Microseismicity of the Béarn range: Reactivation of inversion and collision structures at the northern edge of the Iberian plate

Abstract

The Béarn range, located to the north of the Axial Zone in the Western Pyrenees, is affected by numerous small-magnitude seismic events. These events overlap an area characterized by specific geological structures which are interpreted to have resulted from multistage extensional and compressional deformation. An analysis of surface geology draped over digital elevation model, together with field investigations, allow identification of two main shortening episodes with differing direction of contraction: D1 represents the inversion of the North Pyrenean Basin, whose Mesozoic infill was detached from highly extended crust and transported southward over the necking zone and the northern margin of the Iberian plate; D2 corresponds to the collision stage and is characterized in the study area by backfolding and backthrusting deformation coeval with uplift in the axial part of the chain due to thickening of the Iberian plate. The microseismicity appears to concentrate along the basal part of the inverted basin units (D1) where this initially low angle thrust has been tilted and steepened during collision (D2). We propose that local steepening of this ancient inversion structure, which should not be named "North Pyrenean Fault," provided the suitable dip for extensional solicitation in association with the present uplift of the Axial Zone, whatever the driving mechanism of this uplift could be.

The crustal evolution of the west-central Pyrenees revisited: Inferences from a new kinematic scenario

“…Thrusting was accommodated in the basin by continued slip of the NPFT as well as by the Lakora-Larra thrust, which experienced its period of major displacement. It should be noted that, at variance with a previous interpretation of Teixell (1998) and Dumont et al (2015), the Lakora thrust is envisaged as a detachment structure, not rooted in a south-directed crustal ramp. It is difficult to ascertain the precise time when the protocollision initiated.…”

“…The deep crustal structure is based on the interpretation of the ECORS-Arzacq seismic reflection profile and on receiver functions analysis of the PYROPE and older surveys (see text). The stippled area represents the cluster of microseisms characteristic of the North-Pyrenean Zone projected after Souriau et al (2014), where the vertical precision of the seismic event location is of 3 km (Dumont et al, 2015). Inset boxes represent key features of the Mesozoic and Cenozoic basins for the reconstruction of the preorogenic architecture shown in …”

The pull apart-type Tardets-Mauléon Basin, a key to understand the formation of the Pyrenees

“…This folding phase which was known long ago (Canérot, 1964) has been recently outlighted in the Ossau North Pyrenean Zone where it is interpreted as a north-verging D2 phase following the south-verging D1 phase (Dumont et al, 2015). This second step of deformation generating NW vergent structures is in fact known in different points of the Western Pyrenees specially to the west of Adour Valley.…”

“…Their geometry and general organisation evidence, as in the central Pyrenees, the continental subduction of the Iberian crust under the European one which, here in the Western Pyrenees, occurred near the beginning of the Tertiary period. Their recent interpretation (Dumont et al, 2015) as a result of a southward oriented thrust D1 is not observed. The European crust is in fact involved in a regional northern check structure and the considered southward thrust folds and imbrications observed in the "Chaînons Béarnais" zone, within the Iberian crust, correspond actually to a superficial response to the northward underthrust of southern units.…”

“…It develops eastward towards the Bénou Plateau and westward towards the Hasparren area, emphasizing the influence of the Mesozoic crust thinning on the following Pyrenean fold and fault structures. Therefore, this geodynamic interpretation differs from those considering that the diverging zone between Europe and Iberia must be located within the Iberian margin of the Tardets-Mauléon trough, more precisely within the Bedous small basin (Choukroune and Mattauer, 1978;Dumont et al, 2015). These authors aim to draw in this area the limit between the North Pyrenean Zone and the Axial Zone (Central High Range), i.e., the North Pyrenean transform fault which would separate the European and Iberian plates.…”