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MORPHOLOGY AND SUBSURFACE STRUCTURE OF WRINKLE RIDGES ON MARS

Introduction: Wrinkle ridges are compressional tectonic features that are produced through a combination of folding and faulting in the subsurface [1]. These features exhibit variability in ridge height, length, and morphology. On Mars, wrinkle ridges are common throughout the volcanic surfaces, particularly within the tectonically active region of Tharsis. They are observed to be tens to hundreds of kilometers long, with typical widths of ten kilometers [2]. There are at least four distinct morphological types of ridges: symmetric, asymmetric ramp, arch, and double ridges [3] with arches being less common. Many wrinkle ridges also demonstrate an offset between the elevation of the surrounding surface beyond the forelimb and backlimb, interpreted as evidence for a lithosphere-scale thrust fault [2]. Alternatively, other researchers interpret wrinkle ridges as thin-skinned structures confined to the volcanic plains [1]. However, the depths to which the faults propagate, and the dips associated with these faults remain unknown. Previous studies have applied elastic dislocation models to constrain the depth and geometry of faults in the subsurface [1,3-4]. The aim of this study is to first examine the variability of wrinkle ridge morphology within individual ridges, and then apply a simpler geometric approach to constrain the geometry of faults. We find that ridge morphology can vary significantly within a single ridge. Most ridges demonstrate a topographic step and have profiles consistent with a steeping of the fault dip at shallow depths. Methods: Using Mars Orbiter Laser Altimeter (MOLA) topography data, the morphology of a set of ridges in Solis and Lunae Plana was analyzed through hundreds of profiles produced perpendicular to the ridges. The profiles for a given ridge were then grouped into clusters of similar morphology using a k-means classification algorithm, which iteratively adjusts the clusters based on the RMS difference between each profile and the average profile of the clusters. By assigning three or four distinct groupings, the variability in the morphology of individual ridges becomes clear. The average profiles of the clusters can be classified according to common morphological types as symmetric, asymmetric ramp, and double ridge (Fig. 1). Double ridges are classified as having asymmetric forelimb and backlimb, with a secondary ridge (also described as a broad arch and narrow ridge, or primary and secondary antiform) [1,3-5]. Fig. 1. Three common classifications of wrinkle ridge morphology. Symmetric (a), asymmetric ramp (b), double ridge (c).