Activity of the Mill Creek and Mission Creek strands of the San Andreas fault through the San Gorgonio Pass region

Alex E. Morelan, Michael E. Oskin, Judith S. Chester, & Daniel F. Elizondo

Published August 15, 2016, SCEC Contribution #6965, 2016 SCEC Annual Meeting Poster #131

We present new observations that constrain the recent slip history of the Mill Creek and Mission Creek strands of the San Andreas fault. These faults are the northern strands of a complex series of strike-slip and thrust faults through the San Gorgonio Pass stepover, an important structural barrier that affects seismic hazard in southern California. Understanding the recent activity on each strand of this complex system will help define potential paths for future large, throughgoing San Andreas fault ruptures. In the Raywood Flat area, the Mill Creek fault cuts the base of the upper Raywood Flat fill, a ~50 m thick package of debris-flow deposits. The upper section of the Raywood Flat deposits, however, are not cut by the fault. On the surface of this deposit, a 15 m-wide channel, flanked by boulder-rich, debris-flow levees, crosses the projection of the Mill Creek fault without evidence of offset. We collected boulder-top samples for cosmogenic exposure age-dating of these levees and present preliminary results. Additionally, we mapped inset terraces along the incised channel of the East Fork Whitewater River drainage that also do not show evidence of fault offset, and we collected a depth profile through the uppermost Raywood Flat fill in order to further assess its age. To the south, across Yucaipa Ridge and between the Mill Creek and Mission Creek strands, the basement rocks are cut by an extensive system of smaller faults and associated distributed shear zones that record a transfer of displacement from the Mill Creek to Mission Creek fault systems. Along several transects through this transfer system, east of Forest Falls, we also find no evidence of recent movement. Along the Mission Creek strand, newly devegetated B4 airborne lidar data reveal fault scarps cutting across hillslopes and alluvial fans between the San Bernardino strand and lower Raywood Flat for a distance of 4 km. We identified a lateral offset of 4-6 m in an alluvial fan deposit within a tributary of Banning canyon, and sampled a suite of boulders to estimate the age of this deposit. This site documents that the Mission Creek fault is active and serves as a potential rupture path through the San Gorgonio Pass, bypassing the structural complexity of the San Gorgonio Pass thrust to the south and the eastern portion of the Mill Creek fault to the north, the latter of which appears to be inactive since the Latest Pleistocene.

Citation
Morelan, A. E., Oskin, M. E., Chester, J. S., & Elizondo, D. F. (2016, 08). Activity of the Mill Creek and Mission Creek strands of the San Andreas fault through the San Gorgonio Pass region. Poster Presentation at 2016 SCEC Annual Meeting.

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