Wang, KaiJiang, ChengxinYang, YingjieSchulte‐Pelkum, VeraLiu, Qinya2022-02-032022-02-030094-8276http://hdl.handle.net/1885/259037We build a new radially anisotropic shear wave velocity model of Southern California based on ambient noise adjoint tomography to investigate crustal deformation associated with Cenozoic evolution of the Pacific‐North American plate boundary. Pervasive positive radial anisotropy (4%) is observed in the crust east of the San Andreas Fault (SAF), attributed to subhorizontal alignment of mica/amphibole foliation planes resulting from significant crustal extension. Substantial negative anisotropy (6%) is revealed in the middle/lower crust west of the SAF, where high shear wave speeds are also observed. The negative anisotropy could result from steeply dipping amphibole schists in a shear zone developed during Laramide flat slab subduction. Alternatively, it could be caused by the crystal preferred orientation (CPO) of plagioclase, whose fast axis aligns orthogonally to a presumed subhorizontal foliation. The latter new mechanism highlights potentially complex CPO patterns resulting from different lithospheric mineralogy, as suggested by laboratory experiments on xenoliths from the region.K. Wang and Q. Liu are supported by the NSERC Discovery Grant 487237. Computations for this study were performed on hardware acquired through the combined funding of Canada Foundation for Innovation (CFI), Ontario Research Fund (ORF), and University of Toronto Startup Fund and partly hosted by the SciNet HPC Consortium. Y. Yang is supported by Australian Research Council Future Fellowship (FT130101220) and Discovery Project (DP190102940). Schulte‐Pelkum's contribution was supported by NSF Grants EAR‐1251193, 1735890, and 1927246, and SCEC Grant 17097. This is contribution 1509 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 1393 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au).application/pdfen-AU© 2020. American Geophysical Union202010.1029/2020GL0885802020-12-13