Bach Quadrangle - Geologic History

Geologic History

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. Although these divisions have withstood well the assessments of subsequent investigators, they do not define a stratigraphy. Because the geologic map of the Bach region constitutes a synthesis of observation with interpretation, we shall explore several aspects of the region’s geologic development.

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. Because it is the planet nearest the Sun, Mercury represents one extreme in possible cosmochemical models of planet formation. 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. This core, which formed partly as a result of radiogenic heating, produced additional heating, leading to global expansion and the formation of extensional fractures in the lithosphere (Solomon, 1976, 1977). These fractures may have provided egress for the eruption of the oldest plains material during the period of heavy bombardment. Also about this time other structural lineaments developed, possibly as a result of stresses induced by tidal spin-down from a more rapid rotation rate (Burns, 1976; Melosh, 1977; Melosh and Dzurisin, 1978). 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. However, no unambiguous evidence for tensional faults occurs in the Bach quadrangle.

A population of large, very indistinct, degraded craters, (first noted in stereoscopic images by Malin), occurs within the oldest (intercrater) plains material and is thought by most workers to be coeval with or older than that material. The intercrater unit, presumably volcanic extrusions through tensional fractures, is the most voluminous plains material in the map region. Many large c1 and c2 craters have shallow interiors but moderately well preserved rim features, suggesting that at least some of these craters have undergone topographic adjustment due to isostatic phenomena (Schaber and others, 1977). This adjustment may have been facilitated by a high-temperature mantle that was conducive to “crustal plasticity” (Malin and Dzurisin, 1977). The lesser amount of intermediate plains material indicates decreasing plains formation, some localized within older basins.

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 at about the time that c3 craters and smooth plains material were formed. Following core formation, lithospheric cooling and consequent contraction may have closed the conduits, restricting formation of plains material (Solomon, 1977). By c4 time, such formation was greatly reduced.

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. The scarps occurring in the polar regions do appear to be the result of thrust faulting, which substantiates the suggestion that contraction occurred concurrently with spin-down. Linear structures (other than some ridges) are thus interpreted to form as a result of these two active processes. Fracture and lineament patterns around the Caloris basin suggested to Pechmann and Melosh (1979) that Mercury’s despinning period began before global contraction started and ended during the contraction’s early phases.

Plains formation and cratering continued at reduced rates during the early phases of planetary cooling and contraction. c3 craters are distinguishable by partial retention of secondary craters and by locally prominent morphologic features (McCauley and others, 1981). These characteristics suggest a decreasing rate of resurfacing and of crater modification (Malin and Dzurisin, 1977). The smaller extent of the smooth and very smooth plains units, compared with that of older plains materials, suggests considerable heterogeneity of mercurian crustal materials. 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.

The period of tectonic adjustment of the mercurian lithosphere lasted at least through the time of formation of smooth plains material; c4 craters that formed during this period are cut by scarps and are superposed on them. Some very smooth plains material, most of which postdates c4 craters, appears to postdate the scarps that it commonly embays. Superposition relations of scarps in other regions of Mercury indicate that tectonic activity may have continued into c5 time (Leake, 1982).

However, the time of formation of c5 craters and very smooth plains material has, for the most part, been tectonically quiescent. 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. The youngest smooth plains and the very smooth plains materials that occur within c5 craters may be impact melts.