Dating Lower Crustal Features in France and Adjacent Areas from Deep Seismic Profiles
- Robert F. Mereu,
- Stephan Mueller and
- David M. Fountain
Published Online: 9 APR 2013
Copyright 1989 by the American Geophysical Union
Properties and Processes of Earth's Lower Crust
How to Cite
Bois, C., Pinet, B. and Roure, F. (1989) Dating Lower Crustal Features in France and Adjacent Areas from Deep Seismic Profiles, in Properties and Processes of Earth's Lower Crust (eds R. F. Mereu, S. Mueller and D. M. Fountain), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM051p0017
- Published Online: 9 APR 2013
- Published Print: 1 JAN 1989
Print ISBN: 9780875904566
Online ISBN: 9781118666388
France's geological history may be broadly subdivided into three main phases. the first one corresponds to the consolidation of Western Europe during the Variscan orogeny. the second one is related to the formation of large cratonic basins such as the Paris and Aquitaine ones and the opening of new oceanic domains such as the Tethys ocean and the North Atlantic. the third phase corresponds to the Tertiary closure of the Tethys and the formation of the Pyrenees and Alps collisional ranges.
The deep seismic profiles shot in SW Britain and northern France across the Variscan orogenic belt show a comparatively flat Moho and a highly reflective lower crust displaying prominent layering over 2 to 4.5 s TWT. On the profiles shot on the Aquitaine shelf (Parentis basin) and across the Pyrenees and Alps ranges, the Moho displays definite deformation and the layered lower crust shows local changes in thickness, especially beneath the Parentis Basin. the crust is partly or entirely cut by dipping seismic events or vertical discontinuities that are interpreted as thrust or strike-slip faults related to the last major orogenic event having occurred in the area.
The Paleozoic thrusts and faults seem to have been obliterated by the lower crustal layering which, on the other hand, is involved in the Tertiary deformation. A study of the layered crust beneath various sedimentary basins suggests that the layering was emplaced either during the late Carboniferous-Permian, or the late Triassic-early-middle Jurassic. Both these periods correspond to major geodynamic events associated with regional increases of the thermal flow: reequilibration of the crust following the Variscan orogeny and opening of the Tethys ocean. To assign one of these events to the layering formation depends on the model of formation chosen for the SW Britain sedimentary basins.
Two geological causes may be proposed for the emplacement of the layering in the lower crust: metamorphism and mafic magmatic intrusion and/or cracks filled by fluids associated with moderate extensional shearing.
The seismic Moho was formed at the same time as the layering in the lower crust. Its emplacement across former crustal features suggests that rocks presently beneath the Moho may have been a part of the crust which was largely metamorphosed into higher velocity rocks that cannot be differentiated from those of the original mantle by the usual seismic methods. Some relics of these rocks show the former roots of the Variscan mountain range. Such a metamorphic process might be in progress in the deepest part of the crust beneath the Pyrenees and the Alps.