Abstract
The rheology of the upper mantle impacts a variety of geodynamic processes, including postseismic deformation following great earthquakes and post-glacial rebound. The deformation of upper mantle rocks is controlled by the rheology of olivine, the most abundant upper mantle mineral. The mechanical properties of olivine at steady state are well constrained. However, the physical mechanism underlying transient creep, an evolutionary, hardening phase converging to steady state asymptotically, is still poorly understood. Here, we constrain a constitutive framework that captures transient creep and steady state creep consistently using the mechanical data from laboratory experiments on natural dunites containing at least 94% olivine under both hydrous and anhydrous conditions. The constitutive framework represents a Burgers assembly with a thermally activated nonlinear stress-versus-strain-rate relationship for the dashpots. Work hardening is obtained by the evolution of a state variable that represents internal stress. We determine the flow law parameters for dunites using a Markov chain Monte Carlo method. We find the activation energy 430pm 20 and 250pm 10 kJ/mol for dry and wet conditions, respectively, and the stress exponent 2.0pm 0.1 for both the dry and wet cases for transient creep, consistently lower than those of steady-state creep, suggesting a separate physical mechanism. For wet dunites in the grain-boundary sliding regime, the grain-size dependence is similar for transient creep and steady-state creep. The lower activation energy of transient creep could be due to a higher jog density of the corresponding soft-slip system. More experimental data are required to estimate the activation volume and water content exponent of transient creep. The constitutive relation used and its associated flow law parameters provide useful constraints for geodynamics applications.Graphical
Highlights
Our current understanding of the rheology of the upper mantle comes mainly from rock deformation studies in the steady-state creep regime, where olivine and olivinebearing rocks are deformed in the relevant temperature and pressure conditions (e.g., Carter and Avé Lallemant 1970; Chopra and Paterson 1981, 1984; Karato et al 1986; Karato and Wu 1993; Mei and Kohlstedt 2000a)
We present an analysis of laboratory data for transient and steady state creep to establish the effect of grain size, stress, and temperature for dry and wet conditions
We find that the activation energy and stress exponent of steady-state creep under hydrous condition are EM = 450 ± 60 kJ/mol and nM = 3.6 ± 0.5, respectively, in agreement with the previous results for dislocation-accommodated grain-boundary sliding (DisGBS) (e.g., Hansen et al 2011; Ohuchi et al 2015; Hirth and Kohlstedt 2003)
Summary
Our current understanding of the rheology of the upper mantle comes mainly from rock deformation studies in the steady-state creep regime, where olivine and olivinebearing rocks are deformed in the relevant temperature and pressure conditions (e.g., Carter and Avé Lallemant 1970; Chopra and Paterson 1981, 1984; Karato et al 1986; Karato and Wu 1993; Mei and Kohlstedt 2000a). DisGBS has the characteristics of both dislocation creep and diffusion creep with a nonlinear relation between stress and strain rate and a dependence on grain size (e.g., Hirth and Kohlstedt 1995; Hansen et al 2011; Wang et al 2010; Ohuchi et al 2015). The deformation of olivine under both laboratory and asthenospheric conditions is close to the boundaries between dislocation creep, diffusion creep, and DisGBS, and the physical conditions may vary either due to grain-size evolution at geological time scales (Karato 1989; Rozel et al 2011) or due to varying loading conditions during the seismic cycle (Lambert and Barbot 2016; Barbot 2018)
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