Strength of Martian lithosphere beneath large volcanoes

This paper provides an estimate of the thermal state of Martian lithosphere established since the formation of the shield volcanoes. Comparison of the spherical harmonic models of Martian topography derived using harmonics tr uncated at degrees 10, 11, 12, and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degr ee 11. Therefore intermediate-scale surface topography and gravity anomalie s of Mars, specified by harmonics of degree 11-50, are considered in this s tudy. These harmonics are dominated by shield volcanoes, Alba Patera, and I sidis basin. The central part of Valles Marineris is also well defined by t hese harmonics. The strong correlation of the topography and gravity anomal ies over these harmonics and the positive Bouguer anomalies of the volcanoe s suggest significant excess masses beneath these surface features in addit ion to their topographic masses. This indicates that the Martian lithospher e has been strong enough to support both the topography and the internal ex cess masses. We determine the rigidity of the lithosphere beneath a given s urface feature. The lithosphere is modeled by a thin elastic spherical shel l that overlies an inviscid interior of higher density and is subjected to both surface and internal loads. The thickness of the shell and the magnitu de of the internal load are calculated such that the deformed structure res ulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly. It is shown that the lithosphere beneath Elysium Mons, Alba Patera, and the central part of Valles Marineris can be modeled by elastic shells subjected to topographic loads alone, whereas the lithosphere beneath Olympus and Tharsis Montes ca nnot. Internal excess masses of at least 40%, and more likely 50%, of the t opographic masses are required, and the lithosphere beneath must have an el astic core of similar to 100-80 km to support these volcanoes. The mean the rmal gradient beneath this region of Tharsis is estimated to be similar to7 -10 K/km on the basis of the strength envelopes calculated using diabase rh eology for the crust of 50 km thickness and olivine rheology for the underl ying mantle.