Synergy of Experimental Rock Mechanics, Seismology, And
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RESEARCH ARTICLE Synergy of Experimental Rock Mechanics, Seismology, 10.1029/2020JB019896 and Geodynamics Reveals Still Elusive Key Points: Upper Mantle Rheology • A new probabilistic geodynamic modeling approach is presented to tighten existing constraints on Chhavi Jain1,2 and Jun Korenaga1 olivine rheology • Radial anisotropy beneath the 1Department of Earth and Planetary Sciences, Yale University, New Haven, CT, USA, 2Department of Earth and Pacic plate provides an important Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA constraint on mantle rheology • Our analysis reinforces the possibility of strong shear flow beneath the Pacific plate Abstract We present a novel geodynamic approach that can potentially tighten existing constraints on mantle rheology. This new approach, which we call probabilistic geodynamic modeling, is applied here to the rheology of the upper mantle. We combine the numerical modeling of plate-driven corner flow and the Supporting Information: • Supporting Information S1 seismic observation of radial anisotropy, aiming to reduce substantial uncertainties associated with experimentally derived flow laws, but our results also highlight the complex competition among different deformation mechanisms under mantle conditions. Despite the remaining rheological uncertainty, our Correspondence to: C. Jain, study suggests that significant background shear flow is required near the lithosphere-asthenosphere [email protected] boundary to explain the strong radial anisotropy observed at 100–200 km depth underneath the Pacific plate, and the plausible nature of this background flow is characterized using our new probabilistic Citation: approach. Our analysis also provides a new insight into the asthenospheric water content and the grain Jain, C., & Korenaga, J. (2020). size distribution in the upper mantle, but these results are also subject to nontrivial nonuniqueness. The Synergy of experimental rock merit of our probabilistic approach lies in its ability to assess the extent of such nonuniqueness, and we mechanics, seismology, and geodynamics reveals still elusive demonstrate this by quantifying the robustness of some of our results. upper mantle rheology. Journal of Geophysical Research: Solid Earth, 125, Plain Language Summary A detailed understanding of how silicate minerals deform under e2020JB019896. https://doi.org/10. various conditions is central to conducting realistic simulations of mantle dynamics, but the currently 1029/2020JB019896 available experimental constraints on the flow laws of olivine, which is the dominant phase of the upper mantle, suffer from considerable uncertainties. We present a new kind of geodynamic modeling, which Received 2 APR 2020 can reduce such uncertainties by assimilating geophysical observations. Unlike previous similar attempts, Accepted 6 NOV 2020 Accepted article online 12 NOV 2020 our approach properly handles parameter uncertainties such that the flow laws under consideration remain consistent with experimental deformation data. Most important, our new approach provides a unifying framework to combine experimental rock mechanics, observational geophysics, and theoretical geodynamics in a self-consistent manner. 1. Introduction The knowledge of upper mantle rheology is crucial to our understanding of mantle convection and related tectonic processes that have shaped our environment. Because olivine is the most abundant and usually the weakest phase in the upper mantle (Karato & Wu, 1993), the rheology of olivine aggregates is considered to be representative of upper mantle rheology. Olivine primarily deforms by diffusion and dislocation creep mechanisms, and flow laws for these mechanisms have often been used to implement geodynamic models (e.g., Kneller et al., 2005; Rose & Korenaga, 2011; Zhong & Gurnis, 1996). Flow law parameters for these mechanisms are usually obtained from experimental deformation data of olivine aggregates. Because both diffusion and dislocation creep operate in parallel, however, delineating these two mechanisms from experi- mental data has been difficult. Our recent reanalysis of existing experimental data using a rigorous statistical framework, implemented by Markov chain Monte Carlo (MCMC) sampling, has revealed that, owing to the highly nonlinear nature of parameter estimation as well as large data uncertainties, some flow law param- eters are only poorly constrained (Jain et al., 2018, 2019). If these experimentally constrained flow laws are applied directly in geodynamic modeling, their uncertainties would yield too wide a range of viscosity pre- ©2020. American Geophysical Union. dictions to be useful. To design more realistic and reliable mantle dynamic models; therefore, it is necessary All Rights Reserved. to improve existing constraints on olivine rheology. JAIN AND KORENAGA 1of21 Journal of Geophysical Research: Solid Earth 10.1029/2020JB019896 Whereas experimental rock mechanics provides direct estimates on olivine rheology, additional constraints may be obtained from some geophysical observations. Olivine rheology influences many properties of the lithosphere and the upper mantle, such as seismic anisotropy and attenuation, convection planform, heat transport to the surface, and lithospheric flexure. Consequently, geophysical observations of these phenomena may be used to draw some inferences on upper mantle rheology. Some previous studies have indeed used geophysical data to constrain flow law parameters for olivine rheology. The most straightforward approach to conduct geophysical inversion is to run multiple forward geodynamic models assuming different rheological parameters for the upper mantle. The subset of param- eters that yield the smallest misfits to geophysical data may provide useful constraints on the flow law parameters for olivine rheology. A number of studies have implemented this approach with different geo- dynamical settings (e.g., Lee et al., 2011; van Hunen et al., 2005; Watts & Zhong, 2000). In particular, Alisic et al. (2012) tested a range of values for the stress exponent of dislocation creep in their global mantle flow model and compared their results with surface observables, such as plate velocity and the angle of subducting plates. The stress exponent was varied between 2 and 3.75 to explore the entire range of val- ues previously suggested for this parameter by experiment studies. Because each geodynamic simulation is time-consuming, only a few parameter values could be tested. Consequently, the uncertainty in the inversion result is difficult to assess. Recently, Baumann and Kaus (2015) presented a more intensive geodynamic inversion scheme based on Bayesian statistics, in which a diverse set of data, including topography, gravity, and surface velocity mea- surements, were simultaneously used to constrain all the flow law parameters for dislocation creep. A range of laboratory-derived estimates on the flow law parameters served as a priori bounds within which each parameter was allowed to vary. The multidimensional model space so defined was finely explored with the help of MCMC sampling. Approximately 2 × 106 forward models were conducted, and the resulting a posteriori probability distributions were used to determine the optimum values of all flow law parameters and the associated uncertainty. Despite its high computational cost, this approach allows for an exhaus- tive search of the multidimensional model domain and also provides a way to account for uncertainties in geophysical data. Worthen et al. (2014) suggested a computationally less expensive, adjoint-based inversion method that uses surface plate velocities as constraints to estimate the flow law parameters for dislocation creep in olivine. Instead of sampling a range of values for the unknown flow law parameters, the adjoint-based approach iteratively solves for the minimum in the cost or the misfit function and computes the gradients of the cost function to guide its search for the “best fit” solution. This reduces the number of iterations needed to reach a convergent solution. Li et al. (2017) made improvements to the adjoint-based inversion technique and elucidated its application by modeling time-dependent mantle convection with nonlinear rheology. Their inversion scheme could successfully reproduce the expected values of the stress exponent and the scalar coefficient for dislocation creep in the upper mantle. However, it is difficult to assess the efficiency and the success of the technique in the case where more realistic mantle rheologies, such as a composite flow law with both diffusion and dislocation creep, are assumed and all the flow law parameters, including the activation energies and volumes, are inverted for. Moreover, in these studies, little importance is given to laboratory-derived estimates on the flow law parameters. Even though experimental results are associated with large uncertainties, they are nonetheless the most direct measure of olivine rheology, and it would be beneficial for a geophysical inversion scheme to give some weightage to this information. For example, the adjoint-based inversion scheme of Ratnaswamy et al. (2015) has a provision to incorporate this information as a priori knowledge. Yet another approach to geophysical inversion was presented by Yang and Gurnis (2016). Here, the radial and lateral viscosity variation in the mantle were investigated based on the observations of long-wavelength geoid, free-air gravity anomaly, gravity gradients, and residual topography.