COMPRESSIONAL TECTONISM ON MARS

Contractional features on Mars were identified on the basis of photogeologic evidence of crustal shortening and comparison with terrestrial and planetary analogs. Three classes of structures, wrinkle ridges, lobate 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 feature in Martian highland material. The pattern of contractional features in the western hemisphere reflects the hemispheric-scale influence of the Tharsis rise. Although no comparable hemispheric-scale pattern is observed in die eastern hemisphere, prominent regional-scale patterns exist, the most notable of which occurs in Hesperia Planum. Contractional features that locally parallel the trend of the crustal dichotomy 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 in compressional deformation corresponds with Early Hesperian volcanic resurfacing of a large portion of the planet. Thermal history models for Mars, based on an initially hot planet, are inconsistent with estimates of die timing of peak compressional tectonism and the rate of volcanism. A pulse of global volcanism during the Early Hesperian may have resulted in a punctuated episode of rapid cooling and global contraction that contributed to compressional tectonism. Although global contraction may have contributed a significant component of die total stress 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 features.