INVESTIGATING ROTATION OF THE MIOCENE SEARCHLIGHT PLUTON, NV: HOW EXTENSIONAL TILTING PROMOTES RAPID COOLING AND STRENGTHENING OF THE UPPER CRUST

Magmatism, deformation, and exhumation critically affect the regional geothermal profile, surface heat flux, and cooling history of the crust. The thermo-rheological evolution of extending crust has previously been quantified assuming a variety of pure- and simple-shear-shear models, which each predict different rates of crustal cooling. Conceptually, pure-shear extension relies primarily on conductive cooling, whereas simple-shear extension, accommodated by rotation of the footwall rocks toward the fault surface, is a dominated by advection. We examined the Miocene Searchlight pluton, southern Nevada, which rotated ~90° on its side during regional extension in an east-directed detachment fault almost immediately following its 17-16 Ma emplacement. We hypothesize that flow-like footwall rotation characterized this tiling process, which we test via integrated field, structural, petrologic, and EBSD analyses across the tilted pluton to document sub-solidus fabrics and deformation temperatures/mechanisms. We observe distributed top-to-the-west sub-solidus deformation that is consistent with a flow-like tilting mechanism. Shear strain rates related to this rotation and regional extensional strain rates are comparable at ~10^-15 s^-1. Recrystallized quartz from the deepest part of the pluton display prism <c> slip conditions, indicative of higher temperatures and hydrous conditions. The extensive mineralization around the Searchlight pluton demonstrates the importance of ore-bearing fluids in the area. Inspired by these observations, we present a simple cooling model that highlights how horizontal-axis rotation-dominated extension more effectively cools the crust than pure-shear extension because the hottest deep materials are brought rapidly toward the cooler surface at the rotation rate (15°/myr in this case). This extensional mechanism efficiently cools the upper crust, potentially causing a negative feedback whereby the rapidly cooled crust becomes strong enough to halt further simple-shear style extension. High-angle normal faults with significantly lower offsets dominate the most recent deformation regime. The extensional tilting we document may be an important process in extensional tectonics across the western US and globally.