Validation d'un modèle numérique de terrain adapté à la modélisation hydrologique régionale sur l'Algérie du Nord

Résumé

L'objectif de ce travail est de fournir aux hydrologues travaillant sur l'Algérie du Nord un modèle numérique de terrain (MNT) permettant la mise en ?uvre de méthodes d'estimation régionales de paramètres hydrologiques tels que les débits de crue instantanés décennaux ou les apports moyens annuels. Il doit permettre une détermination satisfaisante des contours des bassins versants et l'obtention de paramètres morphométriques comparables entre eux sur l'ensemble de la zone d'étude. Le MNT que nous proposons est un compromis entre des tailles de pixels petites donnant une bonne précision et des pixels plus grands permettant de couvrir uniformément la zone. Nous montrons que le MNT à la maille de 100 m est parfaitement adapté sous réserve de l'avoir préalablement corrigé manuellement dans les zones endoréiques et dans les zones relativement planes. Ce MNT permet de retrouver correctement les principaux paramètres morphométriques utilisés en hydrologie. Nous mettons donc à la disposition des hydrologues ce MNT à 100 m dans une projection Lambert bien adaptée aux besoins de l'hydrologie algérienne.

Editeur Z.W. Kundzewicz

Citation Rezak, S., Laborde, J.-P., et Errih, M., 2012. Validation d'un modèle numérique de terrain adapté à la modélisation hydrologique régionale sur l'Algérie du Nord. Hydrological Sciences Journal, 57 (5), 928–941.     

A 100 m.y. record of volcanic arc evolution in Nicaragua

The processes that result in arc magmas are critical to understanding element recycling in subduction zones, yet little is known about how these systems evolve with time. Nicaragua provides an opportunity to reconstruct the history of a volcanic arc since the Cretaceous. Here we present the stratigraphy of the Cretaceous–Eocene volcanic units in Nicaragua and their relationship to the different tectonic units where the arc developed. We discovered an evolution from an arc‐dominated by calc‐alkaline compositions in the Cretaceous–Eocene, to transitional compositions in the Oligocene–Miocene, to finally tholeiitic magmas common in the modern volcanic front. Our petrographic studies confirm that in the Cretaceous–Eocene the olivine + clinopyroxene cotectic was followed by clinopyroxene + plagioclase ± amphibole. Given the abundance of amphibole and the lack of this mineral in the modern volcanic front, the Cretaceous–Eocene Arc melts were likely more water‐rich than modern Nicaragua, suppressing the crystallization of plagioclase after olivine. We also found temporal changes in element ratios that are sensitive to variations in sediment input. The Cretaceous–Eocene Arc is characterized by a lower Ba/Th compared to the Oligocene–Miocene and modern volcanic front samples, suggesting that the sediment input was lower in Ba, possibly analogous to old deep siliceous sediment subducting in the western Pacific. Both U/Th and U/La are higher in the modern volcanics, reflecting higher U/Th in the subducting sediments following the 'Carbonate Crash'. Finally, we found that the orientation of the arc axis also changed, from northeast‐southwest in the Cretaceous–Eocene to northwest‐southeast after the Oligocene. This change probably records variations in the location of the subduction zone as this region shaped into its current geographic configuration.     

Volatile (S,Cl and F) and fluid mobile trace element compositions in melt inclusions: implications for variable fluid sources across the Kamchatka arc

Volatile element, major and trace element compositions were measured in glass inclusions in olivine from samples across the Kamchatka arc. Glasses were analyzed in reheated melt inclusions by electron microprobe for major elements, S and Cl, trace elements and F were determined by SIMS. Volatile element–trace element ratios correlated with fluid-mobile elements (B, Li) suggesting successive changes and three distinct fluid compositions with increasing slab depth. The Eastern Volcanic arc Front (EVF) was dominated by fluid highly enriched in B, Cl and chalcophile elements and also LILE (U, Th, Ba, Pb), F, S and LREE (La, Ce). This arc-front fluid contributed less to magmas from the central volcanic zone and was not involved in back arc magmatism. The Central Kamchatka Depression (CKD) was dominated by a second fluid enriched in S and U, showing the highest S/K₂O and U/Th ratios. Additionally this fluid was unusually enriched in ⁸⁷Sr and ¹⁸O. In the back arc Sredinny Ridge (SR) a third fluid was observed, highly enriched in F, Li, and Be as well as LILE and LREE. We argue from the decoupling of B and Li that dehydration of different water-rich minerals at different depths explains the presence of different fluids across the Kamchatka arc. In the arc front, fluids were derived from amphibole and serpentine dehydration and probably were water-rich, low in silica and high in B, LILE, sulfur and chlorine. Large amounts of water produced high degrees of melting below the EVF and CKD. Fluids below the CKD were released at a depth between 100 and 200 km due to dehydration of lawsonite and phengite and probably were poorer in water and richer in silica. Fluids released at high pressure conditions below the back arc (SR) probably were much denser and dissolved significant amounts of silicate minerals, and potentially carried high amounts of LILE and HFSE. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seismological and SAR signature of unrest at Nisyros caldera, Greece

Nisyros island, a Quaternary volcanic center located at the SE of the Aegean Volcanic Arc, has been in the past characterized by periods of intense seismic activity accompanied sometimes by hydrothermal explosions, the last one being in 1887. The recent long lasting episode of unrest (1995–1998) in the area is the first instrumentally documented providing information on the behavior of the volcano. Evidence from seismicity and SAR interferometry suggests that the presently active part of the Kos–Nisyros volcano-tectonic complex is located at the NW coast of Nisyros island defining an area much smaller than the whole volcano-tectonic area. Seismicity patterns vary both temporally and spatially consistently with different rates of vertical ground deformation inferred from SAR interferometry. These observations help us to discuss the different elements controlling the behavior of the volcanic system such as: the existence, location and timing of magma chamber inflation, the occurrence of tensile failure at the boundaries of the chamber and the possibility of magmatic fluids being expelled to form a shallow magmatic intrusion, the seismic failure and migration of hypocenters indicating shallow magma transport.     

Regional geochemistry of sediments from the Hellenic volcanic arc in regard to submarine hydrothermal activity

Some 600 surface sediment samples from the Hellenic Volcanic Arc region have been analyzed by bulk and HCl selective leach geochemical techniques. Geostatisti‐cal analysis, followed by further selective leaches on a number of samples, has been used in order to identify any regional hydrothermal influence on the sediments and/or any local hydrothermal metal enrichments. The two main sediment components affecting the geochemical variability in the region are biogenic carbonate and volcaniclastic/terrigenous detritus. Proximity to islands appears to be the primary factor controlling the variability in these components. Sediments from deeper water, to the south of the Hellenic Volcanic Arc and in the Anydros Basin, exhibit some elevated levels of Mn, Co, and Cu, probably of hydrogenous origin. No widespread hydrothermal influence to the sediments in the region is recognized, although this could in part be due to the high sedimentation rates in the region. Localized hydrothermal inputs to sediments are, however, recognized proximal to Santorini, in particular within the northern part of the central caldera. These are predominantly of Fe and associated adsorbed/coprecipitated elements along with some Mn enrichment. Several samples proximal to Milos, just off the coast in the northern part of the central Milos embayment, also show enrichments of commonly hydrothermally associated elements, including Mn, Cu, Zn, and Pb. Data suggest that these enrichments could, in part, be due to weathering of “on‐land”; mineralization as well as hydrothermal inputs.     

Strike-slip systems as the main tectonic features in the Plio-Quaternary kinematics of the Calabrian Arc

The oblique and diachronous collision of the Apennine-Maghrebian Chain with the Apulian (in the north-east) and Pelagian (in the south) continental forelands, has determined the characteristic arcuate structure of this orogen. The effects of Plio-Pleistocene deformation of the Calabrian Arc have been analysed on the basis of available reflection seismic profiles and using local time-structural maps reconstructed along the main structures. During this period, internal sectors of the Tertiary chain migrated forward on the oceanic Ionian foreland, and were cut by important strike-slip systems. These last have an orientation approximately coincident with that of the migration of the front, allowing differential movement of the different sectors of the arc, towards the weakly buoyant Ionian oceanic domain. The dataset suggests a clear connection between the development of the strike-slip systems cutting the chain and the direction of tectonic transport, towards the East during Late Messinian/Early Pliocene time, to the ESE during Late Pliocene/Early Pleistocene time, finally to the SSE during the Middle/Late Pleistocene to Present, showing a clockwise rotation in well defined stages during the kinematic evolution of the chain. The origin of the Strait of Messina during the different phases is also interpreted in the context of the analysed regional tectonic setting.     

Constructing the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western Bainaimiao arc terrane,southeastern Central Asian Orogenic Belt

Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt(CAOB).The Bainaimiao arc terrane(BAT) is one of the most important Precambrian microcontinents in southeastern CAOB,however,few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton.This study presents an integrated study of geochronology,zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT.The latest Neoproterozoic(ca.546 Ma) diabases display low SiO_2(46.52-49.24 wt.%) with high MgO(8.23-14.41 wt.%),Cr(66-542 ppm) and Ni(50-129 ppm),consisting with mantle origin.Their highly negative zircon ε_(Hf)(t)(-12.0 to-24.7) and high Fe/Mn ratios(62.1-81.7)further indicate a significantly enriched mantle source.Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root,we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust.The Early Paleozoic(ca.475-417 Ma) arc intrusions in western BAT can be divided into Periods Ⅰ and Ⅱ at approximately 450 Ma.The Period Ⅰ(450 Ma) intrusions contain abundant mafic minerals like hornblende and pyroxene,and show positive zircon ε_(Hf)(t)(+1.5 to+10.9).They are predominantly medium-K calc-alkaline with broad correlations of SiO2 versus various major and trace elements,which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7 kbar.We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts.By contrast,the Period Ⅱ(≤450 Ma) intrusions are characterized by variable zircon e_(Hf)(t)(-15.0 to+11.5) with irregular variations in most major and trace elements,which are more akin to the arc magmas generated in an open system.The general occurrence of elder inherited zircons,along with the relatively high Mg#(45) of some samples,call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components.In combination with the Early Paleozoic tectonic melanges flanking western BAT,we infer that the compositional transition from Period Ⅰ to Ⅱ can be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB.The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT,but also on the associated crustal construction processes before the final arc-continent accretion.     

滇西北红牛-红山矽卡岩型铜矿床石榴子石原位LA-SF-ICP-MS U-Pb定年及地球化学特征

红牛-红山铜矿床是义敦岛弧南段已发现最大的(远端)矽卡岩型矿床,目前其赋矿矽卡岩成岩时代缺乏直接的年代学证据。矽卡岩成岩年龄的精准测定对于该矿床在义敦岛弧南段中生代2期斑岩成矿事件中的定位具有重要意义。红牛-红山矿床矽卡岩中石榴子石可分为2期,早期石榴子石多见于贫矿矽卡岩,粗粒结构,为均质体;晚期石榴子石多见于富矿矽卡岩,细—中粒结构,发育振荡环带,具非均质光性异常。对2期石榴子石开展了原位LA-SF-ICP-MS U-Pb定年和微区成分分析,获得早期石榴子石和晚期石榴子石的U-Pb年龄分别为84.2±3.0 Ma和81.7±3.5 Ma。主量和微量元素含量表明,2期石榴子石属钙铁榴石-钙铝榴石系列,亏损Rb、Ba、K、Sr等,富集Th、U、P;其中,早期石榴子石相对富钙铁榴石组分、LREE/HREE值高,Eu呈高正异常,晚期石榴子石相对富钙铝榴石、LREE/HREE值低、Eu无异常。结合本区燕山晚期碰撞型斑岩成矿事件,认为约80 Ma为本矿床远端矽卡岩成岩成矿时期,成矿作用与同期酸性斑岩关系密切。石榴子石地球化学特征揭示,干矽卡岩阶段水/岩比值高,以渗滤交代作用为主,早期流体为氧化、高温、富铁、弱碱性的相对封闭体系,晚期为相对还原、相对低温、富铝、酸性的开放体系。     

杭州市高温致灾因子危险性风险区划

利用杭州市各气象观测站和自动气象站多年气温资料,以及数理统计方法和Arc GIS空间分析技术,采用极端高温与高温日数作为高温致灾因子风险的主要评价指标,分析杭州市高温天气的气候特征,对杭州市高温灾害致灾因子危险性进行了区划。结果表明,杭州市高温天气灾害较为严重,常出现38.0℃以上的高温,40.0℃以上也不少见,最高达到42.9℃;高温日数较长,最长持续时间超过50 d。杭州市高温灾害危险性风险高值区大部分集中在杭州城区所在的东北部平原,包括上城区、下城区、拱墅区、江干区、滨江区、西湖区西南部、萧山区及余杭区大部。另外,富春江、分水江等沿江两岸,西南部的河谷、盆地以及中部地势较复杂的谷地、盆地等低海拔地区也处于高温危险性中高值区域;北部天目山、西北部昱岭及白际山、西南部千里岗山及龙门山脉高海拔地区均属于危险性风险低值区域。     

浅析江淮区中强震活动与琉球弧强震活动的相关性

江淮地震区地处我国东部地区,属于5-6级地震为主的中强震活动区,琉球弧地处西太平洋地震带的西支,属于7级以上地震为主的强震活动区。本文运用列表和回归分析法对江淮区中强震活动和琉球弧强震活动的相关性进行了分析。结果表明,两者之间的相关性较好,对应率达80％,其平均滞后时间在18个月。并且讨论了产生相关性的原因。分析结论可能对江淮区中强震的中期预报具有一定的作用。