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 (Dmax/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 Dmax/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 Dmax/L ratios.

A Semiautomated Analysis of Displacement‐To‐Length Scaling of the Grabens Affecting Lunar Floor‐Fractured Craters

“…This type of methodology, together with the triangular/elliptical fit used by Atkins et al. (2022), is one of the first attempts to introduce a quantitative classification system for the displacement distributions, and while it constitutes a great contribution in refining displacement‐to‐length analyses, we found that in our case the results are not always reliable. In fact, while Martin and Watters (2022) found a good fit between the classification based on CV and the visual classification of the plots, our data revealed that the trends classified as peaked in some cases showed a constant displacement along the profile, and vice versa, plateaued profile were actually showing distinct peaks (Figure S2 in Supporting Information ).…”

“…Linkage processes have been widely studied in literature (e.g., Atkins et al., 2022; Dawers & Anders, 1995; Kim & Sanderson, 2005; Manighetti et al., 2015; Peacock & Sanderson, 1991; Wilkins & Gross, 2002), and the effect of linkage on D max / L values appears evident both in terrestrial and planetary faults: low D max / L values suggest that a large displacement was not accumulated on segmented faults, while isolated faults tend to exhibit larger displacements and therefore a higher D max / L value. Although Dawers and Anders (1995) propose that faults keep accumulating displacement after the connection of different segments, our results and particularly the widespread low D max / L values suggest that at some point the displacement ceased to develop in depth.…”

“…Fault segmentation can cause a decrease in D max / L ratios: Atkins et al. (2022) and Xu et al. (2016) found that more segmented faults show a lower D max / L ratio, while individual fault segments accumulated more displacement for a given fault length, and this could be due either to an advanced maturity in the fault's evolution or to the fact that such fault grew in isolation and had no chances of linkage;…”

“…The variability of D max / L ratios can depend on multiple factors: Mechanical discontinuities (e.g., due to different materials in contact) can act as barriers for the propagation in depth, while laterally the length of the fault can keep growing, therefore causing a vertical fault restriction (Martin & Watters, 2022; Nicol et al., 1996; Wilkins & Gross, 2002); Fault segmentation can cause a decrease in D max / L ratios: Atkins et al. (2022) and Xu et al. (2016) found that more segmented faults show a lower D max / L ratio, while individual fault segments accumulated more displacement for a given fault length, and this could be due either to an advanced maturity in the fault's evolution or to the fact that such fault grew in isolation and had no chances of linkage; Finally, in case where the fault experiences a reactivation in the opposite sense of slip (and this would be the case if we assume both the inflation first proposed by P. H. Schultz (1976), and the piecemeal caldera collapse suggested by Luzzi et al.…”