DEEP SECRETS OF FAULTS, BLOCKS, AND BASINS REVEALED BY GRAVITY, MAGNETIC, AND GEOLOGIC DATA IN SOUTHERN CALIFORNIA

As part of the Southern California Areal Mapping Project, gravity and magnetic data were compiled for an area extending from the edge of the continental borderland shelf inland to the Salton Trough and the southern Mojave Desert, a region with a population of over 14.5 million. These data have been supplemented by new data collected during the past decade, providing a 3-D view of major geologic features in the region. Major magnetic basement terranes include the western Peninsular Ranges, Verdugo and Cucamonga blocks, and the San Bernardino Mountains-Mojave Desert block. Magnetic anomalies over the western Peninsular Ranges indicate that a major discontinuity within the Peninsular Ranges batholith extends to mid-crustal depths and appears to be truncated at the San Jacinto fault. The absence of its offset equivalent across the fault suggests preexisting crustal control on the development of this young, seismically active fault. Correlation of intense magnetic anomalies of the Verdugo and Cucamonga blocks suggests right-lateral offset of ~50 km on the San Gabriel fault. Magnetic data indicate that the southeast margin of the Verdugo block is truncated by the Raymond fault. Left lateral offset of ~7 km on the extension of the Raymond fault into the San Gabriel Mountains is suggested by an intense magnetic anomaly SE of the Sawpit Canyon fault. Correlation of a broad magnetic anomaly related to the La Cienega block in northern Los Angeles basin with an anomaly associated with a buried magnetic block south of the Simi Hills suggests 35 km of cumulative left lateral offset on the Santa Monica fault zone. Magnetic and gravity data indicate faults wedging Peninsular Ranges into San Gabriel and San Bernardino basement rocks beneath San Gorgonio Pass. Gravity data also show the geometry of basin-bounding and intrabasinal faults. The San Bernardino basin, for example, is a pull-apart basin formed by strands of the San Jacinto fault whose dips and locations are defined by geology, gravity, and detailed aeromagnetic data. Gravity data also provide constraints on basin geometry, from the scale of the Los Angeles basin (30 x 50 km) to that of subbasins like the Hollywood basin (~2 x 12 km) south of the Santa Monica fault zone and paleochannels carved into bedrock of the Perris block. We anticipate additional insights as modelling efforts continue.