SILICICLASTIC RECORD OF RAPID DENUDATION IN RESPONSE TO CONVERGENT-MARGIN OROGENESIS, ROSS OROGEN, ANTARCTICA

Siliciclastic rocks of the upper Byrd Group in the Transantarctic Mountains record rapid denudation and molasse deposition during Ross orogenesis along the early Paleozoic convergent margin of Gondwana. These rocks stratigraphically overlie Lower Cambrian Byrd carbonates and are dominated by fresh detritus from proximal igneous and metamorphic sources within the Ross Orogen. Biostratigraphic evidence indicates that deposition of the syn-orogenic succession is late Botomian or younger (<515 Ma). U-Pb and 40Ar/39Ar mineral ages from detrital zircons and muscovites in the molasse sandstones are as young as ~520 and ~500 Ma, respectively, which restrict deposition to late Middle Cambrian or younger. Deposition and deformation of the molasse deposits terminated by ~490 Ma, based on ages from cross-cutting igneous bodies. The entire cycle from tectonism, denudation, sedimentation, deformation, and magmatism therefore appears restricted to a time interval of up to 13 m.y. in the late Middle Cambrian to earliest Ordovician, within the resolution of available stratigraphic and geochronologic data. Large volumes of molassic sediment were apparently shed into forearc and/or marginal basins at this time, primarily due to erosion of igneous rocks in the early Ross magmatic arc. The short time-lag between tectonism and sedimentary response is similar to that determined for other continental-margin arc systems such as in the Peninsular Ranges. Based on similar provenance for detrital zircon and muscovite in the same samples, we obtain cooling rates of 12-75 °C/m.y. using the youngest age populations. Evidence of rapid cooling and unroofing suggest that crustal thickening associated with both magmatic intrusion and structural shortening was balanced in part by erosional exhumation. Assuming an orogenic geotherm of ~50 °C/km, we obtain a denudation rate of 0.1-0.5 mm/a, which is comparable to that in recent convergent or collision orogens. Rapid erosion and unroofing in the axial Ross Orogen is consistent with a sharp carbonate-to-clastic stratigraphic transition observed in the upper Byrd Group, reflecting a sudden outpouring of alluvial-fan and fluvial-marine clastic detritus, and with denudation rates inferred from mineral cooling ages in the adjacent Nimrod Group metamorphic basement.