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Variations in the P-T-t of Deformation in a Crustal-Scale Shear Zone in Metagranite
  • Tarryn Kim Cawood,
  • John Paul Platt
Tarryn Kim Cawood
University of Southern California

Corresponding Author:cawood.tk@gmail.com

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John Paul Platt
University of Southern California
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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 synkinematic mineral assemblages amenable to traditional metamorphic petrology and petrochronology. We overcome these challenges by studying the normal-sense Simplon Shear Zone 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 hangingwall. The Ti-in-quartz and Si-in-phengite thermobarometers yield deformation P-T conditions, as both were reset synkinematically, 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 (490.2°C, 6.73 kbar) at ~24.5 Ma, to lower greenschist-facies conditions (303.7°C, 1.51 kbar) at ~11.3 Ma, with subsequent slip taken up by by brittle faulting. Comparison to independent constraints on the maximum and minimum P-T-t conditions, and to alternate approaches for estimating P-T, suggests that our results may be reasonable, or may underestimate temperatures by up to ~30-90°C.
Nov 2020Published in Geochemistry, Geophysics, Geosystems volume 21 issue 11. 10.1029/2020GC009384