This study focuses on the western Gale Hills located in the western portion of the Lake Mead domain in southern Nevada. The western Gale Hills preserve a record of the Miocene sedimentation and deformation related to the breakup of the hanging walls of the South Virgin-White Hills detachment fault and the Lime Ridge oblique, strike-slip fault of the Lake Mead fault system, the initiation of the right-lateral Las Vegas Valley shear zone in the western Lake Mead domain, and subsequent middle to late Miocene deformation. This study focuses on the lower Horse Spring Formation north of the Las Vegas Valley shear zone. To better understand the stratigraphy and deformation, a detailed geologic map (1:10,000 scale) was produced, data from primary and secondary structures were collected, and ash-fall tuff deposits were dated and correlated through 40Ar/39Ar geochronology and tephrachronology. The stratigraphy of the Gale Hills records the initial buttressing of the lower Thumb Member of the Horse Spring Formation onto pre-Tertiary topography. Deposition of the lower Thumb Member records a rapid transgression of the basin margin to the north and northwest across the majority of the Gale Hills. This time was period was then followed by a coarsening up interval and progradation of large alluvial fans in the middle to upper Thumb Member. A transition to a marginal clastic lake in the uppermost Thumb Member then abruptly changed to the Bitter Ridge Limestone algal lake.
Many studies have evaluated the exhumation history of the Gold Butte block in the eastern Lake Mead domain, which forms the footwall of the major South Virgin-White Hills detachment fault and the relationship with the Frenchman Mountain block. This study shows that the Frenchman Mountain block was just south of the Gale Hills during the early to peak stages of detachment faulting from ca. 17-14 Ma. Two new 40Ar/39Ar dates (15.35 Ma) from a prominent ash-fall tuff in the Thumb Member in the northern and southern regions of the western Gale Hills is also in the Frenchman Mountain block. In addition, new tephrachronology correlations have tied Proterozoic-clast debris flows in the western Gale Hills to Proterozoic-clast megabreccia deposits in the Frenchman Mountain block, indicating that the two areas were one connected basin during upper Thumb Member time.
This study suggests that the left-lateral Government Wash and Southern Gale Hills faults are reactivated northeast-striking, west-down normal faults that were in the correct orientation to be major Riedel prime shears (R') to the right-lateral Las Vegas Valley shear zone. Map and facies relationships show that the Thumb Member deposits were faulted locally during deposition at ca. 15.5 Ma, with increased fault activity and sedimentation rates throughout the Thumb Member after 15.35 Ma and before ∼14.5 Ma.
An analysis of structures in the western Gale Hills results in a new model of progressive clockwise rotation and faulting along the Las Vegas Valley shear zone that for the first time honors paleomagnetic results and accounts for all major faulting north of the shear zone. The model is primarily based on clockwise vertical-axis block rotation of domains between oblique left-lateral faults that curve progressively toward the Las Vegas Valley shear zone and terminate into major folds or areas of complex deformation. This model predicts that the western Gale Hills began as a north-northeast elongate block that was reduced in length and elongated in an east-west direction from about 20 to 14 km during translation and rotations. Most of this complex faulting occurred from ca. 13.8 to 8 Ma, after deposition of the Bitter Ridge Limestone.
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|Advisor:||Umhoefer, Paul J.|
|Commitee:||Beard, L. Sue, Duebendorfer, Ernest M.|
|School:||Northern Arizona University|
|School Location:||United States -- Arizona|
|Source:||MAI 52/02M(E), Masters Abstracts International|
|Subjects:||Physical geography, Geology|
|Keywords:||Gale Hills, Horse Spring Formation, Lake Mead domain, Lime Ridge fault, Miocene sedimentation and deformation, South Virgin-White Hills detachment fault, Thumb Member|
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