Mesozoic Tectonic Evolution In The Northern Part Of Helan-Chuandian Tectonic Belt

The Helan Tectonic Belt(HTB)is located in the northern part of the Helan-Chuandian North-South Tectonic Belt and sandwiched between the Ordos block and Alxa block from east to west.Its unique geotectonic location and Meso-Cenozoic tectonic activities have attracted more attention from scholars.In the early study of this area,the large-scale east-west thrust and the tectonic pattern of the north-south belt in HTB were obtained,but the time limit and the temporal variation of the tectonic deformation were not further determined.Now,based on more data and scientific research results,it is necessary to analyze and discuss the Mesozoic tectonic evolution of the northern part of the Helan-Chuandian North-South Tectonic Belt.It has undergone multi-stages tectonic evolution since the Mesoproterozoic and retained intricate structural deformation patterns.This tectonic pattern records sufficient information on the tectonic evolution of Helan mountain and its surrounding blocks since Mesozoic and Cenozoic and is a natural window to study the continental tectonic deformation since Mesozoic and Cenozoic in mainland China.This paper analyzes the geometry,dynamics,and kinematics of HTB in detail,and examines the sedimentary and denaturation characteristics of Alxa Block and Ordos Block on both sides of HTB.(1)The Triassic sediments in the helan mountain area are mainly from Alxa block in the west.The Jurassic provenance mainly came from the Alxa block in the west,while the part near the Zhengyiguan fault mainly came from the old metamorphic rocks of the Helan Mountain Group in the north of the Zhengyiguan fault.The cretaceous provenance on both sides of Helan Mountain mainly comes from the central area of Helan Mountain.(2)During the Mesozoic,there were three tectonic deformation stages in The Helan Mountain.The first stage was in the late Jurassic,the northern part of the Helan mountain and the middle part of the Helan mountain belonged to the same tectonic deformation stage.The second stage of tectonic deformation occurred at the end of the late Jurassic.After the first stage of deformation,the main plastic deformation was strike-slip fault,which was also subjected to NNW compression.The third tectonic deformation occurred during the cretaceous and was affected by the WNW extension.Meanwhile,the normal faults on the east and westsides of the Helan Mountain also controlled the uplift of The Helan mountain in the late Jurassic and provided the source for the early cretaceous on both sides.(3)In the Triassic and Jurassic,the western boundary of the Ordos Basin was located to the west of Helan Mountain,and the Alxa block was the western boundary of the Ordos Basin.And The Jurassic Rujigou Basin is a relatively independent local faulted basin.During the cretaceous,the Helan Mountain was the western boundary of the ordos basin and provided the provenance for the western margin of the Ordos Basin.