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


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Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc
Journal of Geodynamics
Dumont T., Schwartz S., Guillot S., Simon-Labric T., Tricart P., Jourdan S.
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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 Dauphine 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 Dauphine 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 Dauphine foreland. Since similar to 32 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 (Brianconnais) 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 similar to 15 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. (c) 2011 Elsevier Ltd. All
rights reserved.
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03/07/2013 21:43
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