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Mare Humorum and Mare Nubium: Basalt Thickness and Basin Formation History: R

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Mare Humorum and Mare Nubium: Basalt Thickness and Basin Formation History: R MARE HUMORUM AND MARE NUBIUM: BASALT THICKNESS AND BASIN FORMATION HISTORY: R. A. De Hon, University of Arkansas at Monticello, Monticello, AR. 71655 The Humorum basin is a moderately well-preserved, Nectarian circular impact basin similar to the Crisium basin in size and general characteristics. Mare Nubium is an irregular area of mare basalts filling a less well-defined basin east of Mare Humorum. The region of the Humorum-Nubium basins is outside the zone of Apollo photography; hence, imaging and altimetry are limited to Orbiter spacecraft and earth-based methods. Basalt thickness is estimated by measurements of partially buried craters. A geologic scenario of the basin-forming events is constructed from the surface distribution of terra and mare materials and the thickness distribution of mare basalts. Thickness estimates in the Humorum basin are confined to the relatively thin edges of the basalt disc, but the thickness probably exceeds 1500 m in the central interior. The basalt in the Nubium basin displays a more irregular thickness distribu- tion which reflects the composite structure of the basin floor. The average basalt thickness exceeds 500 m, and large thick lenses of basalt occur in eastern Nubium and southwestern Nubium. Palus Epidemiarum consists of relatively thin basalts in a small irregular basin south of the terra septum between Mare Humorum and Mare Nubium. The main elements of the Humorum basin are defined by two raised rings concentric to the basin center at 25OS; 40°W. The outer ring (or main ring crest) of the Humorum basin, approxi- mately 230 km radius, passes through the highland region of Rupes Kelvin and the terra region between the Humorum and Nubium basins. An inner ring with a radius of 130 km is defined by mare ridges. The mare basalts fill the central basin and flood the shallow shelf between the rings on the northern and eastern sides of the basin. The Nubium basin consists of two or more impact basins which are greatly modified by later events. A multiple impact origin for Nubium is supported by the following: (1) the multiple scalloped outline of Nubium, (2) circular patterns of mare ridges in more than one area of the basin, and (3) more than one thick lens of mare basalt. The west Nubium basin, centered at 23.5OS; 220W, is characterized by a central thick lens of basalt (750 m) surrounded by an arc of mare ridges 100 km from the center and a partial arc of rugged terra with a radius of approximately 210 km. The basalt lens in the east O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MARE HUMORUM AND MARE NUBIUM De Hon, R.A., et al. Nubium basin, centered at 230s; ~O~W,is over 1500 m thick and is bounded by arc of rugged terra 230 km south and east of the center. A smaller crater, or basi'n, 125 km in diameter is super- imposed on the northeastern rfm of the east Nubium basin at 210s; 7OW. Based on the pattern of mare and terra materials a 380 km diameter basin probably exists at 140s; 120W also. The geologic history of the region can be summarized in terms of the major impacts which produced the basins and craters and the emplacement of mare basalts. The earliest dis- cernable event in pre-Nectarian tfme is formation of the west Nubium basin. The Humorum basin was formed in early Nectarian time and its ejecta materfals (Vitello Fm.) were deposited into the Nubium region. The trough now occupied by Palus Epidemiarum formed at the intersection of broad circum-mare depressions concentric to the west Nubium basin and Humorum basin. Craters such as Gassendi, Doppelmayer, Mercator, and Kies formed in the Humroum ejecta. During late Imbrian time, mare basalts were emplaced within the fmpact basins and associated low lying regions. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System MARE HUMORUM AND MARE NUBrUM De Hon, R.A., et al. L 0. E E s IC LO wN L a V) x .F SF .r a ffl >+' V) L I= @ war -+'O rusn V) a a nsc, U a w a-u L n a 0% Em0 H 't V) 0 s 0 a E .r mL13w E-r a n u EX0 U Z? ru Q Q) Y OLL warn HE E w L 3 07 .I- LL. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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