The Larra thrust of the Pyrenees is a bedding-parallel decollement located within a competent limestone unit. It forms the floor of a thrust system of hectometric-scale imbrications developed beneath a synorogenic basin. The fault rock at the decollement is a dense stack of mainly bedding-parallel calcite veins with variable internal deformation by twinning and recrystallization. Veins developed as extension fractures parallel to a horizontal maximum compressive stress, cemented by cavity-type crystals. Conditions during vein formation are interpreted in terms of a compressional model where crack-arrays develop at applied stresses approaching the shear strength of the rock and at fluid pressures equal to or less than the overburden pressure. The cracks developed in response to high differential stress, which was channelled in the strong limestone, and high fluid pressure in or below the thrust plane. Ductile deformation, although conspicuous, cannot account for the kilometric displacement of the thrust, which was mostly accommodated by slip on water sills constituted by open cracks. A model of cyclic differential brittle contraction, stress reorientation, slip and ductile relaxation at a rheological step in the limestone is proposed as a mechanism for episodic decollement movement. The model accounts for the peculiar microstructural character of the fault zone, for alternating sequences of bedding-parallel shortening (leading to crack dilation) and bedding-parallel shear (leading to decollement slip) and for hanging wall imbrication consequent upon decollement slip. (C) 2000 Elsevier Science Ltd. All rights reserved.