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The ubiquity and significance of extensional grabens on Mercury's lobate scarps
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  • Benjamin Man,
  • David Rothery,
  • Matthew Balme,
  • Susan Conway,
  • Jack Wright
Benjamin Man
The Open University

Corresponding Author:ben.man@open.ac.uk

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David Rothery
The Open University
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Matthew Balme
The Open University
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Susan Conway
Laboratoire de Planétologie et Géodynamique - Université de Nantes
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Jack Wright
The Open University
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Abstract

Mercury’s surface is dominated by tectonic landforms formed by compression. Other than within basins, extensional landforms are not well known and have been presumed to be much rarer, with only a handful reported [1]. To date, two types of extensional grabens associated with lobate scarps have been described in literature: pristine back-scarp grabens associated with small lobate scarps (10s of kms in length and 10s of metres in relief) [2] and crestal grabens found on Calypso Rupes (381km in length and ~1km in relief) [3], [4]. This study identifies that such extensional grabens found on lobate scarps are much more widespread than previously recognised. These form when thrusting produces a hanging wall anticline, and local tensional stresses along the anticlinal axis cause antithetic faults to form in the folded strata, parallel or sub-parallel along the hinge zone, producing a down-dropped fault block. These small-scale features (often less than 1km in width, 10s of kms in length and likely 10s to 100s of metres in depth) are not expected survive 100s of millions of years because of regolith formation and impact gardening masking their signature [1], [2]. Our discovery and documentation of more extensional grabens may indicate that significant movement on many of Mercury’s large lobate scarps persisted until geologically recent times. [1] P. K. Byrne, C. Klimczak, and A. M. C. Sengör, “The Tectonic Character of Mercury,” in Mercury : The View After MESSENGER, 1st Editio., S. C. Solomon, L. R. Nittler, and B. J. Anderson, Eds. Cambridge: Cambridge University Press, 2018, pp. 249–286. [2] T. R. Watters, K. Daud, M. E. Banks, M. M. Selvans, C. R. Chapman, and C. M. Ernst, “Recent tectonic activity on Mercury revealed by small thrust fault scarps,” Nat. Geosci., vol. 9, no. 10, pp. 743–747, 2016. [3] C. Klimczak, P. K. Byrne, A. M. C. Şengör, and S. C. Solomon, “Principles of structural geology on rocky planets,” Can. J. Earth Sci., vol. 56, no. 12, pp. 1437–1457, Dec. 2019. [4] M. E. Banks et al., “Duration of activity on lobate-scarp thrust faults on Mercury,” J. Geophys. Res. E Planets, vol. 120, no. 11, pp. 1751–1762, 2015.