Laboratory Constraints on the Rheology of the Upper Mantle

Abstract

INTRODUCTIONThe dynamics of convection and the mechanical behavior of the lithosphere are controlled by the rheology of upper mantle rocks. For this reason, experimental and theoretical studies on the rheology of olivine aggregates have been fields of active research for at least the last 35 years. In this short-course paper, I briefly review some experimental and theoretical constraints on the rheology of upper mantle rocks and minerals and then discuss the application of these data for understanding the rheology of the upper mantle in different tectonic environments. There is an expansive literature on the subject of mantle rheology; extensive reviews and background can be found in several recent articles and books (Poirier 1985;Ranalli 1995;Karato and Wu 1993;Kohlstedt et al. 1995;Drury and FitzGerald 1998;Hirth and Kohlstedt 2003), as well as numerous references made throughout this chapter.I focus the discussion of this chapter on the application of experimental data for constraining the rheology of the oceanic lithosphere and mantle. Constraints on mantle rheology based on extrapolation of laboratory experiments to deformation conditions in the oceanic lithosphere and asthenosphere are illuminating for several reasons. First, the composition of the mantle is relatively well constrained by analyses of peridotites from ophiolites and mid-ocean ridges, as well as chemical analyses of basalts. Second, the oceanic lithosphere is comprised of rock that cooled from high temperature conditions at high pressure, and is therefore not previously fractured. In this way the lithosphere is similar to the "ideal" rocks we use in our experiments. Third, the temperature of the lithosphere is constrained by a number of geophysical observations. Fourth, microstructural observations on naturally deformed mantle rocks justify applying experimental flow laws at geologic conditions. Finally, several independent geophysical observations, such as constraints on viscosity based on analysis of the geoid and the depth distribution of seismicity, can be compared to predictions derived from extrapolation of laboratory data.I first provide some background information on the fracture and frictional behavior of mantle rocks. I then overview viscous deformation mechanisms including low-temperature plasticity and high temperature creep processes such as dislocation creep and diffusion creep. In the discussion, I outline several important caveats regarding the extrapolation of laboratory data to natural conditions. Finally, I review the implications of laboratory data for understanding the depth distribution of seismicity in the oceanic lithosphere, strain localization and spatial variations in the viscosity of convecting regions of the upper mantle. BACKGROUND Brittle deformation and low-temperature plasticityFracture strength and frictional properties of peridotite at low temperature. Brittle deformation of mantle rocks occurs in a wide range of tectonic environments, including oceanic transform faults, the fore-arc of subduction zones and ...

Geothermal gradient and heat flow data in and around Japan (II): Crustal thermal structure and its relationship to seismogenic layer

“…Hirth (2002) showed that what limits the seismogenic zone is still controversial. And, it is still impossible to differentiate between a transition in sliding dynamics and a deformation regime.…”

Rheologic controls on slab dynamics

“…A combination of experimental and theoretical constraints indicate that at shallow depths the cold crust and lithosphere deform by brittle processes in accordance with Mohr-Coulomb failure criteria [Kohlstedt et al, 1995]. As pressure increases but temperature remains relatively low in the lithosphere, brittle deformation gives way to ductile deformation, in which the material strength is controlled by lowtemperature plasticity [Goetze and Evans, 1979;Hirth, 2003]. In addition, the rheology of the lithospheric mantle is stratified owing to differences in water content that results from the melting processes at mid-ocean ridges [Braun et al, 2000].…”

Grain boundary sliding in San Carlos olivine: Flow law parameters and crystallographic-preferred orientation

“…A large body of work describes the deformation of olivine in the dislocation creep and diffusion creep regimes [e.g., Chopra and Paterson, 1984;Bai et al, 1991;Hirth, 2002;Hirth and Kohlstedt, 2003;Keefner et al, 2011]. Hirth and Kohlstedt [1995a] first described this creep regime for olivine in which strain rate is nonlinear in stress and sensitive to grain size.…”