Paper No. 15
Presentation Time: 9:00 AM-6:00 PM
CONSTRAINING BASIN GEOMETRY AND FAULT KINEMATICS ON THE SANTO TOMAS SEGMENT OF THE AGUA BLANCA FAULT THROUGH A COMBINED GEOPHYSICAL AND STRUCTURAL STUDY
SPRINGER, Adam M.1, CALLIHAN, Sean M.
2, WETMORE, Paul H.
3, FLETCHER, John
4, CONNOR, Charles
5, BEESON, Jeffrey W.
6 and WILSON, James
3, (1)Department of Geology, University of South Florida, 4202 E. Fowler Ave., SCA 528, Tampa, FL 33620, (2)Dept. of Geology, University of South Florida, 4202 E. Fowler Ave, SCA 528, Tampa, FL 33620, (3)Department of Geology, University of South Florida, 4202 E. Fowler Ave, SCA 528, Tampa, FL 33620, (4)Geology, CICESE, PO Box 434843, San DIego, CA 92143, (5)Department of Geology, University of South Florida, SCA 528, 4202 E. Fowler Ave, Tampa, FL 33620, (6)Geology, University of South Florida, 4202 E. Fowler Ave, Tampa, FL 33620, amsprin2@mail.usf.edu
The Agua Blanca Fault (ABF) extends for more than 120km east from Punta Banda, in northern Baja California with a southeast-northwest strike, and represents the southernmost fault in the San Andreas system of faulting. The Santo Tomas basin is located roughly 40km south of Ensenada where the ABF overlaps the Santo Tomas Fault (STF). A detailed geophysical analysis constrains basin geometry, and helps to identify 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 the relative amounts of dip-slip and strike-slip motion on basin-bounding faults.
Gravity data from >300 gravity stations, were collected along northwest-southeast lines that cross the basin perpendicular to its axis, as well as two axial lines. These lines are spaced ~1km apart, with 250 meter spacing between points on each line. Data was also collected in the areas surrounding the basin out to 10km to gain a regional context. Data were modeled using Geosoft Oasis Montaj software to produce forward 2D models of basin profiles.
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 STF 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 whereas the STF is characterized by a single strand throughout the basin and it exhibits a minor dip-slip component. Basin sediments thicken slightly against the STF fault to as much as 1km. A third fault, cutting across the basin southeast from 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 7 km (northern ABF, as indicated by an offset Cretaceous intrusion to the NW of the basin) and <3 km (STF), for a combined ~9 km. This total is intermediate between the total slip on the ABF from studies to the southeast (22 km) and to the northwest (7 km) suggesting that the fault is losing slip toward the northwest.
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