Au–quartz mineralization near the base of the continental seismogenic zone
Published:January 01, 2007
Richard H. Sibson, 2007. "Au–quartz mineralization near the base of the continental seismogenic zone", Deformation of the Continental Crust: The Legacy of Mike Coward, A. C. Ries, R. W. H. Butler, R. H. Graham
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The base of the continental seismogenic zone is defined within individual fault zones by the transition with depth from pressure-sensitive factional (FR) faulting to temperature-sensitive quasi-plastic (QP) ductile shearing. The depth of this FR-QP transition fluctuates principally as a consequence of variations in geothermal gradient and crustal lithology but other factors (e.g. fluid pressure level, strain rate) also play a role. For quartz-dominant and feldspar-dominant lithologies, respectively, it corresponds approximately to isotherms at 300–350°C and c. 450 °C., defining an undulating transition zone in the mid-crust with a relief of the order of 5–10 km. This transition zone correlates with the greenschist-facies metamorphic environment where the bulk of mesozonal Au-quartz lodes form in mixed continuous-discontinuous shear zones. In areas of crustal convergence and thickening, where fluid release results from prograde metamorphic dehydration, especially at the greenschistamphibolite-facies transition in the middle to deep crust, the seismogenic carapace acts as an upper crustal stress guide and low-permeability lid to overpressured metamorphic fluids migrating through shear zone conduits. Under appropriate combinations of stress and fault architecture in the brittle carapace, substantial fluid volumes may be trapped beneath this elastic lid at sufficient overpressure to generate dilatant fault-fracture meshes discharging episodically by fault-valve action following fault rupture, with permeability locally enhanced by aftershock activity distributed about the rupture zones. Topographic irregularities in the seismic-aseismic transition determine rupture nucleation sites and probably play a critical role in focusing the discharge of overpressured metamorphic fluids into the seismogenic layer. This helps to account for the observed spacing of mesozonal lode systems along transcrustal shear zones.
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Deformation of the Continental Crust: The Legacy of Mike Coward
This Special Publication, in memory and celebration of the work of Professor Mike Coward, is about the deformation of the continental lithosphere. The collected papers discuss geometry, structural principles, processes and problems in a wide range of tectonic settings and thereby reflect the breadth of Coward's interests. They encompass the evolution of Precambrian basement gneiss terrains, the geometry and evolution of thrust systems, basement involvement and structural inheritance in basins, syn-orogenic extension, salt tectonics, the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization. Examples are drawn from the Lewisian and Moine Thrust Belt of NW Scotland, the Italian Apennines, NW Himalayas, the Cyclades, Oman, Zagros Mountains, Colombian Cordillera, Carpathians, North Sea, offshore Brazil, regional studies of the Irumide Belt (central Africa), Taurus Mountains (Turkey), greater South America, and from the Witwatersrand Basin of South Africa and the Antler Orogeny of SW USA.