Formation of lithospheric detachments: quantifying the mechanical effect of hydration reactions

International audienceMany authors have published experimentally determined flow laws of rheologically important monophase aggregatesand polyphase rocks. These laws provide good first order constraints on lithology-controlled lithosphericstrength variations. However, since the whole range of mineralogical and chemical rock compositions cannotbe experimentally tested, variations in reaction-controlled rock strength cannot be systematically and fullycharacterized.We here present the results of a study coupling thermodynamical and Thermomechanical modelling aimingat predicting the mechanical impact of metamorphic reactions on the strength of the mantle during its exhumationin rifted zones.Thermodynamic modelling is used for calculating the mineralogical composition of a typical peridotite asa function of pressure, temperature and water content. For a given P-T condition, the calculated modes andflow laws parameters for each phase constituting the paragenesis are then used as input of the MinimizedPower Geometric model for predicting the polyphase aggregate strength. Hence, by considering P-T evolutionscharacteristic of exhumed mantle, we quantify the strength of the mantle as a function of pressure, temperatureand hydration history in a rift zone. The mechanical impact of such metamorphic reactions and hydration is firstquantified in 1D for three simplified hydration schemes and then introduced in preliminary 2D models whichcoupled fluid transfer to the thermodynamically derived rheological parameters. Schemes with limited hydratationare found to keep rocks in condition close to brittle ductile transition for a longer time and to permit more efficientmantle exhumatio