High-pressure and high-temperature torsion experiments on olivine aggregates in dislocation creep show about 15 to 20% strain weakening before steady-state behavior, characterized by subgrain-rotation recrystallization and a strong lattice preferred orientation. Such weakening may provide a way to focus flow in the upper mantle without a change in deformation mechanism. Flow laws derived from low strain data may not be appropriate for use in modeling high strain regions. In such areas, seismic wave propagation will be anisotropic with an axis of approximate rotational symmetry about the shear direction. In contrast to current thinking, the anisotropy will not indicate the orientation of the shear plane in highly strained, recrystallized olivine-rich rocks.

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International audienceThe dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release. High velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0.6 to 0.15 as the slip velocity reaches 1.1 m/s at normal stresses up to 24.5 MPa. The extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding. The rapid heating at asperities causes serpentine dehydration. In impermeable fault zones in nature, serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening. This dynamic fault-weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the San Andreas

The experiments were performed in a Griggs solid-medium apparatus (with NaC1 or NaF assemblies) and a Paterson gasmedium apparatus, respectively. In both studies, samples were deformed after only a low-temperature anneal (oven dry), which removes surface water but not hydrous minerals. Boland and Tullis [1986] did, however, obtain limited dry deformation data by venting several samples to air during the deformation. These high-temperature dry results of Boland and Tullis [1986] seem to be quite consistent with the extrapolation of the lower-13,443

The rheological behaviour of synthetic crystal-bearing magmas containing up to 76 vol.% of crystals (0 ≤ φ S ≤ 0.76) has been investigated experimentally at a confining pressure of 300MPa and temperatures between 475 and 1000°C at shear rates between 10 -4 and 2x10 -3 s -1 .Starting hydrated crystal-bearing glasses were synthesized from a dry haplogranitic glass For pure hydrated melt and for 16 vol.% of crystals, the rheology is found to be newtonian. At higher crystal contents, the magmas exhibit shear thinning behaviour (pseudoplastic). TheEinstein-Roscoe equation adequately estimates viscosities of the crystal-bearing magmas at low crystal contents (φ S ≤~ 0.25), but progressively deviates from the measured viscosities with increasing crystal content as the rheological behaviour becomes non-newtonian. On the basis of a power-law formulation, we propose the following expression to calculate the viscosity as a function of temperature, crystal content and applied stress (or shear rate):where γ& is shear rate (s -1 ), τ is shear stress (MPa), Φ is the crystal volume fraction, T is temperature (K), Φ m is the relative maximum packing density, R is the gas constant, Q = 231 kJ.mol -1 is the activation energy of the viscous flow and A 0 , K, K 1 and K 2 are empirical parameters.