A NEW UNDERSTANDING OF THE UPLIFT OF THE QINGHAI-XIZANG(TIBET)PLATEAU

The uplift of the QinghaiXizang(Tibet) Plateau happened indifferent crustal movements and different time from those of the collision of Asia continent with the allochthonous India crustobody, and the uplift occurred very long after the finish of the collision. According to its temporal evolution, the uplift happened in another active stage of the mantle creep flow after the active stage resulting in the collision and the interruption of the 140Ma′s quiet stage. On the basis of the dynamic analysis, the uplift resulted from the multiple compressing stresses in the reactivation stage after weakening of the colliding stress and the following compressing stress, and after the interruption of the stable stage dominating the vertical movements and represented by formation of the universal QinghaiXizang(Tibet) ancient platform. It was the production of another stress field existing in another crustobody evolution stage and growth age. In the light of the nature of the orogeny, the uplift was caused by the intracontinental Diwa (geodepression)type orogeny after converging connection of the Central Asia Crustobody and the India crustbody which immediately became a part of the Asia continent, and hence after the substitution for the colliding stress and the following compressing stress by the platformtype crustal movements.

Determination of minimum overburden depth for underwater shield tunnel in sands:Comparison between circular and rectangular tunnels

With the development of global urbanization,the utilization of underground space is more critical and attractive for civil purposes.Various shapes of shield tunnels have been gradually proposed to cope with different geological conditions and service purposes of underground structures.Generally,reducing the burial depth of shield tunnel is conducive to construction and cost saving.However,extremely small overburden depth cannot provide sufficient uplift resistance to maintain the stability and serviceability of the tunnel.To this end,this paper firstly reviewed the status of deriving the minimum sand over-burden depth of circular shield tunnel using mechanical equilibrium(ME)method.It revealed that the estimated depth is rather conservative.Then,the uplift resistance mechanism of both circular and rectangular tunnels was deduced theoretically and verified with the model tests.The theoretical uplift resistance is consistent with the experimental values,indicating the feasibility of the proposed equations.Furthermore,the determination of the minimum soil overburden depth of rectangular shield tunnel under various working conditions was presented through integrated ME method,which can provide more reasonable estimations of suggested tunnel burial depth for practical construction.Additionally,optimizations were made for calculating the uplift resistance,and the soil thickness providing uplift resistance is suggested to be adjusted according to the testing results.The results can provide reference for the design and construction of various shapes of shield tunnels in urban underground space exploitation.

Tectonics and Mechanisms of Uplift in the Central Uplift Belt of the Huimin Depression

The Huimin（惠民） depression is a third-level tectonic element of the Bohai（渤海） Bay basin in eastern China.The central uplift belt is the most important oil and gas accumulation zone in the depression,but the lack of adequate geological studies in the area has greatly hindered exploration and development.In this article,using seismic data,fracture mechanics,and a combination of data on fault growth indices and fault throws,we present an analysis of tectonic activity in the central uplift belt and adjacent regions.The amount of extension is calculated along balanced N-S cross-sections,along with the thickness of strata eroded from the fourth,third,and second members of the Shahejie（沙河街） Formation（Es4-Es2） in the uplift belt,by analyzing porosity and stratigraphic correlations.In addition,uplift features are described,and their timing and processes of formation are analyzed and dis-cussed.The results indicate that strike-slip and extensional tectonic movements coexisted,with the effects of the latter most obvious.The spatial and temporal nature of the extensional move-ments is varied：fault activity during the period Ek-Es4 was the strongest on the northern Ningnan（宁南） fault,and activity in the western part of the area was stronger than that in the east,which in turn was stronger than that in the central region;during Es3-Es2,the strongest fault movements were along the eastern part of the middle Linyi（临邑） fault,and activity in the western part of the area was rela-tively weak,whereas in the mid-west it was the strongest.The extensional movements were a response to the activity of the faults.The sediments in the lower part of the fourth member of the Shahejie For-mation（Es4x） show that Es4 was the time when the central belt first began to be uplifted strongly.Uplift was uneven during the Paleogene：the western part of the area was uplifted continuously,while the mid-eastern area underwent alternating periods of uplift and subsidence.During the Paleogene,a number of different tectonic features developed in the central uplift belt at different times and are manifested as follows：during the period Ek-Es4,a gentle slope was formed as part of the Zizhen（滋镇） sag;during Es3-Es2,the northern part of the central uplift belt continued to display a gentle slope as part of the Zizhen sag,but the southern region developed an steep slope as part of the Linnan（临南） sag.There are close interrelationships between uplift,strike-slip,and extension within the central uplift belt of the Huimin depression,as is manifest by the areas of strongest extension being uplifted most rapidly,and also eroded the most.