Matthew H. Silver, Whitman College; Andrew S. Gendaszek, Carleton College
Wrinkle ridges are linear or sub-linear positive landforms above compressional structures (Golombek et al., 1991; Schultz, 2000). On Mars, they are typically found on ridged plains, which are inferred to consist of strong material such as basalt (Watters, 1991). Currently orbiting Mars is the Mars Global Surveyor (MGS) spacecraft which contains, among other instruments, the Mars Orbiter Laser Altimeter (MOLA) (e.g., Zuber et al., 1992; Smith et al, 1998) and the Mars Orbiter Camera (MOC) (e.g., Malin et al. 1992, 1998). The MGS spacecraft is in a near-polar orbit (Albee et al., 1998) and thus MOLA data are in north-south tracks that trend sub-parallel to the ridges. Despite the similarity in trend of MGS tracks and the wrinkle ridges in our study area, the topographic data provided by MOLA have a much higher resolution than any other data previously available.
Important geometric parameters of wrinkle ridges include elevation offset of topography on either side of the ridge, width, height and asymmetry (Golombek et al., 2000). Characteristic features include a broad lobate front and sinuous wrinkles. The broad lobate front can extend for kilometers with a trend sub-parallel to that of the ridge. The topography of these landforms is assumed to reflect their internal structure and provide insight into their mechanics of formation, which has previously been interpreted as the result of a combination of faulting and folding (Golombek, 1991; Schultz, 2000; Watters, 1991). Several mechanical models to account for wrinkle ridge surface expression have been proposed. They vary primarily in the depth of fault penetration (e.g., Golombek et al., 2000; Watters, 1991). However, most models agree that wrinkle ridges are the surface expressions of anticlines and that reverse faulting is also somehow involved (e.g., Golombek et al., 2000; Schultz, 2000; Watters, 1991).
In this paper we characterize the morphology of wrinkle ridges contained within a region (Figure 2) of the lowlands east of Elysium Mons and compare our results to the above models. Most ridges in the study area occur to the north of Orcus Patera (Greeley and Guest, 1987) and trend approximately north-south. The study area as a whole is characterized by fluvial channels, lava flows, and knobby terrain.
Compiled profiles indicate that ridges may be wider than previous studies have indicated. Ridges in our study do not appear to be significantly modified by post-deformational processes and thus we can assume that profiles primarily reflect structural deformation. The lack of consistent spacing as well as the presence of elevation offsets point to faulting as the primary deformational mechanism. Ridge surface geometries can be most easily explained by fault-bend folding or fault-propagation folding but, upon closer examination, neither appears feasible. We therefore contend that no dÈcollement is present, which allows faults to penetrate to great depth, as determined by the width of the ridge and an assumed fault dip. Variations in fault dip and interactions between wrinkle ridges and knobby material, at the surface and at depth, are both issues on which future studies may provide further insight. Wrinkle ridges and fissures indicate that this region has been subjected to at least two similar but distinctive stress fields through its history.