The geomorphic expression, sedimentation, and near-field deformation of a fault system may be characterized to obtain an understanding of its kinematic evolution and potential seismic hazards. The dynamics and deformation history of the Eastern California shear zone (ECSZ), a wide and complex network of right-lateral strike-slip faults, is not well understood, despite hosting three large (>Mw 7.0) earthquake ruptures in recent decades. The low-net slip faults of the ECSZ (each with <10 km) offer a unique opportunity to assess strain distribution in a developing, kinematically immature strike-slip system. To do so, I conducted field-based investigations of these faults within the Mojave Block of the ECSZ.
First, I investigated the morphology, structure, and controls of restraining bend growth along the numerous faults of the ECSZ via field mapping and numerical deformational modeling. I found that the ECSZ restraining bends are small (kilometer-scale), exhibit high-angle, doubly fault-bound geometries with positive flower structures, and have self-similar morphologies characterized by a "whaleback" longitudinal profile and an arrowhead shape in map view. Gradual changes in form with increasing restraining bend size suggest a common growth mechanism influenced more by the kinematics of local fault geometries than by the fault's obliquity to plate motion. Modeling results indicate that concentrated shear strain at single transpressional bends facilitates the development of new secondary faults with cumulative strain as a mechanism to accommodate horizontal shortening via uplift between the faults. The ECSZ restraining bends contribute minimally to regional contractional strain due to their small size, steep fault angles, and shallow crustal penetration (< 5 km), which also suggests that they are unlikely to obstruct large earthquake ruptures. Second, I conducted a spatiotemporal slip rate analysis of the Calico fault with new mapping and geochronology of offset alluvial fans from North Hidalgo Mountain. From this work I obtain several findings. 1) The slip rate along North Hidalgo Mountain ranges from 1.5-2.1 mm/yr in the Holocene and 0.8-2.0 mm/yr in the late Pleistocene. 2) The similarity in slip rates between North Hidalgo Mountain and the Rodman Mountains suggests that this 38 km stretch is a kinematically coherent fault segment with a relatively steady slip rate of 1.7 +0.4/-0.3 mm/yr over the past 60 ka. Faster rates reported from Newberry Springs suggest either a significant increase in slip rate from the Rodman Mountains to Newberry Springs or temporal variations in slip rate. 3) The new rates support previous work which showed the central section of the Calico fault has the highest slip rate in the Mojave Block. However, it does not resolve the discrepancy between ECSZ geodetic and geologic slip rates, implying that transient changes in slip rate, or the contribution of off-fault deformation or other structures may be required. Additionally, the lack of geological slip rate data might contribute to this discrepancy if significant spatial and temporal variations exist on other ECSZ faults. / Doctor of Philosophy / The topography and geology within a fault system may be studied to understand tectonic plate motion over time and assess earthquake hazards. The Eastern California shear zone is a complex network of strike-slip faults within the Mojave Desert, which has hosted three large earthquakes (>Mw 7.0) in recent decades. Despite this significant seismic activity, the mechanisms of motion across the numerous faults in the Eastern California shear zone remain poorly understood. The individual faults have accumulated relatively little strike-slip motion since their inception (less than 10 kilometers), offering a unique opportunity to investigate the early-stage kinematics and seismic hazards of a strike-slip fault system. To do so, I conducted field-based investigations of the faults within the Eastern California shear zone.
First, I investigated the early evolution and controls of compressional strike-slip fault bends in the Eastern California shear zone. From mapping and numerical modeling, I characterized the shape, structure, and uplift of numerous small compressional bends dispersed across the faults. From these efforts, I found that uplifted crust in the fault bends exhibit self-similar forms with shallow crustal depths (<5 km). Small changes in the shape of these structures occur with increasing size indicating a predictable pattern of growth with increasing cumulative slip that appears to be partially controlled by local fault conditions. Numerical modeling of simple compressional fault bends indicate that shear strain concentrates at bend corners, which may facilitate the growth of a new fault that more efficiently accommodates contraction in the bend via uplift of the crust between the two faults. The compressional strike-slip fault bends in the Eastern California shear zone are too small to significantly impact regional contractional strain and are therefore also unlikely to impede large earthquake ruptures. Second, I studied the slip rate (or rate at which the fault moves) of the Calico fault via new mapping and age data of displaced alluvial fans. I found that 1) the Calico fault at North Hidalgo Mountain slips at a rate of 0.8-2.0 mm/yr since ~70,000 years ago. 2) The slip rates from North Hidalgo Mountain and the Rodman Mountains are similar, indicating that the 38 kilometers between them behaves consistently, with a steady rate of ~1.7 mm/yr over the last ~60,000 years. However, faster slip rates reported at Newberry Springs suggest either a significant increase in slip rate from the Rodman Mountains to Newberry Springs or that it varies over time. 3) These findings confirm that the central Calico fault has the fastest slip rate in the Mojave Block but does not reconcile regional differences between rates from geodetic and geological measurements. The difference between the slip rates measured by geodetic methods and those from geological studies in the Eastern California shear zone suggests that there could be temporary changes in slip rates or that deformation might be occurring in areas away from the main fault. Also, the lack of geological slip rate data might contribute to this discrepancy if significant spatial and temporal variations exist on other Eastern California shear zone faults.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/121296 |
| Date | 07 October 2024 |
| Creators | Garvue, Max Martin |
| Contributors | Geosciences, Spotila, James A., Stamps, D. Sarah, Bemis, Sean, Owen, Lewis A., Law, Richard D. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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