Geophysics and Tectonics Seminar spring-2021

One of the most important factors that controls the stress state and rock failure in Earth’s crust is pore-fluid pressure. 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. This depth range is dominated by microseismicity and fluid-driven tectonic tremors such as those along the root zone of the San Andreas fault. In the field, deformation that occurred at brittle-ductile transition depths is commonly expressed by the development of semi-brittle shear zones. A semi-brittle shear zone is characterized by coeval cataclastically (frictional sliding and fracturing) and crystal-plastically (dislocation and diffusion creep) deformed rocks. The mixed 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 paleopiezometry and paleobarometry to determine the stress state (i.e., the differential stress and mean stress) during semi-brittle deformation. Note that the mean stress estimated from paleobarometry differs from lithostatic stress, which represents the value of a principal stress that can be determined by the combination of the mean stress and differential stress values. 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 and rock-fracture experiments), we are able to use the estimated magnitudes of differential and mean stresses to determine the ratio between pore-fluid pressure and lithostatic pressure during the development of the Miocene Whipple detachment shear zone. This exhumed mid-crustal, normal-slip shear zone was formed by semi-brittle deformation, and consists of brittlely deformed amphibolite blocks and ductilely sheared quartzite. Assuming 0.6 for the friction coefficient and 50 MPa for the cohesive strength of the amphibolite blocks in the Whipple shear zone, our results require the pore-fluid pressure ratio of 0.90 to 1.10. The excessive pore-fluid pressure ratio with a value greater than 1.0 is consistent with observed tensile fractures developed during the crystal-plastic deformation of quartzite in the shear zone. Our future work is to determine the distribution of pore-fluid pressure ratios across the Whipple shear zone. This knowledge will allow us to better understand fluid-migration processes and pore-fluid-pressure evolution during semi-brittle deformation in the mid-crustal shear zone. Laboratory measurements of the friction coefficient and cohesive strength of the amphibolite blocks will provide a tighter constraint on the range of the estimated pore-fluid pressure ratios in the Whipple shear zone.