## Tectonism

Tectonic activity includes any crustal deformation caused by surface motion. Planetary tectonism is a result of stress and strain within the rigid lithosphere. Stress (r) is the force per area exerted on material while strain (e) is the measure of how much the material deforms when the stress is applied. An elastic material deforms when a force is applied to it, but returns to its original shape when the force is removed. Elastic materials deform according to Hooke's law:

Hooke's law shows that the ratio of the stress r and the elastic strain ee is equal to a property of the material called Young's modulus (E) which measures the "stiffness" of the material.

Alternately, viscous material remains deformed when the applied force is removed. In a simple Newtonian fluid, the rate of change of fluid strain (ef) is directly proportional to the applied stress:

dfif r

The response of a viscous material to an applied stress depends on the viscosity (g) of the material. Most geologically important materials are neither purely elastic nor viscous, but a combination. Viscoelastic materials do not completely return to their pre-deformation appearance but they also do not stay completely deformed when the stress is removed. The viscoelastic strain (ev) is the sum of the viscous and elastic terms:

Therefore

Viscoelastic materials can approach their initial shape over some period of time called the viscoelastic relaxation time (sv). By setting ev = 0 and integrating Eq. (5.17), we find that

Figure 5.23 The presence of near-surface ice weakens the crustal material, allowing viscoelastic relaxation to occur. Features in the mid-latitude regions of Mars often display rounded edges, like the rims of these two craters. This terrain softening is indicative of near-surface ice which can become warm enough to cause such topography to deform. The image is centered near 43.7°S 357.4°E and the larger crater is approximately 36 km in diameter. (THEMIS image I07166004, NASA/ASU.)

where r0 is the initial stress and t is the time. The stress relaxes to a value of 1/e times its original value in sv:

Viscoelastic behavior of the ice-rich martian crust leads to relaxation of mid-latitude topography, including craters and mesas, in a process called terrain softening (Figure 5.23) (Squyres and Carr, 1986).

A viscoelastic material responds elastically to a stress applied on a timescale which is short relative to the viscoelastic relaxation time and viscously to a stress applied on a timescale which is long compared to sv. The material's temperature is an important consideration when determining how it will respond to an applied stress since the material can be brittle at low temperatures and ductile at high temperatures. Planetary lithospheres behave elastically in response to typical stresses. If the applied force is too strong, the material will fracture, creating a fault. The deeper layers of the asthenosphere behave viscously to applied forces and will exhibit ductile behavior.

The strength of the applied force determines whether lithospheric rocks will fold (bend) or fault (fracture). Materials will experience ductile deformation up to some level of r and e during which folding will occur. Once the stress and/or strain rate become too large, the material experiences brittle deformation and faulting results. If the crust is cracked because it moved in response to stresses in the crust, the cracks

Figure 5.24 Extensional stresses cause downdropped valleys called graben. These graben occur south of the Alba Patera volcano. The crater at upper right is 2.7 km in diameter and located at 36.1°N 255.8°E. (THEMIS image I11999006, NASA/ASU.)

are called faults. However, if the crust is cracked due to stresses but no crustal motion occurred, the cracks are called joints.

In 1905 E. M. Anderson realized that tectonic features on Earth resulted from variations in stress orientations. Faults are the mechanism by which the crust is extended in one direction and compressed in the direction 90° from the extension. Anderson's theory of faulting notes that there are three principal stress directions and that the type of tectonic features which results depend on the relative orientations of those principal stress directions. Two of the principal stress directions lie horizontally within the lithosphere while the third is perpendicular to the planet' s surface. The three principal stress directions correspond to the maximum (r1), intermediate (r2), and minimum (r3) stresses.

Extensional stresses will pull the surface apart and produce normal faults, where material on one side of the fault will be downdropped relative to the other side. Normal faults occur when r2 and r3 lie within the lithosphere and r1 is vertical. Many valleys in the Basin and Range Province of the southwestern United States are the result of downdropped blocks (graben) bordered by normal faults. Grabens are common extensional features seen around the Tharsis Bulge on Mars (Figure 5.24).

Compressional stresses push portions of the crust together, creating thrust faults where material on one side of the fault is uplifted relative to the other side. Thrust faults form when r1 and r2 lie within the lithosphere and r3 is vertical. Blocks bounded by thrust faults which have been uplifted relative to their surroundings are called horsts. Wrinkle ridges, common on the martian ridged plains (Figure 5.14a), are compressional features which typically result from sagging of the crust under the weight of large expanses of lava flows.

Strike-slip faults result when r1 and r3 lie within the lithosphere and r2 is vertical. This results in the crustal blocks sliding past one another. If you stand on one side of the strike-slip fault and look across to the opposite crustal block, you classify the fault as a right-lateral fault if the opposite crustal block moves to your right. The fault is a left-lateral fault if the opposite crustal block moves to the left.

Planets with plate tectonics clearly display these three fault classes at plate boundaries. Divergent boundaries, where two plates diverge, display extensional faults while convergent boundaries, where plates converge, display compressional faults. Strike-slip faults occur at transform boundaries where plates slide past each other. Plate tectonics on Mars is still a controversial topic (Section 2.4.2), but such activity would have been limited to very early martian history if it occurred. While extensional, compressional, and strike-slip faults are observed on Mars, none of the geologic features definitively indicate current plate tectonic activity. Localized stresses and strains, rather than global plate tectonics, have dominated the tectonic history of Mars.

The level of current tectonic activity on Mars is unknown because of the lack of seismic data (Section 3.3). Indications of recent volcanism in the Amazonis Planitia and Tharsis regions suggest that seismic activity should occur in those locations. Transection relationships reveal that extensional and compressional tectonism has occurred throughout martian history, with activity primarily concentrated around Elysium and Tharsis (Anderson et al., 2001, 2006). Anderson et al. (2001) determined that tectonic activity in the Tharsis region has occurred throughout martian history with the centers of the activity shifting over time (Figure 5.25). Tectonic activity associated with Elysium has occurred only recently. Noachian-aged faulting was centered in the Claritas region near 27°S 254°E while Late Noachian-Early Hesperian faulting was concentrated along the margins of Syria, Sinai, and Solis Plana. Early Hesperian graben and wrinkle ridges are centered on Syria Planum and Tempe Terra. Extensional faults radial to Alba Patera dominate in the Late Hesperian-Early Amazonian. The most recent tectonic activity in the Tharsis region has centered on the large volcanic shields, with a center near the southern flank of Ascraeus Mons (8°N 200°E). Elysium activity is concentrated in the Middle to Late Amazonian.

The largest extensional feature on Mars is the Valles Marineris canyon system (Figure 5.26), which stretches along the equator for ~4000km between 250°E and 330°E (Lucchitta et al., 1992). Parts of the canyon are up to 6 km below the 0-km elevation contour and up to 11km below the surrounding plains (Smith et al., 2001a). The canyon is divided into three segments based on morphologic changes. The western end of the canyon consists of a series of interconnected canyons called Noctis Labyrinthus. The central portion consists of roughly east-west trending canyons extending for ~2400 km. The eastern section contains irregular depressions which merge with the chaotic terrain and outflow channels.

Figure 5.25 This map shows all the tectonic structures visible on Mars. Map is centered at 0°N 270°E, near the Tharsis tectonic center. (Image courtesy of Robert Anderson, JPL.)

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Figure 5.26 Valles Marineris is composed of a series of smaller canyons. The Valles Marineris system extends over 4000 km near the equator. (MOC image MOC2-144, NASA/JPL/MSSS.)

Figure 5.27 The interior of Valles Marineris shows layered rocks and landslides, providing insights into its geologic evolution. (a) Layers of bright and dark rocks are seen throughout the canyon. This exposure occurs in the western part ofCandor Chasma. The image is ~3km across and located at 5.7°S 284.2°E. (MOC image MOC2-682, NASA/JPL/MSSS.) (b) Landslides are common within the Valles Marineris canyons. This image shows the toe of a landslide in Ganges Chasma, near 8.0°S 315.6°E. (MOC image MOC2-295, NASA/JPL/MSSS.)

Figure 5.27 The interior of Valles Marineris shows layered rocks and landslides, providing insights into its geologic evolution. (a) Layers of bright and dark rocks are seen throughout the canyon. This exposure occurs in the western part ofCandor Chasma. The image is ~3km across and located at 5.7°S 284.2°E. (MOC image MOC2-682, NASA/JPL/MSSS.) (b) Landslides are common within the Valles Marineris canyons. This image shows the toe of a landslide in Ganges Chasma, near 8.0°S 315.6°E. (MOC image MOC2-295, NASA/JPL/MSSS.)

Valles Marineris originated during the Late Noachian or Early Hesperian either from dike emplacement associated with Syria Planum (Mege and Masson, 1996) or stresses associated with the uplift of Tharsis (Smith et al., 2001a). Subsequent subsidence and normal faulting have continued into the Amazonian (Schultz, 1998). MOLA analysis reveals that the canyon is deepest in Coprates Chasma near 300°E, with the western and eastern sections dipping towards that location. The eastern slope of ~0.03° has existed since canyon formation. Water could only have flowed eastward out of the main canyon if the water depth was >1 km to overcome the observed topography (Smith et al., 1999).

The canyon exposes stratigraphic layers emplaced throughout martian history (Figure 5.27a). Thin (tens of meters) strong layers are interspersed with thicker (hundreds of meters) weak layers (Beyer and McEwen, 2005). The strong layers are probably lava flows (McEwen et al., 1999; Williams et al., 2003), but the weaker layers could be sedimentary (Malin and Edgett, 2000a), aqueously altered products (Treiman et al., 1995), or thin/weak volcanic products (Beyer and McEwen, 2005).

The layers are the source of the numerous landslides displayed in the canyon (Figure 5.27b). These landslides are examples of mass wasting processes caused by the martian gravity exerting a downward force on precariously supported materials. Quantin et al. (2004) use crater analysis to determine that the landslides have been occurring since 3.5 Ga ago, with the youngest slides displaying ages of <50 Ma. Both dry granular flows (Soukhovitskaya and Manga, 2006) and wet flows (Harrison and Grimm, 2003) match the morphologies of Valles Marineris landslides,

Figure 5.28 Giant polygons are seen in Utopia and Acidalia Planitiae. They are formed by extensional stresses resulting from uplift or compaction of overlying sediments. These polygons are located in Utopia Planitia near 47.0°N 129.2°E. Image is ~30km in width. (THEMIS image I10119010, NASA/ASU.)

suggesting both mechanisms have operated in different time periods and locations throughout the canyon.

Extensional stresses also are implicated in the formation of giant polygons observed in the northern plains, particularly in Acidalia and Utopia Planitiae (Figure 5.28). These features range in diameter from 2 to 32 km and are bounded by graben with widths between 0.5 and 7.5 km and depths between 5 and 115 m (Hiesinger and Head, 2000). Although a variety of formation models have been proposed for these features, the two dominate models are the tectonic and drape-fold models. The tectonic model proposes that giant polygons result from uplift of basins following removal of large bodies of liquid water (Hiesinger and Head, 2000; Thomson and Head, 2001). The drape-fold model argues that sedimentary layers (of possible lacustrine origin) deposited over rough topography will lead to differential compaction of sediments and produce polygonal features on scales comparable to the underlying topographic variations (McGill and Hills, 1992; Buczkowski and McGill, 2002).

MOLA analysis of the eastern hemisphere dichotomy boundary reveals both normal and thrust faults, indicating that formation of the dichotomy boundary involved both extensional and compressional stresses (Watters, 2003). Compres-sional stresses occurred in many areas of Mars, as indicated by the widespread distribution of wrinkle ridges. Wrinkle ridges are linear broad arches with a superposed ridge (Figure 5.29). They are typically a few tens of kilometers in width and are 80 to 300 m high (Golombek et al., 2001). Wrinkle ridges are now generally accepted to be surface expressions of subsurface thrust faults (Schultz, 2000; Golombek et al., 2001; Watters, 2004; Goudy et al., 2005), although buried impact craters sometimes contribute to wrinkle ridge patterns. Wrinkle ridges are most

Figure 5.29 Wrinkle ridges form from compressional stresses and are often seen on large expanses of flood basalts. This wrinkle ridge on Lunae Planum, near 13.8°N 296.6°E, exhibits the typical morphology of a broad arch with a superposed ridge. The crater near the bottom is —1.9 km in diameter. (THEMIS image V14119007, NASA/ASU.)

frequent on the Hesperian-aged flood basalts called ridged plains. Buried Hesperian-aged volcanic flows, such as that under the northern Vastitas Borealis formation, also show evidence of wrinkle ridges in MOLA analysis (J. W. Head et al., 2002), indicating large-scale compressional folding and faulting of these units.