Date of Award:

1989

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

James P. Evans

Abstract

Deformation mechanisms in the footwall of the Willard thrust fault, northern Wasatch Range, Utah, change from dominantly plastic to dominantly cataclastic (both microscopically and macroscopically) in the Ophir Formation and Maxfield Limestone before the thrust begins to ramp laterally upsection southward, just to the north of the North Ogden Canyon field area. This transition in compressional deformation style and mechanism is located within a lateral distance of 3.2-kilometers along the 22-kilometer long trace of the thrust fault.

Between Willard Canyon and North Ogden Canyon penetrative deformation is localized within 200 meters of the thrust surface and is characterized by transposed bedding, solution cleavage parallel to bedding, a northeast- to northwest-dipping foliation, and tight isoclinal folds with axes plunging generally northward. A fracture overprint in the footwall is present throughout the study area. The transition in deformation mechanism and style suggests that footwall deformation is dependent on the sensitive response of limestone and shale to increased pressure and temperature conditions and also the presence of a lateral ramp in the footwall of the Willard thrust. Data from a hangingwall sequence diagram and a stratigraphic displacement diagram suggest the Taylor and Ogden thrusts formed prior to the Willard thrust (the roof thrust) and their sequential geometrical evolution may have been influenced by preexisting rifts in the underlying crystalline basement rock.

It is proposed that early Cretaceous movement of the Willard thrust sheet over the structurally lower and older Taylor and Ogden thrust sheets resulted in the formation of a recumbent syncline overturned to the east, a southward rising lateral ramp in the footwall of the Willard thrust, a lateral change in footwall deformation, and the anomalous east-west trending canyons that cut through the Willard thrust complex.

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