The manner in which the Earths crust responds to tectonic and gravitational forces depends on the strength distribution of crustal rocks. It has been widely proposed that in certain settings, rocks in the lower half of the crust may be sufficiently weak that they flow laterally in response to pressure differences induced by topographic gradients. Such behavior has important implications for many processes in the continental crust, for example, the growth of mountain plateaus, formation of basins and continental shelves, and seismicity within the continents. These models, however, need to be rigorously tested through better field-based investigations. The Pioneer Mountains of Idaho, where these PIs will focus their study, expose rocks that may represent a boundary zone between strong rocks and underlying weaker flowing rocks, and if so will provide an excellent opportunity to test models that involve flow of weak lower crustal rocks. Better field documentation of the vertical changes in rocks strength will provide us with a more comprehensive view of continental deformation.
Many recent models of crustal deformation feature a middle to lower crust that is sufficiently weak that it decouples and flows in response to lateral pressure gradients. Such flow has profound implications for several aspects of crustal deformation. Many of the current models for weak decoupled middle to lower crustal flow, however, are derived from indirect observation and the extent to which model rheological layers are represented in the rock record is unclear. Preliminary work shows that the Pioneer core complex in Idaho contains a midcrustal section exposing a decoupling boundary, which presents an opportunity to test models of weak and decoupled flow. In this project the PIs with test models of variable decoupling will be tested through documentation of the strain history in the midcrustal rocks through field-based structural and AMS analysis, combined with U-Pb geochronology, and Ar/Ar thermochronology. Together these data will allow us to compare in detail, the kinematic and strain histories at the various structural levels and investigate the relationships between the mid-crustal rheological layering, domes, and upper crustal extension.
