Interplay between tectonics and compaction in a rift‐margin,lacustrine delta system: Miocene of the Eger Graben,Czech Republic

The Early Miocene Bílina Palaeodelta consists of fluvio‐deltaic and lacustrine clastics deposited along the south‐eastern margin of the extensional Most Basin, part of the Eger Graben in north Bohemia (Czech Republic). The Bílina succession shows evidence of repeated advances of an axial deltaic system across a thick accumulation of organic material and clay in the hangingwall of an active fault. Exposures up to ca 4·5 km long in the Bílina open‐cast mine help bridge the gap between seismic scale and typical outcrop scale of observation and thus allow the relationships between small‐scale and basin‐scale stratal geometries to be evaluated. The Bílina Palaeodelta deposits include sand‐dominated, fluvial channel fills and heterolithic sheets interpreted as delta plain strata, sand‐dominated mouth‐bar wedges and heterolithic sheets of prodeltaic deposits, passing distally into lacustrine clays. The depositional environment is interpreted as a fluvial‐dominated, mixed‐load, lacustrine delta with a high degree of grain‐size segregation at the feeder‐channel mouths. On the largest temporal and spatial scales, variable tectonic subsidence controlled the overall advance and retreat of the delta system. The medium‐term transgressive‐regressive history was probably driven by episodes of increased subsidence rate. However, at this temporal scale, the architecture of the deltaic sequences (deltaic lobes and correlative lacustrine deposits) was strongly affected by: (i) compaction of underlying peat and clay which drove lateral offset stacking of medium‐term sequences; and (ii) growth of a fault‐propagation fold close to the active Bílina Fault. At the smallest scale, the geometries of individual mouth bars and groups of mouth bars (short‐term sequences) reflect the interaction among sediment loading, compaction and growth faulting that produced high‐frequency relative lake‐level fluctuations and created local accommodation at the delta front.     

Crustal volatile release at Merapi volcano; the 2006 earthquake and eruption events

High‐temperature gas in volcanic island arcs is widely considered to originate predominantly from the mantle wedge and from subducted sediments of the down‐going slab. Over the decade (1994–2005) prior to the 2006 eruption of Merapi volcano, summit fumarole CO₂ gas δ¹³C ratios are relatively constant at ?4.1 ± 0.3‰. In contrast, CO₂ samples taken during the 2006 eruption and after the May 26th 2006 Yogyakarta earthquake (M6.4) show a dramatic increase in carbon isotope ratios to ?2.4 ± 0.2‰. Directly following the earthquake (hypocentre depth 10–15 km), a 3–5‐fold increase in eruptive intensity was observed. The elevated carbon isotope gas data and the mid‐crustal depth of the earthquake source are consistent with crustal volatile components having been added during the 2006 events, most probably by the thick local limestone basement beneath Merapi. This ‘extra’ crustal gas likely played an important role in modifying the 2006 eruptive behaviour at Merapi and it appears that crustal volatiles are able to intensify and maintain eruptions independently of traditional magmatic recharge and fractionation processes.     

Continental drift and the global pattern of sedimentation

The breakup of Pangaea through rifting and separation of the continents has special implications for the global pattern of sedimentation. The important initial conditions of Pangaea are area, elevation, the nature of the drainage and climate. The development of interior uplifts associated with rifting caused significant reorganization of drainage systems. Rifting and continental breakup result in unique sediment sequences on passive margins.The initial rift valleys were probably occupied by stratified fresh water lakes due to the equable Mesozoic climate, and during this phase might have accumulated about 7.5×10²¹ g of organic carbon; this would be 14% of the earth's total organic carbon concentrated on only 0.3% of the earth's surface. The sediments rich in organic carbon are expected to be typically overlain by evaporites.Sedimentation on the continental shelf is a complex interplay of thermal subsidence, sea level changes, sediment supply and isostatic adjustment.Beyond the shelf break, sedimentation rates in the world ocean appear to change significantly with time; during the Aptian-Albian, Campanian-Maastrichtian, Middle Eocene and Late Miocene-Quaternary overall sedimentation rates were about an order of magnitude higher than during the intervening periods. This variation is likely to be related to changing sediment supply responding to changes in sea level.

---

Zusammenfassung Das Aufbrechen Pangaeas durch Spaltung und Separation der Kontinente hat spezielle Auswirkungen auf die globale Sedimentationsverteilung. Die bedeutenden Anfangsbedingungen Pangaeas sind Größe, Höhe, Abfluß- und Klimasituation. Die Entwicklung innerer Aufwölbungen, zusammen mit Spaltenbildung, hat eine entscheidende Reorganisation der Abflußsysteme bewirkt. Spaltung und Aufbrechen des Kontinents resultieren in einer einzigartigen Sedimentationsfolge an den passiven Rändern.Die anfänglichen Spaltungstäler waren wahrscheinlich von geschichteten Süßwasserseen eingenommen, hervorgerufen vom ausgeglichenen mesozoischen Klima. Während dieser Zeit können sich dort etwa 7,5 · 10²¹ g organischer Kohlenstoffe angesammelt haben, d. h. 14 % der weltweiten organischen Kohlenstoffe auf nur 0,3 % der Erdoberfläche. Die kohlenstoffreichen Sedimente sind typischerweise von Evaporiten überlagert.Sedimentation auf dem kontinentalen Schelf ist ein komplexes Zusammenspiel von Temperatursenkung, Wasserspiegelschwankungen, Sedimentsangebot und isostatischem Gleichgewicht.Jenseits des Schelfabhanges scheinen die Sedimentationsraten im Weltmeer signifikant mit der Zeit zu schwanken. Während des Aptiums-Albs, des Campaniums-Maastrichts, des Mittleren Eozäns und des Späten Miozän-Quartärs lagen die gesamten Sedimentationsraten um eine Größenordnung höher als in den dazwischenliegenden Zeiträumen. Diese Variation hängt wahrscheinlich mit einem geänderten Sedimentsangebot zusammen, das von Wasserspiegelschwankungen hervorgerufen wird.

---

Résumé La dislocation de la Pangée, par fracturation (rifting) et séparation des continents, entraîne des implications particulières en ce qui concerne les modèles de sédimentation à l'échelle du globe. Les conditions initiales importantes de la Pangée sont la surface, l'altitude, la nature du drainage et le climat. Le développement de soulèvements intérieurs en association avec les processus de fracturation (rifting) fut la cause d'une importante réorganisation du système de drainage. Sur les marges continentales passives, des séquences sédimentaires uniqes résultent de ces processus de caussure (rifting), et de dislocation.Les vallées médianes (rifts) initiales ont été probablement occupées par des lacs à eaux douces stratifiées, dues au climat égal du Mésozoique, et ont pu durant cette période accumuler environ 7.5×20²¹ g. de carbone organique; cette quantité serait l'équivalent de 14% du carbone organique total du globe, concentrés sur 0.3% de sa surface. Les sédiments riches en carbone organique sont supposés être typiquement recouverts par des dépôts évaporitiques.La sédimentation sur le plateau continental se trouve en interaction complexe avec la subsidence thermique, les fluctuations du niveau des océans, les apports en sédiments et les ajustements isostatiques.Au delà de la limite externe du plateau continental, les taux de sédimentation dans les océans semblent varier de manière importante dans le temps; durant l'Aptien-Albien, le Campanien-Maastrichtien, L'Eocène moyen et le Miocène supérieur-Quaternaire, les taux d'ensemble de sédimentation ont été environ d'un ordre de grandeur plus élevé que durant les périodes intermédiaires. Cette variation est vraisemblablement liée aux changements des apports en sédiments, en réponse aux fluctuations du niveau océanique.

---

(—1972 —1977) - . , 7,7 /. . . 10–15 , 7,0–7,4 /. . , , 30 7,5–7,7 /. 50 . 12–18 , . , .

     

The global hydrologic cycle and its continental links

The terms of “the Global Hydrologic Cycle” and “the river flow”, that is the continental stage of this cycle, are made more exact in this article. The possibility to regard the river flow as a consecutive change of the genetically and qualitatively different water masses is based here. Processes of mixing of the river water masses and transformation of their chemical composition take place in lakes and reservoirs. As a result of it some complicated structures, which consist of water masses of various genesis, form there. It was shown by calculations that it is an especially multicomponent one in man-made lakes with the residence time of approximately 0.5 yr. The structures of the continental links of the Global Hydrologic Cycle are compared. It is shown that the greater is the capture of advective moisture by the continent the more intensive is the continental water cycle and the higher is the river flow from the continent. Simultaneously these processes effect the mean chemical composition of water and the quantity of dissolved matter, flowing from each continent. It is supposed that the growing water consumption will cause different trends of water resources increase for different continents.     

Crustal thickness map of Brazil: Data compilation and main features

We present a crustal thickness map of Brazil and adjacent areas based on a compilation of data published in the literature as well as new measurements. We used crustal thicknesses mainly derived from seismic datasets such as deep seismic refraction experiments, receiver function analyses, and surface-wave dispersion velocities. Crustal thicknesses derived from modelling gravity anomalies commonly depend on assumptions, such as constant density contrast across the Moho interface, which are not always easily verifiable and were considered only along the continental shelf to fill large gaps in the seismic data. Our compilation shows that the crust in the stable continental area onshore has an average thickness of 39 ± 5 km (1-σ deviation) and that no clear difference can be observed between low altitude, intracratonic sedimentary basins, NeoProterozoic foldbelts (except for the Borborema Province), and cratonic areas. The thinnest crust is found in the Borborema Province of NE Brazil (30–35 km) and along a narrow belt within Tocantins Province (∼35 km), roughly parallel to the Eastern border of the Amazon craton, while the thickest crust is found in the Amazon and São Francisco cratons (41 ± 4 km), and the Paraná Basin (42 ± 4 km). Both the Ponta Grossa and the Rio Grande Arches are areas of thinned crust, and the western border of the Brazilian platform, near the sub-Andean region, seems to be characterized by a crustal thickness of less than 40 km. Although sparse in data coverage, we expect the resulting crustal thickness map to be useful for future studies of isostasy, dynamic topography, and crustal evolution of the country.     

The key role of global solid‐Earth processes in preconditioning Greenland's glaciation since the Pliocene

After >500 Ma of absence, major Northern Hemisphere glaciations appeared during the Plio‐Pleistocene, with Greenland leading other northern areas. Here, we propose that three major solid‐Earth processes underpinned build‐up of the Greenland ice‐sheet. First, a mantle‐plume pulse, responsible for the North Atlantic Igneous Province at ~60 Ma, regionally thinned the lithosphere. Younger plume pulses led to uplift, which accelerated at ~5 Ma, lifting the parts of the East Greenland margin closest to Iceland to elevations of more than 3 km above sea level. Second, plate‐tectonic reconstruction shows a ~6° northward component of Greenland motion relative to the mantle since ~60 Ma. Third, a concurrent northward rotation of the entire mantle and crust towards the pole, dubbed True Polar Wander (TPW), contributed an additional ~12° change in latitude. These global geodynamic processes preconditioned Greenland to sustain long‐term glaciation, emphasizing the role of solid‐Earth processes in driving long‐term global climatic transitions.     

Crustal zircons and mantle sulfides: Archean to Triassic events in the lithosphere beneath south-eastern Sicily

Three types of zircon coexist in an unusual lower crustal xenolith from the Valle Guffari diatreme (Hyblean Plateau, Sicily): igneous Type 1 (near-euhedral, weakly zoned; Ce/Ce > 1); partially recrystallised Type 2 (ovoid, structureless; weak Ce anomaly); hydrothermal Type 3 (sugary, spongy-textured, probably related to F-rich aqueous fluids). U–Pb dating by LAM-ICPMS, supported by in situ Hf-isotope analysis, suggests that both Type 1 and Type 2 zircons were originally Archean (ca 2.7 Ga), though many of these grains have experienced severe Pb loss. The U–Pb ages of the hydrothermal zircons cluster around 246 Ma, interpreted as the timing of the hydrothermal event. Their ε_Hf (+ 8.5 to − 1.2) indicates the mixing of old crustal components and material from a juvenile source.

In situ Os-isotope analyses of sulfides hosted in peridotite xenoliths from Valle Guffari show Paleoproterozoic–Archean T_RD minimum ages, corresponding to the age of the oldest zircon grains in the crustal xenolith. Other peaks of T_RD ages suggest that multiple metasomatic events have affected the lithospheric mantle.

These observations suggest that the lower crust and the upper part of the lithospheric mantle beneath the Hyblean Plateau represent the northernmost portion of the African Plate. These two units have coexisted since at least late Archean time, and have remained linked through several episodes of crustal modification, including the Permo-Triassic hydrothermal event, which was probably related to the onset of rifting in the Ionian Basin.     

Nature of the diffusion, generation, and drift of the geomagnetic dipole from 1900 to 2010

It will be shown below that the main axial dipole component suffers magnetic diffusion only, so it tends to be diminished by a factor of e per 2110 years. This time interval allows us to specify the value of the core’s conductivity and estimate roughly the configuration for the current source of the axial dipole in the core and the minimal time before the upcoming inversion (excursion). The dipole component perpendicular to the axis of revolution is less than the main axial component by an order of magnitude, and has generating, drift, and diffusion components as well. This generating component is characterized by two intervals of growth by a factor of e, 98, and 540 years, which allow us to estimate the a-effect. The drifting component consists of differently directed waves enveloping the globe for 46 000 years, which leads to estimation of the w-effect. The nature of inversion dynamics is also estimated.     

Soilscape and land‐use evolution related to drift sand movements since the bronze age in Eastern Jutland,Denmark

Quarry faces several kilometers long in the Glesborg area in Denmark show that Bronze Age farmers used a sustainable land‐use system. Despite nutrient‐poor soils, the Glesborg area was under a rotation system in which cropland alternated with grassland. Soil fertility was improved by the addition of household waste and probably also by locally obtained inorganic fertilizer. The soil surface was very stable, and local drift sand movement was limited. Toward the end of the Bronze Age, the landscape changed dramatically with the arrival of overwhelming amounts of drift sand, and farmsteads were abandoned. Subsequent land use on these poor fine sandy soils was no longer capable of maintaining a stable soil surface, and frequent erosion/sedimentation events of more local importance took place. The post‐Bronze Age landscape may have been mainly a shifting mosaic of heathland with some temporary arable fields and deflation/accumulation areas. This landscape persisted up to about 200 years ago, when afforestation programs started. © 2007 Wiley Periodicals, Inc.     

Seeking a Holocene drift ice proxy: non‐clay mineral variations from the SW to N‐central Iceland shelf: trends,regime shifts,and periodicities

Quantitative X‐ray diffraction analysis of the <2 mm sediment fraction was carried out on 1257 samples (from the seafloor and 16 cores) from the Iceland shelf west of 18° W. All but one core (B997‐347PC) were from transects along troughs on the NW to N‐central shelf, an area that in modern and historic times has been affected by drift ice. The paper focuses on the non‐clay mineralogy of the sediments (excluding calcite and volcanic glass). Quartz and potassium feldspars occupy similar positions in an R‐mode principal component analysis, and oligoclase feldspar tracks quartz; these minerals are used as a proxy for ice‐rafted detritus (IRD). Accordingly, the sum of these largely foreign minerals (Q&K) (to Icelandic bedrock) is used as a proxy for drift ice. A stacked, equi‐spaced 100 a record is developed which shows both low‐frequency trends and higher‐frequency events. The detrended stacked record compares well with the flux of quartz (mg cm^?2 a^?1) at MD99‐2269 off N Iceland. The multi‐taper method indicated that there are three significant frequencies at the 95% confidence level with periods of ca. 2500, 445 and 304 a. Regime shift analysis pinpoints intervals when there was a statistically significant shift in the average Q&K weight %, and identifies four IRD‐rich events separated by intervals with lower inputs. There is some association between peaks of IRD input, less dense surface waters (from δ¹⁸O data on planktonic foraminifera) and intervals of moraine building. Copyright © 2009 John Wiley & Sons, Ltd.     

熔积岩及其对中国几处关键构造事件的限定

熔积岩是岩浆物质(岩浆侵入体、熔岩流、热火山碎屑流)与未固结的湿冷沉积物发生同生混合的产物,以浆源(岩浆凝固裂解的浆源碎屑)-沉积物源(宿主沉积物)的二元组分为典型特征。熔积岩独特的熔积结构与其成因机制密切相关,也是鉴定熔积岩的主要依据。根据浆源碎屑的形态,熔积岩主要分为块状(多数浆屑呈多边形状)和流态(多数浆屑具曲面外形)两种类型,并分别对应着不同的熔积作用机制。块状熔积岩主要产生于淬冷、自碎裂、水汽岩浆爆发、岩浆内溢气爆发等机制,而流态熔积岩的形成则得益于岩浆表面稳定蒸汽膜的维持使岩浆碎屑缓慢凝固、免于骤冷。沉积物的流体化是熔积作用得以发生的必要条件,流体化的沉积物为岩浆的驱入腾挪空间并形成对流,因此,固结成岩的沉积岩无法与岩浆物质相互熔积。熔积岩具有重要的地质意义:精确限定岩浆活动与沉积作用的同生等时性,以及利用宿主沉积物的沉积环境精确限定火山活动时的地质背景。近年来我们在国内多处发现典型的熔积岩,并应用于中国关键大地构造问题的研究中。在中亚造山带西准噶尔晚古生代火山-沉积地层中发现熔积岩,据此限定了达拉布特—白碱滩蛇绿岩带两侧上古生界的连续性,推断达拉布特—白碱滩蛇绿岩带不是典型的板块俯冲边界的缝合带,西准噶尔晚古生代是渐浅充填的残余洋盆系统。我们还首次在大火成岩省中识别出熔积岩,如在峨眉山大火成岩省中央带下部火山碎屑层序中发现以灰岩和以橙玄玻璃角砾凝灰岩为宿主沉积的熔积岩,表明中央带早期处于海相环境,从而判定火山喷发之前不具有因地幔柱上涌而诱发的大规模地表抬升效应。在塔里木大火成岩省印干剖面中发现的熔积岩以灰质宿主,证明喷发早期并非完全为陆上环境,塔里木大火成岩省喷发期间(二叠纪早期)海进和海退频繁交替。     

Crustal character and thickness over the Dangerous Grounds and beneath the Northwest Borneo Trough

New deep reflection seismic, bathymetry, gravity and magnetic data have been acquired in a marine geophysical survey of the southern South China Sea, including the Dangerous Grounds, Northwest Borneo Trough and the Central Luconia Platform. The seismic and bathymetry data map the topography of shallow density interfaces, allowing the application of gravity modeling to delineate the thickness and composition of the deeper crustal layers. Many of the strongest gravity anomalies across the area are accounted for by the basement topography mapped in the seismic data, with substantial basement relief associated with major rift development. The total crustal thickness is however quite constant, with variations only between 25 and 30 km across the Central Luconia Platform and Dangerous Grounds. The Northwest Borneo Trough is underlain by thinned crust (25–20 km total crustal thickness) consistent with the substantial water depths. There is no evidence of any crustal suture associated with the trough, nor any evidence of relict oceanic crust beneath the trough. The crustal thinning also does not extend along the complete length of the trough, with crustal thicknesses of 25 km and more modeled on the most easterly lines to cross the trough. Modeled magnetic field variations are also consistent with the study area being underlain by continental crust, with the magnetic field variations well explained by irregular magnetisations consistent with inhomogeneous continental crust, terminating at the basement unconformity as mapped from the seismic data.     

白垩纪中期异常地质事件与全球变化

白垩纪中期(125~90 Ma)是地质历史中一个极端温室时期,集中出现一系列异常事件。异常事件是地球系统内各圈层相互耦合的产物,事件相互之间不是孤立的,单个事件引起的全球变化对其他事件起着明显的正/负反馈机制作用。文中基于对白垩纪中期异常事件的深入解剖和分析,包括大规模海底火山事件、大洋缺氧事件、生物异常更替与绝灭、白垩纪超静磁带、大洋红层出现等,在探讨白垩纪中期各个事件特征基础上,重点阐述异常事件所引起的全球变化及其对海洋、气候的影响;提出异常事件之间的相互关联与反馈机制。研究发现,大规模海底火山作用是引起白垩纪中期异常海洋和气候的最根本原因,直接促进大洋缺氧事件、生物绝灭与更替、沉积记录的转变等事件的发生。     

华南东南部地壳厚度、属性及其与成矿的关系:基于地震接收函数的约束

华南大陆主要由扬子与华夏两大古陆块拼合而成,复杂的构造演化过程使得该区不仅具有多样的几何结构与变形特征,也发育成为中国南部重要的多金属成矿区域,其中包括长江中下游、钦州—杭州、武夷山、南岭、等多个重要成矿带。针对华南东南部及位于该区内的长江中下游、武夷山和南岭成矿带的深部结构与成矿背景,本文利用国家地震台网在该区固定地震台站的远震事件记录信息,通过对各个台站的远震接收函数开展H-κ扫描研究,获得了各台站下方的地壳厚度和波速比。对地壳厚度和波速比的相关性与地壳流变学构造模式进行了对比分析,研究结果显示,华南东南部的Moho面起伏整体较为平缓,自东向西逐渐增厚,波速比分布与成矿带和构造格局有明显相关性,在扬子与华夏块体之间的华南陆内复合造山区呈现明显的低波速比特征;结合该区已有的地质构造等研究分析,认为华南地区地壳减薄与燕山期的强烈岩浆活动和成矿过程密切相关,太平洋板块俯冲以及岩石圈和下地壳拆沉所造成的上地幔热物质扰动上涌或是该区矿产资源集中爆发的驱动力源;本研究所得地壳厚度与波速比分布特征与重力学多尺度边缘检测所刻画的构造界限一致性较好,支持其对扬子与华夏块体南界的划分方案。     

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U     

我国油气资源潜力、分布及重点勘探领域

我国地质结构具有3大板块、3大构造域多旋回构造演化特征,造就多种类型叠合沉积盆地,构成克拉通+前陆、断陷+坳陷、前陆+坳陷3种主要类型。大型叠合盆地是油气资源分布与勘探开发主体。我国常规与非常规油气资源十分丰富,常规石油地质资源量1 075×10⁸ t,常规天然气地质资源量83×10¹² m³;致密油地质资源量134×10⁸ t,致密砂岩气地质资源量21×10¹² m³,页岩油地质资源量335×10⁸ t,页岩气地质资源量56×10¹² m³。陆上油气资源主要分布于渤海湾(陆上)、松辽、鄂尔多斯、塔里木、四川、准噶尔、柴达木7大盆地。海域油气资源主要分布于渤海湾(海域)、东海及南海北部的珠江口、北部湾、莺歌海、琼东南6大盆地。未来我国油气勘探应始终坚持“资源战略,稳油增气”战略,坚持“非常并进、海陆统筹”积极进取勘探思路;常规勘探领域,陆上地层-岩性、前陆、海相碳酸盐岩与潜山领域;海域为渤海海域构造与基岩潜山,深水构造与岩性;非常规油气主要是立足陆上7大含油气盆地,立足致密油气与页岩油气,强化勘探开发与技术配套。     

深反射地震数据揭示的拉萨地体北部到羌塘地体南部地壳厚度的变化

由多个地体拼接而成的青藏高原,有着世界上最厚的地壳。在高原中部,从拉萨地体到羌塘地体,穿过班公湖—怒江缝合带(BNS)的地壳厚度变化长期存在争议。本文主要论述从拉萨地体北端横跨BNS到达羌塘地体的深地震反射剖面探测的结果。探测发现了清晰的Moho反射,揭示了拉萨地体—羌塘地体Moho深度和地壳厚度的变化。探测结果表明,在BNS下方Moho深度由南至北出现了6.2 km的急剧减小,并且与BNS向北28 km处的羌塘地体南部比较,地壳厚度变浅了12.5 km。否定了前人对BNS下方Moho存在20 km显著变化的认识。     

Crustal thickness estimate at AAE (Addis-Ababa, Ethiopia) and NAI (Nairobi, Kenya) using teleseismic P-wave conversions

Long-period teleseismic P waves recorded at AAE (Addis Ababa, Ethiopia) and NAI (Nairobi, Kenya) show comparable P_s conversions on the radial component of ground motion. The timing and amplitude of the P_s conversions are modeled with synthetic seismograms to get an estimate of crustal thickness under both stations. Instrument response and effective source time function are removed from the data using a P-wave equalization procedure. The timing of the P_s conversions, relative to direct P, suggests that both stations have similar crustal thickness. Using constraints on crustal velocities determined by previous surface wave dispersion and travel-time studies, the P_s-P timing suggests a crust of 41 km thickness. This agrees reasonably well with previous crustal estimates. Tangential wave forms exhibit large amplitudes and are consistent at most backazimuths. However, these wave forms could not be explained with models containing simple planar dipping interfaces. The crustal thickness of 41 km taken in conjunction with the close proximity of the stations to the rift zone suggests that crustal thinning is localized to the rift itself.     

Evidence for a mid‐Pleistocene shift of ice‐drift pattern in the Nordic seas

Sediment proxy records from a continuous, 1.5 million year long deep‐sea sediment core from a site in the western Norwegian Sea were used to obtain new insights into the nature of palaeoceanographic change in the northern North Atlantic (Nordic seas) during the climatic shift of the Mid‐Pleistocene Revolution (MPR). Red‐green sediment colour and magnetic susceptibility records both reveal significant differences in their mean values when comparing the intervals older than 700 000 yr (700 ka) with those from the past 500 kyr. The timing and duration of these changes indicates that the MPR in the Nordic seas is characterised by a gradual transition lasting about 200 kyr. Together with further sedimentological evidence this suggests that the mid‐Pleistocene climate shift was accompanied by a general change in ice‐drift pattern. It is further proposed that prior to the onset of the major late Pleistocene glaciations in the Northern Hemisphere a significant proportion of the ice in the eastern Nordic seas originated from a southern provenance, whereas later it dominantly came from the surrounding landmasses. Copyright © 2003 John Wiley & Sons, Ltd.     

Crustal growth during back‐arc closure: Cretaceous exhumation history of Cordillera Darwin,southern Patagonia

The Cordillera Darwin metamorphic complex is unique in the Andes in exposing kyanite–staurolite schist north of the Beagle Channel in southern Patagonia. Garnet in amphibolite facies pelitic schists from Bahía Pia has patchy textures whereby some grains consist of clear, grossular‐rich garnet with fine‐grained S1 inclusion trails truncated by regions of turbid spessartine–pyrope‐rich garnet with biotite, muscovite, plagioclase and quartz inclusions. Micron‐scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress; S2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix. Pseudosection modelling in Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) is used to infer a P–T path dominated by decompression from 12 to 9 kbar at T ≈ 620 °C, coupled with garnet mode decreasing from ～5% to <1%. U–Th–Pb in situ dating of S2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72.6 ± 1.1 Ma. Contact aureoles developed adjacent to late granite intrusions include sillimanite‐bearing migmatites formed at P ≈ 6 kbar after 72 Ma. Metamorphism of southern Cordillera Darwin induced by continental underthrusting beneath the arc, related to closure of the Rocas Verdes back‐arc basin, was terminated by thrusting‐controlled exhumation, with the rocks at P ≈ 9 kbar by c. 73 Ma and 6 kbar by c. 70 Ma.