THE OLD AND THE NEW: MERGING THE ROCK RECORD WITH GEOPHYSICAL DATA TO BETTER CONSTRAIN ANOMALOUS SEISMIC VELOCITIES IN MODERN SUBDUCTION ZONES (Invited Presentation)

Subduction zones host Earth’s largest earthquakes along the interface between plates. Down dip of these megathrust earthquakes, modern subduction zones exhibit enigmatic transient seismic and aseismic slip events that contribute to the global seismic budget and are collocated with a 3-8 km thick zone of anomalous seismic velocities (low velocity zones, LVZs). The relationship amongst LVZs, the subduction interface, and associated transient slip events remains unclear, with two competing models: 1) a fault interface that caps the LVZ and 2) a distributed ductile interface shear zone characterized by anomalous seismic velocities. To distinguish between these two models, we combine structural and microstructural data and geophysical analysis to estimate the geophysical signature of a fossil subduction interface shear zone.

The Condrey Mountain Schist (CMS) is a Late Jurassic to Early Cretaceous subduction complex in northern California and southern Oregon exhumed from 25-35 km (450°C, 0.8-1.1 GPa). Our structural mapping of a CMS subunit documented prograde ductile deformation distributed across a 3 km thick zone of primarily metasedimentary schists intercalated with m- to km-scale mafic and ultramafic lenses. Ductile fabrics are pervasively developed across heterogeneous lithologies, result in aligned seismically anisotropic minerals, and overprint relict quartz veins. Using these rock record constraints for mineral and lithologic proportions, mineral orientations, and fracture porosity, we calculated seismic velocities during prograde deformation using theoretical and empirical formulations and accounting for mineral and fracture anisotropy. Estimated seismic velocities are comparable to those characteristic of modern LVZs.

The thickness and seismic velocities of the CMS fossil subduction interface suggest that LVZs represent the seismic signature of distributed subduction interface shear zones and that further studies of LVZs can provide insight into spatial and temporal variations of modern subduction interfaces. Characterization of interface rheology requires research that merges rock record evidence with remote sensing data from modern subduction zones and is key for understanding transient seismic and aseismic slip events and their effect on subduction processes.