Abstract

The kinematic evolution and along-strike variation in contractional deformation in overthrust belts are poorly understood, especially in three dimensions. The Sevier-age Cordilleran overthrust belt of southwestern Wyoming, with its abundance of subsurface data, provides an ideal laboratory to study how this deformation varies along the strike of the belt. We have performed a detailed structural interpretation of dual vergent thrusts based on a 3D seismic survey along the Wyoming salient of the Cordilleran overthrust belt (Big Piney-LaBarge field). The complex evolution of the thrust faults that parallel the overthrust belt is demonstrated by the switching of the direction of thrust fault vergence nearly 180° from east to west. The variation in thrust-fault geometry suggests additional complexities in bulk translation, internal strains, and rotations. The thrust zone is composed of two sub-zones, each with an opposing direction of fault vergence, located on the eastern toe of the Hogsback thrust in southwestern Wyoming. The northern west-vergent thrust is a wedge thrust and forms a triangle zone between its upper thrust plane and the lower detachment that has formed in a weak shale layer (the Cretaceous K-Marker bed). Thrusts to the south have a frontal ramp geometry and are consistent with the overall thrust orientation of the Cordilleran overthrust belt located immediately to the west. The two thrust sub-zones are small, relative to the main Hogsback thrust to the west, and adjacent to each other, being separated by a transfer zone measuring in the hundreds of meters along strike. The transfer zone is relatively undisturbed by the faults (at the scale of seismic resolution), but reflections are less coherent with some very small offsets. The thrusts are thin-skinned and located above a shallow-dipping single detachment (or décollement) that is shared by faults in both sub-zones. Lateral growth of the thrust faults link along strike to form an antithetic fault linkage. Structural restoration of thrust faults shows varied amounts of shortening along strike as well as greater shortening in stratigraphic layers of the west-vergent fault to the north. Results from a waveform classification and spectral decomposition attribute analysis support our interpretations of how the variations in the detachment may govern the structural development above it. The kinematic evolution of the dual-verging thrust faults is likely controlled by local pinning within the transfer zone between the thrust-fault sub-zones as well as by changes in the competence of the strata hosting the detachment and in the thickness of the thrust sheet. The analysis and interpretation of dual-vergent thrust structures in the Cordilleran overthrust belt serve as an analog to better understand complex fold, fault, and detachment relations in other thrust belts.

Degree

College and Department

Physical and Mathematical Sciences; Geological Sciences

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