0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Volcanic Features - Goclenius

VOLCANO-TECTONIC FEATURES OF CRATER GOCLENIUS. W. B. Bryan, P.A. Jezek, and Mary-Linda Adams, Woods Hole Oceanogra- phic Institution, Woods Hole, Mass. 02543 Crater Goclenius is located on the west side of Mare Fecun- ditatis near the southeastern limit of a series of graben-like rifts of which the principal representatives are Rima Goclenius . I and Rima Goclenius 11. The latter structure can be traced through a central high within the crater and was studied in de- tail by Baldwin (1). The unusual elongated outline of Goclenius, combined with its fractured floor and the angular outline of the northwestern wall make this crater almost unique. Many lunar craters have polygonal or irregular outlines, and a signi- ficant number have fractured floors, but they do not combine both features to the extent observed in Goclenius. Predominantly circular craters with polygonal outlines may be expected for impact craters in terrain with a pre-existing structural pattern, while volcanic calderas formed under similar circum- stances tend to be much less circular while also showing evidence of tectonic control (2,3). The unusual form of ~oclenius and its relation to major structures has invited comparison with the central intrusive complex of Scotland (4) . Goclenius measures about 70 km by 50 km, with the long axis oriented about 300'. However, the most conspicuous structural trends in and near Goclenius are provided by the rimae, which strike into and across the crater on an azimuth of about 315", approximately parallel to the southwest wall. The northwest wall of the crater is almost straight and appears to be defined by fault scarps striking 042" to 066'. These trends establish the gross form of the crater and are both close to principal trends of the "lunar grid" system (5). 6 Although the rimae crosscut nearby craters of similar size, such as Gutenberg, none of these show the rectilinear outlines so conspicuous in Goclenius, and are essentially circular in outline. Goclenius also differs in having a dark, mare-like central fill. These contrasts also imply a special origin for some or all of the features shown by ~oclenius.However, in other respects ~ocleniuscould be regarded as a typical impact crater, showing terraced inner walls and an irregular cluster of internal peaks, features which, along with polygonal outlines and a dark fill, have been held to be compatible with an impact origin for other craters (3,6). using Apollo 16 metric and panoramic photos, we have attempted to determine the nature of the internal dark fill, 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Volcanic Features - Goclenius Eryan, W.B. et al. the central high, and the trends and relationships of rilles, craters and pits to one another and to the major structural directions. We suggest that the following criteria may tend to distinguish craters or cones of volcanic or other endogenic origin from those due to impact: (1) Apparent structural control - cones or pits oriented along rifts or at their intersections. (2) Crater is similar in albedo and degree of degradation to the adjacent surface. (3) Cone with or without a central crater fills and overlaps the channel or rift. A substantial number of features in Goclenius fulfill one or more of these criteria. The dark fill is traversed by a kite- shaped network of narrow channels which are subparallel to the walls of the crater. Dark pits with approximately the same albedo as the floor are especially common, either centered on the angular intersections between these channels, or grouped at in- tervals along them. Toward the southeast side of the floor, a light-colored cone appears to have been built upon a channel, filling it and overflowing it. Near the northwest and southern extremities of the dark floor, lines of pits can be traced along the trends of channels or rimae boundary faults from the dark floor into the lower walls of Goclenius, and also across some of the lighter central fill. Some of these individual small craters are distinctly elliptical, being elongated on the trend of the structure. Two of these pits are centered on mounds sur- rounded by especially dark halos which are apparent in various photos with different lighting and thus appear to be dark ejec- ta related to the craters. The surface of Mare Fecunditatis immediately east of Go- clenius shows some similar linear trough-pit complexes with trends subparallel to those within Goclenius. Albedo, cratering, and the general level of degradation of the surfaces are similar. This suggests the surfaces are of similar age, composition, and origin and could even have been continuous at one time. Such direct evidence as can be seen at present, however, suggests upward faulting and doming of the Goclenius floor. The eastern margin of the floor terminates along a channel that forms part of the "kite" pattern. Within this channel, layering can be observed on the walls and appears to be tilted upward along the outer wall. Also, a channel on the floor which clearly seems to have once connected with a pit at the base of the crater wall is now lifted well above the rim of the pit. Because some lines of pits cross from the floor into the walls apparently undis- turbed, at least the latest pit-forming events occurred 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System Volcanic Features - Goclenius Bryan, W.B. et al. after the floor was established in its present position. The floor itself slopes outward and downward toward these boundary faults, with the highest point centered close to the triangular hill near the center of the crater. The rimae appear to be the youngest features, cutting cleanly across the walls and floor ' of the crater. However, consistency between Rima '~ocleniusI1 and the older structures within the floor suggests that this graben has developed along old fault lines that probably were reactivated by warping of the crust under the load of the Fe- cunditatis mare fill. The weight of evidence thus suggests an internal origin for many of the features of the Goclenius floor, The floor appears to be a mare-type volcanic fill, probably contemporary with the filling of Mare Fecunditatis. Goclenius apparently lies near the intersection of two ancient lines of weakness related to the lunar grid; for this reason it may have been the locus of a major feeder vent for the Fecunditatis lavas. The internal group of hills may be remnants of rims between a complex of several small impact craters that were inundated by these eruptions. A broad shield may have been built above the vent; the outward slope of the present floor may reflect this primary structure rather than tectonic doming. Subsidence of this shield caused the irregular fracturing of the floor, while late-stage gas eruptions continued until the floor settled to its present position. It is necessary to postulate a final uplift along the eastern boundary fault; we are not aware of any comparable fea- ture in a terrestrial volcanic caldera. Many features on the floor can be interpreted as collapsed tumuli, pit craters, or cinder cones, but some may simply represent collapse of loose regolith into tensional openings, and a few appear to be for- tuitously placed impact craters. (1) Baldwin, R.B. (1971) J. Geophys. Res. 76, 8459-8465. (2) Murray, J.B., & Guest, J.E. (1970) Modern Geol. 1, 149-159. (3) Guest, J.E. (1971) In Geoloqy & Physics of the Moon, G. Fielder, Ed., 93-103. (4) Kopecky, L., (1972) In The Moon, I,A,U. Sym. 47,231-245. (5) Fielder, G. (1963) Geol. Soc. London Quart. Jour. 119,65-69. (6) Bryan, W.B. & Adams, M-L. (1974), Proc. Lunar Sci. Conf. 5th, 25-34. 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .

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