Extensional neotectonics around the bend of the Western/Central Alps: an overview

Abstract

The Western Alps' active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive/compressive tectonics along its borders. We examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics, numerical modeling, GPS, morphotectonics, fieldwork, and brittle deformation analysis. Extension appears to be the dominant process in the present-day tectonic activity in the Western Alps, affecting its internal areas all along the arc. Shortening, in contrast, is limited to small areas located along at the outer borders of the chain. Strike-slip is observed throughout the Alpine realm and in the foreland. The stress-orientation pattern is radial for r3 in the inner, extensional zones, and for r1 in the outer, transcurrent/tranpressional ones. Extensional areas can be correlated with the parts of the belt with the thickest crust. Quantification of seismic strain in tectonically homogeneous areas shows that only 10-20% of the geodesy-documented deformation can be explained by the Alpine seismicity. We propose that, Alpine active tectonics are ruled by isostasy/buoyancy forces rather than the 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 r3 directions from ENE-WSW in the Simplon area, to N-S in the Vanoise area and to NNW-SSE in the Briançon area. Minor orogen-perpendicular extension increases from N to S. This second signal correlates with the 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.

Quantification of strain rate in the Western Alps using geodesy: comparisons with seismotectonics

“…This regime fits well with the model proposed and numerically tested by Delacou et al (2004;2005b) and Sue et al (2007a) for the current geodynamics of the Western Alpine Arc. Gravitational re-equilibration of the belt due to buoyancy forces (Sue et al 1999;Delacou et al 2004, Sue at al. 2007a, probably enhanced by erosional processes (Champagnac et al 2007), could account for the generalized extension in the highchain, contrasting with compressional areas at the foot of the belt.…”

“…Our interpolation assumes a continuous velocity field and does not consider the effects of possible fault-related velocity discontinuities. However, active faults are sparse in the Western Alps, with very low displacements to be considered in a large scale analysis (Delacou et al 2004;Sue et al 2007a). Despite these limitations, the interpolated velocity field appears to well correspond to the measured field (Fig.…”

Increase in late Neogene denudation of the European Alps confirmed by analysis of a fission-track thermochronology database

“…Such rotation can also explain the current strain pattern in the Western Alps (Calais et al, 2002). At present, geodetic and GPS data show limited (≤2 mm/yr) east-west extension in the Western Alps (Calais et al, 2002;Sue et al, 2007). The lack of present-day convergence in the Western Alps, together with the observation of sediment-sealed thrusts in the western part of the Po basin (Pieri and Groppi, 1981), and the cessation of thin-skinned deformation in the Jura at ca.…”

“…One of the most important arc-parallel tectonic boundaries, the Penninic thrust, may have been extensionally reactivated (Seward and Mancktelow, 1994) as part of a series of Neogene extensional features observed throughout the axial region of the Western Alps (e.g., Sue et al, 2007;Tricart et al, 2007 and references therein). Most of these extensional features may be caused by a Neogene dextral transtensive event triggered by the anticlockwise rotation of the Apulian plate.…”

Normal Faulting in the 1923 Berdún Earthquake and Postorogenic Extension in the Pyrenees

“…Rebound from erosion and deglaciation is related to topography in an analogous way and induces a similar pattern of extension and compression in the upper crust (Sue et al, 2007;Vernant et al, 2013). Hence, the 1923 Berdún earthquake, as well as the other recent earthquakes associated to the same type of faults in the frontal areas of the northwestern Pyrenees, occurs in settings where thrust faulting would be expected a priori (Chevrot et al, 2014;Genti et al, 2016).…”