Abstract: Strike-Slip Faulting in the California Borderland Region

SACRAMENTOGRILO, I.; S. M. HOWELL; J. WASHBURN; C. NOLAND; and H. WARREN, San Diego State University, San Diego, CA

The southern California borderland may contain more strike-slip faulting than was previously thought. Interpretations of new geophysical data, such as seismic reflection profiles, in conjunction with geological data, indicate that numerous closely spaced strikeslip faults are present off the coast. These strike-slip faults probably account for as much as 4-6 mm per year of offset between the Pacific and North American plates. Many of the faults are active as seen by the numerous small earthquakes of M<4 monitored in the borderland region. In addition, these faults cut the most recent sediments seen on the reflection profiles.

In a reflection profile about 30 km off the coast of Oceanside, the Newport-lnglewood fault zone is clearly defined by an acoustic impedance contrast. Within the same reflection profile, at least one flower structure is present suggesting another strike-slip fault farther off to the west. A topographic high, in conjunction with epicenter locations, suggests yet another fault farther to the west, all within one km or so apart. Offset rock units further support these implications. Many vertically offset units imply that normal faults are coinciding with the strike-slip regime.

These strike-slip faults are believed to be an extension of the San Andreas fault system in the offshore region. They may have accumulated as much as hundreds of metres to a few kilometres of offset in a right-lateral sense.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California