Discovery Quadrangle - Geologic History

Geologic History

Any reconstruction of mercurian geologic history must include the inference that at an early time the planet was differentiated into a core and crust. Mercury has a weak magnetic field coupled with high density. Both facts can most easily be accounted for by the presence of an iron core, possibly liquid, roughly 4,200 km in diameter, overlain by a silicate crust a few hundred kilometers thick. The postulated volcanic origin of a substantial fraction of the Mercurian plains also implies a thick silicate crust, and thereby supports the existence of a large iron core.

Early, rather than late, differentiation of Mercury is attested to by the compressional scarps that are so clearly seen in the Discovery quadrangle. Segregation of the core must have released large amounts of heat, which would have resulted in significant expansion of the crust. However, unambiguous extensional features (very rare on the planet as a whole) are not seen in the Discovery quadrangle; only compressional scarps occur. Thus, core segregation occurred relatively early (before formation of a solid lithosphere) and was followed by cooling and contraction, the last phases of which probably contributed to the formation of arcuate scarps that predated the end of heavy bombardment.

Rotational breaking by solar torques is another process likely to have occurred early in Mercurian history. With the formation of a solid lithosphere, stresses induced by tidal despinning most likely were sufficient to cause widespread fracturing. Melosh has shown analytically that the expected pattern of fracturing includes linear strike-slip faults oriented roughly N. 60° W. and N. 60° E., and a younger set of thrust faults with east-west throw and rough north-south trends. Melosh and Dzurisin have pointed out the similarity between this predicted tectonic pattern and that observed on Mercury, and they have proposed that the global system of lineaments and arcuate scarps, which is well developed in the Discovery quadrangle, formed in response to early, simultaneous planetary contraction and tidal despinning.

The observable stratigraphic record in the Discovery quadrangle starts with formation of the intercrater plains, parts of which may have been coeval with the oldest observable craters. During this period, rates of volcanism were probably high as heat from core formation was being dissipated. If the crust was in a state of extension, there would have been easy pathways for large volumes of magma to reach the surface. The resulting plasticity of the crust probably caused large numbers of c1 and c2 craters to be destroyed by isostatic adjustment, so the present inventory of c1 and c2 craters may not be complete.

By c3 time, the rate of volcanism had declined although the impact rate was still high. The preservation of many secondaries1 to 5 km across around c3 basins indicates that surface flows that would have obliterated them were highly restricted. However, some degradation of c3 basins occurred by isostatic adjustment. Most of the intermediate plains material formed at this time. Smooth plains material appears to be largely coeval with c4 craters and basins. The crust was under compression during c3 and c4 time, inasmuch as the compressional scarps and ridges post-date some c3 and c4 craters, and are cut by some c4 craters and by c5 craters. Formation of intermediate and smooth plains materials may have been abetted by the c3 and c4 crater- and basin-forming events that opened up temporary magma conduits. One of the latest large impacts was the Caloris event, which occurred on the other side of the planet from the Discovery quadrangle and which may have initiated formation of the hilly and lineated material within it.

Subsequent to formation of the smooth plains material, the Discovery quadrangle underwent minor tectonic adjustments that formed scarps on plains within craters. The very smooth plains unit was formed in some young craters. The only other activity was a steady rain of relatively small impacts, apparently at about the same rate as on the Moon.