Year of Award

2016

Document Type

Thesis

Degree Type

Master of Science (MS)

Other Degree Name/Area of Focus

Geology

Department or School/College

Department of Geosciences

Committee Chair

James W. Sears

Commitee Members

Rebecca Bendick, Marc Hendrix, Andrew Ware

Keywords

Neotectonics, Basin and Range, strain accommodation, Centennial shear zone

Subject Categories

Tectonics and Structure

Abstract

Depositional characteristics of the ‘Divide unit’ of the Cretaceous Beaverhead Group within the southern Beaverhead Mountains challenge previous interpretations. This study reassigns the Divide unit to the Neogene. This revised age assignment highlights a major influx of immature sediment at the onset of Basin-Range normal faulting. Syntectonic deposition in an active half-graben continued from Middle Miocene to the Pliocene, attaining a thickness of 800 m. Local clasts of carbonate, siliciclastic and orthoquartzite entered the channel from the immediate west through large mass-wasting events. Dacitic fluvial cobbles dated herein to 98 Ma were transported from an unknown southwesterly source. Clasts of the Ordovician Kinnikinic or Swan Peak Quartzite of Idaho experienced multiple phases of recycling from regional Cretaceous conglomerates such as the Beaverhead Group. Detrital zircon analysis of a feldspathic quartzite cobble identifies the Brigham Group of the Pocatello area as a source. Detrital zircons of anomalous ages suggest long-distance transport from the southern Basin and Range. This major drainage, analogous and likely continuous with that of the Sixmile Creek Formation in SW Montana, was long-lived until uplift associated with passage of the Yellowstone volcanic system halted deposition between 4.5 and 4.1 Ma. A broad volcanic plain persisted until movement along the Middle Creek Butte fault exposed the deposit around 0.365 Ma. Progressive recycling of resistant quartzite cobbles creates a subtle record of sedimentation in the northern Basin and Range, which has long been misidentified or overlooked. A revised interpretation for the Divide deposit allows for investigation of Neogene tectonics in the northern Basin and Range.

The revised age interpretation for the ‘Divide unit’ demands a revised interpretation of structural evolution. Dextral shear has been proposed as a mode of strain accommodation between the Centennial tectonic belt and the eastern Snake River Plain, yet no through-going strike-slip fault has been identified. Deformation features within the ‘Divide unit’ highlight a poorly organized and immature strike-slip system. Strain is distributed across numerous high angle conjugate strike-slip faults which loosely lie sub-parallel to the orientation of maximum principal stress (σ1). Distributed strain occurs in a wide yet discrete primary deformation zone (PDZ). These observed features are consistent with the Centennial shear zone hypothesis. Published shear experiments suggest that inherited fractures suppress formation of through-going strike-slip faults. Subcritical offset and inherited structures have suppressed through-going strike-slip fault formation in the study area. Results of this study illustrate fundamentally unique kinematics of emerging shear zones. Strain is distributed among many high angle, possibly conjugate, strike-slip faults that sub-parallel σ1 and the PDZ. Deformation features are unorganized until total offset collapses strain upon a single linked fault. For these reasons, emerging and immature shear zones lack characteristic offset topography and centralized fault geometries of developed systems, bearing little resemblance to classical models of shear.

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© Copyright 2016 Stuart D. Parker