Geological and isotopic constraints on the timing of movement in the Tan-Lu Fault Zone, northeastern China

Abstract

The Tan-Lu Fault Zone forms part of a major fault system which can be traced for over 5000 km parallel to the present Asian continental margin. A sinistral displacement of about 700 km has been postulated on the zone, and it has been argued that this displacement occurred between late Cretaceous and early Tertiary times. However, the geological and isotopic evidence presented in this paper indicate that there was substantial ductile movement on the fault zone during the middle Proterozoic, and that normal faulting was active during the late Proterozoic. In west Shandong Province undeformed late Proterozoic (Qingbaikouan) sediments lie with marked unconformity on Archaean to early Proterozoic schists and gneisses, which contain mylonitic shear zones associated with strike-slip displacements in the fault zone. The Qingbaikouan rocks of this area are comparable to the transgressive sequences of the type area near Beijing. However, syn-depositional fault movements along the Tan-Lu Fault Zone resulted in confined depocentres and rapid facies changes. The limestone micrites at, and near, the bases of some of these sequences are interpreted as calcretes which formed on an irregular landscape. Isotopic studies of the gneisses and schists, within and close to the Tan-Lu Fault Zone, have shown that mineral growth and isotopic resetting were predominantly Precambrian events. Rb-Sr whole-rock data suggest that some of the gneisses first crystallized during the early Proterozoic ( c. 2340 Ma). Biotites and hornblendes from a gneiss within the Tan-Lu Fault Zone gave K-Ar ages between 2342-2059 Ma, a confirmation of this early event. Disturbance of the mineral isotope systems and the growth of muscovite occurred at the end of the middle Proterozoic ( c. 1700 Ma), in response to movement along the Tan-Lu Fault Zone. Whole-rock K-Ar ages of mylonites range from 1025 to 519 Ma, a result of low-temperature hydrothermal alteration, probably no younger than early Palaeozoic.