Junfeng Zhang, Harry W. Green, Krassimir Bozhilov, and Zhenmin Jin
Inst. Geophys. Planet. Phys. and Earth Sci Dept, Univ. of California, Riverside, CA 92521, USA
jezhang@citrus.ucr.edu
Pore fluid-assisted failure accompanying dehydration reactions has been proposed to explain both intermediate focus (70~300 km) earthquakes and deep-focus (>300 km) earthquakes in the subduction zone. Sufficient sources of fluid preserved to great depth in the interior of subducting lithosphere are required to enable brittle failure or frictional sliding. In addition to dehydration of hydrous phases (e.g. lawsonite, zoisite, phengite, antigorite, phase A), in principle hydroxyl-bearing nominally anhydrous minerals (olivine, pyroxene, garnet) within the subducting lithosphere also can provide a source for fluid release in subducting slabs. We have performed deformation experiments at 3 GPa pressure on a reconstituted natural eclogite that contains no hydrous phases but a significant OH concentration in both pyroxene and garnet. Initially fluid-free samples of this material exhibit faulting phenomena under stress at temperatures between the H2O-saturated and dry solidi. Similar brittle failure was also produced in dry eclogite samples with addition of 0.3~3 wt.% water within similar temperature regime at 3 GPa. The flow behavior of these eclogites is controlled by temperature and strain rate but their failure strength is not temperature or strain rate sensitive. Microstructural observations of faulted specimens revealed ultra-thin glass films along grain boundaries and within abundant Mode I microcracks. Very little fluid is required to trigger faulting (<1%). These results show that dehydration-induced melting of 'wet' eclogite can be a cause of high-pressure faulting and therefore dehydration-induced faulting triggered by exsolution of H2O from hydroxyl-bearing minerals could be a viable new mechanism for triggering intermediate-focus (and conceivably deep-focus) earthquakes. Investigation into the operating mechanisms at higher pressures may have further significance for understanding of earthquakes at depth.