Planetary tectonic styles

Let's call this section a short background to better understand Valles Marineris as a tectonic object


Select your planet

Displacements, forces and stresses

Forces generate stress that generates deformation. In planetary crusts or lithospheres, contact forces, which act on rock surfaces, and body forces, which affect the entire volume of rock, coexist. Contact forces can be generated by a number of ways; for instance, on Earth, the weight of the cooling oceanic crust makes it enter into subduction zones, a process which generates traction forces (slab pull). Those forces generate the global system of stresses that drive the lithospheric plates and produce orogenic belts, rifts and the large strike-slip faults. On the other hand, body forces that can result in rock deformation are generated by gravity. Landsliding is an example of gravity tectonics.

Tectonic styles at planetary scale

Typically, tectonic style is thought to be identical to the crustal deformation style. However, this style may not depend on the crust only. What is instead relevant is the portion of the planetary radius located above the thermal boundary layer above the convective mantle, if any. On Earth, this zone includes the uppermost part of the mantle and the crust, of distinct composition, which define the mechanical lithosphere. Alternatively, and more simply, the lithosphere may also be defined as the region in which heat is transported to the surface by conduction. Both lithosphere definitions give similar thickness estimates, though calculated differently. Another useful definition of lithosphere is based on seismic wave propagation speed contrasts. Whatever the definition, on other planets, it is easy to understand that the absence of deep geophysical data is a major issue to determine with certainty what the lithosphere covers.

The tectonic style in the considered lithosphere also varies with time, following the evolution of the heat flow, as high heat flow promotes ductility. In addition, a lithosphere that does not deform instantaneously may deform at geologic time scales by viscous relaxation. The style of deformation will then depend as well on the lithosphere composition, the water content, and the loading conditions. On Earth, where the lithosphere is made of two compositionally different components, crust and mantle, the question of how the continent's lithosphere behaves on the longer term in response to these factors is not yet clearly established everywhere (e.g., Burov, 2011), in spite of the wealth of data available. Understanding the style and origin of tectonic activity of other planets is therefore a formidable challenge, in the absence of data regarding the composition and mechanical properties of the lithosphere, and taking into account the huge uncertainties on how the heat flow and water content of the crust have evolved through time.


Cited works

Burov EB (2011) Rheology and strength of the lithosphere. Marine Petrol. Geol. 28, 1402-1443.