Tectonic Features,Structures And Accretionary Processes Of The Wandashan Terrane,NE China

The Wandashan Terrane is the only recognized terrane in Northeast China,which is a key area and an important window to investigate the tectonic features and accretionary processes of the continental margin in NE Asia.But there are not enough studies focusing on its tectonics features so far.To make up the regret,we firstly designed several long structural sections which intersect the structural lines of the terrane with high angles.In the next,we observed geological phenomena in these sections,including strata of different ages,rocks of different lithofacies and sediments formed in different environment.Then we concentrated on observing the structure styles,including folds,faults,joints and mélanges,as well as measuring their structural elements.Finally,combining our field work with previous researches from several perspectives,including paleontology,petrography,paleomagnetism,isotopic chronology,geochemistry and so on,we carried out a comprehensive analysis to the tectonic features,structures and accretionary processes of the Wandashan Terrane.The Wandashan Terrane consists of a series of parallel tectonic slices of NNE-trending,which show as a complicated large-scale imbricate fault zone.The Yuejinshan Fault is the front belt of the imbricate fault zone and the westmost boundary fault between the Wandashan Terrane and the Jiamusi Massif.There is a subhorizontal detachment surface at a certain depth under the Wandashan Terrane,above which is the end part shared by folds and faults.The imbricate fault zone above the detachment surface,the indigenous basement under the detachment surface and the Benioff zone under the basement together compose a " three-layer structure ".In the accretionary processes,accretionary complex formed in the trench underwent the accretion toward ocean basin at first,and then turned to an imbricate thrust structure system with the influence of the imbrication of tectonic slices,finally thrust onto the Jiamusi Massif along the large-scale detachment surface.In the thrusting process,dips of fault planes and fold axial planes which formed in the early accretionary stage were rotated,resulting that a large number of fault planes and fold axial planes turned their dips toward the ocean basin instead of the former direction which is toward the continent.The directions of structural lines in the Wandashan Terrane change along with their positions,from north to south,the orientation of structural lines rotate from NNE to NW,and the accretionary complex exhibits a westward bulged arc structure.The original orientation of structural lines,as they formed,is similar to the extending direction of the Jurassic Asian continental margin,whose strike is NE.And then the original structural lines were effected by the strike-slip movement of the Dunhua-Mishan fault zone,bending their southern part into arc-shape.Finally the Wandashan Terrane began to rotate anticlockwise along with the Sikhote-Alin Terrane,resulting in the formation of current structural framework of the Wandashan Terrane.According to the comparison of topographical features,the Wandashan Terrane shares a similar stratigraphic sequence and accretionary age(Oxfordian ~131 Ma)with the Khor Formation of the Bikin Terrane,they belong to a single formation(Hongqiling-Khor Formation).From the Wandashan Terrane to the Bikin Terrane,the Hongqiling-Khor,Ussuri and Ulitka formations appear in order,where we can find that the closer to oceanic crust,the younger the formation is,suggesting that the orientation of accretion is toward ocean basin.In addition,the orientation of structural lines of the Bikin Terrane is commonly same to that of the NNE part of the Wandashan Terrane,indicating that the Bikin Terrane is the NNE-extension of the Wandashan Terrane.From the Paleozoic to the Cenozoic,the Wandashan Terrane underwent several stages of tectonic evolutions each of which has different geodynamic background.The Triassic-late Middle Jurassic is a drifting stage,during which the terrane was transported from equatorial region to eastern margin of the Jiamusi Massif under the effect of northward subduction of the Izanagi Plate.The early Early Cretaceous is an accreting stage,during which the Izanagi Plate continued subducting northward with high speed,resulting that accretionary wedges formed from the continental margin toward the oceanic basin.And then the accretionary wedges was thrusted onto the Jiamusi Massif along the large-scale detachment surface,forming an imbricate fault zone.The early Early Cretaceous is a partially rotation stage,during which the southern part of the Wandashan Terrane was influenced by the left-lateral strike-slip movement of Dunhua-Mishan fault zone,resulting in the formation of arcuate structure.The middle Early Cretaceous is a compression stage between the Wandashan Terrane and the Khanka Massif,during which a near SN intensive compression produced a series of near EW-striking(namely,present-day NE-striking)melanges,thrust nappe faults and superimposed folds.The Late Cretaceous-Paleogene is an entire rotation stage,during which the Wandashan Terrane was influenced by the rotation of the Sikhote-Alin Terrane,resulting in the formation of current structural framework.