel et al. (1994): (1) it contradicts the faunal and paleoenvironmental wana margin, the Ross-Delamerian arc (Fig. 12). differences between the tropical to equatorial Laurentian craton and the bo- Separation of Laurentia and Gondwana had to be sufficiently great by late real Gondwana craton, and (2) it barely permits room for the island-arc Early Cambrian time (ca. 520 Ma) for the Laurentian benthic fauna to de- systems known to have collided with Laurentia in the New EnglandÐCana- velop in isolation. However, a previously unexplained mingling of North dian Maritimes region, and probably farther south, during Ordovician time American and Australo-Sinian biofacies is represented by the Aphelaspis (see Astini et al., 1995; Torsvik et al. 1995; van der Pluijm et al., 1995; zone trilobites and by in the Ellsworth Mountains of Antarctica Mac Niocaill et al., 1996, 1997; Dalziel and Dalla Salda, 1996a, 1996b; (Webers and Yochelson, 1989; Shergold and Webers, 1992; Webers et al., Astini et al., 1996a; Torsvik et al., 1996b ). The Texas plateau hypothesis, 1992b). Hence the proximity suggested here for the southern cone of Lau- I submit, overcomes these problems and at the same time addresses “. . . a rentia and the Cape embayment of Gondwana during the Cambrian (Figs. 12 major problem in Appalachian geology . . . to quantify a realistic causative and 14) has some paleobiologic support. If, as suggested by the molecular bi- mechanism for the Taconic deformation in the central and southern Ap- ology studies of Wray et al. (1996), metazoan divergences between phyla palachians” (Drake et al., 1989, p. 162). date back well into the Precambrian, isolation of the Laurentian trilobite Like the microcontinental model, the Texas plateau hypothesis is com- fauna might be required even earlier than late Early Cambrian time. patible with the following paleontologic facts: (1) a Laurentian benthic trilo- A major difference between the Paleozoic Iapetus ocean and the Meso- bite fauna in the Precordillera through Cambrian, Tremadocian, and early zoic South Atlantic Ocean basin lies in the rapidity with which the South Arenigian time: shallow marginal ridges are characteristic of transform con- American margin of Iapetus became an active consuming margin. This tinental margins like the Falkland-Malvinas Plateau and West Africa happened relatively early in Cambrian time, ca. 530 Ma; subduction-re- (Lorenzo and Mutter, 1988; see Figs. 9 and 10) and would permit a Lau- lated magmatism apparently continued until the appearance of what are in- rentian fauna to exist in suitable habitats along the length of the plateau; terpreted as collisional granitoids in Middle Ordovician time (e.g., Dalla (2) Middle to Late Cambrian cosmopolitan pelagic agnostid trilobites in the Salda et al., 1990; Pankhurst et al., 1996; Sato et al., 1996; Toselli et al., Precordillera: as previously mentioned, the analogous Falkland-Malvinas 1996). A possible explanation of this phenomenon may lie in the nature of Plateau was inundated with deep water from the Southern Ocean during the the present-day Shackleton fracture zone (Fig. 10a). At its northern end, the during subsidence after its separation from Africa (Barker Shackleton fracture zone has started to subduct the Antarctic plate beneath et al., 1977); (3) incoming tropical Gondwanan fauna to the Precordillera by the Scotia plate as South America moves west with respect to the mantle, the Arenigian Epoch: the Texas plateau approached the subducting Pam- Africa, and Antarctica. Gondwana moved over the South Pole during latest pean margin at that time (Fig. 15, a and b), and modern larvae can Proterozoic to Cambrian time (Figs. 12 and 14), and the hypothetical an- migrate for up to ≈1000 km (see discussion in Dalziel et al., 1996; Thomas cestor of the continuation of the Mojave-Sonora megashear along the mar- and Astini, 1996); (4) pelagic “Pacific” graptolites in the Precordillera as gin of the Falkland-Malvinas Plateau, the Ellsworth-Mojave-Sonora trans- late as Llandeilian time: even after collision of the tip of the Texas plateau form (Fig. 12), may have changed to a subduction zone in like fashion, with the Pampean subduction zone, its length would have been open to low- propagating northward (in present coordinates) along the proto-Andean Ia- latitude circum-Laurentian ocean waters (Figs. 15 and 16); (5) benthic petus margin during Cambrian time. As Laurentia moved to the equator Gondwana lichid, pterygometopid, and calyminid trilobites first spread to during the Cambrian, the Gondwana supercontinent moved across the Laurentia in the Middle Ordovician (Droser et al., 1996): that is the time of South Pole. In Vendian time western Amazonia (present coordinates) was suggested convergence of the two continental masses; (6) cool water Hir- located over the pole; during the Cambrian it was the West African craton nantian faunas and evidence of glaciation in the Precordillera by Carado- (Figs. 12 and 14), and by the Early Ordovician the pole lay near the the cian-Ashgillian time: the Precordillera was isolated within high-latitude northern coast of Africa (Fig. 15). Thus if, following Dalla Salda (1992a, Gondwana after collision of the Texas plateau in Middle Ordovician time, 1992b) and Dalziel et al. (1994) and as suggested here, the proto-Appala- and rifting in the Caradocian (Figs. 16, 17, and following discussion). chian and proto-Andean margins were at the same longitude, the interven- Recent criticism notwithstanding, the Texas plateau hypothesis for col- ing Iapetus oceanic lithosphere would have contracted between the two lision of an asperity of Laurenia with the proto-Andean margin of Gond- during the Cambrian and Early to Middle Ordovician epochs by subduc- wana appears to accommodate the known South American and Laurentian tion beneath the Pampean magmatic arc. arc terranes within the Ordovician Iapetus ocean basin, in accordance with According to the scenario suggested here, during the early Paleozoic the faunal and paleomagnetic data of Van der Voo (1993), van der Pluijm

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et al. (1995), Harper et al. (1996), Mac Niocaill et al. (1996, 1997), Torsvik vided into at least three parts (Figs. 15b and 16): (1) an easternmost (rela- et al. (1996a), and Williams et al. (1996). This is demonstrated in Fig- tive to present-day Texas) passive margin that rifted from the Cape embay- ures 15Ð17. The change in facies and fauna of the Precordilleran strata to ment and is now subducted beneath cratonic South America; (2) a horst with cooler water peri-Gondwana conditions (Astini et al., 1995, 1996b; carbonate capping that became the Precordillera after limited subduction of Benedetto et al., 1995; Thomas and Astini, 1996) prior to Middle Ordovi- its passive margin; and (3) the Ouachita-Marathon rift basin. cian docking appears to pose a problem with both microcontinental and The passive margin and the Precordilleran horst would have been short- Texas plateau models, and incompatability of the paleomagnetic and geo- ened in the Ocloyic collision and further shortened during the post-Ocloyic logic evidence. The situation is not, however, vastly different from the pre-Andean deformation along the proto-Andean margin, as well as in the present-day relative proximity of the southern limit of the Great Barrier Mesozoic formation of the Andean Cordillera (Astini et al., 1996b; Ramos Reef and the cold-temperate waters around Tasmania, especially given the et al., 1996). The eastern shoulder of the Ouachita basin may have under- fact that the first Gondwana faunas in the Precordilleran platform were gone some Ocloyic shortening prior to Landeilian-Caradocian rifting that warm water forms. separated the Precordillera. Such shortening could explain the reported The Taconic orogeny is generally regarded as one involving diachronous outboard early to middle Paleozoic sediment sources in the Ouachita- subduction, ophiolite obduction, and arc collision along the almost 4000 km Marathon embayment (Lowe, 1985, 1989; Denison, 1995; Keller and length of the Appalachian mountains without continent to continent colli- Dickerson, 1996), including the influx of sand represented by the Middle sion (Hatcher, 1989). The nearly contemporaneous Ocloyic phase of the Ordovician “Mountain Fork” clastic wedge of the Ouachita allochthon, Famatinian orogen along the proto-Andean margin of South America also Oklahoma (Dix et al., 1996). The Laurentian remnants of the Texas plateau involves subduction, obduction of ophiolites, and arc collision. In this re- were shortened during the Ouachita-Alleghanian collision of Laurentia and finement of the proposal by Dalla Salda et al. (1992a, 1992b), that the two northwestern Africa, and then extended during Pangea breakup when they orogens were once continuous and resulted from Laurentia-Gondwana col- subsided again. It is posssible that some remnants of the Texas plateau lision, I suggest that actual craton to craton contact may have been limited were also incorporated into other pre-Andean terranes such as Chilenia to the docking of the extremity of the Texas plateau, the Precordillera, (Ramos et al., 1986). within Gondwana, and possibly the docking of the Carolina terrane, which Following the separation of the Precordillera, Laurentia continued its has strong Gondwanan affinities (Secor et al., 1983), within Laurentia. Pa- clockwise relative motion around the Pacific margin of Gondwana, as dis- leomagnetic data place the latter at ≈23¡ south latitude during the Middle to cussed by Dalziel et al. (1994), colliding with the various exotic terranes of Late Ordovician (Vick et al., 1987), which means that it could have docked the Canadian Maritimes and southern British Isles (Fig. 17). Collision with as part of the Gondwana margin in the vicinity of the Famatinian-Puna arc Baltica (the Russian craton) at the end of the Silurian formed Laurussia, or arcs (Conti et al., 1996) during the Middle Ordovician collision of the which amalgamated with Gondwana and Siberia (the Angara craton) to Texas plateau (Fig. 16). A major Taconic foredeep is developed to the west form the Pangean supercontinent that can be reconstructed using the mag- of the Carolina terrane (Shanmugam and Lash, 1982). netic anomaly and structural fabrics of the present-day ocean basins. The I regard the proposed Ordovician interaction between the Laurentian apparent rapid relative motion of Laurentia and Gondwana between the and Gondwana plates as analogous to the present-day convergence be- Late Ordovician and Silurian time (Figs. 16 and 17) is somewhat puzzling, tween the Indo-Australian and Eurasian plates north of Australia. The In- and may reflect poor resolution in the Gondwana paleomagnetic record for donesian orogen, with which the Taconic orogen is frequently compared the interval, as discussed elsewhere (McKerrow and Scotese, 1990; Van der by others (e.g., Shanmugam and Lash, 1982) is a complex orogenic collage Voo, 1993; Dalziel et al., 1994). involving subduction and arc magmatism, ophiolite obduction, and the col- An important line of evidence bearing on Paleozoic geography lies in the lision of the New Guinea asperity of the Australian continent (Fig. 11) with distribution K-bentonites. These altered volcanic-ash deposits are common southeastern Asia (Hamilton, 1979). If the Precordillera was still attached in Ordovician strata of eastern and midwestern Laurentia, East Avalonia to Laurentia as part of the Texas plateau at the time of Middle Ordovician (England and Wales), Baltica, and the Precordillera. It has even been sug- docking with Gondwana, the combined Taconic-Famatinian (Ocloyic) oro- gested that particularly well developed Caradocian bentonite layers of Lau- gen would have been every bit as complex, a major orogen (Fig. 16) with rentia and Baltica that correlate very closely in time might have resulted the southern Appalachians continuing into the Famatinian belt (the pierc- from the same “ultraPlinian” explosive eruption. This is difficult to envis- ing points of Dalla Salda et al., 1992b), although perhaps offset by a major age in the geographic scenario presented here, as pointed out in a review by transfer zone along the Malvinas-Alabama-Oklahoma fault that I suggest Huff et al. (1995), unless there is an undiscovered equivalent layer in north- bounded the Texas plateau. I refer to the ephemeral Laurentia-Gondwana ern South America. A search for K-bentonites in the Precordillera has re- supercontinent of the Middle Ordovician (Fig. 1) that would have resulted vealed several layers in the lower Llanvirnian part of the section and one in from the docking of the Texas plateau as “Artejia,” thereby reflecting the the Caradocian (Huff et al., 1995; Bergstrom et al., 1996). They are thus geologic connection that appears to suggest its former existence, as well as older than almost all of those in Laurentia. The possible discovery of K-ben- the hispanic background of both Argentina and Texas. tonites in Ordovician strata of the Marathon region of southwestern Texas, What of the Late Ordovician separation of the Precordillera from Lau- first in the entire state, was reported by Patricia Dickerson at the Geological rentia, and what became of the part of the hypothetical ≈1500-km-long Society of America South-Central Section Meeting in Austin, Texas (see Texas plateau that is not represented in the Precordillera? The only hard ev- Keller and Dickerson, 1996), and has considerable potential significance as idence of rifting involving the formation of oceanic lithosphere between the there is no obvious Laurentian source. Preliminary study of clay minerals, Precordillera and Laurentia is, as mentioned previously, the presence along however, has cast some doubt on the presence of K-bentonites in west Texas its Pacific margin of mafic pillow lavas with mid-ocean ridge basalt chem- (W. D. Huff, 1996, personal commun.). The full paleogeographic meaning istry interlayered stratigraphically with graptolite-bearing Llandeilian- of these important volcanic layers is unlikely to emerge without a substan- Caradocian sedimentary strata (Ramos et al., 1986). By general comparison tial amount of further study on several continents, as emphasized by Huff et with the present-day physiography of the Falkland-Malvinas Plateau, and in al. (1995), and by Samson (1996), in an article pointing out geochemical keeping with the horst and graben interpretation of Keller (1995) and Keller reasons for a possible South American source for Laurentian K-bentonites. and Dickerson (1996) for the Precordillera during the Cambrian and Early A search for K-bentonites needs to be undertaken in the cratonic areas of Ordovician, I therefore suggest that the Texas plateau may have been di- northern South America to fill in this emerging picture.

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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 along the 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 of Delamerian-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 in Conti 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) are shown with arrows. Compare the structure of the early Paleozoic Ouachita embayment (Thomas, 1991; Fig. 6a) and hypothetical Texas plateau with that of the Mesozoic southern cone of South America and the Falkland-Malvinas Plateau (Urien et al., 1995; Fig. 10a). Basins and passive margins 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—Rawson basins; 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 the proto-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 represents the 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.

Figure 17. Latest OrdovicianÐEarly 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.

SPECULATION: ENVIRONMENTAL AND BIOLOGICAL Full discussion of possible tectonic causes of the environmental changes IMPLICATIONS that accompanied the emergence of soft-bodied metazoans during the Neo- proterozoic following nearly 3.0 b.y. of single-celled life, the possible di- “Natural selection, the cornerstone of the Neodarwinian paradigm, con- vergence of chordates from invertebrates early in Neoproterozoic time, the cerns the interaction between heritable variation and the environment.” Af- Cambrian “explosion” of skeletalized , and the Middle Ordovician ter making this opening statement in his discussion of the environmental radiation of marine biota, is far beyond the scope of this review. The pale- context of evolutionary change, Knoll (1991, p. 77) emphasized that the ogeographic reconstructions presented here for Neoproterozoic and early period of major biological innovation at the end of the Proterozoic Eon, Paleozoic time do, however, permit evaluation of the possible magnitude was also a time of “profound tectonic, climatic, and biogeochemical of the major tectonic events that occurred during that critical time interval. change.” Intense multidisciplinary studies, undertaken all around the globe This may eventually lead to a better understanding of the factors responsi- over the past decade particularly as a result of the activities of International ble for the environmental changes, and hence perhaps for the unique bio- Geologic Correlation Project No. 320 on “Neoproterozoic Events and Re- logic revolution. It is only by taking a global view of the paleogeographic sources,” have generated a remarkable faunal, climatic, and geochemical changes that we can start to consider the long-term evolution of the whole record. The marked geochemical changes, in particular (Asmerom et al., Earth system. Is it a coincidence, for example, that the global Grenvillian 1991; Brasier, 1992b; Kaufman et al., 1993; Montañez et al., 1996; orogeny that resulted in the amalgamation of Rodinia at the end of the Nicholas, 1996), but also major climatic change such as the growth of ex- Mesoproterozoic, however that supercontinent was configured, marks a tensive ice sheets (Young, 1995), are often ascribed to tectonic causes such major change in Earth’s biosphere; i.e., an increase in the abundance and as rifting, ocean basin formation, and orogenesis. This link to tectonics, diversity of eukaryotic organisms (Knoll, 1994; Knoll and Sergeev, 1995), however, has remained vague. As pointed out by Brasier (1996), there ap- and the conjectured time of divergence of major metazoan phyla (Wray et pears to be an important underlying dichotomy of opinion: do the litho- al., 1996)? logic changes of the times reflect biologic change, a sort of “Gaian” con- A relationship between environmental changes and Gondwana-wide trol, or are all the changes fundamentally driven by tectonics, by the Neoproterozoic to earliest Paleozoic Pan African and Brazilide (Pannotian) planet’s internal thermal energy? orogenesis has been suggested by many. This appears even more likely

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Figure 14. Figure 15.


Figure 16.


Figure 17.

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

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pheric oxygen (Knoll and Walter, 1992). The timing of discrete events tions in the rise of sea level. Some of these could perhaps be explained by along the East African orogen, as well as the other Pannotian belts, needs the hypothetical Laurentia-Gondwana interaction discussed earlier. The to be refined before their environmental effects can be more thoroughly suggestion of a relationship between orogenesis and regression goes back evaluated. to the views of Stille in the 1920s (for a historical review, see Hallam, There have been two major cycles of eustatic rise and fall in sea level, 1992). Many workers have studied the possibility of a causative relation- the first-order or supercycles, during the Phanerozoic Eon (Fischer, 1984; ship between the Taconic orogeny, epeirogenic movements in the Laurent- Hallam, 1992). The more recent cycle started with the fragmentation of ian interior, and sea-level change during the Ordovician (e.g., Quinlan and Pangea in the Jurassic and extends to the present. The cause of the eustatic Beaumont, 1984; Howell and van der Pluijm, 1990; Coakley and Gurnis, rise during the Cretaceous Period is most likely to have been the rapid vol- 1995). The two main problems are timing and the mechanism. Taconic ume increase of oceanic spreading ridges and volcanic plateaus (Kominz, orogenesis is diachronous, and arc-continent collision may not generate a 1984; Larson, 1991). The fall corresponds in time to the late Cenozoic large enough drop in sea level to explain observations around the . Alpine-Himalayan continent-continent collision and the growth of the The possibility that the Taconic orogen in North America was part of a Antarctic ice sheet, the former resulting in an increase in the volume of the much longer mountain belt that involved at least local continent-continent ocean basins, and the latter, possibly causatively linked, extracting water collision suggests to me that these problems should be reevaluated. My from the oceans. The Paleozoic cycle extends from the time of separation colleagues and I have pointed out the coincidence in time of the sequence of Laurentia from the amalgamating Gondwana, i.e., the breakup of the hy- boundary in Gondwana and Laurentia (Dalziel et al., 1994). We drew at- pothetical Pannotia (Fig. 12), to the amalgamation of Pangea. The conti- tention to the fact that continent-continent collision transmits compressive nent-continent collisions along the Caledonian-Scandian and Ouachita-Al- stress far into cratonic interiors (Craddock and van der Pluijm, 1989). Per- leghanian-Hercynian-Uralian sutures may be causatively linked to the haps the sequence boundaries record the time of direct continent-continent development and growth of the late Paleozoic Gondwana ice sheet (Pow- interaction, while the complexity of events in the orogens themselves blur ell and Veevers, 1987; Raymo, 1991). the time signal. For example, the limited expression of the Sauk-Tippeca- By analogy with the Cretaceous, the global rise in sea level that started noe unconformity near the Ouachita embayment, and its absence in the during the Early Cambrian is most likely to have been the result of the Precordillera, may reflect local flexure related to collision of the Texas growth of new, buoyant oceanic spreading centers. The reconstruction of plateau with Gondwana. Convergence of two major continental masses in- Pannotia suggested here yields a hypothetical length of mid-Iapetus volving not only arc and terrane collision, but also collision of asperities spreading ridges of nearly 10 000 km (LaurentiaÐSouth America, Lauren- like the hypothetical Texas plateau, will have a greater likelihood of in- tia-Baltica, and Laurentia-Siberia). The new lithospheric surface was bal- creasing the volume of the ocean basins and hence of lowering sea level, anced by the loss of surface area along the Ross-Delamerian and Pampean than will arc-continent collision alone. By comparison with the Cretaceous subduction zones, but the volume increase at a new trench is small com- history of the Andes, the deformation in the cratonic interior of North pared with the increase along an equal length of new spreading ridge America and volume of siliciclastic sediments spreading westward from (Southam and Hay, 1977). Hence, the major Cambrian transgression of the Appalachians in Middle to Late Ordovician time seems difficult to rec- ocean water across the continents was probably driven by the tectonic oncile with a mere arc-continent collision. processes responsible for the fragmentation of Pannotia. A marked de- The possibility of a causal link between Middle Ordovician orogenesis crease in the generally rising 87Sr/86Sr ratio of global seawater at the Ne- and the diversification of marine biota through the development of new makit-DaldynianÐTommotian boundary (530Ð535 Ma, Nicholas, 1996; habitats has been raised by Miller and Mao (1995). If the orogenic event Fig. 1) could reflect hydrothermal activity along the newly formed mid-Ia- was on a larger scale than previously envisioned and involved Laurentia- petus spreading ridge. Gondwana collision to at least a limited extent, any such effect would have If supercontinental amalgamation and fragmentation, the latter being ac- been more pronounced. Moreover, an orogenic event of this magnitude companied by decrease in ocean volume and transgression, have occurred could also have a major effect on world climate. The Late Ordovician several times in the history of the planet, why should an explosive burst of glacial event has long been somewhat enigmatic, coming as it did during a multicellular hard-skeletal life have accompanied latest Neoproterozoic “greenhouse” stage associated with the early to middle Paleozoic eustatic growth of new ocean basins? Why did this not occur 250 m.y. earlier dur- sea-level rise (Crowley and Baum, 1991). Nonetheless it has been linked to ing the breakup of Rodinia and the opening of the vast Pacific Ocean the Taconic orogeny in North America through “drawdown” of carbon diox- basin? The explanation must surely lie in the interaction between the tec- ide (Raymo, 1991; Kump et al., 1996). If the Taconic event in North Amer- tonic development of the solid earth, the chemical development of the at- ica was merely part of a more extensive orogen like that between Australia mosphere, and the stage of biologic evolution. At the time of the fragmen- and Asia today, this would perhaps make the causative connection far more tation of Rodinia, were at an early stage of development (Knoll likely and strengthen the case for a global connection between orogenic epi- and Sergeev, 1995) and the atmosphere appears to have contained too little sodes and glaciation through geologic time envisaged by Chamberlin oxygen to support multicellular life (Knoll and Walter, 1992). When (1899). As pointed out by Bickle (1996), however, carbon dioxide is both Pangea broke up, most of the metazoan ecological niches had been filled, released and absorbed in orogenesis, so this is a complex problem. despite the biologic crisis at the end of the Paleozoic Era. However, the Ia- petus ocean basin formed within the Pannotian supercontinental assembly CONCLUSIONS ≈200 m.y. later than the breakup of Rodinia, after the initial development of soft-bodied metazoans, notably the Ediacaran fauna (Grotzinger et al., Geologic considerations have led to testable hypotheses for the config- 1995), and possibly after the divergence of metazoan phyla (Wray et al., uration of pre-Pangea supercontinents in the early to mid-Neoproterozoic 1996). The oxygen content of the atmosphere had passed a critical thresh- (Rodinia), and the latest Neoproterozoic (Pannotia). These were separated old (Knoll and Walter, 1992), and the advancing Cambrian seas appear to by the opening of the Pacific Ocean basin and the amalgamation, or nearly have opened up a myriad of unoccupied habitats, permitting the unique complete amalgamation, of Gondwana. The critical breakthrough in the “explosion” of skeletalized metzaoans of phyla already established. development of the hypotheses was recognition that the ancestral North During the early to middle Paleozoic transgressive phase of the Panno- American craton, Laurentia, may formerly have been juxtaposed with tia-Pangea eustatic sea-level cycle, there were several important interrup- southern continents of the present day.

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Traditional geographic reconstructions that juxtapose the proto-Appala- Laurentia, the proto-Andean margin, Baltica, and Siberia at the Precam- chian margin of Laurentia with northwestern Africa in the latest Protero- brian-Cambrian boundary, initiated the first order cycle of eustatic sea- zoic, and keep the two margins at the same longitude through the early Pa- level change that lasted through the Paleozoic Era. The tectonically driven leozoic, may be incorrect. Recognition that the Cambrian-Ordovician Cambrian transgression, coming at a time following the rise of the oxygen carbonate platform composing the Precordillera of northwest Argentina content of the atmosphere and the evolution of soft-bodied metazoans, may was part of Laurentia, and that it could have been detached from the gen- have been critical in the “explosion” of hard-shelled animals. eral area of the Ouachita embayment, implies some close interaction be- Eustatic sea-level drops and epeirogenic movements during Ordovician tween the proto-Appalachian and proto-Andean margins throughout that time may have resulted from a complex Taconic-Ocloyic collision involv- biologically critical time interval. This would have been the global setting ing the Laurentian and Gondwana cratons; a significantly larger tectonic within which the well-established subduction, ophiolite obduction, and event than the arc-continent collision that is well-documented in Lauren- arc-continent collision of the Taconic orogenesis took place. tia. This collision may also have played a role in the Middle Ordovician ra- The hypothesis that the Precordillera originated as a microcontinent de- diation of marine fauna, as well as in triggering the Late Ordovician glacia- tached from within the Ouachita embayment, may also be incorrect. tion of Gondwana. Rather, when Laurentia separated from the proto-Andean margin of Pan- notia to open Iapetus in the latest Precambrian to Early Cambrian, it may ACKNOWLEDGMENTS have done so in a manner analogous to the Mesozoic opening of the South Atlantic Ocean basin. An ≈1500 km × 800Ð1000 km continental plateau The work was supported by National Science Foundation Grants may have extended into the Iapetus ocean basin from the present site of the EAR-94 18236 and OPP-91 17996 and the Institute for Geophysics and Ouachita embayment. The “Texas plateau” could have originated along- Geology Foundation of the University of Texas at Austin. I thank Robert J. side the Falkland-Malvinas Plateau of the present day, its tip rifted from the Hatcher, Jr. and A. R. (Pete) Palmer for constructive reviews that signifi- Ellsworth-Whitmore crustal block of Antarctica in its restored position ad- cantly improved the manuscript. I also benefitted from informal reviews, jacent to southernmost Africa. This was a unique location along the Pacific comments, and discussion with many colleagues. In particular I thank margin of Gondwana, the site of a Cambrian-Permian passive margin in Richard Buffler, Carlos Cingolani, John Cooper, Luis Dalla Salda, Roger southernmost Africa and the early Mesozoic Gondwanide fold belt. The (Tim) Denison, Patricia Dickerson, Robert H. Dott, Jr., Stanley Finney, faults bounding the Texas plateau can be viewed as analogs to those bound- Anne Grunow, Mark Helper, Warren Huff, Frederick Hutson, Dennis Ko- ing today’s Falkland-Malvinas Plateau. Basins within the Mesozoic Falk- lata, Larry Lawver, Z. X. “Lee” Li, Sharon Mosher, William Muehlberger, land-Malvinas Plateau also appear to be analogous in their tectonic setting Samuel Mukasa, Robert Neuman, Chris Powell, Victor Ramos, Margaret to those of the early Paleozoic Ouachita embayment. Rees, William Thomas, Cees van Staal, and Trond Torsvik. Anne Grunow, Subduction started along the proto-Andean and Antarctic-Australian Conan Mac Niocaill, and their colleagues kindly furnished copies of man- margins early in Cambrian time as Gondwana moved over the south pole. uscripts in press. Responsibility for the interpretations in the article, how- Rapid motion of Laurentia to equatorial latitudes may have been driven by ever, is mine alone. The reconstructions could not have been done without a combination of ridge push from the mid-Iapetus ridge and trench pull the outstanding and generous help and advice of Lisa Gahagan of the from a subduction zone within or bounding the Pacific ocean basin. The PLATES project at the Institute for Geophysics, University of Texas at only well-established convergence zone of the time was the Ross-Del- Austin. Drafting was carried out by Gwen Watson. amerian arc in Antarctica and Australia. 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40 Geological Society of America Bulletin, January 1997

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