CRUSTAL STRUCTURE OF ROSS OROGEN REVEALED BY AEROMAGNETICS AND GRAVITY

Near complete coverage of the East Antarctic shield by glaciers and the polar ice cap hampers geological study of basement terrains in Antarctica. Limited exposures in the central Transantarctic Mountains, however, show that Archean and Proterozoic rocks of the shield are reactivated within the Ross Orogen and that Neoproterozoic-lower Paleozoic successions were deformed during oblique convergence associated with Pan-African-age Gondwana amalgamation. We conducted an aeromagnetic and gravity survey across the orogen near Nimrod and Beardmore glaciers in order to image basement architecture as revealed by potential-field behavior in both exposed and ice-covered areas. Our aeromagnetic survey was flown in two parts, first by helicopters over the Transantarctic Mountains in draped mode, and then by fixed-wing aircraft over the adjacent polar plateau. We flew over 32,000 line kilometers covering an area of nearly 60,000 km² at an average altitude of 600 m with average line spacing of 2.5 km over most areas and 1.25 km over basement rocks exposed in the Miller and Geologists ranges. Magnetic sensitivity was 0.01 nT. Additional lines were flown to true north, south and west in order to extend preliminary coverage and tie with existing surveys. Where exposed, high-grade metamorphic basement shows great magnetic variability, with pronounced central highs and a linear, SE-trending corrugated fabric that correlates with ductile shear structures and map-scale folds. Isolated magnetic highs are commonly associated with retrogressed mafic eclogites in layered gneiss. Outboard siliciclastic units show uniform magnetic character, whereas Jurassic Ferrar sills and inferred dike systems are magnetically intense. Exposed Ross-age granitic plutons show distinctive magnetic lows, permitting identification of several large hidden bodies beneath the ice. A prominent thrust system exposed north of Nimrod Glacier, which places Neoproterozoic-Cambrian platform rocks upon Cambrian-Ordovician molasse deposits, can be traced magnetically several 10’s of km to the south, including places previously mapped erroneously as an unconformity boundary. Ongoing data processing and modeling will result in rapid data release to the solid earth community.