FAULT EVOLUTION IN THE ONSHORE SANTA MARIA BASIN, CALIFORNIA, AS INFERRED FROM GRAVITY AND MAGNETIC ANOMALIES

The Santa Maria basin, located at the intersection of the western transverse ranges (WTR) and Coast Ranges provinces, conceals key structures that accommodate the ~90 degree clockwise rotation of the WTR indicated by paleomagnetic studies. Gravity and magnetic data shed light on basin structure. The basin produces a roughly triangular, 55-mGal gravity low. The southern basin boundary is marked by a gravity gradient that coincides with the Santa Ynez River fault and indicates a steep south dip. The arcuate western edge of the basin gravity low trends north and may represent a concealed normal fault that accommodated early Miocene opening of the basin as the WTR began to rotate, modified by later faulting. The NE basin margin changes character in the gravity field, reflecting the different structures that constitute the margin. The northwest segment of the NE basin margin strikes westerly and the basement surface appears to form a gentle south-dipping slope towards the center of the basin. Gravity and magnetic highs indicate a concealed 12 km long basement ridge forming the central segment of the NE margin, that we interpret as an upwarp caused by transfer of dextral slip from the Little Pine to the Garey and West Huasna faults. The Little Pine fault forms the SE segment of the NE basin margin. A joint gravity-magnetic model indicates that this fault dips 30-40 degrees to the NE in the upper 2 km, placing basin deposits beneath Mesozoic basement, with a concealed strand 5 km to the SW beneath the basin (intersected by a drillhole) and possibly another strand 5 km more to the SW. The dip of the main strand, based on the magnetic data, becomes near-vertical at ~6 km depth and suggests significant dextral offset.

The SW edge of a magnetic high that bisects the basin coincides near the coast with the NE-dipping Lions Heads fault, exposed south of Point Sal where it places ophiolite above Miocene sediments. This fault has been extended SE based on drillhole data, where it marks the NE edge of the magnetic high. The change in fault character suggests that the main sense of offset along the Lions Head fault may be strike-slip, locally reactivated as transpressional as the WTR rotated past 45 degrees (i.e., Dickinson, 1996). The geometry and sense of offset of both basin-bounding and intrabasinal faults have thus evolved as the basin responded to WTR rotation.