Extensional failure and hydraulic valving at Minas da Panasqueira, Portugal: evidence from vein spatial distributions, displacements and geometries

At Panasqueira, Portugal; exceptional exposure and demonstrable vein connec tivity allow robust characterisation of brittle/elastic failure mechanisms in intrusive-related environments. Extensional failure was driven by cycles of fluid injection (hydraulic valving) and vein growth under conditions wi th lambda(v) greater than or equal to 1 and differential stress < 4T. Failu re was episodic and produced a swarm of W-Sn-bearing quartz veins character ised by positive volumetric strain. Worked veins consist of families of co- planar vein-lobes linked at branch-points. Geometrically coherent vein disp lacements constrain an elliptical anomaly (the damage zone) in which values of extensional strain are symmetrically distributed, decreasing systematic ally away from a centrally located maxima to zero at a tip-line loop. Vein textures indicate rapid. episodic, vein opening, mu m- to dm-scale vein ape rtures, spatially and temporally variable rates of vein filling and periodi c baffling of fluid migration pathways. Although the vein swarm represents a single vein cluster, vein thickness and spacing populations are typically non-power law and define anomaly-scale heterogeneous strain with inhomogen eously deformed marginal zones surrounding a homogeneously deformed high-st rain core. Deviations from power-law behaviour were promoted by competitive vein growth that provided mechanisms for (i) inhibiting vein nucleation an d (ii) localising deformation onto a few evenly spaced veins. (C) 2000 Else vier Science Ltd. All rights reserved.