NEW INSIGHTS ONn THE TIMING AND EXTENT OF CRETACEOUS EXHUMATION IN THE WEST ANTARCTIC RIFT SYSTEM, FROM U-PB AND (U-TH)/HE ZIRCON ANALYSIS

The West Antarctic rift system is comparable in size to other active rifts on Earth, but its thermal and exhumation history has been obscured by Antarctica’s continental ice sheet. Sparse exposures along the coast of Marie Byrd Land, 1300 km east of the Transantarctic Mountains, offer direct access to migmatite and granite (Fosdick and Alexandra Mtns) and calcsilicate gneiss (Mt. Woodward) of the rift’s thinned crust. Anatectic granites in the Fosdick Mountains have U-Pb zircon crystallization ages of 116-96 Ma, and their host gneisses record T ≥830°C at depths <22 km (Korhonen et al., Lithos, 2010). Detachment tectonics at ca. 102 Ma led to rapid exhumation and cooling to T ≤200° by 94 Ma, based on ⁴⁰Ar/³⁹Ar data (McFadden et al., this volume).

In order to extend the cooling trajectory and assess the size of the region affected by events at ca.100 Ma, we obtained (U-Th)/He zircon [ZHe] ages for metasedimentary gneiss in exposures distributed along 250 km of coastline. An initial task was to test whether the gneisses share a common protolith, using the Arizona Laserchron for LA-ICPMS U-Pb detrital zircon analysis: the dominant populations for all sites are 1100-1000 Ma and 500 Ma, an established characteristic of lower Paleozoic turbidites of Gondwana’s active margin.

In the Fosdick Mtns, two-mica granite at Mackay Nunatak gave a zircon U-Pb age of 108.9± 9.7 and He cooling age of 98± 4 Ma, while migmatite at Mitchell Peak yields ZHe ages of 90± 4 and 77± 3 Ma. Correlative migmatites in the Alexandra Mtns have ZHe ages of 73±3 Ma (Scott Nunatak), 88± 3 and 83±3 Ma (Prestrud Rock). For Mt. Woodward, situated midway between the two ranges, ZHe ages are 74± 3 and 67±3 Ma. The small time difference between zircon U-Pb ages for anatectic granites in the region and (U-Th)/He closure is consistent with rapid cooling due to upward translation of granite magma within hot middle crust during postulated detachment faulting in the region.