Geodynamics of the France Southeast Basin

Abstract

We investigate the geodynamics of the Southeast Basin with the help of maps of the basement and of major sedimentary horizons based on available seismic reflection profiles and drill holes. We also present a study of the seismicity along the Middle Durance fault. The present seismic activity of the SE Basin cannot be attributed to the Africa/Eurasia shortening since spatial geodesy demonstrates that there is no significant motion of Corsica-Sardinia with respect to Eurasia and since gravitational collapse of the Alps has characterized the last few millions years. Our study demonstrates that the basement of this 140 by 200 km Triassic basin has been essentially undeformed since its formation, most probably because of the hardening of the cooling lithosphere after its 50% thinning during the Triassic distension. The regional geodynamics are thus dominated by the interaction of this rigid unit with the surrounding zones of active deformation. The 12 km thick Mesozoic sediment cover includes at its base an up to 4 km thick mostly evaporitic Triassic layer that is hot and consequently highly fluid. The sedimentary cover is thus decoupled from the basement. As a result, the sedimentary cover does not have enough strength to produce reliefs exceeding about 500 to 750 m. That the deformation and seismicity affecting the basin are the results of cover tectonics is confirmed by the fact that seismic activity in the basin only affects the sedimentary cover. Based on our mapping of the structure of the basin, we propose a simple mechanism accounting for the Neogene deformation of the sedimentary cover. The formation of the higher Alps has first resulted to the north in the shortening of the Diois-Baronnies sedimentary cover that elevated the top of Jurassic horizons by about 4 km with respect to surrounding areas to the south and west. There was thus passage from a brittle-ductile basement decollement within the higher Alps to an evaporitic decollement within the Diois-Baronnies. This shortening and consequent elevation finally induced the southward motion of the basin cover south of the Lure mountain during and after the Middle Miocene. This southward motion was absorbed by the formation of the Luberon and Trévaresse mountains to the south. To the east of the Durance fault, there is no large sediment cover. The seismicity there, is related to the absorption of the Alps collapse within the basement itself. To the west of the Salon-Cavaillon fault, on the other hand, gravity induces a NNE motion of the sedimentary cover with extension to the south and shortening to the north near Mont Ventoux. When considering the seismicity of this area, it is thus important to distinguish between the western Basin panel, west of the Salon-Cavaillon fault affected by very slow NNE gliding of the sedimentary cover, with extension to the south and shortening to the north; the central Basin panel west of the Durance fault with S gliding of the sedimentary cover and increasing shortening to the south; and finally the basement panel east of the Durance fault with intrabasement absorption of the Alps collapse through strike-slip and thrust faults.

Three-dimensional structural modeling of an active fault zone based on complex outcrop and subsurface data: The Middle Durance Fault Zone inherited from polyphase Meso-Cenozoic tectonics (southeastern France)

“…In contrast, another study suggested the involvement of basement faults in the regional deformation [ Sébrier et al ., ]. However, earthquake nucleation to the west of the MDFZ [ Le Pichon et al ., ] allowed the observation that the active seismicity is located at a depth of less than 4 km to 5 km in the Mesozoic cover. For Le Pichon et al .…”

“…The hydrocarbon potential of the SE Basin and the tectonic activity of the MDFZ have stimulated numerous stratigraphic, tectonic, and seismological studies of the Middle Durance Valley area [ Roure et al ., ; Benedicto Esteban , ; Roure and Colletta , ; Cushing and Bellier , ; Cushing et al ., ; Guignard et al ., ; Bellier et al ., ; Cushing et al ., ; Molliex , ; Le Pichon et al ., ; Rangin et al ., ; Molliex et al ., ]. Extensive seismic reflection surveys conducted by oil companies between 1971 and 1986 were mainly based on geophysical data [ Baudrimont and Dubois , ; Debrand‐Passard et al ., ; Bove , , , ], while some studies were focused on the deep structure and the stratigraphic and tectonic evolution of the SE Basin [ Benedicto Esteban , ; Cushing et al ., ; Le Pichon et al ., ; Rangin et al ., ; Molliex et al ., ]. These studies evaluated the thickness of the Meso‐Cenozoic sedimentary units, and they tried to image the depth of the Paleozoic basement.…”

Structural style and evolution of the Pyrenean-Provence thrust belt, SE France

“…This fault was mostly active as a normal fault during Paleozoic to Mesozoic times but also during Oligocene and Miocene times [Gigot et al, 1977]. The Mid-Durance fault zone is currently active as revealed by present-day earthquake distribution in the sedimentary cover [Cushing et al, 2008;Le Pichon et al, 2010;Cushing et al, 2014].…”

Morphostructural evolution of the Luberon since the Miocene (SE France)

“…This folding is related to the reverse faulting during the Alpine phase, that corresponds to inversion of Oligocene normal faults associated with 3000 m of total vertical offset [Roure et al, 1992;Benedicto, 1996;Cushing et al, 2008]. The resulting relief was further accentuated by salt tectonics associated with the thick deposits of rock salt in the core of the basin [Mercier et al, 1972;Le Pichon, 2010]. The Manosque Luberon fold is bordered to the west by a N-S fault that follows the Largue valley and that might correspond to the major structural boundary with the Grand Luberon.…”