CONSTRAINING BASIN GEOMETRY AND FAULT KINEMATICS ON THE SANTO TOMAS SEGMENT OF THE AGUA BLANCA FAULT THROUGH A COMBINED GEOPHYSICAL AND STRUCTURAL STUDY

The Santo Tomas basin, located in northern Baja California, formed at a right step in the dextral Agua Blanca fault (ABF). The ABF extends for more than 120km east from Punta Banda, with an east-west strike, and represents the southernmost fault in the San Andreas system of faulting. The basin is located roughly 40km south of Ensenada where the Agua Blanca fault intersects the Maximos fault. A detailed geophysical analysis defines the basin geometry, and helps to constrain the distribution and offset of mapped and concealed faults. Geophysical and structural data sets are combined to constrain the kinematic evolution of the Santo Tomas basin, including determining the relative amount of dip-slip and strike-slip motion on basin-bounding faults.

Gravity data was collected over seven transects across and along the axis of the basin. Magnetic data were taken over the same lines, and are used in conjunction with gravity data to constrain the locations, geometries and displacements of intrabasinal faults. The combined gravity and magnetic data are modeled using Geosoft Oasis montaj software to create 2 ¾D models along profiles across the study area.

Modeling of the geophysical data combined with structural mapping indicates that the Santo Tomas basin is bound by two major strike-slip faults, the ABF on the northeastern side and the Maximos fault on south, Based on offset markers, most of the strike-slip motion appears to be concentrated on the ABF on the north side of the basin. The ABF fault is characterized by multiple subparallel fault strands that appear to coalesce into single strands to the northwest and southeast of the basin. The Maximos is characterized by a single strand throughout the basin and it exhibits a minor dip-slip component. Basin sediments thicken slightly against the Maximos fault to as much as 1km. A third fault, cutting across the basin southeast of the town of Santo Tomas also exhibits a component of dip-slip motion. The total strike-slip offset on the two basin-bounding faults is inferred to be between 5 and 7 km (northern ABF) and <3 km (Maximos fault), for a combined ~9 km. This total is intermediate between the total slip on the ABF from studies to the southeast (22 km, Allen et al., 1960) and to the northwest (7 km, Callihan et al., 2008) suggesting that the fault is losing slip toward the northwest.