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yancy forces rather than ongoing shortening along the Alpine Europe/Adria collision zone. This interpretation is corroborated by numerical modeling. The Neogene extensional structures in the Alps formed under increasingly brittle conditions. A synthesis of paleostress tensors for the internal parts of the West-Alpine arc documents major orogen-parallel extension with a continuous change in σ3 directions from ENE-WSW in the Simplon area, to N-S in the Vanoise area and to NNW-SSE in the Briançon area (Champagnac et al., 2006). Minor orogen-perpendicular extension increases from N to S. This second signal correlates with present-day geodynamics as revealed by focal-plane mechanisms analysis. The orogen-parallel extension could be related to the opening of the Ligurian Sea during the Early-Middle Miocene and to compression/rotation of the Adriatic indenter inducing lateral extrusion. Interactions between the different geodynamic processes control the balance between intrinsic and extrinsic dynamics and explain the late tectonic evolution of the belt (Sue et al., 2007). Figure 1 presents a synthesis of the tectonic regimes and the links with the related geodynamic engines involved in the Neogene to present evolution of the Western and Central Alps, including boundary conditions (collision, rotation, free Ligurian boundary), deep dynamics (gravitational forces, slab dynamics, vertical indentation), and surface processes (erosion, transfer, post-glacial rebound). Three main processes seem to play a fundamental role in this tectonic evolution. First, the end of the opening of the Ligurian Sea during Late Miocene times would imply a decrease in orogen-parallel extension. Second, a progressive decrease in the rate of continental collision (Europe-Adria) appears to be critical for the change of tectonic regime. The decrease of convergence rates allows buoyancy forces to develop, and to control the actual gravitational spreading. Third, the increase of erosion rates during Pliocene times implies major geodynamic modifications, whatever is the origin of this erosion increase (Champagnac et al., 2008). We propose that the evolution from a tectonic regime characterized by extrusion and convergent plate conditions to a tectonic regime characterized by gravitational re-equilibration is controlled by a modification in the subtle balance between boundary forces and buoyancy forces. This transition probably took place during Pliocene times, contemporaneously with the increase of erosion, and in relation with the decrease of the plate convergence rate. Figure 1. Conceptual diagram showing the interactions between intrinsic (rectangles) and extrinsic (ellipses) forces leading to the develop- ment of orogen-parallel versus orogen-perpendicular extension. The major roles are attributed to boundary conditions (convergence, lack of convergence, free boundary) and their equilibrium with gravitational re-equilibration (body forces). REFERENCES Champagnac JD, Sue C, Delacou B, Tricart P, Allanic C, Burkhard M (2006) Miocene orogen-parallel extension in the inner Western Alps revealed by dynamical fault analyses. Tectonics DOI: 10.1029/2004TC001779. Champagnac, J.-D., van der Beek, P., Diraison, G., Dauphin, S.: Flexural isostatic response of the Alps to increased Quaternary erosion recorded by foreland basin remnants, SE France. Terra Nova 20, 213–220, doi: 10.1111/j.1365- 3121.2008.00809.x. Delacou B, Sue C, Champagnac JD, Burkhard M (2004) Present-day geodynamics in the bend of the western and central Alps as constrained by earthquake analysis. Geophys J Int 158: 753-774. Sue C, Thouvenot F, Fréchet J, Tricart P (1999) Widespread extension in the core of the western Alps revealed by earthquake analysis. J Geophys Res B: Solid Earth 104: 25611-25622. Sue, C., Delacou, B., Champagnac, J.D., Allanic, C., Tricart, P. & Burkhard, M. 2007a: Extensional neotectonics around the bend of the Western/Central Alps: an overview. Int. J. Earth Sci. doi:10.1007/s00531-007-0181-3. 6 Symposium 1: Structural Geology, Tectonics and Geodynamics
66 Symposium 1: Structural Geology, Tectonics and Geodynamics 1. How rose up the Ali Sabieh bloc (Afar triple junction)? New insights from faulting analysis Sue Christian, Daoud Ahmed Mohamed, Le Gall Bernard, Rolet Joel IUEM/UBO/UEB/UMR6538, Place Copernic, 29280 Plouzané, France (sue.christian@univ-brest.fr) Tectonics of triple junction zones, of either continental or oceanic type, often lead to complex strain/stress patterns, generally caused by changes in the relative trajectories of the plates involved in the system. The corresponding far stress/strain fields may also interact with more local fields of either tectonic or magmatic origin. Such a geodynamic configuration occurs in the Afar Depression, Horn of Africa, where three recent (
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