Active strike–slip faulting and macroscopically nonrigid deformation of volcanic rocks in central Japan inferred from a paleomagnetic study

Detailed paleomagnetic investigation of a pyroclastic flow deposit has clarified the deformation mode around an active fault. In central Japan, the early Quaternary Nyukawa Pyroclastic Flow Deposit is cut by the active dextral Enako fault. Activity level of the fault is evaluated on the basis of geological and geomorphological surveys. Then, paleomagnetic samples are collected from 22 sites at exposures located on a lineament that is adjoining and parallel to the Enako fault. Stable thermoremanent magnetization (TRM) of the pyroclastic deposit is isolated through progressive thermal and/or alternating field demagnetization tests. Untilted site-mean directions of the TRMs simultaneously acquired during initial cooling indicate significant clockwise vertical-axis rotation. The lineament was then activated with right-lateral motions through the early Quaternary. Together with the late Quaternary activities along the adjoining Enako fault evaluated by our study, the present result exemplifies a migration of active segments within a fault system during the Quaternary. Paleomagnetic directions on the strike–slip fault are not concordant with uniform deformation predicted by the model of rotation of rigid blocks aligned on a master fault, but suggestive of a periodic deformation as a result of intense fracturing and differential rotation of blocks bounded by nested parallel faults.     

Palaeomagnetic study of the Ronda peridotites (Betic Cordillera, southern Spain)

A palaeomagnetic study of the Ronda peridotites (southern Spain) has been carried out on 301 samples from 20 sites, spread along the three main outcrops of the ultrabasic complex: Ronda, Ojén and Carratraca massifs. Different lithologies and outcrops with different degrees of serpentinization have been sampled and analysed. Rock magnetic experiments have been carried out on a representative set of samples. These measurements include: Curie curves, hysteresis cycles, isothermal remanent magnetization (IRM) acquisition curves, thermal demagnetization of IRM imparted along three orthogonal axes and magnetic bulk susceptibility. Results indicate that magnetite is the main magnetic mineral present in the samples. Stepwise thermal and alternating field (AF) demagnetization of the natural remanent magnetization (NRM) reveals the presence of a characteristic remanent magnetization (ChRM) carried by magnetite, and in some sepentinized samples, a northward component with variable unblocking temperatures up to 250–575 °C. The appearance and the relative intensity of this northward component are strongly related to serpentinization degree. Taking into account the geological history of the peridotites, the ChRM has been considered as a thermo-chemical remanent magnetization acquired during the first serpentinization phase associated to the post-metamorphic cooling of this unit. On the basis of radiometric and fission track analysis, the ChRM is proposed to have been acquired between 20 and 17–18 Ma. The inclination of the mean direction of the ChRM statistically coincides with the expected inclination for stable Iberia during the Oligocene–Miocene. The declination of the ChRM differs from the expected declination, indicating clockwise block rotations of 41±12° about vertical axes since the cooling of the peridotites. When applying a compositional layering correction, the ChRM directions fail to pass this kind of fold test, thus, the compositional layering was not a palaeohorizontal during ChRM acquisition time. Normal and reversed polarities of the ChRM are reported, showing that at least one reversal of the Earth's magnetic field took place during ChRM acquisition process. A tentative polarity zonation within the peridotitic outcrops is also suggested. No evidence is found from these data for the previously proposed simultaneity between post-metamorphic cooling and rotation of the peridotites.     

扬子地块南缘晚古生代洋壳存在的Nd同位素证据

总结我国扬子地块南缘新元古代—早中生代沉积岩目前已发表的Ｎｄ同位素资料发现,这些沉积岩具有随时间变化而其Ｎｄ同位素组成发生漂移的现象。在绝大多数时间里,它们具有２０００Ｍａ左右的Ｎｄ模式年龄,但在１０００～７００Ｍａ的新元古代和＜４００Ｍａ的晚古生代—早中生代时间段中,Ｎｄ同位素模式年龄急剧降低,反映在此时间段内,新生地幔物质已经进入沉积物的物源区。１０００～７００Ｍａ期间的Ｎｄ同位素变化归因于晋宁期的碰撞造山作用而晚古生代－早中生代沉积岩的Ｎｄ同位素变异起因于海盆发育阶段洋壳的出现。即扬子地块南缘在晚古生代—早中生代期间曾经存在过洋壳,且该洋盆可能是古特提斯大洋分支的一部分。     

华北地块北缘中段铅锌银矿床成矿作用讨论

华北地块北缘中段分布着许多大中型铅锌银矿床,依据其成因特征可划分为四种类型:沉积喷气型矿床、叠加改造型矿床、与大红峪期碱性火山活动有关的浅成低温热液矿床、与中生代火山-斑岩活动有关的热液矿床。成矿作用具多期多成因的特点:古元古代火山沉积作用不仅可能形成了区内最早的VMS型矿床,而且成为区内最重要的矿源层;中元古代早期形成了本区最古老的浅成热液银多金属矿床;中元古代中期形成了又一期VMS型矿床;而中生代是本区铅锌银最重要的成矿期,成矿显然受基底的制约。     

金平断块及其矿产

金平断块是哀牢山成矿域东南地区最有利的成矿带，成矿与断裂带关系密切，矿带的展布亦严格受构造控制。金平断块可划分为四个元素异常集中区，集中了金、银、铜、铜镍、铜钼、铅锌、铌钽、铂钯等金属矿产和元素异常。每一区有不同的元素组合及矿产。目前，金、铜镍、铜钼等已进行开发利用，铜、铂钯、铌钽、铅锌等已获找矿信息，矿床潜力巨大，矿业开发已成为地方经济发展的支柱产业。由于成矿理论研究及矿床综合评价滞后，极大阻碍该区矿业开发一体化的发展进程，这种局面必须扭转。     

华北陆块对Rodinia超大陆的响应及其特征

研究揭示华北陆块1300-1000Ma和800-650Ma都存在比较弱的岩浆-变质事件，它们可能对应于华南陆块的碰撞（四堡运动）和裂解事件。华北陆块的四堡期蛇绿混杂岩可能只见于新元古代秦岭造山带中。秦岭造山带北缘识别出了一些花岗质分清入体，它们具有碰撞或碰撞后花岗岩的特征。华北陆块北缘的火山沉积岩生活费不具有离散边界杂岩的特征，它们可能揭示了大陆边缘或者大陆伸展过程。华北陆块与800-650Ma事件相关的岩石主要为来自富集地幔的基性岩墙和来自陆内裂谷的沉积岩，它们很可能与Rodinia裂解有关。沉积学和古生物学特征表明元古宙华北陆块不同华南陆块，而与西伯利亚陆块相似。据此可以认为华北陆块是Rodinia超大陆的一部分，它位于超大陆的边缘，可以不与华南陆块紧邻，而与西伯利亚陆地较近。     

Unwedge(2.35)程序在分析块体稳定性中的应用

Unwedge (2 .35 )程序在原Ver.1.0的基础上做了改动 ,程序的功能增强。根据作者使用该程序的经验 ,结合工程实践介绍了Unwedge (2 .35 )程序的基本原理方法、及该版本的缺陷。     

三峡永久船闸高边坡关键块体的概率分析(英文)

基于传统块体理论对不连续岩体的稳定分析是建立在不连续面贯穿所研究岩体的假定基础上的，而实际的岩体不连续而长度是有限的。因此，本文将随机概率模型张块体理论，研究了三峡永久船闸边坡关键块体的肖落概率和块体的大小及分布密度，结果表明该方法具有重要的工程实用价值。     

西藏中部拉萨地块古生代、中生代的超层序研究

识别划分了西藏南部拉萨地块 (措勤盆地 )古生代、中生代以海相为主的沉积地层相当于二级旋回的超层序11个 (CQ _1～11),其中早古生代 3个 (CQ _1～3),晚古生代 4个 (CQ _4～7),中生代 4个 (CQCQ _8～11);描述了各个超层序的特征;不仅在地块内进行了超层序对比,而且与印度北部边缘 (特提斯喜马拉雅 )显生宙的超层序进行了比较和讨论。研究表明,除早古生代外,藏南特提斯喜马拉雅和拉萨地块 (措勤盆地 )的超层序在数量、延时、结构方面极不相同,与所处地块构造背景和沉积盆地性质发生了变化有关。提出早古生代拉萨地块与印度次大陆同属冈瓦纳相区 (克拉通上的陆表海 ),晚古生代早期以后它们已不属同一大陆,整个晚古生代拉萨地块可能为冈瓦纳与劳亚大陆之间过渡带的一部分,中生代则成为Cimmeria次大陆的南部块体,冈瓦纳大陆在西藏境内的北界应为雅鲁藏布江缝合带。