SIGNIFICANCE OF FLUIDS ON POTENTIAL SLOW SLIP AND TREMOR SOURCES: INVESTIGATING SERPENTINITE RHEOLOGICAL EVOLUTION IN THE CONDREY MOUNTAIN SCHIST, NORTHERN CALIFORNIA (Invited Presentation)

Slow slip and tremor (SST) events occur downdip of the megathrust in modern subduction zones and are collocated with seismic low velocity zones (LVZs) indicative of near-lithostatic pore fluid pressures (P_f). Multiple models exist for SST sources but all require brittle failure and transient accelerated slip. Exhumed subduction complexes, such as the blueschist facies unit of the Condrey Mountain Schist (lower CMS), northern California, can help constrain potential SST sources. The lower CMS records distributed prograde viscous deformation at SST depths (460°C, ca. 1 GPa) across a primarily metasedimentary shear zone. Intercalated m- to km-scale mafic and serpentinized ultramafic lenses are comparable in length scale to modern tremor sources.

To evaluate the serpentinite lenses as potential SST sources, we characterized serpentinization timing (pre-, syn-, or post-subduction) using δD and δ¹⁸O to fingerprint the fluids and serpentine rheology using macro- and microstructural observations and Raman spectroscopy to identify serpentine species. δD and δ¹⁸O are most consistent with seafloor serpentinization. Subsequent deformation within the subduction zone was characterized by an initial phase resulting in m- to μm-scale pods with brittle veins wrapped by anastamosing foliations defined by highly strained antigorite followed by a subsequent phase of strain localization in antigorite at the lens’ structural base. Antigorite microstructures in high strain zones suggest a combination of viscous mechanisms, including glide, kinking, and pressure solution.

The ultramafic protolith metasomatized to a low T serpentine species pre-subduction that subsequently metamorphosed to antigorite syn-subduction. Despite early metasomatism to a weaker phase, a recently published flow law predicts the antigorite brittle-viscous transition at CMS depths and modulated by P_f for λ~0.9, consistent with observed microstructures and LVZs. Coeval brittle and complex (non-Newtonian) viscous deformation of antigorite at SST depths is potentially consistent with tremor and accelerated slow slip due to strain localization along the lens margins, respectively. Estimated tremor M_w for these lenses using vein displacement and lens area (M_w 1-3) is also comparable to modern tremor. These data collectively suggest that the CMS serpentinite lenses are potential fossil SST sources.