Trench-parallel flow and seismic anisotropy in the Mariana and Andean subduction systems

Shear- wave splitting measurements above the mantle wedge of the Mariana(1) and southern Andean(2,3) subduction zones show trench-parallel seismically fast directions close to the trench and abrupt rotations to trench- perpendicular anisotropy in the back arc. These patterns of seismic anisotropy may be caused by three-dimensional flow associated with along- strike variations in slab geometry(1-5). The Mariana and Andean subduction systems are associated with the largest along- strike variations of slab geometry observed on Earth(6,7) and are ideal for testing the link between slab geometry and solid- state creep processes in the mantle. Here we show, with fully three- dimensional non- newtonian subduction zone models, that the strong curvature of the Mariana slab and the transition to shallow slab dip in the Southern Andes give rise to strong trench- parallel stretching in the warm- arc and warm- back-arc mantle and to abrupt rotations in stretching directions that are accompanied by strong trench- parallel stretching. These models show that the patterns of shear- wave splitting observed in the Mariana and southern Andean systems may be caused by significant three- dimensional flow induced by along- strike variations in slab geometry.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62601/1/nature06429.pd

Deformation, stirring and material transport in thermochemical plumes

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95697/1/grl21928.pd

Dynamical Geochemistry: Mantle dynamics and its role in the formation of geochemical heterogeneity

Chemical geodynamics is a term coined nearly forty years ago to highlight the important link between Earth's geochemical evolution and plate tectonics & mantle convection. Significant progress in our understanding of this connection has taken place since then through advances in the analytical precision of geochemical measurements, dramatically improved geophysical imaging techniques, application of novel isotope systems, and great advances in computational power. Thee latter especially has improved geodynamical models and data interpretation techniques. We provide a review of these advances and their impact on chemical geodynamics, or perhaps, dynamical geochemistry. To focus this review we will address primarily the role of whole mantle convection and oceanic crust formation and recycling together with an update on our understanding of noble gas systematics

Thermal modeling of subduction zones with prescribed and evolving 2D and 3D slab geometries

The determination of the temperature in and above the slab in subduction zones, using models where the top of the slab is precisely known, is important to test hypotheses regarding the causes of arc volcanism and intermediate-depth seismicity. While 2D and 3D models can predict the thermal structure with high precision for fixed slab geometries, a number of regions are characterized by relatively large geometrical changes. Examples include the flat slab segments in South America that evolved from more steeply dipping geometries to the present day flat slab geometry. We devise, implement, and test a numerical approach to model the thermal evolution of a subduction zone with prescribed changes in slab geometry over time. Our numerical model approximates the subduction zone geometry by employing time dependent deformation of a B\'ezier spline which is used as the slab interface in a finite element discretization of the Stokes and heat equations. We implement the numerical model using the FEniCS open source finite element suite and describe the means by which we compute approximations of the subduction zone velocity, temperature, and pressure fields. We compute and compare the 3D time evolving numerical model with its 2D analogy at cross-sections for slabs that evolve to the present-day structure of a flat segment of the subducting Nazca plate

Starting laminar plumes: Comparison of laboratory and numerical modeling

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94953/1/ggge1631.pd

Variable seasonal coupling between air and ground temperatures: A simple representation in terms of subsurface thermal diffusivity

The utility of subsurface temperatures as indicators of temperature changes at Earth's surface rests upon an assumption of strong coupling between surface air temperature (SAT) and ground surface temperature (GST). Here we describe a simple representation of this coupling in terms of a variable thermal diffusivity in the upper meter of the subsurface. The variability is tied to daily SAT, precipitation, and snow cover, but does not incorporate the physical details of these and the many other factors that influence the air-ground interface in many high-fidelity land-surface models. Our simple model reduces the difference between observed and modeled temperatures by a factor of 3 to 4 over a model with uniform diffusivity driven only by SAT. This simple representation of air-ground coupling offers a means of simulating subsurface temperatures using only archived meteorological records and creates the potential for examining the long term character of air-ground temperature coupling

A divergence free C0C^0-RIPG stream function formulation of the incompressible Stokes system with variable viscosity

Pointwise divergence free velocity field approximations of the Stokes system are gaining popularity due to their necessity in precise modelling of physical flow phenomena. Several methods have been designed to satisfy this requirement; however, these typically come at a greater cost when compared with standard conforming methods, for example, because of the complex implementation and development of specialized finite element bases. Motivated by the desire to mitigate these issues for 2D simulations, we present a $C^0$-interior penalty Galerkin (IPG) discretization of the Stokes system in the stream function formulation. In order to preserve a spatially varying viscosity this approach does not yield the standard and well known biharmonic problem. We further employ the so-called robust interior penalty Galerkin (RIPG) method; stability and convergence analysis of the proposed scheme is undertaken. The former, which involves deriving a bound on the interior penalty parameter is particularly useful to address the $\mathcal{O}(h^{-4})$ growth in the condition number of the discretized operator. Numerical experiments confirming the optimal convergence of the proposed method are undertaken. Comparisons with thermally driven buoyancy mantle convection model benchmarks are presented

Dynamics of thermochemical plumes: 1. Plume formation and entrainment of a dense layer

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94724/1/ggge779.pd

Multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes

The hypothesis that a single mushroom-like mantle plume head can generate a large igneous province within a few million years has been widely accepted(1). The Siberian Traps at the Permian Triassic boundary(2) and the Deccan Traps at the Cretaceous Tertiary boundary(3) were probably erupted within one million years. These large eruptions have been linked to mass extinctions. But recent geochronological data(4-11) reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years. This observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal, single-stage plume model suggests. Here we present numerical experiments to demonstrate that the entrainment of a dense eclogite-derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations. Such a process may contribute to multiple episodes of large igneous provinces.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62705/1/nature03697.pd

Along‐arc variation in the 3‐D thermal structure around the junction between the Japan and Kurile arcs

The thermal structure in subduction zones has a strong influence on seismogenesis and arc volcanism. Traditional 2‐D models have been used to provide reasonable agreement between models and observations, but in a number of cases clear 3‐D effects are present. One such case is in the Northern Japan subduction system. At the junction between Japan and Kurile arcs, surface heat flow and the occurrence of intermediate‐depth seismicity are different than in the Tohoku and Hokkaido regions. We investigate the effects of 3‐D slab geometry and a local deepening of slab‐mantle decoupling depth on the thermal structure in this region based on 3‐D finite element approach. We find that both effects produce the along‐arc variation of slab surface temperature, which could reach ∼100°C. The warmer region arises through 3‐D effects of thermal conduction and the colder region arises through localized slow incoming flow in the case where 3‐D slab geometry is taken into account. 3‐D flow arises where a local deepening of slab‐mantle decoupling depth is assumed, which leads to both warmer and colder regions. The effects on surface heat flow are small. While intermediate‐depth seismicity in the subducted crust is suggested to be controlled by temperature‐dependent phase transitions, the predicted changes in thermal structure are not sufficient to cause the observed deepening of seismicity. This suggests that the thermal structure of this subduction zone may be more strongly influenced by time‐dependent deformation of the overriding crust and slab. Key Points 3‐D effects on the thermal structure of the subduction zone are investigated The effects on slab surface temperature are moderate The effects on surface heat flow are insignificantPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/108003/1/ggge20481.pd