Crustal and lithospheric structure of the West Antarctic Rift System from geophysical investigations - a review

The active West Antarctic Rift System, which extends from the continental s helf of the Ross Sea, beneath the Ross Ice Shelf and the West Antarctic Ice Sheet, is comparable in size to the Basin and Range in North America, or t he East African rift systems. Geophysical surveys (primarily marine seismic and aeromagnetic combined with radar ice sounding) have extended the infor mation provided by sparse geologic exposures and a few drill holes over the ice and sea covered area. Rift basins developed in the early Cretaceous ac companied by the major extension of the region. Tectonic activity has conti nued episodically in the Cenozoic to the present, including major uplift of the Transantarctic Mountains. The West Antarctic ice sheet, and the late C enozoic volcanic activity in the West Antarctic Rift System, through which it flows, have been coeval since at least Miocene time. The lift is charact erized by sparse exposures of late Cenozoic alkaline volcanic rocks extendi ng from northern Victoria Land throughout Marie Byrd Land. The aeromagnetic interpretations indicate the presence of > 5 x 10(5) km(2) (> 10(6) km(3)) of probable late Cenozoic volcanic rocks (and associated subvolcanic intru sions) in the West Antarctic rift. This great volume with such limited expo sures is explained by glacial removal of the associated late Cenozoic volca nic edifices (probably hyaloclastite debris) concomitantly with their subgl acial eruption. Large offset seismic investigations in the Ross Sea and on the Ross Ice Shelf indicate a similar to 17-24-km-thick, extended continent al crust. Gravity data suggest that this extended crust of similar thicknes s probably underlies the Ross Ice Shelf and Byrd Subglacial Basin. Various authors have estimated maximum late Cretaceous-present crustal extension in the West Antarctic rift area from 255-350 km based on balancing crustal th ickness. Plate reconstruction allowed < 50 km of Tertiary extension. Howeve r, paleomagnetic measurements suggested about 1000 km of post-middle Cretac eous translation between East Antarctica and Pacific West Antarctica. Becau se a great amount of crustal extension in late Cenozoic time is unlikely, a lternate mechanisms have been proposed for the late Cenozoic volcanism. Its vast volume and the ocean island basalt chemistry of the exposed late Ceno zoic alkaline volcanic rocks were interpreted as evidence for a mantle plum e head. An alternative or supplemental explanation to the mantle plume hypo thesis is significantly greater lower lithosphere (mantle) stretching resul ting in greater decompression melting than the limited Cenozoic crustal ext ension allows. Because of very slow rates of late Cenozoic extension in the West Antarctic Rift System, the amount of advected heat is small compared with the conductive heat. Therefore, phase transition probably would not ex plain the large subsidence with low extension observed in the West Antarcti c Rift System. (C) 1999 Elsevier Science B.V. All rights reserved.