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.