Conjugate faults are relatively common features that form in a wide variety of tectonic settings. When observed and well characterized they can be used to infer the principal stress directions during faulting. Such information can be useful in developing structural or tectonic models for a given area. Herein I describe and document a set of right and left-lateral strike-slip faults that occur within the San Felipe Hills, California. Both sets of fault systems involve the largely lacustrine sedimentary rocks of the Pliocene-Pleistocene(?) Borrego Formation. Within the field area right-lateral strike-slip faults strike N37W while left-lateral faults strike N37E. Both sets of faults offset hinge lines of major EW trending asymmetric folds, and have vertical dips. Right-lateral faults are more prevalent than are left-lateral faults. These relationships imply that the maximum principal stress direction is oriented NS while the minimum principal stress is oriented EW. The intermediate principal stress is vertical. The general directions of the principal stresses outlined above are consistent with movement on a NW striking wrench fault that may represent the subsurface extension of the Clark segment of the San Jacinto fault. This and other aspects of this work will be tested and assessed by two masters theses that are currently underway.
