Normal Fault Basin Geometries From Gravity Analyses in the La Paz - Los Cabos Region, Baja California Sur, Mexico
Abstract
The southern cape region of the Baja California peninsula is ruptured by an array of roughly north-striking, left-stepping active normal faults, which accommodate regional transtension. Dominant faults within this system include the Carrizal, San Juan de los Planes (SJP) (and offshore Espiritu Santo fault), La Gata, and San Jose del Cabo (SJC) faults. We conducted gravity surveys across the basins bounded by these faults to gain insight into fault slip rates and basin evolution to better understand the role of upper-crustal processes during development of an obliquely rifted plate margin. The geodetic location of each gravity observation station was measured to cm-scale accuracy with real-time kinematic GPS and the relative gravity was measured with a LaCoste and Romberg Model G gravity meter to an accuracy of 0.01 mgal. Gravity data were modeled as a 2D two-layer model with a bedrock density of 2.67 g/cm3 and a basin fill density of 2.1 or 2.2 g/cm3. The hanging wall of the east-dipping Carrizal fault hosts the La Paz basin. In the subsurface, this basin is a half-graben that is manifest as two smaller basins (few hundred meters deep) separated by a bedrock high, which likely reflects the two main east-dipping splays (Carrizal and Centenario faults). The SJP basin is a graben bound by the SJP fault on the west and the La Gata fault on the east and has a modeled maximum depth of approximately 1.5 km. This basin is marked by a series of relict normal faults dipping toward the basin center. The maximum depth to bedrock is just northwest of center, asymmetric toward the SJP fault, indicating that slip may be greater along the SJP than along the La Gata fault. It might also mark the possible location of basin inception, indicating that as the basin evolved, faulting moved outward to the presently active SJP and La Gata Faults. The SJC basin has a maximum depth of approximately 2.5 km. The favored gravity model depicts the SJC basin as resulting from slip along a series of normal faults predominantly dipping toward the basin center (east). Bedrock topography within the basin could be attributed to paleotopography; however, with up to 1 km of relief, the interpreted faults seem more likely. The SJC basin has the greatest depth to bedrock, signifying that it accommodates a greater slip rate than the other faults within this system or that it accommodates equal slip but was the first to initiate. The adjacent topography is also the highest, indicating that the modern deformation has been stationary spatially for some time in contrast to the SJP fault, which has low footwall topography. It is likely that the SJP basin is a graben that serves to transfer strain between the larger Carrizal and Espiritu Santo faults and the SJC fault. An offshore CHIRP survey completed in late August, 2008 will provide further insight into the extent and interactions of faults within this system.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T13E..02B
- Keywords:
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- 0920 Gravity methods (1219);
- 1209 Tectonic deformation (6924);
- 8002 Continental neotectonics (8107);
- 8105 Continental margins: divergent (1212;
- 8124)