San A ndreas F ault dip, P eninsular R anges mafic lower crust and partial melt in the S alton T rough, S outhern C alifornia, from ambient‐noise tomography

We use ambient‐noise tomography to improve CVM‐H11.9, a community velocity model of southern California. Our new 3‐D shear‐velocity model with x lateral and 1 km vertical blocks reveals new structure beneath the San Andreas Fault (SAF), Peninsular Ranges batholith (PRB), southern Sierra Nevada batholith (SNB), and the Salton Trough (ST). We use 4 years of data recorded on 849 broadband stations, vastly more than previous studies and including our own broadband Salton Seismic Imaging Project, a 40 station transect across the ST, as well as other campaign stations in both Mexico and the United States. Mean lower crust and upper mantle wave speeds (3.6 km/s at 20 km, 4.2 km/s at 40 km) are low by global standards. Across the SAF, southeast of San Gorgonio Pass, we observe vertical to steeply dipping lateral velocity contrasts that extend beneath the Moho. Beneath the western PRB and westernmost southern SNB, we observe relatively high shear velocities ( 3.8 km/s) in the lower crust that we interpret as the mafic roots of the overlying arc. Relatively high‐velocity upper mantle (up to ∼4.5 km/s) may be part of the intact arc, or possibly a remnant of the Farallon plate. Beneath the ST, we observe zones of low shear‐velocity in the lower crust and upper mantle which permit up to ∼4.5% melt in the lower crust and up to ∼6% melt in the upper mantle, depending on the assumed composition and pore geometry. Our results preclude the existence of older continental crust beneath the ST and support the creation of new crust beneath the ST. San Andreas Fault: 3‐D S‐velocity model shows velocity contrasts down to Moho Peninsular Ranges and Sierra Nevada: high‐V s upper mantle is Farallon fragment Salton Trough: low‐V s in lower crust (∼4.5% melt) and upper mantle (∼6% melt)