Tectonics of Mars

Like the Earth, the crustal properties and structure of the surface of Mars are thought to have evolved through time; in other words, as on Earth, tectonic processes have shaped the planet. However, both the ways this change has happened and the properties of the planet's lithosphere are very different when compared to the Earth. Today, Mars is believed to be largely tectonically inactive. However, observational evidence and its interpretation suggests that this was not the case further back in Mars's geological history. At the scale of the whole planet, two large scale physiographic features are apparent on the surface. The first is that the northern hemisphere of the planet is much lower than the southern, and has been more recently resurfaced – also implying that the crustal thickness beneath the surface is distinctly bimodal. This feature is referred to as the "hemispheric dichotomy". The second is the Tharsis rise, a massive volcanic province that has had major tectonic influences both on a regional and global scale in Mars's past. On this basis, the surface of Mars is often divided into three major physiographic provinces, each with different geological and tectonic characteristics: the northern plains, the southern highlands, and the Tharsis plateau. Much tectonic study of Mars seeks to explain the processes that led to the planet's division into these three provinces, and how their differing characteristics arose. Hypotheses proposed to explain how the two primary tectonic events may have occurred are usually divided into endogenic (arising from the planet itself) and exogenic (foreign to the planet, e.g., meteorite impact) processes. This distinction occurs throughout the study of tectonics on Mars. In general, Mars lacks unambiguous evidence that terrestrial-style plate tectonics has shaped its surface. However, in some places magnetic anomalies in the Martian crust that are linear in shape and of alternating polarity have been detected by orbiting satellites. Some authors have argued that these share an origin with similar stripes found on Earth's seafloor, which have been attributed to gradual production of new crust at spreading mid-ocean ridges. Other authors have argued that large-scale strike-slip fault zones can be identified on the surface of Mars (e.g., in the Valles Marineris trough), which can be likened to plate-bounding transform faults on Earth such as the San Andreas and Dead Sea faults. These observations provide some indication that at least some parts of Mars may have undergone plate tectonics deep in its geological past.

Physiographic provinces

Southern highlands The southern highlands are heavily cratered and separated from the northern plains by the global dichotomy boundary. Strong magnetic stripes with alternating polarity run roughly east to west in the southern hemisphere, concentric with the south pole. These magnetic anomalies are found in rocks dating from the first 500 million years in Mars's history, indicating that an intrinsic magnetic field would have ceased to exist before the early Noachian. The magnetic anomalies on Mars measure 200 km width, roughly ten times wider than those found on Earth.

Northern plains The northern plains are several kilometers lower in elevation than the southern highlands, and have a much lower crater density, indicating a younger surface age. The underlying crust is however thought to be the same age as that of the southern highlands. Unlike the southern highlands, magnetic anomalies in the northern plains are sparse and weak.

Tharsis plateau

The Tharsis plateau, which sits in the highland-lowland boundary, is an elevated region that covers roughly one quarter of the planet. Tharsis is topped by the largest shield volcanoes known in the Solar System. Olympus Mons stands 24 km tall and is nearly 600 km in diameter. The adjoining Tharsis Montes consists of Ascraeus, Pavonis, and Arsia. Alba Mons, at the northern end of the Tharsis plateau, is 1500 km in diameter, and stands 6 km above the surrounding plains. In comparison, Mauna Loa is merely 120 km wide but stands 9 km above the sea floor. The load of Tharsis has had both regional and global influences. Extensional features radiating from Tharsis include graben several kilometers wide, and hundreds of meters deep, as well as enormous troughs and rifts up to 600 km wide and several kilometers deep. These graben and rifts are bounded by steeply dipping normal faults, and can extend for distances up to 4000 km. Their relief indicates that they accommodate small amounts of extension on the order of 100 m or less. It has been argued that these graben are surface expressions of deflated subsurface dikes. Circumferential to Tharsis are so-called wrinkle ridges. These are compressional structures composed of linear asymmetric ridges that can be tens of kilometers wide and hundreds of kilometers long. Many aspects of these ridges appear to be consistent with terrestrial compressional features that involve surface folding overlying blind thrust faults at depth. Wrinkle ridges are believed to accommodate small amounts of shortening on the order of 100 m or less. Larger ridges and scarps have also been identified on Mars. These features can be several kilometers high (as opposed to hundreds of meters high for wrinkle ridges), and are thought to represent large lithosphere-scale thrust faults. Displacement ratios for these are ten times those of wrinkle ridges, with shortening estimated to be hundreds of meters to kilometers. Approximately half of the extensional features on Mars formed during the Noachian, and have changed very little since, indicating that tectonic activity peaked early on and decreased with time. Wrinkle ridge formation both around Tharsis and in the eastern hemisphere is thought to have peaked in the Hesperian, likely due to global contraction attributed to cooling of the planet.

Hemispheric dichotomy

Hypsometry

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