Topographic Expressions of Large Thrust Faults on Mars

Abstract

On planets with little erosion, thrust faults produce broad, asymmetric, positive-relief, linear to arcuate ridges -often referred to as lobate scarps- that remain largely unaltered, such that their topographic expressions are a measure of the structural uplift caused by the displacement and associated country-rock deformation of the faults. Here we map and systematically assess the structural relief of 24 thrust faults across Mars to infer their growth behavior. Our mapping indicates that the majority of individual thrust faults have simple, linear map traces with lengths of up to ~450 km, but that some thrust faults form systems of up to 1400 km in length. For the most topographically pronounced landforms, the structural relief developed above the fault is as great as ~3400 m. We then relate topographic measurements to the displacement on the underlying fault planes to study the displacement variations along the fault length. We find a variety of displacement distribution shapes of the fault systems, which we attribute to differences in fault growth that include unrestricted and restricted growth, linkage, and/or fault interaction. Finally, we relate the maximum displacements ( ) determined for each of the faults to their respective fault length (L) to establish a maximum displacement-to-length relationship. The observed scaling characteristics and order-of-magnitude scatter of our data are not uncommon for fault populations on Earth and tie in well with the map patterns, tectonic geomorphology, and systematic along-strike displacement distributions to have grown in a basement-block faulting style found in intra-plate tectonic settings on Earth.

Principles of structural geology on rocky planets

“…Many such structures show smaller, subordinate scarps along their leading edges (Fig. 5B) that have previously been described using generic terms (Watters 1988) as second-and third-order ridges, but Klimczak et al (2018) applied geologic nomenclature to describe them as minor thrust faults and fault splays.…”

“…A classification scheme for shortening structures based only on morphology, and with no interpretation tied to it, cannot capture the broad morphological variations of these landforms and is thus of little help in understanding the tectonic style and geometry of associated faults and folds. Instead, careful structural mapping and descriptions and compilation of observations should be followed by an interpretation that provides testable hypotheses for the shortening landforms in the local, regional, or global geologic context of their planet (e.g., Klimczak et al 2018). Techniques as simple as the Holmesian method (Holmes 1876) of drawing profiles across shortening landforms (Fig.…”

“…(As an aside, the term "contraction" is used here only for an overall decrease in the volume of a planetary body, whereas the resulting structures, thrust faults and folds, are termed "shortening structures", referring to a decrease in surface length in one horizontal dimension.) Many different patterns of thrust faults and associated folds exist on these rocky bodies (e.g., Byrne et al 2014Byrne et al , 2015Klimczak et al 2018; Crane and Klimczak 2019), but frequently individual features have been described with generic terms, without context to other structures, or without taking terrestrial analogues into consideration. For example, interactions of thrust faults and associated folds with obstacles in the lithosphere or with each other, such as mechanical contrasts between geologic units that can function as detachment horizons, give rise to complex tectonic systems, including syntaxes, virgations, and linking (Fig.…”

“…Although not explicitly called out as such in a previous publication (Klimczak et al 2018), we consider Icaria Rupes on Mars as a candidate double virgation with free ends (see fig. 6 from Klimczak et al (2018)).…”

Lithospheric Contraction on Mars: A 3D Model of the Amenthes Thrust Fault System

“…The Amenthes Rupes lobate scarp, as well as most of the lobate scarps modeled on Mars, has been modeled through 2D cross sections analysis (e.g., Egea-González et al, 2017;Grott et al, 2007;Herrero-Gil et al, 2019;Mueller et al, 2014;Ruiz et al, 2008;Schultz & Watters, 2001). This 2D modeling applied to lobate scarps restricts the results obtained to the chosen cross sections of structures that are hundreds of kilometers long and present significant variations along their length (e.g., Herrero-Gil et al, 2020;Klimczak et al, 2018). Besides, Amenthes Rupes is not formed by an isolated fault.…”