DURATION OF MELT-PRESENT TO SOLID-STATE DEFORMATION IN THE SOUTH FOSDICK DETACHMENT ZONE, FOSDICK MOUNTAINS, WEST ANTARCTICA

The Fosdick Mountains form an E-W trending migmatite dome in the northern Ford Ranges of Marie Byrd Land, Antarctica. Pervasively folded migmatites derived from lower Paleozoic greywacke and middle Paleozoic plutonic rocks constitute the dome. New field research documents a transition from melt-present to solid-state deformation upon the south flank of the dome, and a mylonitic shear zone mapped for 30 km between Mt. Iphigene and Mt Richardson. Kinematic shear sense is dextral normal oblique, with top-to-the-SW and -WSW transport, consistent with regional strain axes determined from a Cretaceous regional dike swarm. A U-Pb age of 107 Ma, from a leucosome-filled extensional shear band, provides a melt-present deformation age, and a U-Pb age of 96 Ma, from a crosscutting granitic dike, gives a lower age limit for deformation. The shear zone preserves evidence for melt-enhanced deformation and features indicative of crystallization from melt. 24 km to the south in the Chester Mountains, correlative plutonic rocks are unmetamorphosed and unfoliated, but cut by sparse m-scale dikes of dolerite and two-mica granite and pervasive minor brittle faults that record NNE-SSW stretching. We here propose that the mylonitic zone represents a crustal-scale detachment zone on the south flank of the Fosdick Mountains, separating dome rocks dominated by melt-present deformation in the middle crust from the brittely deformed, hanging wall block of the Chester Mountains. The structure, here named the South Fosdick detachment zone, forms the south flank of the migmatite dome and was in part responsible for the exhumation of mid-crustal rocks. Activity on the South Fosdick detachment zone occurred during a rapid change from convergent to extensional tectonics along the proto-Pacific margin of Gondwana. Correlative structures are the Paparoa metamorphic core complex and deep-level shear zones within the Fiordland orthogneiss (e.g., Mt. Irene shear zone) in New Zealand.