Early volcanic rocks of réunion and their tectonic significance

A great sequence of hydrothermally altered and tectonically disturbed lavas and breccias is exposed in the erosional cirques of Piton des Neiges volcano, Réunion. These rocks are cut by intense swarms of minor intrusives and are overlain by a carapace of younger lavas which date back at least 2 m.y. The older lavas and breccias are believed to have been formed by submarine eruption of basalt magma during the pre-emergent growth stages of the volcano. It is suggested that their subsequent elevation to 2000 m. above sealevel is due to the thrusting of oceanic crust from the north-east, associated with movements along NE-SW fractures in the western Indian Ocean floor.     

青藏高原拉萨地块中部上三叠统多布日组沉积岩古地磁研究及其构造意义

拉萨地块晚古生代至早新生代期间的演化历史对理解青藏高原的形成和演化至关重要.本文报道了青藏高原中部申扎地区上三叠统多布日组沉积岩的古地磁学、岩石磁学和岩相学分析结果.岩石磁学结果显示红层、灰岩和灰绿色砂岩分别以赤铁矿,赤铁矿与磁铁矿,以及磁铁矿为主要载磁矿物.岩相学结果表明红层、灰岩和灰绿色砂岩中的磁性矿物主要为自生成因.对60个采点（602块样品）进行系统热退磁实验,结果揭示出多布日组沉积岩的特征剩磁方向在地理坐标系下为D_g=13.9°,I_g=38.9°（α_95g=0.9°）,在地层坐标系下为D_s=16.7°,I_s=8.4°（α_95s=1.5°）.进一步对其特征剩磁方向进行逐步褶皱展平检验,结果显示精度参数k的极大值在9.9%展平状态下最大,对应的特征剩磁方向为D=14.4°,I=35.9°（α₉₅=0.9°）,古地磁极为72.9°N,216.2°E（dp=0.6°,dm=1°）.这一古地磁极与拉萨地块54~47 Ma的古地磁极接近,表明其重磁化时间可能发生在54~47 Ma期间.结合前人获得的拉萨地块新生代早期古地磁数据以及同期的欧亚大陆古纬度数据,我们推断拉萨地块在56~44 Ma时相对欧亚大陆发生了1676±577 km的南北向构造缩短.

     

K-Ar ages of intrusive rocks in the Oban-Obudu massif and their significance for the tectonic and plutonic history of southeastern Nigeria

Abstract K-Ar ages have been obtained for mineral separates: plagioclases from two dolerites and biotites from one granite and four granodiorites in the Oban-Obudu massif of southeastern Nigeria. Dolerites in the Oban area give K-Ar plagioclase ages of 204.0 ± 9.9 Ma and 219.9 ± 4.7 Ma. The granite of the Obudu area yields a K-Ar biotite age of 507.6 ± 10.1 Ma whilst the granodiorites in the Oban area yield K-Ar biotite ages ranging from 474.6 ± 9.4 Ma to 511.8 ± 10.0 Ma. The dolerites are related to the tholeiitic basaltic magmatism at the early opening of the central Atlantic, and are compatible in age with the formation of the Ring Complexes and the rifting of the Benue Trough of Nigeria. The granites and granodiorites belong to the syntectonic Older Granite series in the Pan-African orogeny. The emplacement timing deduced from the reported Pb-Pb zircon age of 617 ± 2 Ma and the newly obtained biotite ages suggest that these older granites in southeastern Nigeria had a prolonged cooling history of ∼110 Ma.     

Mean densities of Pre-Devonian sedimentary rocks in Poland and their depth dependence

Summary Since 1945 in Poland density measurements have been completed on drilling samples to obtain data for reduction and interpretation of geophysical results. In this paper the developments in density measurements techniques and data processing in Poland are reviewed. Mean density maps have been constructed for the following Pre-Devonian divisions: Volhynian, Valdai and Subholmian Cambrian, Holmian and Protoolenus Cambrian, Middle Cambrian, Lower Ordovician, Middle Ordovician, Upper Ordovician, Lower Silurian, Middle Silurian, Upper Silurian. On the basis of the same data the dependence of mean densities on the present depth of all the above divisions has been examined. This dependence has been approximated by regression equations of the following type: (h)= ah + b , (h) = ah² + bh + c, (h)= alnh + b((h): mean density,h: present depth). In addition to the coefficients of these equations the correlation coefficient, regression standard error, and confidence intervals have been calculated. Analysis of the results obtained revealed a dominant effect of compaction on the mean density values. Hypotheses have been presented concerning other geological factors responsible for the variation of these values. Regression curves constructed for the individual stratigraphic horizons served to estimate the maximum earlier depth of the Palaeozoic rocks of the Holy Cross Mountains and their vertical displacement. Similar studies have been completed for two adjacent boreholes situated in the Bug River depression. Their results indicate that mean density values can readily serve to estimate the amount of vertical tectonic displacement.     

The sylhet traps,their tectonic history,and their bearing on problems of indian flood basalt provinces

Recent studies of the Sylhet Traps (? Jurassic) and the overlying Cretaceous-Tertiary sedimentary cover in the southern part of the Khasi Hills, Shillong Plateau in Assam have led to a reconstruction of the tectonic history of the area since Jurassic times; a clear picture regarding the nature of volcanism has also emerged. The history begins with effusion of tholeiitic basalts, apparently through E-W fissures developed in the peneplaned crystalline basement. One of these fractures became a fault (the Raibah fault) along which the northern non-volcanic block moved up relative to the southern block experiencing volcanism. The fault was active during and after the volcanism till Upper Cretaceous times. The sequence of eruption was as follows: (1) tholeiitic basalts, (2) minor alkali basalts (nepheline tephrite), (3) tholeiitic basalts, (4) localised explosive effusion of minor rhyolites and acid tuffs, and (5) tholeiitic basalts. Neither feeder dykes nor volcanic vents have been noted in the Sylhet Traps. There are no agglomerates among the basic flows; the fragmental rocks are actually flow breccias. The formation of the various structures such as flow breccias, layering and flow folds in many of the basalt flows are thought to have been controlled by the angle of slope and the rate of flow. Thus, the Sylhet Trap flood basalts are characterised by quiet effusion through linear fissures. The effusion was followed by a dyke phase, intruding also along E-W fractures, expecially in the monoclinally bent southern portion; the subsequent tectonic history of the area is also characterised by relative uplift and downsinking of different basement blocks. It is concluded that in the Shillong Plateau uparching of the basement led to fracturing, effusion of basalts apparently along some zones of fissuring along which differential vertical movement of basement blocks was taking place. In the light of the foregoing conclusions, available data on the tectonics of the Rajmahal and the Deccan Traps are examined; both these flood basalt provinces have suffered broadly similar tectonic histories as the Sylhet Traps. The various features of flood basalts, viz., large extent, huge thickness, subaerial nature, a post-volcanic dyke phase are interpreted as a consequence of fusion of the Upper Mantle, development of tensional fractures eruptions apparently along fractures between adjoining basement blocks undergoing differential uplift.     

Isotopic variations in precipitation at Bangkok and their climatological significance

The stable isotopic composition of precipitation from low to mid latitudes contains information about changes of some climatic factors, such as temperature, precipitation and atmospheric circulation patterns. However, the isotopic variations in the area are very complicated because of the combined influences of these factors. Proper interpretation of the patterns of isotopic variations for palaeoclimate reconstructions in this area requires a detailed understanding of these complex stable isotope controls. The isotopic data (δ¹⁸O and δ²D) in precipitation at the International Atomic Energy Agency–World Meteorological Organization Bangkok station were collected and analysed because of the relatively long and unbroken isotopic records and the particular geographical location. The isotopic variations at Bangkok have strong seasonal patterns owing to distinct source regions in different seasons. In summer, the remote sources of water there can influence the δ¹⁸O values significantly, which is verified by the simple Rayleigh model. In winter, the mixing of isotopically distinct air masses with different origins, i.e. the continental and oceanic air masses, accounts for the isotopic variations. In the transition periods of the Asia–Australia monsoon, namely in May and October, the local vapour contribution may play a role in the isotopic ratios. On the interannual time‐scale, the influences of El Niño–southern oscillation (ENSO) and the Indian Ocean dipole (IOD) on isotopic composition are examined. The indications are that both the ENSO and IOD indices have a significant correlation with the δ¹⁸O ratios, and that the δ¹⁸O ratio in summer rains is significantly more enriched (depleted) during the warm (cold) phase of ENSO/IOD events. All the results suggest that it is useful for us in understanding the water cycling process and may be helpful in palaeoclimate reconstruction in this monsoon region. Copyright © 2006 John Wiley & Sons, Ltd.     

Geochemistry of the ophiolite and oceanic island basalt in the Kangxian-Pipasi-Nanping tectonic melange zone, South Qinling and their tectonic significance

Detailed studies indicate that Kangxian-Pipasi-Nanping tectonic zone is a complicated melange zone which includes many tectonic slabs of different origins. Ophiolite (MORB-type basalt), oceanic island tholeiite and alkaline basalt have been identified. Moreover, this tectonic melange zone is eastward connected with the Mianlue suture zone. The deformation characteristics, consisting components and volcanic rock geochemical features for the Kangxian-Pipasi-Nanping tectonic melange zone are much similar to those of the Mianlue suture zone and Deerni ophiolite. Therefore, the Kangxian-Pipasi-Nanping tectonic melange zone should be the westward extension part of the Mianlue suture zone. It indicates that the Mianlue suture zone had extended to the Nanping area.     

藏北东巧地区地幔岩流变学特征及构造演化

本文首次从岩石组合、橄榄石与辉石组构、宏观流变、微观流变及波速和各向异性特征等方面详细阐述了东巧地区地幔岩塑性流变学特征,从微观与宏观角度进一步探讨了班公错一怒江结合带东巧地幔岩形成的地质环境及构造演化过程,认为东巧地幔岩具有超高压成因特点,形成于小洋盆环境,在印度板块向欧亚板块推移过程中就位,最终在喜马拉雅整体隆升阶段完成了近地表的构造调整。     

On the nature of cosmic dust in sedimentary rocks

In the paper, the chemical composition and origin of cosmic dust, as well as its distribution in the geological past, are discussed. Special attention is given to the problem of the difference in cosmic dust from volcanic dust on the basis of indicators such as cosmogenic helium and iridium.     

The tectonic and geomagnetic significance of paleomagnetic observations from volcanic rocks from central Afar, Africa

Deformation throughout Afar over the past 2 myr has been characterized by widespread and intense crustal fragmentation that results from inhomogeneous extension across the region. In eastern Afar, this situation has evolved to localized extension associated with the westward propagation of the Gulf of Aden/Gulf of Tadjurah seafloor spreading system into the Asal–Ghoubbet Rift. During the gradual process of rift propagation and localization, crustal blocks in eastern Afar sustained clockwise rotations of 11°. To better understand the processes of rift propagation and localization and how they affect the rest of Afar, we have collected and analyzed over 400 oriented paleomagnetic samples from 67 lava flows from central and southern Afar. Unlike eastern Afar, the mean paleomagnetic direction from central Afar indicates that vertical-axis rotations are statistically insignificant (3.6°±4.4°), though small clockwise rotations (<8°) are permitted. Thus, propagation and localization in central Afar have not had the same influence in causing crustal block rotations or, perhaps more likely, have not reached the same stage of evolution as seen in eastern Afar. In addition, several of the lava flows record intriguing geomagnetic field behavior associated with polarity transitions, excursions, or large secular variation events. Interestingly, the transitional or anomalous virtual geomagnetic poles (VGPs) tend to cluster in two nearly antipodal regions, one in the northern Pacific Ocean and the other in the southwest Indian Ocean. One lava flow has recorded both of the antipodal transitional components, with the two components residing in magnetic minerals with unblocking temperatures above and below 500°C, respectively. Reheating and partial remagnetization by the overlying flow cannot explain either of the transitional directions because both differ significantly from that of the reversely magnetized overlying flow. The high-temperature component gives a VGP in the northern Pacific, whereas the lower-temperature component gives a nearly antipodal VGP south of Cape Town, South Africa. Hence, the configuration of the geomagnetic field appears to have jumped nearly instantaneously from a northern-hemisphere transitional state to a southern-hemisphere one during this normal-to-reverse polarity transition.     

Spinel peridotite xenoliths in the West Carpathian late cenozoic alkali basalts and their tectonic significance

In the Late Cenozoic West Carpatian alkali olivine basalts spinel peridotite xenoliths have been found. Their mineral composition corresponds to those found in this type of xenoliths in alkali basalts throughout the world (Mg-rich olivine, Cr-diopside, clinopyroxene, spinel). For the studied West Carpathian alkali olivine basalts kaersutite amphibole and clinopyroxene magacrysts are characteristic. The presence of the xenoliths under consideration in the boundary zone between Tatra and Pannonian blocks is the consequence of tectonic conditions (thickness of the Crust) in the area mentioned as distinct from the other West Carpathian areas of alkali olivine basalt occurrences.     

On the theory of depositional remanent magnetization in sedimentary rocks

The effects of various factors such as thermal agitation, coagulation, anisotropy of susceptibility, and shape irregularity on the alignment of magnetic carrier grains during the process of acquisition of depositional remanent magnetization in sedimentary rocks is discussed.     

Jurassic sedimentary features and tectonic settings of southeastern China

Two types of the Jurassic basins are distinguished in SE China based on their geodynamic features: the Late Triassic-Early Jurassic post-orogenic basins and the Middle Jurassic intra-continental extensional basins. The Lower Jurassic sequence shows a change from coarse- to fine-grained accumulation, suggesting a gradually deepening depositional environment from river to shore-lake and to deep-water lake. In contrast, the Middle Jurassic accumulation was changed from claystone to conglomerate along the coastal provinces in SE China, indicative of an initial crustal uplift. The Wuyi Mountains have been a paleogeograghic separating zone since the Middle Jurassic. The Late Jurassic strata are absent in most areas of SE China. A large-scale bimodal intra-continental rift-type volcanism occurred during the Middle Jurassic along a 40–60 km wide and 200 km long area in western Fujian and southern Jiangxi provinces, which is most likely the strongest volcanism in SE China since the Cambrian. The SHRIMP zircon U-Pb analyses on the rhyolite from the Dongkeng basin in the southern Jiangxi area yield a concord U-Pb age of 160±0.5 Ma, providing an upper age limit for the bimodal volcanic eruption. The analyses of the basin features indicate a change of the depositional environment during the interval from Middle Triassic to Late Triassic from a shallow-sea to an intra-continent in SE China in response to the strong collision between the Yangtze and North China Blocks. Sedimentary structures record a southward direction of Early Jurassic paleo-currents, reflecting that their source areas were to the north side. We propose that the Wuyi region was uplifted as early as Middle Jurassic, followed by a wide E-W-trending extended depression and bimodal volcanism in the western foot of the Wuyi Mountains. Presumably the uplift of the Wuyi domain changed the Middle Jurassic paleogeographic outline and formed the transformational tectonic regime from compression to extension as a tectonic response to the Pacific plate subduction.     

The mineral chemistry of pyroxenite xenoliths in the volcanic rocks of Hoh Xil and their significance

The pyroxenite xenoliths in the volcanic rocks of Hoh Xil consist of clinopyroxenes and orthopyroxenes. The mineral composition of these pyroxenes is similar to that of mantle xenoliths including peridotite and pyroxenite from China and abroad, and different from that of granulites. The pyroxenes formed at 1101–1400°C (averaging 1250°C) and under 30–60 kb (averaging 46 kb). We deduced that the magma was derived from the mantle at a depth of more than 150 km, which fits in with the geophysical conclusion that the low-velocity layer existed in the mantle under 150 km.

     

The geochemical characteristics,geochronology and tectonic significance of the Carboniferous volcanic rocks of the Santanghu area in northeastern Xinjiang,China

The Santanghu area is located on the northeastern margin of the Junggar Basin,northern Xinjiang,Northwest China.The Carboniferous volcanic rocks in this area are widely distributed in Kaokesaiergaishan,Santanghu,Daheishan and Naomaohu districts,which are located to the north of the Kalameili Fault.These rocks,sourced from a cognate magma,consist of basic,intermediate,and acidic lavas,and pyroclastic rock.The basic volcanic rocks are enriched with large-ion lithophile elements(LILE),but are relatively depleted in high field strength elements(HFSE),and have an obvious negative Nb-Ta-Ti anomaly.They were most probably derived from a depleted mantle source,and during their ascent,these magmas were not contaminated by the crustal material as they underwent magma crystallization differentiation.Based on the Carboniferous volcanic assemblage and geochemical data,it is apparent that the early Carboniferous volcanism occurred in a subduction-related tectonic setting.New LA-ICP-MS zircon U-Pb analyses constrain the age of the andesite within the volcanic rocks as the early Carboniferous(328.9-331.3 Ma).Combined with the regional geological record,comprehensive analysis of the isotope geochronological data indicates that the subduction of the Junggar Ocean predates the early Carboniferous,and that the Santanghu island arc magmatism was induced by the subduction of the Junggar Ocean in the Carboniferous.     

Pore structure and physical properties of porous sedimentary rocks

Summary Authors' work of recent times on sedimentary petrophysics is reviewed. A report is given on theoretical work about petrophysical relations, based on a versatile model of the pore structure, using statistics and topology; on means for directly measuring pore structure by computerized image analysis of sections; and on an experimental study about an electrical interface conductivity of porous, water saturated rocks and its relations to structure. Some outlook on a subject of very recent interest: recovery of geothermal heat from porous formations, is added.     

Relations between static and dynamic moduli of sedimentary rocks

Static moduli of rocks are usually different from the corresponding dynamic moduli. The ratio between them is generally complex and depends on several conditions, including stress state and stress history. Different drainage conditions, dispersion (often associated with pore fluid effects), heterogeneities and strain amplitude, are all potential reasons for this discrepancy. Moreover, comparison of static and dynamic moduli is often hampered and maybe mistaken due to insufficient characterization of anisotropy. This paper gives a review of the various mechanisms causing differences between static and dynamic moduli. By careful arrangements of test conditions, it is possible to isolate the mechanisms so that they can be studied separately. Non-elastic deformation induced by the large static strain amplitudes is particularly challenging, however a linear relationship between non-elastic compliance and stress makes it possible to eliminate also this effect by extrapolation to zero strain amplitude. To a large extent, each mechanism can be expressed mathematically with reasonable precision, thus quantitative relations between the moduli can be established. This provides useful tools for analyses and prediction of rock behaviour. For instance, such relations may be used to predict static stiffness and even strength based on dynamic measurements. This is particularly useful in field situations where only dynamic data are available. Further, by utilizing the possibility for extrapolation of static measurements to zero strain amplitude, dispersion in the range from seismic to ultrasonic frequencies may be studied by a combination of static and dynamic measurements.     

Absence of extraterrestrial He in Permian-Triassic age sedimentary rocks

Helium concentration and isotopic composition were measured in a suite of samples across the Permian-Triassic boundary at Opal Creek, Canada, to determine whether high extraterrestrial helium concentrations are associated with a possible extinction-inducing impact event at this time. No extraterrestrial ³He was detected, implying that neither fullerene-hosted nor IDP-hosted He is present at or near the boundary. This observation is consistent with similar studies of some Permian-Triassic sections, but contrasts sharply with reports of both fullerene- and IDP-hosted extraterrestrial ³He at other sections.Step-heat experiments indicate rapid diffusion of extraterrestrial helium from sediments heated to temperatures above ∼ 70 °C. Given the likelihood of burial and associated heating in Permian-Triassic age rocks, the initially unexpected absence of IDP-hosted ³He likely indicates thermally induced diffusive loss. Indeed one of the key sections (Graphite Peak, Antarctica) from which extraterrestrial ³He has been reported at and near the Permian-Triassic boundary has been sufficiently heated that the reported preservation of extraterrestrial helium, in both IDPs and fullerenes, is inexplicable. Recent contamination provides a plausible explanation for extraterrestrial ³He in these samples.While no extraterrestrial ³He was detected at Opal Creek, there is a sharp increase in nucleogenic ³He very close to or at the Permian-Triassic boundary. This presumably arises from the major lithologic change at this time, from cherts in the Permian to shales and siltstones in the Triassic. Increased nucleogenic ³He is associated with increases in both lithium and organic carbon content into the Triassic. Either the production rate or the retention of this ³He is higher in the shales and siltstones than in the cherts. Care must be taken to eliminate such artifacts before interpreting changes in ³He concentration in terms of fluctuations in the delivery of ³He from space.     

Controls on the geotechnical response of sedimentary rocks to weathering

Weathering reduces the strength of rocks and so is a key control on the stability of rock slopes. Recent research suggests that the geotechnical response of rocks to weathering varies with ambient stress conditions resulting from overburden loading and/or stress concentrations driven by near-surface topography. In addition, the stress history experienced by the rock can influence the degree to which current weathering processes cause rock breakdown. To address the combined effect of these potential controls, we conducted a set of weathering experiments on two sedimentary lithologies in laboratory and field conditions. We firstly defined the baseline geotechnical behaviour of each lithology, characterising surface hardness and stress–strain behaviour in unconfined compression. Weathering significantly reduced intact rock strength, but this was not evident in measurements of surface hardness. The ambient compressive stress applied to samples throughout the experiments did not cause any observable differences in the geotechnical behaviour of the samples. We created a stress history effect in sub-sets of samples by generating a population of microcracks that could be exploited by weathering processes. We also geometrically modified groups of samples to cause near-surface stress concentrations that may allow greater weathering efficacy. However, even these pronounced sample modifications resulted in insignificant changes in geotechnical behaviour when compared to unmodified samples. The observed reduction in rock strength changed the nature of failure of the samples, which developed post-peak strength and underwent multiple stages of brittle failure. Although weakened, these samples could sustain greater stress and strain following exceedance of peak strength. On this basis, the multi-stage failure style exhibited by weaker weathered rock may permit smaller-magnitude, higher-frequency events to trigger fracture through intact rock bridges as well as influencing the characteristics of pre-failure deformation. These findings are consistent with patterns of behaviour observed in field monitoring results. © 2019 John Wiley & Sons, Ltd.