Tracing the evolution of crustal-scale, transient permeability in a tectonically active, mid-crustal, low-permeability environment by means of quartz veins

摘要

In mid-crustal, low-permeability environments pervasive fluid flow is primarily driven by the production of internally-derived metamorphic fluids, causing a near permanent state of near-lithostatic fluid-pressure conditions. In a tectonically active crust, these overpressured fluids will generate intermittently an enhanced permeability that will facilitate fluid flow through the crust. The High-Ardenne slate belt (Belgium, France, Germany) can be considered as a fossil (late Palaeozoic) analogue of such mid-crustal, low-permeability environment at the brittle-plastic transition (depth range from 7 to 15 km). Low-grade metamorphic (250°C-350°C), predominantly fine-grained, siliciclastic metasediments were affected by a contraction-dominated deformation, materialized by a pervasive slaty cleavage. Quartz veins, abundantly present in the slate belt, are used as a proxy for the enhanced permeability. Detailed structural, petrographical, mineralogical and geochemical studies of different quartz-vein occurrences has enabled to reconstruct the evolution of the crustal-scale permeability , as well as to constrain the coupled fluid-pressure and stress-state evolution throughout the orogenic history. Extensive veining on a regional scale seems confined to periods of tectonic stress inversion, both at the onset (compressional stress inversion) and in the final stages (extensional stress inversion) of orogeny. Firstly, compressional stress inversion is expressed by pre-orogenic bedding-normal extension veins, consistently arranged in parallel arrays, followed by early orogenic bedding-parallel hybrid veins. Fluid-inclusion studies demonstrate near-lithostatic to supralithostatic fluid pressures, respectively. Secondly, discordant veins, transecting the pre-existing cleavage fabric, are interpreted to be initiated shortly after the extensional stress inversion, reflecting the late-orogenic extensional destabilisation of the slate belt. Veining again occurred at high fluid pressures. Thus, periods of tectonic stress inversion, characterised by sustained near-lithostatic fluid pressures and low shear stresses, turn out to be key moments of enhanced permeability in mid-crustal, low-permeability environments, guaranteeing fluid-pressure driven flow of internally-derived metamorphic fluids. Syn-orogenic veining, on the other hand, is relatively uncommon in the slate belt. Quartz veins occupy deformation-controlled, low-displacement, structures (e.g. saddle reefs, dilational jogs, boudin necks). During the main stages of orogeny, rather locally enhanced permeability is thus primarily deformation-controlled. Throughout orogeny, intermittent, crustal-scale enhanced permeability is materialized by the different quartz-vein occurrences. Quartz veins occupy low-displacement structures, reflecting brittle (e.g. fault-fracture meshes) or ductile deformation (e.g. folds). Remarkably, the enhanced permeability is highly anisotropic, with primarily a horizontal connectivity, parallel to the intermediate principal stress or the structural grain. Mixed brittle-plastic deformation behaviour is furthermore responsible for maintaining long-lived permeability structures by a steady-state deformation of fluid-filled cavities, ensuring sustained, crustal-scale fluid flow.