Matthew Silver, Class of 2002

Geology Department, Whitman College, Walla Walla, WA 99362

Faculty Sponsor: Pat Spencer, Whitman College

Andrew Gendaszek
Geology Department, Carleton College, Northfield, MN 55057

Faculty Sponsor: Clint Cowan, Carleton College

Introduction

Wrinkle ridges are linear to sub-linear positive landforms produced by compression (Golombek et al., 1991; Schultz, 2000). They are common on several planetary bodies, including Mars, Earth, Venus, the Moon, and Mercury (see discussion in Schultz, 2000). On Mars they are typically found on ridged plains, which are inferred to consist of strong material such as basalt (Watters, 1991). The topography of these landforms is thought to reflect their internal structure and can thus provide insight into the mechanics of formation, which has previously been interpreted as the result of a combination of faulting and folding (Golombek et al, 1991). Important geometric parameters of wrinkle ridges include width, height and an elevation offset of topography on either side of the ridge (Figure 1) (Golombek et al., 2000). Elevation offsets are created by motion on dip-slip faults.

        Several mechanical models to account for wrinkle ridges have been proposed; all include faulting but vary in the depth of penetration. One model proposes deep fault penetration (and thus requires the absence of a dÈcollement) and does not require systematic spacing of ridges (Golombek et al., 1989; Golombek et al., 1991; Golombek et al., 2000). A contrasting model predicts that faulting takes place above a dÈcollement and is the result of processes such as gravity creep (Watters, 1991) or some form of lithospheric compression (e.g., Suppe and Connors, 1992). Parallel, uniformly spaced ridges can be indicative of either scenario (Zuber and Aist, 1990), but if they occur on a regional slope, gravity creep is considered to be the more likely mechanism.

        Currently orbiting Mars is the Mars Global Surveyor (MGS) spacecraft which contains, among other instruments, the Mars Orbiter Laser Altimeter (MOLA) (e.g. Smith et al., 1998). The MOLA provides much more accurate topographic data for Mars than any previously available from photoclinometry (e.g., Golombek et al., 1991) or Earth-based radar.  MOLA data have a footprint spacing of ~330 m, a footprint size of ~130 m, and a vertical precision of ~30 cm (Zuber et al., 1992; Smith et al., 1999). In this study we use MOLA topographic data to characterize the morphology of north-south trending wrinkle ridges present in the lowlands north of Orcus Patera, and then compare these geometric data to the topography predicted for each type of fault model.

Conclusions

        Our data show that wrinkle ridges north of Orcus Patera are wide features with gentle topography. Due to the absence of a regional slope, we find gravity creep to be an unlikely mechanism for the formation of these ridges. Other possible mechanisms include fault-propagation and fault-bend folding: a strain comparison between faulting and folding shows that the thrust faults must break the surface, thus ruling out fault propagation folding, while the presence of elevation offsets as well as decreases in the width and height along strike lead us to believe that the ridges are not produced by fault-bend folding. We thus see deep-penetrating faults as the most plausible mechanism to account for observed wrinkle ridge morphology.