USING POTENTIAL FIELDS TO REFINE BASIN AND FAULT GEOMETRY IN SALINAS VALLEY, CALIFORNIA

Salinas Valley is an important agricultural basin that is approximately 115-km long and extends from Elkhorn Slough in the NW to just SE of the town of San Ardo. The Salinas Valley basin is bounded by the Gabilan Range on the NE and the Santa Lucia Range and Reliz-Rinconada fault zone on the SW. We combine new gravity data, aeromagnetic data, and well information to refine basin and fault geometry in the Salinas Valley as part of a broader effort to investigate the tectonics and water resources of the Central California Coast Ranges.

Gravity data collected during 2009 in the basin and surrounding mountains show that the Salinas Valley is characterized by an elongate gravity low that widens to the north, east of Monterey Bay. Based on a preliminary inversion of gravity data, basin depths average 2-3 km with a maximum of 6 km in southern Salinas Valley. South of where Arroyo Seco Canyon intersects the Salinas basin, the thickness of the Monterey Formation within the basin increases dramatically and corresponds to an apparent deepening of the basin. However, diatomaceous rocks within the Monterey Formation are often less dense than other basin fill, and would lead to an over-estimation of basement depth in areas with no well control.

Gravity and magnetic gradients help characterize the geometry of basin-bounding structures, such as the Reliz-Rinconada fault zone, which marks the western boundary of the Salinas Valley. For example, the Reliz segment of the fault is located at the base of a 50-km-long gravity gradient, suggesting a SW dip. Fault exposures at the mouth of Arroyo Seco canyon and geophysical modeling confirm a SW dip. At Arroyo Seco canyon, where the Reliz and Rinconada segments meet in a right-step, the fault no longer follows the gravity gradient and instead cuts through lower-density Cenozoic rocks. While rocks of the Sierra de Salinas are essentially non-magnetic, there appear to be magnetic sedimentary deposits within northern Salinas Valley that delineate a possible northern extension of the Reliz fault into Monterey Bay. In contrast to the western margin, the linear gravity gradient along the eastern boundary of Salinas Valley is continuous along the entire length of the valley. While no Quaternary structures are mapped along this boundary, the gravity gradient could be controlled by an older, buried structure.