We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
The reconstruction of the Southern California Continental Borderland is constrained by interpretation of widespread Tertiary fluvial-deltaic systems, which contain a primary record of tectonics and sedimentation. Well known extraregional Poway rhyolite porphyry clasts of the Eocene Ballena Channel of San Diego have been recognized in the Channel Islands far to the northeast and constitute a key piercing point in the palinspastic reconstruction of the borderland. The Tertiary section on Santa Cruz Island records distinct changes in provenance starting with Eocene Poway-clast bearing conglomerates overlain by late Oligocene-early Miocene Vaqueros Formation containing a large fraction of locally derived mafic plutonic detritus, which in turn is overlain by blueschist-bearing Miocene strata of the San Onofre Breccia. This paper presents new whole rock major- and trace element geochemistry (n = 62) and laser ablation ICPMS zircon U-Pb ages (n = 21) from Eocene-Miocene conglomerate clasts as well as basement rocks on Santa Cruz Island. The focus is on rhyolite clasts in late Paleocene to middle Miocene extraregional conglomerates in fluviodeltaic and marine borderland sequences, but mafic plutonic clasts from the Vaqueros Formation on Santa Cruz Island as well as local Willows Plutonic Complex basement were also analyzed as a geologic test of local provenance. Nine rhyolite clasts collected from the Eocene-Miocene section on Santa Cruz Island as well as Santiago-Sespe strata in the northern Santa Ana Mountains all have similar high-K calc-alkaline geochemical characteristics and yield zircon ages ranging from 176.9 ±2.7 to 162.2 ±2.7 Ma which overlap the zircon U/Pb age range of 175-165.5 Ma from eleven San Diego Eocene Poway clasts determined by previous workers. This result firmly establishes a common provenance for the rhyolite clasts in the middle Jurassic magmatic arc of northern Sonora, Mexico and south-central Arizona as proposed by others. Two Cretaceous aged clasts (98.9 ±2.7 Ma and 75.1 ±1.7 Ma) are inferred to have been derived from the Peninsular Ranges and Mojave Desert regions, respectively. Angular mafic plutonic detritus is a major component of the Vaqueros Formation on Santa Cruz Island. These deposits accumulated as a fossil-rich shallow-water debris apron and previous seawater strontium isotope dating of the deposit indicates an age of 19 Ma for the youngest part of the sequence. Whole rock geochemistry and zircon U-Pb ages from Vaqueros plutonic clasts closely match the chemistry and zircon U-Pb ages for Willows Plutonic Complex basement and confirm a local derivation for this debris. The Vaqueros Formation, however, still contains a component of extraregional detritus indicated by the present well-rounded quartzite clasts and a rhyolite clast dated at 166.2 ±2.7. Basement exhumation of the Willows Plutonic Complex is here related to the onset of the transpressional regime leading to vertical axis tectonic rotations of the Transverse Ranges from 18-12 Ma.
