The Collapse of Super-isostacy: Volcanic Intrusions as an Alternative Model for Lunar Mascon Gravity Anomalies

Details The Collapse of Super-isostacy: Volcanic Intrusions as an Alternative Model for Lunar Mascon Gravity Anomalies The Collapse of Super-isostacy: Volcanic Intrusions as an Alternative Model for Lunar Mascon Gravity Anomalies

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p33g..02k&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P33G-02

Other

[5417] Planetary Sciences: Solid Surface Planets / Gravitational Fields, [5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [5480] Planetary Sciences: Solid Surface Planets / Volcanism

Scientific paper

Lunar mascons are large positive free-air gravity anomalies associated with topographically low impact basins. This is an unexpected location for a gravity high, implying the presence of denser than normal material somewhere in the crust or lithosphere. The conventional model for lunar mascons interprets these gravity anomalies as the signature of super-isostatic uplift of the crust-mantle interface, presumably created during the impact process. However, for the likely lithospheric thermal gradients during the impact basin forming epoch on the Moon, hydrocode modeling shows that impact heating raises the mantle temperature in the center of the basin above its solidus out to a distance of several hundred km from the basin center. Thus, the post-impact thermal gradient is quite large and essentially no elastic lithosphere is initially present in the basin center. Thermal evolution calculations indicate that it takes of order 100 Ma for significant lithosphere thickening to occur in the basin center. On the other hand, because the partially molten material has a low effective viscosity, visco-elastic relaxation calculations show that the timescale for the collapse of super-isostatic uplift at the basin center is of order thousands of years. Thus, even if the initial impact created super-isostatic uplift of the crust-mantle interface, it would have collapsed to an isostatic state long before the lithosphere was able to thicken significantly. Any successful model for lunar mascons that requires flexural or super-isostatic support of a load must create that load at least 50-100 Ma after the basin impact in order to allow lithospheric thickening in the basin center. One possible load forming process is volcanic filling. In the case of the Orientale basin, mare basalts began erupting at the surface 60-100 Ma after the impact and the maximum eruption rate occurred about 200 Ma after the basin formed. The surface lava flows are too thin (< few hundred meters) to explain the observed gravity. However, the crust below the impact zone must be pervasively fractured, forming a major reservoir for magma ponding within the crust. Intrusive magmatism filling this pore space could be an important contributor to the gravity anomaly as well as an important component of the Moon's total volcanic flux. Such a model is similar to our recent model for the free-air gravity anomaly in the Marius Hills volcanic dome complex but has not previously been considered as a mechanism for producing lunar mascons. For plausible values of the basalt density, crustal porosity, and rate of porosity closure with depth, a spherical cap of dense intrusives, 300 km in diameter and 15 km thick, can explain the Orientale gravity anomaly. The basalt intrusions may fill the pore space in most of the crustal column beneath Orientale, a condition that may be necessary to permit magma to rise buoyantly through the crust and erupt at the surface. Variations in the relative contributions of intrusive magmatism and of surface lava flows may explain the various types of mascon documented in gravity observations made by the Kaguya mission.

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