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.