SCEC Award Number 14103 View PDF
Proposal Category Collaborative Proposal (Special Fault Study Area)
Proposal Title QUATERNARY FAULT SLIP BEHAVIOR OF THE MISSION CREEK FAULT OF THE SOUTHERN SAN ANDREAS FAULT ZONE IN THE SAN GORGONIO PASS, CA Part I - New Insights from Neotectonics and 36Cl/10Be burial dating
Investigator(s)
Name Organization
Gregory Balco University of California, Berkeley Kimberly Blisniuk University of California, Berkeley Katherine Scharer United States Geological Survey
Other Participants Louis Wersen
SCEC Priorities 1a, 4a, 4b SCEC Groups WGCEP, SoSAFE, Geology
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 The objective of our study is to provide precise, accurate fault slip rates on the southern San Andreas fault zone (SAFZ), more specifically the Mission Creek fault strand, as it enters the San Gorgonio Pass (Fig. 1) on the timescale of 104 to 105 years. These data are needed to evaluate our alternative earthquake rupture model that supports the potential for a SAFZ earthquake rupture through the San Gorgonio Pass on the relatively narrow dominantly strike-slip Mission Creek fault strand, rather than the diffuse zone of deformation of the Banning fault strand. To evaluate this model we are presently

(1) re-evaulating the slip rate history of the Mission Creek fault strand of the SAFZ by determining its long-term and short-term slip rate through detailed field mapping and dating of landforms and deposits offset along the fault strand;
(2) conducting a sedimentary provenance study of mid-Quaternary to late Quaternary alluvial deposits and their potentially displaced source-drainages and
(3) developing, validating, and applying the novel technique of 36Cl/10Be burial dating to determine the age of offset alluvial deposits in the 200 ka – 1 Ma age range.

Project results and pending results:
Our investigation on the Mission Creek fault strand thus far suggests that the fault does not just remain active as it enters the San Gorgonio Pass but that slip rates may remain high at this latitude. These results show that the Mission Creek fault strand is potentially the main plate boundary structure here, contrary to current kinematic and fault hazard models for southern California.

Support from SCEC in 2014 provided funding for fieldwork and geochronologic data to test our hypothesis that the Mission Creek fault strand is active as it enters the San Gorgonio Pass. In short, detailed field work conducted during the past year on the Mission Creek fault strand located west and east of Highway 62 (as the fault enters the San Gorgonio Pass) reveal that the fault at this location consists of not just one fault but numerous active Quaternary oblique, reverse and strike slip fault strands that offset Holocene to mid-Quaternary alluvial deposits, terraces and incised channels. Our sedimentary provenance study, which permits us to correlate deposits across the Mission Creek fault, in the active drainages, on buried fans and terrace deposits at the range front of the Little San Bernardino and San Bernardino Mountains reveal distinct trends in clast lithologies and detrital age populations. Though only part of the data has been compiled, clast count data in the active drainages indicate the percentage of biotite gneiss decreasing, while the percentage of quartz monzanite increasing to the northwest along the range front of the Little San Bernardino Mountains. We are now in the process of comparing these data with clast count data from the buried fans and terrace deposits. Additionally, samples from 4 distinct buried alluvial fans near Mission Creek along the fault were collected and recently analyzed at LLNL for 36Cl/10Be burial dating and we are presently waiting for the final ratios from Susan Zimmerman to calculate ages of these mid-Quaternary buried fans and therefore fault slip rates that will provide slip rates over this time interval.

Our fieldwork east of Highway 62 revealed Holocene offsets of terrace deposits, incised channels and bouldery deposits. Determining the age of these offset deposits will allow us to the calculate a cumulative slip rate (up to ~140 m of offset) across the Mission Creek fault that provides a shorter interval slip rate than the work to the east in the fan complex.
Intellectual Merit Our investigation addresses two long-term science priorities for SCEC4; namely, improving our understanding of (1) stress transfer from plate motion to crustal faults and (2) the structural evolution of fault zones and fault systems. Additional information on slip rates will, help test and refine kinematic and mechanical models of fault slip in California in the San Gorgonio Pass region.
Broader Impacts This proposal provided funding for two young early career scientists, Kimberly Blisniuk and Julie Fosdick, and a graduate student.
Exemplary Figure Figure 1. Location of the study area, (A) the southern San Andreas fault zone in the San Gorgonio Pass and northern Coachella Valley, showing the location of the Mission Creek, Banning and Garnet Hill fault strands.
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