Paleomagnetic definition of crustal fragmentation and Quaternary block rotations in the east Ventura basin and San Fernando Valley, southern California

Shaul Levi, & Robert S. Yeats

Published October 30, 2003, SCEC Contribution #729

Paleomagnetic studies of the Pliocene-Quaternary Saugus Formation in the eastern part of the western Transverse Ranges of California show rotation of reverse-fault blocks, indicating that the crust is fragmented into small domains, tens of kilometers in linear dimension. In an area approximately 35x25 km in the San Fernando Valley and east Ventura Basin, we identified four distinct domains. Two domains, southwest of and adjacent to the San Gabriel fault, are rotated clockwise: 1) The Magic Mountain domain, R = 30∞±5∞; 2) the Merrick Syncline domain, R = 34∞±6∞. The Magic Mountain domain has rotated since 1 Ma. Both rotated sections occur in hangingwalls of active reverse faults: the Santa Susana and San Fernando faults, respectively. Structural data suggest that the fault tip of the Santa Susana fault is the rotation pivot of the Magic Mountain domain. Two additional blocks are unrotated: 1) The Van Norman Lake domain, directly south of the Santa Susana fault, and 2) the Soledad Canyon domain immediately across the San Gabriel fault from the Magic Mountain domain, suggesting that the San Gabriel fault might be a domain boundary. Our results suggest that part of the clockwise rotation of some Miocene and older rocks in this area might have occurred in the Quaternary. The Plio-Pleistocene fragmentation and clockwise rotations continue at present, based on geodetic data, and represent crustal response to diffuse, oblique dextral shearing within the San Andreas fault system.

Citation
Levi, S., & Yeats, R. S. (2003). Paleomagnetic definition of crustal fragmentation and Quaternary block rotations in the east Ventura basin and San Fernando Valley, southern California. Tectonics, 22(5), 15.1-15.16. doi: 10.1029/2002TC001377.