QUANTIFYING PORE-FLUID PRESSURE RATIOS AND ANALYZING DEFORMATION MECHANISMS IN THE WHIPPLE MOUNTAINS BRITTLE-DUCTILE SHEAR ZONE

Pore-fluid pressure is a key factor controlling the stress state and rock failure in Earth’s crust. Although its role in brittle deformation in the shallow crust (< 1-3 km) has been extensively examined, and in some cases quantified by direct bore-hole measurements, how pore-fluid pressure affects crustal deformation at brittle-ductile-transition depths (~15-25 km) remains poorly constrained. Deformation that occurred at brittle-ductile transition depths is commonly expressed by the development of semi-brittle shear zones characterized by coeval cataclastically (frictional sliding and fracturing) and crystal-plastically (dislocation and diffusion creep) of deformed rocks. The distinct deformation styles within the same shear zone require stress continuity across the contact between brittle and ductile structures. This stress-continuity condition in turn allows us to use paleobarometry and paleopiezometry to determine the stress state (i.e., the differential stress and mean stress) during semi-brittle deformation. Because the frictional coefficient (~0.6) and cohesive strength of crystalline rocks (<50 MPa) are well-known from laboratory experiments (i.e. Byerlee’s Law), we are able to use the estimated differential and mean stresses to determine the ratio between pore-fluid pressure and lithostatic pressure during the development of the Whipple detachment shear zone. Our results suggest pore-fluid pressure ratios between 0.92 and 1.11 which is consistent with observed tensile-fracture veins (pore-fluid pressure ratio >1.0) developed during the crystal-plastic deformation of quartzite in the shear zone. New P-T and monazite-in-garnet U/Th-Pb age constraints combined with the pore-fluid pressure ratios provide insights into the evolution of the Whipple detachment shear zone. A new P-T-t path may be established for the rocks in the Whipple shear zone and may suggest a deeper initiation or multiple metamorphic events not previously analyzed.