Bach quadrangle
These combinations provided excellent qualitative control of topographic relief and a good quantitative photogrammetric base.
Its surface consists of craters of a wide variety of sizes and morphologies, as well as plains units, fault scarps, and ridges.
It includes three double-ring basins that range from 140 to 200 km in diameter: Bach (after which the region is named), Cervantes, and Bernini.
Another large crater, Pushkin, is 240 km in diameter and occurs at the map boundary at latitude 65° S., longitude 25° .
[4] These attributes of the mercurian crater population allow stratigraphic sequences to be constructed over large regions.
Therefore, all of the craters within the Bach region are probably the result of impact by meteorites, small planetesimals, and possibly comets.
The two proposed origins for this plains unit, as volcanic or basin-ejecta material, cannot be unambiguously resolved by geologic relations in the Bach region.
However, a volcanic origin is favored because of (1) the widespread distribution of the plains material throughout the imaged regions of Mercury, (2) the apparent lack of source basins large enough to supply such great amounts of impact melt, and (3) the restricted ballistic range of ejecta on Mercury.
Smooth plains material embays the ejecta blanket of a c3 crater on Pushkin's rim at lat 66° S, long 28° (FDS 27402) and fills the interior and part of the outer-ring area of Bach.
The ridges appear to be of volcano-tectonic origin; the fracturing may have provided the means by which lavas reached the surface to form these younger plains units.
The map region displays a wide variety of structural features, including lineaments associated with ridges, scarps, and polygonal crater walls.
Three types are seen in the map region: (1) very small (<50 km long, ~100 m high), irregular scarps that commonly enclose topographically depressed areas; they are restricted to the intermediate and smooth plains units in the eastern part of the map region; (2) small (~100 km long, ~100 m high), arcuate or sinuous scarps, also confined primarily to the intermediate and smooth plains units in the eastern part of the map region; and (3) large (>100 km long, ~1 km high), broadly arcuate but locally irregular or sinuous scarps whose faces are somewhat steeper.
Murray and others (1975) proposed that Mercury's history could be divided into five periods: (1) accretion and differentiation, (2) "terminal heavy bombardment," (3) formation of the Caloris basin (centered off map sheet at lat 30° N., long 195° ; U.S. Geological Survey, 1979), (4) filling of the large basins by "smooth plains," and (5) a period of light impact cratering.
The history of the region begins prior to the formation of any presently visible surface, when Mercury's internal evolution played a key role in determining subsequent landform development.
Even before the Mariner 10 mission, Mercury's high density and photometric properties suggested a large core, presumably iron, and a lithosphere of silicate materials.
Evidence for an intrinsic dipolar magnetic field (Ness and others, 1974) reinforces interpretations favoring a large core.
The major east-west lineament trend in this polar region (noted in previous section) conforms to a prediction of Melosh (1977) for the orientation of normal faults.
The intercrater unit, presumably volcanic extrusions through tensional fractures, is the most voluminous plains material in the map region.
Scarps such as Vostok Rupes (in the Discovery quadrangle adjacent to the north) are apparently the expression of thrust faults; they suggest that planetary contraction may have stressed the lithosphere[7] at about the time that c3 craters and smooth plains material were formed.
Theoretical studies by Melosh (1977), based on observations recorded by Dzurisin (1978), suggested that tidal spin-down combined with core or lithospheric contraction could explain many of the tectonic features of Mercury.
Plains formation and cratering continued at reduced rates during the early phases of planetary cooling and contraction.
Subcrustal zones of tension may have allowed molten materials to reach the surface through fractures beneath craters, even during the period of global contraction (Solomon, 1977).
Ridges of domical cross section cut some c4 craters and, at places, flank areas of young, very smooth plains material.
Thus, possible volcanic extrusions associated with tectonic activity may have continued into the period of formation of c4 craters and the oldest very smooth plains material.
Superposition relations of scarps in other regions of Mercury indicate that tectonic activity may have continued into c5 time (Leake, 1982).
During this period, with the exception of a scattering of extremely fresh craters and some minor mass wasting (Malin and Dzurisin, 1977), almost no geologic activity has occurred near the mercurian south pole.
