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

The WNW-ESE trending Agua Blanca fault zone, which is associated with the North American – Pacific plate boundary, traverses Baja California, Mexico near latitude 31º30'N for ~120 km and is commonly interpreted as a dextral strike-slip zone. The fault is divided into three right-stepping, ~40 km 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 GPS data that supports slip rates of 3-4 mm per year.

Observations in our study area suggest recent fault activity and better constrain the location of the main fault trace, which juxtaposes the Cretaceous El Rosario formation or Quaternary deposits with Cretaceous Santiago Peak volcanics, and subsequent fault strands. Geomorphic expressions of the fault include triangular facets, offset and beheaded streams, pressure ridges and sag ponds. Furthermore, groundwater flow along the main fault led to growth of aligned trees. Rocks near the fault are highly-fractured with intermittent zones of breccia and gouge. Although the geomorphology supports dextral motion, slickensides containing gently to moderately plunging slickenlines and 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 buried or otherwise unexposed structural and stratigraphic relationships beneath the surface. Using 100 MHz frequency antennas, depth penetration of 3-5 m 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.