Thrust fault mechanics and dynamics during a developmental stage of a foreland belt

Abstract

Studies of continental seismogenic zones indicate that deformation during an orogenic stage is episodic and spasmodic down to mid-crustal depths, and that seismic as well as aseismic modes of deformation need to be incorporated in quantitative theories of foreland belts. Such a theory is developed here. It includes a method to help decipher the development of structures such as imbricate fans, frontal ramps and ramp anticlines, as illustrated for the Hogsback frontal ramp and its ramp anticline in the Kemmerer region of the Wyoming Salient. The structure of the Hogsback ramp is consistent with a shear-fracture origin. Limit equilibrium mechanics indicate that shear failure would have proceeded incrementally, possibly in response to recurring fault slip on the basal décollement. Contrasts in elastic constants and rock strength are critical, e.g. current data indicate that shear-failure development of the ramp would have begun (1) fourth-fifth's distance down the ramp in competent Madison carbonates if they compared in strength with the Marianna limestone, or (2) higher in the section if the Madison compared in strength with the Hasmark or Blair dolostones. Whether the initial development of the ramp anticline involved seismic modes is not clear yet, but elastic stiffness data indicate that subsequent imbrication of the anticline did involve seismic as well as aseismic modes of deformation.