RHEOLOGICAL IMPLICATIONS OF EXTENSIONAL DETACHMENTS: MEDITERRANEAN AND NUMERICAL INSIGHTS

Even if the seminal comprehensive descriptions of Metamorphic Core Complexes (MCCs) in the American Cordillera mentionned lower plates constituted of gneiss and intruded by granites (e. g. Snake Range, Whipple Mountains), the actual definition of MCCs : « Cordilleran metamorphic core complexes appear to be bodies from the middle crust that have been dragged out from beneath fracturing and extending upper crustal rocks, and exposed beneath shallow-dipping (normal slip) faults of large areal extent » {as in Lister & Davis, 1989, Journal of Structural Geology, v. 11, pp. 65-94} refers to rocks exhumed from the middle crust whatever their thermal history. The fundamental property of this middle crust resides in its ability fo flow lateraly toward the forming dome, to accommodate stretching of the upper plate and preserve a relatively flat moho. Even though thermal reequilibration can induce weakening of the lower crust, a similar strength profile can also be inherited from pre-extension evolution of the continental crust and promote development of the original structure of MCCs : their detachment. In order to unravel the rheological meaning of detachments, we propose here a review of extensional shear zones described as detachments in the Mediterranean realm, and establish a three end-members typology with « hot MCCs » as one end-member, and two cold MCC end-members with a weak middle crust due to stacking of high pressure metapelitic nappes or a strong upper crust responsible for the strength drop exaggeration between the upper and lower crust. Fully coupled thermo-mechanical modeling experiments allow to test this three end-member typology and determine the critical strength constrast for the perennial development of a detachment zone. An exaggeration of one order of magnitude of the strength contrast between the upper and lower crust appears to be sufficient to promote the development of a MCC, whatever the cause of this exaggeration. We eventually propose an interpretation of extensional detachment zones in the Meditarranean as the index of an intially anomalous strength profile within the crust, and discuss the possible generalization of this interpretation toward syn-orogenic extensional shear zones in the same area.