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Experimental modelling of forearc basin development during accretionary wedge growth 总被引：1，自引：0，他引：1
CHRISTOPHE Larroque SYLVAIN Calassou† JACQUES Malavieille† FRANK Chanier‡ 《Basin Research》1995,7(3):255-268
We present results of three sand-box experiments that model the association between tectonic accretion and sedimentation in a forearc basin. Experimental sedimentation occurs step by step in the forearc basin during shortening of the sand wedge. In each experiment, the development of the accretionary wedge leads to the formation of a major backthrust zone. This major deformation zone accounts for the thickening in the rear part of the wedge. In natural settings this tectonic bulge dams sediments that are transported toward the trench from mountainous terrain behind the forearc. We test the variation of friction along the déollement and note the following: (1) shortening of a low-friction wedge involves a mechanical balance between forethrusts and backthrust propagation and this balance is recorded by the sedimentary sequence trapped in the forearc basin. Indeed, if most of the movement occurs along the backthrust, the deepening of the basin will be larger and consequently the thickness of the sedimentary sequence will be greater. (2) Such balance does not exist in the case of a high-friction wedge. (3) Variation of friction along the décollement during shortening of the sand wedge leads to modification in the forearc basin filling. Thus, for similar increments of convergence, the sequence deposited in the forearc basin shows relatively larger thickness when the wedge is shortened above a high-friction décollement. We suggest that contraction and thickening in the rear part of the wedge is an efficient mechanism to, initiate and develop a forearc basin. Thus, this kind of basin occurs in convergent settings, without collapse related to local extension or tectonic erosion. They represent a sedimentary trap on a passive basement, bounded by a tectonic bulge. The Quaternary Hikurangi forearc basin, southeast of the North Island of New Zealand, is bounded by two actively uplifting ridges. Thus, this basin is considered to be a possible example of the basins modelled in our experiments, and we suggest that the limit between the basin and the wedge could be a complex backthrust zone. 相似文献
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The Tillaberi stone fell in April 1970 in Republic of Niger. In the 157 g piece sent to Paris, a centimeter wide lithic inclusion is found. Contrary to previous assumptions, the stone is an L6 chondrite in which few chondrules can be distinguished; olivine is Fa 25 ± 0.4 percent with minor amounts of Ca, Cr, Ti; orthopyroxene is Fs 21.6 ± 1.2 percent with a rather large scatter of the amounts of minor elements. The feldspar, well developed, contains 10 to 11 percent An, 84 to 85 percent Ab, five percent Or, and 0.77 ± 0.09 percent FeO. The lithic inclusion contains much feldspar which corresponds to 14 to 17 percent An, 75 to 79 percent Ab, four to five percent Or without almost any FeO. This inclusion contains also rounded grains of olivine and minute crystals of chromite. It has a frothy microdoleritic texture with a frozen border against the chondritic material. As the nickel rich grains are either martensite or acicular plessite and the silicates are undeformed, a quick cooling after a short but intense heating is postulated to account for the mineralogical characteristics. 相似文献
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RAPHAËL BOURILLOT EMMANUELLE VENNIN JEAN‐MARIE ROUCHY CHRISTOPHE DURLET VINCENT ROMMEVAUX CHRISTOPHE KOLODKA FREDERIC KNAP 《Sedimentology》2010,57(2):477-512
The Sorbas Member is a late Messinian complex sedimentary system that formed immediately following deposition of the Messinian evaporites in the Sorbas Basin (South‐east Spain). This work describes the sequence architecture and facies organization of a continuous kilometre long, alluvial fan to open platform transect near the village of Cariatiz in the north‐east of the basin. The post‐evaporitic Cariatiz platform was a mixed carbonate‐siliciclastic system composed of four intermediate‐frequency, fifth‐order depositional sequences (Depositional Sequences 1 to 4) arranged in an overall prograding trend. The intense fracturing and brecciation of these deposits is attributed to the deformation and dissolution of an evaporite body measuring several tens of metres in thickness. The four sequences display significant spatial–temporal variability in both architecture and facies distribution, with two main phases: (i) Depositional Sequences 1 and 2 are ooid and oobioclastic dominated, and show normal marine faunas; and (ii) Depositional Sequences 3 and 4 show a higher siliciclastic contribution and are microbialite dominated. These important changes are interpreted as modifications of the primary controlling factors. Following an initial 70 m drowning, possibly linked to increased oceanic input, Depositional Sequences 1 to 3 were controlled mainly by eustatic variations and inherited topography; their progradation destabilized the evaporite body near the end of the Depositional Sequence 2 period. During the second phase, Depositional Sequences 3 and 4 recorded a progressive restriction of the Sorbas Basin related to a 30 to 40 m fall in water level that was driven mainly by regional factors. These regional factors were dissolution and gravity‐induced deformation of the evaporites and correlative evaporative fluid circulation associated with the contrasted arid/humid regional climate that, respectively, controlled sequence geometry and fluctuating water salinity which caused a microbialite bloom. 相似文献
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Synsedimentary faulting and palaeocurrent patterns in the Triassic sandstones of the High Atlas (Morocco) 总被引：1，自引：0，他引：1
The influence of syndepositional fault patterns on palaeocurrents is demonstrated in fluvial to shallow-marine sandstones of Upper Triassic basins in the High Atlas. The synsedimentary nature of faults is deduced from hydroplastic slickensides, thickness variations due to block tilting and dislocation of layers next to the fault scarp. On a regional scale, it is shown that the major normal fault trend of N050–070° controlled the overall palaeocurrent pattern which was directed towards the west-southwest, i.e. in the direction of the future Atlantic ocean. Some anomalies in the palaeocurrent pattern could be related to an increase in subsidence which induced a general coarsening of sediment towards the top of the Triassic. 相似文献
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R. B. SCORZELLI M. CHRISTOPHE MICHEL-LVY E. GILABERT B. LAVIELLE I. SOUZA AZEVEDO V. W. VIEIRA T. V. V. COSTA M. A. B. ARAÚJO 《Meteoritics & planetary science》1998,33(6):1335-1337
Abstract— The Campos Sales meteorite fell close to the town of Campos Sales in the northeastern Brazilian state of Ceará (7°2′ S, 40°10′ W) on 1991 January 31 at 10:00 P.M. (local time). Several fragments were recovered from an area estimated to be 1 × 3 km. The stone is an ordinary L5 chondrite (Fa25.0 and FS21.6) and is lightly shocked (S1). Metal phases present are kamacite, tetrataenite, and antitaenite. Noble gases He, Ne, Ar, Kr, and Xe have been analyzed in two bulk samples of Campos Sales. All exposure ages based on determination of cosmogenic 3He, 21Ne, 38Ar, 83Kr, and 126Xe abundances and on the cosmogenic 81Kr/83Kr ratio agree well, which suggests no gas loss during cosmic-ray exposure. The cosmic-ray exposure age is 23.3 ± 1.0 Ma, which falls in the range observed for L5 chondrites (20–30 Ma). The gas-retention ages indicate He loss that must have occurred prior to or during ejection from the L-chondrite parent body. 相似文献
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A semi-empirical model has been developed to reproduce glacial–interglacial changes of continental dust and marine sodium concentrations (factor of ∼50 and ∼5, respectively) observed in inland Antarctic ice cores. The model uses conceptual pathways of aerosols within the high troposphere; assumes the dry deposition of impurities on the Antarctic surface; uses estimates of aerosol transit times taken independent of climate; assumes a temperature-dependent removal process during aerosol pathways from the mid-latitudes. The model is fitted to the data over the last four climate cycles from Vostok and EPICA Dome C Antarctic sites. As temperature is cooling, the aerosol response suggests different modes of climate couplings between latitudes, which can be continuous or below temperature thresholds for sodium and dust, respectively. The model estimates a southern South America dust source activity two to three times higher for glacial periods than for the Holocene and a glacial temperature over the Southern Ocean 3–5 °C cooler. Both estimates appear consistent with independent observations. After removal of temperature effects, dust and sodium residuals for both sites show orbital frequencies in opposite phase at the precession timescale. Such long-term insolation-related modulation of terrestrial and marine aerosol input, could provide a chemical pacemaker useful for refining ice core chronologies. 相似文献
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CORDIER CAROLE; CLEMENT JEAN-PHILIPPE; CAROFF MARTIAL; HEMOND CHRISTOPHE; BLAIS SYLVAIN; COTTEN JOSEPH; BOLLINGER CLAIRE; LAUNEAU PATRICK; GUILLE GERARD 《Journal of Petrology》2005,46(11):2281-2312
This study is based on a set of coarse-grained igneous rockscollected from two zoned plutons located in the central partof Tahiti Nui and Raiatea. The Ahititera pluton (central depressionof Tahiti Nui) comprises a great diversity of rocks, rangingfrom ultrabasic to felsic in composition. It shows a concentriczonation with nepheline-free rocks in its periphery and nepheline-bearingrocks in its central part. The Faaroa pluton (central depressionof Raiatea) is entirely mafic and includes only gabbros andtheralites. The two plutons have variable Nd–Sr isotopicsignatures, especially the Ahititera rocks, which are subdividedinto three groups based on their mineralogy, geochemistry andisotope composition. The isotopic variability probably reflectslocal heterogeneities in the Society mantle plume. Petrographicand isotopic data have been used to define two magmatic suitesin Ahititera, identifiable from their degree of Si undersaturation.The evolution of the mildly Si-undersaturated suite is controlledby simple fractional crystallization, whereas the strongly Si-undersaturatedsuite requires additional H2O influx. The third isotopic groupincludes only theralites. The rare earth element (REE) compositionsof the mafic rocks from both plutons do not correlate with theirisotopic signature. The REE patterns of the most Si-undersaturatedrocks are systematically characterized by steeper slopes. Suchfeatures are also observed in lavas from seamounts located withinthe present-day hotspot area. It appears that REE concentrationsin Society lavas and intrusives are probably mainly governedby variable degrees of partial melting of a garnet-free mantlesource and are independent of their isotopic signature. KEY WORDS: cumulates; fractional crystallization; partial melting; French Polynesia; plutonic rocks; Society Islands; Tahiti; Raiatea 相似文献