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 (∼490°C, 6.7 kbar) at ∼24.5 Ma, to lower greenschist‐facies conditions (∼305°C, 1.5 kbar) at ∼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. Plain Language Summary: Major shear zones deform material over a wide range of conditions, from low pressures and temperatures near the Earth's surface, where rocks are brittle, to high pressures and temperatures in the deep crust, where they are ductile and flow like warm toffee. The age and conditions of deformation can be estimated using mineral chemistry. However, this is difficult where rocks have been deformed over a range of conditions at different times. It is especially challenging in granite, which commonly lacks traditionally used minerals. In studying the Simplon Shear Zone, we overcome these challenges by carefully investigating its structure to untangle the deformation stages; by using the chemistry of quartz and phengite to estimate deformation conditions, both of which occur in granite; and by linking our temperatures to a published cooling history, to determine when our samples were deformed. We show that exposed rocks deformed between ∼490°C at 6.7 kbar (∼25 km depth) approximately 24.5 Myr ago, and ∼305°C at 1.5 kbar (∼6 km) approximately 11 Myr ago. Subsequent deformation was taken up by brittle faulting. Our estimates compare well to independent constraints and alternative approaches. Key Points: The conditions of deformation in crustal‐scale shear zones may vary spatially (with depth) and temporally (due to dip‐slip motion)We use a multidisciplinary approach to quantify the timing and pressure‐temperature conditions of shearing in the Simplon Shear ZoneExposed rocks preserve evidence of deformation conditions from ∼490°C and 6.7 kbar at 24.5 Ma, to ∼305°C and 1.5 kbar at 11.5 Ma [ABSTRACT FROM AUTHOR]