Abstract | Mercury is a shrinking planet with a surface dominated by compressional tectonic landforms as first identified by Mariner 10 and then confirmed by
MESSENGER. Extensional structures are present but are much rarer, with almost
all reported examples found exclusively in smooth plains material within craters
[1]. The only two reported examples of extensional structures outside of the
aforementioned setting are extensional grabens associated with lobate scarps;
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 $\sim 1$km in relief) [3,4]. These grabens form when
thrusting produces a hanging wall anticline resulting in local tensional stresses
along the anticlinal axis parallel or sub-parallel to the hinge zone. Here, tensional
stresses cause antithetic faults to form, which results in a narrow down-dropped
fault block.
We find that extensional grabens on compressional structures are much more
common than previously recognised. These small-scale landforms (often less
than 1km in width, 10s of km in length and likely 10s to 100s metres in depth)
are not expected to survive 100s of millions of years due to continual regolith
formation and impact gardening masking their signature [1,2]. Our discovery and
documentation of potentially young extensional grabens may indicate that many
of Mercury’s compressional structures have continued to move until geologically |
