The pyrite-type strain fringes from Lourdes (France): indicators of Alpine thrust kinematics in the Pyrenees

A new method is developed to reconstruct polyphase deformation histories from pyrite grains with displacement-controlled fibrous strain fringes. It decomposes the relative displacement between a pyrite grain and a fringe into incremental rotations and pull-apart directions. Application to a classic specimen from Lourdes (France) shows that the rotations determine the fibre curvature sense, and strongly depend on local strain parameters as well as the initial orientation and shape of pyrite grains. In contrast, pull-apart directions appear to be relatively constant and straight, although curved pull-apart trajectories were expected from the heterogeneous, non-coaxial deformation in the matrix. The discrepancy is due to the localization of shear-strain components around pyrite-fringe complexes. The complexes themselves represent ellipsoidal domains of relatively coaxial strain that parallel a regional crenulation cleavage, even in zones lacking this cleavage due to stretching and shearing of a pre-existing foliation (S1). Thus, strain fringes consistently record bulk shortening orientations regardless of local variations in strain parameters and microstructural development. The analysed pyrite-fringe complexes from Lourdes were previously considered as classic shear-sense indicators, but yield hook-shaped pull-apart trajectories in the new method. This indicates two deformation events that can be related to a sub-horizontal (S2) and steep regional crenulation-cleavage (S3). The recorded polyphase deformation history is tentatively interpreted in terms of a change from gravitational spreading to a compressive emplacement of a thrust sheet in the North Pyrenean foreland.