As we don't have any direct measuring tools to measure the effects of tectonics (e.g. Mercury-quakes) and not have observed volcanism in the short while that we have been properly observing the planet - we can only surmise what tectonics have and are occurring by observing the surface features.
To understand the past tectonic features and dynamics is a means to understanding the present.
But, an important distinction, it appears, according to Plate tectonics and planetary habitability: current status
and future challenges (Korenaga, 2012), that plate tectonics is a feature unique to Earth.
According to the chapter Tectonics of Mercury, the mostly compressional tectonic features
A combination of tidal despinning and thermal contraction may
account for equatorial N–S and polar E–W trending lobate scarp thrust faults
in the regions by reactivation of normal faults.
and that the
Local preferred orientations of the lobate scarps and uniform thrust slip dip
directions suggest regional-scale stresses influenced the formation of the thrust
faults. The wrinkle ridges in smooth plains outside of the Caloris basin are
likely due to loading and subsidence of volcanic material that flooded lowland
Recent observations discussed in Thermal evolution of Mercury as constrained by MESSENGER observations (Michel et al. 2013), determined that
that the planet’s core is larger than previously estimated. As Mercury’s
mantle layer is also thinner than previously thought, this result gives greater likelihood to the
possibility that mantle convection is marginally supercritical or even that the mantle is not
Using this data in simulations, they
demonstrate that mantle convection can persist in such a thin mantle for a substantial
portion of Mercury's history, and often to the present, as long as the mantle is thicker than
~300 km. We also find that magma generation in Mercury’s convecting mantle is capable of
producing widespread magmas by large-degree partial melting
MESSENGER also observed large, compressional fold structures and further systems of thrust faults.
So, to summarise, it appears that the tectonic activity on Mercury has been and is from a number of dynamic sources:
- Tidal de-spinning
- Contraction as the core cools - reactivating older faults borne from past impacts and earlier tectonic activity.
- Conversely, possibly due to some mantle convection that might exist, hot spot activity.