Structures crustales et subcrustales deduites des tomographies ECORS et Lithoscope

Abstract

A number of seismological tomographies using both artificial (ECORS) and natural (LITHOSCOPE) sources have been performed in France in the aim of precising the structures of the crust and the subcrustal lithosphere. If the results of the ECORS deep seismic reflection profiles are well-known among the community of geologists, it is not the case for the tomographies based on earthquake records performed within the framework of the LITHOSCOPE program. We present here the main contributions of ECORS to the knowledge of the lower crust in France and compare them to the results of the teleseismic studies performed in the Pyrénées, the Alps, the Rhinegraben and the Massif Central. One of the major results of the ECORS program is the discovery of the very strong reflectivity of the lower crust on most of the profiles recorded in France. With similar discoveries made by the deep seismic reflection programs of neighbouring countries (mainly Germany and Great-Britain), this lower crustal reflectivily appears to be the outstanding characteristics of the western european crust which has been affected by the variscan orogeny. The most typical examples can be found on the profiles "North of France", "Bay of Biscay", "East Pyrénées", "Jura-Bresse" and "Rhinegraben". Under the mountain ranges, the contribution of wide-angle reflection profiling to the Moho location was very important, particularly under the internal Alps. A number of reflection and refraction profiles (in the Eastern and Western Pyrénées, the Rhinegraben) were complemented by LITHOSCOPE experiments to measure P-wave velocities below the Moho and, in particular, precise the geometry of the lithosphere/asthenosphere boundary. In the Pyrénées, the only common feature between the eastern and western parts of the chain is the 15 to 20 km increase in Moho depth observed at the limit between the european and the iberic crusts. Very different models have been proposed to explain the teleseismic residuals and the gravimetric data. In the east, an important thinning of the lithosphere could exist between 25 and 50 km north of the North Pyrenean Fault. In the west, the totality of the high-velocity and heavy anomaly has its origin located entirely within the crust, without any anomaly of the subcrustal lithosphere. In the Rhinegraben, no low-velocity anomaly was found below the Moho. Thus, the graben is not a first order phenomenon at the lithospheric scale. In the Alps, the observations of teleseismic residuals are still rare and they only give a very partial image of a possible subduction of the european lithosphere under the apulian lithosphere. However, this lack should disappear in the next future due to the installation of permanent seismic networks covering all the French Alps, and with more intense collaborations with the swiss and itlhian teams. A european experiment conducted in 1992 in the Massif Central will help to precise the crustal structures in a region that has never been imaged by deep seismic reflection profiles. The first results of the teleseismic experiment show an important low-velocity, warm anomaly as a lithospheric plume rising up as separated diapirs under the volcanic massifs. All these data reveal major crustal and lithospheric discontinuities as well as lateral heterogeneities of the rocks physical properties. The intraplate tectonics is thus most probably governed by the variety of structures and rheologies and it implies the reactivation of ancient discontinuities. In a methodological point of view, all these results confirm the importance of a combined use of different seismological techniques (vertical reflection profiling, refraction, wide-angle reflection, teleseismic tomography) with different resolutions and different investigation depths, in the study of the lithosphere.