Belt-parallel mantle flow beneath a halted continental collision: The Western Alps

Abstract

International audienceConstraining mantle deformation beneath plate boundaries where plates interact with each other, such as beneath active or halted mountain belts, is a particularly important objective of "mantle tectonics" that may bring a depth extent to the Earth's surface observation. Such mantle deformation can be mapped at scale lengths of several tens of kilometers through the analysis of seismological data and particularly by mapping seismic anisotropy from the splitting analysis of vertically-propagating SKS waves that largely reflect the strain-induced crystal preferred orientations of the rock-forming minerals within the upper mantle. In the present study, we analyse data from approximately 50 broadband seismic stations covering the Western Alps and we provide a coherent picture of upper mantle anisotropy beneath the belt. The large-scale anisotropy pattern is characterized by fast split directions that closely follow the trend of the belt. Moreover, the maximum anisotropy magnitude is not located beneath the internal zones of the belt but instead beneath external units. All suggests that the anisotropy is likely dominated by sublithospheric mantle deformation. We propose that the observed anisotropy pattern can be explained by recent or active mantle flow around the Eurasian slab presently plunging beneath the inner parts of the Alps

Deciphering orogenic evolution

“…Paleomagnetic data in the Internal Alps are in agreement with a counterclockwise rotation of about 25° (Collombet et al, 2002). Geophysical imaging of SKS waves in the upper mantle below the Alps also support this view by suggesting mineral anisotropy directions in agreement with an along-belt stretching direction (Barruol et al, 2011). Seismicity is characterized by joint extensional and right-lateral focal mechanisms along the Western arc (Jenatton et al, 2007), which shows that such motion is ongoing at the present.…”

“…1. : Post-collisional crustal and upper mantle strain field of Western Alps with polarities and ages of ductile crustal deformation (references below), and direction of upper mantle deformation inferred seismic anisotropies (Barruol et al, 2011). Age and polarities of displacements are from Rolland et al (2009a) in the Aar Massif, from Campani et al (2010) in the Simplon Fault zone, from Rolland et al (2008) in the Mont Blanc, from Sanchez et al (2011a) in the Mercantour.…”

“…Internal Alps comprise: 3, Briançonnais and Piemontais zones, which are made of variably metamorphosed rocks from the continental European margin and the Alpine Tethys oceanic domain; 4, the Austro-Alpine units comprising the Dent Blanche klippe and the Apulian margin; 5, Molasse sediments deposited during the Oligocene to Pliocene lie in the periphery of the Alps. 6, Upper mantle anisotropy defined from systematic SKS splitting measurements (Barruol et al, 2011). 7 and 8, main ductile to brittle post-collisional shear zones: 7, strike-slip shear zones; 8 extensional shear zones.…”

The AlpArray Seismic Network: A Large-Scale European Experiment to Image the Alpine Orogen

“…Solving this controversy is particularly relevant for the Friuli seismic hazard area, which is located either in the foreland (if the slab beneath the Eastern Alps is Adriatic) or in the hinterland of the Alps (if the slab beneath the Eastern Alps remained European). In general for the entire region, the determination of mantle seismic anisotropy patterns helps to reconstruct the current and past plate motions and dynamics in three dimensions; the broad coverage with the uniform AASN will allow a more comprehensive analysis with respect to previous studies often focused on smaller areas (Margheriti et al 2003;Plomerová et al 2006;Kummerow et al 2006;Fry et al 2010;Barruol et al 2011;Salimbeni et al 2013;Qorbani et al 2016;Subašić et al 2017).…”

Automated analysis of SKS splitting to infer upper mantle anisotropy beneath Germany using more than 20 yr of GRSN and GRF data

“…SKS splitting parameters for some of the GRSN stations incorporated in temporary experiments are from Wylegalla et al (1999) (TOR-1 experiment), from Kummerow & Kind (2006) (TRANSALP experiment), from Walker et al (2005) and from Plomerová et al (2012). As an extension to the southwest, we have also incorporated the results from Barruol et al (2011) for some selected stations in Switzerland. Fig.…”