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MPN-05 Earthquakes, fluids and metamorphism
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Geology of intermediate-deep earthquakes and the strength of rocks at high confining pressure
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Torgeir Bjørge Andersen, University of Oslo (Norway)
Haakon O. Austrheim, University of Oslo (Norway)
Timm John, University of Oslo (Norway)
Sergei Medvedev, University of Oslo (Norway)
Karen Mair, University of Oslo (Norway)
Yuri Y. Podladchikov, University of Oslo (Norway)
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Long term field experience with the high- and ultra-high pressure ((U)HP) rocks in western Norway have resulted in several new discoveries of palaeo-earthquake localities in HP-metamorphic rocks. These fossil earthquakes are characterized by eclogite- and blueschist facies pseudotachylyte fault-veins. These discoveries in the Caledonides and more recently in Alpine Corsica demonstrate that co-seismic deformation at metamorphic conditions normally associated with ductile deformation is common; and that the notion of a 'brittle-ductile transition' and the strength of rocks at such high confining pressures are not well understood. Several exhumed HP terrains show evidence of subduction/collision related fossil earthquakes. They are fossil examples of very common earthquakes seen in present-day seismicity of convergent plate-margins. The field- and mineralogical studies of palaeo-earthquakes, combined with theoretical and numerical models suggest that earthquake faulting at blueschist- to eclogites facies conditions commonly is associated with high- to extremely high temperatures on the faults. Observations show that heating and associated melting of mafic and ultramafic rocks may take place even on very small faults, which have displacement <1 cm.
Melting along the smallest observable faults in a peridotite suggest that the mantle rock was very strong and that the faulting raised the temperature (δT) and melted measurable volumes of refractory material such as peridotite. The minimum stress released by a fossil earthquakes can be quantified if we can determine δT, melt volume and displacements. Detailed work on ca 50 palaeo-earthquakes cutting mantle peridotite in Corsica has enabled calculation of minimum stress-drops associated with these faults. We find that differential stress up to 580 MPa (absolute minimum estimate) was released on a very small fault (45 mm displacement) when it was subducted towards lawsonite-eclogite facies conditions at ∼450°C and 2.5GPa. This is less than strength calculations from theoretical flow-law for cold mantle, but much higher than stresses commonly suggested from seismic data.
Our study proves mantle peridotite can sustain several 100 MPa differential stresses at a geological time scale, and that even very small offsets at such high-stress conditions may generate melt in olivine rich mantle rocks with extremely high melting temperature. We speculate that seismic faults at high confining pressure corresponding to blueschist-eclogite facies conditions, i.e. most intermediate deep earthquakes, produce pseudotachylytes and that the apparent relative scarcity of pseudotachylyte in exposed fault complexes is simply a function of the global sampling depth.
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