KICK AND COOK OF PERIDOTITE: COSEISMIC LOADING AND POSTSEISMIC RELAXATION VERSUS STEADY-STATE CREEP

Experiments comprising a sequence of deformation at moderate temperature (600°C) and annealing at higher temperature (700 to 1000°C) were performed on samples of natural peridotite to simulate the natural stress history in the suboceanic upper mantle just below the seismogenic zone. Analyses of the olivine microfabric development revealed that deformation of olivine takes place by microcracking and dislocation glide in the low-temperature plasticity field. During annealing, recovery and recrystallization in olivine are driven by the reduction of strain and grain boundary energy. High strain zones generated during deformation are replaced by olivine subgrains and recrystallized olivine grains at low and high annealing temperatures, respectively. Furthermore, the microstructures after annealing at high temperature (1000°C) are characterized by large olivine clasts surrounded by small recrystallized grains, which resemble the microstructures found in naturally deformed peridotites in the Western Alps. Such microstructures, commonly described as core-and-mantle structures, have previously been addressed as evidence for incipient dynamic recrystallization during steady-state dislocation creep. The average size of the recrystallized grains is then used in conjunction with experimentally constrained paleo-piezometer relations to infer constraints on paleo-stress. Our deformation and annealing experiments suggest an alternative scenario of initial high stress deformation followed by recrystallization at rapidly decaying stresses (or hydrostatic stress) - considered to mimique the sequence of mechanical states during an earthquake – for the development of such microstructures, so that paleo-piezometers do not apply to the natural stress history.