Synergy of Experimental Rock Mechanics, Seismology, and Geodynamics Reveals Still Elusive Upper Mantle Rheology

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 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 study suggests that significant background shear flow is required near the lithosphere-asthenosphere 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 approach. Our analysis also provides a new insight into the asthenospheric water content and the grain size distribution in the upper mantle, but these results are also subject to nontrivial nonuniqueness. The merit of our probabilistic approach lies in its ability to assess the extent of such nonuniqueness, and we demonstrate this by quantifying the robustness of some of our results. Plain Language Summary A detailed understanding of how silicate minerals deform under various conditions is central to conducting realistic simulations of mantle dynamics, but the currently 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 can reduce such uncertainties by assimilating geophysical observations. Unlike previous similar attempts, 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.

Polarity-reversal subduction zone initiation triggered by buoyant plateau obstruction

“…The absence of elasticity in our simulations, while widely adopted in many other modelling studies (Capitanio and Morra, 2012;Gerya and Meilick, 2011;Schellart, 2017;Stegman et al, 2010), is also potentially relevant for our results since it is arguably expected to influence the dynamics of plate bending in subduction systems (e.g., Capitanio and Morra, 2012;Farrington et al, 2014;Thielmann and Kaus, 2012). Also important is the uncertainty per-taining to upper mantle rheology (e.g., Jain and Korenaga, 2020;King, 2016, and references therein) that allows for a somewhat aprioristic choice of parameters, relatively to which the obtained results are expected to be sensitive. This means that a slightly different choice of mantle rheology parameters (in Equation ( 9) above) corresponding, for instance, to an increase in mantle stiffness, could potentially increase the viscous resistance to slab-pull, thus modifying the underlying governing dynamics of the system and implying considerably different results.…”

Diverse Styles of Lithospheric Dripping: Synthesizing Gravitational Instability Models, Continental Tectonics, and Geologic Observations

“…Because lithosphere rheology is generally poorly known, inverse approaches that combine geodynamic modeling with observations are useful (Jain & Korenaga, 2020). For example, a suite of geodynamic models allowing large rheological variations can attempt to reproduce geophysical observations, and the narrower set of models that satisfy observational constraints can be used to further constrain the rheology of the lithosphere (Jain & Korenaga, 2020). Lithospheric dripping provides a target for such modeling in locations where the timing of drip initiation and detachment are well‐constrained (e.g., Molnar & Garzione, 2007; Molnar & Jones, 2004).…”

The Effects of Planetary and Stellar Parameters on Brittle Lithospheric Thickness

“…The flow law for dislocation creep (e.g., Hirth & Kohlstedt, 2003) is given by: where ε̇ dis is strain rate due to dislocation creep, A dis is a constant, E dis is the activation energy for dislocation creep, V dis is the activation volume for dislocation creep, R is the universal gas constant, T is temperature, and n is the stress exponent, which is a constant. We used values from Hirth and Kohlstedt (2003) of A dis = 1.1 × 10 5 s −1 MPa –n , E dis = 520 kJ mol −1 , V dis = 10 −6 m 3 mol −1 , R = 8.314 J mol −1 K −1 , and n = 3.5, although there is still considerable uncertainty in the olivine flow law and its parameters (Jain et al., 2019; Jain & Korenaga, 2020).…”