LAVA FLOODING OF EARLY PLANETARY CRUSTS: GEOMETRY, THICKNESS, AND VOLUMES CF FLOODED LUNAR HIGHLAND TERRA IN. James W. Head, Dept. of Geol og ica I Sciences, Brown Univ., Providence, RI 02912. Recognition of the volcanic origin of surface deposits on ancient cra- tered planetary surfaces provides important information on the presence and significance of melting in the interior. Establishment of the composition, age, and volume of such deposits provides additional clues concerning the characteristics of the thermal history of the planet.' In addition, the Thickness, geometry, and volumes of volcanic deposits provide important data for understanding tectonics and I i thospheric deformation. Once deposits have been recognized as of volcanic origin, it has often been difficult to estab- , . , I sh thicknesses and volumes because in the processes of emp lacement, l avas cover the initial crustal surface, obscuring the geometry of the pre-volcanic terrain. In addition to geophysical analyses, attempts to establish thick- nesses and volumes have concentrated on four approaches: I ) measuring diam- eters and sxposed rim heights of impact craters protruding through the depos- i TS; ' 2 l @cati ng craters in vo l can ic deposits that have excavated sub-vo l- can i c material ;" 3) using stratigraphic techniques; 7 and 4) using the geom- etry of co~parableunflooded regions as models for the initial topography. 2 P<lihouyh these approaches have provided significant advances in the under- stand ing of the emp lacement of the l unar maria,' there are sti l l basic uncer- ta inties concerning thicknesses and vol umes in many areas. The major difficulty in establ ishing volumes in flooded areas is recon- structing the sub-volcanic topography. The approach taken here is to start with known topography characteristic of early planetary crusts, and to arti- ficial ly flood it with lava, keeping track of the evolving thickness, volume, surface area covered, and map pattern. Upon completion of flooding (no sub- volcanic topography exposed), data are avai lable on maximum and average lava ihicknesses, total volume, and the relationship of area covered (or exposed) to lava thickness and volume. In addition, a series of maps is available to show patterns of exposed sub-vol can ic topography at various stages of f i l l i ng. Since the volume and thickness of lavas associated with each map pattern is known, these maps can be used to estimate the same parameters for flooded planetary areas that show comparable map patterns. A range of unflooded ter- rain typical of early planetary crusts is being investigated (highland cra- tered terrain; basin interiors; basin margins; large craters; upland plains). This paper reports on the results of experiments in several areas of lunar high land cratered terra in. The procedure is as follows: Lunar Topographic Orthophotomaps (1:250,000) are used as base maps. The contours of each map chosen are digitized at 300 m intervals. For this study the assumption is wade that lava sources are ubi- quitous; thus flooding begins at the lowest point on the map, flooding evenly to each new contour level, and begins in separate isolated depressions as the rising lava first encounters the appropriate contour within the depression. Flooding continues until all topography disappears. Lava thickness as a function of area (Fig. I) and volume (Fig. 2) are plotted, and maps of any stage of flooding can be produced by Calcomp plots of digitized topographic data. Four contiguous areas of lunar highland cratered terra in inf l uenced by the lmbrium basin were chosen as representative of unflooded terrain of early highland crusts (Table I, Fig. 1,2). Herschel (LT077A3) is characterized by in- tense lmbrium sculpture, a post-lmbrium 40 km crater, and tile northern part of Ptolemaeus, a large (~150km) crater. Gy lden (LT077B4) shows numerous very degraded craters and upland plains. Albategnius (LlUlICI contains two large 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LAVA FLOODING OF EARLY PLANETARY CRUSTS Head, J.W. deep craters and high rough topography in the Wolemaeus-Albategnius crater r im area. Arnmon i us (LT077D4) i s domi nated by Ptolemaeus crater f loor and r im. The flooding and progressive covering of the background topography is il- lustrated in Fig. I. A small inset shows the relationship between the slope of each 300 m i ncrernent and the percentage of the total area f i l led. For -Her- schel, filling begins at the bottom of Herschel and little area is covered for the first km of fill. As the lava encounters the flat floor of Ptolemaeus and tP,e lmbr i urn scu 1 pture va l leys, over 30% of the surface area is covered by a 600 m increase in lava thickness. From here on (7 - 16 x lo3 krn2) the rough topography of the sculpture is being flooded. An additional 600 m is neces- sary to cover the highest parts of the Ptolemaeus rim crest. For Gylden, the shape of the curve is very similar, with steep initial slopes representing in- filling of discrete craters, followed by a very low slope representing flood- ing of large crater floors (Hipparchus) and intercrater plains. This, in turn, is followed by a steeper slope as sculpture, crater rims, and higher in- tercrater plains are covered, and a very steep slope as the few peaks are covered. The presence of the fresh crater Herschel causes the Herschel curve to plot about 600 m above Gylden. Albategnius presents a considerably differ- ent picture. Several small craters on the floor of Albategnius require over a km of f i l I. Although Albategnius and Klein are pre-lmbrian, they are much fresher (hence deeper) than craters such as Ptol amaeus and Hipparchus, result- ing in the initial plateau (0.2 - 4 x lo3 km3) as their floors are covered, fo I lowed by a steep slope, as about one krn of lava f i l 1 s them to the level of surrounding topography. At this point, an intermediate slope is developed as the relatively steep rim topography of the large craters Albategnius and Pto- lemaeus is flooded. Ammonius is dominated bv the Dresence of the Ptolemaeus crater f l oor. Over 5'mearea i s coverLd by one 300 m I ava increment when the f lat floor is encountered (2 - 12 x 1 o3 km2). The curve steepens unti l the rim crest peaks are covered. Total lava thicknesses required to cover al l topography ranges from 3.3 to 7.2 km (Table I ). The lava volumes associated with the flooding are shown in Fig. 2. Very small volumes are associated with the initial km of fill in each area. Vol- ume increases marked1 y between I and 2 km and a qenera l slope is achieved at about 2 km which characterizes the volume addition for Herschel, Ammonius, and Gylden, until topography disappears. For Albategnius, the filling of Alba- tegnius, Klein, and Ptolemaeus craters causes the slope to remain steeper longer because the craters make up I ess than ha I f the total area. Total vol "me required to cover topography in these areas ranges from about 32-60 x lo3 km (Table I ). Style of emplacement - For the highland cratered terrain used here, the followina stvle characterizes em~lacement: Staae I - A~~roximatelvI km of u, 8, lava thickness is used to fi l I ih deep depressions usually formed'by relative- ly fresh craters. The volume of lava and area covered during this stage is minima I. In Stage I I, the highest rates of incremental areal coverage are achieved as the relatively flat floors of large craters are flooded. Although volumes are increasing dramatically during this stage, Stage II is not the highest rate of volume addition since the surface area associated with the flooded craters is usually less than half the total region. Approximately an additional kilometer is added during this stage. In Stage Ill, crater floors are largely covered and lava floods crater interiors and intercrater areas for approximately another ki lometer. The area/vol ume relationship is reversed from Stage I I. Area increases at a slower rate but more volume is added because each i ncrement is we1 l over 50% of the surface area. In Stage -IV an additional 300-900 m is added to complete flooding of isolated peaks, 0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System 7 - 7 ALBATEGNIUS 6- -30% 6 5 - -5 3 -4 cn V) W Z Y $3 I- 2 I (7784) FIGURE I. FIGURE 2. I 0 i 0 2 4 6 8 10 12 14 16 18 0 10 20 30 40 50 60 70 AREA COVERED (KM* x lo3) VOLUME (KM~x lo3) usual ly forming rim crest remnants of degraded craters. This is essential ly the inverse ot the volume si~uz- tion in Stage I; large volumes are being added although the area covered is minimal. Albategnius illustrates these four stages but the presence of the large relatively fresh pre-lmbrian crater Al bategni us produces mark- ed increases in the thicknesses involved in each stage because of the more pronounced topography. bdditional highland areas are being analyzed to define envelopes for area/volume relationships so that volume thickness estimates can bemade of naturally flooded regions by measuring the percent of the area remaining to be cov- ered. For reference, the average volume of three of the areas is about 36 x lo3 km3 which is 1/10 the volume of the Columbia River plateau basalts, and less than the volume of the young Mare lmbrium flows.