Mathematics – Logic
Scientific paper
May 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agusm.p52a..03h&link_type=abstract
American Geophysical Union, Spring Meeting 2002, abstract #P52A-03
Mathematics
Logic
5410 Composition, 5420 Impact Phenomena (Includes Cratering), 5464 Remote Sensing, 5480 Volcanism (8450), 5494 Instruments And Techniques
Scientific paper
Accurate estimates of lava flow unit thicknesses are necessary to constrain estimates of the volcanic flux. In previous studies the thicknesses of individual flow units have been investigated using a variety of techniques, including: (1) shadow measurements in high-resolution images obtained under low-sun conditions, (2) in situ observations of flow units within the walls of Hadley Rille, and (3) studies of the chemical kinetic aspects of lava emplacement and cooling. However, shadow measurements of flow unit thicknesses are made difficult by (1) the limited availability of high-resolution topography and near-terminator images necessary for the recognition of flow fronts, (2) regolith formation processes, which can obliterate flow fronts of up to 15 m, and (3) the composition and the eruption style of lunar lavas which are thought to be responsible for the sparseness of mare flow features. Thus, we developed in more detail an alternative remote sensing technique described by Neukum and Horn [1975] and applied it to numerous mare basalt flow units using recently obtained impact crater size-frequency distribution (CSFD) data. We use the shape of CSFD curves to estimate the thickness of individual lunar mare flow units. A characteristic knee frequently observed in CSFD curves is reasonably interpreted to represent the presence of two lava flow units separated in time. The diameter at which this knee occurs is directly linked to the thickness of the overlying flow unit. We found that this technique expands considerably the ability to assess lava flow unit thicknesses and volumes on the Moon and planets. We examined 58 CSFD curves of basalt flow units in Oceanus Procellarum, Imbrium, Tranquillitatis, Humorum, Cognitum, Nubium, and Insularum that show this characteristic knee. With this technique we were able to identify flow units that have not been detected in low-sun images. We found that the range of flow unit thicknesses is ~20-200 m and the average is ~30-60 m. The volumes range from ~30-7700 km3. The minimum average volume of all investigated flow units is ~590 km3 and the maximum average volume is ~940 km3. We found that the greater abundance of flow fronts and units with evidence of resurfacing in the shallow non-mascon maria and in the younger units in Imbrium. This may be related to two factors. First, because mascon maria tend to undergo subsidence lava units are more likely to pond and have greater thicknesses. Secondly, later flows that are emplaced when topography is smoothed by earlier events theoretically tend to lateral spreading, rather than ponding. Initial analyses also indicate that the range of separation ages between the 26 units analyzed is ~100-2000 m.y. with a mean value of ~700 million years. These values imply that mineralogically similar source regions at depth can be long-lived or repetitively active.
Gerhard Neukum
Head James W.
Hiesinger Harald
Jaumann Ralf
Wolf Ursula
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