Tectonism and volcanism of the Tharsis region of Mars

The Tharsis region of Mars, where the planet's largest shield volcanoes are located, is at the center of a fracture system that extends over almost half the surface of the planet. The fractures are tensional and aligned roughly radial to Tharsis and form two opposing fanlike arrangements that converge on the line of the Tharsis volcanoes. The fractures are attributed to a broad asymmetric updoming of the Martian crust centered to the southeast of the volcano line. Formation of the dome took place before the formation of the shield volcanoes. The location of the shields was controlled by the fractures, and volcano formation may have reactivated some of the fractures. The updoming, thought to result from mantle convection, separates eras of different volcanism and appears to have been a discrete and unique event in the history of Mars.