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mars is a planet of extremes, and no region epitomizes this more than the Tharsis province, long

recognized as a nexus of volcanic and tectonic activity1. This province seems to be a giant volcanic dome built up from low-viscosity lava flows at the surface and igneous intrusions at depth. The enormous weight of this volcanic load generated stresses within the lithosphere, resulting in global deformation2 and widespread tectonism3,4. Tectonic features outside Tharsis align with the predicted stress patterns4,5, but the picture becomes more complicated within the rise. The triangular Thaumasia plateau forms the southern part of Tharsis, and is something of a geophysical ‘Bermuda Triangle’ of poorly understood tectonic features. It is flanked by the enormous Valles Marineris canyon system to the north, the Claritas Fossae assemblage of faults and rifts to the west, and the ancient Thaumasia Highlands to the southeast (Fig. 1). Montgomery and colleagues6 now propose that the complex structure of the plateau margins originates from a giant landslide: Valles Marineris and Claritas Fossae opened up as gashes where the slipping slab pulled away from the adjoining crust, while the Thaumasia Highlands piled up at the other end.

Valles Marineris and Claritas Fossae are usually considered to be two very different manifestations of extensional tectonism, and the Thaumasia Highlands may have formed by compressional buckling7. However, the mechanisms behind the origin of these three features and their relationships to each other are not entirely clear.

Montgomery and colleagues6 present detailed topographic and structural maps of the Thaumasia plateau region by using images obtained by Mars-orbiting spacecraft. From their analysis, it emerges that a 10- to 15-km-thick slab of material from the upper crust is bounded by distinct zones of crustal deformation. To the northwest, the slab seems to have broken loose from the surrounding crust and slid to the southeast along a vast, gently sloping surface, forming an immense landslide.

The Valles Marineris and Claritas Fossae at the northern and western end of the plateau accommodated the motion of the slab relative to the adjoining crust. The Thaumasia Highlands lie at the toe of the slab, and the large-scale thrusting and buckling there probably formed as the sliding slab ploughed into the crust at its leading end. The pattern of compressional thrust faults, called wrinkle ridges, in the Thaumasia plateau suggests

a downward-and-outward movement of material as it slides from the northwest to southeast, supporting this model.

To explain how such a massive slab could have overcome frictional resistance and moved down the very low slope in this region, the researchers invoke an exceptionally weak crust. They cite evidence to show that the region consists of interbedded salt and volcanic deposits8–10, and suggest that the salt layers assisted the initial detachment of the slab, as well as its sliding motion (Fig. 1b). Heating due to Tharsis-related volcanic activity, or a failed plume beneath this region, could have further weakened the salt layers. Slides similar to those proposed by Montgomery and colleagues are seen on the Earth in low-sloping regions underlain by thick salt deposits. A particularly striking example is the slide at Whiting Dome in the Gulf of Mexico.

The patterns observed in the Thaumasia plateau and Whiting Dome seem suggestively similar at a glance, but the mega-slide model faces considerable challenges. Although the existence of salt-rich deposits on Mars is not debated, and the researchers discuss several lines of evidence to argue for the existence of interlayered salt and volcanic material pre-dating the

PlaNetary scieNce

a mega-landslide on mars The vast Thaumasia plateau on Mars is fringed by extensive zones of deformation. Topographic and structural analysis suggests that the plateau may have slipped in a massive landslide, deforming its margins in the process.

Jeffrey c. andrews-hanna

Figure 1 | Thaumasia’s secrets. Montgomery and colleagues6 propose that the Thaumasia plateau on the southern part of the Tharsis rise on Mars is a giant landslide. a, Key tectonic features in the Thaumasia region, such as the Valles Marineris and Thaumasia Highlands, are shown on a Mars Orbiter laser altimeter map. The Thaumasia plateau is bounded by and includes the Claritas Fossae, Valles Marineris and the Thaumasia Highlands. Cross-section in b is along the white line A–B. b, Cross-section through the Thaumasia plateau showing the suggested stratigraphy and geometry of the mega-slide.

a bb

Claritas Fossae

Thaumasia

Highlands

Arsia Mons

?

A

B

Buried salt layers

North

Claritas Fossae

Thaumasia Highlands

Valles Marineris

Arsia Mons

500 km

A

B

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formation of the slide7–10, this evidence is by no means unambiguous. Even if such layers existed, the slopes in the Thaumasia plateau region (~0.25°) are much lower than those associated with analogous salt-lubricated slides such as Whiting Dome (~1.7°).

Furthermore, the 2,000-km-long detachment surface proposed to exist in the Thaumasia region is two orders of magnitude larger than any terrestrial counterparts. The scale and extent of motion proposed in this paper is comparable to having the entire eastern United States slide into the sea. But like a little brother with something to prove, Mars often mimics the features observed on the Earth at an exaggerated scale bordering on absurdity. Questions of scale can be

countered with the multitude of examples of oversized flood channels, volcanoes and canyons that have been documented on the Earth’s smaller sibling. Continental-scale salt tectonics may just be another addition to this list.

The Thaumasia Highlands are the oldest surface within Tharsis and thus may be the key to understanding the early history not only of Tharsis, but of Mars as well. The model proposed by Montgomery and colleagues6 helps to link some of the key features in this region. More conclusive evidence for the feasibility of a giant landslide is needed, however, to make this hypothesis more than another interesting interpretation of the complicated Tharsis region of Mars. ❐

Jeffrey C. Andrews-Hanna is in the Department of Geophysics, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, USA. e-mail: jcahanna@mines.edu

references1. Carr, M. H. J. Geophys. Res. 79, 3943–3949 (1974).2. Phillips, R. J. et al. Science 291, 2587–2591 (2001).3. Anderson, R. C. et al. J.Geophys. Res. 106, 20563–20586 (2001).4. Banerdt, W. B. & Golombek, M. P. Lunar Planet. Sci.

31, abstr. 2038 (2000).5. Andrews-Hanna, J. C., Hauck, S. A. & Zuber, M. T.

J. Geophys. Res. 113, E08002 (2008).6. Montgomery, D. R. et al. Geol. Soc. Am. Bull. 121, 117–133 (2009).7. Schultz, R. A. & Tanaka, K. L. J. Geophys. Res.

99, 8371–8385 (1994).8. Montgomery, D. R. & Gillespie, A. R. Geology 33, 625–628 (2005).9. Malin, M. C. & Edgett, K. S. Science 290, 1927–1937 (2000).10. Okubo, C. H. & Schultz, R. A. Geol. Soc. Am. Bull.

116, 594–605 (2004).

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