Assessment And Zonation Of Regional Crustal Stability In Zhangjiakou Region

Zhangjiakou is one node city of Beijing-Tianjin-Hebei synergy development region,is one of the important cities in the Beijing-Tianjin-Hebei urban agglomeration,also is co-host city of the Beijing Winter Olympic Games in 2022.On the one hand,Zhangjiakou starts undertaking outsourcing Beijing industry with the development of economy and construction in Beijing-Tianjin-Hebei synergy development region.On the other hand,host winter Olympics promote the growth of economic growth and social development of this region.Zhangjiakou region is located at the intersection of Zhangjiakou-Bohai Structral Belt,Shanxi Graben Zone,Taihang Shan Uplift and Yanshan Uplift,where exist complex tectonic deformation,a great mangy active faults,and intense historical earthquakes activities.Therefore,crustal stability assessment and zonation in Zhangjiakou region is an important research subject,meanwhile,is one necessary factor which must be considered in natural resources regionalization,development program and construction.Apparently,crustal stability assessment and zonation in Zhangjiakou region is significant for social development,economic growth and scientific research.In this book,crustal stability assessment and zonation in Zhangjiakou region was studied by using some theories and technical methods of structural geology and engineering geology.The emphasis of this study was focused on the assessment of tectonic stability.Therefore,this study highlighted the research of tectonic evolution,active faults,current in-situ stress state and seismic risk in t process of crustal stability assessment.Meanwhile,the stability of partial foundations(rock and soil masses)and surface slopes(which include landslides,collapses,debris flow)were also taken into consideration.The evaluation and zonation of regional crustal stability was conducted on two scales and scopes:the Zhangjiakou region and the vicinity of the site of the winter Olympics.A great deal of approaches,which include field investigation;fission track(FT)dating;optically stimulated luminescence(OSL)dating;in-situ stress measurement;integrated geophysical exploration;and drilling work,were taken to accomplish this study.This study provides a better understanding of crustal stability in Zhangjiakou region,in addition,this paper arrived at the following main conclusions and understandings.(1)Apatite and zircon fission track data from Zhangjiakou region at the southwestern margin of the Yanshan mountain suggest that this region experienced a seven-stage tectonic evolution,including:1)rapid uplift during early Cretaceous;2)a stage of slow cooling during late Cretaceous-Paleocene;3)Himalayan movement I episode during late Paleocene-early Oligocene;4)a stage of slow cooling during early Oligocene-late Oligocene;5)Himalayan movement II episode during late Oligocene-Miocene;6)a stage of slow cooling during late Miocene;7)Himalayan movement ? episode during Pliocene.The characteristics of Zhangjiakou region inherited the neotectonic movement since the late Quaternary,crustal movement is given priority to with vertical movement,and this region is still in the stage of relative activities.(2)The strike direction of most active faults in Zhangjiakou region is NW-NWW and NE-NEE.These faults control a series of the late Cenozoic sedimentary basins,which are distributed with mainly sinistral and en echelon form.Active faults with NE-NEE strike direction are normal faults,primarily,and active faults with NW-NWW strike direction are normal and sinistral slip.The activity style of faults with NE-NEE strike direction changed from thrust during late Mesozoic into a normal fault since Pliocene,it recorded the process of regional crustal shortening;extension;and destruction of lithosphere.Late Mesozoi,regional crustal shortening in North China maybe the response to multidirectional gathering system among different continents in East Asia.Since Neogene,development of normal fault in North China may be the first far-field response to the collision and convergence of the Indian and Eurasian continents.(3)In order to better understand in-situ stress environment of Zhangjiakou region and to evaluate the seismic risk,deep borehole in-situ stress measurement is necessary.Data was gathered from 12 in-situ stress sensors installed in deep borehole(600 m)at the east of Zhangbei county.The in-situ stress data reveals that the geostress level of the region is relatively high and both horizontal principal stresses in two tests are larger than the vertical stress(SH>Sh>Sv).The direction of the current maximum horizontal principal stress is N59°E.The results of direction of the current maximum horizontal principal stress reflect that the present activity property of Zhangjiakou Fault is sinistral slip,and that the present activity property of the faults with NE-NEE direction is normal.(4)The main engineering geological types of rock and soil masses in Zhangjiakou region are metamorphic rocks and magmatic rock;sedimentary rocks are mainly distributed in the south Zhangjiakou region.Four engineering region of rock and soil masses can be classified,thay are hard rock engineering region;sub-hard rock engineering region;sub-weak rock engineering region;and weak rock engineering region respectively.According to the structure characteristics of different rock group,13 engineering subregions are further divided.Thay are hard intrusive rock group;hard thick layer-block metamorphic rock group;hard block volcanic rock group;hard medium-thick carbonatite group;sub-hard volcanic rock group;sub-hard carbonatite group;clastic rock group;sub-weak clastic rock group;and weak clastic rock group.(5)Geohazards in Zhangjiakou region is given priority to with debris flow,which are mainly distributed in the bedrock mountains or transition parts between mountain and basin.Collapses and landslides are mainly distributed in the bedrock mountains,bank of valley and slope construction by human.The formation reasons of collapse,landslide and debris flow mainly are topography and geomorphology,lithology,geological structure and rainfall.The influence of the rainfall is the main inducing factor.(6)The results of the assessment and zonation of the crustal stability can be summarized as follows:(1)the general crustal stability of theZhangjiakou region is in a less-stable-stable state,there was little less-unstable-unstable area;(2)the main unstable area located in North Yan-fan Basin fault belt and North Huai-zhuo Basin fault belt,where are key fortification region for the construction of Beijing-Zhangjiakou high-speed rail;and(3)the general crustal stability of vicinity of the site of the winter Olympics is in a less-stable-stable state,therefore,assessment and zonation of regional crustal stability provided provides geological and technology support for winter Olympics in 2022.