Deep seawater circulation on oceanic transform faults controlled by the seismic cycle

Arjun Kohli, Stanford University

Wednesday, September 4, 2019 at 10:30 AM

Location
Building 3, Rambo Auditorium
Host
Nicholas Beeler/David Lockner

Oceanic transform faults (OTFs) represent abundant, yet relatively unexplored pathways for fluid flow in the mantle lithosphere. Current models limit seawater circulation to 600 °C, the thermal limit of earthquakes recorded by teleseismic surveys. However, recent geologic and geophysical studies indicate that seismicity extends beyond 600 °C, creating a pathway for deep seawater circulation.

Peridotite mylonites dredged from OTFs on the Southwest Indian Ridge show evidence of brittle deformation and fluid flow concurrent with ductile deformation of olivine. Fluid-rock interactions resulted in fault weakening, grain size reduction, and growth of syn-deformational amphibole. The high chlorine content of amphibole and its association with brittle features suggest that seawater was the fluid source. Geothermometry and thermodynamic analyses of the mylonite mineral assemblage indicate that fluid flow and brittle-ductile deformation occurred at temperatures ~600-900 °C (~14-22 km depth). We interpret the shallow limit of the brittle-ductile transition zone as a fine-grained, hydrated shear zone, and the deeper limit as the transition to distributed, dry deformation of coarse-grained peridotite.

Our results are supported by ocean-bottom seismic studies of OTFs on the East Pacific Rise that have located earthquakes at depths corresponding to temperatures >1000 °C. The presence of earthquakes at the conditions of mylonite hydration and brittle-ductile deformation suggests that deep seawater circulation is controlled by the seismic cycle. We propose that feedbacks between seismicity, fluid flow, and mantle rheology determine how plate motion is accommodated on OTFs.

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