Kinematics of the Western Alpine arc: insights from paleomagnetic data and vertical axis rotations.

Orogenic arcs result from processes ranging from structural inheritance, to molding around arcuate indenters, and/or oroclinal bending of an initially straight mountain range. Most kinematic models for the formation of the Western Alpine arc (WAA) propose a syn-collisional development under the effect of NW- or Wward Adriatic indentation (Brunsmann et al., 2024, for review). This indentation is sometimes associated with a 25° counter-clockwise rotation of the Adriatic indenter, based on GPS- (e.g. Nocquet, 2012), seismotectonic (Bauve et al., 2014), and paleomagnetic data (e.g. Collombet et al., 2002). Paleogeographic reconstructions based on retro-deformation of collisional shortening imply the existence of a pre-collisional proto-arc (e.g. Bellahsen et al., 2014).

Paleomagnetic analyses allow us to study vertical axis rotations, and to discuss them in the frame of orogenic arc development. We present the analysis of an exhaustive compilation of Alpine paleomagnetic data highlighting that on the 1^st order vertical axis rotations affect the formation of the WAA as follows:

1) in the Adriatic plate (Southern Alps, Istria, Pô plain) paleomagnetic data contradict a post-Miocene counter-clockwise rotation of 20-25° of the indenter, showing that the latter does not undergo significant rotation during Alpine collision (<10°).

2) The orogenic internal, subduction wedge undergoes counterclockwise rotations that increases southward of the arc, following the progressive rotation of the main tectonic structures, striking NE-SW in the north and WNW-ESE in the south. The relation between the direction of the main structures and vertical axis rotations of post-Oligocene age in the Internal Zone suggests that the arc was amplified during collision. However, the rotation of the main tectonic structures is greater than the rotation of the Oligocene paleomagnetic directions, implying the existence of a pre-collisional, proto-arc.

3) There is no significative relation between main tectonic structures of the External Zone and the rotation of the paleomagnetic directions along the WAA, and no rotation is measured in the Permian rocks of the Argentera massif. This implies that the arcuate morphology in the European margin is mainly inherited from a pre-collisional phase.

Vertical axis rotations in the western Alps therefore indicate the collisional amplification of an early arc whose morphology is inherited from the subduction period. It also shows that E-W structures in Ligurian region are controlled by the orocline formation but also by left-lateral shear and Apennine slab retreat.

References

Bauve, V., Plateaux, R., Rolland, Y., Sanchez, G., Bethoux, N., Delouis, B., & Darnault, R. (2014). Tectonophysics, 621, 85-100. https://doi.org/10.1016/j.tecto.2014.02.006

Bellahsen, N., Mouthereau, F., Boutoux, A., Bellanger, M., Lacombe, O., Jolivet, L., & Rolland, Y. (2014). Tectonics, 33(6), 1055-1088. https://doi.org/10.1002/2013TC003453

Brunsmann, Q., Rosenberg, C.L., and Bellahsen, N., (2024). Comptes Rendus. Géoscience, 356, 231-263. https://doi.org/10.5802/crgeos.253

Collombet, M., Thomas, J. C., Chauvin, A., Tricart, P., Bouillin, J. P., & Gratier, J. P. (2002). Tectonics, 21(4), 14-1. https://doi.org/10.1029/2001TC901016

Nocquet, J.-M., 2012. Tectonophysics 579, 220–242. http://dx.doi.org/10.1016/j.tecto.2012.03.037