Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 29-1
Presentation Time: 1:30 PM


CLEMENS-KNOTT, Diane1, GEVEDON, Michelle L.2, DUCCINI, Kalie M.1, AVILES, Rodrigo1, BOESHART, Erin1 and KOHLER, Zoe Christine1, (1)Department of Geological Sciences, California State University Fullerton, 800 North State Blvd., Fullerton, CA 92831, (2)Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712

New field and geochronologic data (29 zircon U-Pb LA-ICP-MS dates) elaborate upon existing models for the development of an extensive, NW-trending, Jurassic rift-graben in the southern Sierra Nevada arc (e.g., Busby-Spera, 1988; Saleeby and Dunne, 2015). In the Kern Plateau, the ~50-km-wide rift-graben is bracketed by the Kern Plateau shear zone (KPsz) to the SW and the Independence dike swarm to the NE. We add 12 U-Pb dates to the published record of the Sacatar igneous complex (SIC), a 1000 to 1500 km2 exposure of an quartz dioritic to granitic complex that was assembled incrementally throughout the Jurassic. Age data suggest SIC emplacement in 3 phases punctuated by movement along the bounding Kern Plateau shear zone: 181 - 174 Ma (phase 1; 2 foliations), 170 - 168 Ma (phase 2; 1 foliation), and 165 - 161 Ma (phase 3), plutons of this latter phase being previously mapped as Cretaceous.

Additionally, we add nine U-Pb dates of the newly recognized Tubatulabal hypabyssal-volcanic package and the Osa Creek ring complex. Basaltic andesites and rhyolites directly overlie a dated region of SIC phase 2, and have ages of 167.7 ± 2 Ma and 167.2 ± 2 Ma. Plutonic rocks of SIC phase 3 are intruded by a 159.6 ± 2 Ma andesite dike and overlain by additional hypabyssal rocks. Lastly, near the eastern Sierra crest, SIC phase 3 is intruded by a 146.4 ± 2 Ma porphyritic rhyolite dike. This dike is coeval with the 9-km-diameter, compositionally bimodal, Osa Creek Ring complex (148 - 145 Ma; n=3), which intruded into the SIC near the NW end of the KPsz. This latest Jurassic bimodal volcanism coincides with emplacement of the Independence dike swarm, recognized as marking an arc-wide ca. 148 Ma regional extension event.

Within the rift graben, 8 new U-Pb dates of the Summit gabbro span ca. 150-145 Ma, definitively linking emplacement of the Summit gabbro to this extensional regime. Considered together, intrusion of the SIC, the Summit gabbro, and associated volcanic rocks occurred during punctuated Jurassic extension-transtension in the southern Sierra Nevada arc, which culminated with emplacement of the Independence dike swarm and was closely followed by eruption of the Osa Creek stratovolcano.