P–T–t–D records of Early Palaeozoic Andean-type shortening of a hot active margin: The Dunhuang block in NW China

High-pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra-subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time-deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N-S trending sub-vertical fabric (S2) preserved in low-strain domains and (3) upright folds (F3) associated with a ubiquitous steep E-W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re-equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re-equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange-like association during the late stages of orogeny. The high-pressure event D1-M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre-orogenic crust. The high-temperature (HT) re-equilibration D2-M2 is interpreted as a result of Mid-Devonian shortening of the previously thickened crust, possibly due to ‘Andean-type’ underthrusting. The D3-M3 event reflects Late Devonian supra-subduction shortening and continuous erosion of the sub-crustal lithosphere. This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a ‘Cascadia-type’ active margin, which corresponds to a supra-subduction tectonic switching paradigm.     

高放废液处置场选址勘察特大构造破碎带的钻进工艺

高放废液处置的特殊性决定了它对选址勘察中钻探施工有极其严苛的要求。在钻探工艺方面受到了多方面的限制,对钻探施工影响极大,尤其是遇到特大构造破碎带、地层极为复杂的情况下,对钻探是极大的考验。在甘肃北山BS02号孔施工中,通过采用多种不同技术措施,钻穿了十月井特大构造破碎带和复杂地层,满足了设计要求,保证了工程质量和水文地质试验效果。     

Horizontal crustal movement in Chinese mainland from 1999 to 2001

Introduction In the Crustal Movement Observation Network of China (CMONOC) there are 25 fiducialstations, 56 basic stations and 1 000 regional stations. They are scattered on 10 major blocks inChinese mainland with high density of observation stations on the blocks of high seismic activityin the regional networks. 10 major blocks or regions (they will be referred to as blocks in the paper,a letter is used as a symbol for each block) were divided during the design of the regionalnetwo…     

鄂尔多斯块体西南定边-大罗山大地电磁剖面解释

鄂尔多斯块体定边—大罗山段大地电磁结果表明，鄂尔多斯块体内部电性结构简单，成层性好，上地幔第一高导层顶面埋深基本在１０９ｋｍ左右，相当平坦，块体内部不存在壳内高导层。而靠近大罗山处，上地幔第一高导层向上隆起，隆起最高处距地表９２ｋｍ，此处测点壳内有电性分层，但由于断裂带的切割使测点之间的壳内分层不易连接，形成不连续层。分析认为该处为深大断裂带，是鄂尔多斯块体的西部边界     

六盘山断裂带及其邻区地壳结构

新生代期间,中国大陆西部受印度—欧亚板块碰撞和青藏高原隆升影响,以地壳缩短、增厚、陆内造山和强烈地震活动等为主要特征.在青藏高原东北边缘,高原物质侧向移动被鄂尔多斯地块所阻,在六盘山地区发育了一系列左旋斜冲断裂.断裂带周缘构造变形强烈,地震活动频繁,是研究青藏高原横向扩展控制大陆内部弥散变形的理想场所.本文对穿越青藏高原东北缘—六盘山断裂带—鄂尔多斯地块的宽角反射与折射地震资料使用层析成像和射线反演算法进行成像,获得了研究区地壳速度结构模型,其结果反映出六盘山断裂带两侧地壳结构、构造特征差异显著：1）上地壳层析成像结果显示鄂尔多斯盆地一侧地壳上部速度较低,等值线呈近水平状,具有典型的沉积盆地特征,而青藏高原东北缘一侧上地壳速度相对较高,横向变化剧烈,呈褶皱状,二者的分界为海原—六盘山逆冲走滑断裂;2）全地壳射线反演结果显示鄂尔多斯地块地壳速度梯度大,下地壳底部速度高由铁镁质物质组成,具有典型稳定古老克拉通的特征,青藏高原东北缘地壳速度总体较低,主要由长英质及长英-铁镁质过渡物质组成,具有典型造山带的特征,而六盘山断裂带下方地壳速度结构复杂,层面呈拱形,部分层出现速度逆转,为两个构造单元的接触过渡带;3）青藏高原东北缘一侧地壳厚度~50 km,鄂尔多斯地块地壳厚度~42 km,六盘山断裂带下方莫霍面发生叠置,揭示出青藏高原东北缘、鄂尔多斯地壳在六盘山下汇聚,较薄且刚性的鄂尔多斯地壳挤入较厚且塑性的青藏高原东北缘地壳中的构造模式.

     

Elementary Research on Activity Behavior of the Crust in Funan-Huoqiu Area, Southern Border of the North China Active Block Region

1.ESSENTIAL GEOLOGIC GEOMORPHIC CHARACTERIN THE STUDIDED REGIONThe studied region of the paper,the Funan-Huoqiu area,extends across the Jianghuai hills andHuaibei plain of Anhui Province,located in the south border area of the North China active blockregi…     

大兴安岭莫尔道嘎地区新元古代巨斑状碱长花岗岩地球化学特征

莫尔道嘎地区巨斑状碱长花岗岩是额尔古纳地块最早的岩浆活动的产物.对巨斑状碱长花岗岩的地质学、岩石地球化学特征进行了阐述,确定岩体形成于新元古代,属同碰撞期陆壳改造型钙碱性花岗岩.认为自新元古代开始,额尔古纳地块进入了强烈碰撞增生演化阶段.     

Numerical modeling of block structure dynamics: dependence of a synthetic earthquake flow on the structure separateness and boundary movements

A seismically active region is modelled as a system of absolutely rigid blocks separated by infinitely thin plane faults. The interaction of the blocks along the fault planes and with the underlying medium is viscous-elastic. The system of blocks moves as a consequence of prescribed motion of the boundary blocks and of the underlying medium. When for some part of a fault plane the ratio of the stress to the pressure exceeds a certain strength level a stress-drop (a failure) occurs (in accordance with the dry friction model), and it can cause a failure in other parts of the fault planes. In the model the failures represent earthquakes. As a result of the numerical simulation a synthetic earthquake catalog is produced. The numerical modelling was carried out for three types of structures with increasing of the structure separateness inside of each type and for two types of boundary movements. A synthetic earthquake flow is characterised by several features including the frequency-magnitude relation (the Gutenberg-Richter curve). When the structure separateness increases the slope of the curve changes monotonously in the same direction for all considered types of structures if the boundary movement is the same. The directions of changing of the slope are opposite for two considered boundary movements.     

扬子地块西北缘石门湾铅锌矿的发现及其意义

紫阳石门湾铅锌矿位于扬子地块西北缘的活动大陆边缘,其构造环境为古生代的弧后盆地。该矿处于马元地区大型铅锌矿带与湖北竹溪朝阳铅锌矿带之间,备受关注。经地表追索及工程控制,共圈出铅锌矿体6条。矿体呈似层状、透镜状,长度72~292 m,厚度0.78~4.06 m;Pb平均品位为1.47%~2.83%,Zn平均品位为2.38%~8.45%。矿体产于上震旦统第二岩性段上亚段白云岩中,矿体与地层产状基本一致,受一组顺层断层破碎带控制。初步认为矿石中的微量元素来源于成矿围岩,矿床中的原始Pb主要来源于壳幔混合的造山带。紫阳石门湾铅锌矿与沉积有关,后期有不同程度的改造。初步认为本区铅锌矿成因类型属于沉积-改造型。     

广西北流地震震源区短周期密集台阵背景噪声成像

作为构造上较为稳定的华南地块,其地震活动相对中国西部明显偏低,但也发生过多次中强地震.华南中强地震震源区深部结构对于深入研究华南板内地震成因具有重要科学意义.2019年发生在两广交界的北流5.2级地震是近年来华南最大的一次中强地震,该次地震的发生也为我们研究华南地震震源区深部结构提供了理想场所.本文基于120个短周期地震仪组成的密集台阵背景噪声资料,通过互相关提取了1~5 s高频面波频散信号,然后采用面波直接反演法,对北流地震震源区进行地下6 km深度范围内浅层地壳速度结构成像.结果显示：浅层地壳剪切波速度横向变化强烈,速度分布不均匀;低速异常主要分布于三条断裂带周围;研究区存在NNW-SSE向的高速异常,该高速异常与震中区高速异常相交在水平方向上呈"L "形环绕分布于石窝断裂南东侧;震中上方为上凸形态的高速异常,这些高速异常可能为中生代花岗岩基,结合研究区现今NW-SE向的构造应力场分析：石窝断裂南西盘向南东方向运动时受到" L"形高速体阻挡,可能导致应力积累,进而引发地震.此外,这种局部异常高速体可能是华南"马赛克"结构中微陆块在上地壳中的延伸,上地壳结构的不均匀性和大尺度构造相互作用,会在局部区域导致应力积累,从而引发中强地震,这也可能是华南板内地震发生的一种重要成因机制.