ALBERT

Description: The new release AIUB-RL02 of monthly gravity models from GRACE GPS and K-Band range-rate data is based on reprocessed satellite orbits referring to the reference frame IGb08. The release is consistent with the IERS2010 conventions. Improvements with respect to its predecessor AIUB-RL01 include the use of reprocessed (RL02) GRACE observations, new atmosphere and ocean dealiasing products (RL05), an upgraded ocean tide model (EOT11A), and the interpolation of shallow ocean tides (admittances). The stochastic parametrization of AIUB-RL02 was adapted to include daily accelerometer scale factors, which drastically reduces spurious signal at the 161 d period in C 20 and at other low degree and order gravity field coefficients. Moreover, the correlation between the noise in the monthly gravity models and solar activity is considerably reduced in the new release. The signal and the noise content of the new AIUB-RL02 monthly gravity fields are studied and calibrated errors are derived from their non-secular and non-seasonal variability. The short-period time-variable signal over the oceans, mostly representing noise, is reduced by 50 per cent with respect to AIUB-RL01. Compared to the official GFZ-RL05a and CSR-RL05 monthly models, the AIUB-RL02 stands out by its low noise at high degrees, a fact emerging from the estimation of seasonal variations for selected river basins and of mass trends in polar regions. Two versions of the monthly AIUB-RL02 gravity models, with spherical harmonics resolution of degree and order 60 and 90, respectively, are available for the time period from March 2003 to March 2014 at the International Center for Global Earth Models or from ftp://ftp.unibe.ch/aiub/GRAVITY/GRACE (last accessed 22 March 2016).

Description: The Yellowstone–East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume–lithosphere interactions. We show that fast (~300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that ~150 km large and ~100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.

Description: The H.U. Sverdrupfjella is part of the high-grade Maud Belt in Dronning Maud Land (East Antarctica), which was located in a central position of the Gondwana supercontinent. Here we study high-pressure granulites from the eastern H.U. Sverdrupfjella and present a detailed reconstruction of the P–T–t history based on a combination of Zr-in-rutile and Ti-in-zircon thermometry, zircon U–Pb dating, monazite chemical dating, garnet diffusion modelling and petrological modelling. Peak metamorphic conditions of ~930°C and 1·45 GPa persisted for less than 6 Myr and were attained at 570 ± 7 Ma, embedded in a well-documented clockwise loading, heating, and decompression path. The rocks had already been rapidly exhumed to a crustal depth of ~30 km at 556 ± 7 Ma. In addition to the very short-lived ultrahigh-temperature peak, zircon preserves evidence for protracted granulite-facies conditions with temperatures above 800°C from as early as c. 590 Ma, persisting for c. 40 Myr. Constraints on prograde metamorphism are recorded in zircon and in rutile inclusions in garnet. Zr-in-rutile thermometry using rutile included in different generations of garnet is used to reconstruct the prograde P–T path, documenting burial followed by heating to ultrahigh temperatures at peak pressures. Complementary Ti zonation in prograde cores of zircon grains documents and dates heating and peak temperatures, whereas younger zircon rims show lower Ti-in-zircon temperatures and date the retrograde stages of metamorphism. Our results provide the first evidence for Neoproterozoic high-pressure granulite-facies metamorphism and ultrahigh-temperature conditions for this region. The clockwise loading–heating path and the peak P–T conditions strongly indicate that the rocks preserved in Dronning Maud Land were part of the lower plate during a continent–continent collision event related to Gondwana assembly at c. 570 Ma. The metamorphic evolution determined in this study and the correlation with similar P–T evolutions documented in adjacent terranes favour the continuation of the c. 580–560 Ma Mozambique Belt into Dronning Maud Land. Furthermore, the striking contemporaneity of the metamorphism in the different parts of central Gondwana suggests that the Coats Land Block was part of greater India prior to this collision.

Description: We present a new, computationally efficient numerical method to simulate global seismic wave propagation in realistic 3-D Earth models. We characterize the azimuthal dependence of 3-D wavefields in terms of Fourier series, such that the 3-D equations of motion reduce to an algebraic system of coupled 2-D meridian equations, which is then solved by a 2-D spectral element method (SEM). Computational efficiency of such a hybrid method stems from lateral smoothness of 3-D Earth models and axial singularity of seismic point sources, which jointly confine the Fourier modes of wavefields to a few lower orders. We show novel benchmarks for global wave solutions in 3-D structures between our method and an independent, fully discretized 3-D SEM with remarkable agreement. Performance comparisons are carried out on three state-of-the-art tomography models, with seismic period ranging from 34 s down to 11 s. It turns out that our method has run up to two orders of magnitude faster than the 3-D SEM, featured by a computational advantage expanding with seismic frequency.

Description: Measurements of the magnetic field are one of the most used methods in geophysical exploration. In order to reduce the degree of ambiguity of this technique during inversion and modelling, data acquired by newly available gradiometer systems based on Superconducting Quantum Interference Devices (SQUIDs) are used. These systems provide measurements of the full magnetic gradient tensor of the Earth’s magnetic field, which offers a higher directional sensitivity than conventional total field magnetometers. A magnetization vector inversion (MVI) approach has been applied on data sets acquired over a dolerite intrusion in central Germany in order to model the full magnetization vector including remanent and induced components. Two different models have been created: one using only the magnetic total field anomaly (TFA) and the other based on five components of the magnetic gradient tensor. The two models show in principal the same structure, but the model based on the gradient tensor shows better defined structures. Also, magnetization amplitudes are closer to those measured on rock samples in this area. A comparison of the total magnetization vector of the rock samples and the models shows a better agreement in the vector direction of the gradient model compared to the total field model. A separation of induced and remanent contributions to the total magnetization has been performed and again shows better results when the gradient-based model is used. The effectiveness of the separation procedure will be discussed herein. The usage of gradiometer systems in an airborne geomagnetic exploration provides additional directional information, which is very helpful for MVIs. Compared to our model based on conventional TFA data, the gradient-based model features a much better agreement of the shape and magnetization of subsurface structures with those obtained from geologic−1al studies. The same, FTMG derived estimates of magnetization are more consistent with the results of measurements on rock samples.

Description: In global-scale seismic tomography, teleseismic P and PP waves mainly constrain structures in the upper two thirds of the mantle, whereas core-diffracted waves (Pdiff) constrain the lower third. This study is the first to invert a very large data set of Pdiff waves, up to the highest possible frequencies. This results in tomographic resolution matching and exceeding that of global S-wave tomographies, which have long been the models of choice for interpreting lowermost mantle structure. We present three new global tomography models of 3-D isotropic P-wave velocity in the earth’s mantle. Multifrequency cross-correlation traveltimes are measured on all phases in passbands from 30 s dominant period to the highest frequencies that produce satisfactory fits (≈3 s). Model DETOX-P1 fits ≈2.5 M traveltimes from teleseismic P waves. DETOX-P2 fits the same data, plus novel measurements of ≈1.4 M traveltimes of Pdiff waves. DETOX-P3 fits the same data as DETOX-P2, plus ≈ 1.2 M PP traveltimes. Synthetics up to 1 s dominant period are computed by full wave propagation in a spherically symmetric earth using the spectral-element method AxiSEM. Traveltimes are linked to 3-D velocity perturbations (dVP/VP) by finite-frequency Fréchet kernels, parametrized on an adaptive tetrahedral grid of ≈400 000 vertices spaced by ≈80 km in the best-sampled regions. To complete spatial coverage, the waveform cross-correlation measurements are augmented by ≈5.7 million analyst-picked, teleseismic P arrival times. P, Pdiff and PP traveltimes are jointly inverted for 3-D isotropic P-velocity anomalies in the mantle and for events corrections, by least squares solution of an explicit matrix–vector equation. Inclusion of Pdiff traveltimes (in DETOX-P2, -P3) improves the spatial sampling of the lowermost mantle 100- to 1000-fold compared to teleseismic P waves (DETOX-P1). Below ≈2400 km depth, seismically slow anomalies are clustered at southern and equatorial latitudes, in a dozen or more intensely slow patches of 600–1400 km diameter. These features had long been classed into two large low shear velocity provinces (LLVP), which now appears questionable. Instead, patches of intensely slow anomalies in the lowermost mantle seem to form a nearly continuous, globe-spanning chain beneath the southern hemisphere, according to our increased resolution of LLVP-internal subdivisions and newly imaged patches beneath South America. Our tomography also supports the existence of whole-mantle plumes beneath Iceland, Ascension, Afar, Kerguelen, Canary, Azores, Easter, Galapagos, Hawaii, French Polynesia and the Marquesas. Seismically fast structure in the lowermost mantle is imaged as narrowly elongated belts under Eastern Asia and the Americas, presumably reflecting the palaeo-trench geometries of subduction zones and arcs that assembled Eastern Asia and the American Cordilleras in Palaeozoic and early Mesozoic times. Mid-mantle structure is primarily constrained by teleseismic P waves, but Pdiff data have a stabilizing effect, for example, sharpening the geometries of subducted slabs under the Americas, Eurasia and the Northern Pacific in the upper 2000 km. PP traveltimes contribute complementary constraints in the upper and mid mantle, but they also introduce low-velocity artefacts beneath the oceans, through downward smearing of lithospheric structure. Our three new global P-wave models can be accessed and interactively visualized through the SubMachine web portal (http://submachine.earth.ox.ac.uk/).