THE MERCURY CATALOG OF SHORTENING STRUCTURES (MerCatSS): THE MOST COMPLETE AND ACCURATE TECTONIC MAP OF MERCURY

Introduction: The global distribution of shortening structures on Mercury as well as geographic and temporal trends of their morphometric and structural parameters are key to learn more about the planets’ history of contraction, tidal despinning, and lithologic/rheologic variations in its current crust [1–6]. Mapping efforts carried out over the past decade by [4,5,7,8] had access only to limited topographic data [9–12] which prevented comprehensive morphometric analyses other than displacement-length (D/L) assessments of only the largest scarps. Furthermore, as recently elaborated on by [8], the association of shortening structures with larger tectonic complexes needs to be assessed to derive a more realistic inventory of actually implied faults. We have produced a new global map and morphometric parameter catalog of shortening structures (i.e., lobate scarps, high-relief ridges, wrinkle ridges) on Mercury (“MerCatSS” – Mercury Catalog of Shortening Structures) using the final MDIS mosaics (166 m/px) in combination with novel stereo-imaging-derived DTMs (222 m/px) that we completed for the entire globe based on preliminary earlier work [10–12] (Fig. 1). The goal of MerCatSS is to provide the community with an agnostic, yet comprehensive dataset for a multitude of further analyses and to serve as vantage point for refinements enabled by future data by Bepi-Colombo.

Data & methodology: We established a thorough step-by-step procedure [13] to map and parametrize each identified shortening structure (including individual front and back scarps of wrinkle ridges). To ensure consistency, polyline digitization is carried out with a fixed vertex spacing of 2 km. Parameters that have been measured or derived for every single fault scarp include geodetic length, maximum relief, D/L (derived from relief), maximum width and width at point of maximum relief, assignment to a scarp complex/cluster if possible, vergence, lobateness, whether they are concentric to an ancient basin, or if they are (partially) within a crater.

Select results: MerCatSS is the most detailed and comprehensive map to date of tectonic shortening structures on Mercury excluding the northern plains (which have already been thoroughly mapped by [1]). We identified 6,873 fault scarps (1,462 of which are uncertain), 705 ridges bound by antithetic fault scarps (including wrinkle ridges on smooth plains), and 471 tentative, undifferentiated high-relief ridges of potential tectonic origin but not associated with any fault scarps. Approximately 57% of our mapped fault scarps had not been recognized by [4,14] (does not include partially recognized structures). No longitudinal or latitudinal trends in fault scarp density or average length are apparent although both parameters are geographically very heterogeneous (Fig, 1). While average scarp length shows no clear geographic trends, scarp density (outside smooth plains) is significantly higher along curvilinear bands around the Caloris basin and the ancient basins “b56”, “b57”, and “b90” as mapped by [15] (Fig. 1). This might indicate areas of structurally weakened crust by specific basin-forming events, potentially a function of basin size and age.

Across the globe, over a fifth of fault scarps outside of smooth plains are concentric (quasi-parallel and within one basin radius to its outline) to large, ancient basins as mapped by [15], e.g., the Andal-Coleridge, Sobkou, or Vincent-Yakolev basins. This indicates that global scarp orientation patterns need to be re-examined as they have been significantly affected by basin-related crustal structures. In general, MerCatSS confirms that N-S orientations dominate along the equator, which is indicative of tidal spindown [5,14,16,17].

Release: MerCatSS is available for download at https://www.geol.umd.edu/~hbernhar/.

 

Figure 1: Robinson projection of MerCatSS, the most detailed and comprehensive tectonic map of Mercury to date. Here, a color-coded density map of all 6,873 mapped fault scarps (black lines with sawteeth) is superposed on two heatmaps: One green-to-red of the fault scarp, i.e., line, density, and one grey-to-white of the average fault scarp length within 300 km-sized bins. Grey areas are smooth plains (which include the northern plains, i.e., Vastitas Borealis) as mapped by [18] which have been cut out in this figure. Wrinkle ridges and undifferentiated ridges mapped as part of MerCatSS are also not shown. White circles show ancient impact basins as mapped by [15].

 

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