Geochemical Features, Age, and Tectonic Significance of the Kekekete Mafic-ultramafic Rocks, East Kunlun Orogen, China

The Kekekete mafic-ultramafic rocks are exposed in the Kekesha-Kekekete-Dawate area,which are in the eastern part of the East Kunlun Orogenic Belt.It outcrops as tectonic slices intruding tectonically in the Paleoproterozoic Baishahe Group and the Paleozoic Nachitai Group.The Kekekete mafic and ultramafic rocks is located near the central fault in East Kunlun and lithologically mainly consists of serpentinite,augite peridotite,and gabbro.The LA-ICP-MS zircon U-Pb age of the gabbro is 501±7 Ma,indicating that Kekekete mafic-ultramafic rocks formed in the Middle Cambrian.This rock assemblage is relatively poor in SiO2 and(Na2 O+K2 O) but rich in MgO and SFeO.The chondrite-normalized REE patterns of the gabbro dip slightly to the right;the primitive mantle and MORBnormalized spidergrams of trace elements show enrichment of large-ion lithophile elements(Cs,Rb,Ba,etc.) and no differentiation of high field strength elements.The general dominance of E-MORB features and the geochemical characteristics of OIB suggest that the Kekekete mafic-ultramafic rocks formed in an initial oceanic basin with slightly enriched mantle being featured by varying degrees of mixing of N-MORB depleted mantle and a similar-OIB-type source.From a comprehensive study of the previous data,the author believes that the tectonic history of the East Kunlun region was controlled by a geodynamic system of rifting and extension in the late stages of the Neoproterozoic to early stages of the Early Paleozoic and this formed the paleo-oceanic basin or rift system now represented by the ophiolites along the central fault in East Kunlun,the Kekekete mafic-ultramafic rocks and Delisitan ophiolite.     

东秦岭-大别山及邻区挠曲类盆地演化与碰撞造山过程

东秦岭-大别造山带是3 个板块沿两条缝合带俯冲碰撞而形成的近东西向不对称的反向多层次构造叠置的复合型造山带。在泥盆纪至三叠纪板块构造阶段中不同陆块间由于俯冲碰撞作用形成了多种挠曲类盆地。盆地时空演化充分体现了商丹古洋盆俯冲消减过程、北秦岭弧后区弧陆碰撞过程以及勉略古洋(海)盆斜向的、由东向西的碰撞造山过程。     

Scale and timing of Rare Earth Element redistribution in the Taconian foreland of New England

Clastic sedimentary rocks, deposited on eastern North America in response to the Taconian Orogeny, commonly have Sm/Nd isotope relationships indicating substantial isotope disturbance near or subsequent to the time of sedimentation that may be associated with severe depletion in light rare earth elements (LREE). Affected units [Normanskill Formation (Austin Glen and Pawlet Members), Frankfort Formation and Perry Mountain Formation] are widely separated both geographically (western New York to western Maine) and stratigraphically (Middle Ordovician to Silurian). A model is proposed for the most likely explanation of the observed REE and Sm/Nd isotope relationships involving a two‐stage process. In the first stage, REE are redistributed on a mineralogical scale (dissolution/precipitation on a sample scale) often with the involvement of REE‐enriched trace phases such as apatite and monazite. This stage typically takes place during diagenesis but may also take place later during metamorphism and/or recent weathering, and results in isotope re‐equilibration on a sample scale. The second stage occurs when one or more of these phases is redissolved and REE are transported on large advective scales. Where LREE‐enriched phases are involved, this gives rise to LREE depletion in whole rocks. The timing of this second stage cannot be constrained from Sm/Nd isotope data and may take place at any time subsequent to the isotope re‐equilibration. Such complex histories of REE redistribution may result in serious errors in estimating Nd model ages but not in estimating the Nd isotope composition at the age of sedimentation. Thus, Sm/Nd ratios even of unmetamorphosed sedimentary rocks have to be carefully evaluated before the calculation of depleted mantle model ages for the provenance.     

华南陆域内古特提斯形迹、二叠纪造山作用和互换构造域的东延

华南大陆由扬子克拉通和华夏陆块拼合而成,后者具有滇缅泰马(Sibumasu)、印支等互换构造域内诸地块共同的特点。古特提斯洋的一个分支从两广-浙闽东部通过,沟通了以西越南黑水河(SongDa)带和以东环太平洋晚古生代大洋亲缘诸地体。这个分支始于华南中泥盆世的陆内裂陷作用,古生代末扩展成广海。晚二叠世的龙潭组含煤地层和钦防地区的放射虫硅质岩分别代表浅水台地和远海盆地两个不同地形台阶的沉积,中间隔着一个向南倾斜的大陆斜坡。东吴运动在两广交界地区产生磨拉石、混杂堆积,并伴生六万大山等S型花岗岩基,代表云开地块向北的拼贴。类似事件在闽浙东部可能一直持续到晚中生代。     

库车再生前陆盆地的构造演化

库车再生前陆盆地位于塔里木盆地的北缘,其沉积和构造特征具有典型的前陆盆地性质.库车再生前陆盆地开始形成于吉迪克组沉积早期(距今25Ma),叠置于晚二叠世-三叠纪前陆盆地之上,是始新世末印度-西藏碰撞的远距离构造效应所致.其中的前陆逆冲带是由浅部和深部两个层次的构造组成的,其构造特征具有不一致性和不协调性.库车再生前陆逆冲带内的台阶状逆断层及其相关褶皱都是伴随着中新世以来的造山运动形成的,由山前向盆地以背驮式渐次连续扩展,自渐新世晚期一直持续到现在.印度-西藏碰撞作用引起的陆内俯冲及壳内拆离-缩短作用是库车再生前陆盆地的形成机制.     

扬子西缘泛非造山与绵阳－长宁克拉通裂陷的沉积充填过程研究

四川盆地安岳震旦—寒武系特大型气田的发现,掀起了探索四川盆地深层油气勘探新领域的热潮。然而对于川西北深层原型盆地沉积充填过程及其动力学机制的认识尚处于探索阶段。本文通过对扬子西北缘及盆内典型钻井震旦—寒武纪地层序列的综合调查与区域对比研究,基于关键事件序列的系统厘定,重建了扬子西北缘及邻区新元古代—寒武纪原型盆地沉积充填格架;对比全球冈瓦纳大陆汇聚与泛非造山过程,系统厘定了泛非运动在扬子西缘的沉积－构造－岩浆作用记录,进一步明确了华南扬子克拉通为新元古代末期冈瓦纳大陆的一部分。同时,结合盆内最新深层钻井和二维地震资料解析,研究认为绵阳－长宁克拉通边缘裂陷的成生、发展、充填与折返隆升过程主要受控于扬子西缘的泛非期弧后伸展、弧陆碰撞及陆陆碰撞造山过程,研究提出绵阳－长宁克拉通边缘裂陷经历了如下6个演化阶段：①陡山沱早期克拉通边缘裂陷开启,表现为克拉通边缘的裂陷与快速沉降;②陡山沱晚期—灯影早期为克拉通裂陷的持续沉降与克拉通碳酸盐台地的补偿性生长,区域沉积分异明显;③灯影晚期为克拉通裂陷扩展、定型与碳酸盐台地的活化与重建;④麦地坪期为克拉通边缘的差异隆升与碳酸盐台地的风化剥蚀;⑤筇竹寺期为克拉通增生陆块边缘的构造活化与克拉通边缘的挤压挠曲沉降;⑥沧浪铺期为陆陆碰撞造山与克拉通边缘裂陷的快速充填。至沧浪铺末期,绵阳－长宁克拉通裂陷沉积充填结束,并快速折返隆升为天井山－川中古隆起。因此,本文对扬子西缘克拉通边缘裂陷的盆地动力学过程重塑,不仅为华南板块在冈瓦纳古大陆重建中的构造定位提供了新资料,亦将对探索川西北乃至四川盆地深层油气勘探新领域、新方向具有重要的启示意义。     

阴山晚中生代板内造山特征及其动力机制——以内蒙古大青山为例

内蒙古大青山属我国典型的板内造山带阴山山脉的南部山系,其西段缺少大型低角度推覆构造及大型深成岩对前期演化历史的干扰,是研究阴山板内造山特点及过程的理想区域。通过对大青山西段的构造进行几何学和运动学的分析表明,古老结晶基底以逆冲推覆及基底褶皱的形式广泛地?入中生代的构造变形,以及先存构造样式的广泛复活并对后期地层沉积和断层发育的控制作用是内蒙古大青山地区中生代板内造山的两个基本特征。这些变形特征反映了阴山带板内造山过程中,是以结晶基底为受力层,并控制上覆盖层进行构造变形的,进而表明板内造山主要是由水平挤压应力造成的。结合研究区构造变形特点及邻区中生代构造地质情况的分析认为,晚侏罗世时期之所以在阴山带形成强烈的板内造山运动,是由其北部西伯利亚板块与蒙古褶皱带碰撞产生的板缘应力的远程传递,以及其南侧坚硬的鄂尔多斯地块的阻挡共同作用而形成的。     

Subsurface imaging,TAIGER experiments and tectonic models of Taiwan

The seismicity, deformation rates and associated erosion in the Taiwan region clearly demonstrate that plate tectonic and orogenic activities are at a high level. Major geologic units can be neatly placed in the plate tectonic context, albeit critical mapping in specific areas is still needed, but the key processes involved in the building of the island remain under discussion. Of the two plates in the vicinity of Taiwan, the Philippine Sea Plate (PSP) is oceanic in its origin while the Eurasian Plate (EUP) is comprised partly of the Asian continental lithosphere and partly of the transitional lithosphere of the South China Sea basin. It is unanimously agreed that the collision of PSP and EU is the cause of the Taiwan orogeny, but several models of the underlying geological processes have been proposed, each with its own evolutionary history and implied subsurface tectonics.TAIGER (TAiwan Integrated GEodynamics Research) crustal- and mantle-imaging experiments recently made possible a new round of testing and elucidation. The new seismic tomography resolved structures under and offshore of Taiwan to a depth of about 200 km. In the upper mantle, the steeply east-dipping high velocity anomalies from southern to central Taiwan are clear, but only the extreme southern part is associated with seismicity; toward the north the seismicity disappears. The crustal root under the Central Range is strongly asymmetrical; using 7.5 km/s as a guide, the steep west-dipping face on the east stands in sharp contrast to a gradual east-dipping face on the west. A smaller root exists under the Coastal Range or slightly to the east of it. Between these two roots lies a well delineated high velocity rise spanning the length from Hualien to Taitung. The 3-D variations in crustal and mantle structures parallel to the trend of the island are closely correlated with the plate tectonic framework of Taiwan. The crust is thickest in the central Taiwan collision zone, and although it thins toward the south, the crust is over 30 km thick over the subduction in the south; in northern Taiwan, the northward subducting PSP collides with Taiwan and the crust thins under northern Taiwan where the subducting indenter reaches 50 km in depth. The low Vp/Vs ratio of around 1.6 at a mid-crustal depth of 25 km in the Central Range indicates that current temperatures could exceed 700 °C. The remarkable thickening of the crust under the Central Range, its rapid uplift without significant seismicity, its deep exhumation and its thermal state contribute to make it the core of orogenic activities on Taiwan Island.The expanded network during the TAIGER deployment captured broadband seismic data yielding enhanced S-splitting results with mainly SKS/SKKS data. The polarization directions of the fast S-waves follow very closely the structural trends of the island, supporting the concept of a vertically coherent Taiwan orogeny in the outer few hundred kilometers of the Earth.     

Palaeontological age and correlations of the Tertiary deposits of the NW Iberian Peninsula: the tectonic evolution of a broken foreland basin

Ages of Cenozoic sedimentary basins yield information that can be used to infer detailed spatial and temporal evolution in the Alpine foreland. The Tertiary deposits of the NW Iberian Peninsula comprise the remains of a broken foreland basin (the West Duero Basin). This work constrains the timing of tectonic fragmentation and the evolution of the western termination of the Alpine Pyrenean–Cantabrian Orogen (NW Iberian Peninsula). The discovery of Issiodoromys cf. minor 1 and Pseudocricetodon in the lower formation of the Tertiary depression of Sarria (the Toral Formation) constrains its age to the late Early Oligocene (MP23–MP25), similar to its age in the El Bierzo depression (MP24–MP25). Sedimentation initiated in the NE of the study area at Oviedo during the Middle Eocene (Bartonian–Early Priabonian MP16–MP17) and migrated towards the west and south during the Early Oligocene. The Toral Formation was deposited in a foreland basin that connected the present day outcrops of the El Bierzo, Sarria and As Pontes Tertiary depressions. The basin was segmented during the westward migration of structural deformation associated with the Orogen, and the subsequent uplift of the Galaico–Leoneses Mountains. The present‐day height above reference level of the base of the Toral Formation has been used to quantify Alpine segmentation that took place after Early Oligocene times. Minimum tectonic uplift assessed is 960 m in the Cantabrian Mountains and 1050 m in the Galaico–Leoneses Mountains. Copyright © 2013 John Wiley & Sons, Ltd.     

The Afeição augen-gneiss Suite and the record of the Cariris Velhos Orogeny (1000–960 Ma) within the Riacho do Pontal fold belt,NE Brazil

Early Tonian (∼1000–920 Ma) rocks occur within the Transversal Zone of the Borborema Province in Northeast Brazil comprising the 700 km-long sigmoidal Cariris Velhos belt. The Afeição augen-gneiss Suite crops out in the internal zone of the Riacho do Pontal fold belt, about 100 km southwestward of the closest Cariris Velhos occurrence within the Transversal Zone, and has been proposed to represent a continuation of this belt within the Southern subprovince of the Borborema Province. Several plutons included within this unit intrude or are thrust upon metavolcanosedimentary sequences of three different units (Santa Filomena, Paulistana and Morro Branco complexes). The Afeição Suite is composed mostly of calc-alkaline, high-K, peraluminous, high-silica ferroan and magnesian granites. Chondrite-normalized REE patterns are moderate to highly fractionated, with a pronounced negative Eu anomaly. Incompatible element spidergrams show a negative Nb–Ta anomaly, akin to convergence setting (Cordilleran-type) granites. U–Pb zircon data constrain the age of crystallization between 1000 and 960 Ma, thus confirming chrono-correlation with the Cariris Velhos belt. Values of εNd(t) between −1.0 and +3.1 and T_DM of 1.2–1.5 Ga, similar to other Cariris Velhos occurrences, suggest variable mixing of Tonian juvenile sources with older crustal sources, the latter involving Archean/Paleoproterozoic basement. Although there are currently no reliable geochronological data for the supracrustal sequences of the internal zone, cross-cutting relationships indicate that the Santa Filomena and Morro Branco complexes are older or of similar age to the Afeição Suite, and thus could be related to the Cariris Velhos Orogeny. Based on petrographic, lithogeochemical, geochronological and isotope data, and according to previous models proposed for the Cariris Velhos belt, we interpret the Afeição Suite as the southwestern edge of a continental margin magmatic arc accreted to this portion of West Gondwana during the Early Tonian. Late-Brasiliano dextral displacement through the western branch of the Pernambuco shear zone separated these Cariris Velhos occurrences from its equivalents within the Transversal Zone of the Borborema Province (Recanto-type augen-gneiss of the Alto Pajeú Terrane).