SEDIMENTARY BASIN TAPER AS A FACTOR CONTROLLING THE GEOMETRY AND ADVANCE OF THRUST BELTS

摘要

Application of the critically tapered wedge model to the study of the geometry and kinematics of thrust belts suggests spatial and temporal relationships among sedimentary basin taper (the dip of underlying basement), styles of deformation, magnitude of internal shortening, widths of thrust belts, rates of thrust-belt propagation, and the location of foredeeps. The model predicts basins or portions of basins with higher basement dips will produce thrust belts with the following characteristics: (1) fewer thrusts with larger individual displacements, (2) lower magnitude of internal shortening, (3) greater width, and (4) faster rates of frontal advance. Thrust belt initiation is favored in regions of maximum basement dip. As a consequence, areas of greatest crustal subsidence and the position of synorogenic depocenters will lie cratonward of regions with highest basin taper. Since the basement of most sedimentary basins is convex upward, the initial stages of tectonism should produce larger and fewer thrust sheets and result in high rates of thrust advance. As the thrust belt propagates cratonward into segments of the basin with progressively lower basement dip, the rate of thrust belt advance diminishes and the magnitude of internal shortening increases. Lateral variations of taper within sedimentary basins should likewise affect variation in structural styles and rates of thrust belt advance in the strike dimension. Higher tapered portions of the basin initially exhibit less internal shortening and more rapid thrust advance. Lateral and cross-strike variations in basement dip produce variations in the rate of thrust belt advance and temporal changes in the geometry of the advancing thrust belt. Such structural variations, observed within Rocky Mountain thrust belts, are compatible with predictions of the critical-wedge model.