SCEC Award Number 16194 View PDF
Proposal Category Collaborative Proposal (Special Fault Study Area)
Proposal Title Late Quaternary slip rate of the western Pinto Mountain Fault, Morongo Valley, CA
Investigator(s)
Name Organization
Doug Yule California State University, Northridge Richard Heermance California State University, Northridge Katherine Kendrick United States Geological Survey
Other Participants Kate Lewis (CSUN graduate student) and at least one CSUN undergraduate student. USGS collaborators include Jon Matti and Bob Powell.
SCEC Priorities 1a, 4a, 2a SCEC Groups Geology, SoSAFE, WGCEP
Report Due Date 03/15/2017 Date Report Submitted 11/15/2017
Project Abstract
 In the northern Coachella Valley, the San Andreas Fault (SAF) system is characterized by complex overlapping and intersecting faults that raises questions about how slip is transferred through the region. Recent work in this area proposes that strain may be transferred from the Mission Creek strand of the SAF to the Eastern California Shear Zone (ECSZ), at least partly via the Pinto Mountain Fault, a major east-west trending left-lateral transverse fault that intersects the Mission Creek strand of the SAF. Geodetic and geologic slip rates reported for the PMF vary from 1 to 12.5 mm/yr and are poorly constrained because of a lack of geologic data. This report describes a geologic slip rate from faulted alluvium in Big Morongo Canyon in Morongo Valley, California. A best-constrained strath contact between late Pleistocene alluvium (Qoa) and underlying bedrock (ggm) is offset in a left-lateral sense a total of 228 – 303 m. Cosmogenic exposure ages of six monzo-granite boulders from an prominent terrace yield ages ranging from ~63 ka to ~88 ka. The younger ages probably reflect significant surface deflation and erosion. Three older ages overlap within error and give a weighted average of 86.9 +/- 4.5 ka as the preferred, minimum age for the terrace. This age and offset give a preferred maximum slip rate of 3.0 +0.6/-0.4 mm/yr for western PMF system for the last ~87 ka.
Intellectual Merit This project documents sinistral offset and ages of that offset, and provides a late Pleistocene geologic slip rate of ~3 mm.yr for the western Pinto Mountain Fault. This information therefore contributes to the geologic slip rate database for the San Andreas and related faults in southern California and can be incorporated into earthquake hazard models and fault slip rate models. This new slip rate information can also be compared to geodetic fault models in the region.
Broader Impacts This project formed the basis for an MSc thesis project by Kate Gabriel, completed in August of 2017. Yule and Heermance also used the field site as mini field mapping projects during teaching of their respective courses in Quaternary Geochronology (Heermance) and Geology of Earthquakes (Yule). This site will continue to be used for teaching purposes in coming years.
Exemplary Figure Figure 2. Geologic map of study area. Blue boxes are Cosmogenic 10Be sample locations and are labeled with sample name. Offset piercing points are labelled with capital letters. Arrows indicate offset direction. Multiple strands of the PMF are shown cutting through old alluvium (Qoa), young alluvium (Qa) and offsetting bedrock (ggm). Blue lines show modern stream flow and dashed blue lines show paleo stream flow.