4-D EVOLUTION OF THE APPALACHIAN OROGEN : FATE OF PALEO-LITHOSPHERES

The evolutionary history of the Appalachian Geologic Province (1200 Ma to present) has been studied for more than 200 years, including use of tools applicable for imaging the subjacent lithosphere and deeper mantle. The region preserves a complete geologic record of two successive supercontinent assemblies and dispersals, including evidence for rifting and development of passive margins, accretion of island arcs, as well as continental and oceanic blocks. High resolution SIMS U/Pb zircon ages from igneous rocks provide the events chronology that can be related to each of these records. To extend the observed geologic events to mantle lithosphere requires both geometric and temporal constraints for the evolution of the subjacent lithosphere mantle (SCLM). SCLM ages based on lead isotope modeling of Mesozoic and Cambrian basalts, has been estimated to be approximately 1 Ga. Where are the other SCLM that must have been involved in the many accretionary events and what is the fate of such paleo-lithospheres? A continental scale tectonic model able to explain this physical decoupling of the complex crustal column with the SCLM is related to the geodynamic interaction of plates, where one of the plate margins contains well developed depocenters associated with passive margins. Following discoveries in the Canadian Cordillera that accretion of terranes against thick sedimentary prisms effectively decoupled the terranes from their deep crust and mantle (Snyder.et.al. 2002), it is proposed that interaction of accreting terranes against paleo-depocenters (Thomas, 1993) also removed their SCLM as well. The model provides a possible explanation for the presence of only Grenville age mantle lithosphere under the complex collage of terranes observed at the surface. Existing seismic data suggests that all terranes are rooted above the MOHO, and the available model ages support a mechanical decoupling of the crustal column from the subjacent mantle. The consequences of such an age discontinuity between the crustal column and the SCLM has significant impact on interpreting EarthScope data, and requires the application of new radiometric technologies to further understand the temporal relationship between crust and mantle.