CONTROLS ON STRUCTURAL VARIABILITY IN LARAMIDE BASEMENT-INVOLVED ARCHES, U.S.A

Laramide basement arches of the Rocky Mountain foreland consist of structural culminations linked by complex transition zones with diversely-oriented secondary structures. Two arch segments, the southern Beartooth arch near Cody, Wyoming, and the northeast Fort Range arch north of Fort Collins, Colorado, were studied using fracture analyses and 4D restoration of surface and subsurface geometries to understand the controls on basement-involved arch geometries.

In both arches, minor faults indicate unidirectional, subhorizontal shortening, with regional shortening oriented 065° in the southern Beartooth arch (n=1581) and 085° in the northeast Front Range (n=3326). Map-scale structures are more variable in orientation. Minor faults adjacent to structures that are not orthogonal to regional shortening directions indicate slip variably deflected from regional trends in a manner consistent with vertical axis rotations and/or stress refraction during of strike slip.

The importance of pre-existing basement weaknesses is apparent in both areas. In the southern plunge of the Beartooth arch, structural geometries and balancing suggest that a lack of connecting weaknesses between the east-directed Beartooth and Oregon Basin thrusts may have forced west-directed, hanging-wall backthrusting on the Rattlesnake Mountain and Pat O'Hara structures, which appear to follow pre-existing weaknesses themselves. Eventually, the Line Creek thrust broke through, connecting the Beartooth and Oregon Basin thrusts. In the northeast Front Range, dextral strike-slip faulting along the NE-striking, Proterozoic Skin Gulch Shear Zone allowed the transition from west-directed thrusting near Fort Collins to east-directed thrusting at the Wyoming border.

Interestingly, areas of strike-slip faulting in both structures are marked by saddles in the arches, suggesting that arch amplitude is maximized where thrusting, and thus crustal thickening, predominated. Where strike-slip faulting occurred, typically along pre-existing basement weaknesses, shortening was maintained without the crustal thickening, creating a saddle in the arch.