The Making of Texas

This atlas presents plate reconstructions from the early Neoproterozoic (1000-750 Ma) to the present-day as derived by Ian Dalziel for the PLATES Project, Institute for Geophysics, The University of Texas at Austin.PLATES Project, Institute for Geophysics, The University of Texas at AustinInstitute for Geophysic

The global relevance of the Scotia Arc: An introduction

The Scotia Arc, situated between South America and Antarctica, is one of the Earth’s most important ocean gateways and former land bridges. Understanding its structure and development is critical for the knowledge of tectonic, paleoenvironmental and biological processes in the southern oceans and Antarctica. It extends from the Drake Passage in the west, where the Shackleton Fracture Zone forms a prominent, but discontinuous, bathymetric ridge between the southern South American continent and the northern tip of the Antarctic Peninsula to the active intra-oceanic volcanic arc forming the South Sandwich Island in the east. The tectonic arc comprises the NSR to the north and to the south the South Scotia Ridge, both transcurrent plate margins that respectively include the South Georgia and South Orkney microcontinents. The Scotia and Sandwich tectonic plates form the major basin within these margins. As the basins opened, formation of first shallow sea ways and then deep ocean connections controlled the initiation and development of the Antarctic Circumpolar Current, which is widely thought to have been important in providing the climatic conditions for formation of the polar ice-sheets. The evolution of the Scotia Arc is therefore of global palaeoclimatic significance. The Scotia Arc has been the focus of increasing international research interest. Many recent studies have stressed the links and interactions between the solid Earth, oceanographic, palaeoenvironmental and biological processes in the area. This special issue presents new works that summarize significant recent research results and synthesize the current state of knowledge for the Scotia Arc

South Georgia microcontinent: displaced fragment of the southernmost Andes

The mountainous, glaciated island of South Georgia is the crest of one of the most isolated fragments of continental crust on Earth. It is located approximately 1700 km east of the southern termination of the Andean Cordillera of South America. The island is primarily composed of Lower Cretaceous turbidites, the infill of a marginal basin floored by stretched continental and ophiolitic crust. Remnants of a volcanic arc are preserved on offshore islands to the southwest. The Pacific hinterland of the southernmost Andes is missing in Tierra del Fuego, terminating at a submarine escarpment forming the continental margin immediately east of Cape Horn. The arc and back-arc basin infill rocks of South Georgia correspond exactly to part of the missing Cordilleran hinterland. The mechanism of transport of the South Georgia microcontinent eastward relative to South America remains obscure, but likely involved some form of ‘escape tectonics’ during mid- to Late Cretaceous counterclockwise rotation of the arc that led to closure and inversion of the marginal basin

Upper Mantle Seismic Anisotropy Beneath the West Antarctic Rift System and Surrounding Region from Shear Wave Splitting Analysis

We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozen-in’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS

Upper Mantle Structure of Central and West Antarctica from Array Analysis of Rayleigh Wave Phase Velocities

The seismic velocity structure of Antarctica is important, both as a constraint on the tectonic history of the continent and for understanding solid Earth interactions with the ice sheet. We use Rayleigh wave array analysis methods applied to teleseismic data from recent temporary broadband seismograph deployments to image the upper mantle structure of central and West Antarctica. Phase velocity maps are determined using a two–plane wave tomography method and are inverted for shear velocity using a Monte Carlo approach to estimate three-dimensional velocity structure. Results illuminate the structural dichotomy between the East Antarctic Craton and West Antarctica, with West Antarctica showing thinner crust and slower upper mantle velocity. West Antarctica is characterized by a 70–100 km thick lithosphere, underlain by a low-velocity zone to depths of at least 200 km. The slowest anomalies are beneath Ross Island and the Marie Byrd Land dome and are interpreted as upper mantle thermal anomalies possibly due to mantle plumes. The central Transantarctic Mountains are marked by an uppermost mantle slow-velocity anomaly, suggesting that the topography is thermally supported. The presence of thin, higher-velocity lithosphere to depths of about 70 km beneath the West Antarctic Rift System limits estimates of the regionally averaged heat flow to less than 90 mW/m2. The Ellsworth-Whitmore block is underlain by mantle with velocities that are intermediate between those of the West Antarctic Rift System and the East Antarctic Craton. We interpret this province as Precambrian continental lithosphere that has been altered by Phanerozoic tectonic and magmatic activity

A Tight fit-Early Mesozoic Gondwana, a Plate Reconstruction Perspective

Gondwana, with East Antarctica as its center, began to break up during Late Triassic to Early Jurassic time. Use of the satellite derived gravity map to approximate the ocean-continent boundary allows us to generate a much tighter fit for the reconstructed supercontinent then previously attempted. Major mantle plumes such as the Karoo-Ferrar Plume that first split Gondwana at about 182Ma, the Parana-Etendeka plume at 132Ma that split South America and Africa, the Marion plume at 88Ma that split Madagascar and India and finally the Reunion hotspot that split the Mascarene Plateau from India at 64Ma, were all critical events in the break-up of Gondwana. Our tight-fit produces overlap between cratonic East Antarctica and the Limpopo Plain of Mozambique but there is no evidence that the crustal material underlying the Limpopo Plain pre-dates the break-up of Gondwana. Likewise Madagascar has been reconstructed so that it substantially overlies coastal East Africa in the vicinity of the Anza Trough, an early Jurassic rift in Kenya. The western margin of the island of Madagascar may in fact be crustal material that is younger than the break-up. It may have been produced as a result of the Karoo mantle plume or some may have been the result of the Marion hotspot. Between South America and Africa there are three significant overlaps. Two of them are deltaic, and the third is the Abrolhos and Royal Charlotte banks which post-date Gondwanide breakup by 80 to 100 million years

South Georgia Enigma using PALEOMAP (Paleoceanographic Mapping Project Progress Report No. 28-1287)

This report presents plate reconstructions of the Scotia Sea.UT Institute for Geophysics Paleoceanographic Mapping Project (POMP)Institute for Geophysic