Browsing by Subject "Faults (Geology)"

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Lineament analysis and inference of geologic structure: examples from the Balcones Ouachita trend of Texas
(University of Texas at Austin. Bureau of Economic Geology, 1982) Caran, S. Christopher; Woodruff, C. M.; Thompson, Eric J.
Lineaments perceived in remotely sensed images are reliable indicators of geologic structure. Lineaments on ten Landsat multispectral scanner images (band 5; 1:250,000 scale) were mapped covering the Ouachita/Balcones-Luling-Mexia-Talco structural trend between the Rio Grande and Red river in Texas. More than 5,000 lineaments were perceived in these images. Maps depicting the lineaments (individually and in various combinations) were compared with maps of structural/ tectonic features and geothermal gradient contours, noting instances of apparent correlation among these themes. Lineaments are correlative with the individual faults and the aggregate fault patterns of the Baicones, Luling, Mexia, and Talco fault zones, Transverse lineaments, which trend almost perpendicular to these fault zones, mark the northernmost extent of the Balcones fault system and outline carbonate piatforms, such as the Belton High/Moffat Mound trend and the San Marcos arch. Transverse lineaments are coincident also with the axes of the buried Chittim and Preston anticlines and with the flanks of the Sherman and Round Rock synclines. Numerous salt domes occur at depth in the western part of the east Texas basin near the trend; many of these domes, particularly those in Henderson, Anderson, and Freestone Counties, are found along and at the intersection of major lineament zones where the concentration of individuai lineaments is greatest. Most of the buried Late Crelaceocs volcanoes of central Texas near Austin lie along northeast southwest- trending lineament zones; the altered pyroclastic rocks and associated beachrock facies at many of these volcanoes are hydrocarbon reservoirs. The orientation and spacing of geothermal gradient contour lines ("isograds") also correspond to major structures and thus, to the pattern of lineaments throughout the region. Correlation of (1)individual lineaments, zones of cortiguous or nearly parallel lineaments, and areas of homogeneous lineament density and orientation to (2) surface and subsurface structure and (3) geothermal,'isograd" patterns indicates that lineament analysis has many potential applications to regional mineral resource assessment.
Physical models and natural examples of fold interference
(1995-08) Johns, Mary Katherine; Mosher, Sharon, 1951-
Fold interference is used to identify areas of polyphase deformation and to evaluate their deformation history. In this study, dynamically scaled physical models were shortened in two orthogonal directions in a centrifuge to test the effects of material properties and deformation history on the style of fold interference. Models shortened sequentially demonstrate that rheological contrast strongly controlled the interference style. Models with low competence contrast layering had circular to elliptical interference patterns. In addition to folding, the models accommodated strain by significant layer-parallel shortening. Models with high competence contrast layering had folded early hingelines and axial surfaces. In plan view, the second-generation folds were lobate-cuspate to box-style, and the axial traces of the box folds formed conjugate pairs. Models shortened coevally had distinct structural styles from the sequentially shortened models, as illustrated by serial Computed Tomographic (CT) X-ray scans. High competence contrast models were dominated by irregular elliptical to crescent map-shapes in the center of the models. Near the model edges, folds had straight hingelines, parallel to the boundaries. Coevally shortened models faulted and fractured less commonly than did the sequentially shortened models. Although some of the structures resembled superposed folds, the regionally inconsistent fold orientations and overprinting relationships, as well as the predominance of dome-and-basin over crescent-style interference differentiated coeval from sequential fold interference. For both deformation histories, gravitational body forces effectively damped the vertical amplification of folds, thereby accentuating the change in fold style with depth. Comparison of results from these models with regional-scale natural examples demonstrates that buckle-fold interference occurs in a wide range of rock types and tectonic settings. Comparison of the sequentially shortened, high competence contrast models with the Narragansett Basin, Rhode Island, suggests an alternative tectonic model for its third phase (D3) of Alleghenian deformation. According to previous tectonic models, D 3 sinistral shear produced both E-W trending folds and sinistral kinematic indicators. I propose that the same features could be caused by N-S shortening, which is locally partitioned into a NE-trending megakink band.
The role of fractures in regional groundwater flow : field evidence and model results from the basin-and-range of Texas and New Mexico
(1995) Mayer, James Roger, 1960-; Sharp, John Malcolm, Jr., 1944-
This study integrates fracture mapping and groundwater flow modeling to assess the role of fractures in regional groundwater flow. This is an important topic because fractures play a prominent role in groundwater flow in many aquifers. Furthermore, few studies have addressed quantitatively the regional hydrogeological implications of fractures. The study area is located in west Texas and southern New Mexico, between the Salt Basin and the Tularosa Valley. The region is largely undeformed, but the Permian carbonate bedrock is cut by many extensional faults and fractures. Air-photo analysis and field mapping reveal a broad fracture zone extending from the Sacramento Mountains to the Salt Basin near Dell City, Texas. Most fractures roughly parallel major normal faults and are oriented approximately N20W. The most intense fracturing coincides with a prominent trough in the potentiometric surface and an apparent "plume" of relatively fresh groundwater. Flow simulation and chemical modeling suggest that fracturing has created a high permeability zone that funnels recharge from the Sacramento Mountains at least 80 km southeastward to discharge points in the Salt Basin and the Dell City irrigation district. To estimate the regional transmissivity and to test the role of fractures in regional flow, a steady-state finite-element flow model was constructed in which fracture data are used to constrain a spatially distributed transmissivity. Given the probable range of recharge, discharge and other hydrologic parameters, fractures are the most important single constraint on the configuration of the potentiometric surface. Major results include: (1) fracturing can control groundwater flow over large (>1000 km²) areas, (2) effective recharge areas and regional groundwater chemistry trends are strongly influenced by fractures, and (3) through fracture studies, a priori inferences about aquifer properties and regional flow are possible. Finally, this study demonstrates one mechanism by which the timing and nature of tectonic events can affect regional subsurface fluid flow and, perhaps more importantly, related processes such as hydrothermal mineralization, diagenesis, and hydrocarbon transport and entrapment.