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111.
Karim Kelfoun Tim Druitt Benjamin van Wyk de Vries Marie-Noëlle Guilbaud 《Bulletin of Volcanology》2008,70(10):1169-1187
One of the most remarkable features of the exceptionally well preserved 26 km3 Socompa debris avalanche deposit is the evidence for topographically driven secondary flow. The avalanche formed by sector
collapse of Socompa stratovolcano and spread 40 km across a pre-existing basin, forming a sheet of ∼50 m average thickness.
As the avalanche impinged on the western and northern margins of the basin, it was reflected back, forming a secondary flow
that continued to travel 15 km down a gentle slope at an oblique angle to the primary flow, the front of the return wave being
preserved frozen on the surface of the deposit as a prominent escarpment. Satellite images, aerial photos, digital elevation
models and field observations were used to reconstruct the sequence of events during avalanche emplacement, and in particular
during secondary flow. The avalanche sheet was divided into distinct terrane groups, each believed to have experienced a particular
strain history during emplacement. Evidence for avalanche reflection includes clearly recognizable secondary slide masses,
sub-parallel sets of curvilinear shear zones, headwall scarps separating the (primary) levée from the secondary terranes,
extensional jigsaw breakup of surface lithologies during return flow, and cross cutting, or deflection, of primary flow fabrics
by secondary terranes. Reflection off the basin margin took place in an essentially continuous manner, most major return motions
being simultaneous with, or shortly following, primary flow. The secondary flow occurred as a wave that swept obliquely across
the primary avalanche direction, remobilizing the primary material, which was first compressed, then stretched, as it passed
over and rearward of the wave front. As return flow occurred, surface lithologies were rifted in a brittle manner, and the
slabs were sheared pervasively as they glided and rotated back into the basin; some sank into the more fluidal interior of
the avalanche, which drained out into a prominent distal lobe. Extension by factors of up to 1.8 took place during return
flow. Secondary flow took place on slopes of only a few degrees, and the distal lobe flowed 8 km on a slope of ∼1°. Overall
the avalanche is inferred to have slid into place as a fast-moving sheet of fragmental rock debris, with a leading edge and
crust with near-normal friction and an almost frictionless, fluidal interior and base. The avalanche emplacement history deduced
from field evidence is consistent with the results of a previously published numerical model of the Socompa avalanche. 相似文献
112.
Gelare Moradi Bart Vermeulen Colin D. Rennie Romain Cardot Stuart N. Lane 《地球表面变化过程与地形》2019,44(14):2903-2921
Secondary circulation in river confluences results in a spatial and temporal variation of fluid motion and a relatively high level of morphodynamic change. Acoustic Doppler current profiler (aDcp) vessel-mounted flow measurements are now commonly used to quantify such circulation in shallow water fluvial environments. It is well established that such quantification using vessel-mounted aDcps requires repeated survey of the same cross-section. However, less attention has been given to how to process these data. Most aDcp data processing techniques make the assumption of homogeneity between the measured radial components of velocity. As acoustic beams diverge with distance from the aDcp probe, the volume of the flow that must be assumed to be homogeneous between the beams increases. In the presence of secondary circulation cells, and where there are strong rates of shear in the flow, the homogeneity assumption may not apply, especially deeper in the water column and close to the bed. To reduce dependence on this assumption, we apply a newly-established method to aDcp data obtained for two medium-sized (~60–80 m wide) gravel-bed river confluences and compare the results with those from more conventional data processing approaches. The comparison confirms that in the presence of strong shear our method produces different results to more conventional approaches. In the absence of a third set of fully independent data, we cannot demonstrate conclusively which method is best, but our method involves less averaging and so in the presence of strong shear is likely to be more reliable. We conclude that it is wise to apply both our method and more conventional methods to identify where data analysis might be impacted upon by strong shear and where inferences of secondary circulation may need to be made more cautiously. © 2019 John Wiley & Sons, Ltd. 相似文献
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Nicolas Bourguet Madeleine Goutx Jean-Franois Ghiglione Mireille Pujo-Pay Genevive Mvel Andr Momzikoff Laure Mousseau Catherine Guigue Nicole Garcia Patrick Raimbault Romain Pete Louise Oriol Dominique Lefvre 《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(18):1454-1469
This study investigated the relationships between dissolved organic matter (DOM) composition and bacterial dynamics on short time scale during spring mesotrophic (March 2003) and summer oligotrophic (June 2003) regimes, in a 0–500 m depth water column with almost no advection, at the DYFAMED site, NW Mediterranean. DOM was characterized by analyzing dissolved organic carbon (DOC), colored dissolved organic matter (CDOM) and lipid class biotracers. Bacterial dynamic was assessed through the measurement of in situ bacterial lipase activity, abundance, production and bacterial community structure. We made the assumption that by coupling the ambient concentration of hydrolysable acyl-lipids with the measurement of their in situ bacterial hydrolysis rates (i.e. the free fatty acids release rate) would provide new insights about bacterial response to change in environmental conditions. The seasonal transition from spring to summer was accompanied by a significant accumulation of excess DOC (+5 μM) (ANOVA, p<0.05, n=8) in the upper layer (0–50 m). In this layer, the free fatty acids release rate to the bacterial carbon demand (BCD) ratio increased from 0.6±0.3 in March to 1.3±1.0 in June (ANOVA, p<0.05, n=8) showing that more uncoupling between the hydrolysis of the acyl-lipids and the BCD occurred during the evolution of the season, and that free fatty acids contributed to the excess DOC. The increase of lipolysis index and CDOM absorbance (from 0.24±0.17 to 0.39±0.13 and from 0.076±0.039 to 0.144±0.068; ANOVA, p<0.05, n=8, respectively), and the higher contribution of triglycerides, wax esters and phospholipids (from <5% to 12–31%) to the lipid pool reflected the change in the DOM quality. In addition to a strong increase of bacterial lipase activity per cell (51.4±29.4–418.3±290.6 Ag C cell−1 h−1), a significant percentage of ribotypes (39%) was different between spring and summer in the deep chlorophyll maximum (DCM) layer in particular, suggesting a shift in the bacterial community structure due to the different trophic conditions. At both seasons, in the chlorophyll layers, diel variations of DOM and bacterial parameters reflected a better bioavailability and/or DOM utilization by bacteria at night (the ratio of free fatty acids release rate to bacterial carbon demand decreased), most likely related to the zooplankton trophic behaviour. In mesotrophic conditions, such day/night pattern was driving changes in the bacterial community structure. In more oligotrophic period, diel variations in bacterial community structure were depth dependent in relation to the strong summer stratification. 相似文献
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Estelle Leroux Daniel Aslanian Marina Rabineau Romain Pellen Maryline Moulin 《地学学报》2018,30(3):207-214
A review of geological and geophysical observations points towards a worldwide kinematic change at around 6 Ma. The synchronicity of many manifestations (tectonics, magmatism, kinematics, ecological events, among others) at ~6 Ma, similar to those recognized from time to time on the geological time‐scale, argues for a global geodynamic event that has led to many regional consequences on Earth's surface. In particular, we propose that this global event was the main trigger for the three fold increase in sediment deposits in the world ocean over the last ~5 Ma, but also for the onset of the Messinian Salinity Crisis in the Mediterranean area, one of the most severe ecological crises in the Earth's history. We suggest this Messinian revolution to be the last occurrence of cyclic successions of global events. 相似文献
115.
Talc + olivine in metaperidotites result from the serpentinite breakdown due to increasing temperature in the Bergell contact aureole. Jack‐straw olivine textures are present in close proximity to the serpentine breakdown reaction. As the intrusion is approached, the number of olivine crystals increases while the size of the crystals decreases; this feature documents increased overstepping with increased heating rates. Talc veinlets are observed in the outer parts of the talc–olivine zone and are interpreted to be pathways of fluid produced during devolatilization of serpentine. None of the talc–olivine oxygen isotope pairs analysed are in isotopic equilibrium with respect to the peak contact temperature. This implies that escaping fluids cannot be in equilibrium with both phases. Hence, the fluid produced by serpentine reaction does not directly reflect the protolith composition, and attention must be given to the reaction mechanism before interpreting fluid isotope composition. 相似文献
116.
The sequestration of CO2 occurs naturally in (ultra)‐mafic rocks by carbonation processes and is commonly noted in areas of the seafloor where mantle lithologies are exhumed. As well as carbonation, mantle exhumation is also responsible for rock brecciation. The relationship between carbonation and brecciation is not well constrained. A temporal evolution from syn‐ to post‐tectonic carbonation and brecciation is proposed in line with progressive mantle exhumation. Using a petrological study of brecciated material from IODP drill cores of the Iberia–Newfoundland conjugated margins, we relate crack–seal veins to tectonic brecciation, authigenic calcite with scalenohedral structure to hydraulic brecciation and reworked clasts within cement to (tectono)‐sedimentary processes. Oxygen isotope compositions reveal late‐staged < 50°C carbonate generation in the proximal part of the ocean–continent transition, which have followed an earlier phase of sub‐seafloor carbonate generation. The results are crucial to understand CO2 exchange within the reworked sub‐seafloor in passive margins and oceanic systems. 相似文献
117.
Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling 
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下载免费PDF全文 Valentin Laurent Pierre Lanari Inès Naïr Romain Augier Abdeltif Lahfid Laurent Jolivet 《Journal of Metamorphic Geology》2018,36(6):769-798
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. 相似文献
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