Douwe J.J. van Hinsbergen , Thomas Lamont , Carl Guilmette

Accretionary orogens often contain upper crustal nappes derived from subducted continental lithosphere that display (ultra-)high-pressure, low-temperature ((U)HP-LT) metamorphism. Surprisingly, such orogens also contain continent-derived nappes that underwent ‘Barrovian’ (MP-HT) prograde metamorphism instead. Here, we show that these Barrovian nappes were transported at a low angle below the orogenic crustover 150 km or more within ~10 Ma after the inception of its underthrusting. We show for three Barrovian nappes in the eastern Mediterranean region (Kırşehir Block, Menderes massif, Naxos Basal Unit) that they form the deepest exposed structural levels of the orogen and that they are still underlain by 20-35 km thick crust but not their pre-orogenic lithospheric mantle, which forms part of steeply subducted slabs instead. We propose that these Barrovian nappes were accreted by a process of lithospheric unzipping, whereby during continental subduction the crust decoupled around the Moho and underplated the accretionary orogen at low angle while the mantle lithosphere subducted steeply, as a slab. The unzipped crust, unprotected by mantle lithosphere, heated up quickly as it underthrusted the orogen. We propose that continental subduction has three modes: (i) Formation of thin (U)HP-LT nappes during subduction of stretched continental margins; (ii) underplating of thicker, MP-HT continental crust by unzipping; and (iii) eventual arrest of continental subduction with the arrival of unstretched continent. Finally, the process of lithospheric unzipping may have been the default geological response to continental subduction in a hotter, younger Earth, possibly explaining enigmatic hot Proterozoic orogenesis, such as in the Trans-Hudson orogen of Canada.