Continental Dynamics in High Tibetan Plateau: Normal Faulting Type Earthquake Activities and Mechanisms

Various earthquake fault types were analyzed for this study on the crust movement in the high region of the Tibetan plateau by analyzing mechanism solutions and stress fields. The results show that a lot of normal faulting type earthquakes are concentrated in the central High Tibetan plateau. Many of them are nearly perfect normal fault events. The strikes of the fault planes of normal faulting earthquakes are almost in an N-S direction based on the analyses of the Wulff stereonet diagrams of fault plane solutions. It implies that the dislocation slip vectors of the normal faulting type events have quite great components in the E-W direction. The extensions probably are an eastward extensional motion, being mainly a tectonic active regime in the plateau altitudes. The tensional stress in the E-W or NWW-SEE direction predominates earthquake occurrences in the normal event region of the central plateau. The eastward extensional motion in the high Tibetan plateau is attributable to the gravitational collapse of the high plateau and the eastward extrusion of hotter mantle materials beneath the east boundary of the plateau. Extensional motions from the relaxation of the topography and/or gravitational collapse in the high plateau hardly occurred along the N-S direction. The obstruction for the plateau to move eastward is rather weak.     

Active Faulting Pattern,Present‐day Tectonic Stress Field and Block Kinematics in the East Tibetan Plateau

Abstract: This paper examines major active faults and the present-day tectonic stress field in the East Tibetan Plateau by integrating available data from published literature and proposes a block kinematics model of the region. It shows that the East Tibetan Plateau is dominated by strike-slip and reverse faulting stress regimes and that the maximum horizontal stress is roughly consistent with the contemporary velocity field, except for the west Qinling range where it parallels the striking of the major strike-slip faults. Active tectonics in the East Tibetan Plateau is characterized by three faulting systems. The left-slip Kunlun-Qinling faulting system combines the east Kunlun fault zone, sinistral oblique reverse faults along the Minshan range and two major NEE-striking faults cutting the west Qinling range, which accommodates eastward motion, at 10–14 mm/a, of the Chuan-Qing block. The left-slip Xianshuihe faulting system accommodated clockwise rotation of the Chuan-Dian block. The Longmenshan thrust faulting system forms the eastern margin of the East Tibetan Plateau and has been propagated to the SW of the Sichuan basin. Crustal shortening across the Longmenshan range seems low (2–4 mm/a) and absorbed only a small part of the eastward motion of the Chuan-Qing block. Most of this eastward motion has been transmitted to South China, which is moving SEE-ward at 7–9 mm/a. It is suggested from geophysical data interpretation that the crust and lithosphere of the East Tibetan Plateau is considerably thickened and rheologically layered. The upper crust seems to be decoupled from the lower crust through a décollement zone at a depth of 15–20 km, which involved the Longmenshan fault belt and propagated eastward to the SW of the Sichuan basin. The Wenchuan earthquake was just formed at the bifurcated point of this décollement system. A rheological boundary should exist beneath the Longmenshan fault belt where the lower crust of the East Tibetan Plateau and the lithospheric mantle of the Yangze block are juxtaposed.     

我国的一些造山带的侧向挤出构造

尽管大陆只占地球表面的三分之一,但是人类生活在大陆上,大部分资源也来自于大陆,因此大陆构造研究有特别的意义,我国的地质前辈们为此做出了重要的贡献。然而,陆壳具有高度的非均质性,因此大陆构造要比大洋构造复杂的多,认识其演化规律极其困难,但是人类正在通过不同的途径朝这个目标前进。地块的侧向挤出是大陆构造的主要形式。尽管大规模的地块侧向挤出是否发生在青藏高原主体存在很大的争议,但是有证据显示地块的侧向挤出广泛地发生在青藏高原周边以及我国其它的一些造山带内,呈现出不同的规模、位移量和变形特征。位于滇西三江断裂带内的兰坪-思茅盆地在印度和华南第三纪的压扭性相互作用下向南挤出; 沿喜马拉雅西构造结发生的地块侧向挤出形成于早第三纪印度与欧亚大陆之间的南北向碰撞,最新的挤出地体是塔里木盆地; 雪峰地块向南的侧向挤出受控于华南地区北西-南东向区域性扭性构造作用; 沿扬子地块北缘发生的地块侧向挤出形成于扬子地块与秦岭造山带中生代晚期的南北向挤压,造成四川盆地发生向西的侧向挤出; 沿秦岭-大别山发生的地块侧向挤出发生在中生代,经历了超高压变质作用的下地壳随扬子地块的挤入向东运动,最后在桐柏-大别山隆升到地表,而中上地壳包括留凤关复理石沉积和碧口地块向西挤出。桐柏-大别山和青藏高原均形成于大陆的碰撞,地壳都曾发生过大规模的增厚。因此,有理由相信青藏高原的下地壳和桐柏-大别山的下地壳结构和构造是一样的,要研究两者物质组成和赋存状态以及运动和变形特征可以互相参考和借鉴。例如： 5·12汶川大地震的发生引发了对高原下地壳流变的关注和争论。上述桐柏-大别山中生代下地壳的侧向挤出就是通道流,由此证明青藏高原下地壳通道流是存在的; 而青藏高原下地壳和桐柏-大别山一样,一定是由壳内花岗岩、活化的前寒武结晶基底、变质核杂岩以及混入的上地幔物质组成。     

夏季青藏高原低涡研究进展述评

高原低涡是造成我国暴雨的重要天气系统之一。本文在简要介绍第一次青藏高原科学试验及以前研究工作的基础上,重点介绍了第二次青藏高原科学试验及近年来的一些主要研究成果,包括高原低涡的活动特征、高原低涡发展东移的机理、高原低涡发展东移的大尺度条件及高原低涡的结构特征等。并指出了目前高原低涡研究的局限性和待改进的方向。     

HDD挤压式扩孔钻头拉力-扭矩模型建立及验证

在水平定向钻进中,作用在钻头上拉力和扭矩是设计钻进和施工的重要依据。通过对挤压式钻头的力学分析,提出了挤压式钻头的拉力－扭矩模型并较全面地分析了影响模型结果的因素。还结合现场施工数据对模型进行实践验证,结果与现场数据较为吻合。因此,该模型对挤压式钻头水平定向钻进施工设计优化起到了很重要的指导作用。     

Normal Faulting Type Earthquake Activities in the Tibetan Plateau and Its Tectonic Implication

Abstract: This paper analyzes various earthquake fault types, mechanism solutions, stress field as well as other geophysical data to study the crust movement in the Tibetan plateau and its tectonic implications. The results show that a lot of normal faulting type earthquakes concentrate in the central Tibetan plateau. Many of them are nearly perfect normal fault events. The strikes of the fault planes of the normal faulting earthquakes are almost in the N-S direction based on the analyses of the equal area projection diagrams of fault plane solutions. It implies that the dislocation slip vectors of the normal faulting type events have quite great components in the E-W direction. The extension is probably an eastward extensional motion, mainly a tectonic active regime in the altitudes of the plateau. The tensional stress in the E-W or WNW-ESE direction predominates the earthquake occurrence in the normal event region of the central plateau. A number of thrust fault and strike-slip fault type earthquakes with strong compressive stress nearly in the NNE-SSW direction occurred on the edges of the plateau. The eastward extensional motion in the Tibetan plateau is attributable to the eastward movement of materials in the upper mantle based on seismo-tomographic results. The eastward extensional motion in the Tibetan plateau may be related to the eastward extrusion of hotter mantle materials beneath the east boundary of the plateau. The northward motion of the Tibetan plateau shortened in the N-S direction probably encounters strong obstructions at the western and northern margins. Extensional motions from the relaxation of the topography and/or gravitational collapse in the altitudes of the plateau occur hardly in the N-S direction. The obstruction for the plateau to move eastward is rather weak.     

Exhumation mechanisms of melt‐bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates

A series of 2D petrological–thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high‐temperature–ultrahigh pressure metamorphic (HT–UHPM) rocks. To this end, the models include fluid‐ and melt‐induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large‐scale crustal stacking, shallow crustal delamination, trans‐lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid‐ and melt‐induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid‐ and melt‐related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large‐scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high‐temperature range of P–T conditions documented from UHPM rocks, including those retrieved from HT–UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100–1200 °C, are still to be achieved in experiments using numerical models.     

2017年九寨沟Ms7.0地震揭示青藏高原东缘岷山地区一条新的左旋走滑断裂

2017年8月8日,青藏高原东缘川西九寨沟地区发生Ms7.0地震。基于震源机制解和余震分布特征,结合历史地震和区域新构造分析,揭示了一条新的左旋走滑断裂:九寨沟-虎牙断裂,该断裂斜切岷山隆起,呈弧形展布,北接塔藏罗叉断裂,南连虎牙断裂,是东昆仑左旋走滑断裂的最新东延。新构造研究结果表明,第四纪以来,青藏高原东部巴颜喀拉地块向东挤出,在其东缘岷山地区边界运动学性质发生了明显的转换:早-中更新世之交的昆黄运动时期（1.0~0.6Ma）,向东挤出运动主要转换为地壳缩短变形和块体隆升,沿岷山隆起东西两侧发生逆冲断裂作用;而自晚更新世晚期以来（ca 120 ka）,地块向东挤出伴随着块体顺时针旋转,构造变形以左旋走滑活动为主,主要集中在塔藏-九寨沟-虎牙断裂带。新构造运动性质的转换使得一些早期强烈活动的断裂,如岷江断裂、漳扎断裂等,其现今活动性明显减弱,而新的左旋走滑断裂正在孕育诞生。这一活动断裂演化图像的新认识为本地区未来强震评估和预测提供了新的视角。     

Geology of the Kokchetav UHP-HP metamorphic belt, Northern Kazakhstan

Abstract Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure–high-pressure (UHP–HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP–HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.