John H. Spang

Influence of layering and boundary conditions on fault-bend and fault-propagation folding

The influence of heterogeneous layering and boundary conditions on the structural development of fault-bend and fault-propagation folds has been investigated through petrographic study of nonscaled rock models. The models are deformed in a triaxial rock-deformation apparatus at room temperature and a 50-MPa confining pressure. The models consist of a single layer of sandstone containing a saw-cut ramp that is inclined 20° to the layering, and an overlying, intact, thinly layered unit that is composed of limestone interlayered with lead or mica. Analysis of the fold-thrust structures generated in sequentially shortened models with different loading conditions and layer types suggests that the mode of fold-thrust interaction activated upon shortening will depend on fault zone drag, bending and shearing resistance of the hanging wall, shear strength of layer interfaces, and loading conditions. For the models, these parameters may be expressed as a strength ratio describing the resistance to foreland translation relative to the resistance to internal deformation of the thrust system. Low strength ratios favor fault-bend folding. High strength ratios favor internal shortening of the sheet; isotropic and thick (relative to ramp height) units above a propagating thrust tip will shorten primarily by faulting, whereas thinly layered, anisotropic units will shorten by fault-propagation folding. During both modes of fold-thrust interaction, the dips of the fold limbs increase, interlimb angles decrease, and imbricate faults form in the hanging wall or footwall with shortening. In one model suite, the imbrication is associated with a transition from fault-bend folding to fault-propagation folding and produces a highly asymmetric ramp anticline similar to a second-mode fault-bend fold or to a transported fault-propagation fold. The model data suggest that fault-propagation folding in heterogeneously layered rock occurs by the discontinuous formation, growth, and linkage of faults below the growing fold. Amplification of the fault-propagation fold is affected by the amount of slip transferred out of the deforming region, imbrication, and buckling. The changes in the mode of fold-thrust interaction and modifications in the local geometry and strain distribution that occur during shortening result from slip hardening on faults, or from rotation- or strain-induced variations in the strength of the layers.

Kinematic models of deformation at an oblique ramp

Kinematic models for the deformation of hanging wall material moving over a footwall oblique ramp are developed by considering two end members of assumed mechanical behaviour, vertical shear and layer-parallel shear. In the former case, material is sheared vertically and displacements remain within the tectonic transport plane; the deformation is accommodated by thinning of the hangingwall over the ramp. In the later case, material is deflected out of the transport plane such that the pitch angle of the hangingwall particle path in the plane of the oblique ramp is equal to the angle between the transport direction and the strike of the oblique ramp. As a result, shear strains above the oblique ramp are non-zero in both the transport and transport-normal planes. The deflection and transport-normal shear strains are a minimum for the special cases of pure frontal and lateral ramps, and maximum at an intermediate orientation, depending on oblique ramp dip. Fault-bend folds are similar in most respects for both vertical shear and layer-parallel shear mechanisms. At frontal ramp — oblique ramp intersections, synformal or antiformal multiple bends in the footwall generate, respectively, second order hangingwall synclines or anticlines, which terminate along strike into simple fault-bend folds. For the layer-parallel shear mechanism along the pure oblique ramp, deflected hangingwall material passes through the transport plane, conserving area and volume. At the rearward intersection zone (concave toward the transport direction), hangingwall material diverges resulting in local strike-parallel extension. This extension may be a mechanism for the generation of transverse faults (or ‘tear faults’) in the hangingwall. At the forward intersection zone (convex toward the transport direction), displacement paths converge resulting in local strike-parallel shortening. The attitude of the oblique ramp and the amount of displacement significantly affect the map geometry and magnitude of lateral strains.

Interaction of basement uplift and thin-skinned thrusting, Moxa arch and the Western Overthrust Belt, Wyoming: A hypothesis

Immediately north of Big Piney in western Wyoming, the Moxa arch is modeled as emplaced during the Late Cretaceous along an east-dipping, low-angle thrust (Moxa thrust) that has Precambrian basement and Paleozoic and younger cover rocks in the hanging wall. The west-verging Moxa thrust cut up-section from the basement-cover contact and flattened to the west along a detachment in Lower Triassic rocks (Thaynes detachment). During motion on the Moxa thrust, the leading edge of the hanging wall wedged westward along the Thaynes detachment, peeling back Triassic and younger rocks and thrusting them relatively eastward along the ancestral Prospect thrust. The 5.3 km of westward movement along the Moxa thrust was matched by eastward movement along the west-dipping ancestral Prospect thrust. The ancestral Prospect thrust moved an additional 5.1 km during the late Paleocene when thrust-belt deformation progressed eastward to the Moxa arch. It appears that in the Snider Basin area, the Prospect thrust does not share a ramp with the Darby thrust and that the emplacement of the Moxa arch and Prospect thrust determined the position of the later and more westerly Darby thrust.

Effect of initial fault geometry on the development of fixed-hinge, fault-propagation folds

Abstract This paper describes how a model of fixed-hinge, basement-involved, fault-propagation folds may be adapted to apply to thin-skinned thrust faults to generate footwall synclines. Fixed-hinge, fault-propagation folding assumes that the fold-axial surfaces diverge upwards, fold hinges are fixed in the rock, the fault propagated through the forelimb, thickness changes occur in the forelimb and the forelimb progressively rotates with increasing displacement on the underlying fault. The original model for fixed-hinge, fault-propagation folds was developed for the case of a planar fault in basement with a tip line that was at the interface between basement and the overlying sedimentary cover rocks. The two geometries applicable to thin-skinned thrusts are for the cases where a fixed-hinge fault-propagation fold develops above an initial bedding-parallel detachment, and an initial fault ramp of constant dip which flattens down-dip into a bedding-parallel detachment.