Nanpanjiang basin: A window on the tectonic development of south China during Triassic assembly of the southeastern and eastern Asia

As the largest siliciclastic sink in the South China block, the Triassic Nanpanjiang basin is generally thought of as a foreland basin related to Indosinian collision between the South China and Indochina blocks. Our basin analysis, spanning an eight years period, provides details of Lopingian to Middle Triassic stratigraphic fill and tectonic subsidence of the basin. Field observation and sedimentological analyses reveal that the basin was not filled by a monotonous turbidite system and suggest instead the development of five primary depositional units constituted by deep-water turbidite systems, mass-transport deposits, condensed drapes, shallow-water isolated carbonate platforms, and volcano-volcaniclastic rocks. The stratigraphic completeness of the basin, the coexistence of widespread but localized carbonate platforms within axially filled deep-water basin, tectonically generated cyclothems of transverse mass-transport deposits along intrabasinal faults and basin fill of extrusive rocks of bimodal composition together apparently call in question the claim that crustal shortening prevailed during the Late Permian to pre-Norian Triassic. Patterns of basin fill, differential tectonic subsidence and very high subsidence rate of the region adjacent to master fault are more closely match the pattern expected for extensional to transtensional basin in a back-arc setting. The crustal extension in the southwestern South China was not consistent with hypothesized Indosinian collision between South China and Indochina blocks, but more likely developed in response to northwestward subduction of the paleo-Pacific plate. Our results, combined with regional considerations, lead to a new model for the Triassic tectonic evolution of South China during Triassic assembly of the southeastern and eastern Asia.     

塔里木地块上石炭统的古地磁研究

本文对塔里木地块西北缘柯坪地区的3条上石炭统康克林组剖面进行了古地磁研究。95个定向岩心样品的逐步热退磁和交变退磁揭示出3个磁性组分,第一个是具有低解阻温度谱或低矫顽力谱在现代地磁场作用下形成的粘滞剩磁A(D=10°;I=56°,α95=4°);第二个是红色石英砂岩的具有反极性的高温组分B(D=217°,I=-43°,λ=56°,Φ=184°E;A25=6°详细研究表明,该分量受附近晚二叠世脉岩侵入的重磁化;第三个是灰岩高矫顽力谱的反极性组分C（D=241°、I=-51°,λ=41°N,Φ=160°E,A（?）=4°）。这一分量通讨倾斜检验,在95%概率水平上明显不同于塔里木地块目前已有的石炭纪以后的古地磁方向,它代表了原生剩磁。研究结果表明:（1）晚石炭世塔里木地块已和哈萨克斯坦地块、苏联西天山地块以及西伯利亚地块发生碰撞:（2）苏联西天山地块相对塔里木地块曾逆时针旋转了70°（欧拉极位置λ=8°N,Φ=69°E）,而塔里木地块则相对西伯利亚地块逆时计旋转了21°（欧拉极位置λ=20°N,Φ=34°E）。从该区已有的晚古生代至早三叠世占地磁数据看,这些地块之间的相对旋转运动发生于早三叠世之后。     

蚌埠-凤阳地区构造格架与找矿方向研究

在蚌埠-凤阳浅覆盖区,依据重磁异常进行地质构造解释,基本查明了构造格架,确定了新的找矿方向。研究结果表明,蚌埠复背斜构造轴线为向南凸出的弧形,西部地区走向北西西,与蚌埠隆起构造线一致,中部凤阳县城地区走向近东西,东部地区走向北东。郯庐断裂构造的左行牵引作用是引起轴线变化的主要原因。由此圈定了蚌埠岩体隐覆边界线和凤阳县-总铺构造岩浆热液活动带范围。编制了蚌埠-凤阳地区物探推断基岩构造纲要图。提出了蚌埠岩体接触带、构造岩浆热液活动带和重要断裂带交汇部位为找矿有利地区,为该区指明了找矿方向。     

青海东昆仑哈日扎多金属矿区构造活动的锆石裂变径迹定年分析

青海省哈日扎多金属矿区位于东昆仑东段,属东昆中多旋回岩浆弧带,是新的矿产勘查基地,成矿作用主要受NE和NW向两组构造蚀变带控制。本文通过不同类型样品的锆石裂变径迹定年分析,探讨区内的构造活动。所获得的10个样品年龄为116~204 Ma,并由3个组年龄构成,即204~181 Ma,142~168 Ma和116~120 Ma。第1组年龄204~181 Ma反映印支晚期三叠纪末羌塘地块与昆仑地块碰撞的地质事件及其时限;后两组年龄主要是晚侏罗世~早白垩世冈底斯地体向北与羌塘地体碰撞汇聚的响应,活动时限为168~116 Ma。第2和3组年龄同时表明本区燕山期构造活动存在强度差异,即有两个强作用期。同时,3组年龄也揭示本区具有多期次成矿活动。     

Geochronological and geochemical evidence for a Late Ordovician to Silurian arc-back-arc system in the northern Great Xing'an Range,NE China

The early Paleozoic tectonic evolution of the Xing'an-Mongolian Orogenic Belt is dominated by two oceanic basins on the northwestern and southeastern sides of the Xing'an Block,i.e.,the Xinlin-Xiguitu Ocean and the Nenjiang Ocean.However,the early development of the Nenjiang Ocean remains unclear.Here,we present zircon U-Pb geochronology and whole-rock elemental and Sr-Nd isotopic data on the gabbros in the Xinglong area together with andesitic tuffs and basalts in the Duobaoshan area.LA-ICP-MS zircon U-Pb dating of gabbros and andesitic tuffs yielded crystallization ages of 443-436 Ma and 452-451 Ma,respectively.The Early Silurian Xinglong gabbros show calc-alkaline and E-MORB affinities but they are enriched in LILEs,and depleted in HFSEs,with relatively low U/Th ratios of 0.18-0.36 andεNd(t)values of-1.6 to+0.5.These geochemical features suggest that the gabbros might originate from a mantle wedge modified by pelagic sediment-derived melts,consistent with a back-arc basin setting.By contrast,the andesitic tuffs are characterized by high MgO(>5 wt.%),Cr(138-200 ppm),and Ni(65-110 ppm)contents,and can be termed as high-Mg andesites.Their low Sr/Y ratios of 15.98-17.15 and U/Th values of 0.24-0.25 and moderate(La/Sm)_n values of 3.07-3.26 are similar to those from the Setouchi Volcanic Belt(SW Japan),and are thought to be derived from partial melting of subducted sediments,and subsequent melt-mantle interaction.The Duobaoshan basalts have high Nb(8.44-10.30 ppm)and TiO2 contents(1.17-1.60 wt.%),typical of Nb-enriched basalts.They are slightly younger than regional adakitic rocks and have positiveεNd(t)values of+5.2 to+5.7 and are interpreted to be generated by partial melting of a depleted mantle source metasomatized by earlier adakitic melts.Synthesized with coeval arc-related igneous rocks from the southeastern Xing'an Block,we propose that the Duobaoshan high-Mg andesitic tuffs and Nbenriched basalts are parts of the Late Ordovician and Silurian Sonid Zuoqi-Duobaoshan arc belt,and they were formed by the northwestern subduction of the Nenjiang Ocean.Such a subduction beneath the integrated Xing'an-Erguna Block also gave rise to the East Ujimqin-Xinglong igneous belt in a continental back-arc basin setting.Our new data support an early Paleozoic arc-back-arc model in the northern Great Xing'an Range.     

朝鲜半岛古元古代摩天岭群的碎屑锆石U-Pb年龄及其地质意义

克拉通内部活动带变质沉积建造蕴藏了早期超大陆形成中地体聚-散动力学过程的重要信息,碎屑锆石年代学则是揭示这种信息的最佳工具之一。针对华北(中-朝)克拉通最具代表性的古元古代活动带(辽-吉-朝活动带)中巨量沉积建造精细表征方面的空白区域,本文开展了朝鲜半岛检德地区摩天岭群的碎屑锆石年代学研究。SIMS U-Pb定年结果显示,北大川组变火山碎屑岩样品的锆石年龄谱记录了约2.53Ga的主要年龄峰值和约2.19Ga的最年轻峰值,说明北大川组开始沉积的时代应该晚于2.2Ga。南大川组变沉积岩样品碎屑锆石记录的最年轻峰值年龄为约2.02Ga,说明南大川组的沉积时代应该不早于2.02Ga;其锆石年龄谱记录了一个主要年龄峰值区域(2.02~2.18Ga)、一个次要年龄峰值(约2.46Ga)和一些2.78~3.30 Ga的古老年龄,指示其物质源区主要为同区古元古代地质体和邻近地体上的太古宙变质基底。区域对比表明,摩天岭群与中国境内辽吉活动带中的辽河群及集安群/老岭群具有基本一致的碎屑锆石年龄谱,说明它们不仅是同一时代的产物,而且分享类似的物质源区。这些时代一致、物源相近的沉积建造可以为甄别古元古代辽-吉-朝活动带的大地构造属性提供重要参考。与沉积年龄相近的碎屑锆石占据主导的碎屑锆石年龄谱反映了汇聚板块边缘各类盆地的典型特征,而相悖于大部分碎屑锆石年龄远大于地层沉积年龄的裂谷型盆地。     

Andean earthquakes triggered by the 2010 Maule,Chile (Mw 8.8) earthquake: Comparisons of geodetic,seismic and geologic constraints

The Maule, Chile, (M_w 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the M_w 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), M_w 5.8 Catamarca (1400 km; nine days), M_w 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.     

Tectonomagmatic origin of Precambrian rocks of Mexico and Argentina inferred from multi-dimensional discriminant-function based discrimination diagrams

Several new multi-dimensional tectonomagmatic discrimination diagrams employing log-ratio variables of chemical elements and probability based procedure have been developed during the last 10 years for basic-ultrabasic, intermediate and acid igneous rocks. There are numerous studies on extensive evaluations of these newly developed diagrams which have indicated their successful application to know the original tectonic setting of younger and older as well as sea-water and hydrothermally altered volcanic rocks. In the present study, these diagrams were applied to Precambrian rocks of Mexico (southern and north-eastern) and Argentina. The study indicated the original tectonic setting of Precambrian rocks from the Oaxaca Complex of southern Mexico as follows: (1) dominant rift (within-plate) setting for rocks of 1117–988 Ma age; (2) dominant rift and less-dominant arc setting for rocks of 1157–1130 Ma age; and (3) a combined tectonic setting of collision and rift for Etla Granitoid Pluton (917 Ma age). The diagrams have indicated the original tectonic setting of the Precambrian rocks from the north-eastern Mexico as: (1) a dominant arc tectonic setting for the rocks of 988 Ma age; and (2) an arc and collision setting for the rocks of 1200–1157 Ma age. Similarly, the diagrams have indicated the dominant original tectonic setting for the Precambrian rocks from Argentina as: (1) with-in plate (continental rift-ocean island) and continental rift (CR) setting for the rocks of 800 Ma and 845 Ma age, respectively; and (2) an arc setting for the rocks of 1174–1169 Ma and of 1212–1188 Ma age. The inferred tectonic setting for these Precambrian rocks are, in general, in accordance to the tectonic setting reported in the literature, though there are some inconsistence inference of tectonic settings by some of the diagrams. The present study confirms the importance of these newly developed discriminant-function based diagrams in inferring the original tectonic setting of Precambrian rocks.     

建平地区太古宙变质杂岩构造变形及演化特征

建平地区太古宙变质岩石发育,地质构造复杂。近年研究表明:区内太古宙变质杂岩是由多期变质、变形的中酸性深成侵入岩和少量上壳岩组成,可划分出变质上壳岩组合、叶柏寿片麻岩、斜长片麻岩和二长片麻岩4个岩石单位。本文主要对区内太古宙变质杂岩的构造变形及构造演化进行讨论,初步确定了早期韧性变形阶段(鞍山期划分4个变形幕)和晚期脆性变形阶段(印支—燕山期)。构造层次由早期深部层次向晚期浅层演化;其变形由流动机制—压扁机制—弯曲机制—韧性剪切机制到剪切破裂机制的演化。     

准噶尔盆地南缘中-新生界碎屑成份特征与构造期次

准噶尔盆地南缘晚侏罗世-早白垩世早期、晚白垩世及晚新生代发育的近源粗碎屑沉积显示构造活动的存在。野外剖面及镜下碎屑成份统计表明:砾岩的砾石成份、砂岩碎屑成份的物源属性主要是再旋回造山带和晚古生代的岩浆弧,但盆地南缘东段与西段的岩屑组成及物源属性存在较大的差异。其中,沉积岩岩屑在晚侏罗世—早白垩世早期、晚白垩世和晚新生代发生了相应的增加,显示盆缘沉积岩物源的隆升—剥蚀作用和构造活动的相对活跃。砂岩碎屑特征、重矿物相对含量及重矿物组合特征证明盆地南缘东、西两段的物源属性存在较大差异,特别是不稳定重矿物的增加显示晚侏罗世-早白垩世早期、晚白垩世和晚新生代为构造相对活跃的构造环境。综合中-新生界沉积碎屑特征及差异分析,准噶尔盆地南缘中-新生代盆山格局发生了3次较大的转变过程,分别对应于上述3个时期。中-新生代3次构造活动对含油气系统形成具有重要控制作用,构造活动期次与油气藏形成、调整的期次也有良好的对应关系。