ELSEVIER Tectonophysics 262 (1996) 321-348 rKrllltlmllntlll I IIH li Fluid pressure and salinity variations in shear zone-related veins, central Pyrenees, France: Implications for the fault-valve model I.H.C. Henderson, A.M. McCaig Department of Earth Sciences, Universi~' of Leeds, Leeds LS29JT, UK Received 7 February 1995; accepted 17 January 1996 Abstract Fluid inclusion microthermometry data is presented from quartz veins associated with high-angle reverse shear zones cutting the N6ouvielle Massif in the central French Pyrenees. A complex series of veins were formed during both normal and reverse movement on brittle faults within the shear zone. Brittle structures developed contemporaneously with quasi-plastic deformation in the shear zones. Fluid inclusion data are presented from a variety of vein types and shows that a wide variation in fluid density and salinity is present in the fault system. These variations occur both within individual veins and between different vein types. Temperatures at the time of vein filling are constrained to be in the range 310-360°C by chlorite microthermometry. The variation in homogenisation temperature is too large to be accounted for by variations in the fluid temperature, and is interpreted to reflect variations in fluid pressure between lithostatic (~ 500 MPa) and hydrostatic (~ 200 MPa) values at the time of quartz growth in the veins. The shear zones carry modest displacements and may not themselves have been seismogenic. A model is presented in which fluid pressure variations and stress cycling within the steeply dipping shear zones is controlled by the seismogenic cycle on large underlying thrust structures. The model predicts that the highest maximum trapping pressures should occur in fluid inclusions from low-angle veins formed in a high-Ptaui d compressional regime immediately before earthquake rupture. Lower maximum pressures are expected in steep veins related to extension and normal fault movement across the shear zones immediately after rupture. The fluid inclusion data support this model when maximum trapping pressures are considered, but the majority of inclusions in all vein types formed at pressures well below lithostatic values. This suggests that the dominant factor causing quartz precipitation was the sudden pressure drop immediately after earthquake rupture, regardless of vein type. 1. Introduction Faults and shear zones are acknowledged to be major conduits for fluid movement through the mid- dle crust (Kerrich, 1986; Carter et al., 1990; McCaig et al., 1990; Knipe, 1993; Knipe and McCaig, 1994). In orogenic belts the main driving force for fluid movement in the upper part of the crust where fluid pressures are hydrostaic are probably topographic effects (Garven et al., 1993). At deeper levels, over- pressuring due to the stacking up of nappes (Lobato et al., 1983; Oliver, 1986; McCaig et al., 1995) or metamorphic dehydration reactions (Fyfe et al., 1978) is likely to be more important. Earthquakes have been shown profoundly to influence fluid movement patterns in the upper crust (Briggs and Troxeil, 1955; Nur and Booker, 1972; Sibson, 1981; Muir-Wood and King, 1993) and may also have important effects 0040-1951/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0040-1951(96)00018-2