The Chaihe Thrust Faulting In The Central Great Xing'an Range And Sandstone Provenance Analysis

The Great Xing'an Range passes through NE China,and is tectonically located in the eastern part of the Central Asian Orogenic Belt.It forms a characterized basin and range topography with the Songliao Basin in the east and Hailar-Erlian Basin in the west.The Great Xing'an Range is not only a topographic gradient zone,an important gravity anomaly gradient zone,it is also a belt of crustal thickness.The tectonic evolution in the area began in the Paleozoic,and subsequently underwent the transformation in the Mesozoic,closely related to the subduction and closure of Paleo-Asian Ocean and Mongo-Okhotsk Ocean and the subduction of Paleo-Pacific Ocean regimes.The superposition of different tectonic regimes make the Great Xing'an Range a key area to understand the NE Asian tectonic evolution,and provide important evidences for the study of Central Asian Orogenic Belt.The tectonic background of the widely exposed Mesozoic volcanic rocks in the NE China may be caused by the continental extension after the Mongol-Okhotsk Ocean closed,or the slab rollback of the Paleo-Pacific Ocean.However,there are still widespread disputes about how those two regimes work and whether the superposition happen,which also extends to the question of the closure time of the Mongol-Okhotsk Ocean.Research from thrust fault in the Chaihe area in the middle part of the Great Xing'an Range may provide new ideas.This paper performed field geological observations and finite strain analysis on the thrust fault,combined with the provenance analysis of the Murui Formation sandstone,which formed at the same time with the thrust fault,to reveal the tectonic background of the study area in the late Mesozoic.The study area is located near the Chaihe town,southwest of Zhalantun,Inner Mongolia,between the Siberian plate and the North China Craton,the intersection of the Paleo-Asian Ocean and Paleo-Pacific Ocean tectonic regimes,and also enclosed by a series of late Paleozoic accretionary orogens between the Erlian-Hegen fault and Deerbugan fault.The Chaihe thrust fault extends dozens of kilometers northeast in the area.Detailed analysis of the macro-micro deformation structures shows that the fault plane is inclined to the northwest(about 310°)and the angle of dip is between 45°-50°.The footwall of the fault is Middle Jurassic andesite of Tamulangou Formation,and the hanging wall is Ordovician metamorphic siltstone of Luohe Formation.The oldest layer covering the fault is the Lower Cretaceous Manketou Ebo Formation.It is inferred from the stratum contact relationship that the activity time of thrust is the Late Jurassic.The rocks near the fault had been strongly deformed,and structures such as folds and cleavages are very developed.Based on the occurrence of cleavage plane of the fault,we found its movement direction is from northwest to southeast(about 135°).At the same time,the finite strain analysis of andesite of the Tamulangou Formation with strong deformation from the footwall shows that the type of rock strain is SL,mainly occurred pancake-shaped deformation,which means compression.The upper and lower strata(the Tamulangou Formation and the Manketou Ebo Formation)of sandstone of the Murui Formation in the Chaihe area are same as the thrust fault,indicated that they were in the same tectonic setting.Following the Dickinson method,most of the fragments monominerals in the Murui Formation are lithic,which may be derived from the nearby foreland uplift,and accompanied by a rapid deposition process,without long distance of transportation.This result is consistent with the intensity analysis of chemical weathering.The CIA and CIW indices of the sandstone are both in the lower range,indicating that the sedimentary clasts were quickly in place and consolidated into rocks after undergoing short-term and close-range transportation.The whole rock geochemical analysis of 8 sandstone samples of the Murui Formation indicates that their source rock is mainly composed of felsic igneous rocks,and they were formed in the active continental margins.Combining current research in the region and the study of the Chaihe thrust fault and the Murui Formation sandstone,we concluded that the Chaihe thrust fault was formed in the late Late Jurassic.During this period,the tectonic plates in the Great Xing'an Range were moving very fast.The block located northwest of the study area was uplifted by compression,and then gradually pushed southeast over the study area,causing the formation of thrust fault.The dynamic background of this tectonic process may be the continental collision after the closure of Mongol-Okhotsk Ocean.During the Middle Jurassic and early Late Jurassic,the subducted Mongol-Okhotsk oceanic slab occurred rollback,and the Great Xing'an Range area was in the extension tectonic setting,which also explained the alkaline rocks found in this period.The Mongol-Okhotsk Ocean was closed in the late Late Jurassic to early Early Cretaceous,and the continental collision produced a strong compression,forming the Chaihe thrust fault.Subsequently,the extensional setting after the continental collision affected most of the NE China,resulting in strong magma eruption and acid magma intrusion,forming the wide Great Xing'an Range volcanic belt.