| dc.description.abstract | This study investigates the late Tertiary and Quaternary tectonic, structural and sedimentologic history of the central Garlock fault (GF) in Pilot Knob Valley (PKV) and southwestern Searles Valley, and its interactions with the cross-cutting Eastern California shear zone (ECSZ). The interaction of these two active, orthogonally striking strike-slip fault systems has long been somewhat of a tectonic enigma. However, this work provides considerable insight into how the kinematic and structural relationships between these structures have evolved since ~5 Ma. This in turn, furthers our understanding of how the North American-Pacific transform plate margin evolves over thousand to million-year timescales. The first contribution of this dissertation is the idea that the central GF is shown herein to have experienced large temporal variations in strain release since the late Pleistocene. A long-term, late Pleistocene slip rate of 5-7 mm/yr (e.g., McGill and Sieh, 1993) is comprises a ca. 13.2-4 ka period of low slip (4.3-5.1 mm/yr) followed by a recent, ca. 4-0 ka, period of elevated slip rate (10.2-14.3 mm/yr). Evidence for elevated slip rates in the late Holocene are from a 3.5-3.2 ka alluvial fan deposit offset 43-50 m against a shutter ridge. Two soil profile development index values and an optically stimulated luminescence age from the deposit establish robust age control. High-resolution, EarthscopeTM airborne LiDAR imagery, a boulder levee on the upstream late Holocene deposit, elevation profiles of the site and three hand-excavated trenches establish tight control on fault displacement magnitudes. Periods of elevated strain release on the GF are correlated with higher strain release rates on the San Andreas fault and shortening in the Los Angeles basin, but are anti-phased with the ECSZ (e.g., Dolan et al., 2007). A second contribution of this dissertation is the stratigraphic reorganization of late Cenozoic sedimentary rocks in PKV and Searles Valley, and the tectonic implications for the GF, Searles Valley fault and the newly identified Marine Gate fault. Here, a 1000+ m package of exposed Pliocene and Pleistocene strata have been uplifted and tilted to the northeast. Based on new age and provenance data, we adopt the name Pilot Knob Formation to describe much of these rocks north of the GF and east of the Christmas Canyon gate. The Pilot Knob Formation comprises three distinct lithologic members, from oldest to youngest, the Eagle Crags Member, Randsburg Wash Member and Slate Range Member. The Eagle Crags Member is a ~5-3.7 Ma conglomerate, sandstone, siltstone, claystone and evaporite derived from the Eagle Crags volcanic field to the south of PKV. The Randsburg Wash Member is a ~3.7-3.1 Ma siltstone, claystone and evaporite deposit that grades laterally into a sandstone and is locally interbedded with a rockfall deposit. The Slate Range Member is a ~3.1-0.3 Ma sandstone and conglomerate that grades, in the eastern study area, into a siltstone, claystone and evaporite deposit. Sediments comprising the Slate Range Member are derived from the Slate Range north of PKV. Outcrop relations within the Pilot Knob Formation reveal three stages for the post-Miocene tectonic development of PKV. An initial ~5-3.1 Ma stage characterized by an active sinistral-oblique-normal Marine Gate fault and GF, resulting in a transtensional pull-apart basin in northern PKV. A second, 3.1-1.2 Ma, stage of minimal transtension or transpression, corresponding to a cessation of sinistral slip on the Marine Gate fault at ca. 2.5 Ma, and a continuation of sinistral slip on the GF. Finally, a 1.2 Ma-present stage of significant N-S shortening across the northern PKV, as evidenced by uplifted and incised Pilot Knob Formation and younger sediments. Together, these three tectonic stages record the development and evolution of the Panamint Valley fault, located ~20 km east of PKV, since ~3.5-3 Ma. A third contribution of this dissertation involves quantifying the magnitude of modern N-S shortening in PKV and relating it to decreased slip on nearby strands of the ECSZ (e.g., Panamint Valley fault and Paradise fault zone). New detailed neotectonic mapping, EarthscopeTM airborne and ground-based LiDAR imagery, and chronology of Quaternary deposits exposed along the GF and Marine Gate fault, including (1) three 10Be terrestrial cosmogenic nuclide (TCN) depth profiles and (2) one soil description, place bounds on the vertical components of recent deformation. A ka 10Be TCN profile age for a 16-m-high terrace tread adjacent to the GF (site PKV-1) suggests a differential incision (uplift) rate of 0.40 ± 0.13 mm/yr. A ka 10Be TCN profile age from a 12.5-m-high tread located 4.5 km west on the GF (site PKV-5) suggests a differential incision (uplift) rate of 0.23 ± 0.05 mm/yr. A 25.5-m-high terrace adjacent to the southern Slate Range (site PKV-3) was dated using a soil profile development index (PDI) technique. A soil PDI age estimate of ~162 ka brackets the maximum age of uplift, and suggests a minimum uplift rate of ~0.16 mm/yr here. Using these ages and assuming dips on the GF between 90° and 80° to the north, resultant shortening magnitudes of 0-2.2 m and 0-2.8 m at sites PKV-5 and PKV-1, respectively, and 0-0.04±0.01 mm/yr at PKV-5 and 0-0.07±0.02 mm/yr at PKV-1 are estimated. At PKV-3, we estimate the dip of the newly recognized Marine Gate fault to be 70-80° to the south, resulting in 4.4-8.7 m of N-S shortening, suggesting a 0.03-0.05 mm/yr shortening rate. The shortening rate estimated at PKV-1 corresponds to 2-4% of Panamint Valley fault slip rate. Shortening at PKV-1 and PKV-3 can be summed to partially integrate 0.08-0.12 mm/yr shortening between the GF and Slate Range, or approximately 3-7% of Panamint Valley fault slip rate. The presence of numerous active reverse faults between PKV-1 and PKV-3 suggests that this is a minimum estimate. A fourth contribution of this dissertation is a 1:12,000 scale geologic map of the central GF, northern Blackwater fault, southern Searles Valley fault and the Marine Gate fault. covering approximately 280 km². Additional contributions of this work include: (1) quantification of the thermal history of the southern Slate Range through apatite and zircon (U-Th[SM])/He thermochronometry, and (2) enhancement of a Mojave-wide soil calibration tool developed and used by Eric Kirby and Eric McDonald that will be useful for future tectonic-geomorphic applications, dating climate-induced surficial processes and archeological dating. | |
