Folding In The Evolution Of The Detachment Fault Zone

Under the extension system,the detachment faults formed in the middle-lower crust.Folds(especially the folds that hinge parallel to the stretching lineation)is atypical structure that is unique and typically have some important indicative significance.This paper selects the folds in detachment fault zone of Liaonan metamorphic core complex as the research object.After a large number field observation,the folds are divided into pre-detachment,syn-detachment and post-detachment three types according to the relationship between the timing of folds formed and detachment fault's movement.This paper emphasizes particularly on the study of syn-detachment folds,using microstructure observation,finite strain analysis,quartz grain morphology and quartz c-axis fabric analysis.The syn-detachment folds can be divided into early detachment(al),medium-term detachment(a2)and late detachment(a3)according to their formation time.The main characteristic of early detachment fold is the strict parallelism between the axial plane and the mylonite foliation,and the foliation is superimposed on the axial plane.Medium-term detachment folds mainly are inner lens folds and intraformational rootless folds,large number of folds gathered become fold group.Late detachment folds show curved mylonite foliation,including sheath folds(such as asymmetric or ?-shaped folds).The differences of field morphology and foliation characteristics of different types folds,and their SPO and CPO,provide evidence to recognize them and recover their formation mechanism.They revealed that the folds in detachment fault zone are the outcome of progress deformation,under the interaction of the buckling and bedding squeeze.During the detachment process,all the folds underwent shearing in X direction and buckling on YZ plane.Early detachment(al)and medium-term detachment(a2)folds are mainly by buckling,and late detachment(a3)by flexural-sliping.Folding documents the deformation of the detachment fault within a temperature range(mainly in the 380-500 ?),which is the same with the formation of mylonite foliation,and it demonstrates the genetic and temporal relationship between the folds formation and detachment process.During the detachment process,folds were formed under stable shearing state.The bend of the fold was affected more obviously,showing that its anisotropy is more obvious,and low-temperature deformation superimposed on it.