Geologic map of the western equatorial region of Mars

The regional topographic swell centered in the Tharsis Montes and Syria Planum extends over one-third of the map area (U.S. Geological Survey, 1976). Within this high region are the four largest and youngest volcanoes on Mars: Olympus Mons, Arsia Mons, Pavonis Mons, and Ascraeus Mons. The latter three collectively form the large northeast-trending, volcanic mountain chain of the Tharsis Montes. This volcanic chain lies athwart the global highland-lowland boundary that transects the western equatorial region. The boundary, where not covered b young lava flows, is marked by a gentle, irregular scarp bordered by clusters of low knobby hills. It separates the relatively smooth, flat, sparsely cratered northern plains from the higher, rougher, more densely cratered plateau terrain of the southern highlands. A vast system of canyons, the Valles Marineris, originates in Noctis Labyrinthus, just east of the Tharsis-Syria swell, and extends eastward for several thousand kilometers. Ancient rivers channels appear to have sources in canyons and chaotic terrain north of Valles Marineris and in a large fissure at the head of the Mangala Valles. Visible remnants of large circular basins are not as common on Mars as on the Moon, probably because of more active erosion and deeper burial on Mars by eolian and fluvial activity and by widespread volcanism on both the highlands and plains. The youngest and best preserved impact basin on Mars, lying in the western hemisphere, is more than 1,000 km across. It has a broad flat floor (Argyre Planitia) covered by eolian materials and volcanic flows; the floor is several kilometers below its encompassing rough mountainous rim.