Date of Award:
Master of Science (MS)
James P. Evans
This paper presents new interpretations of two normal-slip, low-angle faults near Smithfield and Richmond, Utah. The faults have previously been interpreted as landslides, gravity slides, slide blocks, and depositional contacts. Recent work in the Basin and Range province allows new interpretations concerning the origins of the low-angle faults.
Working hypotheses used to interpret origins of the faults are classified as folded thrust fault, rotated high-angle normal fault, gravity slide, listric normal fault, and low-angle normal fault Among these general categories are several subhypotheses. The evaluation of each hypothesis includes a description of the geologic requirements of the hypothesis, a comparison of field data to the requirements, and a conclusion regarding the hypothesis. Field maps, computer analyses of fault orientations, geophysical surveys, well logs, and published discussions of low-angle-fault origins provide the data base from which to derive conclusions.
The data best fit a low-angle-normal-fault hypothesis which states that low-angle normal faults in the study area represent a pre-Basin and Range style of extensional tectonism in which principal stress axes were in a transitional state between compressional tectonism and modern Basin and Range extensional tectonism. The northern low-angle normal fault formed as early as the late Eocene, followed by the southern low-angle normal fault in the early to middle Miocene(?). Episodes of high-angle normal faulting followed formation of the southern low-angle normal fault. The faulting history indicates that two distinct stress states existed resulting in two different styles of normal faults.
Schematic cross-sectional reconstructions based on two other low-angle-normalfault subhypotheses and the gravity-slide subhypothesis 2 indicated that these subhypotheses could be valid However, the two low-angle-normal-fault subhypotheses cannot account for transitional stress states, and the gravity-slide subhypothesis explains only the southern low-angle normal fault. On the basis of geologic simplicity, the best hypothesis should explain both low-angle faults because of their similarities in deformation, orientation, and age.
The applicability of the low-angle-normal-fault model to the rest of the Basin and Range province is somewhat limited. Too many local variables are involved to allow one model to be regionally applied. (112 pages)
Brummer, Jon E., "Origins of Low-Angle Normal Faults Along the West Side of the Bear River Range in Northern Utah" (1991). All Graduate Theses and Dissertations. 6784.
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