NEW IDENTIFICATION OF ANCl ENT MULTI-RING BASINS ON AND IMPLICATIONS FOR GEOLOGIC EVOLUTION. P. D. ~~udis'~~and Me E. Strobel i1 1. U.S. Geological Survey, Flagstaff, AZ 86001 2. Dept. of Geology, Arizona State Un iv., Tmpe, AZ 85287

initial examination of photographs of Mercury provided by almost a decade ago revealed numerous Impact features, lnclud lng large basin-s Ired structures I11. In subsequent studies of these photographs, many workers have attempted a systemtic inventory of the large basln population of Mercury 12-71. Recent geologic mapping of the ~lchelan~elo(H-12) quadrangle 181 indicates that its regional geology is strongly controlled by four large, nearly obllPerated multi-ring basins 191. Under thls Impetus, a systsmatic survey of the entlreMariner 10 coverage of Mercury was made to determine the number, distribution, and dimensions of additional ancient basins on the (Table 1). Ancient mult I-ringed baslns on Mercury can be recogn ired by the fol lcuing criteria: 11 arcs of msslf chains and isolated massifs that protrude through younger units; 2) arcwte segmnts of lobate ridges (rupes) that align wlth massifs In circular patterns; 3) arcuate scarps that align wlth ridges and msslfs; 4) segments of ancient crater rims that appear, fran evidence of relief greater than adjacent segments to have been reJuvenated; and 5) Isolated regions of ananalously high topography within the Intercrater regions of heavily cratered terrain. All of the newly identified basins predate the mercurlan intercrater plains, previously held to be tho oldest geologic unit m the planet 1101. The degradatlonal state of these features is canparable to that of ancient lunar basins such as Al-KhwarlzmiXing 111, 121 and sane nearly obliterated martian muiti-ring basins 1131. Thus the current inventory (Table 1) is probably an underestimate due to the relatively poor Mariner 10 photographic coverage of Mercury. The new multi-ring baslns are named here for unrelated, superposed craters and named plalns reglons, following a similar practice for naning degraded lunar baslns 1121. Although ancient bast ns are recogn ized pr imr i ly by photogeoiogy, topograph lc i nforiwt ion f run Earth-based radar provides physical proof of their presence 114,151. Radar prof I les of Mercury are available for only a narrow region between -5' and +15* latitudes, so only baslns located within thls band can be studled (-Moltere, -Theophanes, Budh, and Tlr basins; Table 1). Basin rings are discriminated by the radar as topographic hlghs with relief typically 1 km or less, that are found in otherwise nondescript terrain. Highs on profiles that occur at +8.5*, 21e, +9*, 27*, and -0.5*, 18. lie along portlons of rlngs 2, 3, and 4, respectively, of the Donne-Moliere basln. Ring 3 of the Budh basln is made evident by a 1.5 km peak at+12*, 140. and by a somewhat lower peak at +12*, 163.. Topographic hlghs at +11*, 128' and 132' on rlng 4 of the Mena-Theophanes basin are rejuvenated segments of older crater rims. Subsequent structural evolution of various regions has been influenced by these basins. A global network of lobate ridges (rupes), interpreted as thrust faults, had been recognized previously 1161. In many cases, the arcuate trend of these ridges outlines an ancient basin ring (e.g., Hero Rupes, part of ring 2 of Vincente-Yakovlev). In other instances, a more-or-less continuous ridge network may encounter a pre-existing basin rlng obi iqueiy, follow the outline of that ring, deflect in towards the next ring, and re-establish the circular outline of an inlying ring (e.g., Adventure- and rlngs 3-5 of -). These relations suggest that although the ridge system probably is related to global canpression, local trends are control led by pre-existing structural patterns provided by ancient multi-ri ng basins. The new ldentiflcations of mercurian baslns may change concepts of craterlng historles for the terrestrial . All previous studies of mercurian baslns have emphasized the apparent def lciency of baslns on Mercury relative to the Moon 12-4, 71. Cumulative basin density on Mercury (D > 200 km) is reported by 171 to be only 37 percent of that of the Moon. The Moon has 6 at least 62basins (D > 200 km; 141 1, resulting In an average density of 1.72 + 0.13 per 10 2 km . Only about 35 percent of Mercury's surface was photographed with I lghting conditions adequate 1171 to recognize ancient basin structures. The newly identified baslns suggest that Mercury has at least 50 basins (D > 200 km) within thls area. Thus, the average density of 6 2 mercurian basins is 1.92 -+ 0.14 per 10 km . This value suggests that Mercury Is not def iclent in multi-ring basins and may possibly have mre per unit area than the Moon. The presence of a large population of multi-ring baslns that pre-date the mercurian intercrater plains material

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Spudis P. D. and Strohe'll M. E. suggests that basin production was a more-or-less continuous process during the early history of planets, and lends support to hypotheses of continual 1 y decl in1 ng crater1 ng rates I181 rather than to a "late cataclysm" model of basln production. The recognition of many previously unrecognized ancient baslns on Mercury has several imp1 ications for the geol ogl c evolution of terrestrial planet crusts. It now appears that a1 1 terrestrial bodies with preserved ancient surfaces (Moon, Mercury, Mars) have a large population of mu it 1-r i nged impact structures. Al 1 of the newly dlscovered mercur Ian basins pre-date the lntercrater plains planetwlde. This relation suggests a dlstlnct epoch of intercrater-plains formation that at least partly erased the pre-exlstlng crater population, leaving rannants of only the largest structures. Thus, the process responsible for formation of the lntercrater plains material must have operated planetwlde. The hypothesis that the intercrater plains material Is a prirordlal crust 1101 appears untenable; a primordial crust would preserve a spectrum of multi-ring basln states. The mrcurlan lntercrater plains material may be at least partly volcanic 1191, Its surface morphology disrupted by the last phases of heavy banbardment. The tendancy of global tectonic patterns to be locally Influenced by multi-rlng basin structure seems analogous to similar patterns on the Moon I201 and Mars 1131. Thus, basins appear to provide the broad-scale structural pattern of early terrestrial planetary crusts.

REFERENCES Ill Murray 8. --et al.(1974) Science 185, 169. I21 Wood C. and Head J. (1976) PLSC 7, 3629. I31 Ma1 in M. (1976) PLSC 7, 3589. I41 ~chxrG. et at. (1977) PEP1 15, 189. 151 Ddon (1978) NRSA TM 79729, 150. I61 Croft S. (1979) Ph.D. Thesls, UCLA. I71 ~rey~.and Lawry B. (1979) PLPSC 10, 2669. I81 Spud1 s P. and Prmser J. (In press) USGS Map, H-12 quadrangle. I91 Spudis P. (19z) GSA Ann. Mtg. 16, 694. 1101 Trask N. and Guest J. (1975) JGR 80, 2461. I111 El- Baz F. (1973) Science 180, 1173.1121 Wilhelms D. and El-Baz F. (1977) USGS ~arl-~8.I131 Schultz P. --et at. (198.~~87, 9803. 1141 Zohar S. and Goldstein R. (1974) Astron J. 79, 85. I151 Zohar, S. and ~oldstein,y, M. (1974) unpublished JPL report, The 1972, 1973 probeaf Mercury. I161 Stran R. --et al. (1975) JGR 80, 2478. I171 Stran R. (1979) Space Sci. Rev. 24, 3. I181 Hartmann W. (1975) lcarus 24. 181. 1191 Stran R. (1977) PEP1 15, 156. I201 WI 1he1msD. (In press) Geol ogic h istory of the Moon, USGS Prof. Paper.

Rlng 01anat.rs3 (lun) 0arl n1 cant ld.' Cantar 1st 2nd 3cd 4m 5m -nts4 Boreal lr 2 73'. 53. - 860 15% (22%) Partlal ly mppd bt 1161 -Holbsln 3 35.. 19. 240 500 950 - Derzharl rrSor JUM 2 51'. 27' - 560 740 890 - Partlsllv mapped by 141 Sobkou 1 34.. 132. - 490 850 14X) - hrtlsl arcr: 810, 1010, 1280; partlslly mppd Q 161 Chong4auguln 3 57'. 106' 220 350 580 940 - Brahm- 1 59',172' 340 620 840 (10801 - Partlal aro: 503, 740, 940 Donnku3lI or. 3 4, 0 375 100 (8251 1060 1500 HIr~lhlge+ahler I -IS', 23' 150 355 (7001 - Ygne-Theqhanes I -1'. 129' 260 475 770 1200 - Partlslly mopped Q 141 Tlr 1 6.. 168' 280 660 950 1250 - Budh 2 l7', 151- - 580 850 1140 - l bserrPetrarch 2 I 30 425 640 9x1 1175 - Andal-Colsrldge 1 -43, 49 (420) 700 IOY) 1330 I750 Matlrse-Rspln 1 -24.. 75- 410 850 1250 (15501 (1900) "Haydndsphsel"ot161 Bartok-lves 3 -3, 1 480 7W 1175 (15001 - Hartbr~Rlsnensch~~Ider 2 -56'. 105' 270 5M) 780 1050 - VlncsntsYakwlar 1 -52.. 162' 350 725 950 1'230 (17001 EltokrrHllton 1 -2, 7 280 590 850 1180 - Boccacclc-Scopas 3 -82.5'. 44' 350 600 9% (13101 -

I Basln nonenclaiura after sysh dsrsloped tor arelent lu~rbsslns 1121 Confldenc. level of Idsntlf lcatlon: I - daflnlts; 2 - probable; 3 - posslbls.

' Numbers In parentheses reflect uncertain rtrg contlwlly; srrlgrment to the flve rlq rank1 qs Is arbitrary.

Partlal arcr are concentric structures conflnd to one slde of basln; prsvlous shldles mat are reference3 mappad ?ortlom of larger. multi-rlrg structures recognized here.

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