Variations in the P-T-t of Deformation in a Crustal-Scale Shear Zone in Metagranite

Deformation in crustal-scale shear zones occurs over a range of pressure-temperature-time (P-T-t) conditions, both because they may be vertically extensive structures that simultaneously affect material from the lower crust to the surface, and because the conditions at which any specific volume of rock is deformed evolve over time, as that material is advected by fault activity. Extracting such P-T-t records is challenging, because structures may be overprinted by progressive deformation. In addition, granitic rocks, in particular, may lack syn-kinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal-sense Simplon Shear Zone (SSZ) in the central Alps, where strain localization in the exhuming footwall caused progressive narrowing of the shear zone, resulting in a zonation from high-T shearing preserved far into the footwall, to low-T shearing adjacent to the hanging wall. The Ti-in-quartz and Si-in-phengite thermobarometers yield deformation P-T conditions, as both were reset syn-kinematically, and although the sheared metagranites lack typical petrochronometers, we estimate the timing of deformation by comparing our calculated deformation temperatures to published thermochronological ages. The exposed SSZ footwall preserves evidence for retrograde deformation during exhumation, from just below amphibolite-facies conditions (similar to 490 degrees C, 6.7 kbar) at similar to 24.5 Ma, to lower greenschist-facies conditions (similar to 305 degrees C, 1.5 kbar) at similar to 11.5 Ma, with subsequent slip taken up by brittle faulting. Our estimates fall within the P-T-t brackets provided by independent constraints on the maximum and minimum conditions of retrograde ductile deformation, and compare reasonably well to alternative approaches for estimating P-T.