A PASSIVE SEISMOLOGICAL VIEW OF THE COSO VOLCANIC SYSTEM

Passive seismological experiments over the past 20 years have illuminated the Coso volcanic system from upper crust into the upper mantle. Frank Monastero funded our focused deployment of arrays of seismometers in 1998-2000 (Wilson et al., 2003, JGR), which, when analyzed for converted (P to S) arrivals, yielded tight controls on the lateral extent of a low-velocity body interpreted to be the main silica-rich magma reservoir beneath the volcanic field. This body was inferred to have partial melt amounts of 5-20% from depths of ~5 to 15 km. The lateral extent of melt was limited to be under the rhyolite dome field at the center of the volcanic field, confirming ideas that a rhyolitic magma chamber intercepted any rising basalt, thus creating the geometry of the bimodal field at the surface. A broader analysis using the same data recovered shear wave speeds from ambient noise analysis (Yang et al., 2011, G³). Low wavespeeds between 6 and 12 km below sea level overlap with the inferred magma body from the converted wave analysis, but wavespeeds at this depth grow slower farther east into Death Valley, suggesting some connection to magmatic centers there. Our recent teleseismic study (Bernardino et al., 2019, Geosphere) of the broader Sierra Nevada region recovers variations in vp/vs ratios, which are profoundly high in only three places: the Clear Lake volcanics field, the Mono-Inyo volcanic field, and Coso. Unlike the other two, the deeper levels of this anomaly in the mantle plunge to the southeast at upper mantle depths (c. 70 km), yielding a similar impression as from the ambient noise tomography that the Coso volcanic field lies at the western edge of a broad region of elevated temperatures. The profound modification of the continental lithosphere near Coso suggests that the volcanic field and especially the silicic magma chamber must strongly influence the modern strain field. Creation and evolution of this magmatic center is an important link in understanding the diverse tectonics of the Coso and Indian Wells region.