From early transpression to late extension: The post-orogenic history of the southeastern Grenville Province

The Grenville Province is a remnant of a ca. 1.1 Ga orogen, exposed as a continuous belt in northeastern North America. Rocks showing evidence of Grenville-aged deformation occur all over the world and represent the final amalgamation of the Proterozoic supercontinent Rodinia. By studying the timing and nature of motion along crustal-scale, ductile shear zones, it is possible to place constraints on the tectonic history of this part of the Grenville orogen. This, in turn, provides information on how the lithosphere evolves after a major orogenic event and what kinds of driving mechanisms play a major role in post-orogenic modification and stabilization of the crust.; To constrain the temporal evolution of these shear zones, a detailed structural and thermochronologic study was undertaken across the Metasedimentary Belt in the southeastern portion of the Grenville Province in Ontario and New York State. Ages are based on radiogenic isotope analysis, using the decay of potassium to argon in the minerals hornblende, biotite, and potassium feldspar, and uranium to lead decay in the mineral sphene. Primarily because of differences in diffusion, different minerals will close to diffusion of radioactive elements at different temperatures. Assuming a general geothermal gradient, these temperatures can be assigned corresponding depths in the crust, and a measurement of ages across a shear zone gives an estimate on the amount and timing of displacement. Coupled with field structural analysis, it is then possible to determine the type of displacement and tectonic significance of these boundaries.; In this portion of the Grenville Province, analyses show a protracted tectonic history after the latest episode of contraction. After an initial transpressive event, most shear zones in this part of the Grenville Province are reactivated as extensional zones. Ca. 300 myr later, by 780 Ma, extension has ceased and the region uplifted as a coherent block. Using these time constraints, it is possible to develop quantitative models to explore the physical evolution of the crust. Numerical analyses give some constraints on the behavior of the lithosphere after orogenesis and may give insights on the driving mechanisms that govern post-orogenic extension in mountain belts.