Summary The Cyclops massif (Irian Jaya - Western Indonesia) displays all components of an ophiolitic sequence including residual mantle peridotites (harzburgites and dunites), cumulate gabbros, dolerites, normal mid-oceanic ridge basalts (N-MORB) and minor amounts of boninitic lavas. This ophiolitic series tectonically overlies high temperature (HT)-high pressure (HP) mafic rocks metamorphosed during the Miocene.Mineral chemistry and bulk rock rare-earth element (REE) abundances of the peridotites are characteristic of highly residual mantle rocks. The high Cr# [Cr#=100*Cr/(Cr+Al)] of spinel (up to 60) and very low heavy rare-earth element (HREE) concentrations of peridotites (< 0.1 time the chondritic values) are in agreement with residues of 25 to 35% melting as expected for peridotites from supra-subduction zone environments. Ti-enrichments in spinels and secondary clinopyroxenes (up to 1%, and 0.5%, respectively) are likely a consequence of reaction between mantle-derived melts and the host peridotites. High light rare-earth element (LREE) concentrations reaching up to chondritic values and high field strength element (HFSE) anomalies suggest that the initial composition of the residual peridotites has been previously modified by the passage of boninitic melt(s). The associated basalts and related cumulate rocks display major and trace element contents with Nb-negative anomalies typical of back-arc magmas.New40K/40Ar isotopic ages obtained from the back-arc basin basalts (BABB - 29 Ma) and boninites (43 Ma) combined with the geochemical signatures of the rocks studied here, indicate that the Cyclops Mountains may have formed in a single suprasubduction environment. This implies southward plunging subduction of the Australian oceanic lithosphere beneath the northern part of the Australian margin. The ultramafic rocks and related lavas (boninites) likely formed during the Eocene in a forearc environment, before their southward obduction onto the island arc crustal welt during the early Miocene. The Pliocene back-thrusting event has led to the slicing of the backarc basin series onto the arc and fore-arc sequences.
Le massif des Cyclops (Irian Jaya- Ouest Indonésie montre tous les termes d'une séquence ophiolitique comprenant des péridotites résiduelles (harzburgites et dunites), des cumulats gabbroïques, des dolérites, des basaltes de type N-MORB et de rares boninites. Cette série, ophiolitique repose tectoniquement sur des roches mafiques métamorphisées à haute température au Miocene.
Résumé Les données pétrologiques et géochimiques montrent clairement que les péridotites ont un caractère fortement résiduel. Les fortes teneurs en Cr# [Cr#= 100*Cr/(Cr+Al)] du spinelle (> 60) associées aux très faibles concentrations en terres rares lourdes sur roche totale (<0.1 aux valeurs chondritiques) témoignent de fort taux de fusion (25 à 35%) que l'on rencontre habituellement dans les contextes de subduction. Les enrichissements importants en TiO2 des spinelles et clinopyroxènes secondaires des peridotites (> 1 % et 0.5%, respectivement) sent interprétés comme résultant de phénomènes d'imprégnations importants entre les péridotites et des liquides magmatiques. Les fortes concentrations en terres rares légères des péridotites (proches des valeurs chondritiques) associées aux fortes anomalies en Nb, Sr, Zr, et Hf suggerent que ces liquides étaient de nature boninitique. Les basaltes et les cumulats gabbroïques dérivent de la cristallisation de liquides tholéiitiques de type MORB. Leurs fortes anomalies en Nb, suggerènt cependant une origine dans un bassin arrière-arcDe nouvelles datations isotopiques40K40Ar obtenues sur les basaltes arrière-arch (29 Ma) et les boninites (43 Ma) montrent que le massif des Cyclops s' est probablement formé dans un contexte de zone de subduction impliquant une subduction vers le Sud de la lithosphere océanique australienne sous la marge nord australienne. Les péridotites et laves associées (boninites) se seraient formées à l'Eocène dans un bassin avant-arc, avant d'être obductées au Miocène sur l'are situé plus au sud. Les rétrochevauchements Pliocène ont conduit aux charriages tardifs du bassin arrière-arc sur l'arc et le bassin avant-arc.
In this study, we analyse the susceptibility to liquefaction of the Pozzone site, which is located on the northern side of the Fucino lacustrine basin in central Italy. In 1915, this region was struck by a M 7.0 earthquake, which produced widespread coseismic surface effects that were interpreted to be liquefaction-related. However, the interpretation of these phenomena at the Pozzone site is not straightforward. Furthermore, the site is characterized by an abundance of fine-grained sediments, which are not typically found in liquefiable soils. Therefore, in this study, we perform a number of detailed stratigraphic and geotechnical investigations (including continuous-coring borehole, CPTu, SDMT, SPT, and geotechnical laboratory tests) to better interpret these 1915 phenomena and to evaluate the liquefaction potential of a lacustrine environment dominated by fine-grained sedimentation. The upper 18.5 m of the stratigraphic succession comprises fine-grained sediments, including four strata of coarser sediments formed by interbedded layers of sand, silty sand and sandy silt. These strata, which are interpreted to represent the frontal lobes of an alluvial fan system within a lacustrine succession, are highly susceptible to liquefaction. We also find evidence of paleo-liquefaction, dated between 12.1–10.8 and 9.43–9.13 kyrs ago, occurring at depths of 2.1–2.3 m. These data, along with the aforementioned geotechnical analyses, indicate that this site would indeed be liquefiable in a 1915-like earthquake. Although we found a broad agreement among CPTu, DMT and shear wave velocity “simplified procedures” in detecting the liquefaction potential of the Pozzone soil, our results suggest that the use and comparison of different in situ techniques are highly recommended for reliable estimates of the cyclic liquefaction resistance in lacustrine sites characterized by high content of fine-grained soils. In geologic environments similar to the one analysed in this work, where it is difficult to detect liquefiable layers, one can identify sites that are susceptible to liquefaction only by using detailed stratigraphic reconstructions, in situ characterization, and laboratory analyses. This has implications for basic (Level 1) seismic microzonation mapping, which typically relies on the use of empirical evaluations based on geologic maps and pre-existing sub-surface data (i.e., age and type of deposits, prevailing grain size, with particular attention paid to clean sands, and depth of the water table). 相似文献
In radioastronomy the interferometric measurement between radiotelescopes located relatively close to each other helps removing ionospheric effects. Unfortunately, in case of networks such as LOw Frequency ARray (LOFAR), due to long baselines (currently up to 1500 km), interferometric methods fail to provide sufficiently accurate ionosphere delay corrections. Practically it means that systems such as LOFAR need external ionosphere information, coming from Global or Regional Ionospheric Maps (GIMs or RIMs, respectively). Thanks to the technology based on Global Navigation Satellite Systems (GNSS), the scientific community is provided with ionosphere sounding virtually worldwide. In this paper we compare several interpolation methods for RIMs computation based on scattered Vertical Total Electron Content measurements located on one thin ionospheric layer (Ionospheric Pierce Points—IPPs). The results of this work show that methods that take into account the topology of the data distribution (e.g., natural neighbour interpolation) perform better than those based on geometric computation only (e.g., distance-weighted methods). 相似文献
During the Cretaceous, high global sea-level and low latitudinal temperature variations led to the growth of epeiric carbonate platforms. Platform-scale dolomitization of these platforms is not common, reflecting the low Mg/Ca ratio of seawater and a humid climate. This study describes the processes governing pervasive dolomitization of a land-attached carbonate platform within the Iberian Basin. Dolomite is planar to sub-planar with a geochemical signature consistent with dolomitization from penesaline seawater. Dolomitization was most pervasive during a 1 Myr period in the middle Cenomanian, by repeated reflux of seawater from brine pools formed on the top of a southward-prograding carbonate platform. Tilting and structural reorganization in the Upper Cenomanian led to a reversal in polarity of the platform, and dolomitization was restarted by the northward reflux of seawater. Rising relative sea-level and oceanic acidification led to back-stepping of the platform such that the supply of dolomitizing fluids was cut off. In the Lower Turonian, pervasively dolomitized rudist rudstone facies in the south of the study area indicate that dolomitization restarted, either penecontemporaneously or later, from highly evaporated Campanian–Maastrichtian seawater. A systematic increase in dolomite crystal size up-section ties broadly, but not entirely, to stratigraphy. It is possible that these textural differences reflect changes in fluid chemistry, limestone permeability or precursor rock texture. However, the lack of stratigraphic conformance, and the preservation of the earliest-formed dolomite only in the oldest sediments, could indicate a progressive recrystallization of early-formed dolomite through repeated reflux of brines. As such, the succession appears to preserve a fossilized record of dolomite recrystallization through time during the Cenomanian–Turonian. The results of this study therefore provide a record of the progressive dolomitization of a carbonate platform and demonstrate the important interplay of climate and basin-scale tectonics on dolomite distribution and crystallinity. 相似文献
The mineral water of Vilajuïga village in Alt Empordà (NE Catalonia, Spain) owes its uniqueness to an emanation of geogenic CO2 that modifies groundwater hydrochemistry to produce a differentiated HCO3–Na- and CO2-rich groundwater among the usual Ca–HCO3 type found in this region. A hydrogeological conceptual model attributes its occurrence to the intersection of two faults: La Valleta and Garriguella-Roses. The former provides a thrust of metamorphic over igneous rocks, formed during the Paleozoic, over a layer of ampelitic shale that, from a hydrogeological perspective, acts as a confining layer. The Garriguella-Roses normal fault, which originated during the Neogene, permits the degassing of geogenic CO2 that is attributed to volcanic activity occurring in the Neogene. Groundwater mixing from the metamorphic and igneous rock units plus the local occurrence of CO2 creates a HCO3–Na water that still holds free-CO2 in solution. Interaction with the gas phase is restricted at the intersection of the two faults. Radiocarbon dating, after correcting for geogenic dead carbon, estimates an age of 8,000 years BP. The low tritium content (0.7 TU) indicates that Vilajuïga water is a mix of “older” groundwater recharged in the metamorphic rocks of the Albera range and “younger” groundwater from the igneous rocks of the Rodes range, over a recharge area of 45 km2 and a maximum elevation of 600 m. Given its origin as rare groundwater in the southern slope of the Eastern Pyrenees, purposeful monitoring is necessary to evaluate the groundwater vulnerability and anticipate impacts from nearby wells and climate-change effects.