Post contributed by Valentina Galluzzi, INAF, Istituto di Astrofisica e Planetologia Spaziali (IAPS), Rome, Italy
Any celestial body that possesses a rigid crust, be it made of rock (e.g. terrestrial planets, asteroids) or ice (e.g. icy satellites), is subject to both endogenic and exogenic forces that cause the deformation of crustal materials. As a result of the mass movement, the brittle layers often break and slide along “planes” commonly known as faults. In particular, tensional, compressional and shear forces form normal, reverse and strike-slip faults, respectively. On Earth, plate tectonics is the main source of these stresses, being a balanced process that causes the lithospheric plates to diverge, converge and slide with respect to each other. On Mercury, there are no plates and therefore the tectonics work differently. Instead its surface is dominated by widespread lobate scarps, which are the surface expression of contractional thrust faults (i.e. reverse faults whose dip angle is less than 45°) and this small planet is in a state of global contraction.
Mercury’s interior is cooling down, thus causing a shortening of the planet’s lithosphere [Strom et al., 1975]. Image 1 shows the Endeavour Rupes – Antoniadi Dorsum area on Mercury, characterized by several lobate scarp segments that cross each other, sometimes dipping in opposite directions and defining topographic bulges. This clustering of lobate scarps is encompassed in a 1500 km long N-S array that starts with Victoria Rupes (53.2°N; 34.5°W) and ends near Geddes crater (27.1°N; 29.7°W). Byrne et al.  mapped almost 6000 contractional features on Mercury and based on their length, and a dip angle in the range of 25°-35°, calculated a radius change (i.e. shortening) of up to 7 km. Planetary fault dip angles are usually assessed by means of mechanical models using the available elevation data [e.g. Watters & Nimmo, 2010]. However, the high crater and fault density on Mercury allows us also to assess the geometry and kinematics of faults by using faulted craters as kinematic indicators [Galluzzi et al., 2015], as shown in Image 2. With respect to the numerical fits, this method has the advantage of showing the natural variability inherent in fault geometry with the aid of local scale direct measurements [see also Massironi et al., 2015].The age of lobate scarps on Mercury can be assessed by means of 1) the buffered crater counting technique and/or 2) cross-cutting relationships with dated geological units or craters. The first method consists of counting un-deformed craters superposing the faults (thus younger than the faults themselves) and fitting the crater size-frequency distribution with specific production functions to assess an absolute age [e.g. Marchi et al., 2013]. Results obtained with this method date lobate scarps as active 3.7-3.5 Ga ago [e.g. Giacomini et al., 2015]. In contrast, the second method takes into consideration the relative age of the faults with respect to the cut/superposed units or craters. As shown in Image 3, many fresh and young craters are cross-cut by small lobate scarps, and based on this evidence, several works conclude that the contractional deformation of Mercury extends up to the present day [Banks et al., 2015; Watters et al., 2016]. Watters et al.  also found evidence that some small lobate scarps in the northern latitudes of Mercury might be younger than 50 Myr, arguing that there might be still an active coseismic slip. Although an ultimate answer concerning “mercuryquakes” can only come from direct measurements by seismometers, these new insights into Mercury’s tectonics from the MESSENGER mission will be further advanced once the ESA/JAXA BepiColombo mission arrives providing further high-resolution images. Further reading
Banks, M.E., Xiao, Z., Watters, T.R., Strom, R.G., Braden, S.E., Chapman, C.R., Solomon, S.C., Klimczak, C. & Byrne, P.K. (2015), Duration of activity on lobate-scarp thrust faults on Mercury, Journal of Geophysical Research: Planets, 120, 1751–1762. doi:10.1002/2015JE004828
Byrne, P.K., Klimczak, C., Şengör, A.C., Solomon, S.C., Watters, T.R. & Hauck, S.A. (2014), Mercury’s global contraction much greater than earlier estimates, Nature Geoscience, 7 (4), 301–307. doi: 10.1038/ngeo2097
Galluzzi, V., Di Achille, G., Ferranti, L., Popa, C. & Palumbo, P. (2015), Faulted craters as indicators for thrust motions on Mercury, Geological Society, London, Special Publications, 401, 313–325. doi: 10.1144/SP401.17
Giacomini, L., Massironi, M., Marchi, S., Fassett, C. I., Di Achille, G. & Cremonese, G. (2015), Age dating of an extensive thrust system on Mercury: implications for the planet’s thermal evolution, Geological Society, London, Special Publications, 401, 291–311. doi: 10.1144/SP401.21
Marchi, S., Chapman, C.R., Fassett, C.I., Head, J.W., Bottke, W. F. & Strom, R.G. (2013), Global resurfacing of Mercury 4.0–4.1 billion years ago by heavy bombardment and volcanism, Nature, 499, 59–61. doi: 10.1038/nature12280
Massironi, M., Di Achille, G., Rothery, D. A., Galluzzi, V., Giacomini, L., Ferrari, S., Zusi, M., Cremonese, G. & Palumbo, P. (2015), Lateral ramps and strike-slip kinematics on Mercury. Geological Society, London, Special Publications, 401, 269–290. doi: 10.1144/SP401.16
Preusker, F., Oberst, J., Head, J.W., Watters, T.R., Robinson, M.S., Zuber, M.T. & Solomon, S.C. (2011), Stereo topographic models of Mercury after three MESSENGER flybys, Planetary and Space Science, 59 (15), 1910–1917. doi: 10.1016/j.pss.2011.07.005
Strom, R.G., Trask, N.J. & Guest, J.E. (1975), Tectonism and volcanism on Mercury, Journal of Geophysical Research, 80 (17), 2478–2507. doi: 10.1029/JB080i017p02478
Watters, T.R. & Nimmo, F. (2010), The tectonics of Mercury, in: Watters, T.R. & Schultz, R.A. (eds), Planetary tectonics, Cambridge University Press, ISBN: 9780521749923.
Watters, T.R., Daud, K., Banks, M.E., Selvans, M.M., Chapman, C.R. & Ernst, C.M. (2016), Recent tectonic activity on Mercury revealed by small thrust fault scarps, Nature Geoscience, 9, 743–747. doi:1 0.1038/ngeo2814