COMPRESSIONAL TECTONISM ON MARS

Contractional features on Mars were identified on the basis of photoge ologic evidence of crustal shortening and comparison with terrestrial and planetary analogs. Three classes of structures, wrinkle ridges, lo bate scarps and high-relief ridges, were mapped and their spatial and temporal distribution assessed. Wrinkle ridges account for over 80% of the total cumulative length of the mapped contractional features and occur in smooth plains material interpreted to be volcanic in origin. Lobate scarps, not wrinkle ridges, are the dominant contractional feat ure in Martian highland material. The pattern of contractional feature s in the western hemisphere reflects the hemispheric-scale influence o f the Tharsis rise. Although no comparable hemispheric-scale pattern i s observed in die eastern hemisphere, prominent regional-scale pattern s exist, the most notable of which occurs in Hesperia Planum. Contract ional features that locally parallel the trend of the crustal dichotom y boundary in the eastern hemisphere suggest the influence of stresses related to the evolution of die dichotomy. Compressional deformation apparently peaked during the Early Hesperia, if the tectonic features are roughly the same age as the units in which they occur. This peak i n compressional deformation corresponds with Early Hesperian volcanic resurfacing of a large portion of the planet. Thermal history models f or Mars, based on an initially hot planet, are inconsistent with estim ates of die timing of peak compressional tectonism and the rate of vol canism. A pulse of global volcanism during the Early Hesperian may hav e resulted in a punctuated episode of rapid cooling and global contrac tion that contributed to compressional tectonism. Although global cont raction may have contributed a significant component of die total stre ss that resulted in compressional deformation on Mars, nonhydrostatic horizontal stresses derived from local and regional-scale sources are necessary to account for the uniform orientations of die tectonic feat ures.