Lithospheric Unzipping Explaining Hot Orogenesis During Continental Subduction

Abstract 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 nappes, 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 nappes—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. However, they are missing their pre‐orogenic lithospheric mantle, which forms part of steeply subducted slabs instead. We propose that these Barrovian nappes were accreted by a syn‐subduction delamination process dubbed “lithospheric unzipping”, whereby continental crust decoupled around Moho depth from its steeply subducting mantle lithosphere, and underplated the accretionary orogen at low angle. The unzipped crust, no longer protected from the asthenosphere by mantle lithosphere, heated up quickly while underthrusting the orogen, pushed by the slab. We propose that continental subduction may thus have three modes: (a) formation of thin (U) HP‐LT nappes during subduction of stretched continental margins; (b) underplating of thicker, MP‐HT continental crust by unzipping; and (c) 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.