Is the lithospheric flexure strong enough to hold up the Tharsis Rise? A re-analysis of flexure on Mars

The largest volcanos in our Solar System are part of a huge volcanic complex, named Tharsis Rise, which is located on the Martian surface several kilometers higher than the average topography. Moreover, the gravitational field of Mars shows a strong and large signal centered on top of the region, a positive anomaly (+300 mGal) surrounded by a negative ring (-300 mGal). Flexural theory is commonly used to understand the relationship between observed topography, crustal structure and gravity, revealing structures that support the volcanic complex.

The new information about the Martian lithosphere thanks to NASA’s Insight mission deserves a re-analysis of the lithosphere flexure models. The Martian lithosphere can be modeled by infinite plate and the thin shell flexure models. The latter takes into account the curvature effect responsible for supporting extra surface loads. We see that the need for compensation based on buoyancy is even lower at long wavelength than that of the classic infinite plate model. This has consequences for the interpretation of density structure underneath the volcanic regions.

After conducting spectral analysis on the topographic and gravity results from the flexural models, we found that the gravitational signal of Martian topography with thin shell compensation fits well with the observed free-air anomaly for degrees n≥2 . The best-fit elastic thickness (T_e) is found to be 105 ±5 km and we observe a crustal density of 3050 ± 50 kg/m³. Despite the use of the thin shell flexure model, we notice a mismatch between modeled and observed gravity field between n=2-4 degrees, which suggests an active large-scale dynamic support of the Tharsis Rise. This could explain relatively the young geologic evidence for surface volcanism on Mars.