FAULT-ZONE ARCHITECTURE OF SMALL- DISPLACEMENT OBLIQUE-SLIP FAULTS ADJACENT TO THE SAN JACINTO FAULT, BORREGO MOUNTAIN, CALIFORNIA, USA

The Borrego Mountain area in the western Salton Trough region of southern California contains exceptional exposures of extensive secondary deformation associated with slip along the Coyote Creek fault of the southern San Jacinto fault zone. Abundant youthful oblique-slip faults in this area provide an excellent opportunity to examine damage-zone evolution along small-displacement faults that are associated with a mature, seismogenic fault zone. This research utilizes integrated field studies and microscale analyses to examine the chronological and spatial pattern of damage-zone deformation. The detailed characterization of fault-zone architecture along small-displacement faults allows for an investigation of the early stages of fault-zone development.

The undeformed host rocks are conglomeratic, zeolite-cemented, poorly- to moderately-sorted sandstones of the Pliocene West Butte Conglomerate. Microstructural analyses indicate that the abundance of zeolite cement in some undeformed sandstones can reach up to 25% of rock volume; the high minus-cement porosity suggests that the zeolite started to precipitate during the early diagenetic stage of shallow burial before substantial compaction. The deformation features present include cataclastic fault rocks (with moderate grain-size reduction), cataclastic fault rocks combined with zeolite-cemented dilational fractures, zeolite-cemented dilational fractures, selenite-cemented dilational fractures, and calcite-cemented dilational fractures. Zeolite-, selenite-, and calcite-cemented dilational fractures post-date the formation of quartz overgrowth and cataclastic fault rock development. These observations are used to test theoretical models that make contrasting predictions about the relative timing of deformation in different fault-parallel damage-zone domains.