Global catalog of thrust-fault geometries on Mercury

Introduction
On a global scale, thrust-faults kinematics and geometry remain debated and under-constrained. This is especially important because constraints on the basal geometry of Mercury's thrust faults, in particular their dip angles, directly control estimates of global radial contraction. Previous work based on Andersonian fault theory assumes moderate to high dip angles (25° to 40°), yielding contraction estimates ranging from ~1.3 to 2 km (Watters et al., 2021) to up to 7 km (Byrne et al., 2014), depending on which structures are included. If a significant fraction of Mercury's lobate scarps are low-angle thrust faults (dips below 25°–30°), total shortening and inferred global contraction would be substantially larger. Moreover, current models assume pure dip-slip kinematics, while mixed kinematic behavior has been observed regionally (Galluzzi et al., 2015, 2019; Massironi et al., 2015). In this context, this project focuses on building a global catalog of thrust-fault geometries on Mercury using the morphologic method developed by Galluzzi et al. (2015, 2019), combined with Mercury's global crater catalog (La Grassa et al., 2025) and existing tectonic feature catalogs (e.g., Man et al., 2023; Byrne et al., 2014). A key component is an automatic detection method to identify craters intersected and deformed by thrust faults, which serve as passive strain markers. Using the highest-resolution DTMs available, we will compute fault geometry and kinematic parameters including dip, strike, plunge, horizontal and vertical dislocation, and slip trend.

Objectives
Our global catalog will provide a powerful tool to (i) quantify the amount of global contraction based on direct geometric measurements; (ii) constrain spatial variations in crustal mechanical properties by correlating fault geometry, such as faulting depth, with different surface terrains; (iii) produce regional and global strain estimates; and (iv) assess the global distribution of  fault kinematics. The results of this project will serve as a baseline for the upcoming BepiColombo SIMBIO-SYS data, whose higher-resolution products will enable direct refinement and expansion of the catalog.

Methods and Analytical approach
The main dataset consists of the global crater catalog by La Grassa et al. (2025), published tectonic feature catalogs (Byrne et al., 2014; Man et al., 2023; Bernhardt et al., 2025) the available 222 m/pixel DTM (Preusker et al., 2017) and independently made higher-resolution DTMs, and the MESSENGER's MDIS Global Basemap BDR at 166 m/pixel. These datasets will be evaluated for consistency with the topography since fault traces can occasionally be spatially offset relative to the used surface imagery. The automatic detection pipeline implements the method of Galluzzi et al. (2015, 2019), which assumes that craters were originally circular in plan view and rigidly deformed solely by the fault, such that the offset between circles fitted to the footwall and hanging-wall rim arcs reflects the true vertical slip component. The pipeline proceeds through five main steps: (1) geometric spatial intersection of crater and fault trace catalogs using GIS and Python; (2) candidate selection based on rim circularity, fault crossing geometry, and elevation profile quality; (3) extraction of hanging-wall (HW) and foot-wall (FW) rim elevation profiles from the DTM and flagging of craters exceeding a minimum detectable offset; (4) independent least-squares circle fitting to each rim arc, with rejection of craters whose fitted radii are mutually inconsistent; and (5) derivation of fault geometry and kinematic parameters from the displacement between circle centers, following Galluzzi et al. (2015).

Figure 1. Flowchart of the automatic detection pipeline. Craters from the global catalog are progressively filtered by fault intersection geometry, suitability criteria, and minimum offset threshold, before undergoing circle fitting and fault parameter calculation following Galluzzi et al. (2019).

Expected results
The global catalog of thrust-fault geometries will provide the first observationally constrained, planet-wide dataset of fault dip angles, heave, throw, and slip trends on Mercury. This will enable more robust estimates of global radial contraction, possibly reducing the uncertainty on assumed dip angles that currently drives the large spread in published estimates (~1-7 km). Spatially resolved fault geometries, integrated with geological maps and crustal thickness models, will allow us to assess regional variations in crustal mechanical behavior and constrain faulting depth across compositionally distinct terrains. Combined strain estimates at regional and global scales will shed light on whether shortening is uniformly distributed or concentrated within specific tectonic provinces. Finally, systematic classification of fault kinematics will reveal the global distribution of dip-slip versus oblique-slip behavior, providing a new observational basis for models of Mercury's interior and thermal evolution.

Acknowledgments
This research is funded from the Italian Space Agency (ASI) under ASI-INAF agreement 2024-18-HH.0.

References
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