Strong earthquake occurrence (M ≥ 6.0) onshore and offshore the Cyprus Island constitutes significant seismic hazard because they occur close to populated areas. Seismicity is weak south of the Island along the Cyprean Arc and strong events are aligned along the Paphos transform fault and Larnaka thrust fault zone that were already known and the Lemessos thrust fault zone that defined in the present study. By combining the past history of strong (M ≥ 6.0) events and the long-term tectonic loading on these major fault zones, the evolution of the stress field from 1896 until the present is derived. Although uncertainties exist in the location, magnitude and fault geometries of the early earthquakes included in our stress evolutionary model, the resulting stress field provides an explanation of later earthquake triggering. It was evidenced that the locations of all the strong events were preceded by a static stress change that encouraged failure. The current state of the evolved stress field may provide evidence for the future seismic hazard. Areas of positive static stress changes were identified in the southwestern offshore area that can be considered as possible sites of future seismic activity. 相似文献
Both adakitic and shoshonitic igneous rocks in the Luzong area, Anhui Province, eastern China are associated with Cretaceous Cu–Au mineralization. The Shaxi quartz diorite porphyrites exhibit adakite-like geochemical features, such as light rare earth element (LREE) enrichment, heavy REE (HREE) depletion, high Al2O3, MgO, Sr, Sr / Y and La / Yb values, and low Y and Yb contents. They have low εNd(t) values (− 3.46 to − 6.28) and high (87Sr / 86Sr)i ratios (0.7051–0.7057). Sensitive High-Resolution Ion Microprobe (SHRIMP) zircon analyses indicate a crystallization age of 136 ± 3 Ma for the adakitic rocks. Most volcanic rocks and the majority of monzonites and syenites in the Luzong area are K-rich (or shoshonitic) and were also produced during the Cretaceous (140–125 Ma). They are enriched in LREE and large-ion lithophile elements, and depleted in Ti, and Nb and Ba and exhibit relatively lower εNd(t) values ranging from − 4.65 to − 7.03 and relatively higher (87Sr / 86Sr)i ratios varying between 0.7057 and 0.7062. The shoshonitic and adakitic rocks in the Luzong area have similar Pb isotopic compositions (206Pb / 204Pb = 17.90–18.83, 207Pb / 204Pb = 15.45–15.62 and 208Pb / 204Pb = 38.07–38.80). Geological data from the Luzong area suggest that the Cretaceous igneous rocks are distributed along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China and were likely formed in an extensional setting within the Yangtze Block. The Shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of > 50 km (i.e., 1.5 GPa), possibly leaving rutile-bearing eclogitic residue. The shoshonitic magmas, in contrast, originated mainly from an enriched mantle metasomatized by subducted oceanic sediments. They underwent early high-pressure (> 1.5 GPa) fractional crystallization at the boundary between thickened (> 50 km) lower crust and lithospheric mantle and late low-pressure (< 1.5 GPa) fractional crystallization in the shallow (< 50 km) crust. The adakitic and shoshonitic rocks appear to be linked to an intra-continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along NE fault zones (e.g., Tanlu and Yangtze River fault zones) in eastern China. Both the shoshonitic and adakitic magmas were fertile with respect to Cu–Au mineralization. 相似文献
Fractional crystallization of peraluminous F- and H2O-rich granite magmas progressively enriches the remaining melt with volatiles. We show that, at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity. These melt fractions also fractionate the incompatible elements to significantly different degrees. Coexisting melt fractions have differing chemical and physical properties and, due to their high density and viscosity contrasts, they will tend to separate readily from each other. Once separated, each melt fraction evolves independently in response to changing T/P/X conditions and further immiscibility events may occur, each generating its own conjugate pair of melt fractions. The strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained. Consequently, the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions.
We demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore-forming and accessory minerals, and are visible on scales from a few micrometers to hundreds of meters. 相似文献
We report three new localities of corundum and sapphirine-bearing hyper aluminous Mg-rich and silica-poor ultrahigh-temperature granulites formed during Late Neoproterozoic-Cambrian times within the Palghat–Cauvery Shear Zone system in southern India. From petrologic characteristics, mineral chemistry and petrogenetic grid considerations, the peak metamorphic conditions of these rocks are inferred to lie around 950–1000 °C (as suggested by Al in orthopyroxene thermometer) at pressures above 10 kbar (as indicated by the equilibrium orthopyroxene–sillimanite–gedrite ± quartz assemblage). These rocks preserve several remarkable reaction textures, the most prominent among which is the triple corona of spinel–sapphirine–cordierite on corundum, with the whole textural assembly embedded within the matrix of gedrite, suggesting the reaction: Ged + Crn = Spl + Spr + Crd. The formation of sapphirine–sillimanite assemblage/symplectite associated with relict corundum and porphyroblasitc cordierite is explained by the reaction: Crd + Crn = Spr + Sil. The association of sapphirine cordierite symplectite with gedrite–sillimanite assemblage as well as with aluminosilicate boundaries indicates the gedrite consuming reaction: Ged + Sil = Spr + Crd. Extensive growth of sapphirine–cordierite observed on the rim of gedrite porphyroblasts with spinel occurring as relict inclusions within the sapphirine indicates the reaction: Ged + Spl = Spr + Crd. The pressure–temperature (P–T) path defined from the observed mineral assemblages and reaction texture is characterized by anticlockwise trajectory, with a prograde segment of initial heating and subsequent deep burial, followed by retrograde near-isothermal decompression. Such an anticlockwise trajectory is being reported for the first time from southern India and has important tectonic implications since these rocks were developed at the leading edge of the crustal block that was involved in collisional orogeny and subsequent extension during the final phase of assembly of the Gondwana supercontinent. We propose that the rocks were subjected to deep subduction and rapid exhumation, and the extreme thermal conditions were attained either through input from underplated mantle-derived magmas, or convective thinning or detachment of the lithospheric thermal boundary layer during or after crustal thickening. 相似文献
The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P–T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism.
Abundant H2O–CO2, H2O- or CO2-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H2O- and/or CO2-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation. 相似文献
The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil.
Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the εNd values and TDM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components.
The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, TDM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga.
The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block. 相似文献
Comparative analysis of taxonomic diversity dynamics of condontophorids from Boreal (Arctic regions of Russia) and Tethyan (Northwest Pacific) paleobasins showed that they had most favorable habitat environments in tropical seas. In the Boreal realm, condontophorids went through three stages of evolution comprising probably four substages and four phases, whereas three stages with six substages and twelve phases are distinguished in the Tethyan realm. The most important abiotic factors that controlled development of conodontophorids are paleotemperature of seawater and paleogeographic settings. Renewals in taxonomic composition conodontophorids and diversification of their assemblages have been confined to moments of paleotemperature and/or sea level rise. The comparative analysis of stages in evolution of conodontophorid and bivalve assemblages has been carried out. As is established, the peak taxonomic diversity of bivalves in Boreal seas was in the Late Triassic after the diversity minimum of the Early Triassic time. In contrast, conodontophorids were most diverse in the Olenekian Age. 相似文献
Wave-cut pluvial shoreline scarps are ideal natural experiments in hillslope evolution because the ages of these scarps are often precisely known and because they form with a range of heights, alluvial textures, and microclimates (i.e., orientation). Previous work using midpoint-slope methods on pluvial scarps in the Basin and Range concluded that scarp evolution is nonlinear and microclimatically controlled. The purpose of this study was to further examine the influence of scarp height, texture and microclimate in an attempt to calibrate a nonlinear model of scarp evolution. To do this, over 150 profiles of the Bonneville shoreline in the adjacent Snake and Tule Valleys, west-central Utah were collected and analyzed by fitting the entire scarp profile to diffusion-equation solutions, taking into account uncertainty in the initial scarp angle. In contrast to previous studies, this analysis revealed no evidence for nonlinearity or microclimatic control. To understand the reason for this discrepancy, we undertook a systematic study of the accuracy of each scarp-analysis method. The midpoint-slope-inverse method was found to yield biased results, with systematically higher diffusion ages for young, tall scarps. The slope-offset method is unbiased but has limited resolution because it requires many scarp profiles to yield a single diffusion age. A method that incorporates the full scarp profile and uncertainty in the initial scarp angle was found to be the most accurate technique. The application of the full-scarp method to the Bonneville shoreline supports the use of a linear diffusion model for scarps up to 20 m in height. Scarp orientation had no discernable effect on diffusivity values. Soil texture was found to have a weak but significant inverse relationship with diffusivity values. 相似文献
The barrier islands of Inhaca and Bazaruto are related to the extensive coastal dune system of the Mozambican coastal plain, south-east Africa. Optically stimulated luminescence (OSL) dating of key stratigraphic units indicates that accretion of sediment within these systems is episodic. Both islands appear to have been initiated as spits extending from structural offsets in the coastline. Superposition of significant quantities of sediment upon these spits during subsequent sea-level highstands formed the core of the islands, which were anchored and protected by beachrock and aeolianite formation. At least two distinct dune-building phases occurred during Marine Oxygen Isotope Stage (MIS) 5, tentatively attributed to marine transgressions during sub-stages 5e and 5c. Although some localized reactivation of dune surfaces occurred prior to the Holocene, large quantities of sediment were not deposited on either island during the low sea-levels associated with MIS 2. Significant dune-building and sediment reworking occurred immediately prior to and during the Holocene, though it is not clear whether these processes were continuous or episodic. Significant erosion of the eastern shoreline of Bazaruto suggests that it is far less stable than Inhaca and may suffer further large-scale erosion. A model is presented for the formation of barrier islands along the Mozambican coastal plain. 相似文献
