The structure of the present-day Walles Marinetis rift canyons and related features is a consequence of episodic crustal uplift, dominated by two major events. The first of these occurred near the present-day Thaumasia crustal dome and produced a structural pattern in old plains-forming units which is principally transverse to the main trend of the Valles Marinetis canyons. This pattern has provided some structural control over later tectonic and volcanic events. The major crustal deformation in this region is associated with the younger Tharsis uplift, which is responsible for the major trends observed in the Valles Marinetis. These two distinct structural patterns are best observed in the resistant, least eroded parts of the canyon walls, linear trends in such resistant points provide a closer approximation to the original bounding scarps of the Martian rifts than do the modern, highly eroded canyon walls. Crustal deformation was dominated by the Tharsis uplift but began much earlier in Martian history.