1,720,975 research outputs found
A comparison of tomographic and geodynamic mantle models
No full textWe conduct a comprehensive and quantitative analysis of similarities and differences between recent seismic tomography models of the Earth's mantle in an attempt to determine a benchmark for geodynamic interpretation. After a spherical harmonic expansion, we find the spectral power and radial correlation of each tomographic model as a function of depth and harmonic degree. We then calculate the correlation, at the same depths and degrees, between all possible pairs of models, to identify stable and model-dependent features ( the former being usually of longer spatial wavelength than the latter). We can therefore evaluate the degree of robust structure that seismologists have mapped so far and proceed to calculate ad hoc mean reference models. Tomographic models are furthermore compared with two geodynamic subduction models that are based on plate motion reconstructions. We find systematically low intermediate-wavelength correlation between tomography and convective reconstruction models and suggest that the inadequate treatment of the details of slab advection is responsible. However, we confirm the presence of stable, slab-like fast anomalies in the mid-mantle whose geographic pattern naturally associates them with subduction. This finding, in addition to our analysis of heterogeneity spectra and the absence of strong minima in the radial correlation functions besides the one at similar to 700 km, supports the idea of whole mantle convection with slab penetration through the 660 km phase transition, possibly accompanied by a reorganization of flow. RI Becker, Thorsten/A-6665-201
Vertical coherence in mantle heterogeneity from global seismic data
No full textThe vertical coherence of mantle structure is of importance for a range of dynamic issues including convective mass transport and the geochemical evolution of Earth. Here, we use seismic data to infer the most likely depth ranges of strong, global changes in the horizontal pattern of mantle heterogeneity. We apply our algorithm to a comprehensive set of measurements, including various shear- and compressional-wave delay times and Love- and Rayleigh-wave fundamental mode and overtone dispersion, so that tomography resolution is as high as possible at all mantle depths. We find that vertical coherence is minimum at similar to 100 km and similar to 800 km depths, corresponding to the base of the lithosphere and the transition between upper and lower mantle, respectively. The D '' layer is visible, but not as prominent as the shallower features. The rest of the lower mantle is, essentially, vertically coherent. These findings are consistent with slab stagnation at depths around, and perhaps below, the 660-km phase transition, and inconsistent with global, chemically distinct, mid-mantle layering. Citation: Boschi, L., and T. W. Becker (2011), Vertical coherence in mantle heterogeneity from global seismic data, Geophys. Res. Lett., 38, L20306, doi:10.1029/2011GL049281
On the statistical significance of correlations between synthetic mantle plumes and tomographic models
No full textIn a recent article, [Boschi, L., Becker, T.W., Stemberger, B., 2007. Mantle plumes: dynamic models and seismic images. Geochem. Geophys. Geosyst. 8, Q10006. doi:10.1029/2007GC001733] (BBS07) have reevaluated the degree to which slow seismic tomography anomalies correlate with the possible locations of plume-like mantle upwellings connected to surface hotspots. They showed that several, but not all, hotspots are likely to have a deep mantle origin. importantly, they found that when advection of plume conduits in mantle flow is considered, such correlations are significantly higher than when conduits are assumed to be vertical under hotspots. The validity of these statements depends, however, on the definition of statistical significance. BBS07 evaluated the significance of correlation through simple Student's t tests. Anderson (personal communication, July 2007) questioned this approach, given that the true information content of published tomography models is generally unknown, and proposed, instead, to evaluate the significance of correlation by comparing tomographic results with Monte Carlo simulations of randomly located plumes. Following this approach, we show here that the correlation found by BBS07 between advected plumes and slow anomalies in S-velocity tomography is less significant than previously stated, but still significant (at the 99.7% confidence level). We also find an indication that the seismic/geodynamic correlation observed by BBS07 does not only reflect the natural tendency of plumes to cluster in slow/hot regions of the mantle: although realistically advected, and thereby biased towards such regions, our random plumes correlate with slow tomographic anomalies significantly less than the plume models of BBS07. A less significant correlation with plume models characterizes P-velocity tomography; the correlation is, however, enhanced, if flow is computed from tomographic models with amplified heterogeneity, possibly accounting for the known resolution limits of global seismic data. In summary, the conclusions of BBS07 are confirmed: even at relatively long wavelengths, tomographic models are consistent with the presence of a number of tilted, whole-mantle plume-shaped slow anomalies, connected to surface hotspots. (C) 2008 Elsevier B.V. All rights reserved. RI Becker, Thorsten/A-6665-201
Mantle structure and dynamic topography in the Mediterranean Basin
No full text[1] We study the contribution of mantle flow to surface deformation within the Mediterranean Basin. Flow is modeled numerically based on lateral changes in mantle temperature estimated from tomography models. We find that modeling results are significantly affected by the properties of the selected tomography models. Shear-velocity models based on surface-wave observations achieve the highest resolution of upper-mantle structure, and, as a result, are most successful in predicting microplate motion and dynamic topography. Citation: Boschi, L., C. Faccenna, and T. W. Becker (2010), Mantle structure and dynamic topography in the Mediterranean Basin, Geophys. Res. Lett., 37, L20303, doi:10.1029/2010GL045001. RI Becker, Thorsten/A-6665-201
Mantle plumes: Dynamic models and seismic images
No full textDifferent theories on the origin of hot spots have been debated for a long time by many authors from different fields, and global-scale seismic tomography is probably the most effective tool at our disposal to substantiate, modify, or abandon the mantle-plume hypothesis. We attempt to identify coherent, approximately vertical slow/hot anomalies in recently published maps of P and S velocity heterogeneity throughout the mantle, combining the following independent quantitative approaches: (1) development and application of a "plume- detection'' algorithm, which allows us to identify a variety of vertically coherent features, with similar properties, in all considered tomographic models, and (2) quantification of the similarity between patterns of various tomographic versus dynamic plume- conduit models. Experiment 2 is complicated by the inherent dependence of plume conduit tilt on mantle flow and by the dependence of the latter on the lateral structure of the Earth's mantle, which can only be extrapolated from seismic tomography itself: it is inherently difficult to disentangle the role of upwellings in "attracting'' plumes versus plumes being defined as relatively slow, and thus located in regions of upwellings. Our results favor the idea that only a small subset of known hot spots have a lower-mantle origin. Most of those that do can be associated geographically with a few well-defined slow/hot regions of very large scale in the lowermost mantle. We find evidence for both secondary plumes originating from the mentioned slow/hot regions and deep plumes whose conduits remain narrow all the way to the lowermost mantle. To best agree with tomographic results, modeled plume conduits must take into account the effects of advection and the associated displacement of plume sources at the base of the mantle. RI Becker, Thorsten/A-6665-201
Length scales, patterns and origin of azimuthal seismic anisotropy in the upper mantle as mapped by Rayleigh waves
No full textWe measure the degree of consistency between published models of azimuthal seismic anisotropy from surface waves, focusing on Rayleigh wave phase-velocity models. Some models agree up to wavelengths of similar to 2000 km, albeit at small values of linear correlation coefficients. Others are, however, not well correlated at all, also with regard to isotropic structure. This points to differences in the underlying data sets and inversion strategies, particularly the relative 'damping' of mapped isotropic versus anisotropic anomalies. Yet, there is more agreement between published models than commonly held, encouraging further analysis. Employing a generalized spherical harmonic representation, we analyse power spectra of orientational (2 Psi) anisotropic heterogeneity from seismology. We find that the anisotropic component of some models is characterized by stronger short-wavelength power than the associated isotropic structure. This spectral signal is consistent with predictions from new geodynamic models, based on olivine texturing in mantle flow. The flow models are also successful in predicting some of the seismologically mapped patterns. We substantiate earlier findings that flow computations significantly outperform models of fast azimuths based on absolute plate velocities. Moreover, further evidence for the importance of active upwellings and downwellings as inferred from seismic tomography is presented. Deterministic estimates of expected anisotropic structure based on mantle flow computations such as ours can help guide future seismologic inversions, particularly in oceanic plate regions. We propose to consider such a priori information when addressing open questions about the averaging properties and resolution of surface and body wave based estimates of anisotropy. RI Becker, Thorsten/A-6665-2010; Ekstrom, Goran/C-9771-201
Radial anisotropy in the European mantle: Tomographic studies explored in terms of mantle flow
No full textPrevious studies have shown that radial seismic anisotropy as estimated from flow models is in good agreement with results from tomography at global scale, in particular underlying oceanic basins. However, the fit is typically poor at smaller scale lengths, particularly in tectonically complex regions. We conduct a comparative analysis of tomographically mapped and dynamically modeled radial anisotropy at the scale of Europe and the Mediterranean Basin, including three tomographic models based on different observations and/or methods. We find that adaptive-grid surface-wave tomography, with parametrization density depending locally on the spatial and azimuthal density of data coverage, leads to the seismic model closest to (albeit still far from) geodynamic predictions. The ability to map regional-scale seismic anisotropy may provide a new constraint, complementary to isotropic tomography, to the nature of upper mantle flow. Citation: Schaefer, J. F., L. Boschi, T. W. Becker, and E. Kissling (2011), Radial anisotropy in the European mantle: Tomographic studies explored in terms of mantle flow, Geophys. Res. Lett., 38, L23304, doi:10.1029/2011GL049687. OI Becker, Thorsten/0000-0002-5656-456
On the relevance of Born theory in global seismic tomography
No full textDoes the application of seismic Born theory, as opposed to simpler ray theory, lead to an improvement in tomographic images of the Earth? In recent publications, Montelli et al. (2004a, 2004b) and van der Hilst and de Hoop ( 2005) among others have expressed opposite opinions. We propose a quantitative approach to the comparison of tomographic images, which we apply to the case of surface-wave phase velocity maps derived with Born vs. ray theory. RI Becker, Thorsten/A-6665-201
Morpho‐Tectonics of Transpressional Systems: Insights From Analog Modeling
No full textTranspressional margins are widespread, and their dynamics are relevant for plate boundary evolution globally. Though transpressional orogen evolution involves a topographic response to deformation, many studies focus only on the structural development of the system ignoring surface processes. Here, we present a new set of analog models constructed to investigate how tectonic and surface processes interact at transpressive plate boundaries and shape topography. Experiments are conducted by deforming a previously benchmarked crustal analog material in a meter-scale plexiglass box while controlling erosion through misting nozzles mounted along the transpressional wedge. To analyze the experiments, we generate digital elevation models from laser scans and conduct image correlation analysis on photos taken during experiments. We focus on three experiments that cover a range of erosional conditions and shortening stages (two end-member erosion models and a dry reference). In all experiments, a bivergent wedge forms, and strain partitioning broadly evolves according to previously established models. Regarding drainage networks, we find that the streams in our models develop differently through feedback between fault development and drainage rearrangement processes. Differences between end-member erosional models can be explained by the varying response of streams to structure modulated by rainfall. Additionally, erosion may influence the structural evolution of transpressional topography, leading to accelerated strike-slip partitioning. From these results, we create a model for developing structures, streams, and topography where incision and valley formation along main structures localize exhumation. We apply insights from the models to natural transpressional systems, including the Transverse Ranges, CA, and the Venezuelan Andes
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