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Analog Modeling of Doubly Vergent Thrust Wedges
K. R. McClay, P. S. Whitehouse
Fault Dynamics Research Group, Geology Department, Royal Holloway University of London, Egham, Surrey, U.K.
Research presented in this chapter was supported by the Fault Dynamics Research Group, Geology Department, Royal Holloway University of London. K. McClay also gratefully acknowledges support from BP Exploration. Mike Craker assisted with maintenance and modifications to the deformation apparatus. Fault Dynamics Publication No. 112.
Two-dimensional scaled analog models of doubly vergent thrust systems produced a two-stage evolution for the development of asymmetric thrust wedges. Stage I was the rapid elevation of an axial zone bounded by a major retrovergent thrust system together with closely spaced, low-displacement provergent thrusts in its hanging wall. When a critical elevation of the axial zone was attained, Stage II deformation was characterized by the nucleation and propagation of high-displacement, provergent thrusts to form a critically tapered prowedge. The prowedge taper angle was typically 11–12, whereas the retrowedge maintained a steep surface taper of 38–42. During Stage II, the doubly vergent asymmetry increased. This was characterized by the formation of well-developed provergent thrust sheets and slower uplift of the retrowedge system. Overlapping prowedge thrusts showed synchronous displacements.
Addition of synkinematic erosion and/or sedimentation produced dramatic changes to the doubly vergent wedges in the models. There were fewer prowedge thrusts, prolonged prowedge thrust activities, out-of-sequence thrusting, and reactivation of preexisting thrusts. Synkinematic growth strata recorded the progressive evolution of the wedge system and also the activities on individual thrusts. The final geometries of the analog models of doubly vergent thrust wedges closely replicated those of naturally occurring convergent and collisional orogens, such as the Pyrenees, the western Alps, and the Himalayas. The models provide important geometric and kinematic templates for understanding the development of orogenic thrust-and-fold belts. They emphasize the dynamic feedback between surface processes—synkinematic erosion and sedimentation—and thrust-belt evolution. Synchronous thrust activities, out-of-sequence thrusting, and interaction with erosion and sedimentation all need to be taken into account when one evaluates hydrocarbon generation, migration, and entrapment in thrust belts.
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