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Southward Continuation of the San Jacinto Fault Zone Through and Beneath the Extra and Elmore Ranch Left-Lateral Fault Arrays, Southern California

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Utah State University DigitalCommons@USU All Graduate Theses and Dissertations Graduate Studies 5-2013 Southward Continuation of the San Jacinto Fault Zone through and beneath the Extra and Elmore Ranch Left-Lateral Fault Arrays, Southern California Steven Jesse Thornock Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/etd Part of the Geology Commons Recommended Citation Thornock, Steven Jesse, "Southward Continuation of the San Jacinto Fault Zone through and beneath the Extra and Elmore Ranch Left-Lateral Fault Arrays, Southern California" (2013). All Graduate Theses and Dissertations. 1978. https://digitalcommons.usu.edu/etd/1978 This Thesis is brought to you for free and open access by the Graduate Studies at DigitalCommons@USU. It has been accepted for inclusion in All Graduate Theses and Dissertations by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. SOUTHWARD CONTINUATION OF THE SAN JACINTO FAULT ZONE THROUGH AND BENEATH THE EXTRA AND ELMORE RANCH LEFT- LATERAL FAULT ARRAYS, SOUTHERN CALIFORNIA by Steven J. Thornock A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology Approved: ________________ ________________ Susanne U. Janecke James P. Evans Major Professor Committee Member ________________ ________________ Anthony Lowry Mark R. McLellan Committee Member Vice President of Research and Dean of the School of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2013 ii ABSTRACT Southward Continuation of the San Jacinto Fault Zone through and beneath the Extra and Elmore Ranch Left-Lateral Fault Arrays, Southern California by Steven J. Thornock, Master of Science Utah State University, 2013 Major Professor: Dr. Susanne U. Janecke Department: Geology The Clark fault is one of the primary dextral faults in the San Jacinto fault zone system, southern California. Previous mapping of the Clark fault at its southern termination in the San Felipe Hills reveals it as a broad right lateral shear zone that ends north of the crossing, northeast-striking, left-lateral Extra fault. We investigate the relationship between the dextral Clark fault and the sinistral Extra fault to determine whether the Clark fault continues to the southeast. We present new structural, geophysical and geomorphic data that show that the Extra fault is a ~7 km wide, coordinated fault array comprised of four to six left-lateral fault zones. Active strands of the Clark fault zone persists through the Extra fault array to the Superstition Hills fault in the subsurface and rotate overlying sinistral faults in a clockwise sense. New detailed structural mapping between the San Felipe and Superstition Hills confirms that there is no continuous trace of the Clark fault zone at the surface but the fault zone has uplifted an elongate region ~950 km. sq. of latest Miocene to Pleistocene basin-fill in the field iii area and far outside of it. Detailed maps and cross sections of relocated microearthquakes show two earthquake swarms, one in 2007 and another in 2008 that project toward the San Felipe Hills, Tarantula Wash and Powerline strands of the dextral Clark fault zone in the San Felipe Hills, or possibly toward the parts of the Coyote Creek fault zone. We interpret two earthquake swarms as activating the San Jacinto fault zone beneath the Extra fault array. These data coupled with deformation patterns in published InSAR data sets suggest the presence of possible dextral faults at seismogenic depths that are not evident on the surface. We present field, geophysical and structural data that demonstrate dominantly left-lateral motion across the Extra fault array with complex motion on secondary strands in damage zones. Slickenlines measured within three fault zones in the Extra fault array reveal primarily strike-slip motion on the principal fault strands. Doubly-plunging anticlines between right-stepping en echelon strands of the Extra fault zone are consistent with contraction between steps of left-lateral faults and are inconsistent with steps in dominantly normal faults. Of the 21 published focal mechanisms for earthquakes in and near the field area, all record strike-slip and only two have a significant component of extension. Although the San Sebastian Marsh area is dominated by northeast-striking left- lateral faults at the surface, the Clark fault is evident at depth beneath the field area, in rotated faults, in microseismic alignments, and deformation in the Sebastian uplift. Based on these data the Clark fault zone appears to be continuous at depth to the Superstition Hills fault, as Fialko (2006) hypothesized with more limited data sets. (178 pages) iv PUBLIC ABSTRACT Steven J. Thornock The Clark fault is a significant fault within the southern San Andreas fault system. The Clark fault abruptly ends where it intersects a second, smaller fault that crosses it called the Extra fault zone. In this study we investigate the possibility of the Clark fault continuing beneath and beyond the Extra fault zone. Based on field mapping and other geological data that we present, we determine that the Clark fault continues below the Extra fault and is not evident on the surface. Over time, earthquake slip along the Clark fault has caused the Extra fault zone to rotate in a clockwise direction. Both fault zones have high potential for causing a high magnitude (M >6.0) earthquake in the near future. A second objective of this study is to identify the direction of motion along the Extra fault zone and related faults. Data collected from the field indicate that the fault is a strike-slip fault that moves in a left-lateral sense. Previously published geophysical data support our data collected in the field. v ACKNOWLEDGMENTS Foremost my thanks go to the Southern California Earthquake Center (SCEC) for funding this research and to Utah State University for giving me opportunities to teach. I am immensely grateful for this opportunity of learning provided by Susanne Janecke. Without Susanne’s insights, motivation, truck, and plain hard work, this project would never have reached fruition. So many others assisted in this process including the other members of my committee, Jim Evans and Tony Lowry. My thanks go to Erik Mustonen for allowing me to encroach on his generous hospitality during weeks of fieldwork. Also to Tammy Rittenour for graciously accepting a structure student into her OSL short course and for her patient help. Huge thanks go to my fellow students who were always supportive and encouraging. Especially to Mitch Prante who, regardless of his busy schedule, always had time to help and talk. Finally to my wife, Christie, who sacrificed in often unnoticed ways over the course of this life-changing experience. Steven J. Thornock vi CONTENTS ABSTRACT ........................................................................................................................ II PUBLIC ABSTRACT ...................................................................................................... iv ACKNOWLEDGMENTS ..................................................................................................V LIST OF TABLES ..........................................................................................................VIII LIST OF FIGURES .......................................................................................................... IX LIST OF PLATES ............................................................................................................. xi INTRODUCTION AND MOTIVATION .......................................................................... 1 Geologic Setting...................................................................................................... 1 Stratigraphy and Sedimentology ............................................................................. 3 Major structure of the San Jacinto fault zone ......................................................... 9 Previous mapping and generalized findings ......................................................... 12 Motivation ............................................................................................................. 15 METHODS ....................................................................................................................... 31 Field Methods ....................................................................................................... 31 Structural Analysis ................................................................................................ 34 Geophysical Data and Analyses............................................................................ 39 Morphometric data sets ......................................................................................... 41 RESULTS ......................................................................................................................... 51 Structural results ................................................................................................... 52 Geophysical results in the Extra fault array ........................................................ 105 Geomorphic aspects of the study area ................................................................ 114 DISCUSSION ................................................................................................................. 125 Continuation of the Clark fault beneath the Extra fault array ............................. 125 Dextral
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