Abstract

In the French sector of the Tertiary Alpine mountain belt of southern Europe, the early stages of compression established an initial thrust front a long distance into the foreland, ∼30–40 km rearward of the terminal thrust front. An extensive region of thrust-sheet-top basins was established behind this initial thrust front, particularly after a regional marine transgression in late Eocene time (Priabonian). These basins were initially connected, but became increasingly compartmentalized as the thrust belt evolved. The basins are now preserved as a series of structurally isolated, synclinal remnants in the external zones of the Alpine thrust belt. One of these remnants, known as the Barrême basin, has a preserved fill that is ∼900 m thick, spans ∼18 m.y., and is exposed over ∼50 km2. The Tertiary basin fill comprises initial nonmarine deposits overlain by a marine succession and subsequent continental strata, and includes evidence of submarine gravity slides, growth structures, and abrupt changes in sediment transport directions and provenance. Evolution of the basin fill was influenced by both far-field and more localized near-field deformation events, the latter being dominated by the progressive growth of a steep, thrust-related anticline that defined the eastern margin of the basin. This basin fill records several key steps in the evolution of the Alpine mountain belt, such as the first incursion of clasts derived from the emplacement of major thrust structures in the inner parts of the mountain belt ca. 32–30 Ma, synsedimentary volcanic activity, and the growth of an important out-of-sequence anticline that progressively separated the Barrême basin from the Grès d'Annot basin to the east. In addition, the basin preserves the progression from an early underfilled flysch basin to a subsequent overfilled molasse basin in early Oligocene time (Rupelian) between ca. 33 and 30 Ma. Records of these events are not available from the fill of the younger foreland basin preserved beyond the final thrust front, and this remnant of the thrust-sheet-top basin province therefore provides a unique record of the early structural and stratigraphic evolution of the Alpine mountain belt.

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