Award Date

12-1-2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Michael L. Wells

Second Committee Member

W. Adolph Yonkee

Third Committee Member

Terry L. Spell

Fourth Committee Member

Wanda J. Taylor

Fifth Committee Member

Clemens Heske

Number of Pages

296

Abstract

The Sevier fold-thrust belt of the Cordilleran orogen in the western United States is a well-studied geologic province, yet our understanding of the history of thrust faulting is still far from complete. Thermochronological methods are applied to major thrust systems from two distinct sections of the Sevier fold-thrust belt (SFTB): the Wheeler Pass thrust in the Mojave Desert section of the southern SFTB, and the Willard thrust in the Idaho-Utah-Wyoming section of the SFTB. The new thermochronological data greatly improve our understanding of the structural evolution of this extensive geologic province.

The zircon (U-Th)/He (ZHe) system records thrust-induced uplift and erosional exhumation and cooling of three crustal sections from the hanging wall of the Wheeler Pass thrust in the Mojave Desert section of the southern SFTB. The results constrain two separate episodes of thrust-induced exhumation and cooling. A Late Jurassic cooling event at ca. 158 to ca. 145 Ma is recorded along the western limb of the Wheeler syncline in the northwest Spring Mountains, interpreted to record erosional exhumation as the Wheeler Pass thrust sheet was translated up a major ramp. Late Jurassic shortening is consistent with proposed correlations of the Wheeler Pass thrust to the south with the Pachalka thrust in the Clark Mountains, and is coeval with deformation along the East Sierran thrust system and high-grade metamorphism associated with crustal thickening in the Funeral Mountains. Initial motion along the Wheeler Pass thrust by ca. 158 Ma pre-dates motion along similar quartzite-dominated thrust sheets in northern sections of the SFTB by up to 30 m.y.; instead, northward continuation of the Wheeler Pass/Gass Peak thrusts into the Central Nevada thrust belt is supported. Mid-Cretaceous cooling of the Wheeler Pass thrust sheet at ca. 98 to ca. 85 Ma, evident only in the southern Nopah Range, is interpreted to represent cooling related to passive uplift of the Wheeler Pass thrust sheet during footwall imbrication and formation of a duplex at the former ramp in the Wheeler Pass thrust. Mid-Cretaceous cooling was coeval with slip along the frontal thrusts including the Contact-Red Springs-Wilson Cliffs and Keystone thrusts.

The muscovite 40Ar/39Ar system records the timing of syntectonic mica growth in the upper part the footwall of the Willard thrust, exposed on Antelope Island in the Idaho-Utah-Wyoming section of the Sevier fold-thrust belt. Mica ages from a stratigraphically-localized shear zone, measured by in situ laserprobe and step heating methods, record muscovite neocrystallization resulting from strain and fluid induced chemical alteration of feldspar linked with deformation and fabric development during slip along the Willard thrust. Results record a main period of mica growth at ca. 115 Ma to ca. 90 Ma, which post-date currently accepted estimates of initial thrust motion by ~10-20 m.y. and are interpreted to date motion on the Willard thrust during the later part of its slip history. By ca. 90 Ma, the newly developed Ogden thrust system may have focused fluids away from the footwall, shifting fluid flow towards the foreland. Diminished fluid flow resulted in the termination of mica neocrystallization and ductile deformation, causing rheological strengthening of the Willard footwall. The rheological contrast generated between a weakened basal fault zone and a strengthened internal wedge reduced critical taper, which may have driven initial duplex faulting along the Ogden thrust system, uplift of the Wasatch anticlinorium, and motion along the Crawford thrust in the foreland at this time.

Keywords

Geochronometry; Orogenic belts; Orogeny; Sevier Orogenic Belt; Tectonics; Thermochronology; Thrust; Thrust faults (Geology); West (U.S.)

Disciplines

Geology | Tectonics and Structure

Language

English


Share

COinS