The Phaethontis quadrangle is dominated by the cratered highlands of Terra Sirenum, which display prominent, marginal basins and tectonic structures, reaching thousands of kilometers in length. Except for the interiors of larger craters, elevations are generally 1–3 km above datum. Tharsis lava-flow materials inundate and partially cover the rugged, ancient terrain in the northeast corner of the quadrangle. Some of the structural basins in the northwestern part of the quadrangle display disrupted floors, referred to as chaotic terrain, most notably Atlantis Chaos and Gorgonum Chaos. The segmented, narrow graben systems of Sirenum Fossae and Icaria Fossae extend southwestward from the Tharsis rise, northeast of the quadrangle, cutting both ancient cratered highland materials and some of the older Tharsis lava flows.