Structural and Geomorphic Characteristics of Landslides at Coyote Mountain, Anza-Borrego Desert State Park, California

Abstract

Coyote Mountain is an 8-mi-long (13 km) elongate fault block made up of granitic and metamorphic rocks in northeastern San Diego County, California. A series of landslides, most of which have distinct morphology and failure mechanisms, occurs in the tonalite and gneiss underlying the steep southwest slope of the mountain. The southernmost landslide area is the Peg Leg Smith landslide complex, which is composed of several translational slides and a unique remnant of a long-runout rock avalanche. In the central portion of the mountain, two distinct landslide types underlie the slopes near Coyote Peak. The first is represented by a pair of rock-block landslides, the Coyote Peak landslides, which failed along foliation planes in metamorphic rock. The second is the Coyote Ridge landslide, a 2-mi-wide (3.2 km) area of incipient landsliding in highly fractured tonalite. The Alcoholic Pass landslides, located at the northwestern end of the mountain block, are situated in tonalite. This complex consists of two juxtaposed landslides that failed at nearly right angles to each other. The base of the northernmost landslide is not exposed, and the failure mechanism is postulated to have been block sliding along a well-developed fracture system. The basal rupture zone of the southern landslide is composed of coarse, matrix-rich breccia. The southern flank of the slide grades into linear scarps which define the head of the Coyote Ridge landslide. The Alcoholic Pass landslides are concluded to be rare examples of fully developed translational failure resulting from formation of a through-going rupture surface created by incremental movement along interconnecting fractures.

When Landslides Are Misinterpreted as Faults: Case Studies from the Western United States

“…Because the rock exposed in the canyons flanking the west side of the ridge is highly fractured and lowangle faults that could form a basal rupture surface are not evident or suspected, Hart (2008) suggested that failure occurs along interconnecting fracture systems in step-like fashion because of high transient shear stresses produced during earthquakes along the Coyote Creek and San Jacinto Faults. This ongoing process ultimately results in catastrophic slope failure.…”