Collaborative Research: Experimental Study of the Origin and Nature of High Pressure Faulting Relevant to Earthquakes in Subducting Lithosphere

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Green EAR-0125938 Conventional brittle failure is impossible at significant depth within Earth because the combination of pressure and temperature ensures that the flow strength of rocks is exceeded before the fracture strength. Nevertheless, earthquakes occur abundantly within descending slabs of oceanic lithosphere in subduction zones, in several cases to depths as great as 680 km, where they stop abruptly. Although the mechanism by which these earthquakes occur is unknown, their seismic signals ensure that they represent sudden failure of rock along a fault. There are two mechanisms presently known that can enable faulting to occur at high pressure. The first, dehydration embrittlement, involves generation of a free fluid phase by breakdown of hydrous minerals; the free fluid assists in opening of microcracks which is a crucial step leading to shear failure. So far as is known, this mechanism could potentially explain all earthquakes if appropriate hydrous phases are present in the mantle and if their stability fields are such that they dehydrate under appropriate conditions to yield the observed depth distribution. The second, phase-transformation-induced faulting can be triggered during the phase transformations of olivine to its denser polymorphs, which occur with increasing depth. This mechanism can potentially explain the bimodal depth distribution of earthquakes and their abrupt termination at the base of the upper mantle if metastable olivine is preserved in the cold cores of subduction zones at transition-zone depths. However, given current understanding of the phase distribution within subducting lithosphere, neither mechanism can comfortably explain the occurrence of very large earthquakes at depths exceeding 500 km. The investigators will apply their collective expertise in high-pressure technology, experimental deformation, and seismology to achieve the following: (1) test the hypothesis of reactivation of hydrated faults; (2) monitor experiments on dehydration embrittlement and transformation-induced faulting by detection and location of acoustic emissions; (3) develop improved high-pressure experimental assembly designs for in situ synchrotron experiments on shear failure; (4) use faulting experiments on both dehydration embrittlement and phase-transformation-induced faulting to place new constraints on these two faulting mechanisms.