Strength of Martian lithosphere beneath large volcanoes

Abstract

Abstract.This paper provides an estimate of the thermal state of Martian lithosphere established since the formation of the shield volcanoes. Comparison of the spherical harmonic models of Martian topography derived using harmonics truncated at degrees 10, ll, 12, and 14 shows that the volcanoes have almost no contribution from harmonic coefficients of the topography less than degree l l. Therefore intermediate-scale surface topography and gravity anomalies of Mars, specified by harmonics of degree 11-50, are considered in this study. These harmonics are dominated by shield volcanoes, Alba Patera, and Isidis basin. The central part of Vailes Marineris is also well defined by these harmonics. The strong correlation of the topography and gravity anomalies over these harmonics and the positive Bouguer anomalies of the volcanoes suggest significant excess masses beneath these surface features in addition to their topographic masses. This indicates that the Martian lithosphere has been strong enough to support both the topography and the internal excess masses. We determine the rigidity of the lithosphere beneath a given surface feature. The lithosphere is modeled by a thin elastic spherical shell that overlies an inviscid interior of higher density and is subjected to both surface and internal loads. The thickness of the shell and the magnitude of the internal load are calculated such that the deformed structure resulting from the flexure of the shell under both surface and internal loads gives rise to the observed topography and gravity anomaly. It is shown that the lithosphere beneath Elysium Mons, Alba Patera, and the central part of Vailes Marineris can be modeled by elastic shells subjected to topographic loads alone, whereas the lithosphere beneath Olympus and Tharsis Montes cannot. Internal excess masses of at least 40%, and more likely 50%, of the topographic masses are required, and the lithosphere beneath must have an elastic core of-•100-80 km to support these volcanoes. The mean thermal gradient beneath this region of Tharsis is estimated to be -•7-10 K/km on the basis of the strength envelopes calculated using diabase rheology for the crust of 50 km thickness and olivine rheology for the underlying mantle.

Geologic evolution of the Martian dichotomy in the Ismenius area of Mars and implications for plains magnetization

“…Although there is no surface manifestation of volcanism, it is possible that either a high‐density intrusion formed without any associated surface flows, or that any such flows have been buried by later plains fill material. Analysis of gravity and topography data for a number of large volcanoes indicates that they are constructed from high‐density material, ranging from 3000–3150 kg/m 3 [ Arkani‐Hamed , 2000; McGovern et al , 2002; McKenzie et al , 2002]. Kiefer [2003, 2004] examined the gravity data for Syrtis Major, Tyrrhena Patera, and Hadriaca Patera.…”

“…These areas may indicate subsurface intrusions. Analysis of gravity data for a number of large volcanoes on Mars indicates that they consist of rocks with considerably higher density than normal crust [ Arkani‐Hamed , 2000; Kiefer , 2003, 2004; McGovern et al , 2002; McKenzie et al , 2002]. Such intrusions may be localized along the dichotomy boundary due to a difference in lithospheric thickness, as proposed for the Earth [ King and Anderson , 1995, 1998].…”

Effects of lithosphere on the long‐wavelength gravity anomalies and their implications for the formation of the Tharsis rise on Mars

“…One potential way to resolve this issue on how much a plume may contribute to the elevated Tharsis topography is to carefully infer the elastic thickness for surface loads with similar tectonic age but shorter wavelengths than those for the Tharsis rise [ Zuber et al , 2000; McGovern et al , 2000; Arkani‐Hamed , 2000]. With their short wavelengths, the topography and gravity anomalies of surface loads should not be affected by the plume.…”

Pole wandering of Mars: Evidence from paleomagnetic poles

“…Assuming that the source bodies were magnetized by a dipole core field, he found that most of the pole positions were clustered within a 30 • radius circle centered at 25 • N and 230 • E. Moreover, both north and south magnetic poles were found in the cluster, suggesting at least one reversal of the core field polarity. The anomalies were extracted from a 50 • spherical harmonic model of the magnetic potential field (Arkani-Hamed, 2000) that had been derived using all three vector components of the science-phase and aerobraking-phase low-altitude data acquired within 100-200 km altitudes. There were only a few satellite tracks over a given small anomaly along which magnetic field was actually measured.…”