The contribution of major impact processes to lunar crustal evolution

Abstract

Large terrestrial impact craters provide structural models for the interpretation, of lunar craters and basins and petrological data for comparison with complex lunar breccias. The terrestrial examples illustrate the mixing processes operative in the production of impact melts and breccias and the considerable volumes of impact melt rocks in large impact craters. Three forms of craters are found on Earth, Moon, Mars and Mercury; Simple bowl-shaped craters (smallest), central uplift, and ring structures (largest). The variations in form are interpreted as different degrees of gravitational modification which immediately follow the initial excavation. On Earth the transient cavity excavated has diameter/depth ( D/d) near 3/1 in non-porous, essentially homogeneous rocks. From Orbiter and Apollo photographic data transient cavities in the most competent lunar materials have D/d of approximately 4/1. Experiments indicate ejecta come from as deep as two-thirds rds the transient cavity depth: disturbed material from greater depths remains as the crater lining. Application to the Imbrium basin indicates a portion of the ejecta at the Apollo 15 site may have come from as deep as 100 km, and from somewhat shallower depths at the Apollo 14 site. Ultramafic green and howarditic glasses are possible candidates for these materials of deep origin. Uplift of upper mantle in the centre of the basm would contribute to the Imbrian mascon.

Basin-ring spacing on the Moon, Mercury, and Mars

“…Corresponding raw ratios are approximately either 1.4 or 2.0. Ratios including rings ranked III and V, as for the least-squares groups described in Section 5.1, have a larger increment, x Impact-driven wave mechanism at penetration 01 excavation stage; Dominantly surficial Boon and Albritton, 1936McCauley, 1968Baldwin, 1963Hartmann and Wood, 1971Van Dorn, 1968Head, 1974, 1977Chadderton et al, 1969Howard et al, 1974Murray, 1980Mackin, 1969Pike, 1981a Grieve et al, 1981Croft, 1981b (1) Terrestrial geologic evidence for structure associated with rings.…”

“…(2) Does not explain basin outer rings slump feature surrounding an initially smaller excavated crater (Table XI: Group A), was one of the earliest ideas to be advanced for the origin of basin rings on the Moon (Mackin, 1969;Gault, 1974;Head, 1974;Dence, 1976Dence, , 1977McCauley, 1977;Melosh and McKinnon, 1978;and Croft, 1981b). The structural hypothesis is based in part on purportedly analogous rim failure evident in smaller lunar and terrestrial complex craters (Dence, 1964;Quaide et al, 1965), although it has yet to be shown that such modest slumping can be scaled directly to basin-sized events.…”

“…The basin workers above all agreed on a megaterrace origin for the outer ring(s) of basins, although not necessarily for all the rings shown here. Gault (1974) and Dence (1976Dence ( , 1977 envisioned this mechanism forming all outer rings around a small, proportional-growth transient cavity. Those authors who equated the intermediate ring (usually our rank III) with the transient basin cavity suggested a dual mechanism whereby the outer ring was produced by megaterracing, but the inner rings formed by rebound, analogous to the central peaks of lunar craters (Head, 1974;McCauley, 1977;Scott et al, 1978).…”