Timing of extension, NS shortening, and conjugate strike-slip faulting in the evolution of the Chocolate Mountains anticlinorium: Evidence from the Gavilan Hills, SE California

ABSTRACT
The Gavilan Hills contain the complex record of how North American continental crust responded to the switch from low angle subduction to the embryonic development of the San Andreas fault system.  Within this area, major detachments include the Chocolate Mountains and Gatuna faults.  Though published Ar-Ar thermochronology indicates that movement along the Gatuna fault occurred sometime between about 28 and 24 Ma, our field data and published geochronology shows that the hanging wall block of the Gatuna fault includes a thick (~1.2 km) highly faulted and extended section of ~23 Ma volcanic and epiclastic rocks.  Though this relationship does not rule out movement between 28 and 24 Ma, it does indicate that final displacement along the Gatuna fault occurred after ~23 Ma. 
Following movement on the Gatuna fault, the ~23 Ma volcanic and epiclastic sequence, along with all older units and structures, were folded about the Chocolate Mountains anticlinorium.  Subsequent to this initial phase of fold growth, three unconformably bounded alluvial sequences of the Bear Canyon conglomerate were deposited and then successively folded about the anticlinorium as it grew episodically.  During this interval of fold growth, a conjugate set of dextral and sinistral strike-slip faults and shear zones transected the ~23 Ma section of volcanic and epiclastic rocks and all older structures and units.  Some of these faults offset the axis of the anticlinorium or associated second order folds.  Because the Indian Pass fault, the western most fault in the set of dextral strike-slip faults, cuts the ~9 to ~13 Ma basalts of Black Mountain, we interpret the conjugate set of faults and shear zones to have formed after this date, and to represent a period of pronounced NS shortening.
The Eastern California Shear Zone (ECSZ) is a broad zone of dextral strike-slip faults lying in the Mojave Desert and Death Valley region. It has played a major role in accommodating the dextral shear between the Pacific and North American plates prior to the formation of the San Andreas fault. According to published literature faults of the ECSZ cut and displace ~20 Ma elements of an early Miocene extensional belt and transect and displace a tuff that was deposited about 13.4 ± 0.2 Ma. This history is remarkably similar to that summarized above for the Gavilan Hills.  Hence, we argue that the growth of the Chocolate Mountains anticlinorium following eruption of the ~9 – 13 Ma basalts of Black Mountain reflects NS shortening as the ECSZ extended southward to the latitude of the Gavilan Hills.  In short, our work suggests that the Gavilan Hills contains the complete record of the response of continental crust to the transition from low angle subduction of the Farallon plate to the early embryonic stages of the modern San Andreas fault system.