Alpine deformation at the western termination of the axial zone, Southern Pyrenees

Abstract

Detailed structural and sedimentological research has been conducted in order to unravel the local tectono-sedimentary history of an area located at the western termination of the axial zone, in the Southern Pyrenees. Two cross-sections have been constructed perpendicular to the axis of the orogen, situated above the westward dipping axial zone antiform and the northern part of the south Pyrenean zone, striking perpendicular to the axis of the orogen. The sediments of this area date from Late Cretaceous to Middle Eocene age and display syntectonic characteristics. The cross-sections, together with detailed analysis of key outcrops, reveal that there have been two main phases of deformation. The first phase (D1) is related to the movement along the Lakhoura basement thrust and can be subdivided into three sub-phases, related to activity of two main thrusts, which splay of the basement thrust. The first sub-phase is related to activity along the Lakhoura thrust (D1a), the second sub-phase to the Larra thrust (D1b) and the third sub-phase to reactivation of the Lakhoura thrust (D1c). The second phase (D2) is related to the Gavarnie basement thrust, which resulted in the formation of the axial zone antiform. D1 and D2 show their distinct type of deformation. The first phase is characterised by south vergent ramp-flat thrusting and fault propagation folding, whereas the second phase is characterised by upright to overturned folding and steep reverse faulting. One main structure, the Urzainqui fault propagation fold, shows a synsedimentary relationship and its activity can be dated at Early and/or Middle Lutetian. The total amount of shortening has been estimated at ~ 16.5 km, which represents ~ 49% of the undeformed length. The Lakhoura thrust (D1a, D1c) accounts for ~ 3.5 km of the total amount of shortening. Approximately 8.0 km of shortening can be attributed to the Larra thrust (D1b), whereas the remaining 5.0 km of shortening can be related to the Gavarnie thrust (D2).