Late Mesozoic ductile detachment zones in continental NE Asia: implications for polarity and processes of the crustal extension

Abstract:

Crustal extension in deep levels is usually characterized by localizations of extension strain, i.e., ductile detachment zones. These zones provide available information to insight into timing and mechanisms of crustal extension. Many ductile detachment zones, i.e., master detachments zones of metamorphic core complexes, occur widely across the northeastern section of continental Asia, from Transbaikalia (Russia), to SE Mongolia and to NE China. They are the major features in this province and resulted in the development of one of the largest extensional provinces in the world. This paper summarizes ages and major characteristics of these ductile detachment zones, and use them to constrain the polarity and processes of the deep crustal extension in NE Asia.

Most Late Mesozoic ductile detachment zones 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. Our new and published zircon U–Pb ages and Ar–Ar ages of biotite and hornblende for pre- or syn- (mylonitized) and post-kinematic (unmylonitized) intrusions from these detachment zones suggest that the mid-lower-crustal extension probably initiated at ca. 150–145 Ma with a peak occurring 145–130 Ma and continued to 130–120 Ma.

We interpret the extension in the Transbaikalia, Sino-Mongolia tract to reflect late-orogenic collapse of thickened crust following Middle-Late Jurassic collision along the Mongol-Okhotsk suture. The southeastward extension is probably controlled by crustal-scale top-to-the-SE tangential shear. 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 China might reflect antithetic sub-extensional zones or Mesozoic back-arc extension as a far-field effect of Cretaceous Pacific plate subduction.