Mushwads (Ductile Duplexes) in Thrust Belts: Thermal and Structural Constraints

A mushwad is a distinctive style of thrust-fault-related fold, in which tectonic thickening of the weak-layer rocks uplifts and distorts the geometry of the stiff-layer roof, and tectonic shortening greatly exceeds that of other styles of thrusting. Shortening of the stifflayer is detached trom that of the weak layer and may be accommodated by some combination of excess translation over the foreland and brittle thrusting of the roof above the weak-layer mushwad. Where the stiff-layer mushwad roof is eroded, discrimination of the alternatives is not dear. Translation over the foreland creates excess cover, which predictably produces a thennal anomaly that will remain in an eroded footv/all. The proposed research will test the relation of mushwads to the extent ofthennal anomalies in the foreland footv/all as a measure of the magnitude of thrust translation for comparison with that detennined ITomconstruction of balanced structural cross sections. The areal domain of mushwads is bounded abruptly by other structural styles, and the proposed research will further consider the critical components for mushwad generation and boundary conditions \\'hich appalDent]yinclude thickness of weak-layer rocks, abrupt stratigraphic gradients at mushwad boundari,es, and basement faults in the footwall. The uplifted, fractured stiff-layer roofofa subsurface mushwad offers significant potential as a petroleum reservoir. Because ofllie importance of mushwads in the context of both structural geometry of thrust belt