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Stress induced by the Mio-Pliocene Alpine collision in northern France

¹ IRSN, BP17, 92 262 Fontenay-aux Roses cedex.

In most rocks, tectonic stress induces crystalline deformation, such as mechanical twinning. The inverse analysis of calcite twinning allows reconstruction of both directions and values of the paleostress field. The Etchecopar inverse method using calcite twinning has been improved in this paper, lowering the uncertainties on the calculated stress values. Calcite was sampled in the foreland of the western Alps, along a SE-NW section from the Jura Mountains to the Isle of Wight. The calcite twinning inversion has identified the successive Cenozoic tectonic events, named "Pyrenean" compression, "Oligocene" extension and "Alpine" compression.

The distribution of the Mio-Pliocene Alpine orogenic stress was specified. This stress field varies in terms of stress regime, directions and values. The horizontal principal stress trends E-W in southern France, WNW in the centre, and NW in the North, which can be attributed to the Alpine indenter phenomenon. The tectonic stress regime roughly corresponds to a pure compression in the Jura and rapidly evolves to the NW to a strike-slip state of stress, then beyond the Paris basin’s centre to a perpendicular extension. Unlike the Pyrenean or Appalachian foreland stress, the Alpine differential stress does not significantly decrease from the Jura front to the far field (30 to 25 MPa). Moreover, stress values vary from one area to another, low in the Burgundy high, fractured and uprising during this tectonic event, and high in Paris basin centre, poorly fractured and subsiding during this event. Three possible explanations are proposed : variation in crust thickness, crustal buckling during the Mio-Pliocene, and pre-existing fractures.

Key Words: Alpine orogenesis • Paleostress • Calcite twinning • West-European plate • Crust