The Evolution of Volcanism, Tectonics, and Volatiles on Mars: An Overview of Recent Progress

Among the principal accomplishments of the M E W project are several important and widely accepted scientific findings about Mars. The global and regional volcanic flux has been established from systematic geological mapping using V i g images, providing a relative volcanic chronology for most of martian history. Petrologic and chemical analyses of SNC meteorites, inferred to have been derived &om Mars, have revealed an abundance of volatile materials, including hydrous minerals (amphiboles), oxidized sulfur, possible carbonates, and various salts; these results provide direct evidence that Mars is likely to be ~olatile~rich. Geological mapping and temporal groupings of major fault systems have provided strong new c o n s b t s on the sequence and timing of martian tectonic events, particularly in the vicinity of the Tharsis region. Substantial progress was also forthcoming on several fundmental but incompletely resolved aspects of martian evolution during the MEVTV program. The origin of the crustal dichotomy on Man was the subject of intensive investigations, and two classes of hypotheses have emerged: One relates the northern lowlands to the effects of l q e impact(s); the other calls for tectonic foundering subsequent to subcrustal erosion by mantle convection. The kinematics and mode of formation of wrinkle ridges are the subjects of continuing research. While it is clear that these features represent compressional deformation, whether they are predomhantly the expression of buckling or faulting remains problematic, as do the cause and implications of their remarkably straight trends and periodic spacing in some locales. Isotopic and petrologic analyses have revealed significant variations within the group of SNC meteorites that, if these objects come from Mars, suggests heterogeneous sources for the parental martian magmas. Finally, several intriguing new ideas have resulted from project-sponsored research and workshop discussions. A number of tectonic features have been identi£ied as probable transcurrent faults documenting horizontal offsets of at least several tens of kilometers. Such faulting, along with the large extensional strains required to form Valles Marineris, could indicate an early episode of significant horizontal motions of lithospheric blocks. One speculative hypothesis is that such motions were accommodated in a very early episode of plate tectonics on Mars, during which the original lithosphere of the northern lowlands was subducted beneath the Tharsis region and new lithosphere with thinner crust was generated at a now-extinct spreading center. This hypothesis, which remains to be tested rigorously, links the formation of the crustal dichotomy to the formation of Tharsis.