Deformation and fluid processes in thrust sheets from the central Pyrenees

Deformation and fluid processes in thrust sheets are studied using a small thrust system n the footwall of the Gavarnle Thrust, central Pyrenees. This system comprises a set of blind thrusts that Imbricate a thin Mesozoic cover succession (Triassic red beds and Upper Cretaceous limestone) in the Cirque de Barroude and produce a culmination in the Gavamle Thrust. An Intense cleavage developed within the culmination associated with the oblique tipping of thrusts as they climb from the basement into the cover strata. The distributed strain associated with the fault tips locally reactivated earlier folded
thrusts by the development of upper strain detachments. The main detachment occurred along the Pic de Port Vieux thrust which forms the roof of the culmination. The reactivation occurred along the back-limbs of folds in the thrust and produced faults that cut across the fold crests. The movement on these upper strain detachments was intermittent and the faults were gently folded when inactive. The faults that formed within the zone of distributed strain are discontinuous and their development did not conform
to foreland or hinterland propagation and more than one fault moved simultaneously. The overlying Gavamie Thrust sheet extended sub-parallel to transport during the early
stages of the culmination development. The concurrence of shortening in the footwall and extension in the hanging wall of the thrust implies that the it acted as mechanical
boundary between two differently deforming plates. The extension of the upper plate (the Gavarnie Thrust sheet) Is considered to have occurred during gravity spreading in
response to an Increased critical taper In the south Pyrenean thrust wedge. The increased taper possibly resulted from the shortening associated with the underplating of the lower thrust sheets in the central Pyrenees.

The composition and source of the ambient fluid in the thrusts and the fluid flow processes that operated during the fault activity have been studied using fluid inclusion
mlcrothermometry from cavity-fill quartz and from the chemistry of secondary phases in the cavities. The cavities formed by the hydraulic jacking of fault planes and they
indicate the presence of syntectonic high fluid pressures. These pressures appear to have been generated in situ during the distributed deformation in the culmination and
seismic pumping does not appear to have operated. The fluid is a high salinity Ca-Na-CI brine. The presence of bitumen in the inclusions suggests the brine is a connate basinal
brine and not of metamorphic origin. Heterogeneity of brine compositions between the different samples studied suggests that the fluid reservoir in the culmination was heterogeneous and produced by limited dilution of the connate brine with meteoric water. Fluid-rock ratios were low and the close correspondence between vein mineralogy and wall rock composition suggests that the fluids were equilibrated with the strata in the culmination. Local fluid flow pathways have been mapped using Mgvariations
within chlorite. These show that limited fluid transfer occurred between the Cretaceous limestone and the adjacent basement or the Triassic red beds. This transfer appears to have been related to dilatancy pumping associated with the hydrofracture events. Large-scale fluid migration along the active faults did not occur even though local fracture permeabilities were high.