What do we learn from extensional tectonics in the Western Alps

摘要: The Western Alps’ active tectonics are characterized by ongoing widespread extension in the highest parts of belt and transpressive/compressive along its borders (Sue et al., 1999; Delacou 2004). We examine these contrasting tectonic regimes, as well role erosional processes, using a multidisciplinary approach including seismotectonics, numerical modelling, GPS, morphotectonics, fieldwork, brittle deformation analysis. Extension appears to be dominant process present-day activity Alps, affecting internal areas all arc. Shortening, contrast, is limited small located at outer chain. Strike-slip observed throughout Alpine realm foreland. stress-orientation pattern radial for 3 inner, extensional zones, 1 outer, transcurrent/tranpressional ones. Extensional can correlated with thickest crust. Quantification seismic strain tectonically homogeneous shows that only 10 20% geodesy-documented explained seismicity. show ruled buoyancy forces rather than shortening Europe/Adria collision zone. Numerical modeling corroborates this interpretation. Neogene structures Alps formed under increasingly conditions. A synthesis paleostress tensors West-Alpine arc documents major orogen-parallel continuous change directions from ENE-WSW Simplon area, N-S Vanoise area NNW-SSE Briancon (Champagnac 2006). Minor orogen-perpendicular increases N S. This second signal correlates geodynamics revealed focal-plane mechanisms could related opening Ligurian Sea during Early-Middle Miocene compression/rotation Adriatic indenter inducing lateral extrusion. Interactions between different geodynamic processes control balance intrinsic extrinsic dynamics explain late evolution 2007). Numerous agents involved present Central boundary conditions (collision, rotation, free boundaries), deep (gravitational forces, slab dynamics, vertical indentation), surface (erosion, transfert, post-glacial rebound). Three main seem play fundamental extension. First, end Late times would imply decrease Second, progressive rate continental (Europe-Adria) critical regime. convergence rates allows develop, gravitational spreading. Third, Plio-Quaternary increase erosion implies mass transfert inside outside may have deeply changed alpine tectonics, motions, denudation structure (Willett 2006; Champagnac 2007; Vernon 2008).