Transect across the West Antarctic rift system in the Ross Sea, Antarctica

In 1994, the ACRUP (Antarctic Crustal Profile) project recorded a 670-km-lo ng geophysical transect across the southern Ross Sea to study the velocity and density structure of the crust and uppermost mantle of the West Antarct ic rift system. Ray-trace modeling of P- and S-waves recorded on 47 ocean b ottom seismograph (OBS) records, with strong seismic arrivals from airgun s hots to distances of up to 120 km, show that crustal velocities and geometr ies vary significantly along the transect. The three major sedimentary basi ns (early-rift grabens), the Victoria Land Basin, the Central Trough and th e Eastern Basin are underlain by highly extended crust and shallow mantle ( minimum depth of about 16 km), Beneath the adjacent basement highs, Coulman High and Central High, Moho deepens, and lies at a depth of 21 and 24 km, respectively. Crustal layers have P-wave velocities that range from 5.8 to 7.0 km/s and S-wave velocities from 3.6 to 4.2 km/s. A distinct reflection (PiP) is observed on numerous OBS from an intra-crustal boundary between th e upper and lower crust at a depth of about 10 to 12 km. Local zones of hig h velocities and inferred high densities are observed and modeled in the cr ust under the axes of the three major sedimentary basins. These zones, whic h are also marked by positive gravity anomalies, may be places where mafic dikes and sills pervade the crust. We postulate that there has been differe ntial crustal extension across the West Antarctic rift system, with greates t extension beneath the early-rift grabens. The large amount of crustal str etching below the major rift basins may reflect the existence of deep crust al suture zones which initiated in an early stage of the rifting, defined a reas of crustal weakness and thereby enhanced stress focussing followed by intense crustal thinning in these areas. The ACRUP data are consistent with the prior concept that most extension and basin down-faulting occulted in the Ross Sea during late Mesozoic time, with relatively small extension, co ncentrated in the western half of the Ross Sea, during Cenozoic time. (C) 1 999 Elsevier Science B.V. All rights reserved.