I recently started a study of Late Cenozoic deformation and sedimentation within the San Andreas fault zone along the NE margin of the Coachella Valley, in collaboration with Bernie Housen (WWU), Jim Spotila (Virginia Tech), and Michele Cooke (U. Mass). UO graduate student James McNabb is also working on this project. In this study we seek to test two hypotheses for controls on 3D strain through time: (1) that plate-motion obliquity exerts the primary control on the 4D evolution of the fault zone, as reflected in the spatial distribution, geometries, scales and rates of vertical crustal motions through time; and (2) that the southern San Andreas fault zone experienced a major change at ~1.1-1.4 Ma in response to initiation of the San Jacinto fault and reorganization of the plate boundary. Each hypothesis makes unique predictions about geometries, rates, distribution, and timing of fault-controlled uplift, erosion, transport, and basin subsidence. Methods to be integrated include geologic mapping, stratigraphic and structural analysis, paleomagnetism, detrital zircon dating, low-T (U-Th)/He dating of bedrock sources, and geomorphic analysis of alluvial fans and streams. Numerical modeling by Michele and her students will help us understand the mechanical controls on fault geometries, vertical motions, and kinematic changes through time.
Figure 1 (above). Geologic map of the Coachella Valley region. Light blue line is the catchment boundary. Red lines are contours of sub-surface sediment thickness from Langenheim et al. (2005), dashed where inferred from Langenheim et al. (2007). Compiled from Jennings et al. (1977), Matti et al. (1992), Powell (1993), Axen & Fletcher (1998).
The southern San Andreas fault (SAF) zone in the Coachella Valley is well suited for testing hypotheses about the controls on kinematic evolution and vertical crustal motions in an active strike-slip fault system. The valley is nearly parallel to the relative plate motion and lies between a large releasing stepover to the southeast (Salton Trough) and restraining bend in the northwest (San Gorgonio Pass). Its complex structural evolution has resulted in sequential episodes of subsidence and uplift, exposing a world-class stratigraphic record of Late Cenozoic deformation, erosion, and depocenter migration due to fault zone development. Potential source areas contain unique rock lithologies and ages, catchments are accessible for new work on low-temperature thermochronology, and sedimentary rocks are superbly exposed and well suited for new dating and analysis. The unique geomorphic, geologic, and depositional setting thus allows us to study the 4D crustal evolution of a continental transform system as it responds to changes in slip obliquity, regional stresses, geometric complexities, and plate-boundary kinematics through time.
Figure 2 (above). Hypotheses to be tested in this study. A. Diagram showing oblique wrench-style deformation predicted for larger plate-motion obliquity (alpha > 20°), and strain-partitioning at margins where alpha < 20° (modified from Miller, 1998). B. Strike-slip duplexes may be transtensional or transpressional; transtension is likely to cause subsidence while transpression is expected to create uplift in the fault zone.
In addition to topical questions of vertical motions and strain partitioning, the southern San Andreas system poses a unique problem related to initiation of strike-slip faults and re-routing of plate-boundary strain. A regional tectonic reorganization is believed to have caused the end of slip on the West Salton detachment fault and initiation of the modern San Jacinto, San Felipe and Elsinore strike-slip faults at ~1.1-1.4 Ma (Morton and Matti, 1993; Matti and Morton, 1993; Lutz et al., 2006; Steely et al., 2009; Janecke et al., 2010; Dorsey et al., 2012). This event created rapid uplift in the new fault zones and transferred about half of the slip on the Coachella Valley strand of the SAF (~35 mm/yr) to the San Jacinto and San Felipe fault zones (e.g., Bennett et al., 2004; Behr et al., 2010). A tectonic reorganization of this scale should have influenced the basinal and structural evolution in Coachella Valley, yet the effects of this event on Plio-Pleistocene stratigraphy and faults in the area remain unknown and untested.
Figure 3 (above). C. Secular change at ca. 1.2 Ma is believed to have initiated the San Jacinto fault zone and widened the southern San Andreas fault system. D. Oblique view looking ~north at the southern Santa Rosa Mts, showing geomorphic evidence for hypothesized northeastward crustal tilting, which includes steep range-front topography and large recent landslide on the faulted western margin of the range. Blue lines show contours of sediment thickness in the Coachella Valley (Langenheim et al., 2005, 2007).
Figure 4 (above). Geologic map of the Mecca Hills, compiled from Sylvester and Smith (1976), Rymer (1991, 1994), Boley et al. (1994), and Weldon (unpubl. map data). Blue line near center shows where the contact between lower and upper members of the Palm Spring Fm is concordant and conformable (confirmed, January 2012). The Upper Palm Spring Fm (Qpu) displays abundant lateral variability that reflects local sub-basins, perhaps created during deformation that formed the angular unconformity between the lower and upper members.
Left: UO graduate student James McNabb exploring a slot canyon in the Mecca Hills. James has completed a huge amount of field work in his first season on this project (winter 2012). He did lots of geologic mapping, section measuring, and collected a ton (literally!) of sand samples for detrital zircon dating. In addition, he and Bernie Housen and Bernie's M.S. student Graham Meese collected paleomagnetic samples to be used in rotation anaylsis and dating of the section.
This study seeks to fill large gaps in our understanding of the geologic evolution of the southern San Andreas fault system, a complex network of seismically active faults that define the Pacific-North America plate boundary in California. The history of deformation over geologic timescales (millions of years) is relatively poorly known, despite its critical role in shaping the crustal architecture and fault geometries that control earthquakes in this setting. Our approach benefits from a unique collaboration of academic researchers and students from four universities with earth scientists at the U.S. Geological Survey. We also are collaborating with geophysicists investigating processes of continental rupture beneath the Salton Sea, and scientists studying paleoseismology and fault slip rates on the San Andreas fault over shorter timescales.
Above. Angular unconformity between the lower and upper members of the Palm Spring Formation, Mecca Hills. This contact can be traced to an area between the Painted Canyon and Platform faults where it is concordant and conformable (see Fig. 4, above), making it possible to establish a reliable chronology for the entire Palm Spring Formation in this area. Above the contact we have mapped and measured a thick (ca. 500 m) dipping section of upper Palm Spring Formation that records substantial subsidence NE of the Painted Canyon fault during Pleistocene time. This fault block has been uplifted rapidly since 740 ka (age of the Thermal Canyon ash, near the top of the upper Palm Spring Fm). Compilation of stratigraphic thickness and age data will be used to generate a map of uplift rates in the San Andreas fault zone over the past 740 k.y. Ultimately, the results of this study will generate new insights into dynamic linkages between 4D crustal deformation, fault-zone complexity, growth of topography, erosion, and sediment dispersal within the southern San Andreas fault system over the past ca. 2-4 million years.
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This page was last updated March 2012, by Becky Dorsey.