The tectonic architecture of wrinkle ridges on Mars

The geodynamic history of Mars is written largely in the form of its tectonic structures. A global population of compressional structures records a history of interior cooling and contraction, in combination with volcanic loading and flexure of the lithosphere. The most common of these structures are wrinkle ridges, interpreted as having formed by folding of near-surface layered volcanic materials above blind thrust faults. Topographic profiles of wrinkle ridges are found to dominantly fall into three classes: symmetric ridges, arch-style ridges characterized by broad convex upward backlimbs, and double ridges characterized by a broad arch and narrow ridge. A Monte Carlo boundary element model was used to constrain the underlying geometry of faults associated with the different ridge types. Symmetric ridges are pop-up structures formed between quasi-symmetric thrust and backthrust faults, in some cases above a horizontal decollement or low angle fault at depth. Arch-style ridges formed above a dominant primary thrust fault of near constant dip, with a secondary backthrust playing a lesser role in generating subtle inflections in the backlimb. Double ridges require two backthrust faults to localize strain within the narrow ridge at the ridge crest and to generate the secondary antiform on the backlimb. In these ridges, both the upper primary thrust and backthrust are contained within the layered volcanic materials, while the lower portions of the primary thrust can extend deep into the crust or upper mantle. A shallower depth of fault penetration in areas of thicker crust is consistent with the depth of faulting being limited by the brittle-ductile transition. Dips vary widely, but are typically steep in the upper faults and quite gentle in the lower faults. Thus, while wrinkle ridges do provide important records of tectonic and geodynamic processes on Mars as well as other bodies, the associated tectonic structures are more complex and varied than previously considered.