The mechanisms of creep in olivine

Abstract

We summarize the progress made in providing experimental verification for the deformation map of polycrystalline olivine published by Stocker & Ashby in 1973 ( Rev. Geophys . 11, 391). Porosity-free polycrystalline deformation data, applicable to the mantle, were found to be obtainable only from high-pressure deformation studies. Combination of the results of such studies with hardness measurements and single crystal deformation studies on olivine provides narrow constraints on the flow of olivine resulting from dislocation mechanisms from room temperature to the melting point along a band of experimentally accessible strain rates. A good fit is obtained combining a Dorn law above 2 kbar differential stress, e/s -1 =5.7x10 11 ex{-128kcal/mol/RT (1-cr 1 -cr 2 /85000) 2 }, with a power law below 2 kbar, e = 70(cr 1 —cr 3 ) exp{— 122(kcal/mol)/RT}, where stress is measured in bars ( l bar = 10 5 Pa). Indirect data on a mechanism phenomenologically resembling the Goble creep regime are now available from two sources. The observed strain rates are only slightly faster than those predicted by Stocker & Ashby (1973). The ' wet’ data, previously believed to show hydrolytic weakening, are found to fall within this Coble field. The asthenosphere is still expected to deform by the dislocation mechanism summarized by the two formulae given above, but higher stress deformation within the lithosphere is almost certainly dominated by this Coble creep regime once dynamic recrystallization sets in.

Effects of temperature-dependent rheology on large-scale continental extension

“…The comparisons beween observations and calculations (Sections 5.1 and 5.2) reinforce this suggestion, and show that the present pattern of strain and strain rate in the Aegean are consistent with the deformation of lithosphere whose rheology is controlled by the properties of olivine determined by laboratory experiments (e.g. Goetze, 1978), subjected to an extensional boundary moving at 30-60 mm yr" 1 .…”

Olivine friction at the base of oceanic seismogenic zones

“…[26] Motivated by the microstructural observations, we hypothesize that deformation of the asperities occurs according to flow laws determined from indentation creep tests on olivine single crystals [Goetze, 1978;Evans and Goetze, 1979]. At the temperatures and axial strain rates used in our experiments, single crystal strengths are a factor of two to four times greater than frictional strengths at the same conditions.…”

“…We use microstructures to constrain deformation mechanisms at grain-to-grain contacts and discuss the processes controlling the transition from unstable to stable deformation in the context of rate and state friction. Finally, we extrapolate our laboratory results to geologic conditions using flow laws for low-temperature plasticity of olivine [Goetze, 1978;Evans and Goetze, 1979].…”

Localization of shear along a lithospheric strength discontinuity: Application of a continuous deformation model to the boundary between Tibet and the Tarim Basin

“…Note, however, that Goetze [1978] and Evans and Goetze [1979] point out that for T % 1000 K, dislocation creep, which is described with n $ 3 -3.5, does not apply. Although power law creep with n = 3-3.5 may lead to a situation where strain localization occurs in our calculations, it requires a temperature at the top of the strong layer that is so low that it is inconsistent with the assumption of dislocation creep.…”

“…where q is the second invariant of the deviatoric stress tensor, A and Q (the activation energy) are experimentally derived constants for rock-forming minerals, and R is the universal gas constant [e.g., Goetze, 1978;Karato et al, 1986]. The depth-averaged lithospheric strength is then quantified by…”