Constraints on the elastic thickness, heat flow, and melt production at early Tharsis from topography and magnetic field observations

摘要

The southern half of the Tharsis region of Mars displays enigmatic variations in the crustal magnetic field that are correlated with topography and the distribution of volcanism. Radial magnetic field anomaly values (B r ) at 400 km altitude are greater than 25 nT over areas at elevations less than about 6 km, B r = 0 over areas at higher elevations where volcanism is also concentrated. Assuming that much of the uplift and magmatism of Tharsis postdates the crustal magnetic field, we hypothesize that the absence of radial field in the central part of the rise is related to enhanced heat flow from an underlying mantle plume. A parameterized mantle convection model, combined with a heat transfer requirement for crustal demagnetization and analysis of the dynamic topography provides restrictive and self-consistent constraints on the plume structure, excess temperature ΔT ex and heat flux q p as well as the average elastic thickness d e at Tharsis at the time of uplift. These results lead, in turn, to bounds on the long-term average magma supply from the mantle. ΔT ex is found to be in the range 205–240°C and upper bounds for d e are 29–40 km, depending on the magnitude of the dynamic topography. Assuming magnetite is the dominant magnetic carrier, these results imply also that the lower 50–60% of the crustal column is raised above the Curie temperature and thermally demagnetized. The plume heat flux q p ≈ 60–100 mW m2 and corresponds to melt production rate Q ≈ 0.01–0.03 km3 a?1, comparable to minimum estimates determined from geological data for the rate of Noachian-Hesperian volcanism.