Lunar and Planetary Science XXXI 1280.pdf

PYROCLASTIC ERUPTIONS ASSOCIATED WITH THE FLOOR-FRACTURED LUNAR FARSIDE CRATER OPPENHEIMER IN THE SOUTH POLE AITKEN BASIN. J. W. Head III1, L. Wilson1,2, and C. M. Pieters1; 1Department of Geological Sciences, Brown University, Providence, RI 02912, 2Environ. Sci. Dept., Lancaster Univ., Lancaster LA1 4YQ, UK ([email protected]).

Abstract: Pyroclastic deposits of Imbrian age are potential rise, have encountered the transition from relatively associated with floor-fracturing in the farside crater coherent country rock to the breccia lens underlying the floor Oppenheimer in the South Pole-Aitken basin and are unit of an impact crater. The change in wall rock strength and consistent with sill emplacement, floor uplift, and vulcanian in the density contrast between the magma and the style eruptions. surroundings is interpreted to have led to conditions in which lateral intrusion of magma to form a sill at some depth below Introduction: Pyroclastic volcanic activity is a significant the crater floor is preferred to continued vertical rise. The style of extrusive activity in lunar history [e.g., 1] and occurs elastic bending stresses exerted on the crater floor materials at in a variety of different styles and settings [2,3,4,5]. One style the margin of a sill-like intrusion would eventually lead to is that associated with large floor-fractured craters [6], such as failure, forming graben in the locations observed in Alphonsus, in which numerous vents are associated with Oppenheimer. We assume that the magma intrudes at the base concentric fracture systems on the uplifted and deformed of the breccia lens and supports the entire column of breccia crater floor [7]. Here we report on similar vents associated forming the crater floor unit. In reality, intrusion would lead to with floor fracturing in the crater Oppenheimer, located on the flexure of the upper part of the floor unit and eventual failure lunar farside in the South Pole-Aitken basin. We map and of rocks at the edge of the thickening sill so that the load characterize the distribution and association of vents, model would be accommodated progressively, not abruptly. the magma emplacement, and assess the significance of this We have determined the excess magma driving pressures type of occurrence on the lunar farside. needed to produce such features. No excess pressure is needed Geologic setting and deposit description: The Nectarian- to drive magma to within ~4 km of the nearside surface and ~6 aged crater Oppenheimer (205 km diameter, 35.4S, 166.0W) km of the farside surface. Shallower magma penetration does is located just west of the Apollo basin and just north of the require magma overpressure, but the levels required to Imbrian-aged, mare-filled crater Maksuto [8]. Its floor is produce floor-fractured crater morphologies by shallow composed of Imbrian-Nectarian plains and it contains three intrusion (up to 16-23 MPa depending on crustal thickness) smaller Imbrian-aged craters on the floor and rim [8]. are less extreme than those required to produce surface Clementine image data reveal the presence of distinctive floor eruptions at the same locations (21-28 MPa); they are also fracturing associated with the crater floor margins (Fig. 1) in a extremely similar to those found to be necessary to cause manner reminiscent of the nearside crater Alphonsus [6,7]. eruptions partially filling the floors of craters and basins when The rilles are typically less than a km wide, show linear no account was taken of the presence of brecciated zones [1]. segments that are 30-50 km in length and parallel the crater This treatment assumes that intrusions begin at the base of the walls and rims, and often branch to form parallel segments. brecciated zone in an impact crater and the resulting laccolith Small craters are preferentially located along the rilles, often raises all of the brecciated unit. The exact level at which sill forming a bead-like appearance. Located within Oppenheimer formation might be preferred to continued upward dike are seven major occurrences of dark mantle deposits (low propagation beneath the floor of such a crater is no easier to albedo material that obscures but does not flood underlying predict than in the case of sill injection into sedimentary layers topography). The deposits are generally equidimensional, on Earth. We infer that a number of scenarios are plausible in rather than linear, show a range of sizes (<10 to >40 km), are which only part of the breccia unit is elevated (and the excess widely distributed, are very closely associated with the rille magma pressure require is only slightly different from the system, and are often very closely associated with craters less values deduced above). These scenarios correspond to a range than about 3 km in diameter. In detail, the deposits are of values for the depth of the uppermost part of the magma sometimes seen to occur along a linear part of the rille intrusion below the surface, its cooling rate, its physical associated with several small rille-related craters. evolution, and the nature of its subsequent interaction with the Interpretation and modelling: These characteristics are overlying rocks. In particular, the stress field around the edge very similar to rille systems and associated dark mantling and of this magma body is likely to encourage the formation of dark halo craters seen along the floor fractures of the 118 km secondary intrusions penetrating to shallower depths within diameter crater Alphonsus, a floor-fractured crater interpreted the breccia unit. These secondary dikes may cause local to be the site of a sill-like intrusion [6], and to have involved shallow extensional deformation of the crater floor leading to the formation of vulcanian eruptions to emplace the dark halo the peripheral fractures and narrow graben commonly seen in crater deposits [7]. We have modeled the ascent and floor-fractured craters [6]. Because of the low absolute emplacement of sill-forming bodies to assess whether this pressure in the melt near the tips of such dikes they would be model is potentially applicable to the observations in the optimum locations for gas generation and release (e.g., to Oppenheimer. Floor-fractured craters are attractive candidates form collapse craters or vulcanian eruptions [7]) in exactly the for being locations at which upward-propagating dikes have same way described earlier for primary dikes penetrating to failed to penetrate to the surface but, before ceasing their within ~3 km of the surface [9,10]. Lunar and Planetary Science XXXI 1280.pdf

Summary and conclusions: The farside crater Oppenheimer is a distinctive and typical floor fractured crater References: [1] J. Head and L. Wilson, G & CA, 56, 2155, and contains pyroclastic deposits of Imbrian age. A model of 1992; [2] C. Coombs et al., PLPSC 20, 161, 1990; L. Gaddis sill emplacement below the crater floor is consistent with the et al., Icarus, 61, 461, 1985; [3] B. Hawke et al., PLPSC 19, 255, 1989; [4] C. Weitz et al., JGR, 103, 22725, 1998; [5] C. floor fractures, their locations, and the correlation of dark halo Weitz and J. Head, JGR, 104, 18933, 1999; [6] P. Schultz, craters due to shallow gas production and vulcanian style Moon, 15, 241, 1976; [7] J. Head and L. Wilson, PLPSC 10, eruptions from the emplaced magma for many of the dark halo 2861, 1979; [8] D. Stuart-Alexander, USGS Map I-1047, occurrences. The proximity of the Oppenheimer deposits to 1978; [9] J. Head and L. Wilson, LPS 27, 519, 1996; [10] J. other lava-flooded craters [11] and associated mare-like Head and L. Wilson, PSS, 41, 719, 1993; [11] A. Yingst and J. deposits [12] in this region indicates that volcanism is an Head, JGR, 104, 18957, 1999; [12] C. Pieters et al., LPS 31, important process on the lunar farside in the South Pole- 2000. Aitken basin.

Figure 1. Clementine image mosaic of Oppenheimer crater and sketch map showing location of the major features and dark mantle deposits.