Showing papers on "Gondwana published in 1999"

Paleogeography of the Caribbean region : implications for Cenozoic biogeography. Bulletin of the AMNH : no. 238

Abstract: "This paper presents a series of detailed paleogeographical analyses of the Caribbean region, beginning with the opening of the Caribbean basin in the Middle Jurassic and running to the end of the Middle Miocene. Three intervals within the Cenozoic are given special treatment: Eocene-Oligocene transition (35-33 Ma), Late Oligocene (27-25 Ma), and early Middle Miocene (16-14 Ma). While land mammals and other terrestrial vertebrates may have occupied landmasses in the Caribbean basin at any time, according to the interpretation presented here the existing Greater Antillean islands, as islands, are no older than Middle Eocene. Earlier islands must have existed, but it is not likely that they remained as such (i.e., as subaerial entities) due to repeated transgressions, subsidence, and (not incidentally) the K/T bolide impact and associated mega-tsunamis. Accordingly, we infer that the on-island lineages forming the existing (i.e., Quaternary) Antillean fauna must all be younger than Middle Eocene. The fossil record, although still very poor, is consistent with the observation that most land mammals lineages entered the Greater Antilles around the Eocene-Oligocene transition. Western Laurasia (North America) and western Gondwana (South America) were physically connected as continental areas until the mid-Jurassic, ca. 170 Ma. Terrestrial connections between these continental areas since then can only have occurred via landbridges. In the Cretaceous, three major uplift events, recorded as regional unconformities, may have produced intercontinental landbridges involving the Cretaceous Antillean island arc. The Late Campanian/Early Maastrichtian uplift event is the one most likely to have resulted in a landbridge, as it would have been coeval with uplift of the dying Cretaceous arc. However, evidence is too limited for any certainty on this point. The existing landbridge (Panamanian Isthmus) was completed in the Pliocene; evidence for a precursor bridge late in the Middle Miocene is ambiguous. We marshal extensive geological evidence to show that, during the Eocene-Oligocene transition, the developing northern Greater Antilles and northwestern South America were briefly connected by a "landspan" (i.e., a subaerial connection between a continent and one or more offshelf islands) centered on the emergent Aves Ridge. This structure (Greater Antilles + Aves Ridge) is dubbed GAARlandia. The massive uplift event that apparently permitted these connections was spent by 32 Ma; a general subsidence followed, ending the GAARlandia landspan phase. Thereafter, Caribbean neotectonism resulted in the subdivision of existing land areas. The GAARlandia hypothesis has great significance for understanding the history of the Antillean biota. Typically, the historical biogeography of the Greater Antilles is discussed in terms of whether the fauna was largely shaped by strict dispersal or strict continent-island vicariance. The GAARlandia hypothesis involves elements of both. Continent-island vicariance sensu Rosen appears to be excludable for any time period since the mid-Jurassic. Even if vicariance occurred at that time, its relevance for understanding the origin of the modern Antillean biota is minimal. Hedges and co-workers have strongly espoused over-water dispersal as the major and perhaps only method of vertebrate faunal formation in the Caribbean region. However, surface-current dispersal of propagules is inadequate as an explanation of observed distribution patterns of terrestrial faunas in the Greater Antilles. Even though there is a general tendency for Caribbean surface currents to flow northward with respect to the South American coastline, experimental evidence indicates that the final depositional sites of passively floating objects is highly unpredictable. Crucially, prior to the Pliocene, regional paleoceanography was such that current-flow patterns from major rivers would have delivered South American waifs to the Central American coast, not to the Greater or Lesser Antilles. Since at least three (capromyid rodents, pitheciine primates, and megalonychid sloths) and possibly four (nesophontid insectivores) lineages of Antillean mammals were already on one or more of the Greater Antilles by the Early Miocene, Hedges' inference as to the primacy of over-water dispersal appears to be at odds with the facts. By contrast, the landspan model is consistent with most aspects of Antillean land-mammal biogeography as currently known; whether it is consistent with the biogeography of other groups remains to be seen"--P. 3.

Neoproterozoic continental arc magmatism in west-central Madagascar

Abstract: Published geologic maps, regional geological relations, and new U-Pb ages for intrusive igneous rocks in west-central Madagascar define a 450-km-long belt of rocks emplaced in middle Neoproterozoic time. We report precise U-Pb zircon and baddeleyite ages for 11 coeval gabbro and granitoid plutons from the Itremo region, located in the southern part of this belt. The ages for all gabbroic and granitic plutons and deformed equivalents define an ∼25 m.y. period of magmatic activity between 804 and 779 Ma (at maximum uncertainty). Granitoids intrusive into the Quartzo-Schisto-Calcaire series provide a minimum depositional age of 791 Ma for this Mesoproterozoic platformal sedimentary sequence. Our results, combined with other recent U-Pb age determinations, define a belt of plutonic rocks in west-central Madagascar emplaced between 804 and 776 Ma. We propose that these middle Neoproterozoic rocks constitute the root of a continental magmatic arc emplaced at the time of, or slightly preceding, the breakup of the Rodinian supercontinent. Neoproterozoic plate reconstructions place Madagascar on the putative margin of Rodinia, and therefore the plutonic belt in west-central Madagascar provides important constraints on the timing and extent of middle Neoproterozoic tectonic events in Rodinia and the critical period of Rodinia9s transformation into Gondwana.

Chronology of deformation within the turbidite‐dominated, Lachlan orogen: Implications for the tectonic evolution of eastern Australia and Gondwana

Abstract: Ar-Ar data from fabric-forming white mica in slates, syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism, deformation, and exhumation in the Lachlan orogen, Australia These data also help define the tectonic evolution of the Tasmanides during Paleozoic time The Lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to Gondwana during the closing of a small marginal ocean or back arc basin This tectonic setting was similar to the present western and southwestern Pacific region Accretion of the Lachlan orogen to Gondwana occurred by closing of the basin system by subduction-accretion processes and some translation The process is typified in the western Lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement/melange zone and is recorded by Ar-Ar mica ages ranging from ∼455 Ma in the western part to ∼390 Ma in the eastern part In the central Lachlan orogen, deformation migrated southwestward from ∼440–430 to 405 Ma away from the high-temperature Wagga-Omeo metamorphic complex, where deformation/metamorphism occurred between >440 and 400 Ma In the north, ∼400 Ma mica ages record deformation and inversion of structures in the Cobar basin In the eastern Lachlan orogen, Ar-Ar mica dates range from 450 to 340 Ma Ages of 455–445 Ma are yielded by the Narooma accretionary complex, 405–390 Ma ages are found along the major thrust faults bounding high-grade metamorphic complexes, and 360–340 Ma cooling ages are found in the inverted extensional basins (eg, Hill End) and related structural zones The Ar-Ar results also document periods of reactivation on early-formed structures during later deformation elsewhere in the orogen

Late Ordovician–Early Silurian continental collisional orogeny in southern Mexico and its bearing on Gondwana-Laurentia connections

Abstract: New zircon and monazite U-Pb data, tectonic mapping, and petrologic studies in key units of the Acatlan Complex show a previously undocumented phase of continental collision orogeny of Late Ordovician–Early Silurian age in southern Mexico. The event involved the partial eclogitization of oceanic lithosphere and continental crust, which traveled westward more than 200 km over siliciclastic metasedimentary rocks of the trench-forearc of an opposing continental margin. The overriding eastern margin was the Oaxaquia microplate attached to Gondwana, and the western overridden margin is considered to have been the eastern margin of Laurentia. This event, which we name the Acatecan orogeny, was roughly synchronous with the possible closure of Iapetus along the Appalachian margin, which involved, according to current models, either the docking of peri-Gondwanan terranes such as Avalonia and Carolina or the direct collision between Gondwana and Laurentia. The permanence of Oaxaquia in northwestern Gondwana until the end of the Silurian, as suggested by Tremadocian to Silurian marine faunas in the cover of Oaxaquia, is more consistent with the direct collision of Gondwana and Laurentia at the end of the Ordovician, forming the Acatlan Complex between.

An evolutionary perspective on marine faunal connections between southernmost South America and Antarctica

Abstract: The origins of present day benthic marine faunas from both the Magellan and Antarctic provinces may lie as far back as the Early Cretaceous (approx. 130 Ma). This was the time of the first significant marine incursion across the Gondwana supercontinent and isolation of a high-latitude group of continents. It was also the probable time of formation of the temperate, Pacific-margin Weddellian Province, which extended from Patagonia, through Antarctica and New Zealand, to south-eastern Australia. Both palaeontological and phylogenetic evidence suggest that a number of living taxa (i.e. genera and families) from both provinces can be traced back to the Late Cretaceous-earliest Cenozoic interval. At this time there was no discernible gradient in taxonomic diversity from either southernmost South America or Australasia into Antarctica. The long, essentially temperate, Eocene epoch was followed by a period of major change during the ensuing Oligocene. At some time during this interval the Antarctic circum-polar current was fully formed and this led to a vicariant event between the Magellan and Antarctic faunas. However, it is important to stress that the intensification of circumpolar circulation also promoted at least some dispersal between various Subantarctic and Antarctic sites. In all probability, it was as late as the late Miocene (some 10-12 m.y. ago) before an intense pattern of thermal zonation (in both horizontal and vertical senses) was established in the world ocean. This may be the true time of full differentiation between the Magellan and Antarctic provinces. Although certain major groups, such as the bivalve molluscs and decapod crustaceans, have obviously declined within Antarctic regions through time, others, such as the bryozoans, echinoderms, amphipods and isopods appear to have flourished. The key to evolutionary success in cold polar waters may be not so much resistance to low temperatures, but the ability to exploit novel habitats and trophic regimes. Rates of speciation are not necessarily lower in cold, polar waters, or rates of extinction higher. The Antarctic fossil record suggests that there is no simple relationship between the onset of glaciation and the extinction of certain key bivalve and decapod groups.

Hercynian, Pan-African, Proterozoic and Archean ion-microprobe zircon ages for a Betic-Rif core complex, Alpine belt, W Mediterranean - consequences for its P-T-t path

Abstract: Ion-microprobe analysis of zircons from an andalusite-bearing orthogneiss within the major Alpujarride nappe complex in the central part of the Betic Cordilleras has yielded a Hercynian age of 285 ± 5 (2σ) Ma for euhedral rims, interpreted as the magmatic age of the andalusite-bearing biotite granite parent rock for the gneisses. Zircon age zoning systematics suggest a Paleozoic sedimentation age for the parent material for the anatectic source rock. Zircon cores represent several groups of ages: (1) Archean, c. 2.7 Ga; (2) Early Proterozoic, 2.2–2.0 Ga; (3) Middle Proterozoic, 1.1–0.9 Ga; (4) Pan-African, 0.8–0.5 Ga; including a well-defined event at 612 ± 13 (2σ) Ma. Paragenetic and textural relations indicate that gneissification took place during a high-P (12–13 kbar) low-T (450–500 °C) collisional event during which the primary Alpine nappe pile was produced. The second and final Alpine tectono-metamorphic event led to reorganization of the primary nappe pile by extensional tetonics with coeval very fast rock uplift and cooling (from c. 8? to c. 1 kbar and c. 600 to 100 °C within the period 19.5–18.5 Ma). The fast uplift/cooling stage was triggered by slab break-off which is thought to have induced diapiric underplating by high-T asthenospheric material. This may have heated the collisional wedge, causing thermal weakening which might have advanced the late stage fast uplift/cooling. The Alpine events did not leave a zircon crystallization record. The inherited, Archean–Pan-African zircon age pattern corresponds to that established for the Hercynian basement in central and southern Europe which is considered as reworked Gondwana crust. Deeper levels of core complexes within the Betic-Rif belt thus belong to the pre-Triassic basement of the Tethyan realm (Betic-Ligurian lithosphere) and represent reworked material ultimately derived from Archean and Proterozoic rock complexes from the Gondwana crustal domain. This study implies that thermobarometry of rock complexes which went through several phases of tectono-metamorphic reworking may render ambiguous results if based upon field observations and petrography alone. Zircon ion-microprobe dating may provide additional constraints required to arrive at a feasible tectono-metamorphic history, that is P-T-t trajectory, for such rock complexes.

U/Pb dating of detrital zircons: Implications for the provenance record of Gondwana margin terranes

Abstract: SHRIMP U/Pb age data for more than 300 detrital zircons from late Mesozoic samples of the Torlesse and Waipapa arc-trench terranes in New Zealand range from ca 100 Ma (Early Cretaceous) to 3140 Ma (Archean) More than 65% of the analyzed zircon grains are Permian or Mesozoic age The remaining detritus is largely of Paleozoic age with progressively smaller amounts of Proterozoic and Archean debris Cathodoluminescence imaging indicates that the younger grains are exclusively of igneous origin, whereas the older grains show evidence for a more complex history including metamorphic overprints and inherited cores The youngest zircon grains in most of the samples approximate the age of deposition of the rock units, suggesting input into the depositional basins from contemporaneous igneous activity The overall age profile of the detrital zircons is consistent with sediment accumulation adjacent to the Gondwana margin rather than in exotic blocks accreted to the margin The bulk of the detritus is derived from a late Paleozoic to Mesozoic Gondwana margin, Andean-style magmatic arc Elements of this arc extend from Marie Byrd Land in Antarctica, through New Zealand (Median tectonic zone) to New England in eastern Australia Paleozoic and older grains form a minor but significant component of all samples and have an age signature indicative of derivation from the Paleozoic and Neoproterozoic fold belts of East Australia and Antarctica (Gondwana) A characteristic feature of the older grains is ages in the range 500–650 and 1000–1200 Ma, which is also a feature of the zircon age spectrum for early Paleozoic graywackes from the Lachlan-Tuhua fold belt, suggesting derivation from these sedimentary rocks or from the same original source rocks

Geology and thermochronometry of the east edge of the Median Batholith (Median Tectonic Zone): a new perspective on Permian to Cretaceous crustal growth of New Zealand

Abstract: We report new field, petrological and isotopic data and interpretations from one of New Zealand’s major basement geological boundaries, the contact between the east side of the Median Batholith (formerly Median Tectonic Zone) and the allochthonous Mesozoic terranes of the Eastern Province. In the Nelson and Hollyford–Eglinton areas this contact is a Cenozoic fault, the Median Tectonic Line of earlier workers. However, in the Longwood Range, unfaulted pre-Cenozoic geological relations are preserved intact. Our new Ar–Ar, U–Pb and isotopic data show that the Median Batholith in the Longwood Range consists of two suites. (i) Eastern, isotopically primitive (87Sr/86Sri = 0.702 to 0.703; ɛNdT = + 7 to + 8) trondhjemite and gabbroic rocks of Permian age that we believe are part of the intraoceanic Brook Street arc of the Eastern Province. (ii) Western, isotopically more evolved (87Sr/86Sri = 0.703 to 0.704; ɛNdT = + 3 to + 5) quartz diorites, quartz monzodiorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Batholith. Field observations in the Longwood Range indicate intrusive, not faulted, contacts between these units and constrain accretion of the Brook Street Terrane to Gondwana to have occurred 230–245 Ma. Intra-batholith shear zones (T ~ 600°C and P ~ 0.2–0.3 GPa) were active at approximately 220 Ma. Modelling of K-feldspar Ar incremental heating ages indicate that most of the Longwood Range had cooled below 175°C by the Middle Jurassic and experienced no subsequent reheating. Significant additional post-accretionary Early Cretaceous and Cenozoic thermotectonic activity in Median Batholith in the Hollyford-Eglinton area is indicated by a new 140 ± 2 Ma U-Pb zircon date on a Largs ignimbrite, as well as by Cenozoic K-feldspar Ar–Ar ages in the Middle Triassic Mistake Diorite.

Magmatic sources and tectonic setting of Gondwana margin Ordovician magmas, northern Puna of Argentina and Chile

Abstract: Understanding the sources of Ordovician magmatic rocks in the broad western Faja Eruptiva Occidental and eastern Faja Eruptiva Oriental magmatic belts in the northern Puna of Argentina and Chile is important to Ordovician geodynamic models for western Gondwana evolution and Gondwana-Laurentian terrane interactions. A critical evaluation of existing chemical and age data, along with new major trace element data, and field observations leads to a working model in which magmatism in the western belt progressively occurred in an active to waning arc to collisional regime, whereas that in the eastern belt occurred in an oblique fault regime transitional to a subduction zone to the south. The model is hampered by data gaps in all areas, and particularly by lack of ages in western and southern regions. The best understood area is the northern Faja Eruptiva Oriental where dacitic and mafic units with published ages of 476‐467 Ma occur in linear fault-controlled trends. Their chemistry is consistent with dacitic magma sources being dominated by sedimentary-type protoliths melted in association with emplacement of mantle-derived alkaline mafic magmas. These dacitic units represent lava-dome complexes emplaced contemporaneously with outer shelf and slope basin sedimentation. Farther south where deeper crustal levels are exposed, volcanic/subvolcanic units grade into poorly studied plutonic facies. Trondhjemitic bodies are reported to the east. Contemporaneous units in the Faja Eruptiva Occidental are plutons and mafic to rhyolitic lavas in sequences of volcaniclastic sediments. Chemical signatures in the poorly dated Cordon de Lila lavas and Choschas dioritic to leucogranodioritic pluton in Chile are consistent with emplacement in a magmatic arc on thinned continental or oceanic crust. Mafic volcanic units in volcanic-sedimentary sequences just to the east could represent arc volcanism behind the front. An Arenig-Llanvirn change to bimodal and predominantly silicic magmatism, the emplacement of shoshonitic and mafic andesitic flows with weak arc signatures followed by alkaline dikes in the eastern Faja Eruptiva Occidental, and the formation of the Faja Eruptiva Oriental dacitic-mafic sequences could signal a change to a very oblique subduction regime. This change would be roughly contemporaneous with the arrival of the Laurentia-derived Precordillera terrane to the south. A post‐Arenig-Llanvirn compressional regime seems required to explain Ocloyic deformation in the east and late plutons to the west. The nature of the eastern boundary of a periGondwana Arequipa terrane and the existence of a peri-Gondwana Famatina-Puna Oriental terrane remain unclear.

Post-Gondwana drainage and the development of diamond placers in western South Africa

Abstract: The post-Gondwana history of the major rivers in the western part of South Africa is important because these rivers were instrumental in the development of diamond placers along the west coast of southern Africa. The evolution of the drainage systems that developed after breakup of west Gondwana can be viewed in three time-slots: the middle to Late Cretaceous, the early to middle Cenozoic, and the late Cenozoic periods. During the middle to Late Cretaceous there were two main river systems draining the interior. The one in the south, also referred to as the Karoo River, had its source in the present upper Orange/Vaal drainage basin and its outlet was at the present Olifants River mouth. The second and more northerly system, also known as the Kalahari River, drained southern Botswana and Namibia and entered the Atlantic Ocean via the lower Orange River. Erosion dominated the period immediately after breakup of west Gondwana and most of the diamonds released during erosion of Cretaceous kimberlites in central South Africa were transported by the Karoo River to the coast. By early Cenozoic times, the lower Kalahari River had captured the upper part of the Karoo River and the broad configuration of the present Orange River network was established. This capture and northerly shift of the Orange River, on the newly exhumed pre-Karoo surface, was the result of an accelerated uplift of the southern and eastern subcontinental margins ca. 100 to 80 Ma. During the early and middle Cenozoic, the climate was arid to semiarid. This resulted in a substantial reduction in erosion rates and hence few diamonds were released from the primary bodies during that time. Late Cenozoic fluvial gravels, however, dated as either middle Miocene or Plio-Pleistocene, contain diamonds that were reworked out of older Tertiary fluvial deposits. Sediments at the base of the Koa Valley and in the upper terraces in the Sak Valley formed the Koa River, a major tributary of the Orange River during the Miocene, and drained most of the area previously occupied by the lower Karoo River. The Koa River thus reworked diamonds trapped in the Cretaceous Karoo River deposits or terraces. Younger sediments of the Carnarvon Leegte were never part of the Koa system. In fact, the Sak River captured the upper Koa River by late Pliocene times and the Plio-Pleistocene lower terraces in the Sak Valley and the paleo-Carnarvon Leegte joined as the paleo-Hartbees River--another major tributary of the Orange in the Plio-Pleistocene. Although climatic changes were the major controls that initiated the alluvial pulses during the Cenozoic, asymmetric uplift of the subcontinent was ultimately responsible for the northwesterly shift of the Orange River.

Odyssey of terranes in the Iapetus and Rheic oceans during the Paleozoic

Abstract: Geologic, paleomagnetic, and faunal data indicate that Paleozoic terranes bordering the Iapetus and Rheic oceans may be classified as native or exotic with respect to adjacent cratons. Native terranes include the Notre Dame-Shelburne Falls arc lying along the eastern margin of Laurentia, Cadomia situated adjacent to North Africa, and the Famatina, Puna and Arequipa-Antofalla Terranes occurring adjacent to western South America. Exotic terranes include the western South American terranes, Cuyania and Chilenia, which are inferred to have been derived from southern Laurentia; and the North American terranes, Oaxaquia, Florida, Carolina, Avalonia, and associated terranes, which have a Gondwanan provenance in northern South America-West Africa. Early Cambrian opening of Iapetus was followed in Late Cambrian-Early Ordovician by extensional subduction that produced volcanic arcs and backarc basins on both margins (Notre Dame-Shelburne Falls and Famatinian arcs), which changed to compressional arcs collapsing the backarc basins during the mid-Late Ordovician. During the Ordovician, Cuyania was transferred from southern Laurentia to western Gondwana, and Carolina/Piedmont and Avalonia moved from northwestern Gondwana to eastern Laurentia. This exchange indicates that the southern part of Iapetus was preferentially subducted beneath South America as the central part was mainly subducted beneath eastern Laurentia, and suggests that southern and central Iapetus were separated by a major transform fault. After the Late Ordovician demise of Iapetus, the Rheic ocean persisted between Laurentia and Gondwana, and terranes appear to have remained adjacent to their neighboring craton until the terminal amalgamation of Pangea in the Permo-Carboniferous. During the Mesozoic break-up of Pangea, the Oaxaquia, Chortis, Florida, and Cadomia Terranes were left adjacent to Laurentia and Baltica.

SHRIMP U/Pb Zircon Dating of Neoproterozoic Granitic Magmatism and Collision in the Pelotas Batholith, Southernmost Brazil

Abstract: The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasiliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent Except for associated small schist belts and post-orogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons Three petrotectonic associations were selected for detailed isotopic investigations—the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and t

Aeromagnetic legacy of early Paleozoic subduction along the Pacific margin of Gondwana

Abstract: Comparison of the aeromagnetic signatures and geology of southeastern Australia and northern Victoria Land, Antarctica, with similar data from ancient subduction zones in California and Japan, provides a framework for reinterpretation of the plate tectonic setting of the Pacific margin of early Paleozoic Gondwana. In our model, the plutons in the Glenelg (southeastern Australia) and Wilson (northern Victoria Land) zones formed the roots of continental-margin magmatic arcs. Eastward shifting of arc magmatism resulted in the Stavely (southeastern Australia) and Bowers (northern Victoria Land) volcanic eruptions onto oceanic forearc crust. The turbidites in the Stawell (southeastern Australia) and Robertson Bay (northern Victoria Land zones) shed from the Glenelg and Wilson zones, respectively, were deposited along the trench and onto the subducting oceanic plate. The margin was subsequently truncated by thrust faults and uplifted during the Delamerian and Ross orogenies, leading to the present-day aeromagnetic signatures.

Evolution of the late Paleozoic accretionary complex and overlying forearc‐magmatic arc, south central Chile (38°–41°S): Constraints for the tectonic setting along the southwestern margin of Gondwana

Abstract: Stratigraphic, structural, metamorphic, and geochronologic studies of basement rocks in the Andean foothills and Coast Ranges of south central Chile (39°–41°S) suggest a protracted late Paleozoic to middle Mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern Gondwana margin. In the study area the late Paleozoic paired metamorphic belt, coeval magmatic arc, and overlying Triassic sedimentary units preserve a record of Late Carboniferous to Early Permian subduction and arc magmatism, subsequent deep exhumation of the Western Series subduction complex, and diminished uplift and erosion of the Eastern Series arc-forearc region by the Late Triassic. Late Paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism, collectively referred to as Dl, are largely erased in the Western Series by the dominant D2 schistosity and lower greenschist grade metamorphism. D1 structural features, as well as original sedimentary textures, are relatively well preserved in the less penetratively deformed Eastern Series. The regional distribution of late Paleozoic arc magmatism suggests that the late Paleozoic convergent margin deviated from a N–S trend north of this area to a NW–SE trend near this latitude and faced an open marine environment to the southwest. A transition from F2 isoclinal folding to more open, larger-scale F3 folds, interpreted as change in ductility during differential uplift of the Western Series, is not apparent in the Eastern Series. Despite a lesser degree of uplift during the main exhumational D2 event, delineation of unconformities and U-Pb dating of detrital zircons and intrusions into the Eastern Series allow tighter constraints to be placed on timing of uplift and denudation of the Eastern Series than on that in the Western Series. A regional unconformity exposed in the Lake District that separates more highly deformed Eastern Series lithologies from Late Triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the D2 event. We propose that dextral-oblique convergence, initiated during the middle Permian along this segment of the Gondwana margin, resulted in the transpressional uplift and juxtaposition of high pressure/temperature (P/T) Western Series against low P/T Eastern Series lithologies and culminated with deposition of Late Triassic, continental to shallow marine, coarse clastic sedimentary rocks in fault-bounded strike-slip basins adjacent to the exhumed Western Series. Large-scale dextral transpression and northward displacement of the accretionary complex during Late Permian to Late Triassic time along the Chilean margin of Gondwana are synchronous and kinematically compatible with widespread regional transpression, extension, and silicic magmatism inboard of the southern Gondwana margin at this time. We thank C. Mpodozis, M. Gardeweg, and J. Munoz of the Servicio de Geologia y Mineria de Chile (SERNAGEOMIN) for their support of this work. Fruitful discussions with N. Blanco, F. Herve, H. Moreno, C. Mpodozis, and F. Munizaga have aided in our understanding of the geology of the region. The hard work by the staff of SERNAGEOMIN’s Puerto Varas office is graciously appreciated. We thank J.D. Walker and W.R. Van Schmus at the University of Kansas for allowing MWM use of their U-Pb and mass spectrometer facilities and J. Vargas and the staff of SERNAGEOMIN’s geochemistry laboratory for their assistance in this project. F. Munizaga allowed us to cite an unpublished 40Ar-39Ar date. We thank G. Ya˜nez for access to aeromagnetic data. T. Kato wishes to thank W. G. Ernst. Comments by I. Dalziel, S. Kay, and V. Ramos helped clarify ideas presented in this paper and are greatly appreciated. This work is dedicated to our friend and colleague Alberto Campos C., who died in a climbing accident on Calbuco Volcano, 1996.

Biogeographic analyses of the Ediacara biota; a conflict with paleotectonic reconstructions

Abstract: Paleotectonic reconstructions for the late Proterozoic have differed over the timing of the Cordilleran rifting between Laurentia and the East Gondwana cratons. Parsimony Analysis of Endemism (PAE) and phenetic clustering of the "Ediacara biota" were carried out, for comparison with competing paleotectonic hypotheses. All analyses show a common pattern of similarities among biotas. The biotas of Charnwood Forest and Newfoundland consistently group together, while the south Australian biota is closest to those of Baltica, northern Laurentia, and Siberia. The biota of southwest North America, though still poorly known, strikingly resembles that of Na- mibia-not that of northwestern Canada. This pattern is not obviously due to facies-related or pres- ervational bias and is very different from Cambrian biogeographic patterns. The overall pattern is most consistent with the hypothesis that the western margin of Laurentia was in close contact with East Gondwana, with rifting taking place either during or just before the Vendian. This arrange- ment has been previously proposed as a paleotectonic hypothesis; however, most recent paleo- magnetic and structural studies support the alternate hypothesis that this rifting took place more than 100 million years before the Vendian. Resolving this contradiction will require much more data on both organismal distribution and cratonal position.