A Morphometric Investigation of Large-Scale Crustal Shortening on Mars

Abstract

Mars' surface exhibits abundant topographic expressions of large thrust fault-related folds that have been attributed to global planetary contraction. Morphometric analyses of such structures provide insight into their growth history. With global THEMIS imagery and HRSC-MOLA topographic data, 49 thrusts with lengths between 35 and 544 km were mapped across Mars' surface. Assuming planar fault geometries with dips of 30°, the average maximum displacement-length ratio (D_max/L) of these structures is 6.1 × 10⁻³ ± 1.4 × 10⁻³, with smaller ratios observed for faults within the northern lowlands (2.9 × 10⁻³ ± 0.9 × 10⁻³) compared to the southern highlands (9.2 × 10⁻³ ± 1.9 × 10⁻³). However, these differences may be accounted for if mechanical layering in the northern lowland crust promotes either a shallowing of the fault dip angle relative to the southern highlands or the development of ramp-flat geometries such that the topographic scarp height may under-estimate the total fault displacement or a combination of these two scenarios together. Alternatively, these D_max/L patterns may reflect hemispheric differences in the brittle-ductile transition (BDT) depth; however, the observed pattern is stratigraphically inconsistent with the Martian crustal dichotomy, whereby the northern lowlands have thinner (or denser) crust and therefore presumably a deeper BDT than the southern highlands. Fault displacement-length profiles are commonly asymmetric, with multiple local minima observed along their lengths. Spectral analysis of these profiles, using Fourier- and S-Transforms, indicates power at a range of spatial frequencies, reflecting complex growth and linkage histories, with peak spectral frequency, or number of segments, being negatively correlated with the D_max/L ratios.