Kinematic interpretation of folds in alpine-type peridotites

From a general understanding of the flow mechanisms in alpine-type peridotites, it is possible to describe without ambiguity the general flow regime and its directions in a massif. This result provides the means for an investigation of the origin of the folding in pyroxenitic layers independent of any preconceived theory on folding.The folds are usually isoclinal and of the flexural-flow type as demonstrated by petrofabric studies in hinges. Their axes are always parallel or subparallel to a mineral lineation which in turn is parallel or close to the orientation of the fabric elements defining the flow line. Their axial plane, which usually coincides with the foliation, is parallel to or close to the flow plane. This conclusion, also supported by paragenetic observations, shows that the folds were formed or transposed during the plastic flow responsible for the development of structures (foliation and lineation), textures and preferred mineral orientations. In the case of the Lanzo Massif and a few other Iherzolite massifs, the flow occurred during the intrusion from the mantle. The mapping in Lanzo yields evidence of a large-scale U-shaped fold with a remarkable pattern of mesoscopic folds attached to it: the tight isoclinal folds are restricted to the limbs of the largescale structure, and the open folds locally refolding former isoclinal ones to the hinge area where the angle between the folded pyroxenitic layering and the axial-plane foliation is large. Stereograms of the field structures in this hinge area clearly illustrate the geometric relations mentioned above.This folding, characterized by its axis and axial plane respectively close to the flow line and flow plane, can be explained either by rotation towards the flow line of non-cylindrical-fold axes or by direct formation in a non-plane flow when the flow line is initially contained in the layering or close to it. In this respect, the folding may bring information on the minor flow component, complementary to that given on the major flow component by considering the textures and fabrics. Finally this folding is shown to be ubiquitous in plastically deformed peridotites. It is proposed that these conclusions be extended to other domains submitted to intense non-plane flow.     

Thrusting of young lithosphere in subduction zones with special reference to structures in ophiolitic peridotites

Two contrasted types of structures have been recognized in peridotites from ophiolites and from the oceanic environment. The first one, typical of high-temperature/moderate-stress conditions, is observed in the upper part of ophiolitic peridotites and has been ascribed to plastic flow in an oceanic ridge environment. The second one, typical of moderate-temperature/high-stress conditions, is more specially dealt with here. It is printed in the peridotites above the basal metamorphic aureole found in many ophiolites. The strain increases downward over 1–2 km to produce peridotite mylonites at the contact with the metamorphic aureole. Similarities with rocks from trench and island arc environments suggest ascribing this deformation in ophiolites to a trench environment. We propose that shear fracturing in a young oceanic lithosphere is initiated by the compressive elastic stress in its lower part which is produced by bending of the subducted plate. An externally applied compressive stress is responsible for subsequent overthrusting of the fractured lithosphere. This interpretation is in good agreement with the available geophysical data on young subducted plates and with the physical data on ophiolitic peridotites.     

K-Ar ages from Tertiary lavas of the northernmost Chilean Andes

Radiometric ages for lava flows of northern Chile indicate that a regional Oligocene erosion surface was faulted when the Cordillera de la Costa was uplifted at the end of the Paleogene. Uplift of the coastal mountains caused the longitudinal depression to infil with sediment until aggradation virtually ceased in the Middle Miocene. Ignimbrites of the Rhyolite Formation were deposited in the Pampa del Tamarugal and across the Cordillera de la Costa in the earliest Miocene, whereas volcanoes of the “Andesite” Formation were locally active at least as early as the Middle Miocene.     

Reviewing the discoverability and accessibility to data and information products linked to Essential Climate Variables

ABSTRACT

Essential Climate Variables (ECVs) are geophysical records generated from systematic Earth Observations associated with climate variations, changes, and impacts. ECVs products support the data and information needs of international frameworks and policies such as the work of the United Nations Framework Convention on Climate Change (UNFCCC) and the Intergovernmental Panel on Climate Change (IPCC). We map the main networks and initiatives publishing ECVs, by presenting an overview of existing satellite-based ECVs, their general data creation characteristics, discoverability and accessibility methods from an end-user perspective. We investigate key initiatives providing or coordinating access to ECV data records, such as the Global Climate Observing System (GCOS), the Committee on Earth Observation Satellites (CEOS), the Coordination Group for Meteorological Satellites (CGMS), Joint Working Group on Climate (WGClimate), the Remote Sensing Systems (REMSS), and the European Space Agency Climate Change Initiative (ESA CCI). We find that ECV data discovery and access is difficult and time consuming due to the lack of common data and metadata catalogues. In addition, the selection of fit-for-purpose data records by end-users requires the implementation of interoperable standards and scalable data infrastructures to allow the generation of tailored applications and data-driven information products in support of decision-making processes.     

Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling

The Northern Mediterranean Current is the return branch of the cyclonic circulation of the northwestern Mediterranean Sea. Because of geostrophic constraints, this warm and oligotrophic current is forced to flow westward along the continental slope of the Gulf of Lion. But, occasionally, it penetrates on the shelf and strongly impacts the local biogeochemistry and in turn the primary production. By combining in situ observations and high-resolution modelling, it is shown that intrusions on the eastern part of the gulf are mainly forced by easterly or northwesterly wind events, through physical mechanisms that are very different in nature. Easterlies induce a piling of water along the Gulf of Lion coast that drives, through geostrophy, an alongshore shelf-intruding current. This intrusive current occurs independently of the stratification and is concomitant with the wind forcing. On the other hand, intrusions due to northwesterlies only occur during stratified conditions and are related to the development of upwellings along the Gulf of Lion coasts. When the upwelling develops, a northwestward alongshore pressure force balances the Coriolis force associated with the onshore flow at depth. When the winds drop, the upwelling relaxes and the onshore flow weakens. Consequently, the Coriolis force no longer counterbalances the pressure force that ultimately dominates the momentum balance, causing the displacement of the Northern Current on the Gulf of Lion shelf approximately 1 day after the wind relaxation. This time lag between the northwesterlies decrease and the intrusions permits to anticipate possible changes in the biogeochemistry of the Gulf of Lion.     

A practical review of geostatistical processing applied to geophysical data: methods and applications

Nowadays, geostatistics is commonly applied for numerous gridding or modelling tasks. However, it is still under used and unknown for classical geophysical applications. This paper highlights the main geostatistical methods relevant for geophysical issues, for instance to improve the quality of seismic data such as velocity cubes or interpreted horizons. These methods are then illustrated through four examples. The first example, based on a gravity survey presents how a geostatistical interpolation can be used to filter out a global trend, in order to better define real anomalies. In the second case study, dedicated to refraction surveying, geostatistical filtering is used to filter out acquisition artefacts and identify the main geological structures. The third one is an example of porosity being integrated geostatistically with a seismic acoustic impedance map. The last example deals with geostatistical time to depth conversion; the interest of performing geostatistical simulations is finally discussed.     

Operational performance of current synthetic aperture radar sensors in mapping soil surface characteristics in agricultural environments: application to hydrological and erosion modelling

Synthetic aperture radar (SAR) sensors are often used to characterize the surface of bare soils in agricultural environments. They enable the soil moisture and roughness to be estimated with constraints linked to the configurations of the sensors (polarization, incidence angle and radar wavelength). These key soil characteristics are necessary for different applications, such as hydrology and risk prediction. This article reviews the potential of currently operational SAR sensors and those planned for the near future to characterize soil surface as a function of users' needs. It details what it is possible to achieve in terms of mapping soil moisture and roughness by specifying optimal radar configurations and the precision associated with the estimation of soil surface characteristics. The summary carried out for the present article shows that mapping soil moisture is optimal with SAR sensors at low incidence angles (<35 ). This configuration, which enables an estimated moisture accuracy greater than 6% is possible several times a month taking into account all the current and future sensors. Concerning soil roughness, it is best mapped using three classes (smooth, moderately rough, and rough). Such mapping requires high‐incidence data, which is possible with certain current sensors (RADARSAT‐1 and ASAR both in band C). When L‐band sensors (ALOS) become available, this mapping accuracy should improve because the sensitivity of the radar signal to Soil Surface Characteristics (SSC) increases with wavelength. Finally, the polarimetric mode of certain imminent sensors (ALOS, RADARSAT‐2, TerraSAR‐X, etc.), and the possibility of acquiring data at very high spatial resolution (metre scale), offer great potential in terms of improving the quality of SSC mapping. Copyright © 2007 John Wiley & Sons, Ltd.     

A 2000 year midge-based paleotemperature reconstruction from the Canadian Arctic archipelago

A lake sediment core recovered from Lake V57 on Victoria Island, Nunavut, Canada, spanning the last 2000 years, was analyzed for sub-fossil midge remains and organic-matter content (estimated by loss-on-ignition (LOI)). Significant changes in midge community composition occurred during the last 2000 years, with a distinct midge community appearing after 1600 AD. The chironomid community between 0 and 1600 AD was dominated by Heterotrissocladius, Tanytarsus, Abiskomyia, and Paracladius. At approximately 1600 AD, Heterotrissocladius decreased in relative abundance and taxa such as Corynocera ambigua, C. oliveri, Psectrocladius sordidellus type, and Pentanneurini increased in relative abundance. Previously published midge-based inference models for average July air temperature (AJAT) and summer surface–water temperature (SSWT) were applied to the subfossil midge stratigraphy. The AJAT reconstruction indicates relatively cool conditions existed between 1100 and 1600 AD, with exceptional warming occuring after 1600 AD, as lake productivity inferred from organic-matter content increased concomitantly with midge-inferred AJAT and SSWT. The cooler conditions between 1200 and 1600 AD, and the pattern of warming over recent centuries inferred from Lake V57 is broadly consistent with temperature-sensitive biogenic silica records from other sites in the central Canadian Arctic and the treeline zone to the south suggesting a regionally synchronous response to climate forcing.