Rheological implications of extensional detachments: Mediterranean and numerical insights

Abstract

The Mediterranean realm shaped by extensive back-arc extension after multiple collisions between Europe and isolated continental blocks is the second densest occurrence of Metamorphic Core Complexes (MCC) after the North American Cordillera. The present review aims at determining the factors controlling detachment system development during post-orogenic extension in the continental lithosphere. 26 different detachment zones over 23 different localities are systematically described and show that MCC structures can be positioned in a three end-member classification. Beside the high-temperature lower plate end-member, most of the time considered as the archetypical MCC type, two cold end-members can be identified: a first one preserving high pressure metasedimentary units in the vicinity of the detachment and represented by numerous cases in the Mediterranean and a second theoretical one with strain localized at the bottom of a resistant upper crust. These two end-members show a strain pattern guided by inheritance more than thermal state. The relevance of this three end-member classification is then tested via numerical modelling of extension of layered continental lithosphere. The compilation of our results and recently published similar models show that over a 100 of different simulations the most critical parameter for developing a MCC is the intra-crustal strength contrast (Smax/Smin, where Smax and Smin stand respectively for the maximum – at the brittle-ductile transition – and minimum – at the Moho – crustal strength). This contrast has to be risen only by 1 order of magnitude to change extension mode. Below a critical value of 1000, narrow and wide rifts develop. MCCs with a perennial detachment are produced for values slightly higher than 1000, while higher intra-crustal strength contrasts systematically promote double-domes MCCs, with strain progressively relocalizing within the lower plate. Inherited layering is as efficient as temperature for raising this intra-crustal strength contrast while strain rate is actually a second order parameter. Metamorphic reactions are also to be considered as a first order process in localizing detachments. KeywordsExtensional detachmentMetamorphic core complexMediterranean tectonicsThermo-mechanical modellingRheology of the lithosphere