ORALS

The creep of minerals in Earth’s interior controls a wide variety of large-scale, geodynamic processes. A dominant focus of previous studies has been the steady-state flow of Earth’s upper over long time scales. However, there are a variety of geological processes that involve creep but occur on much shorter timescales, including the rebound of Earth’s surface after melting of ice sheets and the reloading of stresses on seismogenic faults after major earthquakes. Early work rooted in the materials sciences emphasized the likelihood that these short-time-scale processes are dominated by transient creep rather than steady-state behavior [1]. However, relatively little work has subsequently been completed to provide the constitutive laws necessary to incorporate transient processes into large-scale geodynamic simulations. Some experiments have been conducted [2, 3], and theoretical frameworks proposed [4], but the available data do not allow complete calibration of existing models or sufficiently reveal the microphysical processes controlling transient creep. Here we present several data sets that elucidate the mechanisms that control transient creep in olivine, the dominant mineral in Earth’s upper mantle. These data sets consist of uniaxial stress-reduction experiments at temperatures >1200°C, cyclical loading experiments at room temperature, nanoindentation load-reduction experiments at room temperature, and microstructural characterization with high-resolution electron backscatter diffraction. Mechanical experiments reveal evidence for anelasticty at all investigated temperatures and a pronounced Bauschinger effect at low temperatures. Microstructural observations reveal significant stress heterogeneity associated with geometrically necessary dislocations. Taken together, these data suggest that transient creep in olivine is associated with the buildup and evolution of backstresses associated with the dislocation population. We developed a set of constitutive laws based on dislocation density and the evolution of backstresses that are able to explain the yield stress, the magnitude of anelastic strain recovery in stress reductions, the magnitude of the Bauchinger effect, and the characteristic timescales of transient creep in olivine, providing a level of insight into the microphysics of transient creep not yet available for minerals.

References:
[1] Weertman, J. "Creep laws for the mantle of the Earth." Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 288.1350 (1978): 9-26.

[2] Chopra, Prame N. "High-temperature transient creep in olivine rocks." Tectonophysics 279.1-4 (1997): 93-111.

[3] Hanson, David R., and Hartmut A. Spetzler. "Transient creep in natural and synthetic, iron-bearing olivine single crystals: Mechanical results and dislocation microstructures." Tectonophysics 235.4 (1994): 293-315.

[4] Sherburn, J. A., Horstemeyer, M. F., Bammann, D. J., & Baumgardner, J. R. (2011). Application of the Bammann inelasticity internal state variable constitutive model to geological materials. Geophysical Journal International, 184(3), 1023-1036.