Lithospheric Structure in Central California: Towards Identifying the Tectonic Origin of the Isabella Anomaly

Abstract:

The tectonic origin of the Isabella high-velocity anomaly in the upper mantle beneath California’s southern Great Valley is unclear. Previous low-resolution seismic imaging studies of the region have been unable to identify the structural connection between this upper mantle anomaly and the overlying lithosphere. The two dominant hypotheses attribute the Isabella anomaly to a fossil slab or the foundered lithospheric root of the Sierra Nevada batholith. The Central California Seismic Experiment (CCSE) is designed to distinguish between these hypotheses. We present results from the CCSE, which consists of 44 broadband seismometers currently deployed in a quasi-linear array spanning from the Pacific coast, across the Great Valley, to the Sierra Nevada foothills, at an approximate latitude of 36°N. Forward modeling of the 2D structure of the crust is performed using local earthquakes recorded by the CCSE and a finite-difference algorithm to provide constraints on the geometry and velocity of the seismic structure of the Great Valley. This sedimentary basin is suggested to be filled with very low velocity material at shallow depths and partially underlain by a high-velocity ophiolite body. Hence, a well-constrained basin structure will be important in correcting surface wave tomography and receiver function images. The impact of the Great Valley basin structure on body waves is evident by an observed delay in P-wave arrival times on the radial component relative to the vertical component for stations located within the basin. Surface waves along the CCSE array also show a distinct slowing by the valley at periods <10 sec. Data from teleseismic events recorded by the CCSE reveal scattered waves arriving tens of seconds after the S-wave, which we will interpret in terms of the lithospheric structure of the region by identifying the source location(s) of the scatterer(s). We may also gain insights into the structural connection between the Isabella anomaly and the overlying lithosphere with this analysis.