For the smallest of the planets, Mercury is a surprisingly active place with a complex internal structure and magnetic field, however, it’s tectonics is dominated by its size.
Smaller bodies lose heat a lot faster than bigger bodies, due to a higher ratio of surface area to volume, this means that Mercury has cooled relatively rapidly. As the planet cooled it has shrunk. Estimates for this contraction of its radius range from ~2 – 7 km over its lifetime, whilst this is only 0.2% of its total radius, this equates to its diameter shrinking 44 km at its equator.
Mercury’s surface is a single tectonic plate, it doesn’t have the subduction and rifting zones which can accommodate strain. As the planet has cooled and contracted the crust at the surface has become compressed as the larger diameter crust is pulled in to fit into a smaller area. Rocks when under compression buckle and eventually break and form faults. In the case of Mercury, shallow angle faults known as thrust faults are created. As one part of the crust rides over another it forms cliffs (called “Rupes”) which can be 100’s of km long and several hundred meters high.
Where multiple faults interact they can produce complex structures. The Great Valley is 1000 km long valley formed from thrust faults which make up either side and has been bent downwards in the middle.
Smaller scale scarps have also been identified, whilst smaller (10’s of km long and 10’s of meters high) they are often found cutting across small impact craters and smooth younger planes which means that these features are less than 50 million years old and suggests that Mercury is still tectonically active now.
Whilst there are other features such as some structures on its surface which are linked to thicker patches of crust probably linked to mantle structures Mercury is dominated by the side effects of being a small cooling planet which is still slowly shrinking.