Late sinistral shearing along Gondwana's paleo-Pacific margin in the Ross Orogen, Antarctica: New structure and age data from the O'Brien Peak area

New isotopic data from O'Brien Peak, Antarctica, provide constraints on the timing of deformation within the Ross Orogen in the Queen Maud Mountains. U-Pb data, in conjunction with structural data from a mylonitic granite, indicate that sinistral, orogen-parallel ductile shearing occurred after similar to 489 Ma, the granite's crystallization age, and before 478 +/- 3 Ma to 473 +/- 3 Ma, the 40Ar/39Ar cooling ages of undeformed and synkinematic white mica, respectively. These data provide evidence for the youngest sinistral shearing event within the Ross Orogen to date and indicate that left-oblique subduction possibly continued into the Early Ordovician along Gondwana's paleo-Pacific margin.