Gravity studies of the Tharsis area on Mars

The Tharsis rise on Mars with a diameter of about 8000 km and an elevation up to 10 km shows extensive volcanism and an extensional fracture system. Other authors explained this structure by (I) an uplift due to mantle processes and by (II) volcanic construction. Gravity models of four profiles are in accordance with a total Airy isostatic compensation of the whole rise with mean crustal thicknesses of 50 km and 100 km. But two regions exhibit significant mass deficits: (i) the area between Olympus Mons and the three large Tharsis volcanoes and (ii) central Tharsis. This can be explained by (1) a heated upper mantle, (2) a chemically modified upper mantle, (3) a crustal thickening, or (4) a combination of these three processes. Crustal thickening is mainly a constructional process, but the mass deficit should contribute to a certain degree of uplift causing the extensional area of Labyrinthus Noctis. Gravity modelling results in a different isostatic state of the three Tharsis volcanoes. Pavonis Mons is not compensated, Ascraeus Mons is highly or totally compensated, and Arsia Mons is medium or not compensated. The large, flat volcanic structure Alba Patera has been explained by a hot spot with an evolution of a mantle diapir.The results have shown that the Tharsis rise is a very complex structure. The central and eastern part of the rise is characterized by extensional features and a mass deficit (Extensional Province). The western part is dominated by many volcanic features and a central elongated mass deficit (Volcanic Province). The northern part consists of Alba Patera. It seems unlikely that the whole rise has been generated by one stationary large axisymmetric plume or hot spot. There could have been one or more active hot spots with an evolution in space and time.Contribution Nr. 421, Institut für Geophysik der Universität Kiel, Germany.