Radial structures surrounding lunar basins

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Authors Hartmann, William K.

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Link to Item http://hdl.handle.net/10150/319774 RADIAL STRUCTURES SURROUNDING

LUNAR BASINS

by William K» Hartmann

A Thesis Submitted to the Faculty of the

.DEPARTMENT OF GEOLOGY

In .Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

. THE UNIVERSITY OF ARIZONA STATEMENT BY AUTHOR

TtiA ttuesis hag been , submitted in partial fulfillment of require­ ments forvaE.advanced.-degr'ee.at The University of Arizona and is de­ posited in the University Library to be made available to borrowers under rules of the Library®

Brief quotations: from this thesis are allowable without special permission® provided that accurate acknowledgment of source is made®. : Requests, for -permission for. extended quotation fr om Or .reproduction of this manuscr ipt in whole or in part may b e gr anted by the head of the major department or the Bean of the Graduate College w h en in his judg­ ment the proposed use of the material is in the interests of scholarship® In all other Instances, however, permission must be obtained from the author®

SIGNED;

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

S® Bo TITLpT Associate Professor of Geology AC KNOWLEBGME NTS

I wish to thank Dre G.. P. Kuiper? Miss Barbara Ms Middlehurstg

M essrs. Robert Strom, Be W. G. Arthur, and E. A® Whitaker, of. the

Lunar and Planetary Laboratory, and Dr® Evans B® Mayo and Dr.

. Spencer Re Titley of the Bepartment of Geology for many helpful dis­

cussions® I r elied extensively on Mr* . Whitaker1 s help in selection of

photographs from the Lunar and Planetary Laboratory plate collection®

. Some of the rectified negatives were made by Mr®. Harold .Spradley* The

work, was supported by the national Aeronautics and Space Administra­

tion through Grant NsG 161-61® .TABLE. OF CONTENTS

LIST OF ILLUSTRATIONS <..<>. •. .o o = 0 o •»<><,»• eo»» -o .ee..«

Ab s t r a c t ...... 1

UC_y TIQN . o . 0 . . o . o . o .... o .... a ...... o ... o .

THE IMBRIUM STSTE^/I ...... o......

Review of Literature ...... Morphology ...... - o.ue.iu8torie . . @..,..,...... o. *

THE ORIENTALE SYSTEM .v...... '......

Morphology ...... : ' onehie ions ......

The n e c t Aris systeml ......

Morphology ...... -----...... Conclusions .o...... o.o......

THE HUMORUM SYSTEM ^ .

Morphology ...... Conclusions ......

THE CREIUM S YSTEM ......

Morphology ...... Conclusions ......

OTHER. M @ @ .

GRID SYSTEMS ...... - V Page

EADJA.Ii SYSTEMS AROUND CRATERS ..,...... 70

.CONCLUSIONS AND SUMMARY ...... 78

APPENDIX I— PLATES ILLUSTRATING LUNAR .STRUCTURES ...... 82

REFERENCES ...... ___...... _____..... 150 LIST OF ILLUSTRATIONS

Plat© Page

1. View of moon from above Mare Jmbrimm ...... 83

2, LooMng south from above Mare Imbrium ...... 84

3» The Apetinine Mountains and Mare VapOrum ...... 85

4. The Straight Wall ...... 86

•5® The Cauchy scarp ..»* ...... *...... $ 87

6. Haemus Mountains, sunrise ...... 88

T* Haemus Mountains, sunset ^ ...... 89

8e Apennine Mountains and Mare Vaporum, sunrise ...... 90

90 Apennine Mountains and Mare Vaporum, full moon ...... 91

lOo Apennine Mountains and Mare Vaporum, sunset ...... 92

11. Region of Julius Caesar, sunrise ...... 93

12. Region of Julius Caesar, sunset ...... 94

13. Region of Ptolemaeus, sunrise ...... 95

140 . Region of Ptolemaeus, full moon ...... 96

15o Region of Ptolemaeus, sunset ...... 97

160 Region of Ar zachel, sunrise I ...... 98

l'7o - Region of Ar zachel, sunr ise H ...... 9 9

18@ Region of Arzachel, full moon ...... 100

vi vn

Plate Page

19. Region of Arzachel, sunset ...... 101

20. Region of Arzachel with overlay ...... 102

21. Region of Ptolemaeus with overlay ...... 103

22. Region of Fra Mauro? sunrise I . 104

23= Region of Fra Mauro, sunrise II oeo@eeoe©eoooooo 105

24. Region of Fra MaurOj full moon 000ffl®0 ©®00©0©0©0© ©©

25. Region of Fra Mauro^ sunset ... O © ® O ®00®®®® 0 0 0

26. . Carpathian Mountains, sunrise . 108

27. , Carpathian Mountains, sunset .. o©®oeoo®o 109

28. Region of Hansteen ...... e©ee®®ooo©©oeffloo©©o,©p® 110

29. Region southwest of Hansteen ... 'o © o o o o o o o' a ® o o o 111

30. Region of O. Struve ...... 112

, 31. Western reaches of Mare Imbrium . 113

32. Aristarchus plateau and western reaches of Mare Imbrium...... 114

33. Aristarchus plateau, sunrise .... 115

34. Aristarchus plateau, high lighting 116

35. Aristarchus plateau, sunset ... 000©0®00®©00s© 00000©0©0© 117

36. Region of W. Bond, sunrise ...... 118

37. Region of W= Bond, sunset ...... ©• ® ® o o o o o e oo 119

38. Region of J. Herschel..... 'o'eo®6©eoo®oo®©ooo0oeo®o©®®e®o120

39. Parallelism of wrinkle ridges in Mare Tranquillitatis .... 121 viii Plate ■ Page; 40.. The radial system of the. Imbr itrm basin .7 ...... «... 122 41. Regioti of War gentin from above Mare Orientale . 123 42. Region .southeast of Mare Orientale,*'• sunrise '.»,.,»... *•. ' 124 .43 o . Region southeast of Mare. Orientale, high lighting » , 125 44 . Large-scale., view of Schickard and vicinity'-». „ ,.. 126 40.. Region northeast of Mare Orientale ...»...... 127 - 46. The Orientale. Basin System...... , 128

47. . Region south-southeast of Mare Nectar is, sunset I ...... 129

48. . Region south-southeast of Mare. Nectar is, sunrise ... 130

49. R egion south-southeast of Mar e Nectar is, , sunset II ..... 131

50. Limb r egion east-southeast of Mar e Nectar is, sunr ise .. 132

51. Region east-southeast of Mare Neetaris, sunset «. L..... 133

52. The Neetaris .Basin. System u ...... 134

53. Region southeast of the Mare Humorum ...... 135

54. Orthogonal lineament pattern west of Mare Humorum, sunrise ...... 136

55. Orthogonal lineament pattern west of Mar 6: Humorum, sunset...... o...-...... " 137

56. The Humorum Basin.System ...... 138

57. Lineaments northwest of Mare Crisium ...... 139

58. Mare Grisium at sunrise ...... 140

. 59., The-Crisium Basin.System ... 141 60. Region of Mare Humboldfianum ...... 142 ix

. Plate Page

61. ■Region of Mare Huraboldtianum? sunset ,...... 143

62* Radial systems around crater ss showing Aristillus and Bullialdus oo... oos.oo a a... • a ... ooooo o.oo. oo.ooo.. 144

63* Radial systems around craters, showing Copernicus, EratoStheneSg and Ar istar chus ...... 145

64. Comparison of valleys and troughs of radial system s ..... 146

65. Comparison of orthogonal lineaments (a, b# and c), scarps (dg e, and f), and deformed craters (g, h2 and i) of radial systems ...... 147

66. Schematic diagram of lunar history ..... — ...... 148

' 67. The lunar basin systems ...... 149 ABSTRACT

Most of the circular maria on the moon occupy structural ba­ sins surrounded by concentric mountainous arcs and radial lineament systems® Most basin systems are.discussed in detail here, and the structures are illustrated with numerous photographSo, Some remarks are made on the small radial systems around some recent craters and on the nonradial grid patterns®

The radial systems of the oldest basins ar e the least well de­ veloped. The young basins of Mare Imbrium and Mare Orientale have the most prominent radial systems® ; The lineaments vary greatly in age, but conditions for producing them were probably optimum during the relatively short period when the maria were laid down. Many are spatially related to this flooding.

The basins are pr obably the sites of great impacts, accom­ panied by radiating fractur es. Most lineaments are interpreted as the expression of tectonic adjustments in the stressed crust along these fractures as a result of heating of the subsurface. Most of the adjust­ ment is by vertical motion and destruction by flooding. There is little evidence for horizontal motion or for gouging of radial valleys by flying fragments®

. x . INTRODUCTION

As long ago as the 1890s s it was recognized that at least one ©f

the lunar maria is accompanied by a family of radiating surface struc­

tures, Gilbert (1893) described the Imbrium system in a paper which

discusses the ’’Imbrium sculpture, ” Most of the discussion: of such ra­

dial structures has concentrated on the array in the region from the

Haemus Mountains to Ptolemaeus, radiating from Mare Imbrium (Pis.

1-3),

The program of rectified lunar photography at the Lunar and

Planetary Laboratory of the University of Arizona permitted a survey

of the regions around the mare basins without the complications caused

by foreshortening. The results on concentric-ring structures surround­

ing several of these basins were reported in C ommunications of the Lunar

and Planetary Laboratory No. 12, Volume L, ’’Concentric structures

surrounding lunar basins” (Hartmann and Kuiper, 1962), to which the present paper is a companion.

■ In accord with previous Communications, astronautical direc­

tions adopted by the International Astronomical Union (1962) are used.

Most of the photographs are oriented with the basin center downward,

so that radial features fan upward. , References to plates in earlier

Communications are by plate number (e, g ., PL 12.1). Several lightings I of most regions are given to allow optimum interpretation. Original plate number s s:cales. and. the sun’s selenographie colongitude accompany each plate®

The term "system" Is here used in the broad sense of a genetical­ ly r elated grouping* as in definition 4 in the AGI Glossary of Howell

(I960)® .M E IMBEIUM SYSTEM

Review ©f Literature

Because of the extensive liter ature on the Imbrium radial pat-

tern..and the,conflicting, views expressed^. an extensive review is given

here. This review will also serve as an introduction to the subject of

. lunar radial structures®

Many roughly parallel structures were noted by the early

selenographerSe For example,. Beer and Madler ,(183'7$ p® , 250) de­

scribed (translated) a n SE-NW direction of mountain ridges of the

most decided prominence" between Mar e. Tranquillitatis and Mare

Vaporumo However^ the early observers usually discussed individual

formations^ not wider patterns® .

In his classic paper "The Moon’ s F ace*G ilb ert (1893) wrote

(ppe 275 -276 ):

The rim s of certain craters are traversed by grooves or furrows# winch arrest attention as exceptions to the general eonfigurationo In the same neighborhood such furrows exhibit parallelism of direction® Similar furrows appear on tracts be­ tween craters# and are ther e associated with r idges of the same trend* some of which seem to have been added to the surface® Elsewhere groups of hills, have oval forms with smooth contour s and parallel axes* closely resembling the glacial deposits known as drumlins* but on a much larger scale® Tracing out these sculptur ed, areas and platting the 4 trend lines on a chart of the moon, I was soon able to recognize a system in their arrangement, and this led to the detection of fainter .evidences of sculpture in yet other tracts. The trend lines converge toward a point near the middle of the plain called Mare Imbrium, although none of them enter that plain. Asso­ ciated with the sculpture lines is a peculiar softening of the mi­ nute surface configuration, as though a layer of semi-liquid matter had been overspread, and such I believe to be the fact; the deposit has obliterated the smaller craters and partially filled some of the larger. These and allied facts, taken to­ gether, indicate that a collision of exceptional importance occurred in the Mare Imbrium, and that one of its results was the violent disper sion in all directions of a deluge of material— solid, pasty, and liquid.. . .

In addition to the ’’sculpture, ” Gilbert described (pp. 279-280):

... a series of gigantic furrows. In general direction they are remarkably straight, but their Sides and bottoms, with a sin­ gle exception, are jagged, abounding in acute salients and re­ entrants.

o ® o <6 ® © e @ o @ ® o o © © o o jg o e @ ® © o e o e e o ;© o p o o o © o o o © o o o e o o ® ® <9 ® ® ® o o o e It was my first idea that the furrows are the. tracks left by solid moonlets whose orbits at the instant of collision were nearly tangent to the surface of the moon, and for some of them I have still no better explanation to suggest; but when they came to be platted on a chart of the moon* s face it was found that more than half of them accord in direction with the trend lines of the Imbrian outrush.. . .

Gilberts terms are not used here because they are regarded as oversimplified. The Alpine Valley and the Rheita Valley, though classed together as "furrows, " show differences as great as those between the

Alpine Valley and some "sculpture" structures.

It appears that Gilbert may be credited with the discovery of the Imbrium radial system (of. also Shoemaker, 1962; Urey, 1962a) and with the most complete study of it until the last two decades. 5

Edward Suess (1899) supposed that during the solidification of

a molten surface a crust was subjected to considerable fracturing^ cre­

ating linear structures® He discussed the Alpine Valley as follows (pa

,:3’9S, translated):

One might first think it to be a graben structure between two linear faults; it is not that..... The steepness of the linear walls, the. even floor, the regular decrease in width, and the sudden offset allow the supposition that the whole solidified . crust, similar to a huge cake of ice, has been cracked diago­ nally from the W and that both blocks ar e displaced horizon­ tally from one another®

In 1919 Steavenson, apparently independently, proposed two

categories of "linear depressions ®n The "clefts" were irregular, and

common in the low-lying districts and maria® Steayenson implied that

in some cases they are causally associated with nearby crater s® The

"furrowswere broad (usually 2-6 miles.wide), shallow, and "confined,

with very few exceptions, to one region," from the Haemus- Mountains

to Ptolemaeus® His "clefts, " usually referred to now as rules, are in­

deed distinct from the radial structures discussed in the present paper.

His "furrows" are part of Gilbert’s "sculpture" and make up a portion

of the radial system discussed here® While they are most prominent in

the r egion noted, other examples will be shown below® Steavenson made

no mention of a conver gence of his "furrows" toward Imbrium® He

stated:

v „ .it may fairly be: taken that, that origin, whatever it be, is - a common one, for their great similarity in breadth and gen­ eral appearance and their close parallelism seem to demand 6

this. , The only Question is ■whether they -were produced by internal (ie e. volcanic or eeismic) forces or by external agenciesj, such as the grazing impact of meteoric bodies,

Steayenson here juxtaposed the two basic mechanisms which have subsequently been considered for the origin of the radial structures.

Be was inclined to favor a meteoritic origin for his "furrows, n though be was "not an advocate of the meteoric theory as accounting for the craters^"

Fielder; (1961a) and Urey (1962a) refer to additional papers of the early 19O0ls describing the Imbrium radial structur es.

Spurr (1945a? b? 1948) wrote at length On the Imbrium systemo

Be proposed an igneous origin for all the major lunar features^ and his wor k/has been somewhat neglected in recent studies, which emphasize meteoritic processes. Nonetheless, Spurr pointed out many interesting structures and discussed both the Imbrium radial pattern (and included a. map) and the "grids'"' (a term he introduced) which haye recently aroused wide interest. Be wrote (1945b, p, 15):

The ^transverse' faults of the Imbrian system, radiating from the interior of the Mare Imbrium, are extended and identifiable for a long distance away from the mare® . They do not appear on the various mares which lie beyond the Mare Imbrium, and are therefore older than the crust of all of these,. as they are than that of the Mare Imbrium itself® This indicates that all the mar es ar e (but only approximately) of the same age and period® But the faults do appear in the older up­ lands in which these mares lie, far to the south, beyond the equator of the visible moonl. abundantly as far as the .Mare Nectaris,... and more sparingly as far as... the south end of the Mar e Nubium... = This amounts to an extent of a thousand miles, or more than, the diameter ■ of the Mare Imbr ium 7

itiS'elfo.». In. all this field they are plainly .marked by scarps^ ridges^ or grooves,. . . if the teetoaic influence of the Mare Imbrium uplift and subsidence extends as far in all directions as it does southerly^ it must affect a circle with a diameter of nearly .f1?§0§ miles 1.609 selenocentric 1 o‘».

Spurr throughout advocates tectoniti mechanisms to explain the radial system, a proposal for’which the' wr iter ■ finds - considerable' sup­ port (though he'questions Spurrrs. ’’uplift and subsidence” origin for

Mare Imbrium)B The observations here and in Hartmann and Kuiper

■ (1962) concur With Spurr?s relatively uniform ages for the mare Sur­ faces, but indicate more varied ages for the basin systems themselves^

. Baldwin has given considerable •attention to the ■Imbrium radial

System® Considering the elongated structures in the Haemus Mountains, he wrote (1942, p0 366):

Thus their natur e, location, and orientation mark them as a unique type- and are overpowering evidence of a. common Origin® Formations like, these certainly are not volcanic in nature® They Could only have been formed by the almost tangential impacts of tremendous masses of material- -..

He listed four types of radial structure (1943, p. 117):

1 In.Mare Vaporum,.. at least a dozen elongated craters# from thirty to fifty miles in length and perhaps ten m iles broad® They are nearly# but not exactly parallel and their major axes projected in the convergent direction all pass close.to the. center of Mare Imbrium-«,. ■ 2 the strange ap­ pearance of the southern and southwestern rim of Mare - jSerenitatis-... Apparently.. . mutilated by flying material - ® ®from. -. Imbrium- .-•* * 3 near Ptolemaeus grooves.ap­ proximately three miles broad and- from twenty :to seventy miles long. - -. which may be regarded as more elongated versions of the Mare VUporum splash craters# - - • 4 in the Carpathian district a group- of . r idges . .. r oughly twenty m iles long and thr ee m iles wide® 8 These four types are all parts of Gilberts ’’sculpture®n Clear-

,lys the various classification schemes proposed show considerable dif­ ferences; but Baldwin’ s description establishes the nonhomogeneity among the radial features® In his book The Face of the Moon (194% p0

208) Baldwin concluded: ’’This system of valleys is simply an exaggera­ tion of the Structur e surrounding so many of the newer-appearing craters such as Ar istillus and must haye been pr oduced in the same way® ”

Baldwin thus supported his ar gument that the crater s and the mare basins are both products of explosive impacts® His impact hy­ pothesis of basin formation appears to provide the best explanation of the basins® However^ our comparisons between the radial patterns around craters and those of the mare basins indicate that the quoted statement is oversimplified® Baldwin in his recent book The Measur e of the Moon (1963) appears to have maintained his views®

Urey (19 §2) supported the impact hypothesis for basin formation and the flying-fragmemt hypothesis for the origin of the radial features®

He wrote (p® 40):

„.. I wish to suggest that a large object striking near Sinus Jridum at a low angle partially rebounded from the surface, Spreading its substance through approximately 180° between the western Carpathians and the eastern border of Plato® Immediately beyond the point of contact it dropped materials into Mare Imbrium? whose lavas were still to cover its floor. JUst beyond the shores of the mare it dr opped the Alps, Caucasus, Apennines, and Carpathian Mountains in radiating ridges, and in the r egion of the Haemus Mountains, Mare Vaporum, and Mare Nubium further long and short ridges® It also supplied the objects which ploughed out the grooves at still greater distances® 9

These radial patterns are of two distinct kinds and must have been made by materials of different physical properties. The straight narrow grooves suggest materials of high density and tensile strength such as ir on-nickel alloy plowing thr otigh the surface8 while the ridges suggest- silicates of low tensile strength® The Alpine Valley also indicates that part of the meteoric body contained a hlgh-denslty body embedded in the silicate, which because of its greater momentum and energy per unit volume moved on through the region in: which the silicates spread themselves into Mare Frigorls, sinking in . its lavas, or ■ being.subsequently covered by them.

Concerning the mare materials, he concluded (1962a, p9 482) that "o o „ lavas resulted, from.high-energy collisions®.. and are not the result of subsurface melting as is true for the Earth, ”

Urey has maintained these views (1961, 1962a, b)s . They are similar to those expressed here and by Hartmann and Kuiper (1962) as regards the origin of the Imbrium basin (as opposed to Spurr, Firsoff, and von Bulow; see below), but differ in the role of Sinus tridum, the origin of the mere material, and the nature of the radial Structures^

Kuiper has discussed the Imbrium System at length® In 1954

(po 1104) he wrote of isolated ridges west of. Copernicus and others near the center of the disk:

... both groups look like jets of lava or rather lava-covered bits of old crust (or planetesimal) which have been shot from the same splashing center in.Mare Imbrium® Related, but in a different category, are the many grooves cut in the lunar surface, particularly in high regions like old crater walls, and radiating also from Mare Imbrium® They are U-shaped in cross-section and ar e rarely more than five diameters long; the only exceptions I have observed are extremely shah . low grooves, wher e the ratio may approach, ten® It is. natural to assume that they also have been caused by flying fragments.... , 10 For. the Alpine. Valley Kuiper suggested the explanation (pe

IIOS): . - ”«.. that the crust surrounding the impact area in Mare Imbrium split open wher e the valley is now, that; the inter ior lava rose to the level of the present floor, and that the entrance was filled with large solid blocks of crust tossed out of Mare Imbrium, Covered with some lara.. . , "

In 1959 he attributed to Mare Imbrium (p@ 310) rta global Sys­ tem of faults and graben" in whieh were included such features as

»o. the radial and orthogonal fracture system in the north polar ar ea, north and E of Mare Frigoris, several of which became fault planes (and others produced extru­ sions);. . v the Aristarchus Uplift,.. . many graben in the Ptolemaeus area, near the center of the lunar disk (which have sometimes been elassif ied err oneously as grooves. Cut by flying fragments; some genuine grooves occur there as well);.. .-the.Rheita Valley, a beautiful example of a graben;... the Straight wall..

The difficulties of identifying many of these individual features with the Imbrium system are discussed below® With respect to their

Shapes and the tendency for craters to lie along them, the structures called ”graben" show some differences from the features on Earth to which this term is applied. In the sense that the term implies some tectonic origin, it. is compatible with the conclusions reached heres The

. 19#% paper by ■ Kuiper - and the-writer summarizes their views on the nature of the basins themselves,

. Within, the ;last 9 years several discussions: of radial structures have appear ed in the Journal of the British Astronomical Association, FieMer (1955, 1956, 1957) published a series of papers which are sum­

marized in Ms book (Fielder, 1961a)0 He. discussed the grooves near

Piolemaeus and' concluded that blocklike masses along many of them

may be the original projectiles wMch carved them, mentioning a '’clear­

ly defined association between blocks and grooves® n Many of Fielderfs

’’blocks" are described here as ridges, and no unique origin is ascribed

to them® Inan examination of the mechanics involved in penetration of

the flying fragments through the crust. Fielder concluded that this mech­

anism is "wholly plausible®" Fielder’s writings properly stress the

difficulties in any final interpretation of the radial structures®

Firsoff (1956a, b) rejeets the meteoritic hypothesis for basin

formation, preferring a subsidence origin® He thus rejects not only the

flying-fragment theory, but also the existence of radial systems alto­

gether. Summarizing his views, Firsoff (1961) writes (p® 45)° ",. „ the

Apennine valleys are not radial to the supposed focus of explosion in

..Sinus;Iridum fjjrey* S hypothesis] ® In fact they have no common radiant

o o, and consist of several roughly parallel swarms which belong to the

grid systems®""

Firsoff thus related the linear structures discussed here to a

mor e general gr id system of the lunar globe and discussed their origin

in terms of shear stresses resulting from compression of the surface.

However, while it is true that lineaments;exist independent of basins,

there are also genuine radial systems structurally related to various 12 basins® Some Of FirsoffVs deductions may be criticized,, , For example, in his (1956b and 1961) chart of the "Apennine valleys^?' the extrapola­ tion of the long' Ar iadaeus Kiile Is misplaced (its extrapolation , passes near the south, edge ofEratosthenes^ as seen on Plates 3 and 12^ while Firsoff showsit well to the north)® An approximate convergence of a great number of structural features may definitely be established by in­ spection of lunar photographs such as those given below,,

VonBulow (1957) also advocated an internal origin for the ba­ sins,, , What we consider to be a radial system around Imbrium he de­ scribes- as part of a more general grid system®. He believes that the basins lie in subsidence zones r a longitudinal zone north of the; equator ? and.meridional .zones: of basins and uplands, extending.south from It,. He writes (p® 606s translated); n, . - The uplands are dominated by two grid lattices^ which stretch across the meridional zones at sharp angles NE~

SW and NWs.SE® .Departures: appear especially in the central upland spur . . o., n ■

A third north-south pattern is ;:also proposed,. Von.Bulow at­ tributes the linear grid structures to extensive lunar tectonic activity* e, go g faulting, fracturing of the uplands^ and production of graben, domes, crater chains, etc,, which accompanied the production of the maria. The present writer agrees that extensive tectonic activity prob­ ably occurred, but does not concur with the suggestion of a significant alignment of the basins into zones. 13'

Shoemaker (1962)? who examined the ballistics of crater for­

mation, also reviewed the Imtorlum systems He wrote (p$ 349):

...... Trenches? ridges, and scarps in this pattern tend to be approximately aligned along a series of great circles that intersect in the northern part of the Mare lmbrium0 The ridges and troughs of the ..Carpathians are layered over by rocks of the Imbrian System and plunge beneath the Pr ©cel­ lar ian [a division in his stratigraphic sequence] along the northern margin of Mare Imbrium* Individual ridges that rise above the level of the surrounding Imbrian ejecta are the prominent features of the.Carpathians® It is possible that some displacement has occurred along some of these ridges since the Imbrian system was deposited, but there is no apparent displacement of the Procellarian where it over­ laps the ridges and valleys along a series of promontories and bays that constitute the mountain front.... Aside from the fact that the trajectories and strength, of the ejecta re- quired for the plowing of such furrows are improbable, offsets in the walls of the troughs and ridges show that they are more likely to be fault scarpse It appears highly prob­ able that the Imbrian sculptur e is the topographic expr es­ sion of a. radiating ■set of normal .faults that were formed during dilation of the lunar crust by divergent flow behind the shock front generated by the impact that produced the Imbrian ejecta^ The ridges of the Carpathians are thus interpreted as horsts; probably they were scarcely formed before they were partially buried by ejecta. The present writer believes that the set of radiating faults pos­ tulated by Shoemaker is a prime influence in the production of the radial system*. Ths'cenvergent point? , described as being in the northern part of the mare? can be considered the center of the structural basin, as

discussed below and by Hartmann and Euiper (1962)0

The divergent opinions quoted here suggest that a final solution

may have to await surface exploration, though obviously considerable 14 progress is still possible from E:arth-based studies,. Such studies at the same time will prepare for surface exploration^

= Morphology

Associated with the Imbr ium basin is a r ing of isolated peaks

.on.; the'mare surface^ forming sl circle; approximately 0'7© km in- diara- eter0 A surrounding arc of mountains is nearly continuous for 16502- concentric with .the inner' ring, and .approximately 12 ;S40. km. in:diameter®

Several upland areas between these rings define a subsidiary ring about

970 km across®. All of these features find analogues in other basin sys-

. < terns (Hartmann and Kuiper, 1902, pp® 04-05).

The inner ring is often described as being off center® It is true that it does not lie in the center of the mare surface® However, Kuiper

(1959) and Hartmann and Kuiper (1902) have suggested that because of its concentricity with the. C.auc asus-Apennine - Carpathian, arc and in view of the presence of numerous other concentric-ring systems, the inner

/ring,in .Mare. Imbrium may be regarded as defining the center of a' .vast2 structural basin system with concentric mountainous arcs and radial features® The mare itself is considered as a surface phenomenon; it is the mare material which is offcenter in the Imbrium system rather than the ring structur e®

. Baldwin (1942),. Arthur (1902a'), and others have pointed out that although the observed r adial f eatur es do not converge to a point. their projections do intersect within the inner mountain ring of the

imbrium basin0 This ig further evidence that the mountain arcs* not

the - mare material, reveal the fundamental basin-•structure® Rectified

photographs from above the Imbrium Inner Basin assist in defining the

center of the radial system® Plates 1 and.2 are examples, and show

especially well the region from the HaemuS Mountains to Ptolemaeus8

which contains the most prominent radial pattern observable from the

.Earth'(of® also.,Pis® 22® 14.and 21®.!#')® , Plate'S adds a more nearly

vertical view® Just beyond Hipparchus (PL 2) are two valleys whose

projections pass on opposite sides of the center of the Imbrium ring

system, but within the Inner Basin® /These pr ojections inter sect in

Sinus Medii (see PI. 12.25)® Similarly, the Straight Wall, seen in the

upper right of Plate 2? is Somewhat out of alignment with the center of

the Imbr ium Inner Basin® One might thus ask: How far out of alignment

may a linear feature be and yet be considered part of a radial family ?

CleariyP. a. radial .System cannot be defined solely by the direc­

tional properties of its members® Sometimes a parallelism of the fea­

tures, r ather than Strict r adialism, is seen in localized ar eas® This

also applies to the rays of Tycho (see PI® 1 ® 2), though no one has ques­

tioned their association with the crater ® A system is made apparent by

the presence, of an unexpectedly large number of individual structures

aligned with the center of a basin system® These components, range in

dimension from at least tens, of kilometer s to the limit of r esolution (hundreds of. meters)* . Observers with large instruments have .noted that under the best conditions the surface in the midst of these systems is marked, by innumerable^ fine linear structures. Further, it is shown below that a variety of structural fornas composes these system s and that similar forms are found in association with more than one basin.

Thus,, alignment, number of features, and Structural form are all cri­ teria in def ining these systems; and while the projections of the mem­ bers do not meet at one point, they typically intersect within the inner­ most rings of the basin system s. Thus, as Urey (1961) states, "It is not clear whether some of the individual ridges and grooves belong to the System or not, but the overall pattern is entirely convincing . n

.An. example of such .an. individual structure is the . Straight Wall®

The evidence that the Straight Wall belongs to the Imbrium system in­ cludes not only the near alignment of this linear structure, but also the presence, of a. Similar formation nearly aligned® The-Straight Wall, and this second feature, the Cauchy scarp, are shown in Plates 4 and 5 to the same scale.. Their probable association with the Imbr ium system was first pointed out by Kuiper (1959, p® 293).. The-similarity in form and size (length 106+ km) is enhanced by the presence of a parallel rille in each.case. Further, each scarp lies near the edge of a mare sur­ face; the foot of each is on the mare side; each has domes nearby; they are nearly equidistant from Imbrium® Arthur (1962a) has called atten­ tion, to a rille. running along the foot of the Cauchy scarp; a similar, but less prominent, rille has been .reported along the Straight Wall by some observers* although the evidence for the latter is less certain (Whitaker*

1963)o Subsidiary graben along the bases of many terrestrial normal fault searpsare undoubtedly analogous,, The. Cauchy fault also shows en echelon structure and forking, characteristic of many terrestrial nor­ mal faults (de.Sitter* 1956, pa, I51)e

, The: Straight Wall lies nearly on a diameter of anvancient, 20®- - km ring. This ring has been flooded* and its west wall is now shown only as an ar c of wrinkle r idges in the mar e. An ar c of mountains, to the southeast and wrinkle ridges to the west suggest a similar ring for the Cauchy scarpo These faults most probably are post-mare* originat­ ing not long after the mar e surface was laid down, while ther e was still considerable tectonic activity. We might suppose that they f ormed along stress lines set up in the submare surface by the Imbrium impact, lienee the association of these faults with the. Imbrium radial family would be consistent with their appar ent age and would explain their di­ rection. K this be accepted we have a case where faulting* not grooving by projectiles, is part of a.radial system.

The remainder of this section describes the Imbrium struc­ tures* starting with the Haemus Mountains and following around the ba­ sin in a clockwise direction. Reference may be made to. Plates 1* 2, and 40 for orientation purposes. 18

Plates 6 and 7 show the- Haemus Mountains in detail. They form the southerB wall of Mare Serenitatis (to the left), but their linear structure shows no symmetric relationship to the Serenitatis basin.

Bather, each linear component is radial to the inner Imbrium basin.

Because of this and because of the difficulty of tracing mountain walls on the other side of the Serenitatis basin, we conclude that the Serenitatis basin is older than the Imbrium system.

The two plates illustrate several characteristics of the radial system :

(a) The crater Auwers and several others west (lower right) of Menelaus have remarkably rectangular outlines. Often the wall which would lie on the side toward the parent basin is missing. A sequence exists from normal, round craters to these box-like.objects, indicating various stages of modification.

(b) There is a tendency for individual mountain masses to show a central cleavage. Some such cases are marked E in Plate 6. They have at times been considered gashes left by flying fragments, but a faulting mechanism is considered here. The well-known dome H of

Menelaus on the. Serenitatis plain shows a similar cleavage (Pis. 6 and

7) not attributable to a flying fragment. Urey (e. g. , 1902a) has argued that had there ever been extensive melting in subsurface layers, the mountains would have been unstable and subsidence would have occurred.

It is suggested that the Haemus Mountains are to be interpreted in just 19 this way* There is strong evidence (Hartmann and Kuiper* 1962) that most of the basin structures were created well before the formation of the maria, and we may assume that the Haemus Mountains pre-date the melting in this area. Therefore, the fissuring of these mountains may have been caused by subsidence, with the direction of the faults attrib­ uted to fractures generated by the Xmbrium impact.

(c) Individual ridges roughly 8 km by 3 km are also present.

They share in the alignment of the Imbrium radial family, but are dif­ ficult to explain by either gouging or tectonic mechanisms. However, we see much evidence that by some process, probably involving melting in Subsurface layers, mountain masses in flooded regions are broken up. Near major basins the fragmentation is along radial lines. These ridges are therefore likely to be the remains of more imposing pre- mare structures. Alternatively, the ridges could be extrusions along radial fractures.

Baldwin (1963, p. 323) states that the Haemus Mountains "have been almost obliterated by countless valleys, great and small, which have been ripped through this range.... It can only mean that the val­ leys were dug by m issiles from the collision zone." The writer believes that the term "valleys’' is inadequate to describe all the radial structure in the Haemus Mountains, and Questions this general interpretation.

It has been stated that the grooves cut the higher areas and skip the lower areas, indicating that projectiles carved them (e. g ., 20 SteavensoiL, 1919, and Baldwin, 1963, p, 323)® However, examination of, e. g., Plates 6 and 7 shows that, instead, the Imbrium radial strue- tnro in the ’'upland" regions is present at all levels® The smoother areas are avoided, and while it is true that these tend to be at lower levels, it seems clear that the distinction between areas with and without radial structure is hot one of elevation, but of surface type® Not all of these smooth areas are the .dark mare material®, Radial gr ooves of the

Imbrium system are never found in the maria, although a few radial ridges protrude above their surfaces® The maria must be the result of flooding, and their surfaces must have formed after the radial system itself® However, there are relationships between the mare surfaces and the radial structures that suggest that both are the product of a high- temperature stage of lunhr surface history®

These concepts will be used and verified in the discussion of

Some of the r emaining plates®

Plates • 8-1 O ' show the- Apennine. region®, Some ridges in, these mountains exhibit radial structure® The pattern is most prominent along the shores of Mare vaporum where the uplands have been most affected by flooding® The plates show extensive flooding in the Haernus Moun­ tains and reveal several upland areas which are darker than even the maria®, Some radial f eatures are seen inside the Apennine r ing (bottom)®

An example is the rectangular depression flooded With dark material north of Gonone Plates 12s 24=12® 27 also show the radial pattern in this area0

Plates 11 and 12 cover the upward adjacent region,, Various classes of radial features are r epresented here® •. Nearly half the floor of Julius Caesar (left-center on PL 11) is dark, flooded^, and smooth; the other half is light* higher* and grooved parallel to the Imbrium family® Elongated* flooded depressions* similar, to; those north of

Conon* are found south of Manilius (left-bottom on PL 12); especially r emarkable is the r ectangular end of the depr ession near est Manilius®

In the center r e g io n of the Haemus Mountains is an elongated trough roughly 30 km by 110 km (lower left-hand corner of Pis® 11 and 12;

Pis® 6 and 7)* and along the northeast (left) side of Julius Caesar lies a similar formation roughly 17 km by 85 km®

Troughs of this sort usually contain craterform segments®

The northwest branch of the Hyginus Rille (lower-right* PL 12) illus­ trates that crater chains can develop along rilles® Also* visual obser­ vation of the Ariadaeus Rille (above center)* similar in form to the southeast part Of the Hyginus Rille* establishes that it is a graben

(Kuiper* 1959; Fielder* 1960)® Therefore* we have evidence that crater like objects up to 5 km (or 11 km if one includes Hyginus itself) can form in chains along graben-producing faults® Shoemaker (1962* pp® 290* 301-303) suggests that these craters are analogous to terrestrial maars, "opened by piecemeal spalling and slumping of the walls of a 22.

volcanic vent , nand that they r each diameters of 15 km in lunar chainss

The larger j troughlike features: with their crater form segments may he

r elated to these crater chains? Discussing the Stadius chain? Baldwin

(196,3j. pe 378) .states? "These craters were: formed-along an existing

crack, and the eruptions which formed them probably were primarily

gas venting or gas explosions," While this seems the best explanation

of the eraterform segments, the scarcity" of crater chains lu.fhe radial

systems remains puzzling, as does the appearance, instead, of large

troughs. The crater chain in the northwest part of the Hyginus Eille is

exceptional in sharing with the Imbritim alignment,

. High- resolution photographs with low lighting (e0 g„,. Mte Wilson

124) show that the r ille on the east side of Hyginus is continued nearly to

. the highlands in the southwest part of these plates as a fine en echelon

series. The Ariadaeus and Hyginus Rilles define a direction which is

different from the Imbrium system and is not radial to any nearby basin.

The cause of alignment in this direction is unknown.

Fielder (19 61b) has measured the distortion (one m in u s the

ratio, of. the axeS parallel and normal, to the ''grid .system"' axis) of craters

in this region. He finds (p, 3) that "the craters are distorted preferen­

tially, with their longer axes in the direction of the most prominent

I, e,Im brium family of the grid system ,and that '"Segregation of the

craters.,. in accordance with their age shows, that,.. the mean percent­

age distortion of the old craters is considerably greater than that of the young crater s®n No dependence of distortion on size was found® The

two age groups: were, defined jby ",. . characteristics such as ease of

recognition of a crater and the height, or degree of erosion^ of its

wails; but not by considering the degree of distortion of its walls®"

F ielderv- belie ves (pp®. f - f ) n». .that the .oldest craters haye been'de­

formed so greatly from the circular shape that the deviations from ra­

dial symmetry cannot be explained by tensions alone® It is suggested

that the deformations arose as a. consequence of thrust faulting®,r How­

ever, characteristics of certain recent craters and "the rilles—recent

graben features— M lead him to suggest a .more, recent tension® . He; Con­

clude st "All the observations may be explained by supposing that the

crust of the Moon was in compression when the oldest visible craters

were, formed and that, more recently, the stresses reversed in sign,

to become tensions®n

In the writer’s opinion these mechanisms are improbable®

Most of the linearity in this area Fielder takes to be the result of an

early compressiono This implies that either (a) the radial structure of

this area is independent of Imbrium, or -.(b) Imbrium formed before the

expansion began and is a very old feature® Either of these alternatives

is difficult to accept® Fielderfs distortion measures may be criticized®

. For-.example, a.chart of Ms 134,craters shows that the most prominent

elongated depr ession south of Manilius and the trough along the north­

east of Julius: Caesar (already■ discussed) were divided into three' craters .24

eacho We have noted the craterform segments here, but it is difficult

to accept the inclusion of £uch ncraterss ” Fielder's statistics may be

■ interpreted as semiquantitative' proof that the- pre-Imbrium,. pre-mare

features have been deformed along lines radial to Imbrium while the

post-lmbrium features are seen in approximately their initial state®

The evidence for compression is considered inconclusive®

. Returning to Plate 1%. we note: that partially ■destroyed craters

show a relationship to the radial system (e® g®, just northwest of Julius

Caesar)® The front and rear walls* with respect to Imbrium* have van­

ished* leaving the sides standing as radial ridges® Although Beer and

-Madler clearly described this phenomenon in l837? there has been little

comment on it recently® In their words (Beer and Madler? 1837* p®

250)* the cr ater s in the area between the Haemus Mountains and the

. Ariadaeus Rille are ’’bordered by walls only on the particular sides

which coincide with the general lineation® ”

Further light on the relation between crater walls and radial

systems is shed by Plates 13-15* showing the area around Ptolemaeus®

, (An over lay outlining the r adial structures is added in PL 21®) The

walls of nearly all of the craters bordering the south edge of Sinus Medii

are broken Up into linear r idges aligned with the Imbrium system®

Reaumur and Oppolzer present the best examples® One must assume

that these were once ordinary craters* pre-dating both the mare flooding

and the f or mation of the Imbr ium radial System® All that r emains of 25

the wall of Oppolzer, whicli lies on the mare surface^ are a few ridges,

predominantly aligned with Embriumo The north Wall of Reaumur* also

lying .’toward the mare5 though ..not in .a. completely flooded area. (PL 1-4),

is simiiarly broken into ridges ronghly 9 km by 4 km, each aligned to-

ward Jmbriums The south wall of Reaumur, on the uplands? is rela­

tively undamagedo Similar structure is found in other craters,, S.go,

the large ring west (right) of Ptolemaeuss Further, the east and west

. walls of Ptolemaeus show linearities': aligned with the r adial patterno It

is not just the cr ests of these walls which are damaged; they have been

cut Clear down to the level of the mare®

In summary, the walls of pre-mare craters lying on or near

mare surfaces tend to be br oken up into linear segments which are -

aligned with structures in nearby radial systems® The amount of frac­

turing is correlated with the degree of local flooding, whereas the align­

ment of the f ractur es relates to the nearby large basins® These cor »

relations would be unexpected and are unexplained by the hypothesis of

flying: fragments®

Incidentally, Plate 13 shows west of Ehaeticus a groovelike

structure not radial to Imbrium® It is marked by a dotted line on Plate

21 g where it is seen to parallel a local, non-Imbr ium grid pattern® It

is so sim ilar to certain Imbrium grooves; (e® g®, the one which joins it

at its south end) that one -may.ascribe a . similar origin to it® -However ,

one cannot attribute it to a flying fragment unless one assumes that the 26

fragment Game from Serenitatis6 This and other parallel structures ia

this:regiea form part of a, "grid" system marked by dotted lines on Plates

20 and 2L The relation: of this System to Mare Serenitatis will be fur­

ther investigated,;,

., The Southern extension of the area just discussed is shown in

Plates 16-19 ; Plate 20 adds an ower lay map of the r egion® The areas of

Plates 13-19 are known for a large number of grooves or troughlike fea­

tures, similar to those pointed out near Julius Caesar® These are prob­

ably the most discussed of the radial structures and have been considered'

the strongest evidence for the grooving action of flying fragments®

Fielder (1961a) not only suggested "a clearly defined association between

blocks and grooves, " but concluded that "». i there is a marked tendency

for the blocks to be elongated and to be so oriented that their major

axes are generally roughly Coincident with the axes of their associated

grooves ®n We have already noted the tendency for the pre-mare struc­

ture to be broken ,into radial ridges, and -the elongated blocks may be of

such origin® It may be noted on Plates 13-15 that the often-discussed

groove near Herschel, as. well as some others, has more than one ridge

on its floor® ,.. The Sculpture of such ridgesfrom existing terrain, would

require multiple impacts of flying fragments along the same groove®

Similarly, the groove running for some 230 km from near Lalande to

Alphonsus is r esolved into nearly parallel segments and would seem to

. require multiple Impacts along a very narrow azimuth interval radial ■ .27 to Irabriunie Baldwin'.(1963? pe 325) in fact assumes this;: "It is the

result of at least ten... impacts by m issiles... which were ejected on

almost identical paths,,n The high-lighting views of Plates 14 and IB

reveal that.the walls of these grooves are frequently' bright; but the

floors are generally darker ® . If the valleys were carved by flying frag®

mentSy we might expect the walls and floors to be covered by bright^

pulverized material; as is true of recent craters on the maria; al­

though it could be argued that these valleys are analogous to many pre-

mare craters in being dark floored,, On the other hand, many valleys

have raised rimS, ruling out a simple subsidence origin® A comparison

of photographs at varying illumination (cfe Pis® 16 and 17) confirms the

earlier statement that the radial Structures occur at all levels in the

rough uplands, but that other surface types lack them®

Urey (1962b, p® 134) believes that the Imbrium collision should

■ have produced, many '"narrow and closely spaced" fissures in, this area,

and Shoemaker (1962; p® 349) speaks of radiating faults® , The writer

suggests that much of the radial structure has been produced by sub­

sidence and/or volcanic action along such fissures® However, the im­

pact hypothesis presuppeses that material was hurled froni the Imbrium

basin Center, and consequently. Secondary impact structures must ex-

ish The q u e s t i o n is not whether flying fragments existed, but whether

they are responsible for the Varying types of radial structures with

widths from a few to tens of kilometers® The secondary impact craters around recent craters'such ..as Gcpefmicas: are typically slightly oval pits, not grooveSo Perhaps the most likely candidates to be the sec­ ondary impacts of Imbrium ar e small pits about 1 km across with grooves trailing: out of them,, away from Imbriuma These are not seen on the plates here reproduced, but are shown in the USAF-NASA lunar charts (1963; Oo go, LAC-7 7, near Albategnius)*

, Plates' .13-19 show the ridge structure in Alphonsus to be aligned with the Imbrium system* This is another case where align­ ment alone does not prove a simple or dir ect causal relationshipa None­ theless, this deformation of the floor of Alphonsus may be related to the s tr e s s field surrounding Imbrium®

Compar ison of the high- and low-lighting views of the r egion around Ptolemaeus points out that the customary division of the lunar surface into bright uplands and; dark maria is incomplete*, Great ex­ panses of the surface, e® g®, the area of Hipparchus, the floor of

Ptolemaeus, and the region of Flammarlon and north of it, are smooth with a Slightly higher density of crater pits than the mare surf aces®

They ar e usually depressed with respect to the rough uplands and have many "ghost craters®fr They are bright under high illumination and form an intermediate-type surface between, the rough, bright uplands and the smooth, dar k mar ia® Existing theor ies do not clearly explain the form and distribution of these ay ease One might suggest that during

. some stage of lunar history parts of. the crust melted to various, degrees. 29 causing a leveling of relief, and that later many of these areas were flooded by the dark material. This suggestion, of extensive melting as a separate process from flooding, differs from a concept equating melt­ ing with an immediate inundation by dark.lavas. Alternatively, these smooth areas may be covered by ignimbrites resulting from ash flows, as discussed by OfKeefe and Cameron (1962). The area from the

Haemus Mountains to Ptolemaeus contains much surface area of this intermediate type. The suggestion that much of the radial structure in this area was produced by subsidence and breakup of a wide area of stressed crust with associated volcanism during a high-temperature stage is consistent with the above observations. This also would ac­ count for the distribution of the dark mare material in the circular ba­ sins. The darker material was probably produced at greater depth and reached the surface only in areas where the surface layers had been severely damaged, namely, in the large impact basins and their con­ centric subsidence zones.

In the northern Mare Nubium, to the west of Ptolemaeus, lies a region containing some of the most interesting examples of ridges and deformed craters as parts of a radial system. Plates 22-25 show this region, continuing the scale of the preceding plates. The low-lighting views in the first two plates reveal a great amount of low relief, even in the mare. The relatively Smooth but bright surfaces near Fra May.ro

(cf. Pis. 23 and 24) are examples of the surface type noted above. A Study of the relief near the apparently isolated ridges east and northeast

of Fra .Maur o is instructive® For example^ one r idge complex in the

mare*. Lalande n (Flo 23)* is continued by a curved:wrinkle.ridge* giv­

ing the appearance of a crater whose east:wall has been almost com­

pletely destroyedo This is a further case of a partially destroyed crater

whose walls ar e broken down pr ef er entially with r espect to the r adial

System® Another' Clear: example^ is Parry Ms • Her e the front w all (facing

Imbr ium) is missing* the back wall is br oken* but the side walls are in­

tact despite the fact that the mare abuts closely on the east (PL 24)0

. There" can:.be little • doubt that, this was. once an ordinary crater ; , the fact

that its north wall cannot now be traced testifies to the presence of some

agent remarkably effeetiye in reducing relief in localized areas® Again*

Fra '.Mauro has suffered .damage mostly on the east side*, which is near­

est to the mare surface® Many craters in the Sinus Medii area (Flam-

marion* Sommering* etco) show similar structure® Similar evidence

was cited by Fielder (ISila* p® 183) as evidence against modification

by flying fragments® The. resemblance between .some of these disturbed

crater walls and the isolated ridges suggests that these ridges are the

remains of more complex mountain or crater structures which have

been almost completely destroyed^ . They ar e thus further • evidence that

a process of crustal br eakup has left Individual blocks aligned with near­

by radial systems® The northern walls of Hipparchus* Reaumur* Op-

polzer* Flammarion* ■ and Parry M may Show this breakup in. sequence® ■ S-i

The lower lighting views (Pis, 23 and 25) reveal that the floors of Fra

Mauro and its southern neighbor Bonpland are lined by both rilles and ridges, which:-are aligned'with, the radial system. The structure re­

sembles the floor of Julius Caesar (cf. Pis, II.and 12), The same plates show that the mountains north of Fra Mauro are scored by nu­ merous valleys with similar alignment,

• Plates 26 and. 27 show the Carpathian .Mountains* with-the scale remaining the same (ef, also PL If, 19)', These mountains extend the

Apennine arc, but differ from it in being less massive and in showing more pr onounced radial ridges. This is evident fr om a compar ison of

Plates 10 and 27, Baldwin (1949, p, 212) writes of this area:

.,. west of Mare Imbr ium the crust sank considerably. It is even possible that the magma load caused so great an ad­ justment to occur relatively quickly that the western moun­ tain border disappeared beneath the surface except for the scattered Harbinger peaks® The steady westward dip of the Carpathians and the mountains on the northwest of Mare Imbrium support this view....

The pronounced appearance of the r adial r idges in the western

Carpathians, coupled with our association of such ridges in the Haemus

Mountains with crustal breakup, supports in a general way Baldwin's view, Urey$s suggestion that these ridges are masses thrown out from the impact site does not explain the very similar aligned ridges forming the walls of Oppolzer, Reaumur, Parry M, and other craters® A tec­ tonic process of crustal breakup and/or extrusion is more suitable. It

is significant that the part of the Imbrium: outer arc which .shows this . ridge structure most plainly is that very part where' the mountain arc

...dwindle.® out into flooded maria^. a correlation: already noted above con»

cerning the Haemus Mountains,

Farther out from this region^ near the southwest shore of

. Sceanus. Procellarum, 'tee additional structures', which .appear to be aligned with the Imbr ium system. These are shown in Plates 28 and

296 Several ridges are seen northwest (right) of the crater Hansteen

(central on PL 28)g including two which form a boxlike valley typical Of other radial structures discussed earlier® The larger, flooded crater to the northwest shows the characteristic pattern of a missing front wall , (downward) and aligned side walls® As was pointed out by Kuiper

(1 9 5 9 ) 5, the Sirsalis Rille, lying on the opposite side of the basin from tie :Alpine..Talley (PL 40), Is sligned'with.the Imbrium system® Kuiper has suggested (po 304) that the rille ", ...is therefore a major structural feature of the Imbrium impact,... „ ,f and that both features were gen­ erated during a nonvertical Imbrium impact by a horizontal thrust com- ponent toward the Apennines® He points out, however, that this puts the. Sirsalis..Bille. "in. a class by Itself®"

Features similar to those of the Hansteen area, can be found in a similar region further to the north, on the west shore of Oceanus

.Procellarum® The region near' O 0 Struve-is Shown.-in. P late: 'SO® . A linear ridge forming the south wall of O® Struve and a linear valley in the north wall are seen to be aligned with the Imbrium basin® , Similar ridges and valleys are found in the up lands southwest of here,, where the peak marked- P displays: a: Split appearance' sim ilar.io,examples: noted in the-

Haemus Mountainso , These shoreline r egions of Hansteen and -O, Struwe are further examples: of radial structures lying close to mare surfaces«,

- Plate: i l is a rectified view of the. outer' western part of the -

Imhrium system 0 The radiating ridges of the Carpathians ahd similar ridges near Kepler are shown well, and the ring of peaks marking the

Inner Basin is marked. It is interesting to note that several of the elongated blocks of. the ring are oriented parallel to the ring,, not radial to ito .Among: the peaks near Belisle, the uplands near; Sinus..Iridum, and the Harbinger Mountains, little radial structure is seem The sig­ nificance of this is not clear ; it may be that any Imbrium stress pat­ tern in the. tegion of Sinus.:Irtdum was altered-by the formation of that bay. A significant problem is the reason for the differences in form between the - C aucasus, Apennines Carpathian* and Har binger Mountains, which constitute what appears to be a single structural arc Of the Im­ brium system ® Monisoiropic stresses from a nonvertical impact may be. involved.

A prominent featur e of Plate is what Kuiper has called the

’Aristarchus Uplift. ” In 19§9 he-wrote (pp. 296, 2.99):

. o. The Uplift is seen,as a .diamond-shaped'area, that is:Slight­ ly yellowish in color—which is entir ely exceptional on the

Moon. . 0 0 This area itself has clearly been uplifted and cracked into several large blocks*, some of which have been left in a tilted position. „.. ©n the border one finds an isolated ridge (locally composed of three parallel ridges),,, the struc­ tural lines of the Ar istar chus uplift are radial and orthogonal to the. Jmbrium. Center. It is therefore very probable that the Uplift occurred as a result of the Imbrium impact .,,,

Plates 31 and 32 confirm an approximate alignment of the plateau edges

.. with the. Imbrium system* Plates. 33-35 show rectified views under

various lightingSo There is no doubt that the area is elevated relative

to the mare. In view of the previous discussion it seems uncertain

Whether the area has been raised or the surrounding crust has subsided

during a flooding which produced Oceanus Procell arum. Schroter’S

valley may be analogous to the large graben which frequently cut across

major terrestrial uplifts. In either case it is likely that the rectangular

outline is associated with the Imbrium fracture pattern. Evidence in support of thiSg. besides alignments is the ridge structure along the

north edge. Examination of the plates reveals that this ridge forms the

north side of a f looded trough about 34 km by 170 km$ strikingly similar

to others in the r adial system, e, g0, that through the central Haemus

Mountains (about 30 km by 110 km) shown in Plate 12 at the same scale,

Plate 31 also reveals that one of the ridges (marked R in PL 34) near

the middle of this trough and a small mountain just west of Her odotus

Show a split appearance as found among the HaemuS peaks, and that

several parallel valleys about 3-1/2 km wide cut the northern plateau

surface. Some ridges on the southern half of the plateau, near

Herodotus, appear to share the alignment. What appears to be an aligned 35

|3 lock“fauited, region: .nearly- TO km widejtist north of Aristarchus, is also noteworthy^ The Aristarchus plateau eould be interpreted as an upland mass which, while not flooded, is bounded by subsided, flooded regions and shows signs of block faulting and other radial structure along stress lines created by the imbrium impact. The peculiar surface color and the tone, darker' than .most upland -regions, remain unexplained®

The similarity of many structures in Plates 28-35, consider­ ably west of Imbrium, to .those of •earlier plates in the east and south is evidence of the structural unity of the radial System.

The northern reaches of the Imbrium system are shown in

Plates. 36 and ST. This region is marked by the curious upland arm containing the Alps, Plato, and Sinus Iridum (see also Pis. 1 and 40).

The opening remarks of this section leaye uncertain the nature of this upland arm, which is only roughly Concentric with the Imbrium Inner

Basin, and which lies in part in the expected position of an intermoun­ tain zone (PL It® 2.4)® . One might consider the arm made up of three parts: the Alps, which are part of an intermediate, raised arc of the

Imbrium structural system and analogous to those of other basin sys­ tems (Hartmann and Kulper, 1962); a raised area around the post-lm- brlum, pre-flooding Plato impact; and a raised area around the post-

Imbrium, pre-flooding; Sinus. Iridum impact® . We thus suppose that the

Plato and Iridum impacts are principally responsible for the relief in their neighbor hoods. They altered the stress fields set up by the - Imbriura impact and therefore locally altered the symmetric pattern of

Subsidence around the Embrium system 0 The relative lack of tectonical-

ly pr oduc ed Imbf ium r adial structur e in the Plato-and Ir idum uplands is

accounted for if these uplands were molded chiefly by post-Imbrium

fbrees®.. :

In addition to the unusual Plato-lr ldum arm we note the absence

of a mountainous scarp along the northern projection of the Carpathian-

Apennine-Caucasus arc (see . PI® 1.2® 2:4)s The northern bor der of Mare

Frigoris may/correspond to this e-xpected relief® ; At any rate@ as the

plates show® it is north of this border that the Imbr ium r adial system again assumes full prominence® These pictures take us close to the

limb where a streakiness due to elevation differences is introduced by

the. rectification process; this must not be confused with true radial

structure® One of the most interesting structures related to the Im­

brium system is the large square formation, W® Bondr well shown in

Plates 36 and 37® It gives perhaps the most striking demonstration of

. the. alignment of walls of lar ge: craters^. a situation alr eady cited in

Ptolemaeus® ,-Further? it is bounded on the east and west by two remark­

able valleys, about 16 km by 90 km and 28 km by 140 km 2 respectively®

The valleys themselves contribute to the rectangular appearance and

show striking similarity to the large troughs bordering the Aristarchus

Uplift, in the - HaemuS Mountains, and along the Wall of Julius Caesar®

Local curvature of the valley walls in several places produces strong resemblances to craters some 2 0 km acrosse A prominent exampie of the latter forms the north end of the valley east of Bond® The cr ooked shape, the highland mass closihg the Imbrium end, the craterform seg­ ments, and symmetry with the valley on the other side of Bond indicate that this valley is not a groove carved by a flying fragment® The crater­ form segments may therefore indicate nonimpact craters (maars?) of

np to 2 0 km in diameter on the moon®

The entire region of W® Bond is Scored by Imbrinm radial ridges and valleys. Several examples.of craters with the "front'- wall missing are. found on the border between the mar e and upland; one case on a promontory southwest of Bond bears some resemblance to Op- polzer. Plates 30. and 37 show a tendency for the linear structure to exhibit a local parallelism while forming part of a larger radial sys­ tem .

Plate 38 shows the region of J. Her schel, lying to the west of the preceding field. The crater South, identified on the plate, is another remarkable example of a large, damaged "crater" with linear, aligned walls. One may also note the angular outline of J® Herschel, showing alignment with Jmbrium and resembling Ptolemaeus, and the valleys in the surrounding plateau uplands® The striking, similarity of radial struc­ tures in these uplands to those near Ptolemaeus' (of. PL 13) testifies to the structur al unity of the Imbrium radial system. The alignment of linear walls in Ptolemaeus, J. Herschel, South, V . Bond, and many 38

smaller examples contradicts Baldwin’s statement (1963,, • p 0 427) that:

’The polygonalism of many lunar craters is a normal i« e, s intrinsic

aspect and. is not a part ofthe lunar' grid system® ”

Plate 39 shows aligned structures in and near Mare Tranquil-

litatiso Certain features in the uplands are aligned with.the- Imhrium

System and are sim ilar to those noted before® On the mare surface,

however, we see a prominent family of parallel lineations, including

the Cauchy fault and at least .three major, and numerous smaller,

wrinkle ridges® . Rilles in the area exhibit a similar parallelism® This

may be seen near Cauchy and Sabine and is clearly marked in the neigh­

boring Mare Fecunditatis on the chart of rilles by Arthur (1962b)® The ridges may be the- surface expressions: of flooded forrnations under the

mare surface. (This is clearly the case with the flooded crater Lament.)

These structures should be classed either as a local parallel grouping

of the Imbrium system or as part of an independent grid system.

Some other regions not shown here may be mentioned® High-

resolution views of the Taurus .Mountains show many linear features

Which are roughly aligned with Imbrium (Arthur, Whitaker, private com­

munications; cfo also MU Wilson 80)o . Palus Somni shows similar fine

features; some coarser ones canbe seen on some plates accompanying

the Crisium discussion® The.Rhelta. Valley and neighboring linear struc­

tures, often considered to be associated with the Imbrium system, ap­

pear in later plates Showing the Nectaris region. A synthesis of the- linear' features described in this paper is given in Plate 40, The rectified photograph was taken at a distance equivalent to about IQ, 500 km above the center of the Imbrium basin®

'Each.red line represents K Structural- feature and indicates the length and direction of each® This representation should provide a truer pic­ ture of the mature of the system than some earlier charts where lines wer e used that were much too long.. Plate 40 is intended as a summary of the distribution and r elative sizes of the Imbrium radial structures rather than a detailed map showing; every known example.

. Conclusions

There is unquestionably a major system of diver se linear structures radiating from the lmbrium basin. Several Structural types may be listed. In the unf looded uplands, many valleys r oughly 10 by

100km def ine a pr ominent r adial pattern (e. ge, regions of Ptolemaeus and Jo Herschel). Wider, aligned troughs roughly 30 by 120km fr e ­ quently show eraterform segments (Haemus Mountains, north border of

Aristarchus plateau). Walls of polygonal craters (Ptolemaeus, J.

Herschel) and craterlike formations of remarkably rectangular outline

(Auwer s, W. Bond, South) are typically aligned: with .these systems. .In areas characterized both by major mountain masses and extensive flood­ ing, the mountain blocks typically take the form of radially oriented ridges of lengths some 5: to 10km (Haemus, Carpathian .Mountains). 40

Motmtain blocks in such regions often show a split or grooved appear­

ance (Haeraus. Mountains^ north border of A ristarchus plateau); Crater walls lying in proximity to a r e a s of extensive - flooding are often made

up of parallel, ridgelike segments aligned With radial systems

OReaumur, Oppolzer)® In many craters, usually close to areas which

show f looding, the wall toward the basin is missing or damaged; some­

times only side walls remain as ridges radial to the basin (Barry M,

Lalande ^ : ) 61

. The following obseryations are-of special importance r. (1)

Some of the diyerse structures, especially the ridges, cannot be the

r esult of gougingo (2) There is some agent capable of drastically re­

ducing surface relief, as noted in Parry M and Beaumur; (3) The dark

regions described here as "flooded" appear to have a spatial relation to

radial structureo (4) The elongated ridges in the mountain arcs around

-Imbrium,, especially the. Carpathians,. ar e similar- to- those forming the

walls of craters such as.Reaumur* The conclusion that they are of

similar origin contradicts the hypothesis that such ridges are masses

thrown into place by an explosion. (5) Many valleys and troughs show

craterform segmentso ( 6 ) The Imbrium radial family overlies the

majority of craters present and is therefore younger than they are,

though older than the, mare surfaces.

The above observations are, consistent with the hypotheses that

the processes which reduced relief and produced the flooding had a single cause., namely a heating and partial melting at depth after most of the

craters and basins had been formed; and that a major impact at this time produced fractures along which crustal breakup, subsidence, vol-

canism, apd possible extrusion caused most of the Imbrium radial sys­ tem* In analogy with secondary pits near recent craters, there must be numer ous secondar y cr ater lets pr oduced by Imbr ium fr agments among the .field craters,

. Differences in the.types of radial structure in different regions may be attributed to differences in the effects of heating and to varia­ tions, in pre-existing surface structure* Baldwin (1942) pointed out that

the ridges are on the average closer to the basin than the other struc-

tureSe This follows at once if ridges were produced during the breakup

of the mountainous arcs forming the boundaries of the basin* THE OR.IEMTA.XiE .SYSTEM

Morphology

Plates 41. thr ough 46 show the Or ientale System^ When the sunrise terminator lies near longitude 80° We (good sunset pictures are lacking at the narrow crescent phase)* an extraordinary family of linear features is seen northwest of War gentim This aspect is shown in Plate

410 Rectified photography at once reveals the convergence of the linear features. . Plate.46a*. centered over Or ientale'*, reveals the concentric scarp system and confirms that the convergence is toward the Or ientale inner hasina , It is clear that the Or ientale . basin, sec ond only to the Im- brium basin in the magnitude of its concentric ring system (Hartmann and Kuiper* 1962), is also second only to Imbrium in the prominence and extent of its radial system* Plates 41 and 42* with low lighting, demonstrate the great extent of the Orientale radial family toward

Wargentin.

; Oomparlson of Plates 42 (low lighting) and 43 (high lighting) shows, as . did Plates 14 and 18 with the Imbr ium System* that ther e is no prominent ray system radial to Orientale* and that the radial system is structural* not surficiaL Plate 43 incidentally reveals the small

42 43 mare at the center of the basin system*,

Ho radial structure is found inside the Eichsiadt ring* Outside this ring, running to the southeast, are several shallow valleys^ some

30 km acrosso These, valleys merge to the south with arcs of the south- east limb basin.system (Hartmann'and Kuiperj 1902, p, 5%. PI* 12* 45)* hi this same region, and out as far as Inghirami, the mountainous up­ lands haye a pronounced radial and concentric pattern, nearly an or­ thogonal pattern® Some of the radial structures, such as those running toward the northeast rim of highirami (best shown in Pis* 41 and 42), are clearly neither narrow valleys nor mountain ridges, but rather scarps, probably bounding broad depressions,^ The whole region gives the impression of a crustal mass fractured along nearly orthogonal lines®

Farther out, beyond Inghirami and near Bailly, are curious structures formed by 10 to 20 km craters with narrower valleys running southeast out of them away from Orientale* They are well se e n in Plate

42, and some may be seen under the very low lighting and high resolu­ tion of Plate 44* ; The valleys are on the order of 10 km across and have

slightly raised rims (PI* 44)® On the floor of Schickard may be seen

some further examples which appear to be buried by the mare material

in that crater.

. To. the. east and'northeast of the Orientale basin the radial struc­

ture is not nearly so pronounced* Hone is prominent in the region,due east of Mare Orientale® In Plate; 45, showing the northeastern regions from above Grimaldi, we find examples just outside the Eichstadt ring®

It is r emarkable that so many of these lineaments show in spite of being

oriented nearly perpendicular to the terminator® Here again the rough uplands" show -a nearly: orthogonal, pattern symmetric. with .Orlentale®

Locally, for example, northeast of Schluter, the pattern resembles a

Cartesian grid rather than a radial fan, although the relationship to

Orientale is clear from the average orientatione

Just east-northeast of Sc Muter is a striking pair of grooves, typical of radial structure^ but not aligned with .Orientale® These and

several other lineaments are much more closely aligned with Imbrlum

(PL 46b) and may be outlying member s of that system® All prominent lineaments in the regions so far considered appear to be attributable to either Orientale or Imbrium0

The radial structure of the Orientale basin system is sum­ marized- in am.overlay on Plate:46b=,. Comparison of the Orientale. Sys­ tem with the Imbriura System shows the Orientale radial structures to be profuse as far as 48° selenoeentric south- southeast of the basin, while the comparable figure for the Imbr lum System is at least 80°

selenoeentr ic® Thus the ratio of the sizes of the radial systems is roughly 0* 6a . The. Orientale structure begins just outside the. Eichstadt

ring, which is therefore analogous.to Imbrium?s Apennine ring®, - The

ratio of diameters of these rings is rOugMy 0o 7« We may therefore 45

consider the Orientale basin radial and concentric system to be two-

thirds the size of the Imbrium System6

Imbrium is flooded up to and beyond its Apennine ring (diameter

I, 340 km), but Or ientale is flooded only inside its innermost ring (di­

ameter 320 km) and in arcuate patches along the base of the Eichstadt

r ing and its inner companion# .

, There is also a difference in. the/f orm of the major radial struc­ tures^ The well-known Imbr ium gashes? char acteristic of the Ptolemaeus

r egion, ar e rare in the Orientale. Systems The valleys near Schickard

are their closest analogue® The Imbrium structures which I have as­

sociated with flooding, ea go P broken crater walls, split mountain m asses,

mountain ridges, etc0, are absent here® On the other hand, the local

orthogonal patterns present in Orientale are not so characteristic of

Imbriumo

The Or ientale: System shows mor e asymmetry in azimuth than the Imbrium Systemo It has often been .stated that the Imbrium radial

system is highly asymmetric, it. being concentrated in the sector from

the Haemus Mountains to Ptolemaeus^ However, this, is the only well-

observed region of old upland surface close to Imbrium, and radial

systems are well seen only on such surfaces* Other upland regions

around Imbrium are Small, or near the north limb® Plate 40 shows

that some radial structure is found on uplands in all directions from

Imbrium, although the greatest density is indeed in the Ptolemaeus 46 jdireetioia® The:Imbr:ium system' is. tjiss, only sligM ly asymmetric* . In the ease of Qr:ientale9. on the other handj the hasin is bordered by up­ lands on all visible sides* and it is clear that the radial structure is asymmetric and highly concentrated toward the southeast,,

Ba interpreting these photographs one must remember the dis­ tortions inherent in r ectification, hi any depr ession near the west limb* the west inner wall is seen but the east inner wall is mostly hidden; thus the r ectif ied photos do not r e v e a l the whole interior a

. Conclusions

The; Grientale radial and concentric system is second only to that of the Imbr ium basin* but shows signif icant diff er ences in appear- uncoo H the various lunar basins were all caused by basically similar processes* the Question arises as to how one accounts for differences between the basin structures. The most fruitful hypothesis may be that the basins were formed at different stages of lunar evolution. It is as­ sumed that the basins were caused by impacts, I have summarized evidence that, the; Imbrium impact occurred near' a period of maximum surface heatings when the crust was already subject to subsidence into softer subsurface layers, and when the dark mare material had its maximum accessibility to the Surface. , We can suppose, then, that

Orientale is more recent. It has already been pointed out (Hartmann and Kuiper, 1962) that the Orientale basin is a relatively recent feature® 47

Itg 'ring system has; a fresh .appearance^ Its radial system is superim­

posed on the southeast limb basin, and cuts the floors of Schickard and

'Baillyo; Further, the distribution of mare material is explained if the

subsurface was already cooling and material could reach the surface from depth only in the highly disturbed center and along concentric

scarps® The Orientale.basin is therefore supposed to have originated

late in the period of mare formation®

The orthogonal appearance of the. unflooded uplands toward

Inghirami appears to be due to block faulting® It is a different pattern from the orthogonality noted in parts of the Imbrium System, which was related to modification during flooding® It would be nearly impossible to explain either this orthogonality Or the structural differences with

Imbrium by the hypothesis of sculptur e by projectiles from a central

explosion® The hypothesis of tectonic activity allows a greater range

in structures® We can suggest that the Tate mar e crust of the Grientale region was less plastic than the late pre-mare crust near the Imbrium

impactj and that therefore the Grientale System shows more of a shatter pattern than the Imbrium System®

The craters with trailing valleys (shown in Plates. 42 and 44), farther away from Mare. Grientale, may be impact sites of flying frag­

ments® They are similar to som e very small Imbrium valleys, shown

on USAF-NASA chart LAC-77.(1903)® It is clear from the Imbrium

discussion that the larger radial valleys tend to have craterform ; 48

segmentss but do not demonstrate such a systematic tendency to have

craters at one endo Nonradial valleys with craters at one end ar e seen

in Plate 63 d0

We have seen that the Orientale System is highly asymmetric

in azimuth. A frequent explanation for this phenomenon involves non-

isotropic stresses from a nonvertical Impact (ea ga 5 Urey; ■ In this case it suggests that the crust may have. been compressed and broken southeastward when a large body impacted in the present inner ring from the northwest at a low angle., Why# however,, is the concentric ring system so much more isotropic than the radial system ? Possibly the concentric ring system is due more to the isotropic shock waves from a central explosion^ while the radial structures are due more to

horizontal component of momentum imparted to the crust by the impact­

ing body. To produce the concentric arcs, Hartmann and Kuiper (1962, pp. 62-63) consider both gravitationally produced faulting and concentric fractures due to the impact and crustal compression from horizontal

momentum components* Alternative suggestions are Baldwin’s (1963,

p. 317), that the arcs are frozen shock waves, and Fielder’s (1963a),

that they are internally produced thr ough faulting not r elated to impacts* THE NEC TAR IS S YSTEM

Morphology

Plates 47 through 52 Show r egions near Mare Nectarise Plate

5% centered over Nectar isg; is the most useful for purposes of orienta­

tion in the f ollowing discussion®

In the arc from south to northwest of the Nectar is basin lies an

expanse of upland where one would expect to find radial structures anal­

ogous to those of the Imbrium and Orientale Systems® Howeverg no

Nectar is radial system can be found here in spite of the fact that the

concentric system finds its most prominent expression—the Altai

scarp— in this direction®

Nor is there any radial structure on the mare surface north­ west to northeast of Nectaris? as expected from the Imbrium discussion®

The small upland area near Capella and Isidorus* north of Nectariss

Shows no prominent Nectar is radial pattern® The single prominent val­

ley cuts thr ough Capella and is nearly aligned with Imbrium®

In the arc from east to south of the Nectar is basin Is found a

most remarkable family of linear valleys® These valleys are the Rheita

Valley (defined in the lAu catalogue of Blagg and- Mullers 193 5? to ex­

ten d from near Rheita to Voung)2 the narrower valley extending south

49 50 from Yoting through'Mallet^ called here the. Mallet Valley (sometimes incorrectly taken to be a part of the.Bheita Valley), and a long valley running through Snellius, called here the. Snellius Valley (PL 52). In the following paragraphs, some of the characteristics of three of the most prominent valleys are enumerated.

(a) The approximate dimensions are: Bheita Valley, 25 km by 330 km; Mallet. Valley, 12 km by 190 km; Snellius Valley, 23 km by

800 km.

(b) Each of these yalleys is broken by relatively recent craters

(e.g., Bheita) but each disturbs older craters (e,g., Young, whose walls and floor are broken).

(e) The larger valleys (Bheita Valley and Snellius Valley) ex­ hibit eraterforni segments. In the Bheita Valley these are separated by parallel transverse ridges, and the segments so defined do not resemble the normal craterlets of the surrounding uplands. The Snellius Valley most clearly shows the craterform segments and the western part strongly resembles a crater chain.

.(d). The three valleys have rims which are very slightly raised relative to the surroundings (e. g ., see Pis. 47, 50a, and 51).

(e) The interior of the Bheita Valley displays considerable de­ tail in the form of minor ridges and valleys (in addition to the transverse ridges) and the Snellius Valley is even more broken. The Mallet Valley appears to be more regular. (f) The Mallet Valley departs by about 12° from the direction

of the-Bheita Valleyo .Because of this and-the .differences in form* it

should not be assumed that these form a single structural unit0

..(g) The Snellius Valley displays en echelon pattern^ especially

elear at lts,east end (PL- #)*

(h) The Mallet Valley is quite accurately radial to the Nectar is

inner basin* but the R heita Valley is more nearly tangent to it0 The

. Snellius Valley' s sn echelon .pattern definesa parallel family whose

central member is approximately radial to the Nectar is inner basin®

In addition to these three major valleys there are a great num­

ber of smaller, ones* especially near the.Rheita, Valley* east of Janssen®

This area is well shown on the matching Plates 48 and 49 (sunrise and

sunset views* respectively)® The valleys converge approximately to­

ward-the center of the Neetaris basin® . Of the three larger valleys •dis­

cussed above* they most nearly r esemble the Mallet Valley® They do

not exhibit such clear crater form segments as the Rheita and Snellius

.Valleys® . They do have slightly raised rims® , They are. close to the

limb and difficult to interpret adequately* but bear some resemblance

to the Imbrium valleys near Ptolemaeus®

A pair of linear scarps, runs .from Janssen, toward the Neetaris

inner basin® They are well seen on Plates 48 and 49 where the east

side is r evealed to be the higher ® The eastern scarp is about 300 km

long; the western*, at least 120 ,km® The-north end of the .eastern scarp 52 terminates abruptly w h ere it touches the Altai ring just east of Pic- colomini. In its central parts can be seen a resemblance to a valley with craterform segments. Both scarps are nearly tangent to the inner basin (PL 49).

All the structures discussed so far lie outside the outer ring of the Nectaris basin system. The only prominent radial pattern to be found within the Altai, ring is best shown in Plate 47, where a radial trend can be detected in the mountain masses in the arc from south to east of the inner basin. This trend continues outside the Altai ring and it is in this same direction that the major yalleys are found still farther out.

The system of valleys and scarps described above is identified here as a Nectaris System because most of the structures converge to­ ward the Nectaris inner basin. The major valleys have been attributed by Gilbert (1893) and others to the Imbrium System. Plate 12, centered over Nectaris, also shows part of Imbrium1 s Apennine arc, and one sees that the valley system is more nearly symmetric to Nectaris. Nonethe­ less, the region is marked by many outlying members of the Imbrium

System. The degree of convergence of the Nectaris. System is not as great as in the Imbrium or Orientale Systems, but the rule still applies that the trends of the structures pass within or tangent to the inner ring of the basin system. This system, with.its many valleys, resembles the Imbrium System more closely than it does the Orientale System; yet ^ 53 it is analogous to the-Orientale-. System; in its nonuniform distribution in azimuth®

Comparing the extent of Nectaris and Imbrium Systems, we find that the Neetaris radial system can be traced as far as 42° selenocentric from the basin center, 53 percent the comparable distance in Imbrium^

The structures.begin at the Altai ring, which has a radius. S3 percent of the Comparable Apennihe ring of Imbriums Similarly^ the ratio of inner basin diameters is 0,60e Therefore, the Nectaris basin system can be considered slightly larger than half the size of the Imbrium Systems

Conclusions

Although the features discussed do not all show precise con­ vergence toward the center of the Nectaris inner basin, it is considered certain that they define a pattern associated with Nectaris.

The Nectaris radial System is considered to be of an Inter­ mediate pre-m are ages: This is -based.on the obser vation, that the sy s- • tern cuts and is cut by many craters. There is such a large number of superimposed craters that we conclude the Nectaris radial system is older than the Imbrium or -Or ientale. SystemSo The same is true of the concentric systems These results agree with independently assigned ages'based on.pr e-mare ■crater densities in the systems,. as: described by Hartmann and• Kuiper (1962, pp. 5.2-53). 54 Regar ding the f or mation of the valleys* we may rule out a simple splitting=apart of the crust* such as has been suggested for the

Alpine Valley of the Imbrium System (see the r eview of literature in the Imbrium System)0 This hypothesis would not account for (a) the circular outline of Young* not deformed though the Rheita Valley cuts its floor and walls* (b) the transverse ridges* (c) the slightly raised rims* and (d) craterform segments within these valleys^ There has ap­ parently been no horizontal motion in forming the valleyS»

Gouging by flying fragments does not satisfactorily explain (a) the transverse ridges* and (b) the craterform segments. Fielder (1961a* p0 184)* too* lists arguments against an external origin.

Some sort of tectonic, activity and subsidence along major faults more closely accords with the observations. The obser ved en echelon pattern of the Snellius Valley is a known characteristic of terrestrial fault structures, DeSitter (1956* p, 157) states that "normal faulting often shows an en echelon arrangement" and that among wrench faults

(p, 174)* "an en echelon arrangement is not common* but in general the wrench-iaults are accompanied by many smaller parallel faults of the

Same character,Although subsidence alone does not account for the raised rims* subsidence must be involved. The craterform segments do not conflict with this interpretation* for evidence was cited under morphology of the Imbrium System relating crater chains with graben- like features. Furthermore* various other authors (e, ga * Shoemaker* 55

1962, ppo 298-303, Rittmann, 1962, pp8 88-92, and Fielder, 1961a, pp0 210-216) have .discussed the development of lunar and terrestrial craters: and grabens. along deep-seated faults0 , The ;two scarps north of janssen may Mark faults along which full-fledged valleys never devel­ oped.

Because even the terrestrial examples: are not completely understood, it is not profitable to try to describe in further detail the exact processes which occurred along the hypothetical deep-seated lunar faults, except to refer to the geological literature on terrestrial faults, graben, etCc

. There .are fundamental differences in form among the various lunar r illes, crater" chains, graben, and radial valleys. The writer

Suggests that these are in part due to differences in depth of the under­ lying faults and in the accessibility of magma and gas to the surfaceo

These differences may trace in turn to the different epochs during which the particular basin systems formed, as suggested above. The evidence already cited that the Nectar is System formed before Imbrium and

Orientale would thus accord with the differences in form of these Sys- temSo .

The nonuniform distribution in azimuth of the radial structures in the uplands may requir e comments similar to those already made in the Orientale discussion. Nectaris shows evidence that the development of the radial and concentric systems occupied an extended time periods The inner basin itself is old and pre-mare* as evidenc ed by the many super imposed^ post-basin, pr e-mare craters, and the r adial valleys appear on similar grounds to be pre-mare^ However, the Altai scarp must have formed later and may even be post-mare, Judging by its fresher appearance and the relative lack of major craters along its length, Such a, differ­ ence in age and. appearance within a single ring system is inconsistent with Baldwin’s (1963, p« 31f ) interpr etation that the concentric rings are frozen shock waves® THE HUMORUM SYSTEM

Morphology

Plates 53 through 56 show the regiom of Mare Humorum, As expeetedj no radial patterti is yisible in the maria north and east of the

Humorum basin, although Plate 56 shows lineaments in the Riphaens

Mountains and neighboring peakss

Southeast of the basin, as shown in Plate 53, aligned r idges are foundo Study of the plate r eyeals a family of parallel structures, whose central portion is most prominent and on a line radial to Humorum®

There is a resemblance to the Imbrium System in the sense that many of the ridges in this partly flooded region mark portions of older damaged cr ater So

The same description applies south of the basinB The .paral­ lelism here is aligned in a different direction; from that of Plate 53, but again it is most prominent near the line radial to the basin#

Southwest of the basin is found a very striking orthogonal pat­ tern of ridges and broken-down craters# This is shown in the matching

Plates 54 and 55 (morning and evening, respectively)» Most of the struc­ tural lines ar e associated With old cr ater s which have been modified* and there is some localized flooding (cf# PL 43)® . In these respects the 57 region is similar to the- HaemuS.Mountains in the Imbrium System^ e. g .,

the square crater Auwers (Pis. 6 and 7)? and reminiscent of portions of

the Or ientale .System (PL 45). The orthogonal pattern is oriented so

that a line through its central part paralleling; one of the two pr edomi­ nant directions radiates from Humorum,

The phenomenon of local parallelism is well demonstrated by

the Humorum System. That is, each of the r egions cited in the Humorum

discussion exhibits not a strict radial pattern, but rather a more nearly

■parallel subfamily symmetric with the parent basin. The;-width of the

region of local parallelism is roughly equal to the diameter of the inner

basin and in the center of the r egion the direction is radial to the basin.

There are two predominant directions of tr end--northeast-southwest

. and ■northwest-southeast.

, Conclusions

The Humorum basin is accompanied by several families of

nearly parallel lineaments.. The members of each family are strongest

where the .direction is strictly radiaL Together 5 these families; define

a radial system and therefore are related to the basin. The structures

of this radial system exhibit both sim ilarities and differences with struc­

tures of previously discussed systems. The Humorum System exhibits

no major- valleysg but rather a system of ridges and' oriented, crater-wall

remnants in regions of local flooding. The Nectar is System^ on the 59

other handy consists primarily of valleys*, Imbrium shows both types

of structures.. The most prominent feature of the radial system is a

pattern. of near ly orthogonal faults, but flooding is very limited and with

it, the process of crater destructiono

We have already suggested that these differences may reflect

.. varying ages, of the basinse The Hu'morum basin is apparently of inter­

mediate pre-mare agej based on the cr iteria described earlier. In

view of the fact that the major valley systems of Imbrium and Nectar is

appear in the unf looded uplands while the ridge systems of Imbrium and

Humorum appear in partially flooded regions, it appears that the form

which the radial structure takes is also correlated with the degree of

local f looding.

, Fielder's reeent study" (1963b) of the grid system gives a dis­

cussion of Humorum. He lists a number of structures in the Humorum

System which must have been internally produced* including faults along

the edges, concentric rilles, and wrinkle ridges. In this respect Fielder

and the writer are hi close accord: the evidence indicates extensive

tectonic activity. But he continues (p. 83):

All these features are character istic of the rudely circular . type of lunar ■ mare. . The conclusion that Mar e-Humorum is a .sink, like Mare Imbrium, is inescapable®

The sink theory accounts for all the observed features much more naturally than the impact theory® „ o«other mar ia— in particular ? Mar e Nectar is 3 , which is also obviously an igneous sink—might have b een chosen to illustrate many of these pointSo

Fielder's argument apparently is. that if many structures inti­ mately associated with a basin are internally produced^ then the basin itself must have been internally produced® The writer has endeavored to show that, on the contrary, the coupling of the impact hypothesis of basin formation with .the. hypothesis of extensive tectonic activity allows explanation of the various observed features^ It provides for the radial fractures along which activity occurs; it also explains the varying time lags between basin formation and mare formation, which are not pre­ dicted by the-sink hypothesis® - . THE CRJSIUM SYSTEM

Morphology

Ie the vicinity of the Crisinm basin$ shown in Plates 57 through

59? one sees very clear examples of lineaments forming orthogonal pat­ terns® A plot of these reveals a tendency toward symmetry with the foa- sin$„ as seen in Plate 59«

The clearest examples are found northwest of the basin* toward

Posidonius* hi the depressed# partly flooded zone between the inner ring and the outer scarps# are seen ridges and rectilinear outlines of flooded depressions (PI® 57). The lineaments are more nearly parallel than radial# but the pattern is strongest where the common direction is radial to the basin# as has been found with other basins® Ther e is a re­ markable resemblance between, this area and the rectilinear structures of the depressed# partly flooded zone just inside the Apennine ring of the Imbrium System (Pis® 12® 26 and 12*27# and bottoms of Pis® 8-10)®

The same r ectilinear pattern can be traced on other sides of the basin# particular ly on the southwest wher e one of its dir ections is again radial® This is shown by Plate 58# where the southwest arc of the basin wall consists of a steplike pattern of lineaments® The overlay of

61 .62

Plate 59 maps the pattern# showing its presence also in the northeast*,

Additional lineaments: can be seen in. Plates 58 and 59= Many

. flooded depressions to. the •southeast are bounded-by straight sides: radial

to the Crisium center^ and other linear structures can he found in the,

uplands by careful study of various photographs^ not all reproduced

here* These are mapped in Plate I90 Hear the limb, the interpreta­

tion of structure is difficult, but ridges and scarps appear to be the

"most common®

, More than any other basin, Crisium exhibits local parallelism

of lineaments® West, of Crisium the predominant lineaments are not

radial but continue the orthogonal pattern which is more clearly seen

in the northwest and southwest® The Crisium basin resembles the

Humorum basin in the degree of local flooding, in the parallelism of

nearby lineaments, and in the absence of major valleys®

, Conclusions

In' accord with the .discussion of other basins it is concluded

that the lineament pattern near the Crisium basin is causally related

to it® Fielder in examining the grid system (1961a, p® 177) states that:

o ®. careful examinations reveal that near both Mare Humorum and Mare Crisium these localized ridges and valleys are specially placed components of a more gen­ eral family of parallel striations® . Clearly, some.com­ ponents of any family of parallel .lines which .intersect a circle will run along radii of the circle, and it would 63

seem that the two cases cited would be examples of this situation.

The present study shows that there are prominent lineament systems genetically related to neighboring basins | these are not chance juxtapositions of global grid system components and the circular basins.

The supporting evidence is (a) the indisputable presence of accurately radial systems around several major basins, e. g*, Imbrium and

Orientate> (b) the greater prominence of parallel families wher e the direction of parallelism coincides with the radial direction; (c) the close resemblance in- "fine structure” of some members of the Crisium and

Humorum families with members of the Imbrium System (cf. PL 65);

(d) the presence of local parallelism within the radial systems of craters

(see the section on radial systems around craters).

Yet the continuity of the orthogonal pattern even in places wher e it is not radially symmetric with a basin is evidence for the existence of a more widespread independent grid system, especially in the vicinity of the Crisium basin. Other influences, such as crustal stresses, must have caused such a wider grid system. The lines on Plate 59. indicate the direction to the center of the Imbrium basin system. The lineaments under review do not follow the Imbrium direction.

All these facts are consistent with the concepts (I) that the radial systems are the expressions of fractures created at the times of formation of the basins; (2) that the basins are of varying ages, mostly 64 pre-mare; (3) that oti these radial fractures were superimposed the fractures..of the grid.pattern; and (4) that virtually all the visible linea­ ment structure e v o lv e d along these fractures before and during the period of mare formation® OTHER BASINS.

The Mare HumboMtianum basin is seen in Plates 60 and 61.

. Accurately radial to the inner basing, and extending roughly 140 km be­

tween the inner and outer scarps toward the southwest is an unusnally

straight scarp# revealed by its shadow on Plate 61. Comparison of the

opposite-lighting views of 60a and 61 indicates a row of hills on this

line# with the scarp facing Southeast. Plate 60b shows patchy flooding

along the lineament. Our interpretation is that we see a fault scarp

with lava extruded along its base. This interpr etation was also given

to the Orientale scarps# along which one finds flooding (Hartmann and

Kuiper# 1962$ p. 57).

. Between HumboMtianum andCrisium are several linear features*

well shown in Plates 60a and 61.. The clearest example is a scarp facing

east# curiously smoothed# not sharp as the Straight Wall. It might be

termed a "ghost scarpF Its surroundings, are smooth but br ight, neither

rugged upland nor mare. Other minor structures# mostly scarps and

ridges in an area of partial flooding southwest of Humboldtianum, ap­

pear to form a radial patternj however, identification and interpretation

are difficult so close to the limb.

65 66

Grimaldi, Janssen, and. a basin . near Schiller are remaining mmltiring systems dismissed in Commnnlcations NOo .12. Hone of these

Show clear radial structure® . Judging by subsequent damage, we con­ clude they are: early pre-mare structures®

Other mar e basins which g|re little or no evidence of radial structure Include Serenitatis, Nubium, Smyfhii, Marginus, and the basin on the limb northwest of Bailley® Significantly, none Mt the latter shows well-defined concentrlc structure® All of these giye the impression of being relatively old basins, judging by sharpness of the walls and extent of subsequent, damage® , The term basin is applied be­ cause of the size of the circular bounding walls, which can be only par­ tially traced in most cases®

It is .seen that as one-' proceeds, from the-, youngest basins, dating. .from the mare epoch,. back thr ough the' older' pre-mar e .basins,

. the radial systems become harder.to trace, and generally become sub­ ordinate to weak, local grid patterns® To a lesser extent the concentric

scarps become harder to trace, .though they are' less clearly related to the local grid directions® Similar observations, apply as one proceeds from larger basins, through smaller basins to crater-si zed objects®

These observations ars. consistent With, the hypothesis that the radial structures' are. the surface ..expressions of faults which were,'subject to

tectonic activity before and '.during the mare'epoch® CSXD S YSTEMS

The term ’’grid” was apparently first applied by Spurr (1948), who used it to refer to a global system of fractures.. Spurr spoke pri­ marily of a polar-grid fracture system? most prominent near the poles and with nearly meridional member se He suggested symmetry with respect to the mobh’s equator and central meridiano Spurr (1948$ p.

103,) proposed that the polar-grids Were older than the Imbrium Sys­ tem and that they arose when the moon^S rotation became Synchronous with its. revolution^

. The concept of a grid system defined by lineaments has aroused wide interest iix the past 15 years. . Fielder (1961a# pp. 180-

195) gives an .excellent review of-Work which .has. been done#, and re­ produces charts by various authors® He; includes the polygonal shape

of some craters as part of the grid concept and traces such discussions back more than 50 years.

, There is some ambiguity in the use of the term "grid system."

Virtually every author who recognizes the validity of the term considers the systems to be tectonic# internally produced phenomena. Further#

it is usually implied that local gr id patter ns can be combined into one

global system which had its origin in a global phenomenon such as a 68

change in rotation rate, shift in polar position, contraction, or expan®

stone • Nevertheless, all the grld-syStem charts: reproduced by Fielder

(1961a) include the Imbrium radial system, which is definitely a local

system® The present paper shows that systems of lineaments accom­ pany other major basins® Therefore, if "grid system" is to refer tp a

single global pattern only, the. local radial systems should be excluded®

Baldwin (1963, p0 38:5) makes the additional criticism that

merely mapping linear formations is not sufficient for grid-system analysis because the resulting, charts will mix old and young features

of different types, such as wrinkle ridges, valleys, and sections of

crater walls® This mixing has in fact occurred, as can be seen from the charts reproduced by Fielder®

The following additional points are made regarding grid sys- temso

(a) The writer defines a "grid system" as any background pat­

tern of lineaments not clearly related to any individual basin® There­

fore, symmetric radial systems must be subtracted bef or e a study of a

grid system is made, although symmetry in density or direction of radial

structures may be related to grid systems® In fact, there appears to

be a preference for radial structures extending southeast from basin

centers. In these radial systems there is a tendency to favor southeast-

northwest strikes, and to a lesser extent Southwest-northeast strikes,

pr oducing a pattern not unlike that predicted by Vening Meinesz (1947) 69

to result from a change in the planetary rotation axis. This diagonal

preference around basins is probably related to the diagonal global

grid noted by Fielder (1963% pp. 72-75). ,

(b) The lineaments defining the grid systems are for the most

part less prominent than those in radial systems.

(c) Evidence has been found for localized manifestations of

grid systems around basins. We have already seen that in the older

basins the radial pattern is characterised by families of locally parallel

lineaments. The identity of these families is maintained even where

their dominant direction is nonradial. Second, there are lineament

families not radial to any nearby basins. An example is the well-known

lattice pattern near Arzachel mapped in Plates 20 and 21. This system

appears to be very old. Neither Nubium, Serenitatis, Tranquillitatis, nor Nectaris is in a symmetric position to this pattern, and it does not

increase in prominence toward any of these. A more unusual example

is the well-defined lineament pattern which includes the Ariadaeus Rille, the parallel branch of the Hyginus itille, and an en Echelon. extension of the Hyginus Rille westward. This family differs from the typical.linea-^ ment family in being relatively young.

(d) The local grids consist of structures similar to those form­

ing the older radial systems. Both classes of structures probably have tectonic origins. . RAB1&L .SYSTEMS .AR@UHD €EAT1BS

Many craters. display, radial patternSo . This is best seen around young: craters on the nearly featureless mare surfaces® , Here* one ob­ serves a coarse radial pattern in the hummocky* raised rim* grading into a pattern of fine ridges and valleys from a few kilometers width on down* onto the surr ounding mar e0 , It is well known that these sys­ tems exhibit a local parallelism similar to that descr ibed her e in the radial systems of basins (see Warner* 1961* pps 391-392)® Some ex­ amples are shown in Plates 62 and 63® Baldwin (1949* p. 208)* in proposing that the sequence from small crater pits to the large mare basins represents only a size range in a Single family of impact structures, described the radial systems of craters simply as r educed versions of the basin radial systems® The writer regards this description as valid only in a limited sense for the following reasons® The coarse radial structures in the rough rims of these fresh craters indeed resemble the coarse pattern observed in the Apennine rim of Imbrium and perhaps in the rim of the ©rientale basin® We have already identified these as the most recent large basins* and commented on the lack of flooding in the Apennines, and ©rientale region® Therefore*

TO /

71 this type of radial structure can be regarded as the immediate and un­ altered result of buckling of the crust on impact and emplacement of debris on the outer rim. Its presence in both well-preserved craters and basins supports Baldwins conclusion that craters and basins are of similar origin. However, the many other radial structures in large basin sys­ tems, e.g., the breakup of pre-existing mountainous areas broken into ridges, the damaged nearby craters, and troughlike structures, find no counterpart in the radial systems of fresh craters. Yet this is in ac­ cord with the ideas here presented because these structures have been attributed to tectonic processes along fractures related to the high- temperature mare epoch. The recent craters are post-mare, and furthermore it is likely that crater-sized objects are too small to pro­ vide suitable conditions for the growth of such tectonic structures. The most delicate lineaments, e.g., those which radiate at least 160 km from the center of Arlstillus, have no known counterpart in the basin systems because they would like in the rugged uplands where their fineness would prevent detection. Available photos lack sufficient resolution to prove whether these ripples represent fracturing or buckl­ ing of the crust or grooving by fragments blown out by impact. D, W. G. Arthur (1962a) favors the latter view on the basis of high resolution visual studies. Among other - differences between.craters and basins* the pref­ erence of the mare material for the large basins is easily explained if this material is lava? .by . saying :that the-lunar crust was most damaged at the. largest impact sites, allowing/easiest access to the surface,, The most puzzling diff er enc e is the presence of the .concentric r ings ar ound the basins, with.no trace.of. these among:either: the..recent craters with .radial structure or:some pre-mare craters:of basin dimensions^, To explain this, one. may have to appeal to differences inimpact velocity or possible differences. in the impacted crust® CONCLUSIONS AND SUMMARY

The primary result of this paper is the conclusion that a sys­ tem of radially oriented lineaments is an integral part of the typical mare basin .system0 The radial structure can be traced in some cases as far as seyen times the radius of the inner most ring of the concentric ring systems of these basins^ The concept of a basin system with radial and concentric structure has been heavily influenced by the best preserved and largest examples, Imbrium and Orientals^ The basins are of varying ages but the mare surfaces stem fr om mor e nearly one epoch. This has been well documented by Baldwin (1949, pp. 211-212), Shoemaker, Hackman, and Eggleton (1962, Table 2), Hartmann and Kuiper (1902, pp. 62, 65), and Baldwin (1963, pp. 304-309)o The basins can be placed in an age sequence by consider­ ation of: (1) sharpness of concentric walls,\ (2) density of post-basin, pre-mare craters, (3) extent of flooding. When the basins are con­ sidered in this sequence, it is found that the radial systems become less well defined with increasing age. The basins are tentatively classed by increasing age as follows: late-mare: Orientale; late pre­ mare or early-mar e: Imbr ium ; pr e-mar e : Humboldtianum, Nectar is, Crisium, Humdrum,, Ser enitatis, and the smaller basins. 74 Numerous observations of special interest have resulted from this survey of the radial systems: (1) No known radial system is post-mare® A few scattered individual features such as the Straight Wall and Cauchy scarp are prob­ ably post-mar e® ' r ...... (2) The bulk of the structures; cannot be accounted for by the grooving action of flying fragments® Most of the structures form near­ ly orthogonal patterns of ridges and. depressed, often .flooded, zones® Major valleys, which have been considered the prime evidence for grooving, are confined mostly to the upland parts Of the Imbrium and Nectar is systems, and their characteristics, e® g®, craterf or m •seg­ ments, transverse ridges and other structure on their floors, darkness of their floors, en echelon patterns (e® g®, Snellius Valley),, etc®, in­ dicate that not even they or iginated by gr ooving® , (3) No horizontal motion has been foimd in the study of the radial systems (cf® also Baldwin, 1963, p® 371, and others), and the Mheita Talley Shows definite evidence against horizontal displacement® Yertical motion must dominate® This differs from Fielder’s (1963b, p® 87) conclusion from a study of the grid system that "strike-slip faults are of even greater importance on the Moon than are normal faults®,r (4) The en echelon pattern of the Snellius Talley str ongly sug­ gests faulting® (5) Repeated evidence is found in various localized regions that the structures of the crust have been .modified in some cases to the extent of complete destruction^ Best evidence for this is found among structures of the Imbrium and Crisium Systemss Near mare Surfaces there is often a clear association of this phenomenon with the mare material^ but examples can also be found in unflooded upland regions* (6) Some radial systems show an asymmetric distribution in azimuth^ even after allowance for disappearance by flooding* This is most clearly seen in the uplands around Orientale and Nectaris* It is not so definitely known in the case of Imbrium, because of the extent of flooding; but there is probably a concentration toward the southeast* In both Orientale and Nectaris, the concentric system is more uniform in azimuth than the radial system* In the typical basin.system, most of the radial structure' lies beyond the outermost concentric ring* The pattern found within the concentric ring system is usually limited to flooded regions of sub-

sidence® (8)' A certain, unity exists: among the, different radial systems,

as may be seen by noting the similarity of radial lineaments in differ ent parts of the moono Plates. 64 and 65 give such a comparison at uniform scale* The form of the predominant lineaments may nonetheless vary from one basin .system to another* 76 (9) Independent of the radial systems,, there exists patterns of lineaments which define local ’’grid Systems,r# Study of these is beyond the scope of this paper, A consideration of grid systems as a global phenomenon must take into account the presence of independent radial systems around basins. Having listed the chief observations^ we now attempt to fit them Systematically into a hypothesis of the development of the lunar surface. On the basis of telescopic observations alone one is led to the conviction that there was a period of extensive tectonic activity during which many pre-existing structures were damaged and the mare material was de­ posited, Reference to theoretical work of the last decade suggests that this was the period of maximum radioactive heating at the surface. This period conveniently divides lunar history and is called here the mare epoch. The following model is believed consistent with all the observed facts. In the pre-mare period most of the craters formed. The basins

either are unusually large examples of these ordinary craters, or rep­ resent. a separate.class of objects. In support of the.first interpreta- tion, the distribution of crater diameters B with respect .to; cumulative' number of craters n is given by n = OB™-2 " 4 where c is a constant , (Hartmann-, in press), . Thus,. the basins may represent a tail of this distribution. It should be possible to check this quantitatively when a . larger sample of craters has been catalogued. In support of the latter interpretation are the differences in form between basins with their concentric and radial patterns and the ordinary craters® Both basins and craters are probably impact features® As each basin formed, it probably acquired some radial structure around its 'walls, analogous to that seen in r ecent crater rims® But just as importantly many r adiating fractures were formed® The direction and velocity of fall and the pres­ ence of any pre-existing stresses in the surface were probably impor­ tant factors in determining the distribution of these fractures® The nature of the surface, the presence of local stresses, the time interval from basin formation to maximum heating, and the degree of local heat­ ing were probably important factor s in the development of tectonic ad­ justments along these fractures® This .development" must have occurred throughout the interval between basin formation and the close of the mare epoch® This state­ ment is based on the following observations® (1) Coarse radial patterns are thought to be original features of the basin rims; (2) some struc­ tures appear to be pre-mare (e® g®, valleys in the Nectar is. System); (3) other structures in the same systems appear to be younger than those under (2) (e® g®, Altai scarp); (4) even in older basin systems there is a correlation between the degree of local flooding and the presence of certain types of radial structure (e® g®, partially destroyed craters in Imbrlum.and- Humorum.Bystems)s suggesting that these particular struc­ tures formed during the mare epoch; (5) the mare surfaces are 78 virtually never broken by post-mare radial structure® We have noted that radial systems of the older basins are less clearly defined than those of the younger® Not all the differences are due to erosion alone* and the following statement holds.approximatelys The more nearly coincident the basin-forming impact was with the mare epoch, the more pronounced was the resulting radial system,, The period leading into the mare epoch was the optimum period for tectonic activity, because (I) temperatures were maximal in subsurface layers, (2) the viscosity was correspondingly lowered in these layer s* (3) the surface was stressed due to the expansion prior to the mare epoch (MacDonald, 1961), (4) the surface should have been unstable if there was extensive melting below (Drey* 1955)0 The fir st two r easons a l­ lowed tectonic adjustments to occur ; the last two caused them to occur e The observations and theory are thus compatible® It appears observa- tionally that if the interval between basin formation and mare epoch was long, the likelihood of tectonic activity along radial lines during the mare epoch was lessenedo The origin of the grid systems and/or asymmetry and paral­ lelism in the. radial systems remains puzzling® These must relate to stress patterns not associated with the basin systems® The most impor - tant conclusion in this connection reached here is that the radial systems themselves are independent of the proposed global grid system except for possible local modifications of the for mer by the latter® This fact T9 is most clearly shown toy Orlentale and Imbrium« Therefore, linea- ments of a symmetric,. racUal pattern centered on basins should be dis­ regarded in discussions of lunar grids and global stresses^ Most such discussions to date have ignored this aspect; exceptions are Fielder’ s recent paper (1963b) and the work of Strom (in press). The author agrees with the oft-expressed view that the lunar lineaments are inter­ nally produced. During the mar e epoch magma r eached the surface in most of the highly disturbed basins and their fracture systems and in some other localized areas. Much damage was done to pre-existing surface Structures. In view of this hypothesis of an era of strong tectonic ac­ tivity, the following remarks by Dunbar (1963) on Precambrian activity on the Earth are of great interest. ... contemplate the 2,000,000 square miles of granite gneiss that floors the Canadian Shield, and... realize that it all came into place as fluid magma, which, congealed beneath a cover of older rocks now long since removed by erosion® The relatively small areas of sedimentary for­ mations that lie infolded among these batholiths, as rem­ nants of their former cover, convey the impression that during these primeval eras, the crust of the Earth was repeatedly broken and largely engulfed in upwellings of molten material that dwarf all post-Cambrian igneous ac­ tivity . It is doubtful that this represents a true mare epoch on Earth, but the analogy to the hypothesized lunar processes is obvious® The rate of Crater production during the mare epoch did not change much, during the period when the maria formed, as evidenced by the relative constancy of crater leii'Sitlesoh their-yarious .surfaces. (Shoemakerg, Hackman^ and Eggleton^ 1962)e The mare epoch must have occurred early in the moon's history on thef olio wing grounds. The moon is gen­ erally thought- to he at least 4, 5 hillion years, old (Euiper$, 1963; Urey? 1963)o In some models calculated by MacDonald (1961) and others* the temperature at a depth of 200 km reaches a rather flat maximum (and in the moon's center ? the melting point of ir on) in 1 to 2 billion years® But these models assume (X) heat transport by conduction and radiation^ (2) homogeneous distribution.of radioactive material, (3) a chondritic composition Heat transport by convection, concentration of radio- active material near the surface, and the presence of shortlived radio­ isotopes would considerably raise the near-surface temperatures and shorten the time till the mare epoch, (A mechanism for producing shortlived isotopes has recently been discussed by Fowler, Greenstein, and Hoyle, 1962.®) Therefore, it appears that the mare epoch occurred within the first 1-1/2 billion years of lunar history. Shoemaker, Hackman, and Eggleton (1962) propose that the crater density on the maria is consistent with.their age being 4 5 billion years and with a constant flux of meteorites in this period® At this early period, the moon was much closer to the earth, as is known from the theory of tidal evolution, and Baldwin (1963, p® 203) has argued that the elonga­ tion of the earthward axis may represent the tidal bulge frozen into the moon8 s outer layers at the close of the mare epoch with the moon at 81 ® 39 .its present distaBcea , Th^-’correctness of the latter two ideas would imply a mare epoch as long ago as the first «2 billion years of lunar history. Throughout the post-mare period^ the moon must have appeared substantially as it is now#except for the addition of some prominent re­ cent ray craters® The flux of incoming meteorites was much less than in .pre-mare time#. as evidenced, by the sparseness, of craters on the maria and on the earth® Tectonic activity was negligible in its effect upon large-scale surface structure and the moon was essentially dead. On the earth# we can trace only the most recent fraction of this period because of the continued erosional and tectonic activity of the earth# which has. erased its earliest Precambrian structures® Because of this continued activity the presently observed tectonic structure of the earth* s active crust may be fundamentally different from the dead# impact- scarred lunar surface, and the proposed similarity of the mare basins to ocean basins (von Billow# 1958, p. 36), or of faults in the Apennine arc to the San Andreas fault (Fielder, 1963, p® 87) is very question­ able® The history of the moon Sketched on these pages is schematical­ ly outlined in, Plate 66, which is largely self-explanatory® An attempt is made to correlate the three-phase terminology here with the five- phase stratigraphic nomenclature of Shoemaker (1962). Plate 67 schematically summarizes all the radial systems discussed here® APPENDIX I

FLAXES ILLUSTRATING LUNAR STRUCTURES

82

84 85

. J; ‘ YA ’

* ■- f

. < r i

■ V

#

88

AUWERS j f | ► 1 I S

ENELAUS

MANILIU 89 ¥ I

>

m

( k 4 III 90 91 92 93 ■m ih ^ ^ X T < - * A X ^ ? SL v - . v \ A : * 94

96 97

V

r i ■

- % *' ’ ' V " L ^ ^ - x w * - • V x 98 99

ifc

vt rTm > > v - - . ,v - X - tti .4y %P 100 INfc* 101

a ALBATEQNIUS ALBATEGNlUti

ALPHONSUS

PTOLEMAEUS

4ilPPAR^HUS.

son - / FLAMMARION

LALANDE 104 105 i

I

fP A R R Y M if BONPLAND > PARRY

/ I pj mWlLALANDE ' S | f RA MAURO v wMjmk'* LALANDE

k MM O ' f / • ♦ 1 SOMMERING! {

*

B 106 107 108 109 110

Y l > % I Vmersenius^

BILLY HANSTEEN

LETRONNE ■i I l l 112

CARDANUSi

10. STRUVE KRAFFT KEPLER

ARISTARCHUS

HARBINGER

CARPATHIAN

SINUS IRIDUM 114

ARISTARCHU

1R10UM 115 116

- v

HERODOTUS

^ ^ c h r 6t e r ^ , v ALLEY

ARI|TARCHU”

m

% C « 117 118 119 iv\

'i

BOND

MARE FRIGORIS

N f - * k * 4 ( a l p in e v a l l e y f v ,

i * ' • ’«: • 8 120

SOUTH HERSCHEL 121

CAUCHY I

LAMONT SABINE

ARAGO

PLINIUS 122 123

* s . ' -

' ' y rv , / > A

$ >

■ I f- 0 i *■ * 124 125 126 127 128

, ✓ TO IMBRIUM >

4

9 , 129 130 i ; &m mm

' 131

I

MALLET t YOUNG

RHEITA 5 STEVINUS

SNELLIUS

% l . 132

HUMBOLDT

HUMBOLDT

FURNERIUS

PETAVIUS

SNELLIUS 133

PETAVIUS

135

05 # s . W ‘-

-a t,.# < .J-* ■ v 1 136

138 139 140

mm-

y * f * j 0 j

" * ~ . Lrr 141 142

HUMBOLDTIANUM 143 144 145

\ . ' ' v ' r (b) |F # %

t

- '.'f'/v-m, 147

% $ p »: S v

' ' # %

^ W t « I TEMPERATURE NEAR SURFACE

TEMPERATURE NEAR SURFACE RADIUS OF MOON’S ORBIT

IMPACT FREQUENCY

DISTANCE IMPACT FREQUENCY

HISTORICAL ERAS PRE-MARE MARE EPOCH POST-MARE USED IN THIS PAPER

STRATIGRAPHY OF COPERNICUS PRE- PROCEL- REGION AS GIVEN BY IMBRIAN LARIAN ERATOSTHENI AN COPERNICAN SHOEMAKER (1962)'

/ PALEO­ GEOLOGIC TIME SCALE PRE-CAMBRIAN ZOIC

] [ ABSOLUTE TIME SCALE ( 10" YEARS)

Janssen Near Schiller FORMATION OF BASINS Serenitatis Grimaldi Nectaris Humorum Crisium Humboldtianum Imbrium 149 REFERENCES

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