Pattern and kinematic polarity of late Mesozoic extension in continental NE Asia: Perspectives from metamorphic core complexes

Late Mesozoic extension in NE Asia resulted in the development of a large extensional province. Metamorphic core complexes (MCCs) are the major features in this province and have 40Ar/39Ar ages of 130-110 Ma for the mylonites and U-Pb zircon ages of 150-110 Ma for the integral granitic intrusions. Based on this and previous studies, this paper summarizes major characteristics of these MCCs and recognizes a regional kinematic shear sense. Most MCCs in the Transbaikalia region, Sino-Mongolia border tract, and the northwest-central portion of the North China craton (NCC) show a top-to-the-southeast (SE) shear, whereas those in the eastern and southern NCC locally underwent top-to-the-northwest (NW) shear. The three largest basins (Songliao, Huabei and Ordos) in North China are located in the transitional zone between domains of opposing shear sense. We interpret the extension in the Transbaikalia, Sino-Mongolia tract and northwestern part of the NCC to reflect late-orogenic collapse of thickened crust following Middle-Late Jurassic collision along the Okhotsk suture. The southeastward extension is probably controlled by crustal-scale top-to-the-SE tangential shear. The transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage (ecc, i.e., C') in sub-horizontal ductile shear zones late in orogenic crustal thickening. The combined effect of gravitational loading and thermal-uplifting is considered to be the origin of the late-or post-orogenic collapse. The top-to-the-NW extension in the NE of the NCC might reflect antithetic sub-extensional zones or Mesozoic back-arc extension as a far-field effect of Cretaceous Pacific plate subduction.