Young tectonic activity on Mercury

Mercury has been contracting thermally for at least the past 3 billion years. Kilometres-high 'lobate scarps' mark where ancient crustal thrust faults approach the surface. Detailed study by my PhD student Ben Man has now found abundant evidence that many of these structures are still moving.
Published in Astronomy
Young tectonic activity on Mercury

In my experience it seldom disappoints if you task a PhD student with making a geological map of part of a relatively little studied planet such as Mercury,  and encourage them to develop some lines of scientific inquiry along the way. I assigned Ben Man to map Mercury's H-13 (Neruda) quadrangle, one of 15 that cover the entire globe, using data from NASA's MESSENGER orbiter (2011-2015). 

Ben's 1:3M scale map was published a few weeks ago, as part of a pan-European effort to complete the preliminary geological mapping of the globe, and thus set the context for studies to be made by ESA and JAXA's BepiColombo mission that will start producing science from orbit about Mercury early in 2026.

While mapping, Ben's attention was caught by several "lobate scarps" that cross his quadrangle. These are a well-known class of feature on Mercury, being ramps in the terrain, kilometres high and hundreds of kilometres long, where a thrust fault in the crust approaches the surface. Think of them as the wrinkles that form on an apple as it ages and you won't be far wrong, except that an apple shrinks because it is drying out whereas Mercury shrinks because of thermal contraction in its interior.

Mercury began to contract at least 3 billion years ago, and the rate of contraction has probably been decreasing for most of the time since then. Models and observation agree that Mercury's radius has decreased by at least 7 km, hence the thrusting within its crust to produce the lobate scarps.

Ben became interested in lobate scarps because several hitherto unnamed examples cross his quadrangle.  He traced these into adjacent quadrangles and noticed that some of them have small fractures (grabens) piggy-backing on their stretched upper surfaces.  Far from evidencing an episode of expansion, such grabens are a predictable geometric effect where a thrust slice of crust is forced over the step made by the edge of the over-ridden tract of crust. Although the number of superposed impact craters tells us that lobate scarps themselves are billions of years old, these grabens, which are less than 1 km wide and less than about 100 m deep, must be much younger otherwise they would have been erased from view by the processes of regolith generation and impact gardening that toss crater ejecta (at all scales down to the microscopic) across the surface.

Based on the likely rates of infilling, we can calculate that the majority of these small grabens are less than about 300 million years old. Because they were formed in response to motion on the major thrust fault beneath each lobate scarp, this tells us that some movement on those thrusts must have happened equally recently, even if motion began billions of years ago.

MESSENGER image of part of Alvin Rupes
Small grabens manifested as cracks along the crest of the stretched terrain above the lobate scarp named Alvin Rupes. West at top,  field of view about 20 km across.

Previously work by Maria Banks and colleagues had shown that the thrust faults responsible for 14 large lobate scarp thrusts must have moved recently, because those examples cut across crisp-looking <3 km craters which (in order to have been preserved in such a state) must be younger than about 300 million years. Ben realised that extensional grabens atop lobate scarps offered a way to look for recent tectonic activity that did not depend on the rare coincidence of a young crater's position happening to coincide with the location of a fault. He therefore made a global survey of all the high resolution MESSENGER images that intersect known lobate scarps, looking for extensional grabens similar to those that he had first noticed.

Ben found that that 48 large lobate scarps definitely have small extensional  grabens near their crests, and another 244 large lobate scarps are topped by 'probable' grabens. The latter are examples that are seen too indistinctly to be certain on the best MESSENGER images, but that are now prime targets for confirmation by BepiColombo's imaging system.  The map reproduced here shows extensional grabens concentrated at high northern latitudes, probably just a simple consequence of MESSENGER's orbit allowing better imaging there, and also surrounding the Caloris basin (straddling the E and W edges of the map from the equator to 60° N) where basin formation may have thinned and weakened the crust so that it remains particularly susceptible to continued fault movement.

Map of Mercury, showing that young extensional grabens are globally distributed.
Global map of Mercury (Robinson projection) showing shortening structures (lobate scarps) and identifying those that are 'definitely' (triangles) and 'probably' (circles) topped by small, young, extensional grabens.

The low point in BepiColombo's orbit will always be fairly close the equator, allowing equally good imaging of northern and southern hemispheres, unlike MESSENGER where the low point was at northerly latitudes throughout the mission.  As well as likely enabling us to reclassify many of the 'probable' grabens as 'definites',  BepiColombo's orbit will allow it to show whether the current north-south imbalance in both 'definite' and 'probable' grabens is real or just a result of latitude-dependent image resolution.

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