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  • 1
    Publication Date: 2018-07-03
    Description: The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin (SW Ireland). Here, we present two sets of tomographic images obtained with travel-time tomography of two different active-source seismic data sets: ocean bottom seismic (OBS) data and long-streamer data. The study provides new insights into geological processes that occurred at different scales and geological stages during the formation of the Porcupine Basin. OBS-derived images show the Vp structure of the uppermost lithosphere and the geometry of the Moho across and along the basin axis, providing insights into formation processes that occurred during lithospheric extension in the Mesozoic. In particular, these tomographic results together with neighboring seismic reflection lines provide crustal stretching (βc) estimates of ∼2.5 in the north at 52.5N and 〉 10 in the south at 51.7N. These values suggest that no crustal embrittlement occurred in the northernmost region, and that rifting has potentially reached crustal breakup in the southern part of the study area. Tomographic images reveal that mantle velocities decrease across the basin axis from east to west. These variations occur in a region where βc is within the range at which crustal embrittlement and serpentinisation are possible (βc 3-4). Across the basin axis, the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting, indicating fault-controlled mantle hydration. Mantle velocities also suggest that the degree of serpentinisation, together with the amount of crustal faulting, increases southwards along the basin axis. Seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 〉 15 %. This is consistent with laboratory measurements that show that at this degree of serpentinisation, mantle rocks are sufficiently weak to allow low-angle normal faulting. In contrast, the long-streamer tomographic image shows the Vp structure of the post-rift section in much more detail than OBS-derived images providing insights into basin-scale processes that occurred after lithospheric extension during the Cenozoic. The tomographic image reveals faster vertical velocity gradient in the center of the basin than in the flanks. This variation together with a relatively thick sediment accumulation in the center of the basin suggests higher overburden pressure and compaction compared to the margins. This suggests fluid flow driven by differential compaction towards the margins of the basin. The model also reveals two prominent vertical velocity anomalies located at the western margin of the basin, coinciding with the location of a reactivated basin-bounding fault. Comparing the corresponding time-migrated seismic section with the tomographic model, we observe that the hanging wall of the basin-bounding fault is not significantly affected by major normal faulting and yet is associated with comparatively lower seismic velocities. This result together with exploration well data suggests high effective porosities within the hanging wall suggesting potential overpressured areas. Our results suggest that the western basin-bounding fault is acting as a barrier for fluid migration causing overpressured areas over the western flank.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 2
    Publication Date: 2016-01-21
    Description: According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge by impinging at the base of the continental lithosphere shortly before or during the breakup of the South Atlantic margins. However, Tristan da Cunha is enigmatic as it cannot be clearly identified as a hot spot but may also be classified as a more shallow type of anomaly that may actually have been caused by the opening of the South Atlantic. The equivocal character of Tristan da Cunha is largely due to a lack of geophysical and petrological data in this region. We therefore staged a multi-disciplinary geophysical study of the region by acquiring passive marine electromagnetic and seismic data, and bathymetric data within the framework of the SPP1375 South Atlantic Margin Processes and Links with onshore Evolution (SAMPLE) funded by the German Science foundation. The experiment included two ship expeditions onboard the German R/V MARIA S. MERIAN in 2012 and 2013. In our contribution we will present first results on the shear wave velocity structure of the lithosphere-asthenosphere system. We applied the classical two-station method; Rayleigh wave dispersion curves are determined by crosscorrelation of seismograms from a pair of station. We measured interstation phase velocities of (earthquakeexcited) fundamental-mode surface waves in a period range of 10 to 60 s. The selection of acceptable phasevelocity measurements in the frequency domain had to be done manually for each event.We present phase-velocity maps for the study area. Furthermore, we present 1D shear wave velocity models inverted from the highest-quality observations.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2018-02-20
    Description: Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the tem- perature of the lithosphere and asthenosphere beneath it. We measured phase-velocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot. The ocean-bottom dataset presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear-wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. VS in the low velocity zone is 4.1–4.2 km/s, not as low as reported for Hawaii (~4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting. Petrological modeling shows that the seismic and bathymetry data is consistent with a moderately warm mantle (mantle potential temperature of 1420–1440ºC, an excess of about 85–105ºC compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65–70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2016-09-13
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 5
    Publication Date: 2017-04-28
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  • 6
    Publication Date: 2019-09-23
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  • 7
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    In:  Geophysical Journal International ; Year: 2008 ; Volume: 173 ; Issue: 2 ; Pages: 505-518
    Publication Date: 2018-02-09
    Description: We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an Sv-velocity model of the upper mantle (crust–660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamental-mode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface- and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear- and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in S- and P-wave velocities but also allow for S-wave azimuthal anisotropy—in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a ‘spectral-element’ resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-Sv-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 8
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    In:  Lithos ; Year: 2009 ; Volume: 109 ; Issue: 1-2 ; Pages: 96-111
    Publication Date: 2018-02-16
    Description: Depth distributions of seismic velocities and their directional dependence (anisotropy) in the crust and mantle beneath cratons yield essential constraints on processes of their formation and evolution. Despite recent progress in mapping the lateral extent of cratonic roots around the globe, profiles of seismic velocities within them remain uncertain. In this study we employ a novel combination of waveform-analysis techniques and measure inter-station Rayleigh- and Love-wave phase velocities in broad period ranges that enable resolution from the upper crust to deep upper mantle. Sampling a selection of 10 Archean and Proterozoic locations, we derive new constraints on the isotropic and radially anisotropic seismic structure of Precambrian lithosphere. Shear-wave speed VS is consistently higher in the lithosphere of cratons than in the lithosphere of Proterozoic foldbelts. Because known effects of compositional variations in the lithosphere on VS are too small to account for the difference, this implies that temperature in cratonic lithosphere is consistently lower, in spite of sub-lithospheric mantle beneath continents being thermally heterogeneous, with some cratons underlain, as we observe, by a substantially hotter asthenosphere compared to others. Lithospheric geotherms being nearly conductive, this confirms that the stable, buoyant lithosphere beneath cratons must be substantially thicker than beneath younger continental blocks. An increase in VS between the Moho and a 100-150 km depth is consistently preferred by the data in this study and is present in seismic models of continents published previously. We argue that this is largely due to the transition from spinel peridotite to garnet peridotite, proposed previously to give rise to the “Hales discontinuity” within this depth interval. The depth and the width of the phase transformation depend on mantle composition; it is likely to occur deeper and over a broader depth interval beneath cratons than elsewhere because of the high Cr content in the depleted cratonic lithosphere, as evidenced by a number of xenolith studies. Seismic data available at present would be consistent with both a sharp and a gradual increase in VS in the upper lithosphere (a Hales discontinuity or a “Hales gradient”). The VS profile in the upper mantle lithosphere is not shaped by the temperature distribution only; this needs to be considered when relating seismic velocities to lithospheric temperatures. Radial anisotropy in the upper crust is observed repeatedly and indicates vertically oriented anisotropic fabric (VSH 〈 VSV); this may yield a clue on how cratons grew, lending support to the view that distributed crustal shortening with sub-vertical flow patterns occurred over large scales in hot ancient orogens. In the lower crust and upper lithospheric mantle, radial anisotropy consistently reveals horizontal fabric (VSH 〉 VSV); the fabric can be interpreted as a record of (sub-)horizontal ductile flow in the lower crust and lithospheric mantle at the time of the formation and stabilisation of the cratons. We also find indications for radial anisotropy below 200 km depth, corroborating recent evidence for anisotropy in the asthenosphere beneath cratons due to current and recent asthenospheric flow.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 9
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    In:  Lithos ; Year: 2009 ; Volume: 109 ; Issue: 1-2 ; Pages: 96-111
    Publication Date: 2018-02-16
    Description: Depth distributions of seismic velocities and their directional dependence (anisotropy) in the crust and mantle beneath cratons yield essential constraints on processes of their formation and evolution. Despite recent progress in mapping the lateral extent of cratonic roots around the globe, profiles of seismic velocities within them remain uncertain. In this study we employ a novel combination of waveform-analysis techniques and measure inter-station Rayleigh- and Love-wave phase velocities in broad period ranges that enable resolution from the upper crust to deep upper mantle. Sampling a selection of 10 Archean and Proterozoic locations, we derive new constraints on the isotropic and radially anisotropic seismic structure of Precambrian lithosphere. Shear-wave speed VS is consistently higher in the lithosphere of cratons than in the lithosphere of Proterozoic foldbelts. Because known effects of compositional variations in the lithosphere on VS are too small to account for the difference, this implies that temperature in cratonic lithosphere is consistently lower, in spite of sub-lithospheric mantle beneath continents being thermally heterogeneous, with some cratons underlain, as we observe, by a substantially hotter asthenosphere compared to others. Lithospheric geotherms being nearly conductive, this confirms that the stable, buoyant lithosphere beneath cratons must be substantially thicker than beneath younger continental blocks. An increase in VS between the Moho and a 100-150 km depth is consistently preferred by the data in this study and is present in seismic models of continents published previously. We argue that this is largely due to the transition from spinel peridotite to garnet peridotite, proposed previously to give rise to the “Hales discontinuity” within this depth interval. The depth and the width of the phase transformation depend on mantle composition; it is likely to occur deeper and over a broader depth interval beneath cratons than elsewhere because of the high Cr content in the depleted cratonic lithosphere, as evidenced by a number of xenolith studies. Seismic data available at present would be consistent with both a sharp and a gradual increase in VS in the upper lithosphere (a Hales discontinuity or a “Hales gradient”). The VS profile in the upper mantle lithosphere is not shaped by the temperature distribution only; this needs to be considered when relating seismic velocities to lithospheric temperatures. Radial anisotropy in the upper crust is observed repeatedly and indicates vertically oriented anisotropic fabric (VSH 〈 VSV); this may yield a clue on how cratons grew, lending support to the view that distributed crustal shortening with sub-vertical flow patterns occurred over large scales in hot ancient orogens. In the lower crust and upper lithospheric mantle, radial anisotropy consistently reveals horizontal fabric (VSH 〉 VSV); the fabric can be interpreted as a record of (sub-)horizontal ductile flow in the lower crust and lithospheric mantle at the time of the formation and stabilisation of the cratons. We also find indications for radial anisotropy below 200 km depth, corroborating recent evidence for anisotropy in the asthenosphere beneath cratons due to current and recent asthenospheric flow.
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 10
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    In:  Geophysical Journal International ; Year: 2008 ; Volume: 173 ; Issue: 2 ; Pages: 505-518
    Publication Date: 2018-02-09
    Description: We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an Sv-velocity model of the upper mantle (crust–660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamental-mode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface- and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear- and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in S- and P-wave velocities but also allow for S-wave azimuthal anisotropy—in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a ‘spectral-element’ resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-Sv-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere.
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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