SUBSURFACE GEOLOGY OF THE SAN FERNANDO VALLEY, CALIFORNIA

Seismic and well data from the San Fernando Valley (SFV) document evolution of that region from mid-Miocene rifting to north-south contraction. Formations in the western SFV subsurface (Cretaceous to Paleogene strata, and Miocene Topanga and Modelo Formations) trace southward to outcrops in the Santa Monica Mountains that constrain faulting along the valley's south basin edge. Cretaceous strata in the Simi Uplift to the west are >2 km higher than equivalent strata beneath the western SFV across a boundary marked by the Chatsworth Reservoir fault, and Neogene thinning and offlap.

The Santa Susana Mountains and south-vergent Santa Susanna fault (SSF) form the north edge of the SFV and represent Pleistocene inversion of the East Ventura basin, a Miocene rift feature. Contraction related to the SSF formed sub-parallel structures to the south represented by the Mission Hills fault, and active Northridge Hills anticline and blind thrust. Beneath these structures, an older zone of ESE-trending, down-to-the-north faults includes the Devonshire and subsurface Frew faults and marks the northeast edge of the Mio-Pliocene Oak Ridge-Simi Hills shelf. Southeast and en echelon to this, WNW-trending Miocene normal faults form the north edge of the Leadwell granitic high beneath the southeastern SFV.

The Verdugo fault (east edge southern SFV) is a NE-dipping reverse oblique fault in Pacoima oil field (east end Northridge Hills anticline). Projected northwest, this fault coincides with the abrupt transition eastward from the Santa Susana Mountains to the Sylmar basin, where Quaternary sediments exceed 3-km thickness, and from the SSF to basement-involved reverse faults north of Sylmar basin. This may reflect a postulated Miocene connection with the northern San Gabriel fault as a locus of San Andreas displacement 12 to 6 Ma, and of mid-Miocene transrotation. Although this alignment may still represent an important crustal boundary, the near-surface Verdugo fault north of Pacoima field now curves west to join the Mission Hills fault, indicating absorption of dextral slip by thrusting and folding in the Mission Hills zone.