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Estimates of the Neogene to Modern Regional Strain for Northern Walker Lane, Basin and Range Province, Usa

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Estimates of the Neogene to Modern Regional Strain for Northern Walker Lane, Basin and Range Province, Usa ESTIMATES OF THE NEOGENE TO MODERN REGIONAL STRAIN FOR NORTHERN WALKER LANE, BASIN AND RANGE PROVINCE, USA A Thesis Presented for the Master of Science Degree The University of Memphis Norman Richard Mannikko August 1998 ABSTRACT Northern Walker Lane is an intracontinental area of complex faulting. It is comprised of northwest-trending right-lateral strike-slip faults, northeast-trending left-lateral strike-slip faults, and north-trending normal faults. Regional strain causes the motion across active faults in the upper crust. The displacement of the rocks adjacent to these faults is the structural relief. The regional strain, therefore, can be estimated by examining the structural relief and topography assuming that they are a consequence of motion across active faults. The tool used to forward model the development of structural relief is a 3-dimensional boundary element program. An orthogonal far field strain, represented as a displacement gradient tensor, is imposed on a homogeneous elastic half-space with a series of dislocation elements. The relative displacement of these elements give the sense of slip along each fault. A horizontal inspection plane is placed at zero elevation to represent the earths surface. The boundary element algorithm explicitly accounts for the interaction of the faults and permits calculation of the vertical displacement of the inspection plane. This modeled structural relief can be used to discriminate between regional strain orientations. Volume strain and fault slip motion is also used to discriminate between models. The modeled e1 orientations with characteristics approximating northern Walker Lane are E-W through N70˚W. The modeled e1 orientations of N70˚W through E-W show remarkable similarities to the structural relief and topographic features of the northern Walker Lane. The modeled volumetric strain show areas of positive volume strain (locations susceptible to volcanic activity) which correlate with the volcanic activity with few discrepancies. The modeled fault offsets are in agreement with their corresponding faults in northern Walker Lane. 1. INTRODUCTION Northern Walker Lane is an intracontinental area of active deformation over 250 km east of the Pacific-North American plate margin (figure 1). The focus of this research is to estimate the Neogene to modern regional strain for an area of complex faulting within northern Walker Lane. Plate tectonic theory has provided a means to derive kinematics of plate margins, but analysis is limited when working on intracontinental deformation because, by definition, plates are rigid and do not deform. Nevertheless, knowledge of the relative poles of rotation between two plates allows us to derive components of the deformation field in a diffusely deforming zone between two plates (e.g., Jackson and Mackenzie, 1988). Other methods to derive the strain field include the use of high precision surveying (e.g., global positioning satellite system [GPS]) and various geologic markers (e.g., faults, dikes, etc.). The former is limited to time-scales that may be more representative of interseismic deformation and in response to the activity of perhaps a limited number of faults in any particular region. In this case, these observations may not represent the long-term average (~105 - 106 years). The use of geological markers may be limited by the local setting (e.g., influenced by local anomalies in the regional stress field) or, in a practical sense, by the extensive amount of data needed to determine the regional stress field. In this thesis, I present a relatively new method of estimating the regional and long-term (~105 - 106 years) average strain field. If we make the assumption that the structural relief and topography is directly influenced by the motion across active faults, then the structural relief and topography reflects the regional strain. Therefore, we can estimate the regional strain by rigorously looking at the structural relief and topography of a faulted region such as northern Walker Lane. The method I use to make this estimate is with a three-dimensional boundary element model. The numerical model can help distinguish orientations of regional strain by comparing three of the modeled structural characteristics to the actual structural characteristics. One is by comparing the deformation of an originally horizontal surface displacement field to the local topography and structural relief; the second is by comparing modeled fault slip directions and rates to what is observed in the field; and thirdly, the volumetric strains can be compared to areas of volcanic activity. The results obtained here may be useful in two respects. First, they allow us to test the plate tectonic predictions and thereby evaluate the interactions among plates. Second, the boundary element model provides a means of determining the regional strain and provides quantitative estimates about fault slip-rates, which are useful in seismic hazard analysis. Study area The study area is located near the northern extent of Walker Lane (figure 1). The Walker Lane Belt is described by Stewart (1980) as being a fundamental structural unit whose topography contrasts the typical north-northeast-trending ranges of the Basin and Range Province and the north-northwest-trending ranges of the Sierra Nevada. The belt is a diffuse shear zone of comparatively subtle topography and complex structural faults located in western Nevada and parts of California. This northwest trending zone is about 700 km long and 100-300 km wide. The focus of this research is in the area of Reno, Nevada and vicinity and part of California extending 160 km x 120 km (figure 2). The study area runs from the Eastern extent of the Sierra Nevada block of Honey Valley south to Lake Tahoe and extends east to the Walker River and Mason Valley then north to Lake Winnemucca. Geology The geology of the study area is dominated (~ 90%) by Tertiary volcanic rocks and Quaternary deposits (Bonham, 1969). The major portion of Tertiary rocks are volcanic flows, breccias, and water-lain tuff, the remainder are intrusive and non-marine sediments. The Quaternary sediments include glacial deposits, landslide debris, and fluvial and lacustrian deposits. The remaining approximately 10% of surface area is pre-Tertiary (Bonham, 1969). Most of the pre-Tertiary rocks are Mesozoic-age granitoid intrusions. The intrusions are of stock dimensions and intrude questionable Permian to Jurrasic-age metamorphic volcanic and sedimentary rocks. The Mesozoic intrusive rocks that crop out in the area consist of granitoid plutonic rocks. The metavolcanic rocks are locally interbedded with marine sedimentary rocks. Therefore, the volcanic rocks must have been, at least in part, laid down in a marine environment (Bonham, 1969). Physiography Northern Walker Lane belt is characterized by mountain ranges with varying topography and trends and strike-slip faulting breaking up the structural grain. The mountain ranges have moderate to high relief separated by Quaternary alluvial basins that frequently contain playas (Stewart, 1980). The relief, although typically moderate, varies throughout the study area. High relief is associated with the eastern extent of the Sierra Nevada, which is approximately 1150 m above the adjacent valley to the east. The average relief away from the Sierra Nevada is about 600 m +/- 100 m . 2. LINKING NORTHERN WALKER LANE TO PLATE TECTONICS AND DEVELOPMENT OF CENOZOIC STRUCTURES We can use the relative poles of rotation between plates to derive components of the deformation field in a diffusely deforming zone. These velocity vector models can be tested based on regional strain determinations. The topography can be used to determine the best fit model of the regional strain. Therefore, it is prudent to investigate the tectonic history of northern Walker Lane to show that it may be part of a wide diffuse boundary between two plates and that the present topography seems to have developed since the Neogene, and therefore, reflects the motion across Cenozoic faults. Pre-Cenozoic Northern Walker Lane and western Nevada was a deep oceanic basin along the passive margin of North America during the early Paleozoic. This Atlantic-style margin changed to an Andean-style convergent margin in late Devonian time, and convergence continued through the Eocene. Two main thrust systems occurred in central Nevada as a result of this convergence. The Golconda and Roberts Mountain thrusts created a lower Triassic uplands in central Nevada that was a source of detritus to the east and west. Deposition from the highlands to the east had ceased by late Lower Jurassic. Northern Walker Lane was changing from a geosyncline to a subaerially exposed or near-shore marine environment. Moore (1960) pointed out in his discussion on the age of sequences of metabasalt and metarhyolite in Lyon, Douglas, and Ormsby Counties, that although the metavolcanic rocks are interbedded with marine sedimentary rocks and must be, at least in part, of submarine origin, the rarity of pillow lavas within the metavolcanics and the local association of the volcanic rocks with bedded gypsum indicates that a substantial part of the volcanics were formed in a subaerial or near-shore environment. This is also indicated by the occurrence of welded ash-flow tuffs within the volcanic section (Bonham, 1969). Subaerial exposure during the Mesozoic is also indicated by the decrease in abundance of marine sedimentary rocks and the scarcity of Cretaceous sedimentary rocks. Cenozoic The North American western margin makes a drastic change after its long duration of convergence to a new margin dominated by right-lateral strike-slip faulting during the Cenozoic. The convergent plate margin comes to a close along most of the California coast with the consumption of the Farallon plate and the East Pacific Rise. This tectonic change brought about structural changes throughout California. The magnetic anomaly patterns in the northeast Pacific combined with plate theory have been used to determine the tectonic setting and geometry of the western margin (Atwater, 1970; Severinghaus and Atwater, 1990).
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