Mesozoic Structural Characteristics Of The Mishan-Dunhua Fault In Mu Ling,Heilongjiang Province

The Mishan-Dunhua fault zone,northern extending of the Tancheng-Lujiang left-lateral strike-slip fault system,has not been studied clearly.The fault zone cuts through a region of the Mu Ling,Heilongjiang Province.The thesis selects Mu Ling region as a study area to solve some important and basic questions.Meanwhile,it's the key area that the tectonic framework changed from EW-trending to NE-trending.In the region,two stages of granitic intrusions were exposed.Dating of zircon U-Pb LA-ICP-MS method yields weighted mean 206Pb/238U ages of 254±5.0 Ma and 208.8±3.4 Ma,respectively.The isotopic analytical results represent two times magmatic events.moreover,they limit the initial time of the NS-direction compression that formed after 254±5.0Ma.There are two significant deformational stages in Mu Ling-Xia Chengzi section.Firstly,the EW-trending tectonic framework was developed by NS-direction compression.Structural characteristics in the field show that the foliation dips to north,and the folds' hinge trends 95°.By microtectonic analysis,ductile shear deformation was developed,such as a plenty of rotated porphyroclastic augens and quartz's recrystallization with undulatory extinction.Secondly,in south of study area,structural features showing as NW-SE-direction compressional structures were developed.Fold axial planes dip to the southeast,as well,fault planes dip to the southeast.The statistically recovered quartz grains formed during the second-stage deformation,tending to be banded.In addition,latterly,the other two stages of brittle deformation have overprinted those early two-stage deformations:one is a high-angle normal faulting,and the other is brittle sinistral strike-slip faulting.According to both the field geological investigation and microtectonic analysis in Mu Ling,structural stages and tectonic evolution can be re-constructed in different time intervals during the Mesozoic.After regional metamorphism,one stage magmatic intrusion occurred(254±5.0Ma).NS-direction compression existed in this area,after that the compression field's direction changed to NW-SE trending,there are many faults with NE trending and folds with SE-dipping axial surface,and at the same time,also some NE-striking strike-slip deformation occurred.The high-angle normal faulting developed post to the late-stage granitic intrusion,and brittle sinistral strike-slip faulting also developed in the Cretaceous,involving sandstone of the Cretaceous and other older strata and rocks.Since the middle Mesozoic,due to the westward subduction of paleo-Pacific Plate,the EW trending tectonic framework which was formed by north-south-direction compression,changed to NW-SE-direction.The structural feature turns to counterclockwise rotation in a whole east China,or even,East Asia.