The Study Of Waziyu Detachment Fault In The Area Of Mountain Yiwulv

Mountain Yiwulv is a rising mountain range stretching NNE which lies in the east section of Yanshan orogenic belt.Not only does it control the regional tectonic patterns, but also controls the formation and development of Fuxin-Yixian basin in the west and Liaohe basin in the east.Yiwulv metamorphic complex core consists of a footwall of mylonitic Archean gneiss, The Yiwulv Granite, Mesoproterozoic sedimentary strata, a hanging wall of Mesozoic Fuxin-Yixian sedimentary and volcanic units, Waziyu detachment fault that separates a footwall of mylonitic units from a hanging wall of non-mylonitic units.The combination of tectonites in the Waziyu detachment fault zone can be divided into five parts with different deformation behavior and lithologies: mylonitic tectonic schists and tectonic schists band, brecciated mylonite band, fault gouge and microbreccia band, tectonic breccia band and a hanging wall of Sunjiawan Formation consisting of grayish purple polymictic conglomerates.Exposures of the Waziyu detachment fault have dips of 20°-35°Slickensides and step develop on the surface of the fault plane. Stretch fabric such as S-C fabric, stretching lineation defined by alignment of elongated minerals, extensional cleavage develop in the footwall of mylonite, indicating a regional west-northwest sense of extension(ca. 290°).Waziyu detachment fault shows fold-like corrugations geometries in map view.The microtectonic features of tectonites in Waziyu detachment fault zone are as follows: undulatory extinction, deformation band, subgrain, core-and-mantle structure, static recrystallisation and dynamic recrystallisation of quartz and rotating feldspar porphyroclasts. The microscopic and macroscopic tectonic features of tectonites in Waziyu detachment fault zone develop accordingly. First, feldspar porphyroclasts formed during the metamorphic recrystallization of Xiaoqianmaling gneiss.The feldspar cracks in the process of uplifting of footwall and quartz grains around the feldspar develop plastic flow and subgrain in the time, showing ductile deformation of quartz and brittle deformation of feldspar occured simultaneously. Xiaoqianmaling gneiss transformed into mylonitic tectonic schist and tectonic schist in this process. The S-C fabric formed by the plastic flow of quartz aggregates in brecciated mylonite preserved the original characteristics of mylonite, and the cutting of S-C fabric and quartz band by the fractures indicates the superposition of brittle deformation in the later stage.EBSD petrofabric analysis of tectonic schist samples in the footwall shows that maxima is near the Z-axis in quartz c-axis pole figures. This indicates that rocks deformed at low temperature. There are also some secondary maxima in the middle of Z-axis and X-axis, indicating the presence of medium temperature deformation, but its scale and range of influence was very limited. So the deformation temperature is under 400℃.Quartz in quartz veins that injected along foliation of NNE-trending ductile shear belt in the east of Chefang village show microtectonic features such as undulatory extinction and bulging (BLG) recrystallisation. Brittle fracture of feldspar and ductile deformation of quartz happened simultaneously. This indicates the deformation temperature was 250℃-300℃.Bulging recrystallisation of quartz indicates the deformation temperature was 300℃-380℃. Based on the above discussion, we can infer that Waziyu detachment fault formed in a temperature range of 250℃-380℃, undergoing progressive deformation of low to very low temperature.Previous studies on the age of intrusions and mylonites in ductile shear zone in Mountain Yiwulv, volcanic-sedimentary rocks in Fuxin-Yixian basin that overlapped Waziyu detachment fault have two main periods:100-130 Ma and 160-230 Ma.Age of volcanic-sedimentary rocks in Fuxin-Yixian basin is around 120 Ma.Zhang et al. report 40Ar/39Ar plateau ages from mylonitic rocks in the footwall of Waziyu detachment fault;(116土2)Ma(Bi), (127土3)Ma(Bi).Age of the tectonogenesis new minerals in the fault-related mylonite is the most representative for the formation time of the detachment fault.So we think that Waziyu detachment fault formed during Early Cretaceous (ca.127-116 Ma).Through analysis of dynamic mechanism of the footwall tectonic schists, we consider that Waziyu detachment fault formed in the regional extensional tectonic settings. Values of paleo differential stress is estimated at about 31.95 Mpa, indicating Waziyu detachment fault formed by extensional deformation in the shallow crust under the conditions of high stress intensity.