Gravity evidence for an extinct magma chamber beneath Syrtis Major, Mars: a look at the magmatic plumbing system

Syrtis Major is an ancient basaltic shield volcano on Mars with a basal diameter of 1100 km. The free-air gravity anomaly is 126 mGal at spherical harmonic degree 50 and reaches its maximum amplitude over the 2 km deep topographic caldera. The observed gravity anomaly cannot be explained by flexurally supported surface topography and requires the presence of a buried, high-density load. The geologically most reasonable interpretation of this high- density load is that it represents the magma chamber of Syrtis Major, now solidified and filled at least in part by dense igneous cumulates. Pyroxene is likely to be the dominant cumulate mineral in this system, although olivine may also be present. Gravity models presented here define the structure of the buried load and in essence provide a look at the magmatic plumbing system of this volcano. The preferred model involves a buried load that is approximately 300x600 km across, roughly twice as large as the topographic caldera. Both the buried load and the caldera are elongated in the north-south direction. In the center of the buried load, the minimum thickness is 2.8 km for an olivine- dominated cumulate system or 3.9 km for a pyroxene-dominated system. The best terrestrial analog for this structure is the Bushveld Complex, an igneous cumulate body that is similar in size and thickness to the Syrtis Major structure. Assuming that the mean crustal density is 2600 kg m super -3 due to impact brecciation, the elastic lithosphere at Syrtis Major was 10-15 km thick at the time when the topographic load was emplaced. This corresponds to a lithospheric thermal gradient of 28-52 K/km and a surface heat flux of 70-130 mW m super -2. Higher resolution gravity data, such as that which is planned for the 2005 Mars Reconnaissance Orbiter, will permit further refinement of the dimensions of this structure.