This multi-disciplinary study was undertaken to better understand the timing, rates, geometries, and controls on crustal deformation in the southern San Andreas fault zone over the past several million years to the present day.  The project was a collaboration of earth science professors, students, and agency scientists from the University of Oregon, University of Massachusetts, Western Washington University, Virginia Tech University, and the U.S. Geological Survey.  Research methods included geological field mapping, stratigraphic analysis, process sedimentology, U-Pb dating of detrital zircon grains in sedimentary deposits, paleomagnetism, thermochronology, geophysical analysis of gravity data, and boundary element method numerical modeling. All of the collaborating partners shared data, results, and ideas, and co-authored peer-reviewed papers and professional meeting abstracts to disseminate our results to the broader earth science community. 

We documented geologic, geomorphic, and geophysical evidence for active northeastward tilting of a large intact crustal block in the Santa Rosa Mountains and southern Coachella Valley about a horizontal axis oriented parallel to the San Jacinto and San Andreas faults over the past ca. 1.2 million years.  This geometry of strain is unexpected and was not recognized prior to this study.  Of several hypotheses considered, oblique convergence across a northeast-dipping San Andreas fault is the most likely mechanism driving this pattern of crustal strain over geologic time.  Our hypothesis is supported by a boundary-element numerical modeling study that was carried out as part of this study. This result lends further support to a growing consensus on the northeastward dip of the San Andreas fault beneath the Coachella Valley, and shows that a very small angle of plate-motion obliquity can produce surprisingly large vertical displacements in the crust over relatively short timescales (<1.5 million years). 

Stratigraphic analysis reveals a 3-4 Myr history of complex basin development and deformation along the Mecca Hills segment of the southern SAF. The paleolandscape changed dramatically in response to rapid, punctuated, alternating periods of vertical displacements related to changing sense and patterns of slip on the Painted Canyon fault (PCF). Basin evolution was controlled by evolution of local fault-zone complexities superimposed on larger-scale changes in regional subsidence and uplift. Changes in regional fault kinematics likely caused extensive uplift and erosion along the SAF in Coachella valley at ca. 2.5-3 Ma, as recorded in a regional unconformity in the Mecca and Indio Hills, and again from ~0.7 Ma to the present. Laterally extensive cross-bedded sandstone and siltstone in the Palm Spring Formation record deposition in a fluvial system comprised of a migrating channel and adjacent overbank floodplain that occupied a broad basin floor. The Palm Spring lower member was deposited in a large river system that flowed southeast down the fault controlled paleo-Coachella Valley into the Salton Trough, with sources mainly in the Cottonwood and Little San Bernardino Mountains.

Between about 1.5 and 1.1 Ma, the southern San Andreas fault system underwent a major reorganization that included initiation of the San Jacinto fault zone and termination of slip on the extensional West Salton detachment fault. We used three-dimensional mechanical Boundary Element Method models to investigate the impact of these changes in the fault network on deformation patterns. A series of snapshot models explore the role of fault interaction and tectonic loading in abandonment of the West Salton detachment fault, initiation of the San Jacinto fault zone, and shifts in activity of the San Andreas fault. Interpreted changes to uplift patterns are well matched by model results. These results ...

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