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Dear all,

I would like to add my two cents to the discussion on the definition of lithosphere/asthenosphere and on how it relates to THE brittle/ductile transition. As already mentionned by a few, we are dealing here with a bunch of physical and chemical parameters and the answer is not straightforward.

In my lectures, I first insist on the distinction between the chemical and physical layering of the Earth. Chemically the Earth is subdivided into the crust, the mantle and the core. The physical transitions between distinct layers (upper crust/lower crust, lithosphere/ asthenosphere, outer core/inner core) that are implied mostly (i) by the way seismic waves propagate through the Earth and (ii) by evidences of longer term deformation related to plate tectonics in particular, indicate that te Earth is also subdivided into mechanical entities that are not correlated one to one with chemical ones.

If we consider that the brittle/ductile transition refers to the transition between material that deforms by rupture and material that deforms by crystal plasticity, I would like to add that there are other rheological transitions of interest within the Earth that are the solid/partially molten transition and the partially molten/magma transition (a partially molten rock is characterized by a continuous solid framework containing intersticial silicate melt and a magma is a silicate melt that includes solids in suspension).

For a given chemical composition, the brittle/ductile transition, but also the solid/partially molten and the partially molten/magma transitions are a function of temperature and pressure, that are themselves changing with depth. This is why there is a brittle/ductile transition within the crust that does not correspond to the brittle/ductile transition into the mantle. Accordingly, although rheology is a function of the material's chemical composition, it is also a function of physical parameters such as pressure and temperature. Note that strain rate is an important parameter but it is intimately linked to rheology as zones of localized deformation will tend to be controlled by zones of contrasted rheology themselves controlled by temperature, pressure and chemical composition.

With regard to plate tectonics, irrespective of the driving force, it appears that the lithosphere is the mechanical entity that is animated by motion relative to the underlying mantle. The crust is passively dragged with the mantle part of the lithosphere as it moves but mechanical transitions within the lithosphere (the brittle/ductile transition, in some cases, the solid/partially molten and the partially molten/magma transitions, and the crust/mantle transition) might act as decoupling layers that will control the style of deformation at plate boundaries of the different mechanical entities that constitute the lithosphere. It is to note that the deformation of the crust is not solely driven by plate tectonics and that gravity comes into play if there are lateral variations of crustal thickness. Getting back to the lithosphere, it can be defined on the basis of strength, kinematics, seismology, thermal state, density, chemical composition. To be provocative, I am not sure that all would agree that the lithosphere/asthenosphere transition corresponds to the brittle/ductile transition within the mantle! As far as I understand, it is more likely to correspond to some threshold value of the strength of peridotite governed by crystal-plasticity or even by the presence of silicate melts.

After looking at a number of textbooks and scientific papers, I find that the definition of the Asthenosphere is particularly controversial. For some, it is just a layer corresponding to the Low-Velocity-Zone defining the lithosphere/asthenosphere transition, for others it includes the entire mantle besides the lithosphere!! (Here is a good topic for an upcoming meeting!)

To conclude, for first year students, I would stick to a definition of the lithosphere based on kinematic data indicating that it corresponds to the outermost rocky shell of the Earth and that it is subdivided into plates that move relative to each other and relative to the underlying asthenosphere. For higher level students, I would suggest to propose different definitions for the lithosphere based on the propagation of seismic waves, strength profiles determined by experimental work, thermal data and models, density and gravity data and models, chemical composition, kinematics and I would not correlate the lithosphere/asthenosphere transition with the brittle/ductile transition in the mantle but propose to keep this question open mentionning the impact of temperature on crystal-plasticity and the potential role of the presence of silicate melt or even the importance of differences in the chemical composition of the mantle as a function of the geodynamic contexts.

I hope that this will help but also generate some comments.

Cheers!
Olivier.

Olivier VANDERHAEGHE

Professeur
Département des Géosciences
UMR G2R, Géologie et Gestion des Ressources Minières et Énergétiques
Nancy-Université
BP 239
54506 Vandoeuvre-les-Nancy
+33 3 83 68 47 34

Professor
Geosciences department
G2R
Nancy-University
BP 239
54506 Vandoeuvre-les-Nancy
+33 3 83 68 47 34

http://www.geologie.uhp-nancy.fr/
http://www.g2r.uhp-nancy.fr/annuaire/vanderhaeghe