The origin of the overall low and variable topographic character of the northern plains is a fundamental aspect of the Martian crustal dichotomy. A model for the development of the Utopia-Elysium lowlands in eastern Mars in terms of the long-term evolution of large, overlapping impact basins is presented. The effects of relaxation of basin topography, volcanic flooding and loading of the basins with subsequent subsidence and downwarping, and the growth of a long-lived major tectonovolcanic center in the overlap region between the proposed Utopia and Elysium impact basins are qualitatively described. In an early hot, thin (106 years). The later Utopia Basin impact drastically altered the structrue of the earlier Elysium Basin, uplifting portions of what had been the central depression.

The heating and fracturing due to the impacts was most intense in the region of overlap, which probably provided an easy conduit for volcanic eruptions in this area. Loading by Early Hesperian and older basin volcanics caused widespread subsidence which may have extended beyond the basin rim and which was responsible for the generally low character of the northern plains, despite the minimum 2--3 km volcanic fill present there. Prolonged volcanism (through the Early Amazonian) would likely keep the overlap region hotter longer, opposing later general subsidence of the impact basin floor where it had cooled. Slower cooling and thickening of the lithosphere in the overlap region is consistent with lower estimates of the Early-Middle Amazonian elastic lithosphere thickness (50 km) in central Elysium, by comparison with inferred greater thickness in the Early Hesperian near the edge of the Utopia Basin at Isidis (>120 km). Prolonged volcanic construction in the overlap region was also probably responsible for the great height of the Elysium tectonovolcanic complex by comparison with the other portions of the Utopia-Elysium lowland, where most volcanics are Hesperian in age. We suggest this model may be applicable to the origin of the northern lowlands elsewhere on Mars and that overlapping large impacts may be important in the establishment of the fundamental topographic and crystal dichotomy and the origin of major tectonovolcanic complexes on the planet. ¿ American Geophysical Union 1990