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3D Strain at Transitions in Foreland Arch Geometry: Structural Modeling of The Beartooth Arch - Rattlesnake Mountain Transition, NW Wyoming

Thomas G. Neely, Department of Geosciences, Colorado State University, Fort Collins, CO 80524, phone: (970) 218-5168, [email protected]

How is 3D strain accommodated at transitions in foreland arch geometry? End-member hypotheses to explain 3D strain at foreland arch transitions include 1) uniformly directed slip on pre-existing basement structures, and 2) anomalously directed slip necessary for 3D strain accommodation. The Beartooth arch, SW Montana and NW Wyoming, transitions to the SE into a structurally complex zone characterized by a blind master thrust with anomalously oriented and backthrust hanging wall structures. The Rattlesnake Mountain anticline is a SW-vergent backthust within this hanging wall and is linked to the NE-vergent Beartooth arch by the S-vergent Pat O'Hara Mountain anticline. Inferred ideal σ1 orientations from over 1,200 slickensided minor faults and shear bands at the southern Beartooth arch transition show a uniformly oriented 065° stress field at sites located away from anticlinal axes. Inferred σ1 orientations at sites located within anticlinal axes are inconsistent with the regional 065° orientation, and show more northerly orientations (~030° to 045°). If the geometry of the structures is dictated by 3D strain accommodation, slip perpendicular to anticlinal axes would be predicted. Instead, slip is oblique to structural trends and also oblique to the inferred regional stress direction. These observations can be explained by uniformly directed slip on oblique pre-existing structures resulting in material rotation within zones of oblique-slip. The consistency of regional σ1 orientations, abrupt along-strike structural terminations, and documented local extension at structural terminations further support this hypothesis, which is a first-order constraint for current modeling using 3Dmove software.