No description available.

The development of gently inclined faults with large stratigraphic separation has long been enigmatic in the corridor of southeastern Idaho. Recent interpretations have culled examples from across the Basin and Range to suggest that these faults originated at a low dip and represent a regional scale low-angle normal fault system. In contrast, others cite extensive studies from fault mechanics and seismological data that cast doubt on whether these extensional structures could have formed at low inclination in the upper crust. This dissertation reviews the evidence and timing of the proposed Bannock detachment system in the Bannock Range of southeastern Idaho and puts forth a re-evaluation of the styles of extension in the region and a regional framework in which to place them. Chapter 1 re-evaluates gently dipping normal faults in the southern Bannock Range of southeastern Idaho that have previously been interpreted as evidence for a regional detachment system originating and slipping at a low inclination. Previous work was based on geometrical relations between faults and bedding in lacustrine sediments of the upper Miocene to lower Pliocene Salt Lake Formation. The detachment argument was underpinned by three locations on the Oxford Mountain at which Salt Lake Formation was inferred to have been cut by low-angle normal faults. These locations have been re-evaluated. Two of the locations were found to preserve bedding-to-fault geometries that are well explained by offset from a fault of moderately dipping inclination. The third example is re-interpreted as an unconformable contact, not a fault, an observation that by itself precludes the existence of a detachment at that location. Chapter 2 presents a test of tephronchronology by the 40Ar/39Ar isotopic method. This study compares ages obtained by the geochronologic method of tephrochronology to ages obtained by 40Ar/39Ar single grain laser fusion of feldspars. The results of this study suggest certain considerations must be made when employing the method of tephrochronology for chronological work. Chapter 3 presents a regional synthesis for the tectonics of southeastern Idaho expanding on the new data presented in chapters 1 and 2. 40Ar/39Ar ages obtained from the Salt Lake Formation show evidence that extension in this region was underway > 15 Ma. Bedding-to-fault cutoff angles for the low-angle faults with the largest stratigraphic separations in the region suggest that the now gently inclined normal faults developed with moderate to steep dips, then tilted to lower inclination during continued extension. A splay of the Paris thrust is interpreted to account for both geometric relations between Paleozoic age rocks and the Neoproterozoic Pocatello Formation, as well as an unconformable contact between Pocatello Formation and late Miocene to Pliocene lake deposits of the Salt Lake Formation. This dissertation focuses on one example of a detachment system. However, it has implications for low-angle faults in general – particularly in regions like the Basin and Range that have had a protracted deformation history. The examples we have studied are important because they involve strata as young as Pliocene and they provide strong support for the role of tilting in accounting for the present-day attitude of large-offset normal faults, eliminating the need for the well-known mechanical paradox of low-angle normal fault formation.

The Vale KGRA is characterized by high heat flow, two to five times higher than the worldwide average, and by numerous hot springs. The hot springs are aligned along faults. This phenomena is typical of a Basin and Range type geothermal system. The hot geothermal fluids migrate upward along the more permeable, fault planes. The rocks exposed in the Vale area are the Pliocene Chalk Butte formation and the Pleistocene beds of Captain Keeney Pass. Both units are composed of volcaniclastic siltstones, sandstones and conglomerates. The units are differentiated by color, texture and degree of lithification. About 200 meters of the Chalk Butte formation and 100 meters of the beds of Captain Keeney Pass are exposed in the area. Silicification is wide spread in the rocks of the Chalk Butte formation.

The Meers Fault (Oklahoma) is one of few seismogenic structures with Holocene surface expression in the stable continental region of North America. Only the ~37 km-long southeastern section of the ~55 km long Meers Fault is interpreted to be Holocene-active. The ~17 km-long northwestern section is considered to be Quaternary-active (pre-Holocene); however, its low-relief geomorphic expression and anthropogenic alteration have presented difficulties in evaluating the fault length and style of Holocene deformation. We reevaluate surface expression and earthquake timing of the northwestern portion of the Meers Fault to improve fault characterization, earthquake rupture models, and seismic hazard evaluations based on fault length. We use a combination of airborne lidar (0.5--2 m-resolution), historical aerial photos, and new balloon-based photogrammetric (Structure from Motion) topography (0.25--0.5 m-resolution) collected in this study to analyze and characterize the fault scarp and local fault zone geomorphology. In the northwest, complex surface deformation includes fault splays, a left step, subtle monoclinal warping, and a minor change in fault strike. The fault is evident in the landscape as linear escarpments, incised channels on the up-thrown side of the scarp, and closed depressions on the downthrown side. I use topographic profiles, measured perpendicular to the fault scarp to show that the northwest scarp is characterized by decimeter surface offsets. Where the fault traverses the Post Oak Conglomerate the fault zone width rarely exceeds 25 m, in the Hennessey Shale I document an increase in fault zone width with deformation occurring over 20 m to 115 m. I further examined the northwest section of the fault in a paleoseismic excavation where weathered Permian Hennessey Shale and a ~1--2 m-thick veneer of Holocene alluvial deposits have been folded and warped during three surface-folding earthquakes. In an adjacent stream exposure these units are also faulted near the ground surface. Paleoearthquake age modeling (Oxcal) constrained by accelerated mass spectrometry (AMS) dating of detrital charcoal and optically stimulated luminescence (OSL) dating of sandy alluvial beds indicates two earthquakes occurred since ~6152-5550 cal. years BP and one possibly older event along the erosional unconformity along the Hennessey Shale bedrock. This analysis lengthens the Holocene extent of the Meers Fault by ~6 km, to ~43 km, and extends the paleoseismic record of the Meers Fault to ~9598 cal. years BP. These data will improve fault-rupture and earthquake recurrence models used for seismic hazard analysis of the Meers Fault.

The Alpine Fault is the major structure of the Pacific-Australian plate boundary through New Zealand�s South Island. During dextral reverse fault slip, a <5 million year old, ~1 km thick mylonite zone has been exhumed in the hanging-wall, providing unique exposure of material deformed to very high strains at deep crustal levels under boundary conditions constrained by present-day plate motions. The purpose of this study was to investigate the fault zone rheology and mechanisms of strain localisation, to obtain further information about how the structural development of this shear zone relates to the kinematic and thermal boundary constraints, and to investigate the mechanisms by which the viscously deforming mylonite zone is linked to the brittle structure, that fails episodically causing large earthquakes. This study has focussed on the central section of the fault from Harihari to Fox Glacier. In this area, mylonites derived from a quartzofeldspathic Alpine Schist protolith are most common, but slivers of Western Province-derived footwall material, which can be differentiated using mineralogy and bulk rock geochemistry, were also incorporated into the fault zone. These footwall-derived mylonites are increasingly common towards the north. At amphibolite-facies conditions mylonitic deformation was localised to the mylonite and ultramylonite subzones of the schist-derived mylonites. Most deformation was accommodated by dislocation creep of quartz, which developed strong Y-maximum crystallographic preferred orientation (CPO) patterns by prism (a) dominant slip. Formation of this highly-oriented fabric would have led to significant geometric softening and enhanced strain localisation. During this high strain deformation, pre-existing Alpine Schist fabrics in polyphase rocks were reconstituted to relatively well-mixed, finer-grained aggregates. As a result of this fabric homogenisation, strong syn-mylonitic object lineations were not formed. Strain models show that weak lineations trending towards ~090� and kinematic directions indicated by asymmetric fabrics and CPO pattern symmetry could have formed during pure shear stretches up-dip of the fault of ~3.5, coupled with simple shear strains [greater than or equal to]30. The preferred estimate of simple:pure shear strain gives a kinematc vorticity number, W[k] [greater than or equal to]̲ 0.9997. Rapid exhumation due to fault slip resulted in advection of crustal isotherms. New thermobarometric and fluid inclusion analyses from fault zone materials allow the thermal gradient along an uplift path in the fault rocks to be more precisely defined than previously. Fluid inclusion data indicate temperatures of 325+̲15�C were experienced at depths of ~45 km, so that a high thermal gradient of ~75�C km⁻� is indicated in the near-surface. This gradient must fall off to [ less than approximately]l0�C km⁻� below the brittle-viscous transition since feldspar thermobarometry, Ti-inbiotite thermometry and the absence of prism(c)-slip quartz CPO fabrics indicate deformation temperatures did not exceed ~ 650�C at [greater than or equal to] 7.0-8.5�1.5 kbar, ie. 26-33 km depth. During exhumation, the strongly oriented quartzite fabrics were not favourably oriented for activation of the lower temperature basal(a) slip system, which should have dominated at depths [less than approximately]20 km. Quartz continued to deform by crystal-plastic mechanisms to shallow levels. However, pure dislocation creep of quartz was replaced by a frictional-viscous deformation mechanism of sliding on weak mica basal planes coupled with dislocation creep of quartz. Such frictional-viscous flow is particularly favoured during high-strain rate events as might be expected during rupture of the overlying brittle fault zone. Maximum flow stresses supported by this mechanism are ~65 Mpa, similar to those indicated by recrystallised grain size paleopiezometry of quartz (D>25[mu]m, indicating [Delta][sigma][max] ~55 MPa for most mylonites). It is likely that the preferentially oriented prism (a) slip system was activated during these events, so the Y-maximum CPO fabrics were preserved. Simple numerical models show that activation of this slip system is favoured over the basal (a) system, which has a lower critical resolved shear stress (CRSS) at low temperatures, for aggregates with strong Y-maximum orientations. Absence of pervasive crystal-plastic deformation of micas and feldspars during activation of this mechanism also resulted in preservation of mineral chemistries from the highest grades of mylonitic deformation (ie. amphibolite-facies). Retrograde, epidote-amphibolite to greenschist-facies mineral assemblages were pervasively developed in ultramylonites and cataclasites immediately adjacent to the fault core and in footwall-derived mylonites, perhaps during episodic transfer of this material into and subsequently out of the cooler footwall block. In the more distal protomylonites, retrograde assemblages were locally developed along shear bands that also accommodated most of the mylonitic deformation in these rocks. Ti-in-biotite thermometry suggests biotite in these shear bands equilibrated down to ~500+̲50�C, suggesting crystal-plastic deformation of this mineral continued to these temperatures. Crossed-girdle quartz CPO fabrics were formed in these protomylonites by basal (a) dominant slip, indicating a strongly oriented fabric had not previously formed at depth due to the relatively small strains, and that dislocation creep of quartz continued at depths [less than or equal to]20 km. Lineation orientations, CPO fabric symmetry and shear-band fabrics in these protomylonites are consistent with a smaller simple:pure shear strain ratio than that observed closer to the fault core (W[k] [greater than approximately] 0.98), but require a similar total pure shear component. Furthermore, they indicate an increase in the simple shear component with time, consistent with incorporation of new hanging-wall material into the fault zone. Pre-existing lineations were only slowly rotated into coincidence with the mylonitic simple shear direction in the shear bands since they lay close to the simple shear plane, and inherited orientations were not destroyed until large finite strains (<100) were achieved. As the fault rocks were exhumed through the brittle-viscous transition, they experienced localised brittle shear failures. These small-scale seismic events formed friction melts (ie. pseudotachylytes). The volume of pseudotachylyte produced is related to host rock mineralogy (more melt in host rocks containing hydrated minerals), and fabric (more melt in isotropic host rocks). Frictional melting also occurred within cataclastic hosts, indicating the cataclasites around the principal slip surface of the Alpine Fault were produced by multiple episodes of discrete shear rather than distributed cataclastic flow. Pseudotachylytes were also formed in the presence of fluids, suggesting relatively high fault gouge permeabilities were transiently attained, probably during large earthquakes. Frictional melting contributed to formation of phyllosilicate-rich fault gouges, weakening the brittle structure and promoting slip localisation. The location of faulting and pseudotachylyte formation, and the strength of the fault in the brittle regime were strongly influenced by cyclic hydrothermal cementation processes. A thermomechanical model of the central Alpine Fault zone has been defined using the results of this study. The mylonites represent a localised zone of high simple shear strain, embedded in a crustal block that underwent bulk pure shear. The boundaries of the simple shear zone moved into the surrounding material with time. This means that the exhumed sequence does not represent a simple 'time slice' illustrating progressive fault rock development during increasing simple shear strains. The deformation history of the mylonites at deep crustal P-T conditions had a profound influence on subsequent deformation mechanisms and fabric development during exhumation.

"August, 2001" / Bibliography: leaves 173-183. / xvi, 184, 12 leaves : ill. (some col.), maps, plates (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / "The primary objective of the study was to gain a better understanding of the tectonostratigraphic evolutionary history of the Bass Basin. In particular, the study has focussed on mapping and analysing all the faults and fault patterns in the Bass Basin in relation to the subsidence history and its influence on sedimentation and hydrocarbon potential of the basin. The reason why the Durroon area and the Bass area behaved differently in response to extensional stresses was investigated. As a final outcome, it was thought important to clarify some of the existing disagreement about the broad tectonic and structural history of the basin and in particular to separate the influence of the Otway and Tasman Sea rifting episodes on the sedimentation history of the Bass and Durroon area. The study also aimed at investigating the occurence in the basin and nature of a recently recognised fault system, a polygonal fault system." --p. 2. / Thesis (Ph.D.)--University of Adelaide, National Centre for Petroleum Geology and Geophysics, 2002

Two examples of Pacific rim plate boundary deformation are presented. In the first part of the thesis crustal models are derived for the northwestern part of the Vizcaino block in California using marine seismic and gravity data collected by the Mendocino Triple Junction Seismic Experiment. A northwest-southeast trending kink in the Moho is imaged and interpreted to have formed under compression by reactivation of preexisting thrust faults in the paleoaccretionary prism at the seaward margin of the Vizcaino block. The study suggests that the deformation resulted from mainly north-south compression between the Pacific-Juan de Fuca plates across the Mendocino transform fault and predates late Pliocene Pacific-North America plate convergence. In the second part, 195 earthquakes recorded during the duration of the Southern Alps Passive Seismic Experiment (SAPSE) are analysed. Precise earthquake locations and focal mechanisms provide unprecedented detail of the seismotectonics in the central South Island. The short term (6 month) SAPSE seismicity is compared with long term (8 years) seismicity recorded by the New Zealand National Seismic network and the Lake Pukaki network. The seismicity rate of the Alpine fault is low, but comparable to locked sections of the San Andreas fault, with large earthquakes expected. Changes of the depth of the seismogenic zone, generally uniform at about 10-12 km, occur only localised over distances smaller than 30 km, suggesting that thermal perturbations must be of similar scale. This implies that the thermal effects of the uplift of the Southern Alps do not change the seismogenic depth significantly and are not in accordance with most of the present thermal models. Both the Hope and Porters Pass fault zones are seismically active and deformation is accommodated near the fault zones and in the adjacent crust. North of Mt Cook, a triangular shaped region along the Alpine fault is characterised by absence of earthquakes. We interpret this as the result of the plate boundary shift from the Alpine fault to the Hope and Porters Pass fault zones. The study region shows distributed deformation in a 60-100 km wide zone on NNE-SSW trending thrust faults and strike-slip mechanisms on transfer faults. / Graduation date: 2000 / Best scan available for black and white figures.

Nine west-northwest-trending faults on the continental margin of Oregon and Washington, between 43° 05'N and 470 20'N latitude, have been mapped using seismic reflection, sidescan sonar, submersibles, and swath bathymetry. Five of these oblique faults are found on both the Juan de Fuca and North American plates, and offset abyssal plain sedimentary units left-laterally from 2.0 to 5.5 km. These five faults extend 8-18 km northwestward from the deformation front. The remaining four faults, found only on the North American plate, are also inferred to have a left-lateral slip sense. The age of the Wecoma fault on the abyssal plain is 600±50 ka, and has an average slip rate of 7-1 0 mm/year. Slip rates of the other four abyssal plain faults are 5.5 ± 2 - 6. 7 ± 3 mm/yr. These faults are active, as indicated by offset of the youngest sedimentary units, surficial fault scarps, offsets of surficial channels, and deep fluid venting. All nine faults have been surveyed on the continental slope using SeaMARC 1A sidescan sonar, and three of them were surveyed with a high-resolution AMS 150 sidescan sonar on the continental shelf off central Oregon. On the continental slope, the faults are expressed as linear, high-angle WNW trending scarps, and WNW trending fault-parallel folds that we interpret as flower structures. Active structures on the shelf include folds trending from NNE to WNW and associated flexural slip thrust faulting; NNW to N trending right-lateral strike-slip faults; and WNW trending left-lateral strike-slip faults. Some of these structures intersect the coast and can be correlated with onshore Quaternary faults and folds, and others are suspected to be deforming the coastal region. These structures may be contributing to the coastal marsh stratigraphic record of co-seismic subsidence events in the Holocene. We postulate that the set of nine WNW trending left-lateral strike-slip faults extend and rotate the forearc clockwise, absorbing most or all of the arc parallel component of plate convergence. The high rate of forearc deformation implies that the Cascadia forearc may lack the rigidity to generate M > 8.2 earthquakes. From a comparison of Cascadia seismogenic zone geometry to data from circum-Pacific great earthquakes of this century, the maximum Cascadia rupture is estimated to be 500 to 600 km in length, with a 150-400 km rupture length in best agreement with historical data. / Graduation date: 1994

The eastern California shear zone (ECSZ) and Walker Lane belt represent an important inland component of the Pacific-North America plate boundary. Current geodetic data indicate accumulation of transtensional shear at a rate of ~9.2 ± 0.3 mm/yr across the region, more than double the total geologic rate (<3.5 mm/yr) for faults in the northern ECSZ over the late Pleistocene [Bennett et al., 2003, Kirby et al., 2006, Lee et al., 2009, Frankel et al., 2007]. Unraveling the strain puzzle of the Walker Lane is therefore essential to understanding both how deformation is distributed through the lithosphere along this transtensional part of the Pacific-North America plate boundary and how the plate boundary is evolving through time. The observed mismatch between geodetic and geologic slip rates in the central Walker Lane is characteristic of other active tectonic settings, including the nearby Mojave segment of the ECSZ [Oskin et al., 2008] and the Altyn Tagh fault in China [Cowgill, 2007]. In each case, lack of fault slip data spanning multiple temporal and spatial scales hinders interpretation of fault interactions and their implications for lithospheric dynamics. The discrepancy between geodetic and geologic slip rates in the central Walker Lane indicates that if strain rates have remained constant since the late Pleistocene [e.g. Frankel et al., in press], then the "missing" strain is distributed on structures other than the two major dextral faults at this latitude (Death Valley-Fish Lake Valley fault and White Mountains fault). Otherwise the region could presently be experiencing a strain transient similar to that of the nearby Mojave section of the ECSZ [e.g., Oskin et al., 2008], or the rate of strain accumulation could actually increasing over the late Pleistocene [e.g. Reheis and Sawyer, 1997; Hoeft and Frankel, 2010]. The Silver Peak-Lone Mountain extensional complex (SPLM), to which the Clayton Valley faults belong, is the prime candidate to account for the "missing" strain. The down-to-the-northwest orientation of the SPLM faults makes them the most kinematically suitable structures to accommodate the regional pattern of NW-SE dextral shear. We use differential GPS to measure fault offset and terrestrial cosmogenic nuclide (TCN) geochronology to date offset landforms. Using these tools, we measure extension rates that are time-invariant, ranging from 0.1 ± 0.1 to 0.3 ± 0.1 mm/yr for fault dips of 30° and 60°. These rates are not high enough to account for the discrepancy between geologic and geodetic data in the ECSZ-Walker Lane transition zone. Based on geologic mapping and previously published geophysical data [Davis, 1981; Zampirro, 2005], deformation through Clayton Valley appears to be very widely-distributed. The diffuse nature of deformation leads to geologic slip rates that are underestimated due to the effects of off-fault deformation and unrecognized fault strands. Our results from Clayton Valley suggest that the discrepancy between geodetic and geologic strain rates at the latitude of the northern ECSZ is a result of long-term geologic rates that are underestimated. If the true geologic rates could be calculated, they would likely be significantly higher and therefore in closer agreement with geodetic data, as is the case everywhere else in the ECSZ north of the Garlock fault [Frankel et al., 2007a, in press; Kirby et al., 2008; Lee et al., 2009a].

Tian Shan is one of the most important orogenic belts in central Asia. It has been reactivated as a result of the Cenozoic India-Eurasia collision. Dating of the late Cenozoic tectonic deformation of Tian Shan and its piedmonts is important for understanding the mountain building as well as evaluating seismic hazards in the region. This study is focused on the applications of optical dating to the late Quaternary uplift and thrust activity along Manas River, in the northern piedmont of the Tian Shan, China. The sediments on river terraces were dated with optical dating. The elevations were measured with the kinematic global position system (GPS). The results suggest that two phases can be identified according to the significantly different river incision rates. One phase was from ~20 ka to ~4.8 ka, with a much slower incision rate of ~ 2.2 ± 0.6 mm/yr. The other phase was from ~4.8 ka to present, with a faster incision rate of ~ 13.5 ± 0.6 mm/yr. The accelerated incision rate of Manas River was mainly attributed to the tectonic forces, suggesting that the tectonic uplift was significantly intensified since ~4.8 ka in the northern piedmont of Tian Shan. The study region has suffered from multiple thrust activities during the late Quaternary, which led to the intensive deformations of the river terraces. By studying the deformed terraces, I evaluated the timing of the past thrust activities as well as the vertical slip rate of the thrust faults. The results demonstrated that the thrust activity intensified during the late Holocene, as manifested by the more frequent thrust activities and higher vertical slip rates. Both quartz and potassium feldspar can be as dosimeters for optical dating of sediments. However, quartz OSL is sometimes seriously impeded with problems such as very dim signals and insufficient bleaching problems. K-feldspar has attractive advantages over quartz, despite of problem of anomalous fading. K-feldspar was explored in this study, by investigating the relationship between the infrared stimulated luminescence (IRSL) and blue light stimulated luminescence (BLSL) signals. For IRSL and BLSL at 60 °C, it was suggested that most of the IRSL could be bleached by blue light (BL), while the BLSL could only be partially bleached by infrared (IR) stimulation. Besides, the fast and medium components of BLSL were mainly associated with the IRSL. If IR stimulation temperature was raised from 60 to 200 °C, at least two portions of the IRSL signals at 200 °C were observed. One portion could be bleached by BL at 60 °C and the other portion was hardly bleached by BL at 60 °C. Dating of K-feldspar from the various signals provided cross-checking for the reliability of quartz OSL for dating sedimentary samples. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy