The Natural Ductile Deformation Of Marbles From Jing'eryu Formation Of Western Hills Of Beijing And Its Regional Tectonic Significance

As a result of tectonic-magmatic events in response to Early Cretaceous tectonic lithosphere extension,the pre-mesozoic rocks from the Western Hills in the central part of the North China Craton suffered from weak metamorphism but intense top-toESE shear deformation.The prominent features of the deformation structures are the coexisting layer-parallel shear zones and intrafolial folds,and the along-strike thickness variations of the marble layers from the highly sheared Mesoproterozoic Jing'eryu Formation.Detailed microstructural studies on the marbles would help to understand the mechanisms of macrostructure development,and furthermore the Early Cretaceous structural evolution of the Western Hills of Beijing.Macrostructural observation reveals that platy marbles are well-developed in the thinner layers.Meanwhile,the intrafolial folds,a special geologic structural phenomenon are often observed in the thicker layers.Most of the folds are tight recumbent folds confined to some specific layers.Their axial planes are parallel to the foliations and layering of the marbles.The fold hinges are always at directions of ESE-WNW,being parallel to the stretching lineations consistently oriented at 130?-310? directions throughout the entire area.It is thus concluded that these folds are basically A-type folds.SPO studies of deformed calcite grains and microstructural analysis of the sheared marbles suggest that the thicker section suffered from deformations homogeneously,while strain localization can be distinguished in the thinner section.Calcite twin morphological and CPO analysis indicate that the deformation temperature at the range of 300?-400? based on calcite twin morphology and calcite fabrics.The differential stresses calculated by the recrystallized grain sizes vary from 47.755 MPa to 68.437 MPa.And all the values of these conditions of the thicker parts are larger than those of the thinner parts.Vorticity analysis based on the calcite c-axis fabrics defined a simple shear component that varies from 18% and 85%.And the oblique grain shape fabrics method yielded 42%-82% simple shear.Because of the emplacement of Fangshan Pluton,Fangshan Pluton blocked the ESE-ward shearing in the eastern-southeastern part of the marble.The combination of the blocking and temperature caused partial stress concentration in the eastern-southeastern part.Because of the stress concentration,the strains in the eastern-southeastern parts are more homogeneous,and the differential stresses are bigger.Form the view of macrostructure,though the eastern-southeastern parts were stretched and thinned much more strongly,the layers were also folded to make intrafolial fold,which caused the thickness of the eastern-southeastern part are much thicker than the western-northwestern part.Thus,we could observe the prominent features of the deformation structures in the Western Hills which are the coexisting layer-parallel shear zones and intrafolial folds,and the along-strike thickness variations of the marble layers from the highly sheared Mesoproterozoic Jing'eryu Formation.