利用背景噪声反演鄂尔多斯块体及其南缘地区地壳速度结构

文中利用分布在鄂尔多斯块体及其南部周缘地区的53 个宽频带地震固定台站的连续波形记录,采用双台互相关计算 方法由背景噪声提取瑞利波格林函数,经时频分析获得相速度和群速度频散曲线,并分别计算了汾渭地堑、秦岭北缘、鄂 尔多斯块体内部和六盘山地区4 个不同构造区的平均频散曲线,进而反演了各构造区的地壳上地幔一维横波速度结构。结 果显示：地壳厚度在汾渭地堑为34 km,在秦岭北缘地区和鄂尔多斯块体均为40 km,在六盘山地区最厚,达49~50 km;相 应的上地幔顶部横波速度分别为4.20,4.2,4.30 和4.15 km/s;地壳内结构浅部特征差异最大,在地壳中部六盘山地区的速 度较低,下部地壳不同地区的波速较一致。     

Geochemistry and Crustal Evolution of Volcano-sedimentary Successions and Orthogneisses in the São Gabriel Block, Southernmost Brazil — Relics of Neoproterozoic Magmatic Arcs

Precambrian metaplutonic rocks of the São Gabriel block in southernmost Brazil comprise juvenile Neoproterozoic calc-alkaline gneisses (Cambaí Complex). The connection with associated (ultra-)mafic metavolcanic and metasedimentary rocks (Palma Group) is not well established. The whole complex was deformed during the Brasiliano orogenic cycle. Both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the Cambaí Complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the São Gabriel block and its role during the assembly of West-Gondwana. The major element compositions of the igneous rocks (Palma Group and Cambaí Complex) indicate a subalkaline character; most orthogneisses have a calc-alkaline chemistry; many metavolcanic rocks of the Palma Group show signatures of low-K tholeiitic volcanic arc basalts. Trace element data, especially Ti, Zr, Y, Nb, of most igneous samples from both the lower Palma Group and the Cambaí Complex indicate origin at plate margins, i.e., in a subduction zone environment. This is corroborated by relative enrichment in LREE, low contents of Nb and other high field strength elements and enrichment in LILE like Rb, Ba, and Th. The data indicate the possible existence of two suites, an oceanic island arc and a continental arc or active continental margin. However, some ultramafic samples of the lower Palma Group in the western São Gabriel block indicate the existence of another volcanic suite with intra-plate character which possibly represents relics of oceanic island basalts (OIB). Trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks. The patterns of chondrite- and N-MORB-normalized spider diagrams resemble the patterns of the igneous rocks, i.e., LILE and LREE enrichment and HFS depletion. The geochemical signatures of most igneous and metasedimentary samples and their low (⁸⁷Sr/⁸⁶Sr)_t ratios suggest only minor contribution of old continental crust.A geotectonic model for the São Gabriel block comprises east-ward subduction and following accretion of an intra-oceanic island arc to the eastern border of the Rio de la Plata Craton at ca. 880 Ma, and westward subduction beneath the newly formed active continental margin between ca. 750 and 700 Ma. The São Gabriel block represents relics of an early Brasiliano oceanic basin between the Rio de la Plata and Kalahari Cratons. This ocean to the east of the Rio de la Plata Craton might be traced to the north and could possibly be linked with Neoproterozoic juvenile oceanic crust in the western Brasília belt (Goiás magmatic arc).     

华北及其邻区块体转动模式和动力来源

块体的转动是地壳中重要的构造运动形式。根据地质、地球物理和地震活动性等资料，可将华北及其邻区划分为３个亚板块，华北亚板块可进一步细分为多个级块体，这些不同级别的块体或多或少都显示出一定的刚体特性。根据地质构造、地震和古地磁测量等资料，详细地论细地论述了不同级块体的转动问题，即华北及其邻区的黑龙江、华北和华南等３个近东西向亚板块自老第三纪以来相对于新疆地区顺时针转动了１．６°～３．５°；华北亚板块内     

扬子板块北缘下志留统龙马溪组重晶石结核特征及其成因机制分析

在扬子板块北缘城口明中剖面及巫溪徐家坝剖面下志留统龙马溪组底部的硅质岩、泥岩中发现椭球状重晶石结核。其矿物组成主要为重晶石颗粒与作为"基质"的黄铁矿、粘土矿物和石英。通过重晶石岩石学、矿物学及锶同位素分析表明,重晶石结核形成于早期成岩阶段松软沉积物的孔隙水中。上升洋流带来丰富的营养及富钡物质,表层海水的高初始生产力促使生物繁盛,海水中的钡通过生物作用富集形成生物钡,生物钡(bio-barite)在埋藏过程中的硫酸盐耗竭区(sulfate depleted zones)通过硫酸盐细菌作用(BSR)溶解被激活提供了钡的来源。围岩岩性(黑色泥岩和硅质泥岩)表明重晶石结核形成于缺氧的环境中。上述研究对深入理解早志留世时期扬子板块北缘古海洋环境有一定的启示作用。     

RESEARCH ON THE RELATIONSHIP BETWEEN FRACTAL FACTORS AND TECTONIC ACTIVITY——A CASE STUDY OF SOUTHWESTERN YUNNAN BLOCK

Geomorphology could record long-term accumulation of tectonic movement and quantify it by relevant parameters.But because the influences of other factors such as climate and lithology,how to use the relevant parameters to reveal the relationship between geomorphology and tectonics is a research hot spot.In this paper,we utilize the variogram method and the cellular fractal model to estimate parameters such as the fractal dimension (D) and ordinate intercept (γ) from the SRTM3 DEM using a moving window operation.We compare the distribution characteristics of the parameters in different climate and lithology.The results indicate that the correlation between the parameters and lithology or climate is very poor.The fractal dimension (D) reveals a very good correlation with tectonics,which is low in tectonically inactive areas and high in active areas.It implies that fractal dimension (D) may be a new method for research of regional tectonic movement.     

青藏高原西部班公-怒江缝合带下方地壳结构与地块拼合模式

利用远震接收函数偏移成像方法获得青藏高原西部Hi-Climb项目剖面北段地壳结构转换波成像。结果显示班公-怒江缝合带下方拉萨地体上地壳向N仰冲,下地壳向N俯冲,而羌塘地块上地壳向S仰冲,下地壳向S俯冲,可能意味着青藏高原西部拉萨地块和羌塘地块具有复杂的拼合过程。结合前人的岩石学研究成果,建立了新特提斯北洋盆洋壳S向俯冲、距今60～50Ma印度板块与欧亚板块碰撞后,拉萨地块的下地壳向羌塘地块下俯冲,而后印度板块俯冲到羌塘地块下方的地块拼合模式     

浅层结构不确定性对广角地震走时正演模拟结果的影响——以西沙地块OBS2011-1测线为例

海洋地球物理勘探中常使用多道地震数据建立浅部地层模型,结合海底地震仪广角地震数据进行射线追踪和走时反演求取地层速度特征.受采集环境和成本限制,部分海底地震仪测线缺少同步采集的多道地震数据,建立的初始模型其浅层结构有较大不确定性,可能会对正演模拟结果造成严重影响.本研究针对海底地震仪广角地震走时模拟中浅部结构的不确定性因素,使用RayInvr软件构建了理论模型和走时数据体,分析讨论了初始模型沉积层厚度与速度变化对正演模拟结果的影响.之后进行实测数据处理,在MCS2019-3测线数据的约束下建立初始模型,对OBS2011-1测线震相重新进行了走时拟合,将得到的模型结果与先前结果进行对比,得到以下结论：沉积层厚度及速度不确定性对走时模拟结果的浅部结构影响较大,深部结构影响较小;使用错误的速度与基底深度组合来拟合沉积层反射震相PsP,会使台站下方的地壳和地幔折射震相Pg和Pn,以及莫霍面反射震相PmP都出现走时小起伏,走时提前对应沉积层速度与厚度偏大,走时延后对应沉积层速度与厚度偏小;PsP震相缺失时,不同的速度-深度组合都可以很好地拟合震相,但会增加结果的不确定性.

     

华北克拉通南缘鲁山太古宙基底的形成和演化

鲁山地区保存和出露很好的太古宙基底岩石。文章报道了2件魏庄片麻岩的原岩年龄分别为(2867±5)Ma和(2768±5)Ma,变质作用时代分别为(2775±6)Ma和(2775±56)Ma。结合已发表数据,魏庄片麻岩原岩形成时代可大致分为2867~2928 Ma和2765~2768 Ma两个阶段;榆树庄片麻岩原岩形成于2829~2897 Ma、2752~2778 Ma和2723 Ma;斜长角闪岩原岩形成于2838~2845 Ma、2747~2794 Ma和2730 Ma。尽管三者岩性上存在差异,但原岩形成的期次和时代相当,指示它们在成因上可能存在联系。鲁山多地陆续发现2.8~2.9 Ga的岩石,表明该地区存在一定规模的中太古代基底。通过与胶东、霍邱等地对比,笔者认为华北克拉通南缘可能存在中太古代的古老陆块,其重要的初生地壳生长事件发生在2.9~3.0 Ga。     

西藏拉萨地块的乐平统及其古地理意义

西藏拉萨地块乐平统以海相与陆相沉积并存为特征。海相乐平统含复体珊瑚Waagenophyllum、■类Reichelina和有孔虫Colaniella。陆相乐平统含两类植物群,一类发育于措勤盆地中北部地区,为舌羊齿植物群分子Glossopteris、Noeggerathiopsis、Phyllotheca和华夏植物群分子Pecopteris、Sphenopteris共生的混生植物群;另一类主要发现于改则县下岗江和昂仁县贡久布地区,以Lepidophylloides、Paracalamites、Pecopteris等华夏植物群分子为主。根据沉积层序分析,拉萨地块乐平世时表现为整体上升。藏南喜马拉雅特提斯带的乐平统以产有冈瓦纳型Glossopteris植物群、冷水型单体珊瑚Cyathaxonia和腕足动物群为特征,并且从曲布组到曲布日嘎组表现为海进的沉积序列。拉萨地块和喜马拉雅特提斯带乐平世在植物群、动物群以及沉积层序方面存在着本质的差别,因此,拉萨地块在晚二叠世已基本脱离位于冈瓦纳大陆北缘的喜马拉雅特提斯带。     

扬子陆块西缘寒武系砂岩的物源分析：对古地理位置重建和构造背景的指示

扬子陆块西缘寒武系主要为一套碎屑岩- 碳酸盐岩的岩石组合,前人研究多认为形成于相对稳定的克拉通盆地。但同时期出现的大陆岩浆作用显然与前期认定的克拉通盆地性质不符,需要借助扬子西缘的物质来源探讨构造背景。基于野外露头等资料,本文通过对扬子陆块西缘会泽和会东附近寒武系3件砂岩样品进行重矿物分析、电气石电子探针和碎屑锆石U- Pb测年分析,确定扬子西缘寒武纪沉积物的源区;并结合沉积序列等综合探讨扬子陆块西缘寒武纪的构造背景。沉积序列表明,扬子西缘寒武系沧浪铺组、西王庙组和二道水组主要由砂岩和白云岩等组成,沉积环境为滨岸—潮坪。细—粗砂岩碎屑颗粒为次棱角状—次圆状,分选较差;碎屑组分主要为石英,岩屑几乎全部为燧石,长石含量较少。测试分析结果表明：重矿物分析指示扬子西缘寒武系砂岩重矿物主要由锆石、赤—褐铁矿、电气石、钛铁矿、金红石、磷灰石等组成,重矿物组合指示岩浆岩为其主要母岩;电气石电子探针分析结果表明,物源主要来自于贫锂花岗岩和变砂岩、变泥岩;碎屑锆石测年分析表明物源区母岩主要为983~540 Ma岩浆岩。碎屑锆石年龄对比等综合分析表明,寒武系沉积物部分源自康滇古陆983~708 Ma的岩浆岩和变沉积岩,部分源自冈瓦纳大陆东非造山带663~540 Ma的岩石,物源区岩石经历再旋回产物作用。扬子西缘寒武系的沉积序列、碎屑锆石年龄谱图和碎屑组成等特征综合分析表明,扬子陆块西缘寒武系形成于前陆盆地。