| Abstract |
We present finite-element models of interacting brittle and ductile deformation as a method for predicting deep-seated fracture zones developing on time scales of ka to ma (Regenauer-Lieb et al., 2006). These fractures could prove important for the stimulation and extraction of geothermal energy from hot granites. We investigate in particular how deep-seated ductile faults experiencing creep cavitation, emerging from the base of the brittle-ductile transition, propagate upward and interact with brittle faults. Our models use a damage mechanics formulation (Karrech et al., 2011a, b) that considers fractures in the framework of continuum mechanics. Both brittle and creep damage lead to material degradation described by a damage parameter, which in turn is related to energy feedbacks in the model. This approach has been successful in describing seismic behavior in the brittle crust (Lyakhovsky and Ben-Zion, 2008). However, it has been extended only recently (Karrech et al., 2011a) to considering creep damage in the ductile realm. Creep fractures are microscopic cavities that preferentially develop along grain boundaries during ductile deformation (Cocks and Ashby, 1982). During protracted deformation, they can self-organize into macroscopic failure zones. They are commonly observed in the deformation of high-temperature ceramics and metals (Lemaitre, 1985; Tvergaard, 1982) and are expected to occur in deforming rocks (Regenauer-Lieb, 1999; Rybacki et al., 2008). The creep-cavitation phenomenon was documented just recently in natural hot granites deformed at 400-500„aC in the Red Bank shear zone in Central Australia (Fusseis et al., 2009). Here, we model scenarios for fracture propagation in middle and upper continental crust subjected to far-field contraction. Our models predict that brittle fractures are triggered by upward-propagating ductile faults and concentrate near the brittle-ductile transition. These fractures may play a fundamental role in providing fluid pathways in hot rocks. We therefore recommend targeting them specifically for stimulation. |