Michelangelo quadrangle
The Michelangelo quadrangle is in the southern hemisphere of the planet Mercury, where the imaged part is heavily cratered terrain that has been strongly influenced by the presence of multiring basins.
At least four such basins, now nearly obliterated, have largely controlled the distribution of plains materials and structural trends in the map area.
The interaction between basins, craters, and plains in this quadrangle provides important clues to geologic processes that have formed the morphology of the mercurian surface.
In addition, twelve stereopairs cover scattered areas in the quadrangle;[3] these photographs were used to supplement the geologic interpretation.
The results obtained are consistent with a qualitative assignment of relative age that is based on position and size of these ancient basins.
Moreover, the trends of scarp segments, interpreted by some workers to be expressions of thrust faults associated with global compression[4] (Dzurisin, 1978), are deflected into basin-concentric patterns at their intersection with basin rings.
This morphology is similar to that of the lunar basin Grimaldi and is suggestive of an extended period of structural rejuvenation along the margins of the inner ring.
In some areas, the intercrater plains material appears to embay c1 craters, and it is found in all of the degraded basins described above.
The material is most likely polygenetic, including both crater and basin debris and possibly ancient volcanic flows.
At least seven basins in or partly in the Michelangelo quadrangle postdate or are contemporaneous with the last stages of deposition of intercrater plains material.
A small unnamed basin at –48°, 136° may also have formed in this time interval, but its age is uncertain due to its partial burial by ejecta from crater Delacroix (–44°, 129°).
We determined a reference crater density for Caloris in the Shakespeare quadrangle in order to correlate basin ages to that stratigraphic datum.
The Beethoven Basin (–20°, 124°), partly exposed in the Michelangelo quadrangle, consists of one ring 660 km in diameter.
Crater deposits are mapped stratigraphically according to a morphologic degradation sequence devised by N. J. Trask (McCauley and others, 1981).
Although these conditions hold generally, degradation may be accelerated locally by adjacent impact events and flooding by plains materials and, rarely, may be decelerated by structural rejuvenation of topographic elements of craters.
The large basins of the Michelangelo quadrangle have been dated relatively by counting the cumulative density of superposed primary impact craters that have diameters greater than 20 km.
This technique has proven to be of great value in dating lunar basins (Wilhelms, in press), where obvious superposition relations do not exist.
Results of these crater counts indicate that Dostoevskij, presumed to be of c3 age (McCauley and others, 1981), is actually one of the oldest basins in the map area (early c1).
Several of the lobate ridges described by Strom[9] follow arcuate patterns along rings of the Barma-Vincente Basin; Hero Rupes is an example.
These lobate ridges appear to be of compressive tectonic origin and, although global in distribution, may be deflected locally by the presence of preexisting, basin-related structure.
These basins presumably formed during the period of heavy bombardment inferred from lunar history (Wilhelms, in press).
This unit has a complex history of deposition; it was reworked in place and probably includes brecciated plutonic rocks and possibly ancient volcanic flows.
Minor tectonic activity continued as scarps and lunar mare-type wrinkle ridges developed within the smooth plains materials.
