0 Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LAVA FLOODING of EARLY PLANETARY CRUSTS

Home , Albategnius (crater)

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 cratered 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 thicknesses 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 geometry of co~parableunflooded regions as models for the initial topography. 2 P

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 flooding of large crater floors (Hipparchus) and intercrater plains. This, in turn, is followed by a steeper slope as sculpture, crater rims, and higher intercrater 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 relatively 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,

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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 marked 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 covered. 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. 9 Average lava thickness (total volurne/total area) for the four areas is 2.44 km, 2.08 km if Albategnius is excluded. This is considerably greater than the 300 m average TABLE I. mare thickness proposed by H6rz4 and somewhat more than the 1500- T(km) VOL 2000 m val~e~derivedby ~e~on~"for the shallow maria. MAX AVE lo3 km3 References: Head J.W. (1975) Oriqins of Mare Ba!jalt, LSI, 66; Hersche l 77A3 4.20 24 36.65 2Solomon S. and J.W. Head (1978) JGR, in press5 DeHon R. (1974) Gylden 7784 3.3 1.88 32.36 LPSC5, 53; 4~6rzF. ( 1978) unar &. IX. 540; Andre C. et al . Albategnius 77CI 7.20 3.54 60.45 (1978) Lunar Sci. IX, 20; bJ.{. et al. (1978) -Fix Amrnonius 77D2 4.20 2.23 38.85

43; 'HG~J~.~~T(I 974) PLSC5, 207; DeSToni. and J. Waskom ( 12%) + LPSC7, 2729; Schaber G. ( 1973) LPSC4, 73.