A DENSE COMPLEX NETWORK OF FAULTS IN THE COYOTE MOUNTAINS, ADJACENT TO THE ACTIVE DEXTRAL ELSINORE FAULT, SOUTHERN CALIFORNIA

In southern California, the dextral San Andreas plate boundary is distributed across a 250 km wide fault zone marked by (from east to west), the San Andreas, San Jacinto, and Elsinore faults. The south-easternmost segment of the Elsinore fault bounds the Coyote Mountains. There, the fault is curved, striking 270˚- 315˚, and dipping steeply to moderately northeast. It has a significant component of reverse motion, uplifting the Coyote Mountains and forming a steep southwest-facing mountain front.

Detailed geologic mapping within the Coyote Mountains, NE of the Elsinore fault, reveals a dense complex network of faults. Different domains within the range are dominated by different types of faults:

Domain 1 - The westernmost 7 km of the range: In this domain, the range widens gradually eastward from zero at the western tip to 1.5 km. Virtually all of the faults in this domain are sinistral and strike 050˚- 080˚, forming an en-echelon pattern. Left-lateral separation on these faults ranges from 200 m to 1000 m; some of that separation could be caused by a down-to-the NW dip-slip component. Spacing between the faults is regular, averaging ~150 m.

Domain 2 - The central 6 km of the range: In this domain, the range widens toward the SE from 1.5 km to 3.3 km. The main faults in this domain are NW-striking dextral faults with up to 1.4 km of right-lateral separation. Also present are NNE-striking normal faults and ENE-striking sinistral faults, which are typically crosscut by the dextral faults.

Domain 3 - The southeastern corner of the range, forming a roughly 3 km x 3 km trapezoid: In this domain, the southwestern range front makes a 90˚ turn to trend NE, away from the Elsinore fault. The main faults in this domain are 050˚ - 090˚ - striking sinistral faults. Also present are rare NW-striking dextral faults and N-striking normal faults.

Domain 4 - The northeastern part of the range: This domain consists of a 9 km long NW-trending wedge that tapers from 3 km wide in the SE to zero in the NW. The dominant faults strike NW and dip very steeply. Sense of slip on these faults is generally indeterminate, but the most continuous one, the Painted Gorge Fault, has ~1 km of dextral offset. Also present are rare but significant N-striking normal faults.

These complex fault patterns can be interpreted as forming a zone of distributed dextral shear across the entire range.