Lithospheric unzipping explaining hot orogenesis during continental subduction

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

Phanerozoic accretionary orogens typically contain upper crustal nappes derived from subducted lithosphere - oceanic or continental - that display (ultra-)high-pressure, low-temperature ((U)HP-LT) metamorphism. Surprisingly, such orogens often also contain coeval continent-derived nappes that underwent ‘Barrovian’ (MP-HT) syn-burial metamorphism instead. Here, we show examples from the eastern Mediterranean orogen of such Barrovian units, which were transported at a low angle below the orogenic crust over 150 km or more within ~10 Ma after the inception of their underthrusting. These Barrovian units - the Kırşehir Block, Menderes Massif and Naxos Basal Unit - form the deepest exposed structural levels of the orogen and are still underlain by 20-35 km thick continental crust, but are missing their pre-orogenic lithospheric mantle, which is in steeply subducted slabs instead. We propose that these Barrovian units were accreted by a syn-subduction delamination process dubbed 'lithospheric unzipping', whereby continental crust decoupled around Moho depth from its subducting mantle lithosphere, and underplated the accretionary orogen at low angle, pushed by the slab. The unzipped crust, no longer protected from the asthenosphere by mantle lithosphere, heated up quickly during burial. Continental subduction may thus have 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, lithospheric unzipping may have been the default during continental subduction in a hotter, younger Earth, possibly explaining enigmatic hot Proterozoic orogenesis, like in the Canadian Trans-Hudson orogen.