3D evolution of fold and thrust belts formed by oblique convergence

Abstract

This paper presents the results of a series of scaled analogue models designed to simulate thrust belt development formed by orthogonal and oblique convergence. The models were constructed to produce doubly-vergent orogenic wedges with a pro-wedge and a retro-wedge separated by an uplifted orogenic core. Models were constructed from homogeneous layered sandpacks in a 3 m×1.3 m deformation rig. The progressive evolution of the models was recorded using digital photography, and the photographs were animated for analysis. The experiments investigated convergence obliquities from orthogonal (90°) to as low as 15° obliquity. Orthogonal models produced long, linear, critically tapered pro-wedge fold and thrust belts parallel to the convergence margin together with a narrow, uplifted core and a steep retro-wedge thrust system. The taper angle of the pro-wedge was typically 11–12° whereas the retro-wedge maintained a steeper taper of 38–42°. Models where the convergence vector was oblique at angles of 60 and 45° to the margin produced doubly-vergent thrust wedges with thrust faults trending parallel to the margin. There was little evidence of linked, penetrative strike-slip faulting but en-echelon Riedel like shear systems developed in the 45° oblique model. Subordinate oblique-slip motion was observed on some thrust faults. In contrast, for 30 and 15° oblique convergence models, strong deformation partitioning developed with steeply sloping pro-wedges that did not have critical tapers. Through-going linked strike-slip faulting was developed that cut the uplifted axial zones of these models. Analysis of the models by animation of the digital photographs shows how the thrust systems initiated and propagated. In particular it is clear that at any one time several thrusts were moving simultaneously. Serial cross sections through the moderately oblique convergence models (60–45°) are almost indistinguishable from the orthogonal model cross-sections. The results of orthogonal convergence experiments compare well to the map patterns of faults and folds in the Salt Range, Pakistan. 60 and 45° oblique convergence models compare well to the Zagros where some strain partitioning on strike-slip faults occurs. The highly oblique models show very strong strain partitioning with penetrative margin-parallel strike-slip faulting similar to that found in NE Venezuela.