CRUSTAL FRAGMENTATION AND NEOGENE ROTATIONS IN THE EAST VENTURA BASIN AND SAN FERNANDO VALLEY, SOUTHERN CALIFORNIA

The San Fernando Valley and east Ventura basin in southern California, the region of the 1994 Northridge earthquake and the LARSE 2 deep crustal profile, are fragmented into domains up to tens of kilometers in linear dimension. These domains were determined from paleomagnetic studies of the Pliocene-Quaternary Saugus Formation, which was deposited between 2.3 and 0.4 Ma based on a magnetostratigraphic section described by Levi and Yeats (1993, Tectonics). In two of the four domains, southwest of and adjacent to the San Gabriel fault, the Saugus shows significant late Quaternary clockwise rotation (Magic Mountain, 30°; Kagel Ridge, 34°). The two other domains are unrotated (Soledad Canyon, northeast of and adjacent to the San Gabriel fault immediately across the fault from the Magic Mountain domain, and the Van Norman reservoir in the south limb of the Mission Hills syncline, directly south of the Santa Susana fault and southeast of the Magic Mountain domain. The fragmentation and clockwise rotation represent crustal response to complex dextral shear between the Pacific and North America plates. The response of individual domains is dictated by their geometry and zones of interaction along boundaries with adjacent domains. The relatively small size of the domains might impose an upper limit on magnitude of local crustal earthquakes.