Localization of bedding plane slip and backthrust faults above blind thrust faults: Keys to wrinkle ridge structure

Abstract

A mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies, regional topographic offsets suggestive of subsurface thrust faults, and folding of near‐surface layers. This model provides explicit relationships between observed morphologic elements characteristic of wrinkle ridges and plausible mechanisms in the subsurface, a key component that is absent in previous qualitative fault‐based scenarios for these structures. As developed in this paper, wrinkle ridges are the surface expression of anticlines that grow above a blind thrust fault as a result of both flexural slip folding of near‐surface strata and the nucleation and growth of echelon arrays of backthrust faults. Calculations of displacements (both horizontal shortening and vertical uplift) and Coulomb stress change related to slip along blind thrust faults in the model demonstrate physically important spatial inhomogeneities in these quantities, with revealing and useful implications. (1) The ratio of shortening due to folding at the surface to the shortening due to faulting at depth is characteristically small for coupled wrinkle ridge‐blind thrust fault systems and decreases with increasing fault depth; the depth of the blind thrust fault's upper tip thus profoundly influences the surface strains. (2) Folding and uplifted topography, forming the topographic ridge, are produced above the area of the slipping blind thrust fault plane. Horizontal and vertical deformation at the surface extend over several ridge widths, or a total of at least 50 km for a 10‐km‐wide ridge, implying that topographic profiles and geologic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation. (3) Calculations of Coulomb stress changes suggest that fault slip can localize both bedding plane slip in overlying strata and new backthrust faults that propagate upward to become wrinkles on the trailing side of the ridge. Initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whether the resulting surface structure becomes a wrinkle ridge or a lobate scarp. Wrinkle ridge spacing may also be related to stress changes associated with slip along the underlying blind thrust fault.