SEISMIC AND TECTONO-METAMORPHIC CHARACTER OF THELOWER CONTINENTAL CRUST IN PHANEROZOIC AREAS:A CONSEQUENCE OF POST-THICKENINGEXTENSION

P.F REY

Laboratoire des Sciences de la Terre, ENS Lyon, 69364 LYON Cedex 07and Centre Geologique et Geophysique, USTL, 34095 Montpellier Cedex 05,France

in: TECTONICS (1993), Vol.12, 2, pp. 580-590.

ABSTRACT

Deep seismic profiles of Phanerozoic continental crust commonly show ahighly reflective lower crust. Rheological considerations suggest thatthe seismic fabric of the lower crust can be attributed to the tectonictransposition of various petrological heterogeneities in the main flowplane. The Variscan provinces of western Europe have been affected, duringPhanerozoic times, by several extensional and compressional events. Thegeometrical relationships between seismic and geological structures indicatethat the layering of the lower crust was acquired during the Late Carboniferousto Permian when the thickened Variscan crust was affected by gravitationalcollapse. Petrological and geochronological analyses of deep crustal rocks(xenoliths and exposed sections) indicate that the lower crust has recordeda major high-T / medium-P granulite facies metamorphism during the lateVariscan extension, whereas on the surface Upper Carboniferous to Permianbasins were being deposited. A similar scenario characterizes other Phanerozoicorogenic belts. In the Caledonian provinces of the British Islands, thelower crust is seismically reflective; it has undergone medium-P granuliticmetamorphism during the deposition of Devonian sedimentary basins, at theend of the Caledonian orogeny. In the same way, collapse of the Mesozoicbelt in the western part of North America is responsible, during the Cenozoic,for pervasive crustal extension whose consequence is a seismic layeringof the lower crust accompanied by a low-P granulite grade metamorphic event,while in the mid- and upper crusts, low-angle, ductile, normal faults giverise to the Basin and Range Province. Therefore, it is proposed that thereis a genetic relationship between (1) post-thickening crustal extension,(2) low- to medium-P granulite facies metamorphism of the lower crust and(3) seismic layering of the lower crust.

ACKNOWLEDGMENTS

Thanks are due to G. MÚnard, A. Paul, D. Fountain and J. M. Caron,for very stimulating discussions. Special thanks are due to J. P. Burg,C. Teyssier, J. Van Den Driessche, and B.Tikoff for useful reviews, comments,and English improvement. K. D. Nelson and G. Ranalli made helpful suggestionson the manuscript submitted to the journal. This work has been supportedby the INSU-CNRS (ATP-ECORS 891705).

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