The Mercury Catalog of Shortening Structures (MerCatSS): Constraining the Ages of Shortening Landforms on Mercury

Introduction:  The temporal trends of the geographic, morphometric, and structural parameters of shortening structures on Mercury are key to better understanding the planet’s history of contraction, tidal despinning, and lithologic/rheologic variation in its current crust [1-4]. Employing the best resolution image mosaics (166 m/px), as well as a novel, global stereo-DTM with three times the resolution of currently available global DTMs [5,6], we are producing a new global map and parameter catalog of shortening structures on Mercury [20].

To assess the temporal trends of scarp parameters and to better understand global/regional stress, we will determine both relative and absolute ages for the shortening structures. Our database will enable us to discern any spatial as well as temporal trends within any of our gathered values. Here, we present our derived ages for the shortening structures on Mercury,

Data and Methods:  The gathering of relative and absolute ages is carried out on high- and low-incidence angle, ~166 m/pixel mosaics of images by MESSENGER’s Mercury Dual Imaging System [5]. A relative age bracket will be assigned to all our mapped shortening structures that intersect at least one impact crater (or ejecta) with a diameter >1 km. These structures lend themselves to a stratigraphic classification via the degradation state of the superposing/superposed crater(s)/ejecta [7 22] and assigning a chronographic system as defined by [8] (pre-Tolstojan, Tolstojan, Calorian, Mansurian, Kuiperian). This technique has been previously applied to ~400 large scarps (> 100 km in length) [4] and ~6000 scarps in the northern smooth plains [2].

As degradation states of craters and erosional processes have been suggested to regionally vary on Mercury [9], we will further narrow down the stratigraphy-based age brackets for the shortening structures with absolute model ages (AMAs) [10]. These AMAs will be derived via crater size-frequency distribution (CSFD) measurements [10] on the floors and/or ejecta blankets of sufficiently large craters. Recently used by [11] on five mercurian thrust systems, we will employ the method on a subset of scarps where CSFDs on the floor of a crater cross-cut by the fault would derive a maximum age limit, while CSFDs on the floor of a superposing, unfaulted crater or on the ejecta superposing the fault scarp would give us a minimum age limit. We will use the production model by [12] to determine AMAs, but will also offer AMAs derived with the Neukum production model [13] for comparison. For the Le Feuvre and Wieczorek functions [12], both non-porous and porous scaling laws for target materials will be considered [14]. This will incorporate the effects of a porous megaregolith and non-porous hard rock targets. Reporting AMAs using each production and chronology function allows the best representative age to be reported.

Figure 1: Simple cylindrical view of the H-11 Discovery quadrangle of Mercury with 166 m/pix MDIS high-incidence angle mosaic. Line work shows those shortening landforms we have mapped that lend themselves to relative age dating. Tolstojan (red), Calorian (orange), and Mansurian (yellow) relative ages have been given to shortening structures in the H-11 quadrangle.

Although another method to date linear landforms, buffered crater counting (BCC), has been used on Mercury for very large thrust systems [11, 15-17], the technique requires a sufficient crater population that is superposed on the linear feature [18]. For scarps that are not part of Mercury’s largest thrust systems, there might not be enough or no craters to determine a robust AMA, and therefore, the BCC method will not be used in this global study. Utilizing a stratigraphic model [4], with traditional CSFD measurements in large craters crosscut by the faults [11, 15] will permit comparative ages to be determined, where possible.

Initial Observations:  Based on the subset of shortening structures in the H-11 Discovery and H-7 Beethoven quadrangles (n = 239), the majority have a Calorian relative age (52%), with Mansurian being the next largest age subset (39%). Approximately 8% of the structures were labeled as Tolstojan, and only two landforms were labeled as Kuiperian. Those structures that are concentric around the Andal-Coleridge ancient basin in the H-11 quad are mostly Mansurian and Calorian in age.

CSFD measurements were used for a crater cross-cut by a shortening structure in H-11 and produced ages ranging from ~220 Ma to ~3.8 Ga, depending on which production and chronology function was used. We find that the non-porous PF and CF of [12] produce significantly younger ages. The porous function of [12] and [13] produces similar ages. Preliminary AMAs point towards shortening stucture formation starting in early Calorian.

References: [1] P. K. Byrne et al. Nat. Geo. 1 (2014). [2] K. T. Crane & C. Klimczak, Icarus 317, 66 (2019). [3] M. E. Banks et al. JGR. Planets 122, 1010 (2017). [4] M. E. Banks et al. JGR Planets 120, 1751 (2015). [5] S. E. Hawkins et al. SSR, 131, 247-338 (2007) [6] F. Preusker et al., Planet. Space Sci. 142, 26 (2017). [7 22] Trask, N.J. (1975) Proc. Int. Colloq. Planet. Geol., 15, 471-476. [8] Spudis and Guest, in Mercury, pp.118-164 (1988). [9] M. J. Kinczyk et al., In Mercury – Innermost Planet, vol. 2018, 6123. [10] G. Neukum, , Habil. Thesis, Univ. Munich (1983). [11] L. Giacomini et al., Geosci. Frontiers, 18, 15187 (2020). [12] Le Feuvre, M. & Wieczorek, M.A., Icarus, 214, 1-20 (2001). [13] Neukum, G., Ivanov, B.A., & Hartmann, W.K. (2001). SSR, 96 (1-4), 55-86. [14] K. A. Holsapple and K. R. Housen, Icarus, 187, 345-356 (2007). [15] L. Giacomini et al., In Geol. Soc. London, 401, 291-331 (2015). [16] V. Galluzzi et al., LPSC (2016). [17] V. Galluzzi et al., JGR: Planets, 124 (2019). [18] van der Bogert et al., Icarus, 306, 225-242 (2018). [19] K. T. Crane & C. Klimczak, GRL, 44, 3082-3089 (2017). [20] Bernhardt et al., LPSC, Abstract 1406 (2025).