Date of Award:


Document Type:


Degree Name:

Master of Science (MS)




James P. Evans


Fractures and faults in the Jakey's Fork area, northeastern Wind River Range, Wyoming, caused by brittle Laramide deformation in the Precambrian granitic basement have been studied in detail at airphoto, outcrop, and thin-section scales. The study area is bounded on the south by the approximately east-west and vertical Jakey's Fork Fault and on the east by the approximately northwest-southeast and vertical Ross Lakes Fault. Both were active during Laramide deformation. Four distinct structural domains, defined by fracture pat terns and proximity to the two major faults nave emerged in this study. The areas are: 1) Along Ross Lakes Fault granite cores a fold defined by shallowly and steeply east dipping Cambrian Flathead Sandstone. Laramide movement on Ross Lakes Fault appears to have post-dated, Jakey ' s Fork Fault movement and was discordant with Precambrian zones. Fractures at all scales studied strike approximately northeast-southwest, consistent with the inferred maximum Laramide principal stress. 2) Along east-west striking Jakey's Fork Fault, Laramide movement appears to have reactivated Precambrian mylonite zones as evidenced by the chlorite-rich, foliated cataclasite along its trace. Fractures at all scales have an approximate east-west orientation. 3) Near the intersection of the two faults, deformation was intense, as shown by mylonitic, breccia, and veined clasts. Discrete airphoto fractures were not recognized due to intense deformation in this interaction zone. 4) In the central area, away from the two faults, airphoto and outcrop fracture orientations have a north to northeast strike. Fracture orientations at the thin-section scale are more variable and do not agree with macroscopic orientations; they strike west to northwest. The central area is a 'block', possibly divided into 'sub-blocks' , bounded by zones along which much of the deformation occurred. Thus, these zones had an insulating effect at thin-section scale. The Paleozoic rocks were at least partially decoupled from the basement during deformation, suggested by gouge along the contact and different fracture orientations on opposite sides of the contact. At least two fluid systems are represented in the study area. Relatively wide-spread, pre-Laramide chlorite development occurred at temperatures and pressures higher than those present during Laramide deformation. A Laramide (or post-Laramie ) pervasive fluid system (especially near Ross Lakes Fault) is reflected by abundant fracture porosity, advanced feldspar alteration, and kaolinite development.



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