[63] ALTHOUGH DEFORMATIONAL FEATURES are common on the Martian surface, the type of deformation differs from that on Earth. Deformation of Earth's surface is controlled largely by plate motion. Where plates converge, intense folding, overthrusting, and transcurrent faulting result, and mountain chains may form. Where plates diverge, as at midoceanic ridges, tensional features develop, but they are commonly masked by volcanic deposits. Apparently no plate motion occurred on Mars, and the deformational features associated with plate motion on Earth are absent. The dominant type of deformation on Mars is normal faulting; compressional and transcurrent features, although present, are rare.

Most faults are associated with the Tharsis uplift, a 6000-km diameter, 7-km high bulge in the Martian crust roughly centered on Tharsis. Around the periphery of the bulge, and aligned approximately radial to it, are numerous fractures, some of which extend as far as 4000 hen from the center. So extensive are the radial fractures that they are the dominant structural feature of the entire hemisphere. Fracturing seems to vary greatly in intensity and age. In some places, such as in the Ceraunius and Tantalus Fossae (north of Tharsis) and the Claritas Fossae (south of Tharsis), fracturing is extremely intense; other areas are completely free of fractures. The number of craters superposed on the fractures is a measure of their relative ages and indicates a wide range of ages. Thus, fracturing associated with the Tharsis bulge evidently has continued for much of the planet's history.

Although fractures around 'Tharsis include the most prominent tectonic features on the planet, several fracture systems seem unrelated to Tharsis. Some fractures occur around old impact basins and are generally concentric to them. Especially prominent are the Nilae Fossae around the Isidis basin, but less distinct concentric graben and scarps are visible around the Argys and Hellas basins. Dominantly northeast-southwest and northwest-southeast lineaments are detectable throughout much of the old cratered terrain as escarpments or linear sections of crater walls. Where the old cratered terrain is eroded, as in the fretted terrain, the erosion occurred preferentially along these directions.

Mare ridges are other possible examples of deformational features. Such ridges are common on the sparsely cratered plains of Lunae Planum, Syria Planum, Hesperia Planum, and around the site of Viking Lander 1 in Chryse Planitia. In fact, Viking Lander 1 is believed to have landed on a ridge crest. Similar ridges have been studied intensively on the Moon and are considered to be the surface expression of reverse or thrust faults, which formed either contemporaneously with deposition of the lunar mare rocks, or some time [64] after, as a result of accommodation of the Moon's surface to the lava deposition. The ridges on Mars have a striking resemblance to those on the Moon and are probably of similar origin. Terrestrial analogs have not been found, however, and their origin remains uncertain.

Most of the cratered plains in the northern latitudes of Mars exhibit a polygonal pattern of fractures for which there is no terrestrial analog. Individual polygons average approximately 10 km across and extend uniformly in all directions. Ice wedging and contraction by cooling have been suggested as possibilities, but no completely satisfactory explanation has yet been found.

[65] Ceraunius Fossae. Sparsely cratered plains seen here have been intensely fractured by closely space parallel faults. Individual faults can be traced for up to 300 km. The faults are part of the system of fractures radial to the Tharsis bulge. [39B59; 25°N, 101°W]

[66] Chains of Rimless Pits within Graben of Ceraunius Fossae. Rimless depressions commonly occur in the graben of this area. The pits do not seem to be sources of the extensive lava flows visible in the picture, but instead cut across flows and some fractures. The lines of pits are usually located within graben and not on the intervening plains. [224A13; 32°N, 102°W]

[67]

Fractured Terrain of the Thaumasia Region. An escarpment in the center of this picture is at the south extension of the end of Claritas Fossae. The fractures are roughly radial to the Tharsis bulge and cut mostly old cratered terrain. Crater counts indicate that most of the fractures are older than the corresponding fractures north of the Tharsis bulge. [57A04-13;37°S, 103°W]

[68] (a)	Fractured Terrain North of Olympus Mons. (a) This fracture is part of an old arcuate structure that is partly buried to the south by lavas from Olympus Mons. (b) This stereogram shows the area outlined in (a), and gives a greater sense of the steepness of the walls in the fractures. [211-5528; 38°N, 131°W]
(b)

[69]

Grooved Terrain around Olympus Mons. Grooved terrain, the term applied to the fractured surface in the bottom left quadrant of this picture, occurs in a discontinuous ring around Olympus Mons. The origin if the terrain is unknown, but it has been suggested that it occurs at the surface of vast thrust sheets caused by the loading of the crust by Olympus Mons. Another suggestion is that the terrain is formed by erosion of ash flow tuffs that originated from Olympus Mons. To the northeast, young lava flows transect the grooved terrain and an older fractured surface. [48B43-47; 32°N, 132°W]

[70]

Fractured Plains. Large areas of plains in the 40° to 50° N latitude belt have a fracture pattern similar to that shown here. The pattern resembles those that form on some lava lakes as a result of cooling. It also resembles patterned ground formed by ice-wedging in periglacial regions. The polygons on Mars are, however, approximately 100 times the size of the suggested terrestrial analogs, and their origin is unknown. [32A18; 44°N, 18°W]

Fractured Plains. The fracture pattern here is coarser than that shown in the previous photograph. Most of the crevasses have flat floors. The low hills at the bottom of the picture may be erosional remnants of old cratered terrain, the main body of which occurs just to the south. [35A64; 40°N, 14°W]

[71]

Nilae Fossae. The fractures seen here are concentric to what is probably an old impact basin centered on Isidis Planitia. The fractures appear to be very old, as indicated by the superposition of large impact craters. [211-5657; 25°N, 282°W]

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