EXPLORING NATURALLY DEFORMED FELDSPAR MYLONITES FROM A BRITTLE-DUCTILE TRANSITION: IMPLICATIONS FOR CRUSTAL RHEOLOGY

Feldspar dominates the middle and lower continental crust. Models of crustal strength and rheology depend on our knowledge of feldspar deformation mechanisms. Experimental studies provide estimates for feldspar flow-law parameters, but relatively few case studies exist from naturally deformed feldspar-dominated shear zones with well established framework geology, strain rates, and thermal histories. We present EBSD and microstructural observations from an exhumed and deformed Miocene brittle-ductile transition (BDT) exposed in a Cretaceous pluton in the Colorado River Extensional Corridor, southern Nevada. Our sampling traversed the mylonitic BDT allowing us to examine and compare microstructures and deformation mechanisms of feldspar-dominated shear zones under various depth-T conditions. Prior to deformation, the samples across the BDT were near ~10 km depths at background thermal gradients that would have resulted in temperatures at or below quartz plasticity (~350-300°C). Intrusion of two surrounding 15-16 Ma plutons heated the crust significantly. Subsequently, the BDT and its encompassing crustal section exhumed and deformed in the footwall of an east-directed detachment fault. Localized shear zones formed at temperatures of >600°C at ~15.5 Ma. The rocks continued deforming as they cooled both conductively and advectively to ~200°C by ~14 Ma, as constrained by zircon (U-Th)/He ages. In a feldspar-dominated ultramylonite, we observed ~5 micron feldspar with four-grain junctions and weak-to-absent CPO. Larger feldspar porphyroclasts have increasing misorientations toward their rims, and recrystallized feldspar mantling the porphyroclast displayed stronger CPOs than the fine matrix grains. In the same sample, local quartz ribbons and matrix quartz grains show CPO indicative of high-T and low-T deformation. In contrast, the undeformed part of the pluton has coarse cm-mm-sized feldspar grains. We interpret that the feldspar experienced grainsize reduction via dislocation creep and subgrain rotation recrystallization, possibly following established high-T piezometry curves during progressive cooling. Grainsize reduction due to dynamic recrystallization resulted in weakening as the feldspar-dominated rock transitioned to grain-size-sensitive diffusion creep permitting further strain localization in weaker shear zones. We are evaluating other factors such as background stress, strain rate, and the cooling history and how these affect crustal rheology together in feldspar-dominated crust.