CONSTRAINING THE GEOMETRY AND TECTONIC EVOLUTION OF THE MANEADERO BASIN THROUGH COMBINED GEOPHYSICAL AND STRUCTURAL DATA SETS

Maneadero basin, located 5km south of Ensenada, Mexico is bound to the south by the dextral Agua Blanca Fault (ABF). The ABF is the southwestern-most in a network of faults transferring plate boundary strain out of the Gulf of California and around the bend in the San Andreas in southern California. The basin is about 60km² with an axis that trends WNW, subparallel to the ABF. The WNW-trending, western ABF juxtaposes the basin and the Bahia Todos Santos to the northwest, with the 1000m high Punta Banda Ridge.

The goals of this study were to constrain the geometry of the Maneadero basin and the distribution of faults bounding and within the basin, as well as to provide constraints on the kinematic evolution of the basin. A combined geophysical (gravity and magnetic) and structural study were employed to address these issues. Gravity data define a Bouguer anomaly of about 35 mGal across the basin and onto the adjacent up-thrown block, and can clearly identify two gravity highs separated by an intervening gravity low. The highs, irrespective of elevation, are offset by approximately 7km of dextral strike slip. This is consistent with the results of field mapping conducted during the summer 2008 that identifies a Cretaceous intrusion, located just beyond the southeast corner of the Maneadero Basin which is cut by the ABF, which is also displaced by approximately 7km of slip on the ABF.

Gravity data also allow for the estimation of a dip-slip component of displacement across the ABF. As expected, the gravity low appears in the middle of the basin, which is shown to have sediment thicknesses of approximately 1km near the ABF and gradually shallowing northward. There is also a slight gradient that decreases in magnitude from southeast to northwest along the axis of the basin. A density contrast of 0.37 g/cm³ between the basin fill and outcropping Punta Banda Ridge, as described by Florez et al (2004), was used to generate this model. A normal slip component is also documented by Rockwell et al (1989) through a study of uplifted marine terraces on Punta Banda. They documented 260m of uplift since 840ka. The apparent 740m deficit might be explained by strike-slip displacement of the west-sloping coastal margin or the normal component of slip has taken place over a longer time frame than represented by the terraces.