Syncline-Parallel, Thin-Skinned Detachment in Basement-Involved Foreland Structures: A New Mechanism for 3D Folding in the Rockies

Of the smaller, second-order anticlines adjacent to Rocky Mountain basement arches, detachment folds on arch margins are the least well known. Many of these folds trend parallel to the adjoining arch (e.g., Pinedale Anticline, WY) and appear to have compatible arch-normal shortening. But others, like the South Fork detachment complex SW of Cody, WY, display fold axes and thrust faults indicating shortening nearly perpendicular to regional shortening trends.

Beaver Creek Anticline is located along the synclinal axis of a complex series of north-to-northwest-trending, basement-involved faults and monoclines on the western limb of the Bighorn basement arch. This anticline forms the arcuate boundary of an elongate, syncline-parallel detached patch of Jurassic and Cretaceous strata. Over 1500 fractures were analyzed to test whether this detachment formed due to (1) syn-Laramide detachment rooted in mountain-front faulting, (2) syn-Laramide gravity sliding during folding of the mountain front, or (2) post-Laramide gravity sliding away from the mountain-front folds. Kinematic analyses indicate that a complex combination of ENE-WSW regional shortening and south-directed, syncline-parallel detachment resulted in NE-SW shortening along the Beaver Creek Anticline. Independent movement at various levels in the strata shows that local kinematics do not necessarily parallel the regional deformation.

Analog modeling of the basement fault system shows detachment both perpendicular and parallel to synclinal axes. Syncline-parallel detachment may have been facilitated by the bending in the bounding syncline in combination with a southerly regional dip off a basement-cored buttress to the north. The proposed syncline-parallel component of detachment movement may be a new mechanism for the development of foreland second-order structures.