Abrupt shift from trench–parallel to trench–perpendicular backarc extension: Evidence from the Kalaqin metamorphic core complex in the eastern North China Craton

Abstract The driving force of backarc extension remains a subject of debate. In this study, we investigate the Kalaqin metamorphic core complex (MCC) in the eastern North China Craton, to determine the driving mechanism of backarc extension related to subduction of the Paleo–Pacific Plate. The MCC records two phases of extensional structures. D1 structures include the NE–SW–striking Louzidian–Dachengzi and Jinshan–Meilin detachment shear zones on both sides of the core complex. These zones originally constituted a single NE–dipping, low–angle, normal–sense detachment shear zone that records deformation temperatures of 500–600 °C. The shear zone seprarted rifted upper crust from locally deformed middle–lower crust. Together, the D1 structures constitute a Cordillera–type MCC associated with magmatism. The MCC was then overprinted by a second phase of extension (D2) that produced a NE–SW–trending horst between rift basins. The interior of the horst was deformed by early normal shear belts and later normal faults, all striking NE–SW. Our U–Pb zricon dating results from deformed and undeformed igneous rocks, together with previous geochronolical data, constrain D1 at 156–150 Ma (Late Jurassic) and D2 at 135–100 Ma (Early Cretaceous). Kinematic and fault–slip data show an evolution from D1 trench–parallel to D2 trench–perpendicular extension with respect to subduction of the Paleo–Pacific Plate. The D1 trench–parallel, NE–SW extension is considered to have resulted from both west–northwestward subduction of the Paleo–Pacific Plate and southward subduction of the Mongol–Okhotsk Oceanic lithosphere, whereas the D2 trench–perpendicular, NW–SE extension took place after closure of the Mongol–Okhotsk Ocean and is ascribed to the subduction of the Paleo–Pacific Plate only.