The Study Of The Features Of Structural Deformation For The Xu-huai Thrust-and-fold Belt,Se North China Block

The pre-Mesozoic period tectonic setting and the nature of early Mesozoic structural deformation of the southeastern margin on the North China Block have been debated,which are major research targets of this thesis.The pre-Mesozoic stratigraphic sequences and rock assemblages,the deformation features of Indosinian period patterns of structural deformation were detailly analyzed.Results in this study show that the Xu-Huai thrust-and-fold belt on the SE-margin of the North China Block are composed of fault-related folds,imbricated faults and step-type thrust faults(ramp fault+ flat fault).Geologic evidence suggest that this belt was subjected to two-phase structural deformation,namely,an early-phase thrust-fold deformation and late-phase extension with partly sinistral strike-slip component followed by dioritic and monzodioritic porphyrites dated at 131-135 Ma.From 80 drilling holes and geophysical data,three units can be distinguished,(1)the Paleoproterozoic crystalline basement in the eastern segment,(2)the nappe unit or the thrust-and-fold zone in the central segment,which is composed of Neoproterozoic to Ordovician carbonate rocks and Carboniferous-Permian coal-bearing rocks,about 2600 m thick,and(3)the western frontal zone.A major detachment fault occurs in the bottom of the napple unit;all pre-Mesozoic depositional sequences were involved into this thrust-and-fold unit and then were overlain unconformably by the sub-horizontal Cretaceous-Paleogene cover consisting of coarse-grained terrigenous clastic rocks,volcanic rocks and red beds.The km-scale thrust body is composed of allochthonous slices,autochthonous strata and detachment fault.Six uncompetent-rock layers with biostratigraphic ages have been recognized,and each uncompetent-rock layer mainly occurred in the top of the footwall,playing an important role in the development of the Xu-Huai thrust-and-fold belt.Geometry of the major detachment fault suggests that this thrust-and-fold belt was rooted in its eastern side,near the Tan-Lu Fault Zone.Plenty of ramp-flat faults and fault-related folds were identified,from that a shortening of 20.6-29.6 km was obtained,corresponding to the shorting rate of 40.5-46.4%.The formation of the Xu-Huai thrust-and-fold belt was likely derived from a SE-ward continental-crustal underthrust of the North China Block beneath the South China Block during the Mesozoic.