Friday Seminar (23 June 2023) Chengxin Jiang (ANU)

Title: Seismic constraints on subduction termination and lithospheric foundering in central California



The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here, we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a long-recognized high velocity feature in the California upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate, which is a remnant of the former Farallon plate that was captured by the Pacific plate at ∼19 Ma and resides offshore of central California. A dense broadband seismic transect of 53 seismometers was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly and record two years of continuous data. In this talk, I will present multiple seismic observations derived from the dense array and many surrounding stations to decipher the origin of the Isabella Anomaly, including Rayleigh and S-wave tomography, teleseismic receiver function and SKS splitting measurements. They together indicate that the anomaly is more of a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our tomography images support the occurrence of lithospheric foundering beneath the high elevation eastern Sierra Nevada, where a lower crustal low Vs layer consistent with a small amount of partial melt is found. However, the foundering probably has to a more localized extent and lacks connection to the Isabella Anomaly. Another interesting observation regarding the Isabella anomaly is that its surface projection corresponds to the creeping section of the San Andreas Fault. The fossil slab origin of the anomaly might introduce along-strike variations of plate boundary deformation that promote the aseismic creep of the SAF.