Morphology and structure of the taurus-littrow highlands (Apollo 17): evidence for their origin and evolution

Abstract

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The Taurus-Littrow region (Apollo 17 landing area) is located in the northeastern quadrant of the Moon in the mountainous area on the southeastern rim of the Serenitatis basin. The highlands in the Taurus-Littrow region can be divided into three broad terrain types. (1)Littrow massifs - massive, 10-20 km diam, steep-sloped (20°–30°), highland blocks often bordered by linear graben-like valleys. (2)Littrow sculptured hills - a series of closely spaced 1-5 km diam domical hills occupying broad highland plateaus which have been cratered and block faulted. Sculptured hill units stretch along the eastern edge of Serenitatis from the Apollo 17 area north to Posidonius. (3)Vitruvius front and plateau - a long irregular but generally north-trending scarp (occasionally rising over 2 km above the surrounding terrain) and its associated uplifted plateau to the east. This terrain is composed of hills ranging from 2-7 km diam, whose morphology is intermediate between the sculptured hills and the massifs.

It is concluded that the highland units in the Taurus-Littrow region are primarily related to the origin of the Serenitatis basin because of their marked similarity to more well-preserved basin-related deposits in the younger Imbrium and Orientale basins: (1) the massifs and sculptured terra are morphologically similar to the Imbrium basin-related Montes Alpes and Alpes Formation, (2) the relative geographic position of the Taurus-Littrow highlands and Montes Alpes/Alpes Formation is the same, forming the second ring and spreading distally, and (3) the structures are similar in orientation and development (e.g., massifs are related to radial and concentric structure; Alpes Formation/sculptured terra are not).

Interpretation of the massifs and sculptured hills as Serenitatis impact-related deposits lessens the possible role of highland volcanism in the origin and evolution of the Taurus-Littrow terrain, although extensive pre-Serenitatis volcanism cannot be ruled out. The preserved morphology of the sculptured hills suggests that the thickness of post-Serenitatis large basin ejecta (from Imbrium, for instance) is small, compared to the total highland section. This implies that the primary contributions to the highland stratigraphy are from Serenitatis and pre-Serenitatis events. The highland surface, however, may be dominated by ejecta from the latest nearby large event (formation of the Imbrium basin).

Structural elements mapped in the Taurus-Littrow area include lineaments, the Vitruvius structural front, two types of grabens, and scarps. The majority of lineaments, as well as some grabens, appear to be related to a dominant NW trend and subordinate N and NE trends. These trends are interpreted to be related to a more regional lunar grid pattern which formed in the area prior to the origin of the Serenitatis basin, causing distinct structural inhomogeneities in the highland terrain. The Serenitatis event produced radial and concentric structures predominantly influenced by this pre-existing trend. Younger grabens are generally circumferential to the Serenitatis basin and appear to be related to readjustment of Serenitatis-produced structures; those that are oblique to Serenitatis follow the pre-Serenitatis structural grain. No obvious structural elements can be correlated with the post-Serenitatis, Nectaris and Crisium basins. It is believed that the origin and hence the geographic concentration of the Littrow massifs is related to the fact that Serenitatis radials in the massif area coincide with lines of pre-existing structural weakness along a general lunar grid direction (NW). Pre-existing structurally weak lunar grid trends seem to have been structurally reactivated by Serenitatis radials, causing preferential uplift of large blocks in this area. Elsewhere in the region radials would be oblique to this direction. Since Serenitatis and Imbrium radials coincide in the massif area, the post-Serenitatis Imbrium event may have reactivated Serenitatis radial fractures, possibly rejuvenating the massif terrain.

The geologic and tectonic history of the Taurus-Littrow highlands began prior to the origin of Serenitatis in Tectonic Interval I. The strong NW trending structural elements are believed to have formed as part of a global stress pattern (possibly shear) sometime during this period of probable crustal formation and fragmentation. Tectonic Interval II was initiated by the origin of the Serenitatis basin. The basic topography and morphology of the region and most large grabens resulted from this event and their orientations show that they were controlled at least in part by the pre-existing grid. No other large basins forming during this interval appear to have had a major effect on the area. Tectonic Interval III is dominated by the formation of narrow grabens following structural patterns circumferential to the Serenitatis basin and tangential to it where they coincide with pre-existing grid directions. Serenitatis isostatic rebound or early mare fill may have produced this stress system. The scarp in the vicinity of the Apollo 17 landing site is the youngest obvious structural element.