Contrasting thermal evolution within the Ross orogen, Antarctica: Evidence from mineral ⁴⁰Ar/³⁹Ar ages

The Nimrod Group in the central Transantarctic Mountains and the Lanterman Metamorphic Complex in northern Victoria Land of Antarctica constitute internal metamorphic basement terrains of the Ross orogen that share many first-order similarities in lithology, structure, and metamorphism. However, new ⁴⁰Ar/³⁹Ar cooling ages determined for hornblende and muscovite from tectonites in the Geologists (Nimrod Group) and Lanterman ranges (Lanterman complex) indicate that these terrains experienced different post-kinematic cooling histories. Nimrod ductile L-S tectonites derived from igneous and sedimentary protoliths yield ⁴⁰Ar/³⁹Ar hornblende cooling ages of 524-495 Ma and muscovite cooling ages of 499-496 Ma. These ages are interpreted to date the cooling following syn-metamorphic ductile deformation. Combined ⁴⁰Ar/³⁹Ar and existing U-Pb age data yield an average post-kinematic cooling rate for the Nimrod tectonites of ∼ 10°C/m.y. Metasedimentary L-S tectonites of the Lanterman complex yield ⁴⁰Ar/³⁹Ar cooling ages of ca. 486 Ma (hornblende) and ca. 482 Ma (muscovite). These cooling ages indicate an average post-kinematic Lanterman cooling rate of ∼ 30°C/m.y. We interpret the contrasting thermal histories of these terrains to be the result of different modes of accretion along the orogen, due in part to oblique subduction of paleo-Pacific lithosphere beneath the East Antarctic craton. In the central Transantarctic Mountains, Nimrod cooling rates indicate relatively modest post-tectonic denudation rates (∼0.4 mm/a), resulting from crustal thickening in an upper platemargin setting. Contraction of adjacent continental-margin supracrustal sequences did not involve significant underthrusting, possibly as a result of a high component of margin-parallel translation. In contrast, markedly faster Lanterman cooling rates indicate more rapid denudation (∼1.2 mm/a) of thickened crust in northern Victoria Land. Kinematic and cooling-rate data from the Lanterman complex indicate that accretion of outboard lower Paleozoic volcanic arc and continental-rise assemblages involved near-orthogonal underthrusting beneath crystalline basement, leading ultimately to rapid tectonic denudation.