The role of pre-existing weaknesses in intraplate metamorphic core complex formation during slab retreat: 2-D thermomechanical modelling

A number of metamorphic core complexes (MCCs) developed in the North China Craton and adjacent regions in the Early Cretaceous and were characterized by consistent extensional orientations. These MCCs formed in the continental interior and were conceptually attributed to the retreat of the Palaeo-Pacific (Izanagi) Plate, but the exact physical mechanism remains enigmatic. Here we utilize 2-D thermomechanical simulations to study how mechanical conditions of the continental crust respond to stresses derived from oceanic subduction and their roles in the formation of MCCs. Our results demonstrate that pre-existing weaknesses are key for localized formation within the continental interior. These weaknesses first undergo compression to form thrust faults in response to shallow subduction of the oceanic slab. These thrust faults gradually transform into extensional ones as the oceanic slab starts to retreat, eventually causing the synchronous exhumation of middle-to-lower crustal rocks that form the MCCs. The P-T paths of metamorphic rocks in the core of MCCs reveal a two-stage exhumation, with isothermal decompression followed by rapid isobaric cooling. Sensitivity tests show that (1) stronger upper crust and weaker lower crust favour MCC formation, while lithospheric strength could exert an influence on the formation time of MCCs and (2) when the continental crust is hot (TMoho = 800 °C), a new magmatic dome could form along the continental margin. We suggest that pre-existing weaknesses in the North China Craton played a key role in generating the quasi-simultaneous MCC series in response to the retreating Palaeo-Pacific Plate.