Shape, rheology and emplacement times of small martian shield volcanoes

摘要

This paper focuses on the shape of 31 shield volcanoes of small to intermediate size (a few tens of kilometers in diameter) in 5 regions of Mars: Tempe Terra, Syria Planum, Pavonis Mons, Arsia Mons and central Elysium Planitia. A model for the shape of these shield volcanoes is applied, based upon the concept of porous flow of an unconfined aquifer proposed by Lacey et al. [Lacey, A., Ockendon, J.R., Turcotte, D.L., 1981. On the geometrical form of volcanoes, Earth Planet. Sci. Lett., 54(1), 139-143] and developed by Turcotte and Schubert [Turcotte, D., Schubert, G., 2002. Geodynamics. Cambridge University Press, second edition]. The agreement between the topographic profiles and the theoretical shape suggests that the model is appropriate and can be used to derive the product of flow rate and viscosity (Qu). Estimates of Qu are found to be homogeneous within a given volcanic region, but show large differences from one region to another, values generally decreasing with age (from oldest to youngest). Intrinsically, the method cannot separate the flow rate from the viscosity, but independent observations on individual lava flows in Syria Planum and central Elysium Planitia suggest that this evolution may be explained by decreasing magma viscosity with age. Independent constraints on the viscosity are used to interpret Q variations between volcanic regions. Except for Tempe Terra, and within error bars, relative effusion rates are essentially independent of volcano volumes, suggesting that the size of a shield volcano is principally controlled by the duration of volcanic activity. The specificity of central Elysium Planitia is emphasized, with highest flow rates associated with lowest lava viscosity. However, the lower viscosities at central Elysium Planitia cannot totally explain low Qu values. Considering the equation of magma flow through a dike from a magma chamber at depth, to the shield, we suggest that lower volatile contents and/or deeper magma chambers may contribute to the decrease of Qu values with time. Finally, the emplacement times of the 31 shield volcanoes were calculated using a calibration based upon terrestrial volcanoes Mauna Loa, Eldborgir, and Skjalbreiour. Emplacement times of martian volcanoes calculated in this way range from a few hundred thousand years (Tempe Terra) to a few tens of thousands of years (central Elysium Planitia). Despite the simplicity of the model, the quality of the fits and the internal consistency of the results are reasonable, encouraging development of more advanced models linking shield volcano shape to rheological properties of lavas and their characteristic time-scales of emplacement.