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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TThe accretionary wedge of the Bay of Biscay is an east-west compressive belt buried under recent sediments of the abyssal plain at the north Iberian margin. This structure formed through the partial closure of the previously extended Biscay basin during the Cenozoic north-south collision between Europe and Iberia, the same collision that produced the Cantabrian-Pyrenean range on land. Three north-south seismic sections have been prestack depth migrated, showing a narrow-tapered wedge (7Њ-8Њ) whose internal structure corresponds to a set of south-dipping thrusts converging toward a basal decollement. There are differences along strike within the wedge: thrust spacing, the dip of the basal thrust, and the thickness of the sediments at the trough augment toward the east, increasing its overall size. The two-dimensional velocity models obtained through migration analysis reflect values between 2000 km/s at the sea floor (4500 m) and 5000 km/s at 12-km depth. The syntectonic package thickness varies from 1.5 to 3 km, while the posttectonic cover attains 1.5-2 km. A simple analysis based on critical wedge theory approaches suggests that the Biscay wedge formed in similar conditions to active submarine wedges, the strength of the decollement being lower than the strength of the wedge itself. Further comparison with other examples indicates high basal stress, which could be an added factor in the convergence stopping at this margin. The eastward size increase is attributed to the provision of extra sediments by the coetaneous rising of the cordillera on land. This weight steepens the basal angle without affecting the overall taper. Surprisingly, the eastward change from an oceanic to a transitional basement does not seem to be crucial in its geometry.

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Resumen: Durante el Neoproterozoico tardío y el Paleozoico, el actual segmento argentino-chileno de la Cordillera de los Andes y desde finales del Paleozoico la Península Antártica, formaron parte del margen suroccidental de Gondwana. Durante este periodo de tiempo, a dicho margen se fue acrecionando un conjunto de fragmentos continentales de tamaño y aloctonía variable, dando lugar en el Paleozoico a seis orogenias de diferente extensión temporal y espacial: Pampeana (Ediacárico-Cámbrico temprano), Famatiniana (Ordovícico Medio-Silúrico), Chánica (Devónico Medio-Carbonífero temprano), Oclóyica (Ordovícico Medio-Devónico), Gondwánica (Devónico Medio-Pérmico medio) y Tabarin (Pérmico tardío-Triásico). Todas estas orogenias son colisionales, salvo la Tabarin y la Gondwánica al sur de los 38º S.Palabras clave: Evolución geodinámica, Paleozoico, Andes argentino-chilenos, Península Antártica, Orógeno Oclóyico, Orógeno Famatiniano, Orógeno Chánico, Orógeno Gondwánico, Orógeno Tabarin.Abstract: During the late Neoproterozoic and Paleozoic times, the Argentine-Chilean Andes, -and since the late Paleozoic the Antarctic Peninsula,- formed part of the southwestern margin of Gondwana. During this period of time, several continental fragments of variable extensión and allochtonie were successively accreted to that margin, resulted in six Paleozoic orogenies of different temporal and spatial extension: Pampean (Ediacaran-early Cambrian), Famatinian (Middle Ordovician-Silurian), Chanic (Middle Devonian-early Carboniferous), Ocloyic (Middle Ordovician-Devonian), Gondwanan (Middle Devonian-middle Permian) and Tabarin (late Permian-Triassic). All these orogenies had a collisional character, with the exception of the Tabarin and the Gondwanan south of 38º S.Keywords: Geodynamic evolution, Paleozoic, Argentine-Chilean Andes, Antarctic Peninsula, Ocloyic orogen, Famatinian orogen, Chanic orogen, Gondwanan orogen, Tabarin orogen.

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Geochemical measurements have been performed on about 8,000 rock samples collected across bio-event horizons using instrumental neutron activation analysis (INAA) for about 40 major, minor, and trace elements and radiochemical isolation procedures for Ir. On selected samples, Os, Pt, and Au were also radiochemically determined. These studies, some previously reported, have encompassed the time interval from the Precambrian-Cambrian transition to the late Eocene impact (microspherule) horizons. Our early work strengthened the Alvarez impact hypothesis by finding the Ir (platinum group element [PGE]) anomaly at the K/T boundary in continental sedimentary sequences. In collaborations with paleontologists we discovered weak to moderately strong Ir anomalies at the Frasnian-Famennian boundary in Australia, in the Early Mississippian of Oklahoma, at the Mississippian-Pennsylvanian boundary of Oklahoma and Texas, and in the late Cenomanian throughout the western interior of NorthAmerica and on the south coast of England. We have found no compelling evidence for an impact-related cause for these anomalies, although PGE impact signatures in the two late Cenomanian anomalies could be masked by the strong terrestrial mafic to ultramafic overprint. Thus far, our evidence for extinction events older than the terminal Cretaceous does not support recent hypotheses, which suggest that impacts from cyclic swarms of comets in the inner Solar System were responsible for the periodic mass extinctions identified by Raup and Sepkoski. However, much more work on extinction boundaries needs to be done to resolve the problem, especially tedious searching for microspherules and shocked mineral grains, because comet impacts might provide little or no excess Ir.