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Ross Sea mylonites and the timing of intracontinental extension within the West Antarctic rift system

¹ Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA
² Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA
³ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
⁴ Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA

There are few direct constraints on the timing and style of faulting in the Ross Sea sector of the West Antarctic rift system, although Cretaceous plate reconstructions indicate that Ross Sea extension between East and West Antarctica occurred prior to breakup of the Gondwana margin ca. 80 Ma. Mylonitic gneisses dredged from the eastern Ross Sea indicate shear-zone deformation considerably earlier, at 98–95 Ma. Strain analysis of fabrics indicates 85%–100% extension. Overprinting brittle structures record translation of shear-zone gneisses into the upper crust. Samples yield sensitive high-resolution ion-microprobe U-Pb zircon ages of 102–97 Ma, correlated to Byrd Coast Granite onshore, and concordant ⁴⁰Ar/³⁹Ar biotite and K-feldspar ages of 98–95 Ma, indicating that granites were mylonitized soon after emplacement and cooled rapidly. Apatite fission-track data corroborate this rapid cooling event, and reveal a second rapid cooling event ca. 80 Ma. Evidence for contemporaneous deformation and a similar thermal evolution at Deep Sea Drilling Project Site 270 on the Ross Sea central high and for a migmatite dome on land attests to the regional extent of intracontinental extension. Extension occurred at a time of complex microplate interactions along the Cretaceous active Gondwana margin, suggesting that distributed deformation in the overriding Antarctic plate may be related to plate boundary dynamics.

Key Words: West Antarctica • intracontinental extension • thermochronology • U-Pb SHRIMP • mylonite

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