elasticity: code to manipulate elastic constants

<p>elasticity is a set of Fortran and GMT codes for manipulating and displaying sets of elastic constants, relying on the seismo-fortran package (https://github.com/anowacki/seismo-fortran). Follow the master branch for the latest developments.</p

f90rad3: code to deal with GPR data in RAD3 format

<p>f90rad3 is a set of Fortran and GMT codes to deal (in an automated way) with ground-penetratic radar (GPR) data in the RAD3 format. It provides a way to read, filter, write and view 2D and 3D sections. Follow the master branch for the latest developments.</p

Deformation of phase D and Earth's deep water cycle

<div>Talk given at the AGU Fall Meeting 2016. Abstract number DI22A-03.</div><div><br></div><div><b>Abstract: </b>The stability of dense hydrous magnesium silicates such as phase D in subducting slabs provide a potential path for hydrogen transport from the Earth’s surface environment into the lower mantle. Recent analysis of source-side shear wave splitting for rays from deep earthquakes around slabs detected a signal of anisotropy that could be attributed to the deformation of phase D [Nowacki et al. 2015; <em>Geochem. Geophys. Geosyst.</em>, 16, 764–784]. If this is the case these observations could provide an estimate of the hydrogen flux into the lower mantle at depths beyond shallow recycling through the volcanic arc. However, the processes leading to the deformation of phase D and the generation of seismic anisotropy are not well known and this is a barrier to progress. Here we present initial results of simulations designed to reveal how easily different dislocations move in phase D during deformation and lead to the generation of seismic anisotropy measured by shear wave splitting. In particular, we use atomic scale simulations to calculate the energies of generalised stacking faults in phase D, which are used to parameterise Peierls-Nabarro models of dislocation structures and Peierls stresses at pressures up to 60 GPa. We then use results from these calculations as parameters for models of texture development in polycrystalline aggregates during deformation using the visco-plastic self-consistent approach. In combination with measurement of the distribution of seismic anisotropy around subducting slabs, and an analysis of the strain pattern expected as slabs pass through the transition zone, these results could constrain an important part of Earth’s deep water cycle. </div

Models of deformation and texture inheritance at the base of the mantle

<div>A poster presented at the 15th Symposium of the Study of the Earth's Deep Interior (SEDI 2016); 24-29th July, Nantes, France</div><div><b><br></b></div><div><b>Abstract</b></div><div><br></div><div>The profound changes in physical properties across the Earth’s core-mantle boundary makes this region key for the understanding of global-scale dynamics. As well as moderating any interaction between the metallic core and rocky mantle, the lowermost mantle also hosts the basal limb of mantle convection acting as a kind of inaccessible inverse lithosphere. In principle, knowledge of seismic anisotropy permits us to probe mantle flow in this region. However, in order to understand anisotropy in terms of flow, we need to know how the minerals present in the lowermost mantle deform and generate the textures that lead to bulk anisotropy. Previously, by combining predictions of mantle flow with the simulation of texture development in deforming post-perovskite aggregates, we have explored how different slip system activities give different predictions for the long-wavelength anisotropy in the lowermost mantle. By converting these results into models compatible with global scale radially anisotropic seismic tomography we have shown how different predictions correlate with tomographic inversions. We found that the most recent experimental indication of the active slip systems in post-perovksite, where dislocations gliding on (001) are most mobile, give predictions that were anti-correlated with results from tomography at long wavelengths. This means that it is difficult to explain the observed patterns of seismic anisotropy in the lowermost mantle as being due to the generation of lattice-preferred orientation in post-perovskite. A possible resolution to this difficulty is offered by experiments on analogues, which show that texture can be inherited during the perovskite to post-perovskite phase transition. Here we modify our previous approach to include this effect. This results in distributions of predicted seismic anisotropy that are in better agreement with tomography. In particular, we find that models where texture is generated by deformation of post-perovskite dominated by dislocations gliding on (001) followed by texture inheritance during the phase transition to perovskite driven by increasing temperature results in models that correlate with tomography at spherical harmonic degrees 1-5. In particular, texture inheritance in our models results in a better match to tomography in regions where the vertically polarised shear waves propagate more quickly than horizontally polarised shear waves.</div

Can global or regional scale seismic anisotropy in D″

<p>Slides for an invited talk at the 2011 Australian Academy of Science Elizabeth and Frederick White Research Conference "<strong>Minerals at extreme conditions – Integrating theory and experiment</strong>".</p

D" anisotropy derived from models of mantle flow: Predictions of elasticity and comparisons with seismic observations

<p><strong>Slides for invited talk at AGU 2011 Fall Meeting.</strong></p> <p><strong><br>Abstract number DI32A-02: <br></strong>We have developed an integrated model of the development of elastic anisotropy in the lowermost mantle by assuming mantle flow and deformation leads to the development of a lattice preferred orientation in post-perovskite. This texture development is simulated by tracing particles through a range of plausible mantle flow fields derived from the joint inversion of body wave travel times and geodynamic observations. Strain histories for the particles are extracted from the particle paths and these are used as the boundary conditions for polycrystalline models of texture development based on the visco-plastic self-constant approach. The resulting models of elastic anisotropy have been compared to global and regional scale observations of seismic anisotropy attributed to post-perovskite in D". At one extreme, we reduce the modelled anisotropy to one displaying vertical transverse isotropy. This is incompatible with some splitting measurements from ScS and SKS phases but permits a direct comparison with the results of global scale anisotropic tomography using S-waves. In order to assess the extent of azimuthal anisotropy in D" we make use of a global database of S-wave splitting measurements. These are also compared to the models of elastic anisotropy and provide further constraints on the degree to which lattice preferred orientation can be the explanation for anisotropy in D". On the regional scale we make comparisons with recent multi-azimuthal measurements of ScS splitting where the source side and receiver side upper-mantle anisotropy has been measured and removed. These measurements offer the most rigorous test of our predictions for the anisotropy of the lowermost mantle but also lead to difficulties in describing the frequency dependent effect of rapidly varying elasticity on the simulated seismic waveforms. Key findings are that uncertainty in the flow model is relatively insignificant compared to the current uncertainty in the single crystal plasticity, which can lead to model results which are anti-correlated to each other. In models where post-perovskite deformation is accommodated by dislocations moving on (010) or (100), patterns of anisotropy are approximately correlated with the results of tomographic inversions. On the other hand, in models where dislocations move on (001) patterns of anisotropy are nearly anti-correlated with tomographic inversions. If all the seismic anisotropy extracted from global anisotropic inversions is due to the presence of a lattice preferred orientation in post-perovskite in the lowermost mantle, and if the results of the tomographic inversions are not strongly biased by the sampling geometries, these results suggest that deformation of post-perovskite in the lowermost mantle may be accommodated by dislocations moving on (010) or (100)</p

Simulating the seismic signal of phase transtions in the deepest mantle

<p><strong>Slides for invited talk at AGU 2013 Fall Meeting.</strong></p> <p><strong>Abstract number DI34A-02:</strong> The discovery of the perovskite to post­perovskite phase transition in (Mg,Fe)SiO3 explains many of the seismic observations of the lowermost mantle including the presence of multiple seismic discontinuities and significant seismic anisotropy. However, the explanations of many detailed features remain elusive. The recent discovery of a topotactic relationship between the orientation of perovskite and post­perovskite crystals in a partially transformed analogue opens the possibility of texture inheritance through the phase transition [1]. This effect must be captured in simulations designed to explain the seismic anisotropy of the lowermost mantle, especially those which link mantle dynamics with seismic observations.</p> <p><br>We have extended our previous work linking models of flow in the lowermost mantle with simulations of texture development and predictions of seismic anisotropy [2] in order to account for the topotaxy between perovskite and post­perovskite. In particular, we compare four cases: (1) As in [2], anisotropy is only generated in post­perovskite by dislocation mediated deformation dominated by one of a number of slip systems, phase transitions destroy texture and ferropericlase and perovskite dominated rocks are isotropic. (2) Although phase transitions destroy texture, ferropericlase and/or perovskite deform by dislocation motion permitting the generation of seismic anisotropy in warmer regions of the mantle where post­perovskite is unstable. We account for the possibility of the inversion of slip­system activities in ferropericlase at high pressure as suggested by models of dislocation motion based on atomic scale simulations [3]. (3) Allow texture development by dislocation motion in perovskite and post­perovskite and texture inheritance through phase transitions by the mechanism described in [1]. However, we assume that the bulk of the lower mantle deforms by a mechanism that does not lead to the development of texture and so begin the simulation from a random distribution of crystal orientations the first time the post­perovskite stability field is encountered for downward migrating packages of mantle material. (4) Allow the bulk of the lower mantle to deform by dislocation creep such that material entering the lowermost mantle for the first time is already textured, allow this texture to be inherited and further modified by strain and phase transitions.</p> <p><br>These calculations show clear differences in global and local scale elastic anisotropy in the lowermost mantle between cases where texture is allowed to persist through the phase transitions and those where it is not. On a global scale and when radial anisotropy is imposed<br>the inclusion of topotaxy results in a dramatic decrease in the strength of the degree two signal and better agreement between observations and the model for post­perovskite deformation where dislocations moving on (001) dominate. On a smaller scale we see potential signs of reflectors generated by a change in anisotropy between perovskite that has inherited a strong starting texture from post­perovskite and overlaying perovskite that has never undergone the phase transition. These observations suggest that the incorporation of texture inheritance will be an important feature of future models of anisotropy in the lowermost mantle.</p> <p><br>[1] Dobson et al. 2013 Nature Geosci. 6:575–578 [2] Walker et al. 2011 Geochem. Geophys. Geosys. 12:Q10006 [3] Cordier et al. 2012 Nature 481:177­180</p

Modelling the Tectonics of the Lowermost Mantle

<p>Poster outlining method for and recent results of texture modelling from flow in the lowermost mantle. Presented at a meeting in June 2013. Includes textures from the deformation of post-perovskite, perovskite and periclase as well as the possibility of texture inheritance (topotaxy) for retrograde post-perovskite to perovskite transition.</p