Variation of depth to the brittle-ductile transition due to cooling of a midcrustal intrusion

The depth to the brittle-ductile transition in the crust is often defined by the intersection of a shear resistance relation in the brittle upper crust that increases linearly with depth and a power law relation for ductile flow in the lower crust that depends strongly on temperature. Transient variation of this depth caused by a magmatic intrusion at a depth near the regional transition can be modeled by a heat conduction model for a rectangular parallelepiped superimposed on a linear geothermal gradient. When parameters appropriate for the southeastern United States are used, a moderate-sized intrusion is found to decrease the transition depth by as much as 7 km; significant variations last about 10 m.y. Since the base of the seismogenic zone is identified with the brittle-ductile transition and since significant depth variations provide loci for strain energy concentration, these results imply that intrusions of late Tertiary age or younger could be important sources of clustered seismicity.