THE HIGHLAND BOUNDARY SCARP ON URS: DISTRIBUTION OF CHARACTERISTIC STRUCTURES: H. Frey and A.M. Semeniuk. Geophysics Branch, Goddard Space Flight Center, Greenbelt. W 20771

One of the major unsolved problems in the geologic evolution of the martian surface is the origin and development of the prominent boundary between the cratered highlands and northern lov- land plains [I]. This feature is so fundamental and so poorly understood that efforts to explain it and the major crustal dichotomy it represents have ranged from subcrustal erosion and underplating [2] to mega-impact [3]. The present-day boundary "scarp" is a complex line of diverse landforms produced by a variety of processes 14-81. In an effort to shed some light on these procesnes.ve bve mapped the global distribution of knobby terrain, detached and separated.blocks and wsas, and impact craters larger than 10 km in diameter. We represent these data as fractional areal percentages of the sample box. vhich is 2.5' high (latitude) and 5.0' vide (longitude) along north- south profiles between +65* and -45' latitude every 5. in longitude. These limits are dictated by the incompleteness of the 1:2,000,000 controlled photomosaic series vhich is the source for this work. This landform mapping compliments other studies which to date have generally been restricted to eastern Hars where the highland boundary is well expressed [9,10]. and extends a more limited study of this kind previously reported [11,12]. Globally we represent the fractional area of the different structures as contour maps. Figure 1 shows a series of 20 selected profiles derived from the contoured data crossing the highland boundary where it is vell expressed in eastern Hars between 190'W and 350%' longitude. These profiles are oriented along the maximum local topographic gradient [13]. Topography at every 1 km contour level is shown for comparison with the 0-km elevation corresponding to the x-axis in each profile. The fractional area of crater interiors provides a way of quantitatively characterizing the boundary betveen the cratered highlands and relatively uncratered plains. In the vicinity of the boundary 'scarp" there is generally good agreement between the drop-off in fractional area of craters and a decrease in elevation (see Figure 1). The contoured versions of this data show good agreement vith the geologic mapping [I] even at the relatively low resolution imposed by the size of our sample box. Background heavily cratered plateau terrain typically has 30-40% fractional area as crater interiors. The 5% contour line closely follows the highland boundary as shovn on the geologic map of Mars [I], and the profiles in Figure 1 have been aligned at this level for convenience of comparing the distribution of other features with the fractional area of craters. One surprising result shown in Figure I is the variability of the fractional area of craters in the vicinity of the highland boundary. Background values range from as high as 45-50% to as little as 10-15% prior to the rapid decrease in crater area which characterizes the boundary. Knobby terrain includes a variety of irregular, positive relief features from large and generally isolated massifs that make up portions of the rims of the Argyre, Hellas and Isidis impact basins to closely spaced hills only 1-2 km wide. We are unable to distinguish at the scale of the controlled photomosaics any difference between the closely spaced irregular hills that make up part of the chaotic terrain and knobby terrain in many other areas. Assuming that chaotic terrain can be deecribed as a combination of broken blocks and knobby terrain, the largest areal percentage of knobby terrain (45% of the sample box) is found in the chaos at the eastern portion of Eon Chasma at 45.W. 15's. Other major occurances lie at 125*W, 30'N (35%). 2SeW, 50°N (20%) and in ElysiuwAmazonis at 175.to 185.W between 5 and 25'N. Note that this is southeast of and does not include the prominent Phlegra Montes knobby terrain, which despite its obvious nature is thinly distributed at 5-10% of the sampled area. The knobby terrain vhich is mapped along the highland boundary in the Ismenius Lacua-Syrtis Major-Amenthes-Aeolis quadrangles from about 340°W, 45'N to 225OW. 5's likewise has relatively lov spatial density. In Deuteronilus-Protenilus-Nilosyrtis Hensae knobby terrain makes up only 15% of the sampled area. h he knobby terrain constitutes a band about 8' wide (in latitude) and follows the trend of the cratered highland boundary so closely that there appears to be a clear association between the decrease in crater area and increase in the area of knobby terrain. This can be seen in Figure I, profiles 2, 7-11 and 13-16 where the peak of the knobby terrain distri- bution generally lies north of but immediately adjacent to the fall-off in crater area. This 'collar" of knobby terrain does not exist everyvhere along the boundary. however. Noteable gaps occur at 340-360". 215-220, and 180-190"W in Eastern Uars. Detached plateaus may represent broken off pieces of the former cratered highland plateau. Large pieces and large areal percentages of these features ere found in the Deuteronilus- Protonilus-Nilosyrtis Uensae re~ion(see profiles 4-11). Areal percentages reach 30% in the fretted terrain at 320-33O0W, 40'N and LOX at 220°W. 5's along the highland boundary. As shovn in Figure 1, the peak distribution of detached plateaus lies either the cratered terrain where the fractional area of craters begins decreasing (profiles 7, 9, 10, 11, 16, 17) or north of this between the cratered terrain and the knobby terrain "collar" (profiles 1, 4, 5, 6; also note again profiles 7, 8, 9. 10, 11 where the knobby terrain lies north of the detached plateau peak). It is also clear from Figure 1 that these associations are not universal. and in some places the peak of the detached plateau distribution overlaps directly that of the knobby terrain (profiles 2. 15. 19). lies north of the knobby terrain (profiles 4, 14). or does not exist (profiles 3, 13). Topo- graphic gradients may play a role in the relative distribution of knobby terrain and detached plateaus, but the uncertainty of the elevation data in the northern hemisphere [13.14] make such studies difficult.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System 1iI GHLrlND EOUNDARY SCARP OP! PlARS Frey, 11. 2nd Sen~eniuk,A.PI.

References: [I] Scott, D.H. and Carr. H.H. (1978) Geologic Hap of Hars. USGS Hap 1-1083. 121 Vise, D.U., Colornbek, H.P. and HcGill, G.E. (1979) J. Geophya. Res. 84, 7934-7939. [3] Vilhelms, D.E. and Squyres, S.W. (1984) Nature 309, 138-140. 141 Schultz, P.H. et al. (1982) J. Geophys. Res. -87, 9803-9820. (5) Luchitta, B.K. (1984) Proc. Lunar Planet. Sci. Conf. 14th, in J. Ceophys. Res. -89, B409-B418. [6] Squyres, S.W. (1978) Icarus 24, 600-613. (71 Sharp, R.P. (1973) J. Ceophys. Res. 2, 4073-4083. [8] Carr, M.H. and Schaber, C.G. (1977) J. Geophys. Res. 82, 4039-4054. 19) Haxvell, T.A. and Barnett, S.J. (1984) Lunar Planetary Science XV, 521-522, (101 Kochel, R.C. and Peake, R.T. (1984) Lunar Planetary Science XV, 433-434. [Ill Serneniuk, A.M. and Prey, H. (1984) Lunar Planetary Science XV. 748-749. [I21 Haxvell. T.A. et dl. (1984) Geol. Soc. Am. Abstract with Programs 2, 586. [I31 Topog,raphic Hap of Mars. USGS Hap 1-96!. 1141 Dovns,, C.S. et al. (1982) J. Geophys. Res. -87. 9747-9754. (151 Serneniuk. A.M. and Frey, H. (1985) This volume.

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-- FRACTIONAL AREA OF CRATERS ------FRACTIONAL AREA OF KNOBBY TERRAIN -FRACTIONAL AREA OF DETACHED PLATEAUS 0 USGS TOPOGRAPHY [EVERY 1 KMI

FIGURE 1. PROFILE DISTRIBUTIONS ACROSS HIGHLANDILOWLANDS BOUNDARY IN EASTESN MARS

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System