Structural Controls on Volcanic Eruptions: Insights from the Copland-Rachmaninoff Tectonic Regime on Mercury

Tectonic activity from global contraction and its influence on the location of volcanic eruptions (i.e. faculae) continues to elicit diverse interpretations, with the underlying structural controls of many faculae still poorly constrained [1;2]. At the boundary of the Rachmaninoff basin area and the northern smooth plains lies a large 800 km long, 100km wide, and 1000-3500 km high elevation structure which terminates at the 200 km diameter Copland crater. 400 km north of this boundary is a parallel structure of similar dimensions, implying a shared formation mechanism. Topographic profiles perpendicular to these structures reveal that both have asymmetric positive relief indicative of thrust fault scarps (i.e. lobate scarps and rupes), with a steep sloping forelimb followed by a more gently sloping backlimb. Although these structures are generally taller and wider than even the largest thrust fault scarps on Mercury (e.g. Enterprise Rupes with <2500 m of relief), we present evidence that these structures contain a significant amount of shortening and may be unidentified thrust faults which strike east and dip to the south. Specifically, they outline the rims of relic craters (b50 and b72, [3]), meaning that crustal shortening utilized preexisting crater wall bounding normal faults. This shortening is identified from the deformation induced on Copland crater where its southern rim is elevated 1,250 m respect to its northern rim. Mapped faults in the area have noted smaller lobate scarps in the area, and one which passes through the center of Copland and offsets its floor by 400 m [4], however this is dwarfed by the deformation caused by the deflected large thrust which has uplifted the southern rim of Copland crater. Furthermore, the presence of volcanic eruptions (Neidr and Nathair Faculae, [5]) along the southern edge of the scarp, the hanging wall, is typical of thrust fault activity on Earth [6]. The parallel trend shared with the long-wavelength topography (broad troughs and crests, [7]) may also indicate a shared formation mechanism. Revelations from the BepiColombo mission, particularly the updated high-resolution topography, will facilitate more interpretation of the local tectonic regimes on Mercury and may reveal many undetected shortening structures and faculae, and in turn a full appreciation of their geospatial relationships can be achieved.

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Acknowledgements: We gratefully acknowledge funding from the Italian Space Agency (ASI) under ASI-INAF agreement 2024-18-HH.0.