PROTRACTED DEFORMATION WITHIN THE MESOZOIC SIERRAN ARC, CRONESE HILLS, SOUTHERN CALIFORNIA

The Sierra Nevada continental arc formed by episodic magmatism during the Mesozoic era. The emplacement of the plutonic and eruption of volcanic rocks was partly accompanied by intense deformation of the arc, particularly during magmatic flare-up events, and is now recorded in intrusions and their pendant rocks. The East Sierran thrust system (ESTS) is a contractile belt at the eastern margin of the arc and is well exposed from the Sierra Nevada Mountains southward across the Garlock fault and into the Mojave Desert. Deformation occurred from Triassic through Jurassic time with the main activity from around 200 to 140 Ma. Contractional deformation during a middle-late Jurassic magmatic flare-up is well recorded and preserved in the ESTS.

Our work focuses on zircon geochronology of the Cronese Hills area of the ESTS in the Mojave Desert, where deformed rocks and a reverse-sense shear zone are exposed. The field evidence and our new data show that contractional deformation started before the emplacement of an undeformed 200 Ma diorite pluton. This pluton intrudes folded metasedimentary rocks of Triassic age. The second deformation event is fabrics developed in and is cross-cut by plutons, all with ages around 159 Ma. Metavolcanic rocks dated at around 164 Ma rest as deformed pendants in these plutons. The latest deformation episode is the development of the Cronese shear zone, a southeast directed structure. The shear zone is accompanied by a suite of synkinematic felsic plutonic rocks that range in age from 152 to 150 Ma. This last event is responsible for most of the exposed geology in the Cronese Hills. We consider these new dates to document protracted contractional deformation within the Sierran arc, starting in Triassic time and continuing until after the late Jurassic flare-up event.