Grant: Discovery Grants Program - Individual

The repeated assembly and dispersal of supercontinents have profoundly influenced the Earth?s evolution. Pangea, the latest of these supercontinents, formed about 300 million years ago (300 Ma). There is consensus that ?top down? tectonic processes are probably responsible for supercontinent amalgamation. However, if top-down processes are applied to a 500 Ma paleogeography, they fail to yield Pangea in the correct configuration. As a corollary, we have no viable model for the coupling between processes in the Earth?s crust and its mantle, or for understanding long-term global change. Geodynamic and geological models are largely aimed at explaining the crustal record and the role of the mantle lithosphere is not adequately constrained. The oceanic lithosphere is largely consumed by subduction (and so we must use proxies to deduce its evolution) and the continental lithospheric mantle (CLM) is rarely exposed. The main objective of this proposal is to use the preserved geological record to better understand the evolution of the mantle lithosphere during the assembly and amalgamation of Pangea. To achieve these objectives, I will focus on mantle processes that accompanied two time intervals pivotal to the amalgamation of Pangea: (i) the early evolution of the Rheic Ocean (490-440 Ma), a time interval accompanied by global-scale changes in mantle dynamics, and (ii) the demise of the Rheic Ocean in the Late Paleozoic which resulted in the continental collisions that produced the Appalachian-Variscan orogenic belt in the interior of Pangea. The early evolution of the Rheic Ocean coincided with global proxies for ocean dynamics including the highest global sea level and the highest 87Sr/86Sr concentration in ocean water in the Phanerozoic. The hypothesis I want to test is whether the 490-440 Ma evolution of the Rheic Ocean coincided with a mantle superplume event that fundamentally affected Paleozoic plate motions, setting the stage for Pangea to form in the Late Palaeozoic. For Rheic Ocean closure, I will deduce the extent of crust-mantle coupling by determining the isotopic (Lu-Hf, Sm-Nd) signature of rocks over a range of time (before, during, after collisional orogeny) so that mantle replacement events (MREs), which produce more juvenile isotopic compositions in mantle-derived magmas, can be deduced. These analyses, combined with detailed field work by myself and my students will document (i) the CLM evolution and timing of MREs relative to stratigraphic, structural, metamorphic and magmatic responses in the crust, and (ii) the potentially changing relationships between the CLM and overlying crust before, during and after collisional orogenesis. The results of this research will provide greater insight into the role of the oceanic and continental lithospheric mantle in the formation of Pangea, and supercontinents in general.