JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 90, NO. B4, PAGES 3049-3064, MARCH 10, 1985 The Deep Structure of Lunar Basins' Implications for Basin Formation and Modification STEVEN R. BRATT AND SEAN C. SOLOMON Departmentof Earth, Atmospheric,and Planetary Sciences,Massachusetts Institute of Technology,Cambridge JAMES W. HEAD Departmentof GeologicalSciences, Brown University, Providence,Rhode Island CLIFFORD H. THURBER Departmentof Earth and SpaceSciences, State University of New York at Stony Brook We presentmodels for the structure of the crust and upper mantle beneath lunar impact basinsfrom an inversionof gravity and topographic data from the nearsideof the moon. All basin models display a thinner crust and an elevated Moho beneath the central basin region compared to surrounding areas, a signature of the processesof basin excavation and mantle uplift during collapse of the transient cavity. There is a general decreasein the magnitude of apparent uplift of mantle material with increasingbasin age; we attribute this relation primarily to enhanced rates of ductile flow of crustal material early in lunar history when crustal temperatureswere relatively high and the effectiveelastic lithosphere was thin. The more relaxed topographic and Moho relief associatedwith older basinson the central nearsidemay, in particular, be at least partly a consequenceof the extensivesubsurface heating associatedwith the formation of the large Procellarum basin. The deep structure of the youngest basins constrains the geometry of the cavity of excavation and the amount of crustal material ejected beyond the basin rim. From the volumes of the topographic basin, of mare basalt fill, and of uplifted mantle material, the volume of crustal material ejectedbeyond the basin rim for an Orientale-sizedevent was of the order of 107 km3. A near-constantthickness of nonmarecrustal material beneath the centralregions of young basinsof various diametersand preimpact crustal thicknessessuggests that the transient cavity excavated to at least the base of the crust for the largest basins; significant excavation into the mantle may have been impeded by an abrupt increasein strength at the lunar Moho. INTRODUCTION data [Spudis, 1982, 1983; Spudis et al., 1984], but such esti- The formation of multiring impact basins has played a mates depend critically on the accurate identification of pri- major role in the geologicalevolution of the moon. During the mary ejecta and on assumptionsabout chemical layering of first billion years of lunar history the impact of large projec- the lunar crust. In this paper, we apply gravity and topo- tiles onto the lunar surface resulted in the excavation of basin graphic data to infer the three-dimensionalstructure of the cavities hundreds of kilometers in diameter [Wood and Head, crust and upper mantle beneath impact basins on the lunar 1976] and the implantation of large quantitiesof heat into the nearside. With the derived structural models we constrain sev- lunar interior [O'Keefe and Ahrens, 1977]. Impact basins also eral of the important geometrical and physical parameters re- became the focus for volcanic and tectonic activity over a lated to the processesof basin formation and modification, considerabletime period following the basin formation events and we assesstheir variations with basin age and size. [e.g., Head, 1976; Solomonand Head, 1979; Solomon et al., Muller and Sjoqren [1968] were the first to recognize that 1982]. Important constraintson the processesof basin forma- the youngestnearside mare basins are characterizedby posi- tion and modification are provided by the presentvolumes of tive gravity anomalies, which they attributed to "mascons." the topographicbasin, of material ejectedduring basin forma- Since that discovery, a number of efforts have been made to tion, and of mare basalt fill as well as by the degreeof involve- model the gravity anomalies over mascon basins with contri- ment of the mantle in isostatic compensationof basin relief. butions to the anomalous mass placed at the surface IConel The topographic volumes of the basins are reasonably well and Holstrom, 1968; Baldwin, 1968; Booker et al., 1970], at the known. Estimates for the volumes of mare basalt and basin lunar Moho [e.g., Wise and Yates, 1970], or at both locations ejectadeposits have also been obtained from photogeological [e.g., Hulme, 1972; Wood, 1972; Bowin et al., 1975; Sjoqren studies[e.g., Moore et al., 1974; Head et al., 1975; DeHon and and Smith, 1976]. At least some portion of the anomalous Waskom,1976; Head, 1982], but such techniquesare generally mass contributiong to mascon anomalies residesnear the sur- limited in their ability to resolvethe thicknessof the deposits. face [Phillips et al., 1972], but models where mare basalt fill is The depth of excavation of several basin-forming events on the sole source of anomalous mass require an unreasonable the moon has also been estimated from the chemistry and thickness of mare basalt to fit the measured gravity field mineralogy of ejecta deposits inferred from remote sensing [Thurber and Solomon, 1978]. While both mare fill and an elevated Moho likely contribute to the observedgravity, the solution for the distribution of anomalous mass between the Copyright 1985 by the American GeophysicalUnion. two locations given only gravity and topographic data re- Paper number 4B5151. quires additional assumptions. 0148-0227/85/004B- 5151$05.00 Structural models consistentwith gravity and topographic 3049 3050 BRATTET AL.' DEEPSTRUCTURE OF LUNARBASINS NEARSIDE BASINS 30* N - 20* - I0' - O* -- 30' S [AA ,I-"'• • I I IO0*W 80* 60* Fig. 1. Outlines of the lunar basins consideredin this study. The labeled lines indicate the locations of cross sections discussed in text. data have been constructed for a number of individual basins beneath the largest lunar basins,we derive new bounds on the [e.g., Bowin et al., 1975; Sjogrenand Smith, 1976; Phillips and volume of material ejectedfrom each basin, and we evaluate Dvorak, 1981; Janle, 1981a, b]. Most of these models,how- the implicationsof structuraldifferences among basins for the ever, were developedunder dissimilar setsof assumptionsand processesof basin formation and modification as functionsof constraints,thus hinderinga comparisonof the infe..rredstruc- time on the moon. tures beneathdifferent basins.Also, the interpretation of grav- ity data over a singlebasin requiresthat the investigatormake PROCEDURE subjective judgements about regional trends in the gravity To determine the crustal structure in the vicinity of lunar data arising from structures outside the area of interest or basins,we perform a simultaneousinversion of gravity and occurring over wavelengthsgreater than the scaleof the basin. topographyfor the low-latitude portion of the lunar nearside, In contrast, global models for lunar crustal structure [Wood, following a proceduresimilar to that of Thurber and Solomon 1973; Bills and Ferrari, 1977a; Thurber and Solomon, 1978] [1978]. The moon is divided into a grid of blocks, each calculatedunder uniform setsof constraintsand assumptions 5øx 5ø in horizontal extent (Figure 2). These block dimen- permit an internally consistentassessment of structural varia- sions represent the approximate limits of resolution of the bility on a regional scale.These global modelswere developed available gravity data, as discussedfurther below. The dis- to addresscrustal structure at scalesgreater than the dimen- turbing gravitational potential at any point over the nearside sionsof most lunar basins,however, and with the exceptionof can be approximated as the sum of the potentials due to the the study of Thurber and Solomon[1978], none considered distribution of anomalous mass within each block. In this mare basalt as a significantcontributor to the observedgrav- section we describethe adopted procedure for calculation of ity field. gravity anomalies,as a forward problem, given a distribution In this paper we determine modelsfor the crustal structure of topography or anomalousmass that is uniform within each in the vicinity of nine impact basins on the lunar nearside block. We then discussthe constrainingassumptions that we (Figure 1). The models are derived as part of a simultaneous have chosento make the inverseproblem well posed.Finally, inversion of nearside gravity and topographic data, using a we describean iterative, linearized inversionprocedure to de- proceduresimilar to that of Thurber and Solomon[1978]. The termine the crustal structure within each block from gravity modelsinclude a low-densitynonmare crustal layer and a and topography,subject to the adoptedconstraints. mare basalt layer, both of variable thickness. The contri- butionsof Moho relief and mare basalt to the observedgrav- Computationof Gravity Anomalies ity anomaliesare separatedwith the assumptionsthat basin In previousanalyses of the gravity anomaly fieldsof planets topography was isostatically compensated prior to mare using spherical shell segments[Morrison, 1976; Thurber and basaltfill and that crustalsubsidence in responseto loading Solomon,1978], contributions to the anomalousmass within by mare basaltsmay be neglected.These assumptionslead to each segment have been approximated by uniform surface minimum valuesfor mare basalt thicknesses[Thurber and Sol- masses located at a constant radius from the center of the omon, 1978], but becauseof a similarity in density of mare planet. While this method simplifiesthe computation of the basaltsand mantle material, the thicknessof