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

A mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies, regional topographic offsets suggestiv e of subsurface thrust faults, and folding of near-surface layers. This mod el provides explicit relationships between observed morphologic elements ch aracteristic of wrinkle ridges and plausible mechanisms in the subsurface, a key component that is absent in previous qualitative fault-based scenario s 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 nucleat ion and growth of echelon an arrays of backthrust faults, Calculations of d isplacements (both horizontal shortening and vertical uplift) and Coulomb s tress change related to slip along blind thrust faults in the model demonst rate physically important spatial inhomogeneities in these quantities, with revealing and useful implications. (1) The ratio of shortening due to fold ing at the surface to the shortening due to faulting at depth is characteri stically small for coupled wrinkle ridge-blind thrust fault systems and dec reases with increasing fault depth; the depth of the blind thrust fault's u pper tip thus profoundly influences the surface strains. (2) Folding and up lifted topography, forming the topographic ridge, are produced above the ar ea of the slipping blind thrust fault plane. Horizontal and vertical deform ation at the surface extend over several ridge widths, or a total of at lea st 50 km for a 10-km-wide ridge, implying that topographic profiles and geo logic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation. (3) Calculations of Conlomb stress ch anges suggest that fault slip can localize both bedding plane slip in overl ying strata and new backthrust faults that propagate upward to become wrink les on the trailing side of the ridge. Initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whethe r the resulting surface structure becomes a wrinkle ridge or a lobate scarp . Wrinkle ridge spacing may also be related to stress changes associated wi th slip along the underlying blind thrust fault.