Edge tectonics (trench rollback,terrane export) of Gondwanaland-Pangea synchronized by supercontinental heat

From 650–500 Ma assembly, 320 Ma merger in Pangea, to 185 Ma breakup, Gondwanaland developed by the accretion of lithosphere along the convergent edge on the south and by the export of terranes from the divergent edges on the west and northeast. The interior underwent epeirogenic movement except in areas affected during the merger by farfield shortening. Synchronous or near-synchronous events on the edges and interior are linked hypothetically by convection currents in the asthenosphere driven by supercontinent-induced heat. On the convergent edge, currents countered the sinking slab to roll back the trench and generate a backarc basin. On the divergent edge, currents initiated an ocean that prised off continental rims in the form of terranes. In the interior, currents extended the lithosphere in basins and rifts.     

Slab rollback and microcontinent subduction in the evolution of the Zambales Ophiolite Complex(Philippines):A review

New radiolarian ages show that the island arc-related Acoje block of the Zambales Ophiolite Complex is possibly of Late Jurassic to Early Cretaceous age.Radiometric dating of its plutonic and volcanichypabyssal rocks yielded middle Eocene ages.On the other hand,the paleontological dating of the sedimentary carapace of the transitional mid-ocean ridge-island arc affiliated Coto block of the ophiolite complex,together with isotopic age datings of its dikes and mafic cumulate rocks,also yielded Eocene ages.This offers the possibility that the Zambales Ophiolite Complex could have:(1)evolved from a Mesozoic arc(Acoje block)that split to form a Cenozoic back-arc basin(Coto block),(2)through faulting,structurally juxtaposed a Mesozoic oceanic crust with a younger Cenozoic lithospheric fragment or(3)through the interplay of slab rollback,slab break-off and,at a later time,collision with a microcontinent fragment,caused the formation of an island arc-related ophiolite block(Acoje)that migrated trench-ward resulting into the generation of a back-arc basin(Coto block)with a limited subduction signature.This Meso-Cenozoic ophiolite complex is compared with the other oceanic lithosphere fragments along the western seaboard of the Philippines in the context of their evolution in terms of their recognized environments of generation.     

Petrography and geochemistry of the Lopingian (upper Permian)-Lower Triassic strata in the southern Junggar and Turpan basins,NW China: implications for weathering,provenance, and palaeogeography

ABSTRACT

This study investigates three Lopingian (upper Permian)-Lower Triassic terrestrial successions in northwest China, namely the Urumqi and Jimsar sections in the southern Junggar Basin (SJB), and the Taodonggou section in the Turpan Basin (TB). Stratigraphy studies suggest that, in all three sections, the Lopingian-Lower Triassic strata are represented by mixed fluvial and lacustrine deposits. The lithofacies and geochemical indicators (CIA, PIA) suggest that, in all three sections, the Wutonggou Formation (Wuchiapingian) was deposited under subhumid conditions. The Guodikeng Formation (Changhsingian-early Induan) represents subhumid to semiarid conditions. The Jiucaiyuan and Shaofanggou formations (mid-to-late Induan to Olenekian) in the SJB show highly variable subhumid-semiarid conditions, while the two formations in the TB display early episode of fluctuating subhumid-semiarid and later semiarid-arid conditions. Within each section, all four formations display similar petrographic and geochemical characteristics, suggesting consistency in provenance during deposition. However, the provenance characteristics of the Urumqi and Jimsar sections differ from those of the Taodonggou section. Relative to the Taogonggou section, the two sections in the SJB contain more felsic and recycled sedimentary components. This suggests that the greater Junggar-Turpan Basin was in a partitioned setting during the Lopingian-Early Triassic, when different subbasins have relatively independent provenance systems.     

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).     

U-Pb (zircon) ages for the gneissic terrane west of the Nile,southern Egypt

The first U-Pb zircon ages are reported for the gneissic bedrock inliers previously interpreted as part of the Nile Craton. The inliers crop out in the Egyptian Western Desert, east of the Uweinat area and west of the Eastern Desert. Multi- and single-grain zircon analyses of granitoid gneiss and migmatite from Gebel Um Shagir, Aswan, and another locality approximately 160 km south-west of Aswan, yield simple discordia with near modern day Pb loss trajectories, and the following Neoproterozoic crystallization ages: 626+4/–3, 634 ± 4 and 741 ± 3 Ma. In contrast, multi- and single-grain U-Pb analyses (zircon and sphene) from an anorthositic gabbro at Gebel Kamil (22°46N 26°21E) and an anorthosite at Gebel El Asr (22°46N 31°10E) yield Archean and Paleoproterozoic emplacement ages. The former yield a crystallization age of > 2.67 Ga and a metamorphic age of 2.0 Ga; the latter a metamorphic age of 0.69 Ga and an inheritance age of 1.9–2.1 Ga. Because high grade gneiss and migmatite of Neoproterozoic, Paleoproterozoic and Archean age crop out west of the Nile, pre-Neoproterozoic crust should no longer be identified by its metamorphic grade. By contrast, mapping the anorthosite and related rocks might provide first-order estimates for the extension of pre-Neoproterozoic crust in north-east Africa. It is suggested that Archean and Paleoproterozoic crust of the Uweinat and Congo Craton are contiguous because these U-Pb (zircon) data show no evidence for a Neoproterozoic thermal overprint in the Gebel Kamil area and there is no pronounced Neoproterozoic magmatic activity south of the Uweinat inlier and north of the Congo Craton.     

Petrogenesis and Geodynamic Implications of Early Cretaceous (~130 Ma) Magmatism in the Baingoin Batholith, Central Tibet: Products of Subducting Slab Rollback

The Baingoin batholith is one of the largest granitic plutons in the North Lhasa terrane. Its petrogenesis and tectonic setting have been studied for decades, but remain controversial. Here we report data on geochronology, geochemistry and isotopes of Early Cretaceous granitoids within the Baingoin batholith, which provide more evidence to uncover its petrogenesis and regional geodynamic processes. The Early Cretaceous magmatism yields ages of 134.4–132.0 Ma and can be divided into I-type, S-type and highly fractionated granites. The I- and S-type granites exhibit medium SiO2, high K2O/Na2O with negative εNd(t) and εHf(t) values, whereas, the albite granites have very high SiO2 (79.04%–80.40%), very low K2O/N2O, negative εNd(t) and a large variation in εHf(t). Our new data indicate that these granitoids are derived from unbalanced melting in a heterogeneous source area. The granodiorites involved had a hybrid origin from partial melting of basalt-derived and Al-rich rocks in the crust, the porphyritic monzogranites being derived from partial melting of pelitic rocks. The albite granites crystallized from residual melt separated from K-rich magma within the ‘mush’ process and underwent fractionation of K-feldspar. We believe that the Early Cretaceous magmatism formed in an extensional setting produced by the initial and continuous rollback of a northward-subducting slab of the NTO.     

Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.     

A review of Archaean and Paleoproterozoic evolution of the In Ouzzal granulitic terrane (Western Hoggar, Algeria)

The In Ouzzal terrane (Western Hoggar) is an example of Archaean crust remobilized during a very-high-temperature metamorphism related to the Paleoproterozoic orogeny (2 Ga). Pan-African events (≈0.6 Ga) are localized and generally of low intensity. The In Ouzzal terrane is composed of two Archaean units, a lower crustal unit made up essentially of enderbites and charnockites, and a supracrustal unit of quartzites, banded iron formations, marbles, Al–Mg and Al–Fe granulites commonly associated with mafic (metanorites and garnet pyroxenites) and ultramafic (pyroxenites, lherzolites and harzburgites) lenses. Cordierite-bearing monzogranitic gneisses and anorthosites occur also in this unit. The continental crust represented by the granulitic unit of In Ouzzal was formed during various orogenic reworking events spread between 3200 and 2000 Ma. The formation of a continental crust made up of tonalites and trondhjemites took place between 3200 and 2700 Ma. Towards 2650 Ma, extension-related alkali-granites were emplaced. The deposition of the metasedimentary protoliths between 2700 and 2650 Ma, was coeval with rifting. The metasedimentary rocks such as quartzites and Al–Mg pelites anomalously rich in Cr and Ni, are interpreted as a mixture between an immature component resulting from the erosion and hydrothermal alteration of mafic to ultramafic materials, and a granitic mature component. The youngest Archaean igneous event at 2500 Ma includes calc-alkaline granites resulting from partial melting of a predominantly tonalitic continental crust. These granites were subsequently converted into charnockitic orthogneisses. This indicates crustal thickening or heating, and probably late Archaean high-grade metamorphism coeval with the development of domes and basins. The Paleoproterozoic deformation consists essentially of a re-activation of the pre-existing Archaean structures. The structural features observed at the base of the crust argue in favour of deformation under granulite-facies. These features are compatible with homogeneous horizontal shortening of overall NW–SE trend that accentuated the vertical stretching and flattening of old structures in the form of basins and domes. This shortening was accommodated by horizontal displacements along transpressive shear corridors. Reactional textures and the development of parageneses during the Paleoproterozoic suggest a clockwise P–T path characterized by prograde evolution at high pressures (800–1050 °C at 10–11 kbar), leading to the appearance of exceptional parageneses with corundum–quartz, sapphirine–quartz and sapphirine–spinel–quartz. This was followed by an isothermal decompression (9–5 kbar). Despite the high temperatures attained, the dehydrated continental crust did not undergo any significant partial melting. The P–T path followed by the granulites is compatible with a continental collision, followed by delamination of the lithosphere and uprise of the asthenosphere. During exhumation of this chain, the shear zones controlled the emplacement of carbonatites associated with fenites.     

北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

超基性岩是苏鲁超高压变质地体中一类特殊且十分重要的岩石类型,它们通常呈规模不一的块状、条带状或不规则透镜状 (体) 赋存于区域大面积出露的花岗质片麻岩中。锆石中矿物包体激光拉曼测试、阴极发光图像分析和不同性质锆石LA-ICP-MS U-Pb定年等综合研究结果表明,北苏鲁威海地区含橄榄石辉石岩 (样品W1和W2) 中锆石的成因十分复杂,可进一步划分3种不同类型锆石。其中第一类锆石呈自形-半自形晶,阴极发光图像显示清晰的岩浆结晶环带,矿物包体主要为Ol+Cpx+Ap, 记录的²⁰⁷Pb/²⁰⁶Pb年龄为1835~1845Ma,应代表含橄榄石辉石岩的原岩形成时代;第二类为变质重结晶锆石,呈半自形-他形晶,阴极发光图像显示模糊的岩浆结晶环带,矿物包体与第一类完全一致,记录的²⁰⁶Pb/²³⁸U年龄变化范围大,为250~784Ma之间,表明部分继承性岩浆结晶锆石明显受到后期岩浆-变质热事件的影响而发生不完全重结晶和Pb丢失,进而使其记录的年龄相对偏新;第三类锆石呈他形晶,为典型的变质锆石,阴极发光图像十分均匀,矿物包体相对少见,主要为Grt+Cpx,记录的²⁰⁶Pb/²³⁸U年龄为230~234Ma, 且与苏鲁地体榴辉岩及其围岩中含柯石英锆石微区记录的超高压变质年龄 (225~235Ma) 十分一致,应代表含橄榄石辉石岩的峰期超高压变质时代。超基性岩中超高压变质锆石的准确识别表明苏鲁地体在峰期超高压变质阶段的确存在流体,流体的存在对超高压变质锆石的形成起着至关重要的作用。该项研究不仅准确厘定北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代,而且对于深入探讨苏鲁-大别超高压地体流体行为、演化规律及其水-岩相互作用机理具有重要的科学意义。     

Late Jurassic terrane collision in the northwestern margin of Gondwana (Cajamarca Complex,eastern flank of the Central Cordillera,Colombia)

Medium-grade metabasites and metapelites from the Cajamarca Complex (Central Cordillera of Colombia) are in fault contact with the Jurassic Ibague batholith and show a penetrative foliation, locally mylonitic, suggesting intense dynamic–thermal metamorphism. The amphibolites are composed of calcic amphibole + epidote + plagioclase + quartz plus rutile + titanite + apatite + carbonate as accessory phases. Chlorite and albite appear as retrograde replacements. The metapelites are mainly composed of phengite + quartz + garnet + chlorite, plus epidote + albite + apatite + titanite + haematite as accessory phases. Bulk geochemistry of the amphibolites indicates basaltic protoliths with a mid-ocean ridge basalt (MORB) signature, although enrichment in the mobile large-ion lithophile elements compared to MORB suggests pre- and/or syn-metamorphic alteration by fluids. Peak pressure–temperature determinations for both types of rocks are similar, ranging 550–580°C and 8 kbar (approximately 26 km depth and an apparent geothermal gradient of 22°C/km). ⁴⁰Ar-³⁹Ar dating of amphibole from two amphibolite samples and one phengitic mica from a pelitic schist yielded plateau ages of 146.5 ± 1.1 Ma and 157.8 ± 0.6 Ma, and 157.5 ± 0.4 Ma, respectively. These Late Jurassic ages contrast with previously published (Permian)Triassic ages of metamorphism in the Cajamarca Complex. Taken together, our data indicate tectonic-driven burial of oceanic supracrustal sequences down to mid-crustal depths during Late Jurassic times and are best explained as the result of terrane collision-related metamorphism and deformation in a fore-arc/volcanic-arc environment of the active western margin of Gondwana rather than as a result of Jurassic thermal–metamorphic resetting of a (Permian)Triassic metamorphic sequence during intrusion of the Jurassic Ibague batholith. Our results represent the first report of Jurassic terrane collision tectonics involving supracrustal oceanic rocks in the northwestern margin of Gondwana in Colombia.     

Triassic Nb-enriched basalts, magnesian andesites, and adakites of the Qiangtang terrane (Central Tibet): evidence for metasomatism by slab-derived melts in the mantle wedge

New chronological, geochemical, and isotopic data are reported for Triassic (219–236 Ma) adakite-magnesian andesite-Nb-enriched basaltic rock associations from the Tuotuohe area, central Qiangtang terrane. The adakites and magnesian andesites are characterized by high Sr/Y (25–45), La/Yb (14–42) and Na₂O/K₂O (12–49) ratios, high Al₂O₃ (15.34–18.28 wt%) and moderate to high Sr concentrations (220–498 ppm) and ε_ND (t) (+0.86 to +1.21) values. Low enrichments of Th, Rb relative to Nb, and subequal normalized Nb and La contents, and enrichments of light rare earth elements combine to distinguish a group of Nb-enriched basaltic rocks (NEBs). They have positive ε_ND (t) (+2.57 to +5.16) values. Positive correlations between Th, La and Nb and an absence of negative Nb anomalies on mantle normalized plots indicate the NEBs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids. A continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite. The spatial association of the NEBs with adakites and magnesian andesites define an “adakitic metasomatic volcanic series” recognized in many demonstrably subduction-related environments (e.g., Mindanao arc, Philippines; Kamchatka arc, Russia; and southern Baja California arc, Mexico). The age of the Touhuohe suite, and its correlation with Triassic NEB to the north indicates that volcanism derived from subduction-modified mantle was abundant prior to 220 Ma in the central Qiangtang terrane.     

拉萨地块中北部晚白垩世(约90Ma)拔拉扎含矿斑岩地球化学特征及其成因

拉萨地块中北部形成于90~88Ma的拔拉扎含矿斑岩具有明显的埃达克质岩特征:高SiO₂(>69%)、Al₂O₃(平均为15.89%)、Sr (平均为354×10^-6),低Y(平均为12.97×10^-6)、Yb(平均为0.95×10^-6)含量,轻重稀土强烈分异((La/Yb)_N平均为19.8);同时它们有着高Mg^#(平均为65)、Cr(平均为107×10^-6)、Ni(平均为13×10^-6)含量。研究区这些具有埃达克质岩特征的含矿斑岩并非源于俯冲洋壳、底侵或加厚下地壳部分熔融的产物,也不是玄武质岩浆结晶分异的产物,而很可能是拆沉下地壳部分熔融的结果。另一方面,南向俯冲的Slainajap洋壳或班公湖-怒江洋壳的断离也可能诱发板片窗上部的壳幔物质发生部分熔融而形成研究区的含矿斑岩。     

Geologic evolution of the Serrinha nucleus granite–greenstone terrane (NE Bahia, Brazil) constrained by U–Pb single zircon geochronology

U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9 Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15 Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07 Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6 Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement.Group 1 gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100 Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 ± 26 Ma), Araci (3072 ± 2 Ma), Requeijão (2989 ± 11 Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton.Group 2 granitoid plutons were emplaced at 2.16–2.13 Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement.     

Relationships between karst and tectonics: case-study of the cave system north of Lake Thun (Bern,Switzerland)

Abstract

The cave system is situated north of Lake Thun, in the Helvetic border chain. The overall geology is simple: the slightly dipping (15–25° towards the southeast) strata are interrupted by a NE-SW trending normal fault with a throw of 150 m in the NE and about 500 m in the SW. Since a part of the region is covered by flysch, the caves are the only way to observe the geological setting of the underlying Cretaceous and Eocene series. We show that observations in caves may yield valuable information about the onset of the tectonic movements: in particular, observations in the Barenkluft region clearly demonstrate that the beginning of préalpine extension had already begun in the Upper Cretaceous, and that this normal fault has been inverted later during Alpine compression. We also illustrate the influence of tectonic stress and strain upon karstification. The Alpine tectonic phases, with alternating compression and extension, contributed to the development of different karstogenetic levels. Tectonic strains opened and possibly closed some fractures, allowing (or preventing) water to flow through parts of the karst massive. The structural setting, defining the overall geometry of the limestone bed, played an important role in the development of the various phases of the system. Most of the conduits appear to belong to old, deep phreatic systems. Tectonics is only one of a number of factors controlling karstification. Together with lithology, it represents the geological control. Geomorphological factors (mainly spring and catchment positions, but also erosion of the flysch cover), as well as bioclimatical factors (quantity and physico-chemical characteristics of water), and hydrodynamics and transport processes can play a significant role on the genesis of karst systems. © 1999 Éditions scientifiques et médicales Elsevier SAS     

西藏西南部普兰盆地东缘伸展构造初步研究

笔者在西南西南部普兰盆地填１：１０万地质图时,发现了一条沿盆地东缘展布的新生代伸展断层,其上盘为上新世普兰组砾岩：下盘为前寒武纪聂拉木群花岗片麻岩及花岗质糜棱岩和碳酸质糜棱岩。糜棱岩的拉伸线理为ＷＮＷ向,倾角平缓。在上下盘地层中都发育大小不同,近似平行的正断层,其中一条组成盆地东缘的大陡坎。     

The lower carboniferous (dinantian) of the Swords area: Sedimentation and tectonics in the Dublin Basin,Ireland

The litho- and biostratigraphy of the Lower Dinantian succession in a deeper part of the Dublin Basin is described. The sub-Waulsortian Malahide Limestone Formation (emended) is described fully for the first time, and has proved to be very much thicker than was previously suspected, in excess of 1200 m. Succeeding the ‘Lower Limestone Shale’ unit, which is transitional from the underlying Old Red Sandstone facies, the following six new members are recognized: Turvey Micrite Member, Swords Argillaceous Bioclastic Member, St. Margaret's Banded Member, Huntstown Laminated Member, Dunsoghly Massive Crinoidal Member and Barberstown Nodular Member (top). The Malahide Limestone Formation is overlain by ‘Waulsortian’ limestones of the Feltrim Limestone Formation (new name) which form overlapping and isolated mudmounds with complex relationships with their enclosing non-mound facies. Though very much thicker, the Courceyan succession is comparable with that elsewhere on the south side of the Basin, and is part of the Kildare Province (Strogen and Somerville 1984). Isopach maps for the region show that this province and the North Midlands are separated by the deepest part of the Dublin Basin, named the ‘East Midlands Depocentre’, in which a shale-dominant facies is present. The top of the ‘Waulsortian’ is of early Chadian age. Formations younger than this are dominated by basinal calcareous shales (Tober Colleen Formation) and by storm deposits and calciturbidites with appreciable terrigenous input from the east (Rush Formation). The Courceyan main shelf conodont biozones are also greatly thickened in this area. The Pseudopolygnathus multistriatus Biozone (> 300 m thick) is succeeded by a very thick (> 900 m) Polygnathus mehli Biozone. The base of the Chadian is considered to occur below the top of the Feltrim Limestone Formation and, although equivocal, may be diagnosed in the Dublin Basin by the first appearance of the problematic microfossil Sphaerinvia piai and a primitive form of the calcareous alga Koninckopora. In the late Courceyan, the Swords area was part of a gently sloping shelf extending northwards into the basin. During deposition of the Feltrim Limestone Formation there was major deepening and there is evidence of initial break up of the Dublin Basin by faulting into separate blocks. By Chadian time the Basin was definitely subsiding by fault displacements and basinal limestones contain shallow water faunas and littoral sand and pebbles derived by turbidite flows from the margins of the higher blocks. The early subsidence was apparently by pure flexure, but in the Viséan the Dublin Basin was fault-controlled, differing from the adjacent Shannon Basin in having both margins strongly faulted.     

A review of the In Ouzzal granulitic terrane (Tuareg shield, Algeria): its significance within the Pan-African Trans-Saharan belt

The In Ouzzal terrane (IOT) or In Ouzzal granulite unit (IOGU) is an elongated Palaeoproterozoic block within the Neoproterozoic Pan-African belt of north-west Africa. The granulites derive from Archaean protoliths that include a large volume of metasediments which were deposited on a 3.2-Ga gneissic basement. Near-peak granulite facies conditions between 2.17 and 2 Ga were estimated at P=10 kbar and T rising from 800 to 1000°C. Premetamorphic orthogneisses were intruded at 2.5 Ga, and followed by the emplacement of syn- to late-kinematic charnockites, syenites and carbonatites at around 2 Ga. Cooling of the granulites occurred till 1800 Ma. Shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas, the IOT was buried beneath late Palaeoproterozoic and Neoproterozoic cover sequences, and then behaved as a rigid block. Both margins are lithospheric faults, as evidenced by the occurrence of shear-zone-related mafic and felsic plutons. Pan-African tectonothermal events were negligible in the north, but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern Pan-African terranes. The Cambrian molasse, associated with widespread alkaline volcanism and subvolcanic granites, is horizontal in the north. The IOT, which was part of a larger continental mass including its counterpart in northern Mali, is interpreted as an exotic terrane which may have docked during Pan-African plate convergence and lateral collision. The unchanged pediplain since c. 1.7 Ga in the north suggests that the IOT is underlain by thick Palaeoproterozoic lithospheric mantle, whereas its southern part is probably allochthonous and overlies Pan-African structural units.     

Facies architecture and cyclicity in a mosaic carbonate platform: effects of fault-block tectonics (Lower Lias, Causses platform, south-east France)

In the Causses platform (south‐east France), Late Hettangian to Sinemurian deposits were interpreted previously as shallow‐water carbonate ramp deposits. A new look at these deposits has shown a fault‐controlled mosaic carbonate platform that is different from the carbonate ramp models. Within the platform mosaic, 15 lithofacies have been recognized, which are organized in four facies associations, including peritidal, restricted shallow sub‐tidal, sand dunes and sub‐tidal shelf facies associations. The rapid lateral and vertical facies changes, and the lack of consistent landward or seaward direction indicated by the pattern of facies changes, question the existence of a shoreline suggested by the traditional models for this region. Instead, the facies organization and cycle stacking pattern suggest deposition in a mosaic of intertidal islands between which sub‐tidal restricted or open conditions could coexist in very close proximity. Such a platform mosaic would have been defined by tectonic activities along normal faults which segmented the shallow‐water Causses platform. The facies and facies associations are arranged into metre‐scale, peritidal and sub‐tidal cycles that are also variable. Certain cycles show the same stacking pattern in all the sections and seem to be traceable over tens of kilometres. On the contrary, other cycles cannot be correlated; they are present only in specific sections and have a maximum lateral extension of 1 or 2 km. These metre‐scale cycles stack to form four medium‐scale cycles bounded by surfaces that display sub‐aerial exposure features. Medium‐scale cycles stack into two larger‐scale cycles (tens of metres thick) and are bounded by well‐defined karstic surfaces. Based on their lateral continuity and their stacking pattern, the metre‐scale cycles are controlled probably by high frequency eustatic variations overprinting the topographic irregularities formed by differential subsidence of fault‐bounded blocks. Episodic fault activities may reorganize the topography so that, even if eustatic changes may still be the major control of cycles, the expression and number of cycles could be different. Cycles of medium and large‐scale are interpreted as being allogenic, controlled by changes in eustasy and/or subsidence rates as evidenced by their lateral continuity and the correlations of the large‐scale cycles with third‐order depositional sequences.     

Upwarped high velocity mafic crust, subsurface tectonics and causes of intraplate Latur-Killari (M 6.2) and Koyna (M 6.3) earthquakes, India – A comparative study

A unique attempt is made to understand the genesis of intraplate seismicity in the Latur-Killari and Koyna seismogenic regions of India, through derived crustal structure by synthesizing active and passive seismic, magnetotelluric, gravity and heat flow data. It has indicated presence of relatively high velocity/density intermediate granulite (and amphibolite) facies rocks underneath the Deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since Precambrian times. These findings have been independently confirmed by detailed borehole geological, geochemical and mineralogical investigations. The crystalline basement rock is found to contain 2 wt% of carbon-di-oxide fluid components. The presence of geodynamic process, associated with thermal anomalies at subcrustal depths, is supported by a high mantle heat flow (29–36 mW/m²) beneath both regions, although some structural and compositional variations may exist as evidenced by P- and S-wave seismic velocities. We suggest that the stress, caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate. These stresses appear to dominate over and above those generated by the India–Eurasia collision. The role of fluids in stress generation, as advocated through earlier studies, appears limited.     

Evolution of H2O content in a polymetamorphic terrane: the Plattengneiss Shear Zone (Koralpe, Austria)

The Koralpe of the Eastern European Alps experienced high-temperature/low-pressure metamorphism (∼650 °C and 6.5 kbar) during the Permian and eclogite facies metamorphism (∼700 °C and 14 kbar) during the Eo-Alpine (Cretaceous) metamorphic event. In the metapelitic Plattengneiss shear zone that constitutes much of the Koralpe, the second metamorphism caused only partial re-equilibration of the assemblages formed during the first metamorphism. It is shown here that the Eo-Alpine mineral assemblage, garnet + biotite + muscovite + plagioclase + quartz (with or without kyanite), formed under low water activity conditions that are consistent with the level of dehydration that occurred during the Permian event. This implies that the rocks were essentially closed-system from the peak of the Permian metamorphism through the Eo-Alpine event. The evolution of water content of the rocks is traced through time: that prograde dewatering during the Permian metamorphic event terminated at the metamorphic peak with a water content around 3–4 mol.%. The water content remained then constant and led to water-undersaturation during the subsequent Eo-Alpine metamorphism. From the water content and activity evolution a post-peak isothermal decompression path close to the solidus is inferred for the Eo-Alpine event.