Deformation at Lateral Ramps and Tear Faults—Centrifuge Models and Examples from the Canadian Rocky Mountain Foothills

Abstract

Scaled physical models demonstrate how a thrust sheet accommodates itself to displacement over ramp systems consisting of two frontal-ramp segments linked by a vertical tear fault or a gently dipping lateral ramp. Four stratigraphic units of alternating competence, composed of thin layers of plasticine and silicone putty, rest on a rigid base. Ramp systems are cut in the basal (competent) stratigraphic unit during assembly. The models are built at a linear scale ratio of 106 (1 mm 1 km) and deformed in a centrifuge at 4000 g. Matched models are serially sectioned transversely or longitudinally, to constrain the structure in 3-D. Structures in the hanging wall and footwall are diagnostic of the geometry of the ramp system. The thrust sheet develops prominent structural culminations associated with the frontal ramps. The culminations terminate abruptly above the buried transverse structure, and associated folds plunge in opposite directions off the ends of the culminations. The leading edge of the competent lowest unit in the thrust sheet tends to become overturned toward the foreland. In plan view, it mimics the geometry of the ramp system. However, the leading-edge segments become curved, reflecting a decrease in thrust displacement on each frontal-ramp segment toward the lateral structure. The transverse structures appear to be rotated in response to 3-D strain comprising longitudinal gravitational spreading and stretching of the ramp-anticline culminations, as well as local longitudinal spreading of the footwall of the trailing frontal ramp adjacent to the transverse structure. Both frontal ramps propagate laterally past the transverse structure. The leading frontal ramp propagates into the footwall beneath the main thrust surface, and the trailing ramp propagates into the thrust sheet itself. Lateral propagation of the ramps generates local fault-propagation fold structures that are potential hydrocarbon traps. The models suggest that a transverse link between two frontal ramps is unstable. Once it has rotated out of the transport direction, the linking fault is no longer favorably oriented to transfer slip between the frontal-ramp segments, so it is abandoned and the two frontal ramps propagate laterally to form a lap-joint displacement-transfer zone. This begs the question: Can a tear fault or lateral ramp develop as a link between two frontal ramps in laterally uniform strata, or must such a structure be localized by a transverse heterogeneity such as a preexisting fault or facies boundary? Some features of the models are also seen in the Limestone Mountain area, southern Alberta Foothills, where the Brazeau Thrust fault and its splays ramp upward laterally to the southeast through lower Paleozoic carbonates. The Limestone Mountain and Marble Mountain culminations terminate abruptly to the southeast, at these blind lateral ramps. In strike section, the lateral structure appears to change its orientation along strike, from southwest to northeast, from a shallow northwest-dipping lateral ramp to a steep and even overturned (southeast-dipping) tear fault. With its variable dip, the Brazeau lateral ramp resembles a combination of the lateral-ramp and tear-fault physical models, but the variable dip is inferred to be a primary feature rather than a result of out-of-plane rotation. Serial transverse and longitudinal sections of the models provide constraints on 3-D geometry that will aid geoscientists in interpreting wells and seismic sections through these complex structures and other similar situations, in the Canadian Rockies and other fold-thrust belts.