Mesozoic Tectonic Deformation And Paleostress Field Aroud The Junction Of Yanshan And Taihangshan Tectonic Belt

The W-E trending Yanshan belt and the NNE trending Taihangshan belt lying in north part of the North China Craton underwent multi-phase tectonic deformations during Mesozoic time.Although many scholars have recognized the main Mesozoic tectonic events based on systimatic geological survey and investigations on tectonic deformations and related geological records,there are still many controversies regarding to the relationships among different tectonic events and various structural elements as well as their formation time.These tectonic deformations around the junction part of the Yanshan and Taihangshan belts are quite important to a better understanding of the spatial and temporal characters of the destruction of the North China Craton(NCC)and the relationship between the two tectonic belts.The research area is located at the junction of the Yanshan and Taihangshan belts,where are viewed as the northwestern margin of the destructed area of the NCC.This area provides people with an idealistic area for studying on the tectonic history and their interrelations of the Yanshan and Taihangshan belts,and the spatial and temporal extent of the destruction of the NCC.Based on detailed studies on structural deformations,radiometric geochronology analysis and paleostress field inversions form fault-slip data,a four-phase Mesozoic tectonic deformation sequence is established with paleostress fields.Following these findings and research results,the relationships between the two tectonic belts,as well as the spatial and temporal distribution of the destruction of the NCC have been discussed.During Mesozoic time,there developed four major phases of tectonic deformations in the research area.(1)In pre-Nandaling period(Pre-174 Ma),W-E trending contractional deformations developed under a S-N compressional paleostress field.(2)In pre-Tiaojishan period(168-157 Ma),NW trending thrusts and folds developed as a result of a NE-SW compressional paleostress field.(3)During pre-Zhangjiakou period(156-143 Ma),the NW-SE compression resulted in the formation of a series of NE to NNE trending thrusts and folds,and lead to the dextral strike-slip movement along the W-E trending fault zones.These contractional and strike-slip structures constitute a large-scale dextral transform contraction tectonic system.(4)During post-Zhangjiakou period(Post-129 Ma),the a series of NNE trending normal faults and rift basins formed as result of NWW-SEE stretching,accompaning with sinistral strike-slip movement along part of the W-E trending fault zones.Theseextensional and strike-slip fault structures formed a large-scale sinistral transform extension tectonic system.The studies on the tectonic deformations in this paper indicate that,although the Yanshan and Taihangshan belts formed in different geological time and belonged to different tectonic systems,the Taihangshan belt has partially inherited and modified the tectonic feathers of the Yanshan belt in its formation and evolutionary process.Besides,many structural feathers of these two tectonic belts were simultaneously formed under a unified paleostress field.The pre-Zhangjiakou dextral transform contraction tectonic system(156-143 Ma)contains numerous NE to NNE trending contractional deformations.Their westernmost margin locates roughly between the Ordos plateau and Taihang belt and defines the boundary between the undestructed craton or the “Intact North China Craton” and the “Modified North China Craton”.The post-Zhangjiakou sinistral transform extension tectonic system(Post-129 Ma)contains plentiful large-scale extenional deformations.Their westernmost margin of these entension deformations lie roghly along the eastern flank of the Taihang belt and step to east via partial reactivated east extending faults south to the Yanshan belt as sinistral strike-slip faults.They form the boundary between the “Modified North China Craton” and the“Destructed North China Craton”.The timing of the switching from dextral to sinistral transform tectonic system provide an exact age constraints on the modificaltion and destruction of the North China Craton.