Neoarchean Tectonics

The controversy over the Archean tectonic regimes has lasted several decades centering around horizontal and vertical tectonics,the two classical tectonic models for Archean times.Thus,more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian.Anshan which located in the northeastern North China Craton preserves a large amount of ancient basement and abundant structures showing the early earth tectonics.In this study,we have carried out detailed structural analysis of two down-slip ductile shear zones which developed in eastern Anshan area and provided an example for revealing of Neoarchean vertical tectonics in the study area.The Qidashan and Baijiafen ductile shear zones are located along the east and west sides of the Qidashan-Xidabei BIF iron ore belt,and the both limbs of the syncline of greenstone belt,respectively.The ribbon structures formed by intensely elongated felsic minerals are widespread in the deformed gneisses.The gneissosity of the granitic gneisses in the Baijiafen ductile shear zone dip to N50°~70°E with angles of 55°~65°,and the deformed rocks in the Qidashan ductile shear zone exhibit NNE trending foliation with almost vertical angles(265°~330° ? 69° ~88°).Well-developed asymmetric fabrics and mineral stretching lineations indicate an SEE-directed downward shearing in the west limb of the syncline,and nearly W-directed downward shearing in the east limb.Abundant types of middle and low temperature deformed microscopic fabrics are observed in the sheared rocks.Microstructures such as undulatory extinction,subgrain formation,deformation bands,and boundary bulging and subgrain rotation recrystallization are commonly observed in quartz grains.Feldspar porphyroclasts appear elongated,lenticular in shape,showing obvious grain size reduction.The quartz c-axis fabric patterns obtained by electron backscatter diffraction technique imply low to middle temperature non-coaxial deformation with active rhomb <a> slip and basal <a> slip.Deformation behaviors of minerals and quartz crystallographic preferred orientations demonstrate that the rocks underwent mylonitization at a temperature of 400-500 °C under greenschist facies metamorphic conditions.Dislocation creep is the main rock deformation mechanism within the shear zones.Finite strain measurement results suggest that the strain types of the shear zones are generally related to elongate-plane deformation.Toward the iron ore belt,the tectonites change from L=S-to LS-type and the strain intensity exhibits an enhanced trend across the shear zones.Kinematic vorticity values(>0.75)indicate that the deformed rocks in both ductile shear zones were produced by steady-state simple-shear dominated general shear.Based on abundant structural evidence and previous studies,we infer that the vertical tectonics is still the dominant model for Neoarchean crust growth and tectonic evolution in Anshan area.The formation of dome and keel structure,and the deformation of the down-slip ductile shear zones may have resulted from the sagduction of the banded iron formations and synchronous Archean granite dome emplacement,supporting a vertical tectonic regime in Archean times.