THERMAL AND RHEOLOGICAL CONSTRAINTS ON THE EARTHQUAKE DEPTH DISTRIBUTION IN THE CHARLEVOIX, CANADA, INTRAPLATE SEISMIC ZONE

The Charlevoix Zone is the most active seismic area of eastern Canada. For the period 1978-1993, 99% of earthquakes occurred at less than 25 km depth and 80% at less than 15 km. This depth distribution is compa red with the estimated brittle/ductile and velocity weakening-velocity strengthening transitions. Using realistic ranges of thermal paramete rs and a 41 +/- 10 mW/m(2) surface heat flow, one-dimensional thermal models show that 90% of 22,000 computed geotherms fall between 215 and 355 degrees C at 25 km. For the central value of heat flow, this rang e is reduced to 280 and 340 degrees C. These temperatures and the infe rred mafic mid- and lower-crustal composition imply a brittle-ductile transition deeper than 25 km. With a higher than average geotherm, the maximum depth of seismicity could correspond to the velocity weakenin g-velocity strengthening boundary. The basic lower crust of the area p recludes a correlation of this depth with the onset of ductility of qu artz at around 300 degrees C. However, it may correspond to the onset of ductility for hydrated feldspar at about 350 degrees C if the geoth erm is relatively high, Since the maximum possible crustal stress diff erence is unlikely to be larger than 200 MPa, high pore-fluid pressure s and/or low static coefficient of friction are required for the occur rence of lower-crustal earthquakes.