Extreme Nd isotope homogeneity in a large rhyolitic province: the Estérel massif, southeast France     

Franck Poitrasson  Christian Pin 《Bulletin of Volcanology》1998,60(3):213-223

 Previous detailed studies of large rhyolite bodies propose that their elemental and isotopic characteristics were largely acquired in shallow crustal magma chambers. This model explains the common chemical and isotopic zonations of large volumes of rhyolites as well as the less common chemical and isotopic homogeneity of such bodies. We report an intermediate situation (the Estérel massif, southeast France) in which chemical variations contrast with Nd-isotope homogeneity. We thus infer that, in this case, large volumes of rhyolite resided for enough time in shallow magma chambers to develop chemical zonations through differentiation, but this process was not accompanied by crustal assimilation. The subordinate amount of mafic rocks cropping out in the Estérel probably evolved from basalt to trachyte through assimilation and fractional crystallization. The relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust show that it cannot have been generated by partial melting of exposed basement rocks. Several geological similarities with large rhyolitic provinces could suggest that the rhyolite was purely mantle derived or, alternatively, generated by partial melting of an ad hoc crustal component. However, mineralogical, geochemical, and geodynamic connections between the Estérel rhyolite and the hypersolvus anorogenic granites of Corsica, as well as the extreme Nd-isotope homogeneity of the rhyolite, lead us to propose that the rhyolite was generated by mixing between mantle-derived magmas and a mafic lower crust. This scenario accounts for the relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust. The good Nd-isotope homogeneity observed in the rhyolite implies that the mixing process, which occurred in the deep crust, was complete and provided a shallow magma chamber with isotopically and probably chemically homogeneous magmas. Received: 5 December 1997 / Accepted: 16 June 1998  相似文献   

Silicon dynamics within an intense open-ocean diatom bloom in the Pacific sector of the Southern Ocean     

Mark A. Brzezinski  David M. Nelson  Valerie M. Franck  Daniel E. Sigmon 《Deep Sea Research Part II: Topical Studies in Oceanography》2001,48(19-20)

An intense diatom bloom developed within a strong meridional silicic acid gradient across the Antarctic Polar Front at 61°S, 170°W following stratification of the water column in late October/early November 1997. The region of high diatom biomass and the silicic acid gradient propogated southward across the Seasonal Ice Zone through time, with the maximum diatom biomass tracking the center of the silicic acid gradient. High diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of January 1998 (ca. 70 d) driving the gradient over 500 km to the south of its original position at the Polar Front. The bloom consumed 30 to >40 μM Si(OH)₄ in the euphotic zone between about 60 and 66°S leaving near surface concentrations <2.5 μM and occasionally <1.0 μM in its wake. Integrated biogenic silica concentrations within the bloom averaged 410 mmol Si m⁻² (range 162–793 mmol Si m⁻²). Average integrated silica production on two consecutive cruises in December 1997 and January 1998 that sampled the bloom while it was well developed were 27.5±6.9 and 22.6±20 mmol Si m⁻² d⁻¹, respectively. Those levels of siliceous biomass and silica production are similar in magnitude to those reported for ice-edge diatom blooms in the Ross Sea, Antarctica, which is considered to be among the most productive regions in the Southern Ocean. Net silica production (production minus dissolution) in surface waters during the bloom was 16–21 mmol Si m⁻² d⁻¹, which is sufficient for diatom growth to be the cause of the southward displacement of the silicic acid gradient. A strong seasonal change in silica dissolution : silica production rate ratios was observed. Integrated silica dissolution rates in the upper 100–150 m during the low biomass period before stratification averaged 64% of integrated production. During the bloom integrated dissolution rates averaged only 23% of integrated silica production, making 77% of the opal produced available for export to depth. The bloom ended in late January apparently due to a mixing event. Dissolution : production rate ratios increased to an average of 0.67 during that period indicating a return to a predominantly regenerative system.Our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around Antarctica also occur in the deep ocean near the Polar Front. The bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of Antarctica implying a role for such blooms in the formation of those sediments. Comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the Southern Ocean, which have been corrected for sediment focusing, indicate a burial efficiency of 4.6% for biogenic silica. That efficiency is considerably lower than previous estimates for the Southern Ocean.  相似文献   

Volcanic hazards at Mount Semeru,East Java (Indonesia), with emphasis on lahars   总被引：1，自引：0，他引：1  

Jean-Claude Thouret  Franck Lavigne  Hiroshi Suwa  Bambang Sukatja  Surono 《Bulletin of Volcanology》2007,70(2):221-244

Mt. Semeru, the highest mountain in Java (3,676 m), is one of the few persistently active composite volcanoes on Earth, with a plain supporting about 1 million people. We present the geology of the edifice, review its historical eruptive activity, and assess hazards posed by the current activity, highlighting the lahar threat. The composite andesite cone of Semeru results from the growth of two edifices: the Mahameru ‘old’ Semeru and the Seloko ‘young’ Semeru. On the SE flank of the summit cone, a N130-trending scar, branched on the active Jonggring-Seloko vent, is the current pathway for rockslides and pyroclastic flows produced by dome growth. The eruptive activity, recorded since 1818, shows three styles: (1) The persistent vulcanian and phreatomagmatic regime consists of short-lived eruption columns several times a day; (2) increase in activity every 5 to 7 years produces several kilometer-high eruption columns, ballistic bombs and thick tephra fall around the vent, and ash fall 40 km downwind. Dome extrusion in the vent and subsequent collapses produce block-and-ash flows that travel toward the SE as far as 11 km from the summit; and (3) flank lava flows erupted on the lower SE and E flanks in 1895 and in 1941–1942. Pyroclastic flows recur every 5 years on average while large-scale lahars exceeding 5 million m³ each have occurred at least five times since 1884. Lumajang, a city home to 85,000 people located 35 km E of the summit, was devastated by lahars in 1909. In 2000, the catchment of the Curah Lengkong River on the ESE flank shows an annual sediment yield of 2.7 × 10⁵ m³ km⁻² and a denudation rate of 4 10⁵ t km⁻² yr⁻¹, comparable with values reported at other active composite cones in wet environment. Unlike catchments affected by high magnitude eruptions, sediment yield at Mt. Semeru, however, does not decline drastically within the first post-eruption years. This is due to the daily supply of pyroclastic debris shed over the summit cone, which is remobilised by runoff during the rainy season. Three hazard-prone areas are delineated at Mt. Semeru: (1) a triangle-shaped area open toward the SE has been frequently swept by dome-collapse avalanches and pyroclastic flows; (2) the S and SE valleys convey tens of rain-triggered lahars each year within a distance of 20 km toward the ring plain; (3) valleys 25 km S, SE, and the ring plain 35 km E toward Lumajang can be affected by debris avalanches and debris flows if the steep-sided summit cone fails.  相似文献   

Revised seismic history of the El Pilar fault,Northeastern Venezuela,from the Cariaco 1997 earthquake and recent preliminary paleoseismic results     

Franck A. Audemard 《Journal of Seismology》2007,11(3):311-326

In light of the July 9, 1997, Cariaco earthquake, it is clearly understood now that damage in the city of Cumaná – located in northeastern Venezuela and frequently destroyed by the largest earthquakes since the first recorded event in 1530 – is strongly enhanced by poor soil conditions that, in turn, are responsible for site amplification and widespread earthquake-induced effects. Therefore, most previous macroseismic studies of historical earthquakes must be revaluated because those localized high-intensity values at Cumaná surely led to the misestimation of past epicenters. Preliminary paleoseismic results, gathered at three exploratory trenches dug across the surface break of the Cariaco 1997 earthquake in 1998, allow us to associate the 1684 earthquake with this recently ruptured fault segment that extends between the towns of San Antonio del Golfo and Río Casanay (roughly between the two gulfs of Cariaco and Paria, state of Sucre). Other major results from the reassessment of the seismic history of this fault are: (a) the 1766 event seems to have generated in a different source to the El Pilar fault because the size of the felt area suggests that it is an intermediate-depth earthquake; (b) damage to Cumaná produced by the 1797 event suggests that this was a local earthquake, perhaps equivalent to the 1929 earthquake, which ruptured for some 30 km just east of Cumaná into the Gulf of Cariaco; and (c) seismogenic association of the 1530 and 1853 earthquakes still remains unclear but it is very likely that these ruptures occurred offshore, as suggested by the rather large tsunami waves that both events have generated, placing their hypocenters west of Cumaná in the Cariaco Trough. This reassessment also sheds light into the El Pilar fault segmentation and the behavior of its seismogenic barriers through time.  相似文献   

Performance analysis of multi-GNSS static and RTK techniques in estimating height differences     

Ahmed Elaksher  Tarig Ali  Franck Kamtchang  Christian Wegmann  Adalberto Guerrero 《International Journal of Digital Earth》2020,13(5):586-601

ABSTRACT

Establishing reliable elevation differences is imperative for most geoscience and engineering applications. This work has traditionally been accomplished through spirit leveling techniques; however, surveyors have been utilizing satellite positioning systems in measuring height differences for more than a decade. Yet the quality of these heights needs to be evaluated in order to adopt them in different applications. In this article, we present the outcome of an accuracy assessment of height differences obtained with static and RTK surveys. Twenty control points with an average baseline length of 1?km were occupied with dual-frequency GNSS receivers for different time periods. Collected signals were processed using open-source software and verified with an online processing tool. Heights were estimated by processing the GPS and the GLONASS data individually, and combined (i.e. GNSS). Height differences were determined and compared with those measured by spirit levels and corrected through geoid models. Best results were achieved by combining GPS and GLONASS solutions for both static and RTK surveys. Solutions with either GPS or GLONASS satellites were comparable, but in most cases, the GPS solutions performed better. For the static surveys, longer occupation provided much accurate height differences. Inconsistencies among 10 different RTK surveys were minimum for the GPS?+?GLONASS solutions and worst for the GLONASS solutions. The ANOVA, LSD, F, and χ² statistical tests confirmed our findings at the 95% confidence level.  相似文献   

Sonostratigraphy of tropical Indian Ocean giant piston cores: toward a rapid and high-resolution tool for tracking dissolution cycles in Pleistocene carbonate sediments     

Franck C. Bassinot   《Earth and Planetary Science Letters》1993,120(3-4):327-344

During the seymama expedition of the French R/V Marion Dufresne in the equatorial Indian Ocean, we retrieved giant piston cores (30–53 m long) as part of a high resolution palaeo-oceanographic and stratigraphic study of Pliocene-Pleistocene pelagic carbonates. Major changes in the compressional wave (P wave) velocity profiles recorded in these cores appear to be correlatable from the Madingley Rise (western equatorial Indian Ocean) to the southeast of the Maldives archipelago (central equatorial Indian Ocean), about 1700 km away, thus emphasizing the stratigraphic potential of acoustic records in uncemented pelagic carbonates.

As expected in deep-sea carbonate deposits, changes in P-wave velocity parallel past changes in coarse fraction content (> 63 μm). Changes in grain size appear to be mainly controlled by carbonate dissolution, as evidenced by a strong relationship between sand content and a foraminifer preservation index. Thus, in uncemented pelagic carbonates, P-wave velocities provide quick and easy to obtain qualitative information on carbonate dissolution pulses. As diagenesis takes place, however, compaction and cementation change the dynamic rigidity (μ) of the sediments and may conceal the original grain size signal.

Due to the strong positive relationship between P-wave velocity and coarse fraction content in uncemented pelagic carbonates, P-wave velocity profiles can be tied to a precise chronologic framework by correlating them to the composite grain size index curve (CGSI) established by Bassinot et al. for the tropical Indian Ocean [1,2]. This composite curve has been constructed by stacking the normalized coarse fraction records from ODP Site 722 (Owen Ridge, Arabian Sea [3]) and ODP Site 758 (Ninetyeast Ridge, central equatorial Indian Ocean [4]). In these two sites, detailed δ¹⁸O records provide the basis for precise inter-site correlations. They ensure the accuracy of the stacking procedure, which tends to reduce most of the local grain size signals and enhances the regional signal related to carbonate dissolution pulses [1,2]. A detailed chronostratigraphy of CGSI curve was developed by correlating the δ¹⁸O records of Sites 722 and 758 to the orbital chronology recently developed from ODP Site 677 [5]. The CGSI may be used as a reference curve for developing a sonostratigraphy in the tropical Indian Ocean.  相似文献   

Oldest description of a caldera-forming eruption in Southeast Asia unveiled in forgotten written sources     

Mutaqin  Bachtiar W.  Lavigne  Franck 《GeoJournal》2021,86(2):557-566

GeoJournal - As one of the most powerful volcanic events of the Holocene era with a Volcanic Explosivity Index of seven, the 1257 CE eruption of Samalas volcano in the Rinjani Volcanic Complex had...  相似文献   

Assessing post-pliocene deformation in a context of slow tectonic deformation: insights from paleoseismology,remote sensing and shallow geophysics in Provence,France     

Thomas  Franck  Rizza  Magali  Bellier  Olivier  Billant  Jérémy  Dussouillez  Philippe  Fleury  Jules  Delanghe  Doriane  Ollivier  Vincent  Godard  Vincent  Talon  Brigitte 《Natural Hazards》2021,105(2):1453-1490

Natural Hazards - The Provence region, located in the south-east of France, has experienced a few destructive earthquakes during the last centuries, such as the 1909 Lambesc earthquake or the 1509...  相似文献   

Objective assessment of the contribution of the RECOPESCA network to the monitoring of 3D coastal ocean variables in the Bay of Biscay and the English Channel     

Julien Lamouroux  Guillaume Charria  Pierre De Mey  Stéphane Raynaud  Catherine Heyraud  Philippe Craneguy  Franck Dumas  Matthieu Le Hénaff 《Ocean Dynamics》2016,66(4):567-588

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Toward objective rockfall trajectory simulation using a stochastic impact model   总被引：3，自引：0，他引：3  

Franck Bourrier  Luuk Dorren  Franois Nicot  Frdric Berger  Flix Darve 《Geomorphology》2009,110(3-4):68-79

The accuracy of rockfall trajectory simulations depends to a large extent on the calculation of the rebound of falling boulders on different parts of a slope where rockfalls could occur. The models commonly used for rebound calculation are based on restitution coefficients, which can only be calibrated subjectively in the field. To come up with a robust and objective procedure for rebound calculation, a stochastic impact model associated with an objective field data collection method was developed and tested in this study. The aims of this work were to assess the adequacy of this approach and to evaluate the minimum amount of field data required to obtain simulation results with a satisfactory level of predictability. To achieve these objectives, the rebound calculation procedure developed was integrated into a three-dimensional rockfall simulation model, and the simulated results were compared with those obtained from field rockfall experiments. For rocky slopes, the simulations satisfactorily predict the experimental results. This approach is advantageous because it combines precise modelling of the mechanisms involved in the rebound and of their related variability with an objective field data collection procedure which basically only requires collecting the mean size of soil rocks. The approach proposed in this study therefore constitutes an excellent basis for the objective probabilistic assessment of rockfall hazard.  相似文献