Debris flows are very important and widespread mass movements, and represent a remarkable geomorphological hazard. This research deals with debris flows in an alpine environment, studied using dendrogeomorphological dating techniques, outlining their relation with precipitation, and analysing possible changes in their frequency and intensity over time. The study area is the upper Valle del Gallo (Northern Italy), a typical high mountain environment dominated by mass wasting processes, where many debris-flow fans occupy the valley bottom. Dendrogeomorphological research was conducted on twelve of these fans and two channels located on slopes. Tree growth anomalies (abrasion scars, compression wood and abrupt growth changes) were used as dating methods. Two hundred and thirty nine debris debris-flow events between 1875 and 2003 were dated using 757 trees (Pinus montana Mill.). Analysis between dated events and precipitation suggests that debris flows in the study area could be triggered by 20–30 mm of rain concentrated in a few hours. The debris-flow frequency tends to increase gradually, but the highest value seems to have occurred in the period 1974–1983. This trend agrees with the historical occurrence of flooding events in Northern Italy as inferred by literature, and with similar studies conducted in the Swiss Alps. The results of this research are intended as a contribution for understanding the response of geomorphological processes to climatic changes. 相似文献
We discuss the importance of modelling riparian vegetation and river flow interactions under differing hydrologic regimes.
Modelling tools have notable implications with regard to the understanding of riverine ecosystem functioning and to promote
sustainable management of water resources. We present both deterministic and stochastic approaches with different levels of
simplification, and discuss their use in relation to river and vegetation dynamics at the related scale of interest. We apply
such models to both meandering and braided rivers, in particular focusing on the floodplain dynamics of an alpine braided
river affected by water impoundment. For this specific case we show what the expected changes in riparian vegetation may be
in a ‘controlled release’ scenario for the postdam river Maggia, Switzerland. Finally, the use of these models is discussed
in the context of current research efforts devoted to river restoration practice. 相似文献
Mass movements varying in type and size, some of which are periodically reactivated, affect the urban area of Avigliano. The disturbed and remoulded masses consist of sandy–silty or silty–clayey plastic material interbedded with stone fragments and conglomerate blocks. Five landslides that were markedly liable to rainfall-associated instability phenomena were selected.
The relationships between landslides and rainfall were investigated using a hydrological and statistical model based on long-term series of daily rainfall data. The model was used to determine the return period of cumulative daily rainfall over 1–180 days. The resulting hydrological and statistical findings are discussed with the aim of identifying the rainfall duration most critical to landslides.
The concept of a precipitation threshold was generalized by defining some probability classes of cumulative rainfall. These classes indicate the thresholds beyond which reactivation is likely to occur. The probability classes are defined according to the return period of the cumulative rainfall concomitant with landslide reactivation. 相似文献
The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive
presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite
(websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted
major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of
major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile
major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes
exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative
Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene.
Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the
sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical
signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and
Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at
Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained
by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because
of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting
temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with
the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling,
the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics.
In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways
through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process,
which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition
is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence
of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting
in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and
with the occurrence of modal metasomatism before peridotite incorporation in the crust.
Received: 20 July 1998 / Accepted: 28 October 1998 相似文献
Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by
lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption,
ranges in composition from trachyte (SiO2 = 62.5 wt%) to latite (SiO2 ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic
scoriae with fairly uniform composition (SiO2 ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite
scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic
rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital
trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out
syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates
not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma
chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage
and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall
of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more
evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber
were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role
played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1).
Received: 23 March 1998 / Accepted: 11 December 1998 相似文献
In this contribution we evaluate the capabilities of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using a 12 μm spot size. Precision, accuracy and detection limits were assessed on the USGS BCR-2G reference material. We demonstrate that the 12 μm LA-ICP-MS analyses of experimentally-grown amphibole and garnet are in excellent agreement with secondary ion mass spectrometry (SIMS) trace element determinations on the same crystals. The 12 μm spot size configuration was subsequently used to determine trace element crystal-melt partition coefficients (Dc/m) for a wide range of trace elements in amphibole in equilibrium with a basanitic melt. The following strategy to determine accurately and evaluate Dc/m is proposed. One or more major elements determined previously by electron probe microanalysis (EPMA) was used to ensure consistency between EPMA and the composition of the aerosol produced by the laser ablation. Measured Dc/m values were successively evaluated using the lattice strain model. The use of this strategy significantly improved the precision and accuracy of Dc/m determination when a LA-ICP-MS configuration with a high spatial resolution was employed. 相似文献
Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORB-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 ± 20 Ma, which is consistent with a 207Pb/206Pb evaporation age of 549 ± 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter being the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this case, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite).
Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 ± 23, 226 ± 7, 223 ± 10, 214 ± 17, and 203 ± 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 ± 15 Ma ago.
Initial Nd(533) values average +6.3 ± 0.4 for six samples of the Amphibole Peridotite unit and +6.0 ± 1.2 for ten samples of the External Gabbro unit. 87Sr/86Sr ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between 87Sr/86Sr and K2O and weaker correlations between initial Nd and K2O imply a comparatively minor (≤ 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative Nd and high 87Sr/86Sr values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ‘pristine’ and the magmatic body has been extensively contaminated by assimilation of crustal rocks. 相似文献
The inversion routine proposed by Aueret al. (1977), for the determination of vector magnetic fields from Stokes profiles, has been generalized to include magneto-optical and damping effects. Synthetic profiles have then been generated from a sunspot model atmosphere accounting for the depth variation of the relevant physical parameters such as the magnetic field amplitude, inclination angle, etc...., each variation being considered one at a time. Alfvén waves and magnetic inhomogeneities over the field of view have also been considered. These synthetic profiles have been presented to the inversion routine. The results of the fits show that the magnetic field amplitude and direction are always recovered with good accuracy when these quantities are constant in the model atmosphere, and, in those cases where te magnetic field vector is supposed to vary monotonically with optical depth, the values recovered are always intermediate between the values corresponding to the top and bottom of the atmosphere. Moreover, we found that the differences between synthetic and best-fit profiles are able to characterize, in many cases, the particular physical situation considered. 相似文献