Sedimentary provenance constraints on the Quaternary faulting history of the Mission Creek fault strand, southern San Andreas Fault Zone, CA

Julie C. Fosdick, Kimberly D. Blisniuk, & Louis Wersan

Published August 15, 2016, SCEC Contribution #6574, 2016 SCEC Annual Meeting Poster #122

Quaternary alluvial fan deposits along the foothills of the San Bernardino Mountains record sediment dispersal from upland catchments across the Mission Creek fault strand of San Andreas fault in the San Gorgonio Pass, CA, and thereby yield key markers of the fault’s lateral and vertical displacement history. Here, we present an integrated provenance analysis of conglomerate, sandstone, and detrital zircon collected from early Pleistocene (?) Deformed gravels of Whitewater River (Qd), the overlying flat-lying alluvial-fanglomerate (Qo, Cabezon Fanglomerate), and modern drainages to evaluate catchment sources within the San Bernardino Mountains (SBM) and Little San Bernardino Mountains (LSBM). A statistical analysis of our integrated provenance approach from Qd and Qo alluvial deposits and modern drainages suggests Qo deposits are most compatible with a Mission Creek source (or possibly a Morongo Valley source for oldest strata) in the SBM. Qd deposits are incompatible with a SBM source and more closely match the eastern LSBM, where catchments expose mostly Cretaceous and Jurassic plutons of the Sierra Nevada batholith. These data provide evidence of continued dextral fault displacement along the Mission Creek fault strand in the San Gorgonio Pass.

These findings are based on our provenance data which show that modern drainages along the SBM and LSBM are dominated by plutonic and gneissic lithic grains and clasts. Detrital zircon U-Pb ages measured by LA-ICP-MS reveal Mesozoic (65-120 Ma and 130-170 Ma), Mesoproterozoic (1.3-1.5 Ga), and Paleoproterozoic (1.6-1.8 Ga) populations. Sandstone compositions of the Qd are characterized by a transition from basement uplift to dissected magmatic arc provenance fields and increase in plutonic character over time. Detrital U-Pb data reflect dominantly Mesozoic ages, with lesser 1.3-1.5 Ga, and 1.6-1.8 Ga peaks. Clasts compositions are mostly monzonite, granodiorite, biotite gneiss, and hornblende diorite. Matrix sandstone of the Qo is similar to underlying strata, but contains progressively more metasedimentary (phyllite, quartzite, and marble) and volcanic lithic grains and clasts over time. Higher proportions of Proterozoic zircons and sparse Paleozoic zircons, together with conspicuous quartzite and phyllite, may reflect contributions from the Big Bear Group in the northern SBM. The Jurassic age peak covaries with the abundance of hornblende diorite clasts and plutonic lithic fragments, suggesting a diagnostic signal of the diorite exposed in the western LSBM. We note that the modern Whitewater River lacks diorite clasts, precluding it as a source for the Quaternary deposits, all of which contain diorite.

Key Words
Mission Creek fault strand, provenance analysis, Quaternary faulting history

Citation
Fosdick, J. C., Blisniuk, K. D., & Wersan, L. (2016, 08). Sedimentary provenance constraints on the Quaternary faulting history of the Mission Creek fault strand, southern San Andreas Fault Zone, CA. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)