Study On The Late Paleozoic Tectonic Evolution Of The Great Xing’an Ranges

The marine sediments are widely developed in the NE China, and considered to bevery rich in oil and gas resources, therefore become the most promising new areas for theoil exploration. In order to better evaluating the late Paleozoic marine sedimentary oil andgas resource, a reliable tectonic setting is anxiously required. In the paper, we are closelyfocusing on the study of regional tectonic framework and tectonic evolution of the easternInner Mongolian. On the basis of the researches of the late Paleozoic sedimentarycharacteristics, structural characteristics, lithological-paleogeography and geochronologyresearches, to discuss the late Paleozoic tectonic evolution of the eastern Inner Mongolia,further to provide an important basis for the late Paleozoic marine sedimentarybackground, even for the oil and gas resource evaluation.The Great Xing’an Ranges in eastern Inner Mongolia consists of the Ergun massif, theXing’an massif and the Songnen massif (the combined block is termed as ‘North block’ inthe paper). The paleogeostratigraphic study suggests that the eastern Inner Mongolia isdivided into two stratigraphic areas bounded by the Hegenshan-Nenjiang-Heihe suturezone, e.g., Xing’an stratigraphic area including the Ergun and Xing’an massifs, and InnerMongolia grass stratigraphic area including the Songnen massif. The late Paleozoic stratadeveloped in the Xing’an stratigraphic area are distributed in NNE-trending direction, andthey are composed of the early-mid Devonain Niquhe formation, the mid-late DevonainDaminshan formation, the late Devonian Angeeryinwula formation, the early Carboniferous Hongshuiquan formation, the late Carboniferous to early PermianBaoligaomiao formation, the mid Permian Zesi formation, the late Permian Linxiformation and the early Triassic Laolongtou formation. The Paleozoic strata in the InnerMongolia grass stratigraphic area are the late Silurian to early Devonian Xibieheformation, the late Carboniferous Benbatu formation, the late Carboniferous to earlyPermian Amushan formation, the early Permian Shoushangou and Dashizhai formations,the mid Permian Zesi formation, the late Permian Linxi formation and the early TriassicLaolongtou formation.According to the characters of the Paleozoic strata developed in the Ergun, Xing’anand Songnen massifs, and their comparison, the division of the tectonic sequences isfirstly determined as three stages, e.g., Devonian-early Carboniferous tectonic sequence,late Carboniferous-mid Permian tectonic sequence and late Permian-early Triassic tectonicsequence. The Devonian-early Carboniferous strata is characterized by the marinesedimentary, and is conformity with under layered Silurian marine strata and unconformitywith the upper layered late Carboniferous strata, respectively. The late Carboniferousstrata in the study area significantly shows that the north part is continental sedimentary,whereas the south part is marine sedimentary. The late Permian strata is represented by thecontinental sedimentary.In order to better understanding the direction of the stress field in the differenttectonic sequences, the detailed measurements of structural occurrences have been done inthe field investigation. The results show that,(1) the compressive stress direction of theDevonian-early Carboniferous tectonic sequence was NW-SE trending, and the structuralpattern was represented by the broad folds;(2) the stress field was characterized by theextensive stress during the late Carboniferous-mid Permian tectonic sequence, with thedirection of NE-SW trending, and the broad folds and cleavages were widely developed inthe study area; and (3) the compressive stress direction of the late Permian-early Triassicchanged to S-N trending, structural transpositions were well performed in the south part of the study area, in where the schistosity and tight folds were also well developed.The lithological-paleogeographical maps of the study area suggest that,(1) thepaleogeography in early Carboniferous period was the character of continent in the southand sea/ocean in the north. Moreover, the deposit center is located closed to the Yiming inHailar and the Ergun areas;(2) the paleogeography in the late Carboniferous was differfrom former, and showed characters of continent in the north and sea/ocean in the south.In this period, the entire eastern Inner Mongolia uplifted to the surface, indicating theErgun, Xing’an and Songnen massifs had amalgamated as an uniformed block (so called‘North block’);(3) in early Permian, the paleogeography was similar to that of lateCarboniferous, and the Dashizhai marine sediments and the Baoligaomiao continentalsediments were deposited;(4) the paleogeography was not changed till to the mid Permianperiod, but the volcanic activities decreased obviously. The marine basin became more andmore deep, implying the basin underwent settlement stage. The deposit center wasconstrained by the south boundary roughly along the Linxi-Zhalute areas and northboundary along the Huolinguole-Tuquan areas;(4) the paleogeography of late Permiansignificantly changed, showing the continental deposits instead of former marine depositsin the eastern Inner Mongolia, and newly formed a near east-west continental depositcenter along the mid Permian marine deposit center.Combining this study and previous studies, including characteristics of the latePaleozoic strata, structural characteristics, lithological-paleogeography and detrital zirconU-Pb dating results obtained from the eastern Inner Mongolia, we concluded three tectonicevolutional stages, e.g.(1) the Ergun and Xing’an massifs amalgamated in Cambrianalong the suture zone of Xinlin-Xigutu belt. Subsequent to this period, the subduction andcollision between the Ergun-Xing’an block and Songnen massif took place during lateDevonian to early Carboniferous. At the beginning of the subduction process, a‘trench-arc-basin’ system was existed. The final collision was complete at end of earlyCarboniferous along the Hegenshan-Nenjiang-Heihe suture zone. An uniformed block of ‘North block’ was then formed, and underwent the subsequent tectonic evolution together;(2) the post-collision between the Ergun-Xing’an block and Songnen massif was lasteduntil to mid Permian, the study area was therefore belong to the extensive environment;(3)the subduction and collision between the ‘North block’ and North China Craton took placeaccompanied with the closure of Paleo-Asian Ocean, and the final collision was completeat late Permian to early Triassic along the Xar Moron River zone. The study area was thentransferred to the continental evolutional stage, the role of the structural response of themain convergence performed related to the EW trending tight folds and thrusts.