Sedimentary and crustal structure of California and Nevada from joint inversion of multiple passive seismic datasets

Abstract:

Although the California and Nevada region has been the subject of numerous tomographic studies, there is still a lack of a 3-D velocity model that can accurately describe both the shallow sedimentary structures (depths < 8 km) and the deeper crystalline crustal structure. Taking advantage of the outstanding station coverage in this region, we build a new 3-D shear wave speed model by jointly inverting Rayleigh wave phase velocity, Rayleigh wave ellipticity, and teleseismic P waveforms. For the biggest basin system in this region, the forearc basin of Great Valley is imaged as an asymmetric syncline with a steeply east-dipping west flank and a gently west-dipping east flank with Vs less than 2.4 km/s. Below the western limb of the forearc basin, a low-velocity anomaly extends from the basin basement to a depth of 8 km with Vs less than 2.8 km/s, consistent with the previously imaged Coast Range ophiolite and Franciscan Complex mélange from the seismic refraction and gravity data. To the west of this mélange, the imaged Franciscan Complex has Vsless than 3.6 km/s and wedges eastward into the upper crust under the Great Valley Basin basement. The coastal Franciscan Complex to the depth of the middle crust with the overlying accretionary prism at depths less than 4 km are both well resolved for the first time by our passive seismic datasets. Furthermore, the Salton Trough, another important basin system due to continental rifting and extension, has a very thick sedimentary layer (~4 km) and a thin crust (~23 km) beneath the rift axis compared to its flanks. In particular, our model reveals a high-velocity beneath the rift axis with Vs about 4.2 km/s overlying the very shallow Moho, which is interpreted as a rift-related mafic intrusion, consistent with the active seismic survey results. Our results indicate the joint inversion of multiple passive datasets from dense array stations can still recover geologically meaningful structures in the crust, comparable to a smoothed version of active survey results. Future refinement of this new 3-D model will be based on full waveform inversion to further improve the image resolution, which will provide important insight into the crustal growth and modification at the North American Continental margin.