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Department of Geology, University of Otago, P. O. Box 56, Dunedin, New Zealand (e-mail: rick.sibson{at}stonebow.otago.ac.nz)
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.
