The role of E–W basement faults in the Mesozoic geodynamic evolution of the Gafsa and Chotts basins,south-central Tunisia

The Gafsa and Chotts intracratonic basins in south-central Tunisia are transitional zones between the Atlasic domain to the north and the Saharan platform to the south. The principal aim of this paper is to unravel the geodynamic evolution of these basins following an integrated approach including seismic, well log and gravity data. These data are used to highlight the tectonic control on the deposition of Jurassic and Lower Cretaceous series and to discuss the role of the main faults that controlled the basin architecture and Cretaceous–Tertiary inversion. The horizontal gravity gradient map of the study area highlights the pattern of discontinuities within the two basins and reveals the presence of deep E–W basement faults. Primary attention is given to the role played by the E–W faults system and that of the NW–SE Gafsa fault which was previously considered active since the Jurassic. Facies and thickness analyses based on new seismic interpretation and well data suggest that the E–W-oriented faults controlled the subsidence distribution especially during the Jurassic. The NW–SE faults seem to be key structures that controlled the basins paleogeography during Late Cretaceous–Cenozoic time. The upper Triassic evaporite bodies, which locally outline the main NW–SE Gafsa fault, are regarded as intrusive salt bodies rather than early diapiric extrusions as previously interpreted since they are rare and occurred only along main strike-slip faults. In addition, seismic lines show that Triassic rocks are deep and do not exhibit true diapiric features.     

The age of Spitsbergen basement consolidation: U–Pb dating of detrital zircons from the Upper Precambrian and Lower Carboniferous clastic rocks of the northwestern part of Nordenskiöld Land

The age distribution of detrital zircons from Upper Riphean and Early Carboniferous sandstones of the northwest part of Nordenskiöld Land (Spitsbergen Island) was studied. The results obtained supplement the previously known data on the rocks of Precambrian and Paleozoic masses of West Spitsbergen. Integrated consideration of these data showed that the time interval from the Middle Riphean to the Early Carboniferous inclusive might be subdivided into at least five stages characterized by quite different provinces supplying detritus to sedimentary basins. (The relics of these latter are represented in the present structure of western Svalbard.) The most ancient distinguished event of alteration of the sources of sedimentary matter (at the boundary of the Middle and Late Riphean) corresponds to the consolidation time of the ancient basement of Svalbard.     

Early Cambrian palaeobiogeography of the Zhenba–Fangxian Block (South China): Independent terrane or part of the Yangtze Platform?

Early Cambrian small skeletal fossils (SSFs) are studied and revised from the Zhenba–Fangxian Block of the transitional zone between the Yangtze Block and the South Qinling Terrane. The study reveals a diverse fauna with 47 species of various biological affinities, including the new species Gapparodus gapparites sp. nov. The SSFs are assigned to the newly defined Cambroclavus fangxianensis–Rhombocorniculum cancellatum Assemblage Zone. Based on the investigated SSF fauna from Zhenba County, Southeast Shaanxi of China and published data, a palaeobiogeographic study is carried out for the Cambrian Stage 3 (equivalent to the Atdabanian–Botoman of Siberia). A hierarchical Pearson similarity cluster analysis of 295 species from 32 regions of the world indicates a distinct palaeobiogeographic pattern with seven faunal provinces. The result is mostly consistent with existing palaeogeographic reconstructions for the early Cambrian. However, it is also shown that the SSF assemblages of the Zhenba–Fangxian Block have low similarity with those of the Yangtze Block. Instead, they share high similarity with those from Armorica, Tarim and the Karatau–Naryn terranes (South Kazakhstan/North Kyrgyzstan). The Yangtze Block has a unique SSF assemblage dissimilar to most of other regions. The Terreneuvian–Cambrian Stage 3 sedimentary sequence of the Zhenba–Fangxian Block is more consistent with that of the South Qinling Terrane. Besides, sedimentary Ediacaran manganese ore deposits and Cambrian barite/witherite deposits have unique distribution pattern on the Zhenba–Fangxian Block. Derived from the profound dissimilarities in faunal composition, sedimentary sequence and distribution of sedimentary ore deposits, we hypothesize that during the Neoproterozoic–Cambrian transition, the Zhenba–Fangxian Block might have been an independent terrane and more distant from the Yangtze Block. The palaeobiogeographic analysis of SSFs also indicates a closer alliance between Avalonia and Siberia. It corroborates the palaeogeographic reconstruction of North China at the margin of Gondwana, in the vicinity of Australia, Antarctica, and Armorica.     

U–Pb zircon chronology and petrogenesis of Carboniferous plutons in the northern part of the Eastern Pontides,NE Turkey: Constraints for Paleozoic magmatism and geodynamic evolution

Numerous intrusive rocks of varying ages and compositions exist in the Paleozoic to Tertiary periods in the Eastern Pontides. Carboniferous intrusive rocks are commonly observed in the southern part of the Eastern Pontides. The nature of the rocks in the northern part of the region has not been determined because of Upper Cretaceous and Tertiary volcano-sedimentary units. Whole-rock geochemical, isotopic and geochronological data obtained from five different mapped granitoid bodies located in the northern part of the Eastern Pontides allow for the proper reconstruction of Carboniferous magmatism and the geodynamic evolution of the region.According to laser ablation ICP-MS U–Pb zircon dating, the Özdil, Soğuksu, Seslikaya, Kızılağaç and Şahmetlik plutons have similar ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U concordia ages of 340.7 ± 1.8 Ma and 323.1 ± 1.5 Ma, 348.4 ± 1.6 Ma, 335.4 ± 1.4 Ma, 337.2 ± 0.6 Ma and 334.5 ± 1.4 Ma, respectively. The aluminium saturation index (ASI) values of all of the samples from the plutons are between 1.0 and 1.32, which indicate peraluminous melt compositions. The plutons have SiO₂ contents between 59 and 79 wt.% and show low- to high-K calc-alkaline characteristics. The plutons are enriched in large-ion lithophile and light rare earth elements and are depleted in high-field strength elements. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies, with La_CN/Yb_CN = 1.9–46.8 and Eu_CN/Eu* = 0.19–1.76. The studied plutons show considerable variations in ⁸⁷Sr/⁸⁶Sr_(i) (0.70255 to 0.71006) and εNd_(i) values (− 4.8 to − 7.1), as well as Nd model ages (1.15 to 2.47 Ga). The Pb-isotopic ratios are ²⁰⁶Pb/²⁰⁴Pb = 17.11–18.60, ²⁰⁷Pb/²⁰⁴Pb = 15.58–15.64 and ²⁰⁸Pb/²⁰⁴Pb = 36.95–38.62. The crystallization temperatures of the melts range from 676 to 993 °C, as determined by zircon and apatite saturation thermometry.These data suggest that the Carboniferous granitic magmas were produced by the partial melting of meta-mafic to meta-felsic lower crustal source rocks, with minor contributions from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the northern part of the Eastern Pontides.     

From the Libyan border to the Nile – Neoproterozoic magmatism and basement evolution of southern Egypt

ABSTRACT

In southern Egypt, Neoproterozoic granitoids intrude the crust from Libya in the west to the Nile River in the east. Best exposed at Gebel Uweinat, Gebel Kamil, Bir Safsaf, Gebel El Asr and Gebel Umm Shaghir, these rocks represent the transition between the rejuvenated Archean to Paleoproterozoic crust of the east Saharan craton and the juvenile Neoproterozoic crust of the Arabian–Nubian Shield to the east. We present U-Pb zircon geochronology and whole rock geochemistry from 16 Neoproterozoic (c. 628–580 Ma) granitoids across southern Egypt which document the synchronous production of diverse compositions indicative of an intraplate or intracontinental tectonic setting. Our data better constrain the age relationships, petrogenesis, and Pan-African crustal growth associated with the amalgamation of Gondwana and contribute to understanding the crust-modifying processes that led to the development of the Saharan metacraton in the Ediacaran.     

The Neoproterozoic–Early Paleozoic tectonic evolution of the South China Block: An overview

This paper gives a brief review of what I consider as the state of the art regarding the largely accepted data and ideas concerning the Proterozoic to Early Paleozoic tectonic evolution of South China. The South China craton was built by the welding of the Yangtze and Cathaysia blocks, with a different previous history giving a different pre-Neoproterozoic basement composition, due to the Jiangnan (Jinning, Sibao) orogeny. This Jiangnan orogeny was a collisional event, induced by the consumption of an intervening oceanic domain by subduction beneath the Yangzte plate. The evolution involved a volcanic arc on the Yangtze active margin, active from ca. 980 Ma to ca. 850 Ma, the subsequent collision beginning at around 870–860 Ma and responsible for the emplacement of thrust sheets of ophiolitic mélange (dated around 1000–900 Ma) and blueschists (900–870 Ma), followed by late- to post-collisional granitic plutonism (840–800 Ma). The newly amalgamated South China craton suffered from rifting, starting around 850 Ma, marked by mafic–ultramafic magmatism until ca. 750 Ma. The Nanhua rift basin evolved with a thick sedimentation in its middle part until the Ordovician. South China was affected by the Early Paleozoic orogeny (mainly Silurian), characterized by a strong quasi-symmetrical intracontinental shortening, involving the sedimentary cover of the rift and its margins as well as the basement, leading to crustal thickening. This crustal thickening induced an important anatexis and emplacement of peraluminous granites during the Silurian. Unlike the Jiangnan orogeny, which was of collisional type, the Early Paleozoic one was a bit similar to a Pyrenean intracontinental type.Some pending problems need further research for clarification, for example: the location and timing of integration of South China within Rodinia, the triggering factor of the Early Paleozoic orogeny, the mapping of the contacts bounding the Lower Paleozoic thrust sheets responsible for the crustal thickening.     

Samovar: a thermomechanical code for modeling of geodynamic processes in the lithosphere—application to basin evolution

We present a new 2D finite difference code, Samovar, for high-resolution numerical modeling of complex geodynamic processes. Examples are collision of lithospheric plates (including mountain building and subduction) and lithosphere extension (including formation of sedimentary basins, regions of extended crust, and rift zones). The code models deformation of the lithosphere with viscoelastoplastic rheology, including erosion/sedimentation processes and formation of shear zones in areas of high stresses. It also models steady-state and transient conductive and advective thermal processes including partial melting and magma transport in the lithosphere. The thermal and mechanical parts of the code are tested for a series of physical problems with analytical solutions. We apply the code to geodynamic modeling by examining numerically the processes of lithosphere extension and basin formation. The results are directly applicable to the Basin and Range province, western USA, and demonstrate the roles of crust–mantle coupling, preexisting weakness zones, and erosion rate on the evolutionary trends of extending continental regions. Modeling of basin evolution indicates a critical role of syn-rift sedimentation on the basin depth and a governing role of Peierls deformation in cold lithospheric mantle. While the former may increase basin depth by 50%, the latter limits the depth of rift basins by preventing faulting in the subcrustal lithosphere.     

Petrogenetic evolution of the Early Miocene Ala?amda? volcano-plutonic complex, northwestern Turkey: implications for the geodynamic framework of the Aegean region

Extensional-tectonic processes have generated extensive magmatic activity that produced volcanic/plutonic rocks along an E-W-trending belt across north-western Turkey; this belt includes granites and coeval volcanic rocks of the Ala?amdağ volcano-plutonic complex. The petrogenesis of the Early Miocene Ala?amdağ granitic and volcanic rocks are here investigated by means of whole-rock Sr–Nd isotopic data along with field, petrographic and whole-rock geochemical studies. Geological and geochemical data indicate two distinct granite facies having similar mineral assemblages, their major distinguishing characteristic being the presence or absence of porphyritic texture as defined by K-feldspar megacrysts. I-type Ala?amdağ granitic stocks have monzogranitic-granodioritic compositions and contain a number of mafic microgranular enclaves of monzonitic, monzodioritic/monzogabbroic composition. Volcanic rocks occur as intrusions, domes, lava flows, dykes and volcanogenic sedimentary rocks having (first episode) andesitic and dacitic-trachyandesitic, and (second episode) dacitic, rhyolitic and trachytic-trachydacitic compositions. These granitic and volcanic rocks are metaluminous, high-K, and calc-alkaline in character. Chondrite-normalised rare earth element patterns vary only slightly such that all of the igneous rocks of the Ala?amdağ have similar REE patterns. Primitive-mantle-normalised multi-element diagrams show that these granitic and volcanic rocks are strongly enriched in LILE and LREE pattern, high (⁸⁷Sr/⁸⁶Sr)_i and low ε _Nd(t) ratios suggesting Ala?amdağ volcano-plutonic rocks to have been derived from hybrid magma that originated mixing of co-eval lower crustal-derived more felsic magma and enriched subcontinental lithospheric mantle-derived more mafic magmas during extensional processes, and the crustal material was more dominant than the mantle contribution. The Ala?amdağ volcano-plutonic complex rocks may form by retreat of the Hellenic/Aegean subduction zone, coinciding with the early stages of back-arc extension that led to extensive metamorphic core-complex formation.     

The late Mesozoic–Cenozoic tectonic evolution of the South China Sea: A petrologic perspective

This paper presents a review of available petrological, geochonological and geochemical data for late Mesozoic to Recent igneous rocks in the South China Sea (SCS) and adjacent regions and a discussion of their petrogeneses and tectonic implications. The integration of these data with available geophysical and other geologic information led to the following tectono-magmatic model for the evolution of the SCS region. The geochemical characteristics of late Mesozoic granitic rocks in the Pearl River Mouth Basin (PRMB), micro-blocks in the SCS, the offshore continental shelf and Dalat zone in southern Vietnam, and the Schwaner Mountains in West Kalimantan, Borneo indicate that these are mainly I-type granites plus a small amount of S-type granites in the PRMB. These granitoids were formed in a continental arc tectonic setting, consistent with the ideas proposed by Holloway (1982) and Taylor and Hayes, 1980, Taylor and Hayes, 1983, that there existed an Andean-type volcanic arc during later Mesozoic era in the SCS region. The geochonological and geochemical characteristics of the volcanics indicate an early period of bimodal volcanism (60–43 Ma or 32 Ma) at the northern margin of the SCS, followed by a period of relatively passive style volcanism during Cenozoic seafloor spreading (37 or 30–16 Ma) within the SCS, and post-spreading volcanism (tholeiitic series at 17–8 Ma, followed by alkali series from 8 Ma to present) in the entire SCS region. The geodynamic setting of the earlier volcanics was an extensional regime, which resulted from the collision between India and Eurasian plates since the earliest Cenozoic, and that of the post-spreading volcanics may be related to mantle plume magmatism in Hainan Island. In addition, the nascent Hainan plume may have played a significant role in the extension along the northern margin and seafloor spreading in the SCS.     

Structure of the Upper Devonian Boyd Volcanic Complex,south coast New South Wales: Implications for the Devonian‐Carboniferous evolution of the Lachlan Fold Belt

Upper Devonian continental and subaqueous sedimentary rocks and bimodal volcanic rocks of the Boyd Volcanic Complex of the south coast of New South Wales were deposited in a rapidly subsiding, 330°‐trending, transtensional basin. Structural analysis of synvolcanic and synsedimentary deformational structures indicate that basin formation is related to a 330°‐orientated subhorizontal σ₁ and a 060°‐orientated subhorizontal σ₃, which account for the development of the observed intrusion and fracture orientations. Rhyolitic, basaltic and associated clastic dykes are preferentially intruded along extensional 330°‐trending fractures, subordinately along sinistral, transtensional 010°‐trending fractures and along 290°‐trending fractures. One of the implications of such a palaeotectonic reconstruction is that the so called north‐trending Eden‐Comerong‐Yalwal Late Devonian rift does not represent a simple, single palaeobasin entity, but is presently a north‐trending alignment of exposures of sedimentary and volcanic rocks probably emplaced in different basins or sub‐basins, mildly folded during the Carboniferous Kanimblan compression (which also formed the north‐trending Budawang synclinorium) and then extended to the east by the Tasman Sea opening during the Jurassic. The development of scattered, rapidly subsiding, basins characterised by bimodal volcanism during the Late Devonian throughout the Lachlan Fold Belt, can be interpreted in terms of extensional collapse of a forming mountain belt contemporaneous with a sharp decrease of compressional stress after the Middle Devonian Tabberabberan orogenic event. This would promote a reorientation of σ₃ and transition from a compressional to a transtensional tectonic environment, which could also favour block rotation and formation of release basins.     

The miarolitic pegmatites from the K?nigshain: a contribution to understanding the genesis of pegmatites

In this paper, we show that the crystallization of miarolitic pegmatites at K?nigshain started at about 700°C, in melts containing up to 30 mass% water. Such high water concentration at low pressures (1–3 kbar) is only possible if the melts are peralkaline. Such peralkaline melts are highly corrosive, and reacted with the wall rock—here the granite host—forming the graphic granite zone, in part via a magmatic–metasomatic reaction. With cooling, the water concentration in some melt fractions increased up to 50 mass% H₂O. The melt-dominated system ends below 600°C and passes into a fluid-dominated system, the beginning of which is characterized by strong pressure fluctuations, caused by the change of OH and CO₃ ²⁻ in the melt, to molecular water and CO₂. We note two generations of smoky quartz, one crystallized above the β–α-transition of quartz (≈573°C), and one below, both of which contain melt inclusions. This indicates that some melt fraction remains during at least the higher-temperature portion of the growth of minerals into the miarolitic cavity, contradicting the view that minerals growing into a pegmatite chamber only do so from aqueous fluids. We show that the K?nigshain miarolitic pegmatites are part of the broad spectrum of pegmatite types, and the processes active at K?nigshain are representative of processes found in most granitic pegmatites, and are thus instructive in the understanding of pegmatite formation in general, and constraining the composition and characteristics of pegmatite-forming melts. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The large-wavelength deformations of the lithosphere： materials for a history of the evolution of thought from the earliest times to plate tectonics

The notable authority on tectonics and the history of geosciences, Professor Celal Sengor from Istanbul, has produced another remarkable book-which, as he tells the reader, grew rapidly from an initial paper into a massive tome. Just as Georges Cuvier liked the idea of ‘bursting the limits of time‘, so Professor Sengor has again ‘burst the limits of a paper‘!     

Zedong terrane revisited: An intra-oceanic arc within Neo-Tethys or a part of the Asian active continental margin?

Precise timing of the India-Asia collision is important to constrain the evolution history of both the Himalayan orogen and the Tibetan Plateau. It has been proposed that the Indian plate first collided with an intra-oceanic arc at ∼55 Ma, and then the composite terrane collided with the Asian continent at ∼35 Ma. The Zedong terrane has been suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean, as some volcanic rocks with high K₂O have been classified as shoshonites. In this study, we present detailed geochemical and geochronological data of various types of magmatic rocks (including volcanic, cumulate and granitic rocks) widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane. We found that the Zedong volcanic rocks belong to calc-alkaline series rather than shoshonites and high K₂O contents in some volcanic rocks resulted from alteration. The basalts are highly enriched in LREE and LILE, but strongly depleted in HFSE, indicating they were derived from a metasomatized mantle. Presence of hornblende phenocryst in both gabbros and hornblendites indicates that the cumulates were produced from hydrous basalts through crystallization. The granitic rocks have adakite-like compositional characteristics, i.e., high Sr/Y ratios but low Y contents, which were formed by melting of a thickened lower crust. Zircons from six samples, including a volcanic rock (an andesite), three cumulates (a hornblendites, a hornblende-bearing gabbro and a gabbro) and two granitic (a tonalite and a granodiorite) rocks, have been dated to yield identical ages of ∼155–160 Ma. This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedong terrane. Zircons from both the andesite and the cumulates have similar positive ε_Hf(t) values (∼+11.6 to +16.7), indicating they were stemmed from similarly depleted mantle sources. Meanwhile, zircons from the granitic rocks also have positive ε_Hf(t) values of ∼+12.6 to +15.2, implying their derivation from a juvenile lower crust. Therefore, we proposed that the basalts in the Zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab-released fluids/melts. A part of hydrous basalts were underplated in the thickened lower crust beneath the Zedong terrane, which gave rise to the cumulate and granitic rocks. By comparison, magmatic rocks in the Zedong terrane show compositional similarities with the Jurassic rocks exposed in the Gangdese arc. This suggests that the Zedong terrane represents a slice of the active continental margin developed on the southern margin of the Lhasa terrane as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic, rather than the vestige of an intra-oceanic arc.     

An Early Cambrian granodiorite age from the pre-Andean basement of Tierra del Fuego (Chile): the missing link between South America and Antarctica?

Basement rocks of the Magellan basin at the Atlantic margin, in extra-Andean Tierra del Fuego (Chile), are predominantly formed of orthogneisses. They have been recovered only by drilling and their ages are mostly unknown. In this study, using the U-Pb method on zircons, age data have been obtained for a granodiorite gneiss from the basement of the Magellan basin near San Sebastian Bay close to the Chile–Argentina border. Data confirm that zircons of the granodiorite gneiss from the Gaviota 6 drill hole grew primarily during magmatic processes. The Concordia intercept age of unabraded zircons, 529±7.5 Ma, denotes the most probable time of intrusion. Abraded zircons revealed initial zircon growth at 549±6 Ma, probably under primary melt conditions and subsequent closed U-Pb system behaviour. The discordance of data points related to zircon re- and/or neo-crystallization attributed to metamict(?) cloudy domains is contemporaneous with Sr and Pb isotope homogenization in apatite and K-feldspar at 161.6±4.5 Ma. The contemporaneousness of total (Sr) or partial (Pb) equilibration of radiogenic isotopes in minerals of the granodiorite gneiss and formation of the nearly 2000 m thick Jurassic volcanic series leads to discussion of a fundamental heating of the lower crust at that time, controlled by the spatially distributed influx of a fluid phase. The Early Cambrian intrusion age of the precursor rock of the dated granodiorite gneiss suggests that basement rocks of Tierra del Fuego were generated contemporaneously with the Pampean orogenic cycle. The age of 529±7.5 Ma supports the idea of considering Tierra del Fuego as the ‘missing link’ between the Early Palaeozoic orogenic cycles of central and northwestern Argentina and the Late Cambrian/Early Ordovician Ross orogeny in central Antarctica.     

Early Paleozoic geodynamic evolution of the Eastern Central Asian Orogenic Belt: Insights from granitoids in the Xing’an and Songnen blocks

The early Paleozoic tectonic framework and evolutionary history of the eastern Central Asian Orogenic Belt (CAOB) is poorly understood. Here we present zircon U–Pb geochronology, whole rock geochemistry, and Sr-Nd-Hf isotope data of the early Paleozoic granitoids in eastern CAOB to investigate the petrogenesis and geodynamic implications.The early Paleozoic granitoids from the Songnen Block yield zircon U–Pb ages of 523–490 ​Ma, negative ε_Nd(t) values of –6.7 to –0.8, and ε_Hf(t) values of –8.6 to 7.1, indicating they were generated by partial melting of ancient crustal materials with various degrees of mantle contribution. They generally show affinities to A-type granites, implying their generation from an extensional environment after the collision between the Songnen and Jiamusi blocks. In comparison, the early Paleozoic granitoids from the Xing’an Block have zircon U–Pb ages of 480–465 ​Ma, ε_Nd(t) values of –5.4 to 5.4, and ε_Hf(t) values of –2.2 to 12.9, indicating a dominated juvenile crustal source with some input of ancient crustal components. They belong to I-type granites and were likely related to subduction of the Paleo-Asian Ocean. The statistics of T_DM2 Hf model ages of the granitoids indicate that the Erguna and Jiamusi blocks contain a significant proportion of Mesoproterozoic crystalline basement, while the Xing’an Block is dominated by a Neoproterozoic basement.Based on these observations, the early Paleozoic evolutionary history of eastern CAOB can be divided into four stages: (1) before 540 ​Ma, the Erguna, Xing’an, Songnen, and Jiamusi blocks were discrete microcontinents separated by different branches of the Paleo-Asian Ocean; (2) 540–523 ​Ma, the Jiamusi Block collided with the Songnen Block along the Mudanjiang suture; (3) ca. 500 Ma, the Erguna Block accreted onto the Xing’an Block along the Xinlin–Xiguitu suture; (4) ca. 480 Ma, the Paleo-Asian Ocean started a double-side subduction beneath the united Erguna–Xing’an and Songnen–Jiamusi blocks.     

A New Diverse Macrofossil Lagerst?tte from the Uppermost Ediacaran of Southwestern China

A new macrofossil Lagerstatte was discovered from the uppermost Ediacaran Jiucheng Member at Jinning and Jiangchuan of the eastern Yunnan, yielding numerous diverse well-preserved thallophyte macrofossils. These include the previously-known representatives of vendobionts, Vendotaenia and Tyrasotaenia, and the biggish multicellular benthos such as Chuaria-like and Tawuia- like fossiis, as well as Longfengshaniaceaens with diverse holdfast structures. There are still some other problematic macrofossils with peculiar configurations as well as uncertain relatives. The distinct dominance of the giant, unbranching thallophytes occasionally with holdfast structures distinguishes this assemblage from the other Ediacaran macrofossil Lagerstattes in the Doushantuo Formation at Miaohe, Wenghui and Lantian, and the contemporary assemblage in the Shibantan Member of Dengying Formation, Yangtze Gorges area. This paper outlines the characteristics of some of the multicellular macrofossils from the Jiucheng Member at Jiangchuan. They include some macrofossils with different types of holdfast structure, larger Chuaria-like and Tawuia-like morphology and questionable affinities as well. The discovery of greater diverse macrofossil assemblages from the Jiucheng Member of eastern Yunnan has further indicated that an important diversification and evolutionary radiation of metaphytes took place in the latest Ediacaran time. This radiation of largescale, benthic metaphyte along with phytoplankton was likely important contributors to the early Cambrian explosion of metazoans.     

Structural evolution of the Peninsular Gneiss — an early Precambrian migmatitic complex from South India

The Peninsular Gneiss, which is considered by a number of workers to be the basement on which the supracrustal rocks of the Dharwar Group were deposited, is a composite gneiss formed by migmatization of pre-existing metasedimentary and meta-igneous rocks. These gneisses show the same style and sequence of superposed deformation as those in the enclaves of metamorphic rocks and in the linear Dharwar schist belts outside. The main migmatization is broadly coeval with the isoclinal first folding, which is followed by near-coaxial refolding and non-coaxial upright folding. Small inclusions of migmatized amphibolite and granodioritic to dioritic gneiss, with a fabric athwart to, and overprinted by, the earliest deformation affecting the Dharwar Group of rocks in a large part of the gneissic terrane, point to at least one deformation, a metamorphic event and one episode of migmatization antedating the isoclinal first folds in the rocks of the Dharwar Group. The Peninsular Gneiss in its present state, therefore, represents an extensively remobilized basement.

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Zusammenfassung Der Peninsula-Gneis, der von einigen Bearbeitern als das Basement angesehen wird, auf dem die Gesteine der Dharwar-Gruppe abgelagert wurden, ist ein Migmatit aus Komponenten älterer metamorpher Sedimentgesteine und Magmatite. Abfolge der Deformation und Deformationsart dieser Gneise entspricht der Deformation des Dharwar Schiefergürtels. Die Hauptmigmatisierung verlief zeitgleich mit der isoklinalen ersten Faltung, die von einer zweiten fast coaxialen und einer dritten aufrecht coaxialen Faltung gefolgt wird. Es existieren Einschlüsse migmatisierten Amphibolits und granodioritischen bis dioritischen Gneises, deren Gefüge auf die älteste die Dharwar-Gruppe beeinflussenden Deformation zurückgeführt wird. Diese Beobachtungen sprechen für mindestens eine Deformation, eine Metamorphose und eine Migmatisierungsphase, die älter sind als die isoklinalen Falten der ersten Generation der Dharwar-Gruppe. Nach heutigen Erkenntnissen stellt also der Peninsular-Gneis ein intensiv durchbewegtes Basement dar.

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Résumé La formation des «Peninsular Gneiss» est considérée par de nombreux auteurs comme le socle sur lequel se sont déposées les roches supracrustales du Groupe de Dharwar. Il s'agit de gneiss composites formés par migmatitisation de roches méta-sédimentaires et méta-ignées préexistantes. Ces gneiss présentent le même style de déformation et la même succession de déformations superposées que la ceinture des schistes de Dharwar. La migmatitisation est contemporaine d'un premier plissement isoclinal, qui a été suivi d'un deuxième plissement sensiblement coaxial et d'une troisième déformation en plis droits non coaxiaux. Il existe toutefois des inclusions d'amphibolites migmatitiques et de gneiss granodioritiques à dioritiques dont la structure, transverse à la déformation la plus ancienne du Groupe de Dharwar, est remaniée par celleci. Ces observations plaident en faveur de l'existence d'au moins une phase de déformation, de métamorphisme et de migmatitisation antérieure aux plis isoclinaux de première génération du Groupe de Dharwar. A la lumière des connaissances actuelles, les Peninsular Gneiss apparaissent ainsi comme un socle polycyclique intensément remanié.

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The Ohlsbach Plume – Discharge of deep saline water from the crystalline basement of the Black Forest, Germany

 Salt-water discharges from a fault system in the crystalline basement of the Black Forest into the gravel aquifer of the Kinzig River valley near Ohlsbach, upper Rhine River valley, southwestern Germany. The salt water (TDS, 16 g/kg) ascends from a reservoir at  1 3 km depth. Once discharged into the gravel aquifer, the saline deep water mixes with fresh groundwater and is carried along the groundwater flow path to the middle of the Rhine River valley. There, the natural geogene salt-water plume merges with a man-made chloride-rich infiltration zone along the Rhine River. The plume was mapped using (1) chloride data from groundwater observation wells, and (2) resistivity data from geoelectric sounding. Background chloride is about 7 mg/kg. In the central region of the plume, chloride concentration exceeds 200 mg/kg. A continuous area of Cl  1 50 mg/kg is distinguished from the discharge fault to the Rhine River over a distance of 12 km. Resistivities range from  1 50 Ω in uncontaminated regions to <7 Ω in the 700 m-long central region of the plume. A low resistivity plume (7–10 Ω) stretches for  1 7 km into the Rhine River valley. The two plume maps are in good agreement. Received, April 1998 / Revised, November 1998, January 1999 / Accepted, January 1999     

Structural evolution of the Ural–Tian Shan junction: A view from Karatau ridge,South Kazakhstan

The deformation history of the Late Palaeozoic Ural–Tian Shan junction is discussed for the example of the Karatau ridge in southern Kazakhstan. Three deformation events are recognized. The Late Carboniferous D1 event is characterized by Laramide-style thrust-and-fold structures on the southern margin of Kazakhstan with shortening in a NE–SW direction. The Latest Permian and Triassic D2 event is controlled by compression in an east–west direction, which reflects collisional deformation in the Urals. The main structures are submeridional folds and north–west-striking sinistral strike–slip faults. The Triassic D3 event with shortening in a north–south direction reflects collision of the Turan microcontinent against the southern margin of Kazakhstan. The main structures are north–west-striking dextral strike–slip faults. Our new data provides important clues for the reconstruction of pre-Cretaceous structures between the Urals and the Tian Shan.