Discovery of Jurassic ammonoids from the Shyok suture zone to the northeast of Chang La Pass, Ladakh, northwest India and its tectonic significance

Abstract   Callovian (late Middle Jurassic) ammonoids Macrocephalites and Jeanneticeras were recovered from the Shyok suture zone, northeast of Chang La Pass, Ladakh, northwest India. They are the first reliable Jurassic fossils and the oldest chronologic data from the Shyok suture zone. The ammonoid-bearing Jurassic strata, newly defined as the Tsoltak Formation, consist largely of terrigenous mudstone with thin sandstone beds and were probably a part of the continental basement to the Cretaceous Ladakh Arc.     

Exploration of geothermal structure in Puga geothermal field, Ladakh Himalayas, India by magnetotelluric studies

To understand the crustal electric structure of the Puga geothermal field located in the Ladakh Himalayas, wide band (1000 Hz–0.001 Hz) magnetotelluric (MT) study have been carried out in the Puga area. Thirty-five MT sites were occupied with site spacing varying from 0.4 to 1 km. The measurements were carried out along three profiles oriented in east–west direction. After the preliminary analysis, the MT data were subjected to decomposition techniques. The one-dimensional inversion of the effective impedance data and the two-dimensional inversion of the TE (transverse electric) and TM (transverse magnetic) data confirm the presence of low resistive (5–25 Ω m) near surface region of 200–300 m thick in the anomalous geothermal part of the area related to the shallow geothermal reservoir. Additionally, the present study delineated an anomalous conductive zone (resistivity less than 10 Ω m) at a depth of about 2 km which is possibly related to the geothermal source in the area. A highly resistive basement layer separates the surface low resistive region and anomalous conductive part. The estimated minimum temperature at the top of conductive part is about 250 °C. The significance of the deeper conductive zone and its relation to the geothermal anomaly in the area is discussed.     

阿尼玛卿缝合带及两侧上地壳泊松比分布

本文通过对马尔康－古浪宽角反射和折射剖面高精度观测段（200～400 km）的Pg、Sg资料的分析解释,给出了该区段上地壳速度结构和泊松比分布.并利用震相识别误差分析、分辨率计算及不同初始模型筛选等方法,使走时反演结果的可靠性进一步提高.研究表明：对应于阿尼玛卿缝合带的275km～355 km桩号段内平均泊松比高达0.297,P波和S波速度呈横向高低速条带变化,库赛湖－玛沁断裂和舟曲－两当断裂基本上均位于高泊松比和低速带内,并分别处在缝合带与若尔盖盆地和西秦岭褶皱带的分界上.阿尼玛卿缝合带南侧的若尔盖盆地平均泊松比较低,约为0.257,北侧西秦岭褶皱带的平均泊松比为0.264.     

Phengite compositions and post-nappe high-pressure metamorphism in the pennine zone of the French Alps

A systematic study of relict highly substituted phengite micas (Si_3.6–3.7) formed in the less deformed and less recrystallized rocks of the internal zone of the French Alps shows that silica content “isograds” for phengites cut across major thrust boundaries. This indicates that a high-pressure metamorphism was post-nappe in the orogenic sequence. Little substituted, secondary phengites and those found in highly recrystallized rocks reflect metamorphic conditions which occurred during the later mountain building phases of the orogeny.Variations of mica phengite content within mineral facies zones indicate rather important differences in pressure along the Pennine zone in going from low- to high-temperature mineral facies.     

Paleomagnetism of the Xigaze ophiolite and flysch (Yarlung Zangbo suture zone, southern Tibet): latitude and direction of spreading

The Xigaze ophiolite (29.2°N, 89.5°E), which outcrops in the Yarlung Zangbo suture zone, represents the remnants of an oceanic lithosphere formed in middle Cretaceous times between the Lhasa block to the north and the Indian plate to the south. In an attempt to define the paleo-orientation and latitude of the spreading center at which it has been created, a paleomagnetic study has been done on three sites in volcanics and overlying (or interbedded) radiolarites forming the upper part of the ophiolite sequence and also on seven sites in the Xigaze Group flysch which stratigraphically overlies the volcanics to the north. In each site, hand-blocks carefully oriented both with sun and magnetic compass have been sampled. The paleomagnetism data, combined with structural data on the ophiolite dolerite intrusives, allow a partial reconstruction of South Eurasia at the time of formation of the Xigaze ophiolite. The paleolatitude of accretion and deposition of the Xigaze ophiolite and overlying sediments is found to be 10–20°N. Both ophiolite and basin have encountered a 85 ± 20° anti-clockwise rotation. The corresponding ridge was close to the southern margin of the Lhasa block and was oriented N175 ± 25°.     

Sequestration of carbon as carbonate in the critical zone:insights from the Himalayas and Tibetan Plateau

The carbon pool stored in soil carbonate is comparable to the soil organic carbon.Therefore,secondary calcite precipitation in supersaturated catchment could be an important,yet poorly constrained,carbon sink within the modern global carbon cycle.The chemical analysis of some dissolved species transported by rivers,such as elevated Sr/Ca and Mg/Ca ratios but also heavy stable Ca isotopic compositions,witness the formation of secondary calcite in rivers draining arid regions.However,in areas affected by active tectonics and rapid physical erosion,co-variations in the fluvial Sr/Ca and Mg/Ca ratios could also be related to incongruent carbonate weathering processes.Here,we present a model to assess the roles played by incongruent carbonate dissolution and secondary calcite precipitation in modern weathering processes.We tested and applied the model to rivers draining the Himalayan–Tibetan region.The results suggest that regional aridity in the drainage basin promotes carbon sequestration as secondary carbonate but that for a given runoff,incongruent dissolution of carbonate possibly related to rapid physical erosion amplifies such sequestration.The isotopic compositions(~(13)C/~(12)C and~(18)O/~(16)O)of detrital carbonate transported by the main rivers in South and South-East Tibet imply that around 1%of the suspended material transported by those rivers corresponds to secondary carbonate and can represent between 5%and 15%of the alkalinity flux.Most of these alkalinity transported as particulate material is,nevertheless related to the weathering of carbonate lithologies and is also subjected to dissolution prior its final storage in sedimentary basins.However,on glacial-interglacial timescale this will amplify the significant role of mountain weathering on climatic variations.     

Shock metamorphism of plagioclase and amphibole (Experimental data)

The shock metamorphism of plagioclase and amphibole of various chemical compositions from amphibolite and granulite facies schists was studied in experiments with shock wave loading of samples in steel recovery ampoules of plane geometry. A maximum shock pressure was reached after a few circulations of waves in the sample (stepwise shock wave compression) and varied within 26–52 GPa. The recovered samples were examined by the methods of scanning electron microscopy and microprobe and X-ray phase analysis. It is established that an increase in the F, Ti, and K concentrations in amphibole and a decrease in the Ca concentration in plagioclase make these minerals more stable with respect to shock waves. It is shown that the migration of some chemical elements, starting already at the solid phase stage of transformation in plagioclase and amphibole, is intensified at the stage of melting. It is established that isotropization of plagioclase occurs through two different mechanisms. At relatively low pressures, it is caused by the fragmentation of substance at the microlevel and is accompanied by the formation of maskelynite, a typical mineral of meteorites and astrobleme rocks. At higher pressures, isotropization is associated with melting-induced amorphization.     

An alternative tectonic model for the Yarlung Zangbo suture zone

The new field investigations along the Yarlung Zangbo ophiolites zone show that these series underwent low green-schist metamorphism and were then fractured and occurred as slabs in tectonic melanges,without regional tectonic polarity.No large shear zone in north-south direction has been identified between ophiolite bodies and flysch layers on both side and a conformable contact relationship can be observed locally between them.A great mass of tectonic mélange has been substantiated as submarine olistolith ...     

基于大地电磁测深的藏南雅鲁藏布江缝合带电性结构研究

为研究日喀则市活动断裂深浅部构造关系及深部孕震机制,跨雅鲁藏布江谢通门—日喀则段部署了48个宽频大地电磁测深点,剖面长度为108 km。在二维反演的基础上对壳幔200 km深度范围内的电性结构进行了探测研究。剖面自南向北依次经过喜马拉雅地块、雅鲁藏布江缝合带和拉萨—冈底斯地块。喜马拉雅地块地壳表现为高阻特性,其北侧的仲巴—郎杰学陆缘移置混杂地体发育了深达上地幔盖层的巨厚的北倾低阻体;雅鲁藏布江主缝合带表现为喇叭状低阻通道,宽约10 km,存在深浅部两处低阻体,浅部南倾深部北倾,低阻通道南部发育近似直立或南倾的高阻日喀则蛇绿岩,北部发育近直立的高阻冈底斯花岗岩体,整体表现为两个高阻异常体中间夹一个连通壳幔的带状低阻通道;拉萨—冈底斯地块以高阻为主,中下地壳普遍发育低阻体。缝合带附近因板块俯冲作用导致壳幔局部增厚或减薄,表现为电性的梯度变化,表现为低阻特性的部分则是壳幔物质的运移通道。      

龙木错-双湖-澜沧江缝合带中段双湖地区地球物理特征

龙木错-双湖-澜沧江板块缝合带位于青藏高原羌塘中部至藏东一带,相关研究将其厘定为冈瓦纳板块的北界,对探讨青藏高原早期形成与演化历史及区域资源勘查与评价有重要的地质意义.本文结合区域重磁数据、大地电磁数据对该缝合带地球物理特征进行分析论证.首先基于WGM2012全球重力场模型网格化数据,利用垂向一阶导数计算、构造增强滤波等处理方法,推测龙木错-双湖-澜沧江缝合带异常的空间分布.计算的区域莫霍面在缝合带南北两侧深度存在较大差异.重力数据及其处理结果论证了龙木错-双湖-澜沧江缝合带存在的可能性.其次,选取了龙木错-双湖-澜沧江缝合带中段双湖地区的高质量大地电磁数据进行反演解释,电阻率反演结果表明缝合带南北存在较大的电性差异,结合区域地质情况,推测缝合带是不同地质体的分界线.最后,对缝合带地球物理响应特征和地质认识进行了讨论.通过对地球物理数据分析处理,为进一步讨论缝合带形成提供了有利的科学依据.     

Himalayan metamorphism and deformations in the North Himalayan Belt (southern Tibet,China)

The Kangmar Massif which has been attributed to the North Himalayan Belt of two-mica granites in southern Tibet (China) is shown to be an orthogneiss in the core of a dome which displays an intermediate high-pressure type metamorphism, Himalayan in age (40 to 20 Ma) and above the Main Central Sheet. This metamorphism took place during high shear strain deformation, and it is not recorded to the west where North Himalayan plutons have developed contact metamorphism.     

Late Paleozoic radiolarians from the Bentong-Raub suture zone, Peninsular Malaysia

Abstract Radiolarians extracted from marine siliceous sediments from the Bentong-Raub suture zone, Peninsular Malaysia have indicated a range of ages for olistostromal blocks of bedded chert, siliceous argillite and tuffaceous argillite, and chert clasts and lenses within the mélange from the suture zone. Late Devonian (Faniennian), Early Carboniferous (Tournaisian and Viséan) and Early Permian (Wolfcampian and Leonardian) ages are represented by seven radiolarian zones from ten localities along the suture zone. In stratigraphic order these include Holoeciscus 2–3 Assemblage Zones, Albaillella paradoxa Zone, Albaillella dejendrei Zone, Albaillella cartalla Zone, Pseudoalbaillella lomentaria Zone, Albaillella sinuata Zone and Pseudoalbaillella longtanensis Zone. Fifteen genera are represented by 35 species. The range of ages from Late Devonian to Early Permian suggests that an ocean existed between the Sibumasu and East Malaya terranes from at least Late Devonian to late Early Permian time and that closure of the ocean between the two terranes could not have occurred until after late Early Permian time. The range of ages and rock types from different depositional environments, indicate that the Bentong-Raub suture zone includes a disrupted accretionary complex.     

Blueschist: A window into high-pressure/low-temperature metamorphism and subduction zone dynamics

Science China Earth Sciences - Blueschist is a regional metamorphic rock formed under high-pressure (HP) low-temperature (LT) conditions. It is formed in the subduction zone environment with low...     

The Jinshajiang suture zone: tectono-stratigraphic subdivision and revision of age

Integrated study of rock assemblage, tectonic setting, geochemical feature, fossil contained and isotopic geochronology on the metamorphic mixed bodies, exposed in the Jinshajiang suture zone, suggests that one informal lithostratigraphic unit, the Eaqing Complex, and three tectono-stratigraphic units, the Jinshajiang ophiolitic melange, the Gajinxueshan Group and the Zhongxinrong Group, can be recognized there. It is first pointed out that the redefined Eaqing Complex might represent the Meso- to Neo-Proterozoic remnant metamorphic basement or mi-crocontinental fragment in the Jinshajiang area. The original rocks of it should be older than (1627 ±192) Ma based on the geochronological study. The zircon U-Pb age of plagiogranites within the Jinshajiang ophiolitic assemblage is dated for the first time at (294 ± 3) Ma and (340 ± 3) Ma respectively. The Jinshajiang ophiolite is approximately equivalent to the Ailaoshan ophiolite in the formation age, covering the interval from the Late Devonian to the Carboniferous. Dating of U-Pb age from basalt interbeds indicates that the redefined Gajinxueshan Group and Zhongxinrong Group may be considered Carboniferous to Permian and latest Permian to Middle Triassic in age. In geotectonic terms the Jinshajiang suture zone is thought to be a back-arc basin in the eastern margin of the Paleo-Tethys. This back-arc basin started in the Late Devonian, and formed in the Devonian-Carboniferous. The collision event around the Permian/Triassic boundary to the Middle Triassic led to the closure of the back-arc basin and formation of suture.     

扬子与华夏块体缝合带区域的Rayleigh及Love波相速度层析成像

扬子与华夏块体碰撞拼合构成华南大陆, 其间存在多条大型断裂带, 两个块体缝合于哪些断裂尚存争议, 这限制了对华南构造演化的认识.本研究使用布设于扬子和华夏块体之间的184个流动密集台站以及华南地区273个固定台站的三分量连续波形数据(2014年10月至2016年12月), 采用背景噪声层析成像方法获得研究区域内Rayleigh和Love波周期为4~25 s的二维相速度.短周期的相速度与地壳中上部的盆地和褶皱等地质构造单元吻合; 长周期的相速度在东部地区整体表现为高速异常, 可能是受较薄的地壳厚度影响, 西南部地区Rayleigh波呈现高速异常, Love波则呈现显著低速异常, 这种差异可能反映该区域地下介质存在径向各向异性.成像结果显示, 扬子与华夏块体的Rayleigh和Love波相速度存在较大的差异, 结合华南地区已有的地质与地球物理学研究成果, 本研究认为扬子与华夏块体可能在地壳浅层缝合于绍兴—江山—萍乡—永州—河池一线, 深部缝合带则可能由石台—九江—长沙延伸至永州—河池线.

     

Geochemistry of calc-alkaline volcanics in northwestern Nigeria,and a possible Pan-African suture zone

Major and trace element characteristics of dacites and rhyolites overlying and intruding basement rocks in northwestern Nigeria most closely resemble those of intracontinental orogenic volcanic associations. REE patterns point to a deep-seated source for the magmas, perhaps involving garnet fractionation at mantle depths and low-pressure plagioclase fractionation. The occurrence of calc-alkaline volcanics, small basic-ultrabasic complexes and major transcurrent faulting, is consistent with the presence of a Pan-African suture zone in northwestern Nigeria.     

Mianlüe tectonic zone and Mianlüe suture zone on southern margin of Qinling-Dabie orogenic belt

The Mianlue tectonic zone (Mianlue zone), an ancient suture zone in addition to theShangdan suture in the Qinling-Dabie orogenic belt, marks an important tectonic division geo-logically separating north from south and connecting east with west in China continent. To de-termine present structural geometry and kinematics in the Mianlue tectonic zone and to recon-struct the formation and evolution history involving plate subduction and collision in theQinling-Dabie orogenic belt, through a multidisciplinary study, are significant for exploring themountain-building orogenesis of the central orogenic system and the entire process of the majorChinese continental amalgamation during the Indosinian.     

Upper Cretaceous trench deposits of the Neo-Tethyan subduction zone: Jiachala Formation from Yarlung Zangbo suture zone in Tibet,China

The history of convergence between the India and the Asia plates, and of their subsequent collision which triggered the Himalayan orogeny is recorded in the Yarlung Zangbo suture zone. Exposed along the southern side of the suture, turbidites of the the Jiachala Formation fed largely from the Gangdese arc have long been considered as post-collisional foreland-basin deposits based on the reported occurrence of Paleocene-early Eocene dinoflagellate cysts and pollen assemblages. Because magmatic activity in the Gangdese arc continued through the Late Cretaceous and Paleogene, this scenario is incompatible with U-Pb ages of detrital zircons invariably older than the latest Cretaceous. To solve this conundrum, we carried out detailed stratigraphic, sedimentological, paleontological, and provenance analyses in the Gyangze and Sajia areas of southern Tibet,China. The Jiachala Formation consists of submarine fan deposits that lie in fault contact with the Zongzhuo Formation.Sandstone petrography together with U-Pb ages and Hf isotope ratios of detrital zircons indicate provenance from the Gangdese arc and central Lhasa terrane. Well preserved pollen or dinoflagellate cysts microfossils were not found in spite of careful research, and the youngest age obtained from zircon grain was ～84 Ma. Based on sedimentary facies, provenance analysis and tectonic position, we suggest that the Jiachala Formation was deposited during the Late Cretaceous(~88–84 Ma) in the trench formed along the southern edge of Asia during subduction of Neo-Tethyan oceanic lithosphere.