Lunar lineaments and meteorite impact

Abstract

Through the use of terrestrial analogies with regard to the formation of craters, new prominence has been given to the theory that lunar craters are possibly the result of volcanism rather than of random meteorite impacts. The paper examines the validity of using terrestrial parallels and suggests that an impact origin neither precludes volcanic activity nor is incompatible with what is known of lunar surface features. At first sight the apparently random distribution and predominantly circular shape of most lunar craters favours an origin through meteorite impact (Dietz 1946). This is supported by resemblances between lunar craters and those terrestrial features which are generally considered to have been formed by meteorites. The hypothesis that global lunar volcanism is responsible for the cratering has lately regained some prominence, however. Regionally oriented intersecting sets of lineaments are now a well-documented feature of the lunar surface (Fielder, 1963). The relatively close and remarkably even spacing of such lineaments, which have been shown in places to be faults, has led to the reasonable conclusion that the whole lunar surface is under uniform stress, probably tensional (Warner and Fielder, 1962). This eliminates one possible argument against lunar volcanism, namely that because the craters are randomly distributed and not confined to well-defined belts as on Earth, they cannot be of volcanic origin; for if similar geotectonic conditions obtain over the whole globe, there is no reason why related volcanic activity should not be global also. Although the systematic differences of appearance between lunar craters and terrestrial craters of volcanic origin have been used to favour the impact hypothesis, they do not of themselves exclude lunar volcanism. Possibly the reverse in fact, for it has been argued that volcanic activity in vacuo and a low gravitational field would not necessarily give rise to landscapes of terrestrial aspect (McCall, 1966). More important for some, however, are the similarities between lunar and terrestrial craters (Green, 1959; McCall, 1966), which have often been cited as evidence that the lunar structures resulted from caldera collapse or cauldron subsidence. Especial weight is given to integration of lunar crater structures with visible trends in the lunar tectonic environment; features such as associated graben, step faulting, and alignment of crater walls with regional lineaments have been exhaustively documented (McCall, 1966). Following the terrestrial analogy further has led to the conclusion that lunar fracture systems must have controlled the siting of craters, which cannot therefore have resulted from random meteorite impacts. It is here suggested that the terrestrial analogy has been taken too far in the wrong direction and that the strong structural control of lunar surface features is not incompatible with an impact origin. If, as seems likely the moon's outer layer is a uniformly stretched skin tending to fracture along well-defined lines, it clearly resembles on a global scale those terrestrial volcanic belts associated with regional uplift in both orogenic (Healy, 1964; Wyllie, 1965) and non-orogenic (McCall, 1966) regions. If the forces associated with terrestrial volcanism can initiate fracturing along such lines of weakness, the (explosive) impact of large meteorites on a surface under tension can hardly fail to do likewise, despite possible cushioning effects by fragmental material (Jaffe et al., 1966; Urey, 1966). Just as the aftermath of a major volcanic eruption involves extensive movement along prevailing fault systems, so readjustment of the lunar surface following a major impact would be controlled by regional lineament trends. The latter, in short, merely control the physiographic evolution of impact craters and only appear to have influenced their positioning. (Radial fracture systems centred on some lunar craters and depressions (Fielder, 1962) can be reasonably presumed to have been locally superimposed on the regional pattern.) Two possible terrestrial parallels can be cited, but there are almost certainly others: The Nördlinger Ries crater is probably a meteorite impact crater (Stöffler, 1966), but clearly shows some integration with regional structures, which has been cited as evidence in favour of a cryptovolcanic origin (McCall, 1964). The same applies to Arizona's Meteor Crater (Hager, 1953) which in addition is transected by a graben parallel to the regional fault trend. Some other annular structures have been less conclusively identified as astroblemes (Dietz, 1961, 1964), although on evidence quite as adequate as that used to identify primary volcanic structures on the moon. There is no need to invoke large-scale volcanism on what may well be a fairly cold and inactive moon (Dietz, 1946), but volcanic activity following impact fusion is not precluded. On the contrary, the formation of some crater chains, both circumferential and linear, and of some crater floor features, is possibly best explained in this way (Fielder, 1962; McCall, 1966).