Earthquakes in the deep continental crust - insights from studies on exhumed high-pressure rocks

Recent studies on two exhumed anorthositic and gabbroic high-pressure complexes in the Bergen Arcs and Western Gneiss Region of Norway demonstrate that deeply subducted dry rocks in the lowermost continental crust may resist metamorphic re-equilibration for geologically significant periods of time. This persistent metastability exercised a profound control on the mechanical properties of these rocks, as evidenced by abundant pseudo-tachylyte veins, cataclasites and fractures, documenting eclogite facies seismic failure at depths exceeding 60 km in the thickened Caledonian crust. Our observations indicate that the brittle seismogenic state of some lower crustal rocks, and their propensity to remain metamorphically metastable despite prevailing conditions of T = 600-800 degreesC and P = 1.5 to > 2.0 GPa, is largely controlled by their very low volatile contents. Our data find support in present-day earthquake focal depth distributions, such as beneath the Himalayas, and provide a physical basis for understanding the generation of deep-crustal earthquakes in collisional settings.