Sequential Thrusting Beneath the Willard Thrust Fault, Wasatch Mountains, Ogden, Utah

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Schirmer, Tad William
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Willard Thrust Fault; Wasatch Mountains; Ogden; Utah; Earth Sciences; Geology; Physical Sciences and Mathematics
thesis / dissertation description
The downstructure of viewing geologic maps, balanced and cross sections, and hanging-wall-sequence diagrams are applied to produce the first comprehensive synthesis of the structure below the willard thrust sheet.Development of the duplex beneath the Willard thrust may be explained with a "piggyback" thrust model where younger thrust slices form below and fold an older, overlying thrust sheet. Progressive failure of the footwall ramp of the Willard thrust sheet extended the sole thrust eastward and produced a duplex consisting of thrust slices (horses) which adhered to the overriding thrust sheet where it ramped from a lower sole thrust to an upper decollement horizon. The resulting structural culmination produced a distinct antiform in the Willard thrust sheet. The duplex is here named the Ogden duplex.Frontal folds (formed at ramps perpendicular to transport) and lateral folds (formed at ramps parallel to transport) mark the margin of Individual horses within the duplex. Folded thrusts, thrust-splay relationships, and lateral overlap of horses help determine the sequence of thrusting.The involvement of cratonic foreland basement rocks (Farmington Canyon Complex) in thrust slices within the Ogden duplex is similar to the Moine thrust belt in northern Scotland and pinpoints this area within zone III of Boyer and Elliott's (1982) model of a thrust system dominated by a major thrust sheet.The basement rocks form the core of several horses which moved a minimum of 9.6 km. Total shortening within the Ogden duplex is estimated at 8 to 12 km.The sequence of thrusting is proposed from higher to lower: the willard thrust fault moved first, then the Ogden thrust fault and, finally, the Taylor and Weber thrust system (here named). Striking similarities between the Ogden thrust fault, the Weber-Taylor thrust system, and the Durst thrust fault geometries suggest that they are all part of the same system.