The evolution of mesoscopic imbricate thrust faults—an example from the Vermont Foreland, U.S.A.

Abstract An outcrop of a thin (30 cm) bed of micrite surrounded by thicker sequences of well-cleaved, calcareous shale in the Ordovician flysch of western Vermont records a complex history of imbricate faulting and associated folding. The calcareous shale has been shortened by pressure solution. Cross-cutting relations among the floor, ramp and roof faults indicate that faulting progressed from the hinterland (east) to the foreland (west). All the ramp faults developed from arrays of W-climbing enechelon extension fractures. All other faults were controlled by the weaker bedding planes that surround the micrite, and are found at various levels in the shale. Each fault is marked by layers of highly twinned, sparry calcite and black carbonaceous shale selvedge. Simple shear along the floor thrust has rotated the S 1 cleavage toward the foreland and produced, along with volume change, a pressure-solution cleavage in the fault zone. All cleavages have progressively developed from the hinterland to the foreland during folding and faulting of the micrite. Layer-parallel shortening measured in both rock types is between 11 and 16%.