HR: 16:30h
AN: P44A-03    [Abstracts]
TI: Gravity Constraints on the Structure of the Marius Hills Volcanic Complex on the Moon
AU: * Kiefer, W S
EM: kiefer@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, United States
AB: The Marius Hills in central Oceanus Procellarum is the largest volcanic dome complex on the Moon. It is roughly 200 by 250 kilometers across and contains more than 300 volcanic domes and cones and 20 sinuous rilles. Individual domes are up to 25 kilometers across. Volcanic activity in this region is primarily Upper Imbrian in age (3.2-3.8 Ga). On the entire rest of the lunar mare, only about 200 volcanic domes are known, emphasizing the unusual nature of the Marius Hills. Gravity observations provide additional constraints on the structure of the Marius Hills. Individual domes are not resolved in free-air gravity model LP150Q, but the gravity data does resolve the regional structure of the Marius Hills. The overall shape of the gravity anomaly closely corresponds to the outlines of the volcanic field. The maximum anomaly near 14 North, 307 East, is 168 mGal at spherical harmonic degree 65 and 241 mGal at harmonic degree 110. There is a secondary maximum of 113 mGal at 8 North, 309 East, in the southern part of the dome complex. The regional topographic relief is just 500 meters across this region, again at harmonic degree 65. If uncompensated, this topography can only account for at most 40% of the observed free-air gravity anomaly, and even less for reasonable compensation models. Most of the observed gravity anomaly in the Marius Hills must be due to buried, high density material. In this regard, the Marius Hills are similar to some highland volcanos on Mars such as Syrtis Major and Tyrrhena Patera (Kiefer, Earth Planet. Sci. Lett. 222, 349-361, 2004). The density contrast that produces the Marius Hills anomaly is most likely between the surface basalt and the underlying, brecciated anorthositic highland crust. Thus, the gravity observations likely constrain the overall thickness and volume of volcanic activity in the Marius Hills. For plausible density contrasts, the volcanic unit is several kilometers thick. Lunar Prospector Gamma Ray and Neutron Spectrometer observations indicate that basalts in this region have high Ti content, suggesting a high density due to the presence of ilmenite. The densities of high Ti basalts from Apollo samples elsewhere on the Moon are being assessed as possible constraints on the basalt density in the Marius Hills, which will lead to more quantitative constraints on basalt thickness and volume.
UR: http://www.lpi.usra.edu/science/kiefer/home.html
DE: 5417 Gravitational fields (1221)
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 6250 Moon (1221)
DE: 8450 Planetary volcanism (5480, 6063, 8148)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting