Distribution, Morphology, and Origin of Ridges and Arches in Mare Serenitatis

T. A. MAXWELL Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 FAROUK EL-BAZ National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20560 S. H. WARD Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112

ABSTRACT The Apollo Lunar Sounder Experiment (ALSE) flown on Apollo 17 has produced profiles that are useful in the study of mare ridge Lunar mare ridges and arches in Mare Serenitatis were mapped systems (Phillips and others, 1973). A continuous radar profile to understand better their mode of formation. Mapping of these from the VHF mode (150 MHz) is used as a detailed topographic features indicates that several pre-mare impacts in the Serenitatis profile across ridges in southern Serenitatis, from which slopes and area may be responsible for the localization of the circular ridge elevations of the ridges and the amount of east-west crustal short- systems and that the subsurface, pre-mare topography is more ening have been measured. complex than previously recognized. Locations of the Serenitatis ridge systems were mapped, using as Apollo Lunar Sounder cross sections of ridge systems in southern a base the 1:2,750,000 scale Lunar Orbital Science Flight Chart Serenitatis indicate 50 to 100 m of local relief on these features. produced by the Defense Mapping Agency Aerospace Center for Ridges in the southwestern part of the basin mark the boundary of the National Aeronautics and Space Administration. In mapping a bench 200 m above the local mare level. As reflected in their the ridge systems, we used Apollo metric and pan photographs and orientation, arches and ridges are possibly controlled both by rings additional Earth-based photographs from the Consolidated Lunar of pre-mare basins resulting from impacts and by a more wide- Atlas (Kuiper and others, 1967). The purpose of this paper is to (1) spread global stress system. Small-scale features of ridges, such as summarize our observations on the distribution of ridges and medial lineations and lobate margins, do not conclusively define arches in Mare Serenitatis, (2) discuss these features in relation to the origin of the ridges. However, estimates of crustal shortening the Serenitatis basin and possibly more widespread stress systems, from Lunar Sounder data and the coincidence of the major ridge and (3) describe the morphology of ridges and arches in Mare system with the Serenitatis mascon suggest that ridges and arches Serenitatis in order to understand better their origin. were formed by gravitational readjustments of the mare fill along four probable impact structures and along a north-trending frac- DISTRIBUTION ture pattern. Key words: lunar morphology, lunar tectonics, lunar ridges and arches. Ridges and arches in Mare Serenitatis exhibit five dominant trends, four of which are related to circular pre-mare impacts. A INTRODUCTION general northward trend is also evident. The dominant ridge-system orientation (400 km in diameter; A Although lunar ridges occur in the highlands, they are usually in Fig. 1) is coincident with the inner ring of the Serenitatis basin as best exposed in the lunar maria, where they either parallel the mapped by Wilhelms and McCauley (1971), but as has been noted trends of the lunar grid system (Strom, 1964) or form trends con- for ridge systems in other maria, the system is not continuous; seg- centric with the major basins. These topographic forms have been ments of ridges and more continuous arches vary locally in orienta- referred to as "mare ridges" or "wrinkle ridges." tion. In western Mare Serenitatis, the ridges mark the boundary of Four basic hypotheses have been used to explain the origin of a topographic bench, which is raised relative to the center of the mare ridges and arches: a thin lava veneer on high ridges of pre- basin. Phillips and others (1972) showed that these benches coin- mare ground (Baldwin, 1963); tectonic deformation resulting from cide with the edges of a near-surface disk model for mascons in the shrinkage and isostatic subsidence of mare lava flows (Bryan, both Serenitatis and Crisium. Both ridges and arches are associated 1973); intrusive volcanism in the form of laccoliths or sills that lo- with the bench in western Serenitatis, and their occurrence is quite cally dome the surface (Whitaker, 1966; Strom, 1972); and extru- similar to the coincidence of a ridge system and the bench in west- sive volcanism of repeated flows to form both arches and ridges ern Crisium. (Young and others, 1973; Hodges, 1973). Combinations of the Three other circular ridge systems are also evident in Serenitatis. above hypotheses were favored by Tjia (1970), Hartmann and East of Bessel, a small ridge system, 140 km in diameter (B in Fig. Wood (1971), and Colton and others (1972). Recently, a tectonic 1), is coincident in the southeast with ridges that mark the inner mode of formation was advocated for ridge systems in Imbrium ring of the Serenitatis basin. The western limb of the "B ridge sys- (Schaber, 1973) and Serenitatis (Howard and Muehlberger, 1973; tem" extends in a north-south arc through the south-central part of Muehlberger, 1974). the basin. The mare area immediately to the north is smooth, and Adhering to the terminology proposed by Strom (1972), there is no evidence for the continuation of this ring, even when wrinkle-ridge systems in Serenitatis are divided here into mare photographed at lowest sun elevations. Evidence for the B ridge ridges and arches: arches have smooth slopes nearly indistinguish- system consists solely of the northward arc of ridges through Bes- able from the surrounding mare except under illumination at low sel. This trend may be the result of a north-south submare struc- sun elevation; ridges are smaller and have sharper edges. The term ture, but a circular structure as outlined in Figure 1 is preferred. At "ridge system" is reserved for both ridges and arches following the the northern boundary of Serenitatis, two circular ridge systems are same general trend. more clearly distinguished by discontinuous mare ridges and

Geological Society of America Bulletin, v. 86, p. 1273-1278, 6 figs., September 1975, Doc. no. 50910.

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MORPHOLOGY

The apparent difference between mare ridges and arches is most evident at low sun angles in Earth-based and lunar orbital photographs; orbital radar (ALSE) has justified this distinction. Although detailed observations of the ridges and arches do not conclusively define the origin of these features, it is apparent that the ridges have been subjected to several periods of deformation. Lobate fronts (A in Fig. 3) that subdue neighboring topography at the edges of ridges have been used to indicate flows and, therefore, an extrusive volcanic origin for the ridges (Strom, 1972). However, the possibility remains that these features may result from debris slides of regolith on the flanks of the high standing ridges (Bryan, 1973; R. S. Saunders, 1974, oral commun.). Many of these lobate fronts occur on low-angle slopes and are conspicu- ous in the interior of neighboring craters, thus indicating that slumping may be a valid mechanism of formation. Crests of mare ridges are in places characterized by parallel lineaments (B in Fig. 3) that trend in the same direction as the ridges (medial lineations). These lineations are found only on the crests of ridges and contribute to the youthful appearance; they do not appear on mare arches. Slumping on the sides of ridges may be par- tially responsible for local tension that could produce the lineations. However, their dominant orientation, which persists despite the proximity of ridge scarps, suggests a deeper structural control. Similar lineations or parallel fractures appear on the relatively level mare in eastern Mare Serenitatis (Apollo 15 pan frame 9301), I6*N where they may be related to the same stress that produced the 26»E 30*E larger rilles in the dark mare surrounding Serenitatis (Muehlberger,

C-3E? MARE ARCH MARE-HIGHLAND BOUNDARY 1974). Two areas in southern Serenitatis show a variety of relationships MARE RIDGE SCARP (BALL ON LOWER SIDE) between mare ridges and arches. Both the inner-ring ridge system Figure 1. Map of Mare Serenitatis showing distribution of mare ridges and the B ridge system roughly coincide in southeastern Serenitatis and arches. Dotted circles mark trends of ridge systems: A, inner ring; B, C, (Fig. 4). Although both ridges and arches follow the same general and D, three smaller structures. trends, there is no consistent feature that permits the ridges and arches to be assigned to either the inner-ring or the B ridge system. arches. The northeastern system, the "C ridge system" (330 km in Mare ridges in this region may form at the crest of an arch or on the diameter), continues from the mare into Serenitatis rim materials, boundary of an arch, or they may continue in smoother mare mate- but it is marked there by a scarp instead of ridges as in the mare rial that has no apparent relationship to arches (Fig. 4). (Fig. 1). Southwest of Posidonius, the continuation is marked by a Mare ridges in this area may, however, be distinguished as to age subdued arcuate rille, most likely a graben (R in Fig. 1). This sys- on the basis of their sharpness. East of the main ridge system in tem is unique because of changes in its character — grading from Figure 4, a more subdued ridge is present in relatively smooth, ridges in the mare to a probable fault in the highlands. nonarched mare material. This degraded ridge probably formed at To the west of the northeast ring, a circular ridge system 250 km an earlier time than the fresher appearing ridges that lie on the in diameter, the "D ridge system" (Fig. 1), is evident from both edges of the arches. The more degraded profile of this ridge is ap- arches and ridges in northern Serenitatis. These ridges, however, parent on the ALSE image of this region. The profile crosses two are truncated by the northeast system, which indicates that this ring mare ridges and an arch in southeastern Serenitatis (A to A' in Fig. structure formed before the C ridge system. 4). The arch is 90 to 100 m above the local mare level, and its slope Using the 1:2,750,000 scale map as a base, both ridges and arches were divided into three size classes (less than 20 km, 20 to 50 km, and greater than 50 km) according to length and orientation. In plotting the orientations, segments were weighted in order to give lesser emphasis to the smaller segments (Fig. 2). A general northward trend is evident for both arch and ridge orientation. However, the northeastward trend of some arches is not matched by a similar ridge orientation. Instead, ridges show a more pre- dominant northwestward trend. If it is assumed that the ridges formed more recently than the arches (because of their sharper appearance), then this phenomenon may indicate a changing orientation of the stress system with time. 40% 20% 40% 40% 20% 20% 40% If the ridge systems were solely the result of circular structures, a more symmetrical distribution of orientations would be expected. RIDGE ORIENTATION As will be shown, however, different processes may be capable of ARCH ORIENTATION producing the same topographic forms. Possible directional bias (70 measurements) (112 measurements) may be due to incomplete ring structures (such as B in Fig. 1) or Figure 2. Azimuth-frequency diagrams of ridge and arch orientation in east-west sun illumination, but other studies (Elston and others, Mare Serenitatis. Note lack of northeastward trends of ridges as compared 1971) have shown that the northward trend is real. with trends of the arches.

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arch disappear in the darker annulus surrounding the southern part of the Serenitatis basin. Several investigators (for example, Thomp- son and others, 1973) have shown that contrary to earlier beliefs (Wilhelms and McCauley, 1971), this darker material is older than the lighter central-mare material on which mare ridges and arches occur. The disappearance of ridges and arches in this older unit may be due to greater thickness of the deformed younger mare unit, a greater thickness of regolith on the older surface tending to subdue the ridges, or contemporaneous formation of the ridges and arches with the younger mare unit. Arcuate rilles in the dark an- Figure 3. Part of a mare nulus surrounding Serenitatis indicate that the dominant tectonic ridge in eastern Serenitatis. A, stress in the darker material was tensional and that the rilles prob- Lobate front that overlaps a crater resulting from either ex- ably formed as grabens in the early volcanic fill of the Serenitatis trusion or debris slides; B, line- basin (Muehlberger, 1974). As shown by Bryan (1973), there seems ations on the crest of the ridge to be no great age difference (on the basis of crater density) be- (Apollo 15 pan frame 9298). tween mare ridges and arches and nearby "undeformed" mare surfaces. Because the regolith of the dark mantling material is most likely less than 25 m thick (Kovach and others, 1973), at least at the Littrow landing site, it seems likely that even a great amount of mass wasting would not obscure mare ridges. Therefore, mare ridges and arches are most likely the result of processes acting after to the west is a little greater than 1°. A ridge on the eastern part of deposition of the younger mare unit. this arch has a slope of 6° to the east. The degraded mare ridge to In southwestern Serenitatis (Fig. 5), east of the crater Sulpicius the east of the arch is 50 m above local mare level, and its western Gallus, the mare ridge system is associated with the inner ring of and eastern slopes are 4° and 2°, respectively. These values are not Serenitatis. Both ridges and arches are present, and there is also a typical for all arches and ridges. Ridges in eastern Serenitatis may very prominent bench that is further delineated by the mare ridges have slopes as great as 35° (Apollo 15 pan frame 9298), and relief (Fig. 5). Ridges in this area are approximately 50 m above the mare on arches may be visible only on near-terminator photographs. level of the bench, and the bench is 200 m above the level of mare South of the area shown in Figure 4, both a mare ridge and an fill in central Serenitatis.

Figure 4. Apollo 17 metric photograph (frame 0799) showing mare Figure 5. Apollo 17 metric photograph (frame 0806) of southwestern ridges and arches in southeastern Serenitatis. Cross section below is Apollo Serenitatis showing mare ridges and arches. VHF profile below shows break Lunar Sounder Experiment VHF radar profile showing two ridges and an in mare level marked by ridge that corresponds to approximate location of arch. edge of the mascon.

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The coincidence of this bench in western Serenitatis with the that 1 km of collapse for Serenitatis would result in 0.38 km of inner-ring ridge system is topographically similar to the coinci- crustal shortening. In his analysis, however, the marginal ring dence of a bench and a ridge system surrounding Mare Crisium. In faults were assumed to be vertical, and they intersected each other Mare Crisium, the inner-ring ridge system and a topographic bench at a depth of 1,740 km (Bryan, 1973, Fig. 6). may be manifestations of the same subsurface structure, similar to For 450 km across southern Serenitatis (Apollo 17, revolution the situation in northern Serenitatis, where a mare ridge continues 25), ALSE VHF imagery was enlarged to a scale of 1:150,000, and as a fault scarp in the highlands. measurements were made of the topography along the ground track. Ridges, arches, and subtle undulations in the mare along this DISCUSSION track account for an additional 3.5 km of surface topography. Using Bryan's (1973) "model 2" collapse, about 9 km of collapse Although a dominant north-south orientation of ridges and would be required to account for this amount of crustal shortening. arches in Mare Serenitatis is present (Fig. 2), both stress systems If the faults are not vertical, however, and have a shallow dip to- and local gravity anomalies must also be considered with regard to ward the center of the basin (Muehlberger, 1974), then much less the origin of the ridge systems. collapse is needed. By analogy with Crisium, if the amount of sub- Highland and mare ridges, straight rilles, and polygonal crater sidence is 500 m and the crustal shortening is 3.5 km, a 400-km walls on the lunar nearside have been analyzed from Orbiter IV disk would need faults of approximately 16° dipping toward the photographs and were found to trend primarily northward, which center of the basin. indicates a global compression in this direction according to Elston Muehlberger (1974) also suggested a compressional mode of and others (1971). Mare ridge orientations in that study had peaks origin of the ridge systems in Serenitatis, in which a reduction in the at north and northwest, and there was a definite lack of east-west lunar circumference by thermal contraction was seen as the most orientation, which Elston and others (1971) interpreted as the re- likely mode of east-west compression. The crustal shortening of 0.8 sult of east-west tension. In southern Oceanus Procellarum, ridges percent found here is compatible with his estimate of radial (linear) may have been offset to form the northeast- and northwest- shortening of 0.5 percent (Muehlberger, 1974). The association of trending segments (Colton and others, 1972). the inner-ring ridge system of Serenitatis with the best-fit mascon An alternate interpretation has been put forth by Tjia (1970), models, however, suggests that gravitational readjustments may be who proposed that the ridges are drag folds that could have re- a more valid explanation. sulted from east-west regional compression. As will be shown, it On the basis of the distribution and morphology of ridge systems seems likely that east-west compression and subsequent crustal in Mare Serenitatis, the following characteristics must be consid- shortening in that direction could account for lunar-wide, north- ered with regard to their origin: (1) dominant circular distribution trending ridge systems. Also, a linear, north-trending thrust fault along the eastern margin of Serenitatis (as proposed by Muehl- berger, 1974) may also be responsible for enhancing the same trend of ridge systems. The ridge systems in Serenitatis, however, are also circular and in some cases arcuate, thereby suggesting additional stress systems. The largest ridge system, which coincides with the inner ring of the Serenitatis basin, is more prominent than the three other systems, but this does not imply any great age difference. Instead, coincidence with the location of a shallow-disk model for the Serenitatis mascon (centered at lat 23°N, long 18.6°E with a radius of 221 km; Sjogren and others, 1974) indicates that the magnitude of forces responsible for ridge formation is related to the size of the depression that has since been filled with mare material. Phillips and others (1972) showed that the best-fit mascon models for both Crisium and Serenitatis are bounded by the ridge systems, and ALSE radar data further indicate a break in the mare level of 200 to 500 m in both eastern and western Crisium and western Serenitatis. Although the presence of the mascons indicates that little isostatic compensation has taken place in the large multiring basins, there apparently has been some compensation to account for the downdrop of the central portions of Crisium and Serenitatis. The lack of a topographic bench in southeastern Serenitatis may be ex- plained by assuming that no compensation has taken place. Indeed, this seems to be the case, because Apollo 17 gravity data indicate a greater negative anomaly over eastern Serenitatis than western Serenitatis (Sjogren and others, 1974). The coincidence of the major mare ridge systems with the mascon boundaries suggests that the principal stresses were the result of gravitational readjustments. A downward offset of the central mare relative to the benches would result in high-angle normal faulting and may not necessarily produce the amount of crustal shortening necessary to form the ridge systems. If the upper crust of mare materials is considered as a plate bounded by normal faults, then at some depth, the radius of the plate would be smaller than at the surface. With subsidence, the circumferential normal faults Figure 6. Proposed ancient ring structures as delineated by ridge systems dipping toward the center of the basin would have a compressional in Mare Serenitatis (Lunar Orbiter IV frame 86M). Lines are dashed where component of stress. Bryan (1973) used this type of model to show structures are inferred.

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with a subordinate northward trend; (2) coincidence of the western have had an effect on the present configuration of the major lunar topographic bench, mascon boundary, and the inner-ring ridge sys- basins. tem; (3) general correlation between trends of arches and ridges; (4) changes from ridges in the maria to scarps, grabens, and other CONCLUSIONS lineaments in the highlands; and (5) small-scale lineations on the crests and lobate scarps on the edges of the ridges. Ridge systems, composed of both gentle arches and sharp crested The circularity of the inner-ring ridge system in Serenitatis sug- ridges in Mare Serenitatis follow four circular trends: a major sys- gests that this system is structurally controlled by the shape of the tem that marks the inner ring of the Serenitatis basin and three pre-mare basin. Coincidence of the western topographic bench and smaller circular structures. A general northward trend is also pres- the mascon boundary further substantiates this hypothesis and al- ent for both ridges and arches; this trend may be related to a global lows quantitative estimates of the thickness of the mare material. stress system. Locally, medial lineations on the crests of ridges and The inner-ring ridge system, therefore, probably formed at the lobate fronts at their margins give a fresher appearance to the boundary between shallow and deep mare fill. The same correla- ridges, compared to the more gently sloping arches. There is a gra- tion is also evident in Mare Crisium, where a circular topographic dation in the appearance of ridges, which may indicate that these bench is distintively marked by wrinkle ridges. features have formed over a long period of time. With this information on the subsurface control of the inner-ring Coincidence of the inner-ring ridge system with the location of ridge system, it is possible to construct a sequence of pre-mare, the Serenitatis mascon indicates that ridge systems may mark the crater-forming events that have controlled the distribution of other boundary between shallow and deep mare fill. The subsidiary circu- circular ridge systems within Serenitatis. Following the impacts lar trends may mark the sites of pre-mare impacts on the floor of that produced the Serenitatis basin (Scott, 1972) more than 4.0 b.y. the Serenitatis basin. We suggest that these ridges and arches ago, three other depressions were formed, probably by meteorite formed by gravitational readjustments of the mare basalt flows, impact: one on the southeast floor of the basin, about 140 km with dominant trends being controlled by pre-mare impact struc- in diameter (B in Fig. 1), and two in the northern part of Seren- tures. itatis, which produced basins of 330 and 250 km in diameter (C and D in Fig. 1; Fig. 6). Although some flooding of the ACKNOWLEDGMENTS Serenitatis basin by basaltic lava flows may have taken place between the Serenitatis impact and these later events, any evidence We acknowledge the helpful and stimulating discussions with has been covered by basalt flow of Imbrian and Eratosthenian age R. E. Eggleton and R. S. Saunders and thank R. C. Wilson and (Scott, 1972). Formation of the Imbrium basin and its ejecta, in ad- W. E. Glenn for suggesting improvements in the manuscript. We dition to later flooding, obscured the outlines of these other basins, are also grateful to E. A. Whitaker for providing Earth-based which are in evidence now only as ridge systems in the mare and as photography and to the Lunar Sounder Team for permission to use lineations in the Serenitatis rim materials. The circular ridge sys- two radar profiles. This study was supported by National Aero- tems in Serenitatis, therefore, are herein considered as the surface nautics and Space Administration Contract NAS9-12168 and Ex- manifestation of faulting localized by the inner-ring systems of four periment S—222. pre-mare basins (Fig. 6). The postulated depth of mare fill in Serenitatis is greatly affected REFERENCES CITED by these additional depressions (probably impact scars). If the large impacts that occurred to the north of the basin occurred after the Baldwin, R. B., 1963, The measure of the Moon: Chicago, Chicago Univ. 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