Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO₂ inclusions by preferential loss of H₂O. Those inclusions armoured within garnet retained their primary 3-phase H₂O–CO₂ compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H₂O leakage along dislocations and planar lattice defects which results in the predominance of CO₂ inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO₂-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.     

拉萨地区晚第四纪地壳的抬升与拉萨河的向南迁移

根据数字高程模型、遥感资料分析及实地调查, 拉萨地区在近南北向挤压的作用下, 与NWW向的主构造格架断裂走向相一致, 近东西向的分水岭和沟谷沿南北方向相间分布。自分水岭附近的山顶剥蚀面至盆地, 坡度由平缓、较陡、平缓逐渐过渡, 反映了山顶剥蚀区、强剥蚀区、堆积区的依次变化, 表现了一个完整的构造、气候控制的地貌剥蚀、堆积过程。在同样的气候条件下, 拉萨地区晚第四纪以来的新构造运动整体表现为自北向南的掀斜, 造成北侧剥蚀较强, 下游河流得以携带更多的泥沙物质, 北侧冲、洪积扇汇入拉萨河的物质较南侧多, 加之地壳自北向南的掀斜, 使得拉萨河拉萨段向南迁移, 堆龙曲河口冲积扇也向南迁移。      

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km²) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year⁻¹ for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.     

西湖凹陷陆盆冲积体系河道演变规律探讨

西湖凹陷渐新统一中新统陆盆沉积发育了类型丰富的河道沉积，本文根据陆盆充填机制对盆地演化各阶段的控制作用，从沉积层序、沉积构造、岩性组合特征诸方面探讨了盆地演化过程的河道变迁特点。     

西秦岭岷县地区二叠纪裂陷带斜坡沉积及其构造意义

秦岭造山带西段，二叠纪裂陷作用非常活跃。位于西秦岭中部的岷县红崖、梅州一带，二叠系以粉碎屑沉积为主，泥质较少。根据对其岩石类型、沉积构造、岩石组合和生物特征的分析，认为这套地层主要形成于海底扇体系的内扇和中扇环境，外扇不发育，系由碎屑流和浊流沉积组成，包括内扇的水道和水下天然堤沉积及中扇的网状水道和朵叶体沉积。这套地层沿中秦岭南缘呈ＮＷＷ－ＳＥＥ方向延伸，代表了二叠纪西秦岭裂谷的存在及其继续裂陷的活动特征。     

Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex,Czech Republic: Implications for Variscan orogenesis

The Mariánské Lázn complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated _Nd values and initial ⁸⁷Sr/⁸⁶Sr ratios. Five of the eight eclogite samples have high _Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower _Nd values (+ 5.4 to –0.8) and widely variable ⁸⁷Sr/⁸⁶Sr ratios (0.7033 to 0.7096). Such low _Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.     

Decompressional coronas and symplectites in granulites of the Musgrave Complex, central Australia

Abstract In granulite facies metapelitic rocks in the Musgrave Complex, central Australia, reaction between S1 garnet and sillimanite involves the development in S2 of both garnet + cordierite + hercynitic spinel + biotite and hercynitic spinel + cordierite + sillimanite + biotite. The S2 assemblages occur either in coronas and symplectites, mainly around garnet, or, in rocks in which S2 is more strongly developed, as recrystallized assemblages. Ignoring the presence of biotite and ilmenite, the mineral textures can be accounted for qualitatively by a consideration of the model system FeO-MgO-Al₂O₃-SiO₂ (FMAS); the textural relationships accord with decompression accompanying the change from S1 to S2. However, since biotite and ilmenite are involved in the assemblages, the parageneses are better accounted for in terms of equilibria in the expanded model system K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂-TiO₂-Fe₂O₃ (KFMASHTO), i.e. AFM + TiO₂+ Fe₂O₃. The coronas reflect the tectonic unroofing of at least part of the Musgrave Complex from peak S1 conditions of about 8 kbar to S2 conditions of about 4 kbar.     

钱塘江冲海积粉土渗透破坏试验研究

钱塘江冲海积粉土以砂质粉土为主,呈“高粉性、低黏性”的地区特点,在地下水动水压力作用下极易产生渗透破坏。粉土是介于砂性土和黏性土之间的一种过渡类型土,工程性质既与砂性土不同又与黏性土有较大区别。采用自行研制的粉土抗渗强度测定设备,对钱塘江冲海积粉土5种典型样本和微层理原状样本进行了渗透稳定试验。结果表明,钱塘江冲海积粉土渗透稳定性差,抗渗强度低;砂质粉土临界水力比降icr为1.01～1.25,破坏形式为局部流土,从临界水力比降icr发展到破坏比降iF的过程是瞬间的,基坑开挖遇到该类土层极易产生渗透破坏;黏质粉土临界水力比降icr随黏粒含量增加而增加,从临界水力比降icr发展到破坏比降iF有个短暂的过程,是整体流土破坏;钱塘江冲海积粉土渗透稳定各向异性;渗流垂直微层理方向的临界水力比降icr最大,是渗流沿微层理方向icr的2.5倍左右。研究结果为粉土地基渗透破坏防治提供参考。     

Assessment of Debris-Flow Hazards of Alluvial Fans

A strategy is presented for the assessment ofdebris-flow hazards on alluvial fans on the basis ofa case study carried out on the southern foot of astratovolcano named Mt. Yatsugatake. Transformation ofcommercial forests into a golf course was planned ata corner of the Kikkakezawa fan. The case studyinvolves an assessment of hazards due to debris flowsof different magnitudes and their recurrenceintervals. The environmental conditions for therecurrence of these debris flows are discussed as wellas the extent of the areas affected by them. In orderto generalize the case study, concepts of hazardpotential, hazard and risk for debris flows onalluvial fans are established. Accordingly, the hazardpotential is the possible hazards at any location onan alluvial fan in an indefinitely long time period,which can be assessed on the basis of hydrological andother geoscientific investigations. Hazards associatedwith a particular land use can be evaluated on thebasis of the hazard potential considering the locationand the time period associated with the land use. Riskcan be further assessed considering the life styles inthis land and the social conditions.     

塔里木盆地塔东凸起西部中上奥陶统地震层序与海底扇沉积

根据区域地震资料研究塔里木盆地塔东凸起西部中上奥陶统层序地层格架及沉积演化, 在中上奥陶统识别出了2个地震层序, 发现了叠置的丘状前积反射地震单元, 综合岩心观察、岩屑录井和薄片资料, 确认为海底扇沉积体.海底扇沉积主要由块状砂、砾岩, 递变层理砂岩, 平行层理砂岩, 砂纹层理粉砂岩, 变形或包卷层理粉砂岩, 水平层理泥质粉砂岩或粉砂质泥岩, 块状或递变的粉砂质泥岩和泥岩等岩相组成, 形成于中扇和外扇环境, 物源来自研究区南部的岛弧带.海底扇的发现对于塔东凸起乃至整个塔里木盆地中上奥陶统油气勘探具有重要意义.