NEW INSIGHTS INTO THE PROTEROZOIC EVOLUTION OF THE WESTERN MARGIN OF LAURENTIA AND THEIR TECTONIC IMPLICATIONS

A large section of SWLaurentia consists of a poorly constrained mosaic of Archean and Proterozoic crust that accreted to the Wyoming craton (WC) subsequent to its incorporation into Laurentia at ~1.86 Ga. The western margin of the Wyoming craton, which is dominated by crust formed during two distinct Archean cycles (2.7-2.9 and 3.2-3.5 Ga), acted as a rigid and impenetrable lithospheric buttress to the repeated juxtaposition of late Archean and Proterozoic crust. Geo- and thermochronologic data clearly indicate significant, widespread events involving magmatic additions to the crust that were accompanied by metamorphism and crustal melting at 2.2-2.5 Ga. Rocks of this age are found within the western Great Falls tectonic zone (MT-ID) and as far south as the Farmington Canyon complex (UT). Younger Proterozoic tectono-thermal events are not as widespread, but include burial and crustal melting of the NW margin of the Wyoming craton at 1.77 Ga and a widespread tectono-thermal event at ~1.65 Ga that is recorded from UT to southwestern MT/eastern ID. Crust of intermediate ages (e.g., 1.81 to 1.91) has also been documented along this margin. Because none of the currently recognized tectonothermal events associated with Proterozoic accretion resulted in the formation of magmatic systems within the Wyoming craton, we propose that 1) crust accumulated along a transpressional margin that stretched from at least the Uinta reentrant to the northern margin of the Great Falls tectonic zone and that 2) any coeval subduction was directed outboard from the Wyoming craton. Unraveling the spatial and temporal distribution of this crust is critical to understanding the overall plate tectonic regime that characterized Proterozoic accretion along the southern margin of Laurentia and for identifying the specific terranes that may have been joined to Laurentia prior to Neoproterozoic rifting. Present evidence is most compatible with an extended period of largely SW-NE directed convergence that resulted in the accretion of the Wyoming craton and smaller segments of Proterozoic crust to Laurentia in the Paleoproterozoic. Characterization of these numerous crustal components through space and time will be essential to guide and interpret the results of future EarthScope experiments.