An Imbrium pattern of graben on the Moon

ROGER MASON, J. E. GUEST and G. N. COOKE

MA SON . R.. J. E. GUEST & G . N . COOKE. 1976. An Imbr ium pattern of grabenon the Moon. Proc. Grot. .~ .H .. 87 (2 ). l o l - l oX. Many lunar graben form patterns thatare rad ia l and concentric to the major impact basins . The pattern cent red on theImbrium Basin. alt hough previously considered to be local in effect . is shown here tobe widespread. af fect ing at least half the Moon. Th is pattern is considered to resultfrom posthumous movement s on a fracture patt ern formed in the lunar lithosphere bythe Imbr ium impact.R. Mason. Department Q! Geology. University College London. Go....er Street.London WC/ E 6B T.J. E. Guest and G. N . Cook e. University of London Observatory, Mill Hill Park .London N W7 lQS.

I. INTROD UCTION2. METHOD OF STUDYJ . DIS C USSION4. CO NCL USION

ACK NOWLEDGM ENTSREFER ENCES

CONTENTS

1. INTRODUCTION

page161\ 6\

167167167168

Previous tectonic compilations for the Moon (Fielder, 1963; Strom, 1964) have been based onplotting the positions and orientations of various different linear features interpreted as being oftectonic origin. Of all these features, the straight and arcuate rilles are most widely accepted asbeing tectonic in origin (Baldw in, 1963). The y are interpreted as graben bounded by high-anglenormal faults (McGill, 1971). Because of this the present stud y is restricted to these together withsingle scarps of undisputed origin (e.g. the Straight Wall) . These are rarer than graben but theyusually have similar strikes and locations. The rilles are troughs with almost flat bottoms up toabout 450 km in length, 6-7 km in width and up to about I km in depth. The rilles arc offset enechelon at intervals (Plate I) .

Graben are concentrated on the nearside of the Moon (Whitford-Stark, 1974a) and many formconcentric and radical patterns around the circular basins (Baldwin, 1963). The study reportedhere has been concerned to identify patterns of graben quantitatively, particularly in relation tothe Imbrium Basin, the largest circular basin on the Moon . Previous compilations of linearfeatures (e.g. Whitford-Stark , 1974b) have shown that graben patterns may be related to thenorth-east-south-west and north-west-south-east trending grid pattern (Fielder, 1966; Strom,1964). Fielder (1963, 78) has argued that an Imbrium pattern of all types of linear features is oflocal extent. Here it is shown that at least for graben it is widespread, extending over morethan half the Moon's surface.

2. METHOD OF STUDY

Straight and arcuate rilles and faults are plotted for the whole Moon on the Aeronautical Chart

161

162 PROC. GEOL. ASS., VOL. 87 (1976), PLATE I

Full Moon photograph (astronautical convention) of the hemisphere facing the Earth, to illustrate the locations offeatures shown in Fig. I. The centre of the Imbrium Basin is marked, and also the outer rim of the circle of mountainssurrounding the Basin, which corresponds to the dashed circle of Fig. I, a. Other features marked correspond to those ofFig. I. From Consolidated Lunar Atlas, Lunar and Planetary Laboratory, Arizona, U.S.A.

PROC. GEOL. ASS ., VOL. 87 (1976), PLATE 2 163

Arcuate rilles concentric with the Humorum basin, on its eastern side. The rilles cut older highland material as well asyounger mare plains. Examples of the graben offset en echelon are marked by arrows . Orbiter IV photograph 132H I (thetriangular black marks are processing defects of the Orbiter negative). Scale bar 10 km

164 ROGER MASON, J. E. GUEST AND G. N. COOKE

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AN IMBRIUM PATTERN OF GRABEN ON THE MOON 165

and Information Centre (ACIC) LPM maps which are mercator equatorial and polarstereographic projections on a scale of approximately 1 : 5,000,000. The larger rilles are promi­nent features that may be seen on medium resolution Orbiter IV pictures as well as Earth-basedtelescopic photographs, and our mapping probably represents all these features. The recognitionof rilles with widths less than I km is noticeably sensitive to resolution ofthe Orbiter pictures andthe direction of lighting, and it is likely that those on the far side, in polar regions, and those witheast/west trends parallel to lighting direction, may be under-represented. Graben measured havea total length of 48,452 km.

Because the graben directions are often related to circular basins, all the graben and faults wereplotted on a pair of stereographic projections centred on the largest of these basins, Imbrium(36°.0 N, 18°.0W) and the antipode of Imbrium (36°.0 S, 162°.0 E). The latitude and longi­tude of the ends of rille segments were measured to the nearest O· I° from the ACIC mapand the segments plotted by a Calcomp plotter as great circles between these co-ordinatesusing a FORTRAN computer program (written by G.N.C.) on the IBM 1130 computer at MillHill.

Fig. I shows that there is a widespread pattern of rilles radial and concentric to the ImbriumBasin. This is confirmed by plotting a rose diagram (Fig. 2, a) of all graben and fault directionswith respect to the centre of Imbrium. It Was found that 30·7 per cent of the total length ofgraben and fault segments lies within 10° of being radial or concentric to Imbrium. It is notice­able that although close to basins the concentric pattern dominates, in the case of Imbrium themore widespread pattern is mainly radial.

In order to compare the pattern of graben discussed here with the grid system, rose diagramsare also presented for graben directions relative to the sub-Earth point (Fig. 2, b) and the northpole (Fig. 2, c).

Both the sub-Earth and the north polar plots show concentrations in orthogonal directions. Butthese concentrations (respectively 30·5 per cent and 32·7 per cent by length being within 10° ofthe preferred directions) are not stronger than those relative to the centre of Imbrium. They maybe different manifestations of the grid pattern. The north polar plot corresponds to a north­west-south-east and north-east-south-west pattern and the sub-Earth plot shows concentrationsat 45° to the family of great circles converging at the sub-Earth point. This latter pattern mayindicate a relation between the grid and tidal forces.

That none of these patterns is an arbitrary effect is illustrated by plots of directions relative torandom points on the lunar surface (e.g. Fig. 2, d). Furthermore, the Imbrium pattern may bedetected as preferred directions in the graben surrounding older basins such as Humorum (Fig. 2, e)which do not correspond to a north-west-south-east and north-east-south-west grid pattern(Fig. 2, f). Graben within the younger basin Orientale do not show an Imbrium pattern. Rillescutting mare material flooring craters tend to follow trends shown by rilles outside the crater, theImbrium trend being imposed on the crater's own pattern (e.g. craters Janssen and Hevelius).There appears to be a relationship between the Imbrium pattern and the orthogonal concentra­tion patterns illustrated in Figs. 2, band 2, c, which are probably reflections of the lunar gridpattern. If anyone of the patterns is eliminated from the data set, by excluding graben within 10°of the maximum concentration directions, the preferred orientation of graben due to the otherpatterns is no longer visible. The directions of a few prominent rilles are not explained by any ofthe patterns.

Fig. I (opposite). Stereographic projections of Lunar graben and faults centred on Imbrium (A) and the antipode ofImbrium (B). A: Ariadaeus Rille, AV: Alpine Valley, H: Hevelius, HB: Humorum basin, IB: Imbrium basin, J: Janssen,K: Kepler, OB: Orientale basin, OP: Oceanus Procellarum, SW: Straight Wall

166

a.

2000

c.

ROGER MASO N. J. E. GUEST AN D G. N. COOKE

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d.

2000 1000 0 1000

NORTH POLE - all data

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I I

200 100 0 100 200

IMBRIUM - data within 10°01 Humorum

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NORTH POLE - data within 10° 01 Humorum

Fig. 2. Rose diagrams of grab en and fault directions. showing accumulated lengths. gro uped in sectors of 50. The direc­tions are shown relat ive to the system of great circles which converge at the point nam ed: radial directions are par allel tothe ordin ate. concentric directions to the abscissa. Diagrams a, b. c. d plot data for the who le Moon , e and f plot only datafrom within a circle of radius 100

• concentric with Humorum

AN IMBRIUM PATTERN OF GRABEN ON THE MOON

3. DISCUSSION

167

Fractures in radial and concentric patterns are frequently associated with circular and sub­circular structures on Earth. There may be related to volcanic (e.g. Richey, 1961) or shock wavephenomena (Baldwin, 1963). On the Moon, fracture patterns of this type are found aroundcraters interpreted as being of impact origin (Guest, 1973). The large circular basins are thefundamental structural features of the lunar surface and are surrounded by one or more circularmountain chains (Stuart-Alexander & Howard, 1970). For most of the basins the radial and con­centric graben patterns are limited in extent to about the radius of the outermost mountain ring.Here it is argued that for Imbrium the pattern extends far beyond this to include at least thehemisphere of the Moon-centred on Imbrium. This also corresponds to the area where most of thegraben occur.

It appears that, depending on how the observations are treated, more than one pattern for lunargraben may be recognised. Most previous discussions of lunar tectonism have emphasised the im­portance of the grid pattern. The present data suggest that an Imbrium centred pattern accountsequally well for graben directions.

The circular basins are widely regarded as having an impact origin. Schultz & Gault (1974)have shown that there are good reasons for considering that shock and seismic waves caused bybasin-forming impacts would be Moon-wide, particularly for Imbrium. Therefore a pattern offractures such as that observed around the Imbrium basin is explicable in terms of the fracturingof the Moon's lithosphere by shock and seismic waves generated by the impact. Once formed, thepattern of fractures would be retained and would be reactivated by later movements, such as thosegenerated by later impacts, volcanism and tidal forces. The present graben were formed bymovements that occurred long after the formation of Imbrium because they cut mare materialswhich post-date Imbrium by as much as 600 million years (Taylor. 1975). Thus the initiation ofthe fracture pattern occurred at the time of the Imbrium event and the formation of grabenfollowing this pattern was posthumous. The extensive development of graben in the regionbetween Imbrium and Orientale may be due to the enhancement of Imbrium fracturing by theyounger Orientale impact event and this same effect may have caused graben occurring in theregion of young craters such as Kepler.

The concentration of graben on the nearside of the Moon is paralleled by a concentration ofvolcanic materials. This association is particularly strong in the Oceanus Procellarum regionwhere mare material is not obviously related to basin infilling by lavas. Oceanus Procellarurn, aswath of mare surrounding one part of Imbrium, appears in this region to be bordered by frac­tures radial to Imbrium, and it may be this fracturing that has provided the channels forvolcanism in this broad spread of lavas. The association of graben with basaltic volcanism onEarth is well established.

4. CONCLUSION

It can be shown that, whatever the significance of the lunar grid pattern, the circular basins exerta local control on the direction of graben. Here it is shown that in the case of Imbrium. the con­trol is more widespread and extends over at least half the Moon's surface area.

ACKNOWLEDGMENTS

The help of Mrs. M. Lee who punched some 2000 data cards and helped in other ways isgratefully acknowledged. Dr. M. K. Wells suggested improvements to the manuscript. Fundingfrom N.E.R.C. for comparative planetology studies is gratefully acknowledged by J.E.G.

11111 ROGER MASON, J. E. GUEST AND G. N. COOKE

REFERENCES

BALDWIN, R. B. 1963. The Measure of the Moon.Univ, of Chicago Press , 488 pp.

FIELDER, G. 1963. Lunar tectonics. Q. JI geol. Soc .Land.. 119, 65-94.

FIELDER, G. 1966. Convection in the Moon: a boun­dar y condition . Geophys. J. R . astr. So c.. 10,437--43.

GUEST, J . E. 1973. Stratigraphy of eject ion from thelunar crater Aristarchus. Bull. geol. Soc. Am.. 84,2873-94.

McGILL, G. E. 1971. Attitudes of fractures boundingstraight and arcuate lunar rilles . Icarus. New York . 14,53-8.

RICHEY , J . E. 1961. British Regional Geology.S cotland: the Tertiary Volcanic Districts. H.M.S.O.,Edinburgh, 120 pp.

SCHULTZ. P. H. & D. E. GAULT. 1974. Seismic

Received 4 July 1975Revised version received 2I November 1975

effects from major basin formation on the Moon andMercur y. NA SA Technical Memorandum . TM X­62388, 19 pp.

STROM , R. G. 1964. Analysis of lunar lineaments, I:Tectonic maps of the Moon . Com. Lunar & PlanetaryLab. Univ. of A rizona. 39, 205-16 .

STUART-ALEXANDER, D. & K. A. HOWARD.1970. Lunar maria and the circular basins - a review.Icarus. New York . 12,440-56.

TAYLOR, S. R. 1975. Lunar Science: a Post-ApolloView. Pergamon Press , New York, 372 pp.

WHITFORD·STARK, J. C. I974a . Internal origin forlunar rilled craters and the Maria. Nature . 248, 573-5 .

WHITFORD-STARK, J. C. 1974b. A detailed struc­tural analysis of the Deslandres area of the Moon .Icarus. New York. 21, 457-85.