Seismic signature of a fossil subduction interface shear zone in the Condrey Mountain Schist, northern California

Abstract:

Most modern subduction zones have a 3-5 km thick low seismic velocity zone (LVZ) down-dip of the subduction megathrust that is spatially correlated with Episodic Tremor and Slow Slip (ETS) events and characterized by anomalously high V_p/V_s (1.8-2.5). This seismic signature is consistent with near-lithostatic pore fluid pressures in 3-4% fracture porosity. The LVZ could be 1) the oceanic crustal portion of the downgoing slab beneath a narrow interface shear zone or 2) a distributed interface shear zone. To differentiate between these two, we estimated the thickness and seismic signature of a fossil subduction interface shear zone in the Condrey Mountain Schist (CMS) subduction complex in northern California.

The CMS is predominantly hemipelagic sediment with m- to km-scale lenses of mafic and serpentinized ultramafic rocks, all metamorphosed to epidote blueschist facies (460°C, 1 GPa). Protoliths subducted along a sediment-poor erosive margin and record distributed ductile deformation with coeval brittle failure across 3+ km of graphitic mica schist (gms) with periodic strain localization to <1 km in serpentinite. We calculated V_p/V_s assuming isotropic (1), anisotropic (2), fractured isotropic (3), and fractured anisotropic (4) lithologies using the MATLAB seismic anisotropy toolbox (MSAT) and various theoretical formulations. We constrained mineral and lithologic proportions, mineral orientations, maximum fracture porosity (5-13%), and aspect ratios over a range of scales from outcrop to thin section. For cases 2-4, which are most consistent with field and microstructural observations, the estimated V_p/V_s is highly anisotropic and anomalous (1.7-2.3) for near-vertical incidence angles.

Interface thicknesses and V_p/V_s constrained by field observations are comparable to modern LVZs, suggesting that the LVZ includes the distributed interface shear zone, consistent with rock record evidence of ETS source regions collocated with the interface shear zone. Given near-ubiquitousness of modern LVZs despite vast differences in sediment supply at the trench, researchers have previously assumed minor sediment contributions. The CMS, however, reveals that the interface shear zone can be dominantly sediment despite limited sediment input at the trench.