Structural-kinematic parageneses of the basement and cover at the southeastern margin of the Baltic Shield

Through, long-lived structural-kinematic parageneses were established in the southeastern marginal part of the Baltic Shield on the basis of structural studies. These parageneses were formed and periodically rejuvenated from at least the Paleoproterozoic until the neotectonic stage of the evolution of this territory. A series of consecutive tectonic events related to the vertical and horizontal mobility of rocks of the crystalline basement and sedimentary cover had important implications for the formation of present-day structure of the southeastern margin of the Baltic Shield. These tectonic displacements developed for an extremely long time with retention of the main kinematic tendencies. At the end of the Paleoproterozoic, the volcanic and sedimentary rocks of the Vetreny Belt underwent tectonic stacking as a result of the countermotion of the crystalline masses of the Vodlozero Massif and the Belomorian-Lapland Belt. The clockwise rotation and lateral displacement of the Vodlozero Massif to the northeast provided the left-lateral transpression of the Vetreny Belt. Under these conditions, the Paleoproterozoic sequences experienced squeezing in the southeastern direction. This kinematic tendency was retained at the subsequent evolutional stages and eventually was recorded in the structure of the present-day boundary between the Baltic Shield and the Russian Platform.     

Cambro-Silurian magmatisms at the northern Gondwana margin (Penninic basement of the Ligurian Alps)

The Early Paleozoic evolution of the northern margin of Gondwana is characterized by several episodes of bimodal magmatism intruded or outpoured within thick sedimentary basins. These processes are well recorded in the Variscan blocks incorporated in the Ligurian Alps because they experienced low temperature Alpine metamorphism. During the Paleozoic, these blocks, together with the other Alpine basements, were placed between the Corsica-Sardinia and the Bohemian Massif along the northern margin of Gondwana. In this framework, they host several a variegated lithostratigraphy forming two main complexes(Complexs I and II) that can be distinguished by both the protoliths and their crosscutting relationships, which indicate that the acidic and mafic intrusives of Complex II cut an already folded sequence made of sediments, basalts and granitoids of Complex I. Both complexes were involved in the Variscan orogenic phases as highlighted by the pervasive eclogite-amphibolite facies schistosity(foliation II). However, rare relicts of a metamorphic foliation at amphibolite facies conditions(foliation I)is locally preserved only in the rocks of Complex I. It is debatable if this schistosity was produced during the early folding event e occurred between the emplacement of Complex I and II e rather than during an early stage of the Variscan metamorphic cycle.New SHRIMP and LA ICP-MS Ue Pb zircon dating integrated with literature data, provide emplacement ages of the several volcanic or intrusive bodies of both complexes. The igneous activity of Complex I is dated between 507 ± 15 Ma and 494 ± 5 Ma, while Complex II between 467 ± 12 Ma and 445.5 ± 12 Ma.The folding event recorded only by the Complex I should therefore have occurred between 494 ± 5 Ma and 467 ± 12 Ma. The Variscan eclogite-amphibolite facies metamorphism is instead constrained between ~420 Ma and ~300 Ma. These ages and the geochemical signature of these rocks allow constraining the Early Paleozoic tectono-magmatic evolution of the Ligurian blocks, from a middleeupper Cambrian rifting stage, through the formation of an Early Ordovician volcanic arc during the Rheic Ocean subduction, until a Late Ordovician extension related to the arc collapse and subsequent rifting of the PaleoThetys. Furthermore, the ~420-350 Ma ages from zircon rims testify to thermal perturbations that may be associated with the Silurian rifting-related magmatism, followed by the subduction-collisional phases of the Variscan orogeny.     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

佳木斯地块东缘晚古生代大陆边缘构造演化

佳木斯地块位于中国东北微陆块群的最东缘,其东缘地区晚古生代的岩浆和沉积演变进程为欧亚大陆东缘由被动陆缘向活动陆缘构造环境的转化提供了关键证据。年代学和地球化学研究表明,佳木斯地块东缘中泥盆世黑台组砂岩,形成于被动陆缘的构造环境,黑台组上覆的老秃顶子组流纹岩也形成于被动陆缘的构造环境;晚石炭世珍子山组砂岩,形成于活动陆缘的构造环境;早二叠世的二龙山组安山岩以及相邻地区早二叠世的其它火成岩形成于活动陆缘的构造环境。同时,佳木斯地块东缘泥盆-二叠纪的沉积地层也呈现出由浅海相到陆相地层转化的特征。因此,佳木斯地块东缘由被动陆缘向活动陆缘的转化应该发生在中泥盆世到晚石炭世,而该构造环境的转化也为晚古生代时期蒙古-鄂霍茨克洋向欧亚大陆之下俯冲过程的研究提供了关键信息。

     

The geosynclinal pair at the continental margin of Peru

The Andean geosynclinal pair in Peru consists of a eugeosynclinal marine andesitic volcanic trough and a miogeosynclinal sedimentary trough. Both troughs developed on a block-faulted basement of Precambrian crystalline rocks in which movement occurred along faults parallel to the continental margin. Subsidence in the eugeosyncline was most rapid during the Albian during which period about 7000 m of marine volcanics were deposited. In the miogeosyncline the greatest subsidence took place during the Tithonian but continued throughout the Cretaceous to accumulate a total thickness of about 6000 m. From the Late Cretaceous to the mid-Tertiary the Andean granitoid batholith was emplaced, mainly in the eugeosynclinal zone. A Benioff zone has been active at the continental margin from the Early Jurassic until the present so that the entire geotectonic cycle from the geosynclinal stage through to orogeny and uplift has taken place under a regime of active subduction.During the geosynclinal stage the sedimentary troughs developed in fault-bounded blocks which subsided under tension. It is possible to envisage crustal thinning within the subsident blocks by rotational movement on extensional faults or by plastic stretching of the lower crust. Stretching would be facilitated by the high geothermal gradient resulting from the emplacement of plutonics and volcanics and might be analogous to the formtion of marginal basins of western Pacific type. It is more difficult to account for the subsequent uplift for although the granites contributed to the crustal thickening, the main uplift did not occur in the granitic sector but further inland, and was moreover delayed for at least 20 m.y. after the last granites were emplaced.     

Collision processes at the northern margin of the Black Sea

Extended along the Crimea–Caucasus coast of the Black Sea, the Crimean Seismic Zone (CSZ) is an evidence of active tectonic processes at the junction of the Scythian Plate and Black Sea Microplate. A relocation procedure applied to weak earthquakes (m_b ≤ 3) recorded by ten local stations during 1970–2013 helped to determine more accurately the parameters of hypocenters in the CSZ. The Kerch–Taman, Sudak, Yuzhnoberezhnaya (South Coast), and Sevastopol subzones have also been recognized. Generalization of the focal mechanisms of 31 strong earthquakes during 1927–2013 has demonstrated the predominance of reverse and reverse–normal-faulting deformation regimes. This ongoing tectonic process occurs under the settings of compression and transpression. The earthquake foci with strike-slip component mechanisms concentrate in the west of the CSZ. Comparison of deformation modes in the western and eastern Crimean Mountains according to tectonophysical data has demonstrated that the western part is dominated by strike-slip and normal- faulting, while in the eastern part, reverse-fault and strike-slip deformation regimes prevail. Comparison of the seismicity and gravity field and modes of deformation suggests underthusting of the East Black Sea Microplate with thin suboceanic crust under the Scythian Plate. In the Yuzhnoberezhnaya Subzone, this process is complicated by the East Black Sea Microplate frontal part wedging into the marginal part of the Scythian Plate crust. The indentation mechanism explains the strong gravity anomaly in the Crimean Mountains and their uplift.     

Tectonic erosion at the front of the Japan Trench convergent margin

The imaging of a multichannel seismic record was improved by reprocessing using pre-stack techniques. The reprocessed record shows structures that indicate tectonic erosion and gravity collapse at the front of the Japan Trench margin. Much of the lower slope appears to be underlain by a detached, coherent block of continental crust. The lower slope has failed by mass wasting and the resulting apron of slump debris at the base of the slope has become involved in thrust faulting at the front of the subduction zone. Slumping continues as long as debris is removed from the front of the margin by subduction, and the apron cannot build up sufficiently to stabilize the failing lower slope. Truncated beds at the base of the upper plate indicate subcrustal erosion as well, this probably being the main cause of massive subsidence of the margin. Subsidence was the cause of oversteepening, destabilization and subsequent gravity collapse of the leading edge of the upper plate.     

华北克拉通北缘及邻区前燕山期主要地质事件

近年来,华北克拉通北缘及邻区的研究进展集中在前燕山期的主要地质构造格架的廓清,以及晚中生代以来的构造岩浆事件和克拉通岩石圈减薄研究的深化。本文对前者的研究进展作评述和展望。华北克拉通自1.8～1.75Ga形成后,时有岩浆扰动。1.35Ga的基性岩床和岩墙群事件代表了华北克拉通与北美克拉通的裂解,说明华北克拉通曾经是哥伦比亚超大陆的组成部分。华北克拉通北缘大陆边缘的演化也应当从1.35Ga以后开始。早古生代时期,在华北克拉通以北的兴蒙造山带南部发育了白乃庙岛弧岩带,但此时华北克拉通依然记录的是稳定沉积。该岛弧岩带在早古生代末期可能通过弧-陆碰撞形式增生到华北克拉通北部边缘。早中泥盆世期间,在华北克拉通北缘发育了年龄为410～380Ma的碱性杂岩,可能与弧陆碰撞后的伸展有关。从晚石炭世(～320Ma)开始,华北克拉通北缘发展为安第斯型活动大陆边缘,古亚洲洋向南俯冲在华北克拉通之下。在相邻的兴蒙造山带,古亚洲洋还存在向北的俯冲,形成了白音宝力道岛弧岩带。古亚洲洋沿索伦缝合带的最终闭合发生在二叠纪末—三叠纪初期。华北克拉通北缘大量～250Ma以来的后碰撞岩浆活动记录了这一拼合过程。晚三叠世—早侏罗世华北克拉通北缘发生大规模逆冲推覆。早侏罗世时期,华北克拉通北缘已经出现基底结晶岩系的广泛剥露。在燕山期构造岩浆作用之前,华北克拉通北缘的东西向构造格架基本奠定。     

吐哈盆地西南缘新构造控矿规律及十红滩铀矿床外围成矿有利区预测

本根据吐哈盆地西南缘新构造运动的表现特征,结合已知铀矿床、矿点的空间分布,认为该区新构造控矿规律主要体现在3个方面：(1)由新构造复活基底断裂形成的“断裂挠曲”控矿;(2)新构造褶皱形成的“坡中坡”转折区域控矿;(3)新断裂构造形成的地下水局部排泄源也参与了对铀矿化的控制作用。依据这些规律,笔预测十红滩北带之北带仍有成矿远景,并认为白咀山西侧及迪哈尔511线附近区域具有一定的成矿潜力。     

Cenozoic basement structure of the South China Sea and adjacent areas by modeling and interpreting gravity data

The South China Sea and adjacent areas is a large region with diversely complicated geological conditions. In spite of investigations carried out over the past many years, the marine geological structure in many places has remained poorly understood because of the deficient data, a thick seawater layer as well as of the sensitive areas among the countries in the region. In this paper, the authors study and apply a model-based methodology of the modeling and interpretation of the newest gravity data and others, which are 3D modeling, frequency filtering, horizontal gravity gradient and maximum horizontal gravity gradient, in order to determine clearly the basement structure. The basement features such as the main faults systems, uplift-depression zones and seafloor spreading axis, oceanic boundary in the region have been defined. The achieved results are checked by the seismic data available for the region. From the studied results, the authors have brought out some initial remarks on the structure and form of the basement in the South China Sea and adjacent areas.     

Mid-Proterozoic magmatic arc evolution at the southwest margin of the Baltic Shield

Mid-Proterozoic calc-alkaline granitoids from southern Norway, and their extrusive equivalents have been dated by LAM-ICPMS U–Pb on zircons to ages ranging from 1.61 to 1.52 Ga; there are no systematic age differences across potential Precambrian terrane boundaries in the region. U–Pb and Lu–Hf data on detrital zircons from metasedimentary gneisses belonging to the arc association show that these were mainly derived from ca. 1.6 Ga arc-related rocks. They also contain a minor but significant fraction of material derived from (at least) two distinct older (1.7–1.8 Ga) sources; one has a clear continental signature, and the other represents juvenile, depleted mantle-derived material. The former component resided in granitoids of the Transscandinavian Igneous Belt, the other in mafic rocks related to these granites or to the earliest, subduction-related magmatism in the region. Together with published data from south Norway and southwest Sweden, these findings suggest that the western margin of the Baltic Shield was the site of continuous magmatic arc evolution from at least ca 1.66 to 1.50 Ga. Most of the calc-alkaline metaigneous rocks formed in this period show major- and trace-element characteristics of rocks formed in a normal continental margin magmatic arc. The exceptions are the Stora Le-Marstrand belt in Sweden and the Kongsberg complex of Norway, which have an arc-tholeiitic chemical affinity. The new data from south Norway do not justify a suggestion that the crust on the west side of the Oslo Rift had an early to mid-Proterozoic history different from the crust to the east. Instead, they indicate that the different parts of south Norway and southwest Sweden were situated at the margin of the Baltic Shield throughout the mid-Proterozoic. Changes from arc tholeitic to calc-alkaline magmatism reflect changes with time in the subduction zone system, or lateral differences in subduction zone geometry. The NW American Cordillera may be a useful present-day analogue for the tectonomagmatic evolution of the mid-Proterozoic Baltic margin.     

Cenozoic Volcanism and Intraplate Subduction at the Northern Margin of the Tibetan Plateau

Developed in the Mt.Kunlun orogenic belt at the northern margin of the Tibetan Plateau is an active Cenozoic volcanic zone which is more than 1000km in length and some ten to hundred kilometers in width.It extends east-westwards and is roughly parallet to the strike of Mt.Kunlun.The Cenozoic volcanic rocks are divided into the northern(N-)and southern(S-)subzones.Eruptions of volcanic lavas in the S-subzone are related to an initial rift zone within the north Qiangtang terrane,but the volcanic rocks in the N-subzone are relatively close to the contact zone between the Mt.Kunlun and the Tarim terrane.The space-time distribution,petrological and geochemical features can be explained by a model of southward intraplate subduction of the Tarim terrane.     

Unravelling the pre-Variscan evolution of the Habach terrane (Tauern Window, Austria) by U-Pb SHRIMP zircon data

The U-Pb SHRIMP age determinations of zircons from the Habach terrane (Tauern Window, Austria) reveal a complex evolution of this basement unit, which is exposed in the Penninic domain of the Alpine orogen. The oldest components are found in zircons of a metamorphosed granitoid clast, of a migmatitic leucosome, and of a meta-rhyolitic (Variscan) tuff which bear cores of Archean age. The U-Pb ages of discordant zircon cores of the same rocks range between 540 and 520 Ma. It is assumed that the latter zircons were originally also of Archean origin and suffered severe lead loss, whilst being incorporated into Early-Cambrian volcanic arc magmas. The provenance region of the Archean (2.64-2.06 Ga) zircons is assumed to be a terrane of Gondwana affinity: i.e., the West African craton (Hoggar Shield, Reguibat Shield). The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on rocks of the Habach terrane; it is postdated by discordant zircons of a migmatitic leucosome at <440 Ma (presumably ca. 420 Ma). Alpine and Variscan upper greenschist- to amphibolite-facies conditions caused partial lead loss in zircons of a muscovite gneiss ('white schist') only, where extensive fluid flow and brittle deformation due to its position near a nappe-sole thrust enhanced the grains' susceptibility to isotopic disturbance. The Habach terrane - an active continental margin with ensialic back-arc development - showed subduction-induced magmatic activity approx. between 550 and 507 Ma. Back-arc diorites and arc basalts were intruded by ultramafic sills and subsequently by small patches of mantle-dominated unaltered and (in the vicinity of a major tungsten deposit) altered granitoids. Fore-arc (shales) and back-arc (greywackes, cherts) basin sediments as well as arc and back-arc magmatites were not only nappe-stacked by the Caledonian compressional regime closing the presumably narrow oceanic back-arc basin and squeezing mafic to ultramafic cumulates out of high-level magma chambers (496-482 Ma). It also induced uplift and erosion of deeply rooted crystalline complexes and triggered the development of a successor basin filled with predominantly clastic greywacke-arkosic sediments. The study demonstrates that the basement rocks exposed in the Habach terrane might be the 'missing link' between similar units of the more westerly positioned External domain (i.e., Aar, Aiguilles Rouges, Mont Blanc) and the Austroalpine domain to the east (Oetztal, Silvretta).     

Genetic types of metaconglomerates at the Paleoproterozoic basement of the Krivoi Rog iron ore basin

The Paleoproterozoic basement of the Krivoi Rog iron ore basin comprises five genetically different lithopetrographic types of metaconglomerates that make up stratigraphic and lithofacies successions ranging from the proluvial to shallow-basin formations. The dominant type is represented by the basal flow (proluvial and alluvial) metaconglomerates. Variations in the lithopetrography of metaconglomerates reflect the evolution of paleofacies and paleotectonic constraints.     

中天山南缘乌瓦门蛇绿岩形成构造环境

中天山南缘乌瓦门蛇绿混杂岩主要由构造岩块和混杂基质两部分组成,构造岩块主要包括:由变质橄榄岩、辉长岩、玄武岩组成的蛇绿岩残块、中天山基底变质岩系的斜长角闪岩和片麻岩构造块体以及来源于南天山泥盆系的大理岩残块;混杂基质主要为强烈剪切变形的绿泥石英片岩、绢云石英片岩、二云母石英片岩、千枚岩和变砂岩。变质橄榄岩主要为蛇纹石化的橄榄岩,SiO2、TiO2、Al2O3和CaO含量相对较高,而MgO含量相对较低。总体特征类似于二辉橄榄岩。稀土元素总量低,是球粒陨石稀土元素总量的0.3-0.5倍,以强烈亏损LREE为特征。乌瓦门玄武岩属于拉斑系列,主量元素显示低Al2O3、高TiO2、MgO,贫K2O、P2O5,Na2O>K2O性状,并以低∑REE、LREE亏损、高场强元素不分异为特征,类似于N-MORB。同时,部分岩石样品在高场强元素地球化学性状类似的基础上,具有不同程度的LILE、Th富集和Nb、Ta亏损,以及Pb富集特征,并有Zr的轻度低谷,显示岩浆源区曾遭受不同程度的俯冲带流体交代作用的影响。综合分析认为,乌瓦门蛇绿岩形成于弧后盆地环境。     

Debris structures in basal ice exposed at the margin of the Greenland ice sheet

The basal ice of many glaciers contains debris structures that reflect subglacial processes. Presented here is an unusually clear photograph of ice and debris in the lowest 2 m of the basal layer at the margin of the Greenland ice sheet. The photograph shows ice-debris relationships and deformation structures that reflect entrainment processes and flow history.     

Benthic dynamics at the carbonate mound regions of the Porcupine Sea Bight continental margin

A brief review is given of some dynamical processes that influence the benthic dynamics within the carbonate mound provinces located at the Porcupine Bank/Sea Bight margin, NE Atlantic. The depth range of the mounds in this region (600–1,000 m) marks the upper boundary of the Mediterranean outflow water above which Eastern North Atlantic Water dominates. Both water masses are carried northwards by the eastern boundary slope current. In the benthic boundary layer both the action of internal waves, and other tidal period baroclinic waves, may enhance the bottom currents and add to both the residual and maximum flow strength. Both residual and maximum bottom currents vary at different mound locations, with stronger currents found at Belgica (SE Porcupine Sea Bight) mound and Pelagia (NW Porcupine Bank) mound regions, whilst weakest currents are found at the Hovland and Magellan Mounds at the northern Sea Bight margin. The differences may be attributed to the presence of internal waves (Pelagia) or bottom intensified diurnal waves (Belgica). These different dynamical regimes are likely to have implications for the distribution patterns of live coral at the different locations.     

Outline of the Somalian basement

A sequence of events which has been recognized in the two basement regions of Somalia is outlined on the basis of new field and petrographic data and pre-existing published and unpublished reports. The evolution of the Somalian basement took place during the Pan-African event, However, much detailed work is necessary before the history of the basement can be integrated into the more detailed Pan-African picture recognized in the neighbouring regions.

---

Zusammenfassung Eine Folge von Ereignissen, die sich in den zwei Basement-Regionen Somalias erkennen lie\en, wird auf der Basis neuer GelÄnde- und petrographischer Daten und früherer publizierter und nicht publizierter Berichte umrissen. Das Basement von Somalia entwickelte sich wÄhrend des Panafrikanischen Ereignisses.Viel Detailarbeit ist jedoch noch zu leisten, bis die Geschichte des Basements in das detailliertere Bild Panafrikas, wie es aus den Nachbarregionen gewonnen wurde, integriert werden kann.

---

Résumé Dans les deux régions de la Somalie ou le socle affleure, on a reconnu une succession d'événements sur la base de nouvelles données géologiques et pétrographiques et de travaux pré-existants, publiés et non publiés. L'évolution du socle de la Somalie se déroula pendant l'événement Pan-Africain. Toutefois, beaucoup de travail doit Être encore fait avant que l'histoire du socle somalien puisse Être intégrée dans un cadre Pan-Africain plus détaillé, tel qu'on le connait dans les régions voisines.

---

, , , . .

     

Evidence for oceanic subduction at the NE Gondwana margin during Permo-Triassic times

Blueschist was recently recognized within the Lhasa terrane, which is one of the NE Gondwana blocks. In this rock, the Mn and Mg contents of garnet enclosing aegirine-rich clinopyroxene, rutile and quartz decrease and increase, respectively, from core to rim. Amphibole changes from glaucophane through Na–Ca amphibole to Ca amphibole. The Si contents of phengite are high in the centre but low along the rim. The P – T path, starting above 2.5 GPa–450 °C and showing subsequently first a temperature increase to 500 °C and then a pressure release via blueschist conditions to 0.6 GPa, was reconstructed using a P – T pseudosection calculated for the P – T range 0.4–2.8 GPa and 250–650 °C. This path points to deep subduction of a cold oceanic crust probably beneath the NE Gondwana margin during Permo-Triassic times. This finding contributes to a better understanding of the pre-Cenozoic history of major terranes of NE Gondwana.