Louis Briqueu

Nouvelles données géochronologiques, géochimiques et isotopiques sur le volcanisme du Forez : relation avec l'évolution cénozoïque du manteau du Massif central

In the Forez district, two volcanic events are documented. The earlier one, of Paleocene age, predates the Oligocene rifting. The magmas (melilitite) are of carbonatitic affinity and they would correspond to small volume melts responsible for the chemical and mineralogical modifications previously identified in the lithospheric mantle of this region. The later lavas are Lower Miocene and of basanitic composition. Their isotopic signatures (Sr, Nd, Pb) suggest that they partly originated from partial melting of this modified lithosphere. The Paleocene magmatism would indicate the initiation of asthenospheric upwelling beneath the French Massif Central.

Impact of the pre-processings on the fractal properties of the airborne gamma ray measurements: a case study from Hoggar (Algeria)

The analysis of gamma ray measurements provides valuable information about the basement’s lithospherical model and the radioactive elements contained in the subsurface. To be exploited, the raw data need pre-processing operations which consist of corrections of the Compton effect, altimetry and the aircraft background, and 2D interpolations. These transformations may modify the data’s stochastic component which reflects heterogeneities of the investigated medium. In this study, a fractal analysis is carried out on airborne gamma ray data measured in the Hoggar’s area (Algeria). First, it is shown that the analyzed data exhibit fractal properties. Due to the dependence of the latter on the spatial location of the measures, the data can be considered as paths of multifractional Brownian motions. Another result worth noting is that the fractal nature of the raw data is not modified by the pre-processing operations. The fractal analysis can be then performed on the raw spectrometric measurements without submitting them to any pre-processing; thus, the stochastic component of the measurements is kept intact.

Le complexe annulaire d'âge Oligocène de l'Achkal (hoggar Central, Sud Algérie) : témoin de la transition au Cénozoïque entre les magmatismes tholéitique et alcalin. Évidences par les isotopes du Sr, Nd et Pb

The Achkal Oligocene ring complex cross-cuts the Upper Eocene tholeiitic traps located on the top of the Hoggar swell. The plutonic rocks range from tholeiitic gabbros to alkali essexites, monzonites and syenites, whereas the volcanites are restricted to late peralkaline rhyolites. The affinity change linked to the large isotopic heterogeneities (from EM1 to HIMU) suggests that the parental magmas are issued from two distinct mantle sources, first lithospheric then deeper. The Achkal has recorded the magmatic evolution of the Hoggar hot spot, between Eocene and Miocene.

Regularity Analysis of Airborne Natural Gamma Ray Data Measured in the Hoggar Area (Algeria)

The airborne Gamma Ray (GR) measurements have been used since decades in geophysical research. The airborne measurement of gamma radiation emitted by naturally occurring elements finds applications in: geological mapping (Graham and Bonham-Carter, 1993; Jaques et al., 1997; Doll et al., 2000, Aydin et al., 2006; Sulekha Rao et al., 2009), regolith and soil mapping (Cook et al., 1996; Wilford et al., 1997; Bierwirth and Welsh, 2000), mineral exploration (Brown et al., 2000), and hydrocarbon research (Matolin and Stranik, 2006). Potassium (K), Uranium (U) and Thorium (Th) are the three most abundant, naturally occurring radioactive elements. The K element is the main component of mineral deposits, while Uranium and Thorium are present in trace amounts, as mobile and immobile elements, respectively. The concentration of these different radioelements varies between different rock types, thus the information provided by a gamma-ray spectrometer can be exploited for needs of the rocks cartography. The obtained maps allow to localize radioelement anomalies corresponding to zones disrupted by a mineralizing system. The approach presented in this chapter deepens the results derived from the conventional study. It consists on a mono(two)-dimensional fractal analysis of natural radioactivity measurements recorded over the Hoggar area (Algeria). The natural radioactivity measurements, like other geophysical signals, contain a deterministic and a stochastic components. The former part holds information related to the regional aspect, while the latter reflects the local heterogeneities. As the raw spectrometric data need to be processed before any exploitation, the stochastic component can be altered and some information about heterogeneities is lost. Here, we show first the fractal behavior of the analyzed GR measurements. In addition, it is demonstrated that this behavior is not affected by all the pre-processing operations (spectrometric corrections and 2D-interpolations). The corrections are not then necessary. Since the analyzed data exhibit a fractal exponent varying with the spatial position, they are modeled as paths of multifractional Brownian motions (mBms) (Peltier and Levy-Vehel, 1995).