PATTERNS OF LATE CENOZOIC VOLCANIC AND TECTONIC ACTIVITY IN THE WEST ANTARCTIC RIFT SYSTEM REVEALED BY AEROMAGNETIC SURVEYS

Aeromagnetic surveys, spaced less than or equal to 5 km, over widely s eparated areas of the largely ice- and sea-covered West Antarctic rift system, reveal similar patterns of 100- to 1700-nT, shallow-source ma gnetic anomalies interpreted as evidence of extensive late Cenozoic vo lcanism. We use the aeromagnetic data to extend the volcanic rift inte rpretation over West Antarctica starting with anomalies over (1) expos ures of highly magnetic, late Cenozoic volcanic rocks several kilomete rs thick in the McMurdo-Ross Island area and elsewhere; continuing thr ough (2) volcanoes and subvolcanic intrusions directly beneath the Ros s Sea continental shelf defined by marine magnetic and seismic reflect ion data and aeromagnetic data and (3) volcanic structures interpreted beneath the Ross Ice Shelf partly controlled by seismic reflection de terminations of seafloor depth to (4) an area of similar magnetic patt ern over the West Antarctic Ice Sheet (400 lan from the nearest expose d volcanic rock), where interpretations of late Cenozoic volcanic rock s at the base of the ice are controlled in part by radar ice sounding, North trending magnetic rift fabric in the Ross Sea-Ross Ice Shelf an d Corridor Aerogeophysics of the Southeast Ross Transect Zone (CASERTZ ) areas, revealed by the aeromagnetic surveys, is probably a reactivat ion of older rift trends Gate Mesozoic?) and is superimposed on still older crosscutting structural trends revealed by magnetic terrace maps calculated from horizontal gradient of pseudogravity. Long wavelength (similar to 100-km wide) magnetic terraces from sources within the su bvolcanic basement cross the detailed survey areas. One of these exten ds across the Ross Sea survey from the front of the Transantarctic Mou ntains with an east-southeast trend crossing the north trending rift f abric. The Ross Sea-Ross Ice Shelf sun ey area is characterized by hig hly magnetic northern and southern zones which are separated by magnet ically defined faults from a more moderately magnetic central zone. Ae romagnetic data in the south delineate the Ross fault of unknown age. The extension of the southern Central Basin south of the Ross fault is associated with an 825-nT magnetic anomaly over the Ross Ice Shelf re quiring inferred late Cenozoic volcanic rock essentially at the seaflo or at its south end, as shown by magnetic models. Models show that the thickness of magnetic volcanic rocks beneath Hut Point Peninsula at M cMurdo Station is probably <2 km. The detailed surveys, combined with data from >100,000 km of widely spaced aeromagnetic profiles, led to t he interpretation of the mostly subglacial West Antarctic flood basalt s(?) or their subglacially erupted and intruded equivalent. The volume of the exposed volcanos is small in contrast to the much greater volu me (>10(6) km(3)) of late Cenozoic magnetic rock remaining at volcanic centers beneath the continental shelf, Ross Ice Shelf and West Antarc tic Ice Sheet. We suggest as an alternative or supplemental explanatio n to the previously proposed mantle plume hypothesis for the late Ceno zoic volcanism significantly greater lower lithosphere (mantle) stretc hing resulting in greater decompression melting than the limited Cenoz oic crustal extension allows. However, this implies a space problem th at is not obviously resolved, because the Antarctic Plate is essential ly surrounded by spreading centers.