COLLAPSE OF THE EUROPEAN VARISCAN BELT IN THEVOSGES MASSIF AND SEISMIC FABRIC OF THE CONTINENTAL CRUST

P.F. Rey

Laboratoire des Sciences de la Terre, ENS Lyon, 69364 LYON Cedex 07,France and Centre Geologique et Geophysique, USTL, 34095 Montpellier Cedex05, France

ABSTRACT

In the Vosges, collapse of the variscan belt is responsible for the mainstructure of the continental crust. This tectonics is strong enough toerase the structures related to the eo-variscan compression. In this area,the seismic fabric of the continental crust displays the main feature ofphanerozoic provinces. The relative homogeneity of granitic rocks, whichcompose the upper crust, can explain its transparency on deep seismic profiles.In such a medium, low-angle ductile normal faults introduce elastic heterogeneities,whose geometrical feature are sufficient to be detected. However, the detectabilityof such structural heterogeneities depend also on the impedance contrastexisting between the mylonitic level and the protolith and on the transitionzone thickness- wavelength ratio (H/l). Measurements of S wave velocitiesacross ductile shear zones indicate that the impedance contrast is notsufficient for giving rise to seismic reflectors. On the contrary, P wavemeasurements indicate that the impedance contrast is sufficient, but alsothat the reflectivity of ductile shear zones increase when the H/l ratiodecreases. Within the granitic upper crust, low H/l ratios are expectedin low-temperature ductile shear zones, which suggests that these low-temperatureshear zones are more reflective than those developped at higher temperature.The seismic fabric of the lower crust can be discussed upon the confrontationof geological and seismic data. In many Phanerozoic areas, where a layeredlower crust has been observed, petrological and geochronological data indicatethat the major event recorded by the lower crust is related to a medium-pressuregranulite facies metamorphism, whose age argues for a late-orogenic origin.Field studies demonstrate that extension of the upper crust, with developmentof flat-lying ductile normal faults controlling sedimentary basin deposition,is coeval with the granulite-facies metamorphism of the deep crust. Therefore,it is proposed that a relationship exists between lower crustal seismiclayering, medium- to low-pressure granulite facies metamorphism of thelower crust, upper crust stretching and sedimentary basin deposition. Thesecombined processes evolve together in a post-thickening, crustal extensiontectonic setting.

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