As the collision between India and Asia progressed, sediments in the foreland basin were incorporated into the Himalayas between two of its major faults, the Main Boundary Thrust and the Main Frontal Thrust. These sediments are termed the Siwalik Formation and provide a detailed record of the Himalayas’ erosional history, paleoclimate, transitions in paleobotany and biology, and exhumation. One critical question regarding Himalayan evolution that can be addressed by detailed analyses of Siwalik Formation sediments is how its record of metamorphic minerals tracks with our ideas of metamorphic processes recorded by bedrock geology. To address this question, we chemically analyzed garnets from the middle and upper sections of the Siwaliks exposed in the Surai Khola of Nepal. The Surai Khola samples cover the Middle Miocene through Pleistocene time interval (13-1 Ma). Garnets from the sections are abundant at some levels and appear chemically distinct. Their chemistry and compositional zoning allow the affiliation to be made to particular units within the Himalayas. For example, Greater Himalayan garnets have compositions and zoning consistent with higher-grade metamorphism (low Mn, flat zoning). In contrast, Lesser Himalayan Formation garnets preserve more prograde compositions (high Mn cores, bell-shaped profiles). We aim to use the appearance, relative abundance, and composition of metamorphic index minerals in the Siwalik sandstones, including garnets, to constrain the validity of models for Himalayan metamorphism and processes.

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.