Date of Completion


Embargo Period



Julie C. Fosdick, Timothy Byrne, Michael Hren

Field of Study

Geological Sciences


Master of Science

Open Access

Open Access


Tectonic reconstructions of the southern San Andreas Fault (SAF) system (California, USA) have produced conflicting estimates of cumulative strike-slip offset from middle Miocene to present. We propose that the sedimentary record can resolve both spatial and temporal development of this system, allowing us to reconstruct the paleogeography of this system in the past 10 Ma. An important, but unresolved consideration is whether the Miocene Coachella Fanglomerate, located on the northwestward migrating Pacific side of the dextral SAF transform plate can be correlated to the Bear Canyon Conglomerate located ~200 km to the southeast in the foothills of the southerly Chocolate and Cargo Muchacho Mountains. We report new sedimentology, geochemistry, and U-Pb detrital zircon provenance data to test the paleogeographic connection of these two units and introduce new fault restorations. New integrated provenance data show both similarities and differences between the units. For example, units contain basaltic andesite clasts with elevated potassium concentrations, a distinctive composition found in basalt flows in the southeastern Chocolate Mountains. Additionally, detrital zircon U-Pb analyses from both formations exhibit a predominantly Jurassic age peak and smaller Paleoproterozoic peak. Although the complete detrital zircon distributions from both formations are statistically different – the Bear Canyon Conglomerate also reports Miocene and Mesoproterozoic age – our results imply that the Coachella Fanglomerate and Bear Canyon Conglomerate may have had multiple shared sources during deposition. These new findings support a southerly source of the Coachella Fanglomerate consistent with ~215 km of offset along the southern SAF in the past 10 Ma.

Major Advisor

Julie C. Fosdick