Under the Volcano: Gravity Evidence for an Extinct Magma Chamber Beneath Syrtis Major, Mars
Abstract
Syrtis Major is a Hesperian age shield volcano on Mars, 1100 km in diameter and just 1 km high. Two calderas, Meroe Patera and Nili Patera, are set within a broader topographic summit depression. The regional gravity field is well modeled by flexural support of the surface topography. For a crustal density of 2800 kg m^{3} and a mantle density of 3400 kg m^{3}, the best fit elastic lithosphere thickness is 10 to 15 km. Increasing the crustal density requires a decrease in the lithospheric thickness. There is a pronounced freeair gravity anomaly over the summit caldera complex. This gravity high has an amplitude of 100 mGal through spherical harmonic degree 40 and 124 mGal through spherical harmonic degree 50 and is thus robustly determined by the Doppler tracking data from Mars Global Surveyor. The anomaly has a high degree of axial symmetry, with a low amplitude extension to the south. The gravity high occurs over the caldera's topographic low and thus requires the presence of dense material in the subsurface. The spatial association between the caldera and the buried mass anomaly suggests that the subsurface structure is due to the accumulation of dense igneous cumulates in a now solidified magma chamber. Because of the symmetry of the observed anomaly, it is modeled as a buried vertical cylinder in order to minimize the number of free parameters that must be constrained. The width of the observed anomaly constrains the cylinder radius to a maximum of 150 km. Assuming that the intrusive body is olivine similar to the martian meteorite Chassigny (Fo68, density 3500 kg m^{3}), the intrusive body has a minimum thickness of 5 km. A pyroxene rich intrusive, similar to the martian meteorite Nakhla, would have a lower density and thus a greater minimum thickness. The total mass anomaly is between 1.82.7 ⋅ 10^{17} kg. This constitutes only 510% of the total mass of the volcano and its subsurface root. Thus, generating the required amount of cumulate minerals is probably not a difficult petrological problem.
 Publication:

AGU Fall Meeting Abstracts
 Pub Date:
 December 2002
 Bibcode:
 2002AGUFM.P71B0463K
 Keywords:

 5417 Gravitational fields (1227);
 5480 Volcanism (8450);
 6225 Mars