HR: 1340h
AN: P23A-0229    [Abstracts]
TI: Viscoelastic Relaxation of Lunar Basins
AU: * Mohit, P S
EM: pmohit@wustl.edu
AF: Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, One Brookings Dr, St Louis, MO 63130 United States
AU: Phillips, R J
EM: phillips@wustite.wustl.edu
AF: Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, One Brookings Dr, St Louis, MO 63130 United States
AB: The large lunar impact basins provide a unique glimpse into early lunar history. Here we investigate the possibility that the relief of the oldest lunar basins (with the exception of South-Pole Aitken) has decayed through viscous relaxation. We identify nine ancient multi-ring basins with very low relief and low-amplitude Bouguer and free-air gravity anomalies. The characteristics of these basins are consistent with either 1) relaxation of topographic relief by ductile flow (e.g. Solomon et al., 1982) or 2) obliteration of basin topography during crater collapse immediately following impact. Both scenarios require that the basins formed early in lunar history, when the Moon was hot. The latter possibility appears to be unlikely due to the great topographic relief of South Pole-Aitken basin (SPA), the largest and oldest impact basin on the Moon (with the possible exception of the putative Procellarum basin; Wilhelms, 1987). On the other hand, the thin crust beneath SPA may not have allowed ductile flow in its lower portions, even for a hot Moon, implying that a thicker crust is required beneath other ancient basins for the hypothesis of viscous relaxation to be tenable. Using a semi-analytic, self-gravitating viscoelastic model, we investigate the conditions necessary to produce viscous relaxation of lunar basins. We model topographic relaxation for a crustal thickness of 30 km, using a dry diabase flow law for the crust and dry olivine for the mantle. We find that the minimum temperature at the base of the crust ($T_b$) permitting nearly complete relaxation of topography by ductile flow on a timescale $<$ 10$^{8}$ yrs is 1400 K, corresponding to a heat flow of 55mW/m$^{2}$, into the crust. Ductile flow in the lower crust becomes increasingly difficult as the crustal thickness decreases. The crust beneath SPA, thinned by the impact, is only 15-20 km thick and would require $T_b$ $\geq$ 1550 K for relaxation to occur. The fact that SPA has maintained high-amplitude relief suggests that $T_b$ dropped below $\sim$ 1550 K no later than 100 Myr after the formation of SPA. When did these basins form? The formation of a stable lunar crust ($\sim$4.4 Ga) is clearly an upper limit on their age. The precipitation of ilmenite from the magma ocean is predicted to occur at $\sim$1373 K, which suggests that the lunar magma ocean had not fully crystallized when the oldest basins formed. From thermal evolution models (e.g., Solomon and Longhi, 1977) and crystallization ages of Mg-suite rocks (mostly 4.2-4.4 Ga; Nyquist and Shih, 1992; Snyder et al., 1995) 4.2 Ga appears to be a lower bound on the freezing of the magma ocean (although some residual liquid may have remained in the Procellarum KREEP Terrane). Therefore, the basins that relaxed almost completely due to high crustal base temperatures probably formed between 4.2-4.4 Ga.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 6250 Moon (1221)
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting