The Gravitational Signature of Martian Volcanoes

A. Broquet (1) , M. Wieczorek (1)

1 LAGRANGE - Joseph Louis LAGRANGE

By modeling the elastic flexure of the Martian lithosphere under imposed loads, we provide a systematic study of the old and low-relief volcanoes (>3.2 Ga, 0.5 to 7.4 km) and the younger and larger prominent constructs within the Tharsis and Elysium provinces (<3 Ga, 5.8 to 21.9 km). We fit the theoretical gravitational signal to observations in order to place constraints on 18 volcanic structures. Inverted parameters include the bulk density of the load, the elastic thickness required to support the volcanic edifice at the time it was emplaced (T e), the heat flow, the volume of extruded lava, and the ratio of volcanic products that form within (V i) and above the preexisting surface (V e). The bulk density of the volcanic structures is found to have a mean value of 3, 206 ± 190 kg/m 3 , which is representative of iron-rich basalts as sampled by the Martian basaltic meteorites. T e beneath the small volcanoes is found to be small, less than 15 km, which implies that the lithosphere was weak and hot when these volcanoes formed. Conversely, most large volcanoes display higher values of T e , which is consistent with the bulk of their emplacement occurring later in geologic history, when the elastic lithosphere was colder and thicker. Our estimates for the volumes of volcanic edifices are about 10 times larger than those that neglect the flexure of the lithosphere. Constraints on the magnitude of subsurface loads imply that the ratio V i ∕V e is generally 3:5, which is smaller than for the Hawaiian volcanoes on Earth. Plain Language Summary The elastic deformation of the Martian lithosphere from volcanic edifices and magmatic intrusions was investigated for the old and low-relief volcanoes (>3.2 Ga, 0.5 to 7.4 km) and the younger and larger constructs clustered in the Tharsis and Elysium provinces (<3 Ga, 5.8 to 21.9 km). The observed topography is used to define the load and to derive a simulated gravity field that is compared to the observed gravity. The bulk density of the lava that composes all volcanoes is found to be 3,200 kg/m 3 , which is representative of iron-rich basalts as sampled by the Martian basaltic meteorites. The elastic part of the lithosphere, that is, the part maintaining loads over geologic time, is found to have been thin when the small, old volcanoes formed, which implies that the lithosphere was hot early in geologic history. Conversely, larger volcanoes are found to have been emplaced on a cold and thicker elastic lithosphere.

Domaines: Planétologie; Géophysique [physics.geo-ph]

A. Broquet, M. Wieczorek. The Gravitational Signature of Martian Volcanoes. Journal of Geophysical Research. Planets, 2019, 124 (8), pp.2054-2086. ⟨10.1029/2019JE005959⟩. ⟨hal-02324431⟩