A structural interpretation of a section of the Gavarnie nappe and its implications for Pyrenean geology

Abstract

The Gavarnie nappe is a feature of the Tertiary Pyrenean orogen and is shown to consist of at least two thrust sheets of Palaeozoic rocks which are overlain by a southward-dipping sequence of Cretaceous and Eocene sediments, showing folded thrust structures. The Gavarnie nappe covers a basement and Mesozoic cover-rock sequence which is exposed in the tectonic windows of La Larri and the Troumouse Cirque. Here, previously unrecognized thrusts involving basement were responsible for folding the overlying Gavarnie nappe. These basement-involved thrusts climb up section westwards giving a westward lowering of the Gavarnie thrust along strike. The structural evolution of the Gavarnie nappe in a region extending from Heas in France to the Valle de Pineta in Spain can be explained in terms of a piggy-back thrusting sequence. On a regional scale, thrust-tectonic models may be used to explain the double vergence of the Pyrenean chain where early southward-directed thrusting was responsible for structures in the South Pyrenean zone. A later northward-directed back thrusting event, or rotation of southward-directed thrust sheets by the stacking of lower thrust horses, can explain the steepness of structures in the axial zone and the northward-verging North Pyrenean thrust zone. Both models suggest that prior to the Pyrenean orogeny, some of the Hercynian structures in the axial zone were flatter lying, and have been rotated to their present steepness during the Pyrenean orogeny.