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Description
The study area lies in the Yuha desert near the western margin of the Salton Trough structural province and is centered on the northern extension of the Laguna Salada fault zone. The focus of this study is an analysis of the interrelationship between different phases of late Cenozoic structural development and their relationship with the Elsinore and southern Laguna Salada faults which trend towards the study area from the northwest and southeast, respectively. The northern Laguna Salada fault is not comprised of a single break in this area. Rather, the geometry of dextral faulting is complicated and represented by both left-stepping and right-stepping strands, not all of which have been active synchronously. Within the southern part of the field area, the Laguna Salada fault apparently steps left across Lucy Ridge producing a broad area of elevated topography. Just north of the border, the present activity steps right to the Sunrise Butte strand and then steps left again at Yuha Basin to become the Elsinore fault. Older strands are also present and indicate that the complexity of stepping patterns has changed with time. Field mapping identified an area dominated by a northeast structural grain which is rotated clockwise near the Laguna Salada fault. Excellent exposures of detachment-related, normal faults in progressive stages of development were found. A pervasive tectonic fabric of the rocks also attest to the regional extension. Structural analyses indicate the strata has undergone two deformational events resulting in superposed folds. The first-order folding is about a northeast trending axis whereas the second-order folding is about a west-northwest trending axis. Field data, kinematic indicators and the structural analyses support the following interpretation of the deformational events. The first event is related to extension in a northeast-southwest direction. This extension produced northeast trending folds, detachment-related faults, tilted homoclinal fault blocks and a pervasive low-angle tectonic fabric. The second, less pronounced, event was due primarily to wrench-related stresses associated with the present Laguna Salada fault zone. The wrench deformation produced west-northwest trending folds, associated faulting and horizontal rotation of blocks. The magnitude of this rotation is directly proportional to the proximity of the wrench fault. The deformation associated with both events involve the basement rock, the Pliocene Imperial Formation and the Pleistocene Palm Spring Formation indicating the youngest documented evidence of detachment faulting in the Salton Trough region. The Salton Trough and other subsidiary basins, such as the Laguna Salada basin, are postulated to be the source of the regional extension experienced in this area. This idea is also suggested by gravity trends. Therefore, the study area is important as it provides an excellent opportunity to study the complex stress interactions, in particular extensional deformation, generated in oblique pull-apart systems.
