Global tectonics on the terrestrial planets
and why Valles Marineris matters
Plate tectonics, which prevails on Earth since the Proterozoic at least, provides one of the possibilities to evacuate internal heat from the inner planets (Mercury, Venus, Earth, Mars). Internal heat needs to be in sufficient quantity, as well as water in the crust, to make possible plate overthrusting and subduction (like below the Himalayas), and some mechanism to generate rigid plate boundaries. These factors partly depend on how and where the planet accreted in the Solar System, and how it survived the Late Heavy Bombardment.
Mercury, dry due to its proximity to the Sun, illustrates that horizontal plate mobility does not occur in the absence of water. The origin of deformation of the Mercurian crust is conjectural but could lie in shock waves related to a giant impact and volcanic loading tectonics. More on the Planetary tectonic styles page.
Venus is a fascinating case because its diameter almost equals the Earth's diameter, it formed in an orbit close to the Earth's, and has therefore quite a similar composition. There is evidence of intense past crustal deformation, and plate tectonics has been advocated as a mechanism explaining the formation of mountain belts in the Ishtar Terra region, such as Freyja, Danu and Akna Montes around Lakshmi Planum, that some authors compare to the Himalayas/Tibet subduction and collisional system. The existence of deformable plates (tesserae) showing a tectonic style akin to ductile tectonics of Archean cratons, such as the Dharwar in India, has led to the suggestion that these terrains were similarly deformed by sagduction. Transition from sagduction tectonics to plate tectonics is possible by lowering the crustal thermal gradient, causing the crustal brittle-ductile transition to move from the surface downward. The possibility exists, therefore, that the early Venusian crust evolved in a way much similar to the Earth. Then, perhaps some ~1 Gy ago, deformation ceased. The extreme greenhouse effect that exists on Venus nowadays (90 atm and 460°C at the surface) would certainly not allow plate tectonics because of crust dehydration. The origin of the greenhouse effect is not known on Venus, and its understanding has consequences for the Earth within the framework of continuous supply of greenhouse effect gases by human activities. As long as the interplay between causes and consequences is not understood, whether greenhouse runaway is to be anticipated on Earth, causing the loss of mankind, is an open issue.
Mars is still another case, where the dominant stress source is connected to the evolution of the Tharsis volcanic rise, which was shown to control not only the tectonic deformation of the planet, but also the main lines of its geomorphology and hydrology. In 1996 a plume tectonics model was proposed in which Tharsis is very similar to a continental large igneous province on Earth, such as the Ethiopian volcanic province, where flood lava eruption occurs at a "hot spot" in response to a buoyant plume rising through the mantle, impinging on the base of the lithosphere, and partial melting. In the brittle crust, the generated magma is horizontally channelled by huge swarms of vertical magma sheets (dykes). Most of dyke propagation occurs at a depth of several kilometres, controlled by magma buoyancy, and is associated with extensional tectonics ("rifting", somewhat similar to rifting of the East African Rift System, see the Earth picture above) that breaks the weakened lithosphere.
Other stress sources may have existed. For instance, the growing Tharsis magmatic load in the Martian lithosphere, if not built at the equator where it is now located, may have modified the planet's rotation axis, until its present, stable, position. The Tharsis lithosphere would have stored elastic stress during axis reorientation, possibly until failure, producing deformation in a form that still needs investigations.
One of the main problems actually to understand global tectonics on Mars is that most of the deformation of the crust of Mars is extremely old, corresponding to the Archean eon on Earth. Understanding the tectonic events that occurred in this distant past requires examination of exposures of the earliest crust.
What the study of Valles Marineris reveals
The Valles Marineris giant equatorial trough (chasma) system (more on the Valles Marineris summary page), located on the flank of the Tharsis rise, is an unique site to study such exposures. It provides a 700,000 km2 view into crustal processes that occurred over 4 Gy of planetary history through a window up to 10 km deep that exposes the oldest rocks that can be found on Mars. How this window opened is not clear but there is evidence that extensional tectonics played a significant role. Some authors have compared Valles Marineris to a rift in the sense of continental terrestrial rifts, like for instance at the East African Rift System (see the Earth picture above).
The earliest geological events in Valles Marineris have been seldom investigated. Difficulties are diverse. Evidence of the oldest events lies in the deepest parts of chasmata, which are frequently masked by various more recent deposits. The area is really huge, covering it with high resolution orbital images (HiRISE data from the Mars Reconnaissance Orbiter spacecraft, 25-50 cm/pixel) takes time, and the number of geologists interpreting Valles Marineris is very small. As a result, most useful data have not yet been examined in detail. The V-MACS project accepts the challenge of identifying these rocks and interpreting their deformation.
Recent findings in Valles Marineris
Considering that the key to understanding Valles Marineris evolution lies in its earliest rock exposures, a preliminary survey was undertaken in order to identify such outcrops. It revealed the presence of many dykes on chasma floor having an orientation predicted by the plume tectonics model. The model is therefore confirmed, but also its limitations are revealed. The observed thickness as well as the dyke density are indeed evidence that the top of these dykes was removed by erosion of several kilometres of crust, similar to e.g. the Proterozoic Mackenzie dyke swarm of the Canadian shield associated to the opening of the Poseidon ocean, first formed as sub-rift intrusives, then exhumed by erosion. Therefore, the identified dyke swarm not only is consistent with tectonic extension, but also indicates that chasma floor lowering occurred through substantial erosion.
The preliminary survey also revealed additional dyke swarms on the chasma walls that do not follows the orientation predicted by any tectonic model. Dyke swarms are unique tectonic indicators because at regional scale they follow principal stress trajectories. They offer the potential to enrich dramatically our knowledge of Tharsis tectonics and therefore, Martian global tectonics.
Check the Valles Marineris dykes section for some preliminary results.
V-MACS is also interested in tectonics even in the absence of dykes! But there is nothing published yet, check the V-MACS site in a few months.