Studies on ^(40)Ar/^(39)Ar thermochronology of strike-slip time of the Tan-Lu fault zone and their tectonic implications

Samples of mylonite, ultramylonite and phyllonite were collected from 5 localities in the Anhui part of the Tan-Lu fault zone for 40Ar/39Ar chronological studies. Among them 4 samples from 3 localities on the eastern margin of the Dabie orogenic belt yielded 40Ar/39Ar plateau ages of 128 -132 Ma; and 2 samples from the western margin of the Zhangbalin uplift and eastern margin of the Bengbu uplift gave the same 40Ar/39Ar plateau ages of 120 Ma. Isochron analyses and other lines of evidence suggest that the data are reliable. The data are interpreted as cooling ages of sinistral strike-slip deformation of the Tan-Lu fault zone. The younger ages from the north might be related to slower strike-slip rising. These results indicate that the large-scale left-lateral displacement in the Tan-Lu fault zone took place in the Early Cretaceous, rather than in Late Triassic (Indosinian) as proposed by some geologists. Therefore, this fault zone is an intracontinental wrench fault rather than a transform fault or suture line developed during formation of the Dabie orogenic belt.

Structure, evolution and regional tectonic implications of the Queshan metamorphic core complex in eastern Jiaodong Peninsula of China

The Queshan MCC is an important example of a crustal extensional structure in the eastern Jiaodong Peninsula along the southeastern margin of the NCC in the Early Cretaceous. The MCC is a typical Cordilleran-type core complex with a three-layered structure:(1) the upper plate is constituted by the Cretaceous supradetachment basin and Paleoproterozoic basement;(2) the lower plate comprises the Neoarchean high-grade metamorphic complexes and late Mesozoic granitic intrusions; and(3) the two plates are separated by a master detachment fault. A series of late NEN-oriented brittle faults superimposed on and destructed the early MCC. Petrology, geometry, kinematics, macro- and micro-structures and quartz c-axis fabrics imply that the MCC has a progressive exhumation history from middle-lower to subsurface level(via middle-upper crustal level) under the nearly WNW-ESE regional extensional regime. We present structural and geochronological evidence to constrain the exhumation of the Queshan MCC from ca. 135 to 113 Ma. Based on the comprehensive analysis of the different patterns of extensional structures in the Jiaodong and Liaodong Peninsula, we have defined the Jiao-Liao Early Cretaceou extensional province and further divided the crustal extension of it into two stages: the first stage was the intense flow of the middle-lower crust and the second stage was the extension of the middle-upper crust. Combining the tectonic setting, the lithosphere thinning in the Jiao-Liao Early Cretaceous extensional province can be considered a typical model for the response of crust-mantle detachment faulting under regional extension in East Asia.

Structural deformation and fault activity of the Tan-Lu Fault zone in the Bohai Sea since the late Pleistocene

The Tan-Lu Fault zone (TLFZ) is a significant fractural zone in eastern China and also a seismicity belt in North China. Based on total 4000-km-long shallow penetrated single seismic data with high-resolution, structural deformation and fault activity of the TLFZ in the Bohai Sea since the late Pleistocene are discussed in detail. The results show that the TLFZ with a discontinuous distribution and a general NNE-trending consists of 14 active subfaults with an NNE or NE strike in the Bohai Sea. Seismic data reveal that deformation zones along the subfaults in the central Bohai Sea and the Laizhou Bay are wider and more complex than those in the Liaodong Gulf. Related folds and lots of secondary normal faults which are characterized by nearly vertical fault planes and a same or reverse dip construct the fractural zone in the Laizhou Bay and the central Bohai Sea. Usually, micro sym-metrical grabens develop on the top of anticlines. In the Liaodong Gulf, subfault fractural zones usually consist of secondary normal faults with the same inclination or opposite inclination. Ages of seismic sequences and cutting relation between subfaults and seismic sequences suggest that the latest faulting age of the TLFZ is the end of the late Pleistocene in the Liaodong Gulf and the early Holocene in the Laizhou Bay and the central Bohai Sea. There is a good match between distribution of earthquakes and that of the subfaults in the Laizhou Bay and the central Bohai. Statistical result shows that total vertical offset of the TLFZ since the late Pleistocene ranges from 6 to 11 m. On the basis of offsets of the subfaults, the vertical slip rate is calculated and results show that average vertical slip rates in the central Bohai Sea are larger than those in the Liaodong Gulf and the Laizhou Bay. Slip rates more than 0.06 mm/a during 23-10 ka B.P. and 85-65 ka B.P. are larger than those in other stages. The TLFZ was mainly dominated by tensional normal component since the late Pleistocene. Synthesizing shallow deformation, activity and distribution of earthquakes, the TLFZ in the Bohai Sea can be divided into three segments: the Laizhou-Bay segment, the Bozhong segment, and the Liaodong-Gulf segment.