ROTATION OF THE STRAIN FIELD DURING EMPLACEMENT AND EXHUMATION OF THE FOSDICK MOUNTAINS MIGMATITE-CORED GNEISS DOME, WEST ANTARCTICA

The Fosdick Mountains in West Antarctica expose an elongate migmatite-cored gneiss dome exhumed from mid- to lower crustal depths during a transition from oblique convergence to extensional deformation associated with the West Antarctic Rift system. The migmatites and granites underwent crustal melting and deformation during convergent margin tectonics, both in the Devono-Carboniferous and Cretaceous. Crustal-scale heterogeneities formed in the Paleozoic served as sites of strain localization that influenced the distribution and migration pathways of anatectic melts formed during the Cretaceous event, evidenced by ca. granites that occupy distinct structural sites formed during wrench (117–114 Ma: U-Pb zircon) vs. transtensive (109-102 Ma) deformation.

Prior work linked the two generations of granite to a counterclockwise rotation of the maximum strain axes from 265–085° to 235–055° that culminated in development of the oblique South Fosdick Detachment zone. Brittle structures that overprint plastic fabrics provide evidence of unroofing and cooling of dome rocks as the strain axes rotated further counterclockwise into orthogonal extension, oriented 185–005°. We used ⁴⁰Ar/³⁹Ar thermochronology for the detachment zone and syntectonic diorite dikes to determine steps in the cooling history. ⁴⁰Ar/³⁹Ar ages on amphibole and biotite range from ca. 102–100 Ma, with K-feldspar cooling ages at ca. 94 Ma. The biotite and amphibole ages are indistinguishable from U-Pb zircon crystallization ages as young as 102 Ma. Collectively, these data indicate cooling of the Fosdick dome from peak metamorphic conditions of 830 °C at 6.6 kb at a rate of >85 °C/myr, and unroofing at a rate of ~2.2 km/myr. The near-superposition of U-Pb zircon and argon cooling ages is possible evidence of granite emplacement and rapid tectonic exhumation of hot middle crust to shallow levels, a process recognized in new numerical models for orogenic crust undergoing rapid extension.