Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science最新文献

Hercynian metamorphism in the axial zone of the Pyrenees is mainly represented by a low-pressure (LP) episode with andalousite–sillimanite, but a later kyanite-bearing medium-pressure (MP) event has been described in the most oriental part of the chain. This article shows that, in the Canigou region, the main episode was preceded by a MP event characterised by a kyanite–staurolite assemblage, which is in agreement with Early Hercynian tangential tectonics. It appears that the low-pressure event has been framed by two medium pressure episodes during the Hercynian evolution.

The stationarity of the rainstorm characteristics recorded since 1956 in Niamey is studied by statistical tests. Form and intensity variables are defined from hyetographs (graphs of rainfall versus time) with a five-minute time step. These variables show a significant change especially for the events between 20 and 35 mm of rainfall, before and after 1969.

In semiarid western African regions, potential evapotranspiration (ETP) data are often difficult to collect, preventing from using conceptual rainfall–runoff model. The use of loss functions based on the amount of available water in soil and not on ETP allows to sort out this difficulty. The results underline the interest of this approach in numerous southern countries which have got the same climate.

Although developed and optimised for open oceans, satellite altimetry has the potential to monitor level variations of inland surface waters such as lakes and rivers. Here we present results of water level variations of the Amazon River based on eight years (1993–2000) of altimetry data of the Topex/Poseidon satellite. We first discuss methods to detect wet surfaces from the altimetric measurements, discriminate between water and dry land, and quantify the accuracy of altimetric measurements over water. Then we show water level fluctuations at selected locations where the satellite crosses the Amazon River. The dominant signal is seasonal, mostly annual, with an amplitude of up to 10–15 m peak to peak. Comparison with in situ measurements indicates that water levels are well measured by Topex/Poseidon during high-water season, unlike low-water season, which suffers from data gaps. We further discuss the interannual component of the signal, which shows two marked minima in 1995 and 1998. The 1998 minimum is interpreted as an effect of the 1997–1998 ENSO event, causing rainfall deficit in the central part of the Amazon basin, hence decrease in water levels. An EOF analysis of precipitation fields over the basin during the 1993–1999 period confirms the rainfall minimum by the end of 1997 for this region.

A critical revision of published data along with new field data allow to draw up the succession of the fish faunas from the Lower Cenomanian to the Lower Turonian in the Tafilalt basin and surrounding areas (southeast Morocco). The analysis of these faunas shows changes from freshwater to marine palaeoenvironments. The palaeogeographic distribution of some taxa is discussed. It shows that the crossing of strictly freshwater organisms between Africa and South America was likely impossible at the time of the formation of the deposits resting around the Tafilalt basin and named ‘Kem Kem beds’. The Cenomano-Turonian transgression reached the Erfoud–Errachidia carbonate platform from the Central Tethys, and then connected the central Atlantic.

We discuss the seismic cycle in the Himalayas and its relation to mountain building on the basis of geodetic, seismological and geological data collected in the Himalaya of Nepal. On average over several seismic cycles, localized slip on a major thrust fault, the Main Himalayan Thrust fault, MHT, accommodates the ∼21 mm·yr⁻¹ convergence rate between southern Tibet and India. The geodetic data show that the MHT is presently locked from the sub-Himalayas to beneath the front of the high range where it roots into a sub-horizontal ductile shear zone under southern Tibet. Aseismic slip during the interseismic period induces stress accumulation at the southern edge of this shear zone triggering intense microseismic activity and elastic straining of the upper crust at the front of the high range. This deformation is released, on the long term, by major earthquakes on the MHT. Such an event is the M_w 8.4-1934-earthquake that ruptured a 250–300-km long segment. The major seismic events along the Himalayas since the 19th century have released more than 70% of the crustal strain accumulated over that period, suggesting that, if any, aseismic slip on the MHT cannot account for more than 30% of the total slip.

Over periods of thousands of years, active faults tend to slip at constant rates. Pioneer studies of large Asian faults show that cosmogenic radionuclides (¹⁰Be, ²⁶Al) provide an unparalleled tool to date surface features, whose offsets yield the longest records of recent cumulative movement. The technique is thus uniquely suited to determine long-term (10–100 ka) slip rates. Such rates, combined with coseismic slip-amounts, can give access to recurrence times of earthquakes of similar sizes. Landform dating – morphochronology – is therefore essential to understand fault-behaviour, evaluate seismic hazard, and build physical earthquake models. It is irreplaceable because long-term slip-rates on interacting faults need not coincide with GPS-derived, interseismic rates, and can be difficult to obtain from paleo-seismological trenching.

Active normal faults cut the uplifted reef platforms of Grande-Terre and Marie-Galante and the volcanic rocks of Basse-Terre in the Lesser Antilles arc. New marine geophysical data shows that such faults extend offshore, forming two distinct sets. One set bounds graben perpendicular to the arc, attesting to ∼ north–south extension. The ‘en echelon’ faults of the other set, roughly along the volcanic arc, accommodate a component of sinistral motion. The active Soufrière volcano lies inside the western termination of the Marie-Galante graben. Historical and instrumental earthquakes with magnitude ⩾5.5 may have ruptured the Marie-Galante graben bounding faults.

The Mw7.1, Hector Mine, California earthquake of October 16, 1999 produced a 50 km-long surface rupture, ∼20 km east of the surface rupture of the 1992 Landers earthquake. ERS2 radar data acquired before and after the event provide geodetic measurements of two components of the co-seismic surface displacement field. (1) Interferometric processing of the data provides the range (radar line of sight) component of the displacement field with a precision of a few millimeters. (2) The sub-pixel correlation of the before and after amplitude images provides a second component of the surface displacement field, parallel to the satellite track, with a precision of ∼20 cm. The azimuth offset map reveals a ∼50 km long, NNW surface rupture formed of three sections: a N40°W southern section following the Bullion fault, a N10°W central section cutting through the Bullion Mountains, and a northern N40°W section extending up to the Lavic Lake. A north–south-trending, localized gradient in the azimuth offset field suggests that the central section of the fault extends to the north at depth over a distance of ∼10 km. The slip distribution curve constructed using the azimuth offsets has a bell shape, with a maximum of 6.5 m at the north end of the central fault section. The fault slip model derived from the inversion of the azimuth and from range displacement data indicates that 3–5.8 m of right-lateral slip occurred between the depths of 0 and 8 km along the northern and the central sections of the fault. The solution bears a down-dip component of slip of ∼2 m, west side down, along the southern part of the northern fault section. The down-dip component of slip is consistent with compression on a northeast dipping fault.