Slingram EMI prospection: Are vertical orientated devices a suitable solution in archaeological and pedological prospection?

Electromagnetic induction (EMI) is one of the geophysical techniques widely used in soil studies, the slingram devices being held horizontally over the soil surface, i.e. with the coils located at the same height above the ground surface. Our study aims assessing the abilities of slingram devices when held vertically. 1D and 3D modelling have been achieved in order to compare the theoretical responses of vertical devices to the horizontal ones. Some comparative surveys were also undertaken in archaeological contexts to confirm the reliability of theoretical conclusions. Both approaches show that vertical slingram devices are suitable for survey and can constitute an alternative to the usual horizontal orientation. We give a table in Appendix A which contains the calibration coefficient allowing transforming of the values given by some of commercially available devices which would be advantageous to use in vertical orientation     

Tephra stratigraphy and eruptive volume of the May, 2008, Chaitén eruption,Chile

On May 1st 2008 Mount Chaitén (southern Chile) interrupted a long period of quiescence, generating a sequence of explosive eruptions and causing the evacuation of Chaitén town located a few kilometers south of the volcano. The activity was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level. The products were dispersed across a wide area, with the finest ash reaching the Atlantic coast of Argentina. Our field observations in the proximal-medial area (3–25 km from the vent) indicate that the May 2008 tephra deposit consists of numerous layers, most of which can be correlated with individual eruptive events. These layers vary from extremely fine-grained ash to layers of lapilli and blocks, composed of both juvenile and lithic material. Here we describe the stratigraphy and physical characteristics of the May 2008 deposits, and propose a reconstruction of the timing of the May 2008 events. The deposits are mainly associated with the three main explosive phases which occurred on 1st–2nd May, 3rd–5th May and 6th May, with an estimated bulk tephra volume of 0.5–1.0 km³ (integration of both exponential and power-law fitting). For the 6th May event, represented by a layer composed mainly of lithic lapilli and blocks (>2 mm), an isopleth map was compiled from which a 19 km plume height was determined, which is in good agreement with satellite observations.     

巢湖北部地区海相地层黏土矿物特征及其地质意义

对巢湖北部地区以碳酸盐岩为主的寒武系—下三叠统海相地层的黏土矿物分析及伊利石结晶度测定的结果表明:伊利石是本区海相地层的主体矿物,平均体积分数为73%。黏土矿物的成岩转化特征表现为蒙皂石的伊利石化。按照伊利石结晶度的成岩-浅变质程度划分,本区寒武系—下三叠统的成岩作用已达到最高成岩阶段。伊利石结晶度的纵向变化总体上可分为上、下两部分:下部寒武系至志留系的结晶度为0.41～0.60°Δ2θ,结晶度与埋深具有一定的相关性,成岩作用是下部地层黏土矿物特征的主控因素;上部泥盆系至下三叠统的结晶度为0.45～0.77°Δ2θ,呈波动变化,蒙皂石连续出现,地层没有被显著埋藏,沉积因素是上部地层黏土矿物特征的主控因素。下三叠统地层中蒙皂石的分布特征以及上泥盆统五通组顶部高岭石的普遍富集具有重要的古气候研究意义。     

On The Small-Scale Dynamics Of Flow Splitting In The Rhine Valley During A Shallow Foehn Event

Flow splitting in the Rhine valley has been observed with a transportable wind lidar (TWL) during a shallow-foehn event in the framework of the Mesoscale Alpine Programme (MAP). The Doppler lidar recorded in detail flow splitting, foehn wind gusts, and flow reversal. Such structures have not previously been observed with comparable detail by conventional in-situinstruments. In addition to the TWL, boundary-layer processes have been documented by means of rawinsondes and surface stations. This paper presents an analysis of the processes giving birth to flow splitting between the Seez and Rhine valleys during Intensive Observation Period (IOP) 5 (1–3 October 1999) by combining the collected data with hydraulic theory. The study shows thatthe splitting of the channelled flow is associated with (1) the existence of a stagnation point at the intersection of the Seez and Rhine valleys, and (2) the deflection of the flow by the lateral sidewalls of the valleys.     

Typologie et mode de mise en place des roches magmatiques dans les Pyrénées hercyniennes

Résumé L'évolution géodynamique hercynienne des Pyrénées se caractérise par une importante activité magmatique à l'origine de corps plutomques de taille, de composition et de niveaux de mise en place différents. A partir de ces caractéristiques nous avons pu établir une typologie qui nous a permis de distinguer deux types principaux de massifs: 1) les massifs de composition homogène, acides (massifs de type HA) ou basiques (massifs de type HB), localisés dans les grands ensembles métamorphiques régionaux et 2) les massifs de composition hétérogène, formés par l'association de roches basiques et de roches acides dont la répartition spatiale a permis de distinguer les massifs de type CA (roches acides au coeur du massif), les massifs de type CB (roches basiques au coer du massif) et les massifs de type CR (récurrences de lentilles de roches basiques dans un massif zoné de type CA ou CB), mis en place dans le Paléozoïque supérieur généralement loin des grands ensembles métamorphiques. Tous ces massifs se sont mis en place diapiriquement au cours du métamorphisme régional et de la déformation hercynienne majeure.

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One of the main characteristics of the Hercynian geodynamic evolution of the Pyrenees is an intense magmatic activity producing large plutonic bodies. They have different sizes, various compositions and distinct emplacement levels. From these characteristics, a typology for these massifs is defined. Two main types of massifs have been recognized: 1) massifs with an homogenous composition founded in large regional metamorphic areas, and 2) massifs with an heterogenous composition of acid and basic rocks, whose spatial disposition permits the recognition of three sub-types: a) the CA type where acid rocks are at the core of the massif; b) the CB type where basic rocks are at the core of the massif, and c) the CR type where numerous basic xenoliths are scattered within either a CA or a CB type massif. These heterogenous massifs generally occur within Upper Paleozoic rocks and usually far away from regional metamorphic areas. All of these massifs were emplaced diapirically during the regional metamorphism and the main Hercynian deformation.

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Zusammenfassung Ein wichtiges charakteristisches Merkmal der geodynamischen Entwicklung der Pyrenäen während der herzynischen Phase ist eine hohe magmatische Aktivität, auf die Plutone größerer Ausdehnung zurückgeführt werden. Größe, Mineralogie und Erstarrungstiefe der Plutone variieren. Anhand dieser Charakteristika können die Intrusivkörper in zwei Hauptgruppen differenziert werden. Die erste Gruppe (1) sind Plutone homogener Zusammensetzung in regionalmetamorph überprägten Gebieten. Die zweite sind heterogen zusammengesetzte Plutone (2) saurer und basischer Magmatite, deren räumliche Anordnung eine Untergliederung in drei Untergruppen ermöglicht:a) Bei dem CA-Typ befinden sich saure Gesteine im Kern; bei b), dem CB-Typ, liegen basische Gesteine im Kern vor und bei c), einem CR genannten Typ, sind zahlreiche basische Xenolithe entweder in dem CA-oder dem CB-Typ verstreut vorhanden. Diese heterogenen Plutone findet man allein in spätpaläozoischen Gesteinen und in der Regel weit entfernt von Gebieten, die metamorph beeinflußt wurden. Sämtliche Plutone haben eine Diapir-ähnliche Form und intrudierten zeitgleich mit der Regionalmetamorphose und der Hauptphase der herzynischen Deformation.

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Analysis of snow-vegetation interactions in the low Arctic-Subarctic transition zone (northeastern Canada)

Abstract

Recent studies have shown that northern vegetation has been growing in relation to a warming climate over the last four decades, especially across the transition zone between tundra and taiga. Shrub growth affects snow properties and the surface energy budget, which must be better studied to quantify shrub-snow-climate feedbacks. The objective of this research is to improve the characterization of the impact of shrubs on snow evolution, from its accumulation to its melt, using in-situ and satellite measurements. The research is presented for the Umiujaq site, Nunavik, representative of the low Arctic–Subarctic transition zone. Snow depth, measured along numerous transects spanning different land cover types is found to increase by a factor 2.5–3 between tundra and forest, while snow density decreases. This illustrates the trapping effect of vegetation well. Complementary, continuous snow depth measurements using weather stations from two sites (tundra with low shrubs and a small clearing with shrubs within the forest) show different site-dependent behaviors. Spatial analysis from high-resolution Pleiades images combined with Landsat (Normalized Difference Snow Index) and MODIS (Fractional Snow Cover) images suggest a slight delay in melt over open and dense forest areas compared to tundra and dense high shrubs.     

Evidence of quaternary transtensional tectonics in the Nekor basin (NE Morocco)

The geodynamic processes in the western Mediterranean are driven by both deep (mantle) processes such as slab‐rollback or delamination, oblique plate convergence and inherited structures. The present‐day deformation of the Alboran Sea and in particular the Nekor basin area is linked to these coeval effects. The seismically active Nekor basin is an extensional basin formed in a convergent setting at the eastern part of the Rif Chain whose boundaries extend both onshore and offshore Morocco. We propose a new structural model of the Nekor basin based on high‐resolution offshore data compiled from recent seismic reflection profiles, swath bathymetry acquisitions and industrial seismic reflection profiles. The new data set shows that the northern limit of the basin is oriented N49° with right‐stepping faults from the Bousekkour–Aghbal fault to the sinistral Bokkoya fault zone. This pattern indicates the presence of an inherited left‐lateral basement fault parallel to the major inherited Nekor fault. This fault has been interpreted as a Quaternary active left‐lateral transfer fault localized on weak structural discontinuities inherited from the orogenic period. Onshore and offshore active faults enclose a rhombohedral tectonic Nekor Basin. Normal faults oriented N155° offset the most recent Quaternary deposits in the Nekor basin, and indicate the transtensional behaviour of this basin. The geometry of these faults suggests a likely rollover structure and the presence at depth of a crustal detachment. Inactive Plio‐Quaternary normal faults to the east of the Ras Tarf promontory and geometries of depocentres seem to indicate the migration of deformation from east to west. The local orientations of horizontal stress directions deduced from normal fault orientations are compatible with the extrusion of the Rifian units and coherent with the westward rollback of the Tethyan slab and the localization of the present‐day slab detachment or delamination.     

Découverte d’un chevauchement d’âge quaternaire au sud de la Grande Kabylie (Algérie)

Résumé

À l’échelle du Maghreb, la Kabylie (Algérie) est considérée comme une région asismique. L’étude détaillée des données de la sismicité historique et instrumentale de cette région montre une activité sismique modérée depuis au moins un siècle et demi (M₁ = 5,0). Les observations géomorphologiques faites sur le versant sud du massif kabyle, s’appuyant sur l’analyse d’images satellites, de photos aériennes et de la topographie, nous ont permis de démontrer la présence d’une activité tectonique quaternaire le long de la rupture de pente au sud de ces massifs. Elle correspond à la réactivation de chevauchements d’âge miocène qui se manifestent par des escarpements d’au moins 10 m affectant le glacis d’âge quaternaire ainsi que les cônes de déjection. Ces escarpements sont interprétés comme des ruptures probablement associées à une succession de séismes de forte magnitude. Les observations que nous décrivons se situent dans la région de Tazmalt et de Boüira où ces déformations sont les plus spectaculaires. L’ampleur des déformations quaternaires qui affectent l’ensemble de cette région considérée jusqu’à l’heure actuelle comme asismique ou faiblement sismique pose, comme dans la plupart des régions intracontinentales, le problème de la période de retour des séismes de forte magnitude. © Elsevier, Paris