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101.
The study of the geochemical compositions and K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Baja California, the Gulf of California and Sonora allows us to discuss the nature of their mantle or crustal sources, the conditions of their melting and the tectonic regime prevailing during their genesis and emplacement. Nine petrographic/geochemical groups are distinguished: ??regular?? calc-alkaline lavas; adakites; magnesian andesites and related basalts and basaltic andesites; niobium-enriched basalts; alkali basalts and trachybasalts; oceanic (MORB-type) basalts; tholeiitic/transitional basalts and basaltic andesites; peralkaline rhyolites (comendites); and icelandites. We show that the spatial and temporal distribution of these lava types provides constraints on their sources and the geodynamic setting controlling their partial melting. Three successive stages are distinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the subduction of the Pacific-Farallon plate formed the Comondú and central coast of the Sonora volcanic arc. In the extensional domain of western Sonora, lithospheric mantle-derived tholeiitic to transitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Farallon subduction was marked by the emplacement of much more complex Middle to Late Miocene volcanic associations, between 13 and 7 Ma. Calc-alkaline activity became sporadic and was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comendites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and extending from the Pacific coast of Baja California to coastal Sonora. Tholeiitic, transitional and alkali basalts of subslab origin ascended through this window, and adakites derived from the partial melting of its upper lip, relatively close to the trench. Calc-alkaline lavas, magnesian andesites and niobium-enriched basalts formed from hydrous melting of the supraslab mantle triggered by the uprise of hot Pacific asthenosphere through the window. During the Plio-Quaternary, the ??no-slab?? regime following the sinking of the old part of the Farallon plate within the deep mantle allowed the emplacement of alkali and tholeiitic/transitional basalts of deep asthenospheric origin in Baja California and Sonora. The lithospheric rupture connected with the opening of the Gulf of California generated a high thermal regime associated to asthenospheric uprise and emplaced Quaternary depleted MORB-type tholeiites. This thermal regime also induced partial melting of the thinned lithospheric mantle of the Gulf area, generating calc-alkaline lavas as well as adakites derived from slivers of oceanic crust incorporated within this mantle.  相似文献   
102.
A near-global grid-point nudging of the Arpege-Climat atmospheric General Circulation Model towards ECMWF reanalyses is used to diagnose the regional versus remote origin of the summer model biases and variability over West Africa. First part of this study revealed a limited impact on the monsoon climatology compared to a control experiment without nudging, but a significant improvement of interannual variability, although the amplitude of the seasonal anomalies remained underestimated. Focus is given here on intraseasonal variability of monsoon rainfall and dynamics. The reproducible part of these signals is investigated through 30-member ensemble experiments computed for the 1994 rainy season, a year abnormally wet over the Sahel but representative of the model systematic biases. In the control experiment, Arpege-Climat simulates too few rainy days that are associated with too low rainfall amounts over the central and western Sahel, in line with the seasonal dry biases. Nudging the model outside Africa tends to slightly increase the number of rainy days over the Sahel, but has little effect on associated rainfall amounts. However, results do indicate that a significant part of the monsoon intraseasonal variability simulated by Arpege-Climat is controlled by lateral boundary conditions. Parts of the wet/dry spells over the Sahel occur in phase in the 30 members of the nudging experiment, and are therefore embedded in larger-scale variability patterns. Inter-member spread is however not constant across the selected summer season. It is partly controlled by African Easterly Waves, which show dissimilar amplitude from one member to another, but a coherent phasing in all members. A lowpass filtering of the nudging fields suggests that low frequency variations in the lateral boundary conditions can lead to eastward extensions of the African Easterly Jet, creating a favorable environment for easterly waves, while high frequency perturbations seem to control their phasing.  相似文献   
103.
Over the last three decades, many regional studies in mountain ranges under temperate climate revealed that it is possible to discriminate debris-flow and fluvial fans from morphometric indicators measured at the scale of the catchment and the fan itself. The most commonly used indicators are the Melton index (R), a normalized index of the gravitational energy of the catchment, and the fan slope (S). A wide range of thresholds have been proposed for discriminating purpose, but these are generally based on a small population of catchments and may be highly influenced by ambiguous fans included in the data set. A database of 620 upland catchments from several mountain ranges under temperate climate was compiled from the literature to propose robust discriminant morphometric thresholds for debris-flow versus fluvial responses. Linear discriminant analysis (LDA) and logistic regression (LR) were performed using the whole data set, and a leave-one-out cross-validation was used to evaluate performances of the models. Sensitivity and specificity scores obtained for LDA and LR were 0.96 and 0.73, and 0.95 and 0.75, respectively. It is also shown that the channel slope above which debris-flow is observed decreases with the gravitational energy of the catchment. Limitations of the morphometric discrimination are discussed.  相似文献   
104.
We present detrital zircon UPb SHRIMP age patterns for the central segment (34–42°S) of an extensive accretionary complex along coastal Chile together with ages for some relevant igneous rocks. The complex consists of a basally accreted high pressure/low temperature Western Series outboard of a frontally accreted Eastern Series that was overprinted by high temperature/low pressure metamorphism. Eleven new SHRIMP detrital zircon age patterns have been obtained for meta-turbidites from the central (34–42°S) segment of the accretionary complex, four from previously undated metamorphic complexes and associated intrusive rocks from the main Andean cordillera, and three from igneous rocks in Argentina that were considered as possible sediment source areas. There are no Mesozoic detrital zircons in the accretionary rocks. Early Paleozoic zircons are an essential component of the provenance, and Grenville-age zircons and isolated grains as old as 3 Ga occur in most rocks, although much less commonly in the Western Series of the southern sector. In the northernmost sector (34–38°30′S) Proterozoic zircon grains constitute more than 50% of the detrital spectra, in contrast with less than 10% in the southern sector (39–42°S). The youngest igneous detrital zircons in both the northern Western (307 Ma) and Eastern Series (345 Ma) are considered to closely date sedimentation of the protoliths. Both oxygen and LuHf isotopic analyses of a selection of Permian to Neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions. The results suggest that Early Paleozoic orogenic belts (Pampean and Famatinian) containing material recycled from cratonic areas of South America supplied detritus to this part of the paleo-Pacific coast. In contrast, in the southern exposures of the Western Series studied here, Permian detrital zircons (253–295 Ma) dominate, indicating much younger deposition. The northern sector has scarce Early to Middle Devonian detrital zircons, prominent south of 39°S. The sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of Devonian (Achalian) detritus by a topographic barrier formed by the Precordillera, and possibly Chilenia, terranes. Devonian subduction-related metamorphic and plutonic rocks developed south of 39°S, beyond the possible southern limit of Chilenia, where sedimentation of accretionary rocks continued until Permian times.  相似文献   
105.
Soils, as well as most of deformable multiphase porous materials, are likely to suffer from desiccation cracking, leading to the formation of regular crack patterns affecting their permeability. The ensuing crack spacing has often been related to a concept sometimes called “sequential infilling”: it is assumed that desiccation cracks are formed by successive generations. However, such a concept does not consider the pattern of a simultaneous crack formation at a given moment. Using our desiccation cracking test results and their numerical simulation, we propose a consistent explanation for the formation of desiccation crack patterns in soils. We show that the “sequential infilling” concept is suitable only when the position of the crack(s) clearly stems from the stress field. To derive an estimate of the desiccation crack spacing, the overall energy of the system needs to be considered. Statistical variability should be superimposed on the mean deterministic conditions discussed here.  相似文献   
106.
Since the nineteenth century scientists have tried to reproduce natural events in order to study and understand them through the technique of modelling. However, technology has evolved rapidly in the past two decades and now sophisticated numerical models are widely used to reproduce past events or simulate new scenarios. These models are particularly useful to reproduce the large scale and complexity of geological events. To illustrate the use and potential of numerical modelling in geological sciences, we describe a simulation of a large debris avalanche caused by the collapse of the north flank of the Taranaki volcano in New Zealand and the value of this information in the context of disaster planning.  相似文献   
107.
Land cover changes have a great impact on nitrogen (N) and phosphorous (P) fluxes catchments. In this study, we wanted to compare different land covers: deciduous (D), evergreen (E) (both native forests), and exotic Eucalyptus globulus plantation (EP), affected precipitation, and stream discharge on N and P species concentrations and fluxes, under a low deposition climate in south‐central Chile. For this, we collected bulk precipitation, throughfall, and stream water samples after 41 rainfall events in E and EP, during the period June 2009–March 2011, and 31 rainfall events in D, during the period October 2009–March 2011. The highest canopy enrichment of N and P species for throughfall was observed in deciduous forest, while E. globulus showed the minimum enrichment. Total nitrogen (total‐N) discharge in EP was about 8.6 times higher than that of E and D catchments. Total‐N annual retention was positive only in E and D catchments. However, EP catchment showed a net loss (?4.79 kg N ha/year). The biggest difference was observed in nitrate stream concentrations, which showed low values for E (3.4 ± 1.3 μg/L), while EP and D showed higher nitrate concentrations (84.9 ± 16.7 and 134.7 ± 36.7 μg/L, for EP and D, respectively). Total phosphorous (total‐P) discharge flux was low in EP (0.4 kg P ha/year), and negligible in E and D forests (<0.1 kg P ha/year). Total‐P annual retention was near to 1.0 kg N ha/year (on E and D), while a net loss (?1.5 kg N ha/year) was observed for EP. We attribute the observed differences in nutrient throughfall enrichment due to high multi‐stratified canopies in the native forests. Both deciduous and evergreen native forest‐covered catchments showed the highest retention of total‐N and total‐P, in contrast to Eucalyptus‐covered plantation.  相似文献   
108.
Indisputable geochronology of Icelandic volcanism may contribute to improving the use of volcanic proxies for paleoenvironmental studies. Accurate and reliable ages may also provide some useful constraints in studying the possible relationship between volcanism and deglaciation in Iceland. Given the difficulty of dating accurately young and low potassic volcanic samples from Iceland, the relationship between deglaciation and increasing levels of volcanism has been shown only for the last deglaciation. The purpose of this study is to evaluate the potential of the unspiked K–Ar dating method for dating Quaternary Icelandic volcanic rocks, because we consider that reliable K–Ar ages of interglacial and sub-glacial volcanic products may generate useful data sets for paleoclimatic reconstructions, so enabling the link between glaciation and volcanic activity to be better established. Given the ages and precisions obtained, this study demonstrates that the unspiked K–Ar method is a promising tool for reconstructing the recent volcanic activity in Iceland during glacial and interglacial intervals.  相似文献   
109.
Deciphering the complex interplays between climate, uplift and erosion is not straightforward and estimating present‐day erosion rates can provide useful insights. Glaciers are thought to be powerful erosional agents, but most published ‘glacial’ erosion rates combine periglacial, subglacial and proglacial erosion processes. Within a glaciated catchment, sediments found in subglacial streams originate either from glacial erosion of substratum or from the rock walls above the glacier that contribute to the supraglacial load. Terrestrial cosmogenic nuclides (TCN) are produced by interactions between cosmic ray particles and element targets at the surface of the Earth, but their concentration becomes negligible under 15 m of ice. Measuring TCN concentrations in quartz sand sampled in subglacial streams and in supraglacial channels is statistically compliant with stochastic processes (e.g. rockfalls) and may be used to discriminate subglacial and periglacial erosion. Results for two subglacial streams of the Bossons glacier (Mont Blanc massif, France) show that the proportion of sediments originating from glacially eroded bedrock is not constant: it varies from 50% to 90% (n = 6). The difference between the two streams is probably linked to the presence or absence of supraglacial channels and sinkholes, which are common features of alpine glaciers. Therefore, most of the published mean catchment glacial erosion rates should not be directly interpreted as subglacial erosion rates. In the case of catchments with efficient periglacial erosion and particularly rockfalls, the proportion of sediments in the subglacial stream originating from the supraglacial load could be considerable and the subglacial erosion rate overestimated. Here, we estimate warm‐based subglacial and periglacial erosion rates to be of the same order of magnitude: 0.39 ± 0.33 and 0.29 ± 0.17 mm a?1, respectively. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   
110.
Throughout its history, the Earth has experienced global magmatic events that correlate with the formation of supercontinents. This suggests that the distribution of continents at the Earth's surface is fundamental in regulating mantle temperature. Nevertheless, most large igneous provinces (LIPs) are explained in terms of the interaction of a hot plume with the lithosphere, even though some do not show evidence for such a mechanism. The aggregation of continents impacts on the temperature and flow of the underlying mantle through thermal insulation and enlargement of the convection wavelength. Both processes tend to increase the temperature below the continental lithosphere, eventually triggering melting events without the involvement of hot plumes. This model, called mantle global warming, has been tested using 3D numerical simulations of mantle convection [Coltice, N., Phillips, B.R., Bertrand, H., Ricard, Y., Rey, P. (2007) Global warming of the mantle at the origin of flood basalts over supercontinents. Geology 35, 391–394.]. Here, we apply this model to several continental flood basalts (CFBs) ranging in age from the Mesozoic to the Archaean. Our numerical simulations show that the mantle global warming model could account for the peculiarities of magmatic provinces that developed during the formation of Pangea and Rodinia, as well as putative Archaean supercontinents such as Kenorland and Zimvaalbara.  相似文献   
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