Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

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• 作者：T. Dumont ; ^{thierry.dumont@ujf-grenoble.fr} ; S. Schwartz ; S. Guillot ; T. Simon-Labric ; P. Tricart ; S. Jourdan
• 关键词：Western Alps ; Continental subduction ; Collision ; Foreland basins ; Westward escape
• 刊名：Journal of Geodynamics
• 出版年：2012
• 期刊代码：45_02643707
• 类别：ear
• 出版时间：May, 2012
• 卷：56-57
• 期：Complete
• 页码：18-38
• 文件大小：7165 K

摘要

The northwestwards-directed Eocene propagation of the Western Alpine orogen is linked with (1) compressional structures in the basement and the Mesozoic sedimentary cover of the European foreland, well preserved in the External Zone (or Dauphin茅 Zone) of the Western Alps and (2) tectono-sedimentary features associated with the displacement of the early Tertiary foreland basin. Three major shortening episodes are identified: a pre-Priabonian deformation D1 (N-S shortening), supposedly linked with the Pyrenean-Provence orogeny, and two Alpine shortening events D2 (N- to NW-directed) and D3 (W-directed). The change from D2 to D3, which occurred during early Oligocene time in the Dauphin茅 zone, is demonstrated by a high obliquity between the trends of the D3 folds and thrusts, which follow the arcuate orogen, and of the D2 structures which are crosscut by them. This change is also recorded in the evolution of the Alpine foreland basins: the flexural basin propagating NW-wards from Eocene to earliest Oligocene shows thin-skinned compressional deformation, with syn-depositional basin-floor tilting and submarine removal of the basin infill above active structures. Locally, a steep submarine slope scar is overlain by kilometric-scale blocks slided NW-wards from the orogenic wedge. The deformations of the basin floor and the associated sedimentary and erosional features are kinematically consistent with D2 in the Dauphin茅 foreland. Since 鈭?2 Ma, the previously subsiding areas were uplifted and the syntectonic sedimentation shifted westwards. Simultaneously, the paleo-accretionary prism, which developed during the previous, continental subduction stage, was rapidly exhumed during the Oligocene collision stage due to westward indentation by the Adriatic lithosphere, which likely enhanced the relief and erosion rate. The proposed palinspastic restoration takes into account this two-stage evolution, with important northward transport of the distal passive margin fragments (Brian莽onnais) involved in the accretionnary prism before the formation of the western arc, which now crosscuts the westward termination of the ancient orogen. By early Oligocene, the Ivrea body indentation, which was kinematically linked with the Insubric line activation, initiated the westward escape and the curvature of the arc was progressively acquired, as recorded by southward increasing counter-clockwise rotations in the internal nappes. We propose that the present N-S trend of the Ivrea lithospheric mantle indenter which appears roughly rectilinear at 鈭?5 km depth could be a relict of the western transform boundary of Adria during its northward Eocene drift. The renewed Oligocene Alpine kinematics and the related change in the mode of accomodation of Africa-Europe convergence can be correlated with deep lithospheric causes, i.e. partial detachment of the Tethyan slab and/or a change in motion of the Adria plate, and was enhanced by the E-directed rollback of the eastern Ligurian oceanic domain and the incipient Ligurian rifting.