Collaborative Research: Geological Constraints on the Physical State of the Subduction Zone Interface at the Depth of Slow Slip and Tremor

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Subduction complexes such as the Franciscan Complex of California contain large volumes of rock that have been exhumed from up to 40 km depth, and which preserve a record of the ambient physical conditions and deformational processes. In particular, the South Fork Mountain Schist (SFMS) in the Northern Coast Ranges is a 240 km long and up to 5 km thick body of intensely deformed sedimentary and volcanic rock that occupied the subduction zone interface at around 123 Ma, at a depth and temperature corresponding to the source area of present-day episodic slow-slip and seismic tremor (ETS) events. The SFMS preserves an extraordinary high-strain deformational microstructure that indicates the activity of a combination of microcracking, pressure-solution and dislocation creep at relatively high stress, burial depths of around 40 km, and temperatures of around 350 degrees C. The SFMS offers an easily accessible window into the physical conditions and deformational mechanics during ETS events. This project quantifies (a) deviatoric stress using recrystallized grain-size piezometry on rocks affected by dislocation creep, (b) strain and strain-rate using the estimated subduction rate for this period, the thickness of the deformation zone, and microstructural features including microfold geometry and deformed clastic quartz and radiolarians, (c) temperature using laser Raman spectroscopy on carbon and the Ti content of quartz, (d) pressure (and hence depth) using phase assemblages in the blueschist-facies rocks, and multi-equilibrium thermobarometry on coexisting chlorite and white mica, and (e) water activity using cathodo-luminescence and FTIR analyses on quartz. These measurements will allow thermomechanical modeling of the effect of dissipative heating, and calculation of the rate of moment release, which can be compared with the seismic record during ETS events. Elastic modeling of the rate of crack propagation will allow calculation of the rate of fluid transport and the rate of propagation of slip events.The most powerful and damaging earthquakes known occur between 15 and 40 km depth in regions known as subduction zones where oceanic plates are being carried down into the Earth's mantle. Below that, there is a transition downwards from the abrupt movements that cause earthquakes to steady sliding without earthquakes. This transition produces several types of ground motion, including "slow earthquakes", in which slip continues for up to two weeks, but without ground shaking. The transition may play a role in building up the stress that produces normal earthquakes at shallower depths. This project investigates rocks in northern California that were formed in a subduction zone at about 40 km depth, and which preserve a record of the physical conditions (pressure, temperature, stress, and water content) and processes that lead to slow earthquakes and related phenomena in present-day subduction zones.