Analysis of tectonic structures on the Martian surface: a contribution to the understanding of the crustal dichotomy
Authors: Balbi Evandro , Cianfarra Paola , Ferretti Gabriele , Crispini Laura , Tosi Silvano 
Published: July 2021

The Martian crustal dichotomy is the main physiographic feature at planetary scale on Mars and marks the topographic and geological boundary between the northern Lowlands and the southern Highlands. It can be easily followed over the entire surface of Mars except for the Tharsis volcanic region that is presumably superimposed. Concerning the formation of the crustal dichotomy there is not a widely accepted model from the scientific community and at present three main hypotheses have been proposed: i) endogenous/geodynamic origin driven by mantle convection (Wise et al., 1979; Sleep, 1994; Wenzel et al., 2004); ii) exogenous origin, related to a giant impact (Wilhelms &amp; Squyres, 1984; Andrews-Hanna et al., 2008; Nimmo et al., 2008) or multiple large impacts (Frey &amp; Schultz, 1988, 1990); iii) a combination of them (Yin, 2012). None of these hypotheses can totally exclude the others and so the process or processes that led to the development of this planetary structure are still matter of debate. We aim at better understanding the Martian crustal dichotomy especially in an historical moment when the current paradigm of Mars as a tectonically dead planet is weakened by the new important seismic data from the NASA&#8217;s InSight mission (Banerdt et al., 2020; Dahmen et al., 2020;Giardini et al., 2020; van Driel et al., 2021).In order to give a contribution to this open question, here we present a study of the morphotectonic structures at regional and global scale outcropping over the surface of Mars between 60&#176;N and 60&#176;S. We map and analyze tectonic structures with lithospheric relevance at global and regional scale (length exceeding one order of magnitude the average crustal thickness (Neumann et al., 2004, L &#8805; 450 km) using low spatial resolution satellite images from multiple datasets. This allows us to restrict the mapping only to the linear-to-curvilinear morphotectonic elements that are detectable on satellite image mosaic representing both the topography (e.g. Viking dataset and Mars Orbiter Laser Altimeter (MOLA) DEM with multiplelighting conditions and spatial resolution of 200mpp) and alignments of thermo-physical properties of the outcropping lithologies (e.g. night and day thermal infrared - Thermal Emission Imaging System (THEMIS) dataset with spatial resolution of 100mpp). Image processing, including luminosity contrast stretching and convolution filtering for edge enhancement, supported the photo-geologic interpretation for the manual mapping of theidentified morphotectonic structures. Image processing is performed with the software Envi 5.6 and the mapping is conducted in GIS environment (software used: QGIS 3.16_Hannover). The analysis of multiple satellite dataset strengthens the reliability of the mapped structures by reducing the possible bias of the operator subjectivity.The mapped structures are statistically analyzed with the freeware Daisy3 software (http://host.uniroma3.it/progetti/fralab/) in order to explore their clustering in azimuthal family sets. Following Wise et al. (1985) we refer to these azimuthal families as swarm. Each swarm is described in terms of spatial distribution, mean length of the tectonic structures, spacing, sinuosity and self-similar clustering. These properties allow to describe the geological properties of the region where they develop (e.g. spatial variation of the elastic lithospheric thickness). In this contest, the main geological relevance is given to the swarm and not to the single structure, in fact the inclusion, omission, or distortion of few structures will not affect the geologic/tectonic/geodynamic properties and meaning of a swarm.We have started our work, that is still in progress, from the Cerberus Fossae where we are mapping extensional morpho-tectonic structures with en-echelon patterns. They exceed 1000-1200 km of length and are aligned along a NW-SE trend representing a single swarm. These structures are described in literature and are generating a lot of interest in the scientific community for the possibly associated recent volcanic, tectonic and seismic activity (Berman &amp; Hartmann, 2002; Plescia, 2003; Giardini et al., 2020; Horvath et al., 2021 ). Similar morphotectonic structures outcrop approximately at the same longitude but more to the south, at the boundary between Cimmeria and Sirenum Terrae (see also Tanaka et al., 2014). They affect the Highlands for thousands of kilometers reaching Daedalia Planum, Tharsis and Thaumasia Highlands. From the mapping these structures seem similar compared to the previous cited one in Cerberus Fossae with extensional features and en-echelon patterns that lead us to think that strike-slips movements and so the horizontal component over the surface of Mars may have played an important role in the geologic history of the planet. The azimuthal trend of this swarm varies from about W-E in Sirenum Terra to SW- NE approximately in the Daedalia Planum, roughly parallel to the alignment of Arsia, Pavonis and Ascraeus volcanos in the Tharsis rise. Along the Thaumasia Highlands, in particular, we identify three swarms with different azimuthal trends. Some authors ascribe the structures in this region to a possible rift-like system (Dohm &amp; Tanaka, 1998 Hauber &amp; Kronberg, 2005; Grott et al., 2007; Nahm &amp; Schulz, 2010). We will further map and analyze these structures and swarm(s) to highlight their mutual relationships also to frame them into an evolutionary model. If the tectonic origin of the mapped swarms will be confirmed this will be fundamental to investigate the tectonic styles (stagnant lid, fragmented lid, a contribution of both?) that affected or have been affecting Mars since the formation of the lithosphere-asthenosphere system. Furthermore, this will also allow to further constrain the role of tectonic processes on rocky or icy bodies with rigid outer shell. A better comprehension of their formation and evolution can in addition provide important clues for similar rocky exoplanets.