Correlation of gravity with the Maquereau Group, southern Gaspe Peninsula, Quebec

Gravity observations at 150 locations in the Port-Daniel and Chandler area, southeastern Gaspé, Quebec, delineate an E-W to ENE-WSW trending elliptical positive Bouguer gravity anomaly with an amplitude of 19 mGal and a residual gravity anomaly of circa 22 mGal. This anomaly correlates closely with the Lower Cambrian and/or Hadrynian Maquereau Group, the steep gradients defining it coinciding with the contact between the group and the surrounding Ordovician and Siluro-Devonian sedimentary rocks. On the basis of its similar geological setting, this gravity anomaly may be compared with the positive Bouguer anomalies observed over the Fleur de, Lys zone to the east of the Baie Verte lineament in Newfoundland. Tridimensional modelling of the gravity data is inconsistent with a serpentinite dome as originally proposed and it suggests that the Maquereau Group represents an upthrusted slice of deep-seated volcanosedimentary sequences. These were originally located on the lower segment of the continental slope; they constituted a topographic high during the final process of the mountain building at the Taconic phase (Middle to Upper Ordovician time). The most probable way of emplacement of the Maquereau block is through a tear-fault running E-W to ESE-WNW in the southern Gulf of St. Lawrence west of Cape Breton and resulting from Early Paleozoic continental collision between a Eurafrican continent and a North American continent which had a salient located beneath the site of the modern Gulf. The Maquereau may then have been reactivated with a left-lateral motion accompanied of a rotation about a sub-horizontal axis. The presumed Baie Verte-Brompton line ought to be displaced to the north and the Humber zone is narrowed.     

A simple method of computing the variation of annual precipitation over mountainous terrain

A method for computing the distribution of annual precipitation for mountainous areas is presented. As a pilot study, the South Thompson River basin in British Columbia is examined. In addition to data from climatological (valley) stations, annual precipitation is estimated for snow course (mountain) locations from the water equivalent of the snowpack on April 1.An equation for the dependence of precipitation amount on orography is derived from simple physical considerations. Regression equations based on this are then computed and the calculated precipitation is checked with runoff data. A basic predictor is the mean vertical water vapor transport through a horizontal unit area due to flow over undulating terrain. This depends on the average slope of a region 3 deg long by 2 deg lat centered on each grid point. The transport at the earth's surface has a simple correlation coefficient with the combined valley and mountain precipitation data of 0.76. When the transport is computed at the mean cloud base (taken as 1800 m above sea-level), the correlation coefficient rises to 0.87.     

Influence de la subsidence sur la sédimentation

Résumé Des seuils peu profonds et stables, balayés par des courants (stabiliseuils) séparaient en France pendant le Jurassique supérieur des aires subsidentes où s'accumulaient les sédiments; des récifs coralliens pouvaient s'y développer lorsque la subsidence était comprise entre 30 et 70 m. par zone paléontologique, et constituer alors de véritables barrières: la localisation des régions où la subsidence optima était réalisée est par conséquent un élément essentiel de l'abondance et de la répartition des sédiments calcaires.Les variations locales de la subsidence constituent donc un facteur important de la sédimentation, non seulement au point de vue quantitatif, mais encore en ce qui concerne la nature des dépôts.     

Geophysics of the Quebec Appalachians

This paper deals with a synthesis of the geophysics of the Quebec Appalachians. An emphasis is given to the regional geophysical surveys for the final geological interpretation. Regional, residual and downward continued aeromagnetic (AM) data yield information on the tectonic history of the region. The main features are pattern recognition of folded and particularly faulted structures and the boundaries of the tectonic domains. Three main fault systems have been identified and classified using AM information. In addition, depths to the crystallise basement and to the Cambro-Ordovician magnetic layers were obtained for sectors of the Gulf of St. Lawrence-Gaspé region. Nine main geological features are extracted from the processed AM data. Detailed geological modelling of some of the identified structures (e.g. nappes) are presented.Five main geological features are recognized from the gravity data. These features are correlated with the ones obtained from AM data whenever possible. Geological models based on information from surface geology, rock density contrasts and occasional contraints from regional seismic reflection data are presented. The boomerang shape of the Sutton-Bennett anticlinorium and the planar or curved slab-like shape of the obducted ophiolite bodies (upper mantle proto-oceanic floor) may be considered as the significant features.Regional reflection seismic data give a clear geological picture of three (out of four) distinguishable zones of the Quebec Appalachians, that is: autochthonous, parautochthonous, and transition zone: parautochthonous-allochthonous(corresponding approximately to the external domain). The pattern recognition of the allochthonous (approximate internal domain) is somewhat more difficult to decipher from the reflection seismic data. Intraplate seismicity and seismotectonics indicate the presence of four zones of high seismicity in southern Quebec. In almost all zones, the seismotectonic history goes back to Late Precambrian (Hadrynian) times.The scarce and scattered paleomagnetic data of geological formations located on the platform and the external domain of the Quebec Appalachians and in the internal domain are compared with those belonging to the stable North American plate. The paleomagnetic results indicate that the angular difference between the poles from the platform and external domain is large at Cambrian times (40°) and small at Devonian times (5°-10°). This suggests a fast rate of motion and accretion of the internal domain relative to the North American Plate in the Cambrian—Lower Devonian time span (550→-385 Ma).     

Magnetostratigraphy of the lower member of the hadar formation (Ethiopia): Evidence for a short normal event in the mammoth subchron

Summary Rock magnetism and magnetostratigraphy of the lower part of the Hadar Formation (Afar, Ethiopia) is presented after analysis of multiple new collection of samples from over 84 horizons. The Hadar Formation is composed of lacustrine, lake margin, fluvial and flood plain sediments and known for important Pliocene vertebrate faunas including Australopithecus afarensis. Hysteresis measurements, thermomagnetic analysis, growth and decay of isothermal remanent magnetisation are used to unravel the complex magnetic mineralogy of the different representative lithologies. Ferrimagnetic minerals of magnetite or titanomagnetite in composition, in the stable pseudo-single domain (PSD) size range are found to be the main carriers of the remanence. In most sites the characteristic remanence was isolated using stepwise thermal demagnetisation. The overall mean direction for about 72 horizons (434 samples) is D=358·6°, I=7° (k=17·9, α₉₅=4°) implying some 14° of inclination shallowing, related to sediment compaction due to the very rapid sedimentation history of the site. Five successive polarity zones (N1-R1-N2-R2-N3) are identified and correlation with the lower Gauss chron of the astronomically calibrated geomagnetic polarity time scale (GPTS) is proposed using the existing⁴⁰Ar/³⁹Ar ages. This implies the existence of a short normal polarity event (N2), identified on six different sites, within the reversed Mammoth subchron, called the Kada-Hadar event. The age calculated for the Kada-Hadar event, using linear interpolation of the dated horizons, assuming a constant rate of sedimentation is 3.246 Ma and its duration is about 8 kyr.     

Silicate determination in sea water: Toward a reagentless electrochemical method

Silicate has been determined in sea water by four different electrochemical methods based on the detection of the silicomolybdic complex formed in acidic media by the reaction between silicate and molybdenum salts. The first two methods are based on the addition of molybdate and protons in a seawater sample in an electrochemical cell. Cyclic voltammetry presents two reduction and two oxidation peaks giving four values of the concentration and therefore increasing the precision. Then chronoamperometry is performed on an electrode held at a constant potential. A semi-autonomous method has been developed based on the electrochemical anodic oxidation of molybdenum, the complexation of the oxidation product with silicate and the detection of the complex by cyclic voltammetry. This method is tested and compared with the classical colorimetric one during ANT XXIII/3 cruise across Drake Passage (January–February 2006). The detection limit is 1 μM and the deviation between both methods is less than 3% for concentrations higher than 10 μM. Finally a complete reagentless method with a precision of 2.6% is described based on the simultaneous formation of the molybdenum salt and protons in a divided electrochemical cell. This latter method should be very useful for developing a reagentless sensor suitable for long term in situ deployments on oceanic biogeochemical observatories.     

A long pollen record from Lac du Bouchet, Massif Central, France: for the period ca. 325 to 100 ka BP (OIS 9c to OIS 5e)

Pollen analysis was carried out on sediments older than the Würm pleniglacial (OIS 4), in two new sequences (H and I) derived from the centre of Lac du Bouchet, Massif Central. The inferred vegetation history enables, for the first time in France, five temperate episodes to be defined which pre-date the last interglacial. These temperate episodes alternate with episodes during which the changes in vegetation are indicative of glacial climates. Comparison of these climatic episodes with the oceanic isotope record shows that the pollen record of sequences H and I from Lac du Bouchet spans the time interval from OIS 9c (Ussel interstadial) to OIS 5e (Ribains/Eemian interglacial). In the organic sediments from the Amargiers interstadial (OIS 9a), a trachytic layer, Ar/Ar dated to ca 275 ka, enables a correlation to be established with the upper part of a sequence derived from the nearby Praclaux crater, the lower part of this sequence being of Holsteinian age (OIS 11c). The cross-dating of the pollen sequences from Lac du Bouchet (cores H, I and D) and from Praclaux provides a complete record from the Massif Central, southern France, of successive glacial and interglacial episodes that span the last ca. 400 ka, that is the interval from the Holsteinian to the Holocene.     

Découverte d'un flux géothermique anormal au puy de Monténard (chaı̂ne des Puys,France) : résultats préliminaires

Preliminary soil-temperature measurements over 16 months indicate an important geothermal flux of at least 0.4 W m^?2 on the summit of a scoria cone in the Cha??ne des Puys (France). This anomaly can be explained by a residual hydrothermal flux that has begun when the maar and the overlying cone were formed around 30 000 years ago. To cite this article: M. Aubert, C. R. Geoscience 336 (2004).     

Palaeomagnetism of middle Ordovician volcanic rocks from Quebec

This palaeomagnetic study is centered on agglomerates and volcanic rocks from the western margin of the Appalachian belt in the Drummondville-Actonvale-Granby area, Quebec (long.: 72°30′W, lat.: 46°00′N). It involves a total of 36 oriented samples (111 speciments) distributed over eleven sites. Both thermal and AF cleaning techniques were used to isolate residual remanent components. The dispersion of the directions is slightly reduced after AF cleaning and thermal treatment.The palaeopole position obtained is 191°E, 6°N (d_m = 14°, d_p = 7°) after thermal treatment and 164°E, 19°N (d_m = 11°, d_p = 6°) after AF cleaning. The polarity of most of the sites (two exceptions) are reversed. The thermal-treated data appear to be relatively stable and an approximate value of the primary magnetization is extracted from them. The palaeopole obtained does not lie close to the tentatively defined position of the Cambrian and Ordovician poles from rocks of the North American plate; it is located near the Upper Cambrian and Lower Ordovician poles from eastern Newfoundland and the Lower Ordovician pole from the Caledonides in Europe.