Geomorphic Mapping and Ground-Penetrating Radar Survey of the Western Segment of the Agua Blanca Fault, Baja California, Mexico

S. R. Madsen and H. Alsleben
Texas Christian University

The WNW-ESE trending Agua Blanca fault, which is associated with the North American – Pacific plate boundary, traverses Baja California, Mexico near latitude 31º30’N for approximately kilometers and is commonly interpreted as a dextral strike-slip fault. The fault is divided into three right-stepping, 40 kilometers long segments. The westernmost segment, our study area, comes onshore at Punta Banda Ridge and bounds the southern edge of Valle Maneadero. A lack of recent seismicity along the fault zone contrasts global positioning system data that supports slip rates of 3-4 millimeters per year.

Geomorphic expressions of the fault suggest recent fault activity and zones of highly-fractured rocks with intermittent zones of breccia and gouge better constrain the location of the main fault trace, which juxtaposes Cretaceous beach deposits or Quaternary alluvium with Cretaceous volcanics. Although the geomorphology supports simple dextral motion, kinematic indicators suggest an additional normal-oblique component along the fault.

To characterize the near-surface structure of the fault zone, we completed 20 ground-penetrating radar transects across apparently faulted areas. This method reveals unexposed structural and stratigraphic relationships beneath the surface. Using 100 MHz antennas, depth penetration of 3-5 meters was achieved and potentially recently offset stratigraphic markers were recognized.

Our study shows that the western segment of the Agua Blanca fault is an active, dextral-normal oblique fault. The WNW-ESE orientation and the dextral-normal displacement make this fault unique in this plate boundary system. We infer that the fault is currently accumulating strain and is likely to experience seismic activity in the future.

AAPG Search and Discover Article #90087 © 2008 AAPG/SEG Student Expo, Houston, Texas