Two grabens were developed in the Yi-Shu segment of the Tan-Lu fault zone (TLFZ) during its extensional activities, and are now confined by four major NNE-trending normal faults and filled with Cretaceous sediments. These faults were developed due to their reactivities, containing gouge and cutting the graben sediments. Detailed fieldwork demonstrates that the faults experienced sinistral transtensional moment related to regional NE-SW extension during the reactivity. X-ray diffraction (XRD) analysis of the finest gouge samples gives illite crystallinity values higher than 0.42°Δ2θ, indicating temperatures experienced by the gouge samples were less than 150°C. From the relation between K-Ar data and proportions of detrital illite in different size fractions of the gouge samples, we conclude that refaulting for the western boundary fault of the TLFZ, abbreviated to F4, took place at ca. 90 Ma and for the eastern boundary fault, abbreviated to F1, happened from 70 to 60 Ma. During the two phases of reactivity imposed by the same NE-SW extension, the TLFZ experienced uplifting and no sediments were deposited in the two grabens. It is suggested that the TLFZ experienced extension during the Late Cretaceous, which supports the inference that lithospheric thinning was still undergoing in the east of the North China Craton during the Late Cretaceous magmatic hiatus.
WANG YongShengZHU GuangHU ZhaoQiZHANG BiLongXIANG BiWeiXIE ChengLong
The Waziyu metamorphic core complex is situated at the eastern end of the Yanshan tectonic belt.The NNE-striking detachment ductile shear zone in the core complex lies between the Archean metamorphic basement and Fuxin-Yixian rift basin,dips NW gently,and shows corrugation folds.Exposure structures,microstructures,and quartz C-axis fabrics all indicate top-to-the WNW sense of shear,i.e.,ca.285°,for the shear zone.Estimates of the deformation temperatures(ca.550-250°C) demonstrate its mid-crustal origination and progressive deformation from deep to shallow levels.The northern segment of the shear zone shows relatively weak exhumation with exposures of low-temperature mylonites whereas its middle and southern segments have more intense uplifting with exposures of high-temperature mylonites.Biotite and muscovite 40 Ar/39 Ar ages,U-Pb dating results of zircon from dikes and plutons as well as formation ages of the supra-detachment basin all suggest the formation time of 135-100 Ma for the core complex.The formation was also associated with syntectonic emplacement of the Early Cretaceous Shishan pluton.The western margin of the core complex was truncated by the Sunjiawan-Shaohuyingzi brittle normal fault when it uplifted to shallow crust levels,and finally exhumed to near-surface levels.The core complex was developed by the rolling-hinge model under WNW-ESE extension during the Early Cretaceous peak destruction of the North China Craton.Ductile flow did not appear in the lower plate,therefore not supporting the low-crust gravitational collapse.
