STRUCTURAL STYLE AND DEFORMATIONAL HISTORY OF THE FUPING ARCHEAN DOMAL COMPOSITE FOLD GROUP

The Fuping area is located in the middle sector of the Taihang Mrs., in Western Hebei province,where the Fuping Group of Archean age is exposed.The age of formation of this group ranges from 2900m.y.to 2560m.y as determined by the U-Pb method,chronologically belonging to late Archean. The Fuping domal composite fold group is 120kin in a NE direction and approximately 70kin across in a SE direction,with an exposed area of more than 8000 sq km.Its southeastern part is not well exposed due to large-scale faulting on the eastern side of the Taihang Mts.The general geometry of this fold group appears to be an incomplete dome-shaped uplift.Its internal structure is quite intricate,the western part is an isocline which dips gently to the west and northwest.Eastwards there occurs a complex fold group composed of a series of domes,brachy-axis antiforms and structural basins, brachy-axis synforms and residual overturned folds which spread as an arc trending NW-EW-NEE.This domal composite fold group is quite similar to the "gneiss fold ovals"described by L.J.Salop,but it is more complicated than those illustrated by L.J.Salop in respect to its formation. The protoliths of the metamorphic rocks in the Fuping area consist predominantly of sedimentary rocks intercalated with small amounts of volcanic rocks,all of which have experienced regional metamorphism of amphibolite facies（some showing granulite facies）and large-scale polyphase migmatization,with anatectic granite formed locally. Adopting the method of structural analysis initiated by Prof.Ma Xing- yuan in their studies on the deformational history at Houshiding and other places east of Fuping,the authors have identified that this domal composite fold group went through three main stages of fold deformation during the Archean tectonic cycle:small recumbent flow folds were formed under high plasticity with accompanying schistosity in the early stage;overturned tight folds originated in tile second stage;a series of gentle and broad domes （brachy-axis antiforms）and structural basins （brachy-axis synforms）as well as crenulation cleavages came into being in tile late stage,with the whole area doming up to form a domal composite fold group. This fold group is the product of specific tectonism and plutonism in a given area of the Archean crust.A model for its formation is proposed as follows:（1）The flow stage of compression and shearing:during this stage the crustal rocks were in a state of high plasticity,and when mantle material moved horizontally in the subcrust,a sort of shearing and compressive force, nearly horizontal,was generated in the lower part of the earth’s crust due to the difference in viscosity between the crust and the upper mantle,and influenced by this force,small recumbent flow folds and regional schistosity resulted.（2）The crust-mantle detaching and stacking stage:non-uniform shearing and compressive deformation brought about local crustal thinning and breaking and caused a part of the crustal rocks to be subducted along the low-angle shear zone to subcrustal depths and underplate and jack another part of the crust and upper mantle,thus resulting in detaching and stacking of the crust and upper mantle and shear deformation of stratified rocks and forming overturned tight folds.（3）The crustal doming stage due to upward intrusion of subcrustal plumes:the crust thickened due to crust -mantle detaching and stacking;the accamulation of mantle heat and addition of alkaline solutions and regenerated magmas in the depth gave rise to a series of plumes of various scales and considerable activity in the deep crust, whose upward intrusion brought about the doming of the crust in the area as a whole to form a dome-basin system.During the course of crustal doming, lateral flow and rotation also occurred,making the internal structure of the composite fold group more complicated. The dominance of a horizontal structural regime in the early stage and a vertical structural regime in the late stage may be regarded as a model for the structural evolution and crust-making process in the Archean high-grade metamorphic regions of northern China.The authors believe that this model is applicable to other high-grade metamorphic regions in the world.