Eric Debayle

Global azimuthal seismic anisotropy and the unique plate-motion deformation of Australia

Differences in the thickness of the high-velocity lid underlying continents as imaged by seismic tomography, have fuelled a long debate on the origin of the ‘roots’ of continents. Some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth, with horizontally polarized shear waves travelling faster than vertically polarized ones. This azimuthally averaged anisotropy could arise from present-day deformation at the base of the plate, as has been found for shallower depths beneath ocean basins. Such deformation would also produce significant azimuthal variation, owing to the preferred alignment of highly anisotropic minerals. Here we report global observations of surface-wave azimuthal anisotropy, which indicate that only the continental portion of the Australian plate displays significant azimuthal anisotropy and strong correlation with present-day plate motion in the depth range 175–300 km. Beneath other continents, azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans. We infer that the fast-moving Australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy. Simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents.

Seismic evidence for a deeply rooted low-velocity anomaly in the upper mantle beneath the northeastern Afro/Arabian continent

Abstract We present seismic results that support the presence of a small, low shear velocity anomaly deeply rooted in the upper mantle transition zone beneath southern Arabia and the Red Sea. The low shear velocity anomaly persists down to the 660 km discontinuity. It is found from the waveform inversion of 2741 Rayleigh wave seismograms taking into account several higher modes. We use records from the permanent IRIS and GEOSCOPE stations completed with data collected after various field deployments of portable stations in the Horn of Africa (INSU experiment), Tanzania, Saudi Arabia and Tibet (PASSCAL experiments). The complete dataset provides a dense ray coverage of the Afro/Arabian continent and allows shear-wave heterogeneities to be resolved with wavelengths of a few hundred kilometers. To achieve a good vertical resolution in the whole upper mantle, we analyze up to the fourth Rayleigh mode in the period range 50–80 s, in addition to the fundamental Rayleigh mode in the period range 50–160 s. We discuss whether the pattern of upper mantle shear velocity anomaly could be related to local causes or to one or several plume conduits in the region. Our lateral resolution may intuitively not be sufficient to resolve a narrow plume conduit at transition zone depths. However, we show that when a dense coverage is available, a narrow low-velocity anomaly will affect the path-average measurements for a large number of individual seismograms crossing the anomaly. In this case, the low-velocity perturbation is mapped in the tomographic model, even though smoothed by the lateral resolution. We conclude that our observation is difficult to attribute to a shallow origin or to reconcile with a single narrow plume conduit in the region. It can be explained either by several close narrow plume tails or by a broad region of upwelling.

The Australian continental upper mantle structure and deformation inferred from surface waves

We present a new three-dimensional model for the SV wave hetero-geneities and azimuthal anisotropy in the upper mantle of the Australasian region. The model is constrained by the waveforms of 2194 Rayleigh waves seismograms with a dense ray coverage that ensure a lateral resolution of the order of few hundred of kilometers. The use of higher modes allows the resolution of the structure down to depths of at least 400 km. In the upper 200 km of the model, seismic velocities are lower on the eastern Phanerozoic margin of the continent compared to the Precambrian central and western cratons, in agreement with previous results for Australia. The boundary between Phanerozoic and Precambrian Australia is not clear, especially in the south, where a broad positive seismic anomaly underlays the Lachlan Fold Belt. The high-velocity lid beneath the continent shows significant variations in thickness. Locally, it may extend down to a depth of 300 km in the mantle, but for most of Precambrian Australia the lithospheric thickness oscillates around 200 km, while it is thinner on the eastern Phanerozoic margin. We found significant SV wave azimuthal anisotropy in the upper 250 km of the mantle, with a drastic change in the organization of anisotropy between the upper 150 km of the model and the deeper part, as revealed in a preliminary inversion. In the upper 150 km of the mantle, azimuthal anisotropy appears more likely to be related to past deformation frozen in the lithosphere, and in central Australia we found clear evidence that deformation is preserved since the Alice Springs orogeny. Below 150 km, a smoother pattern of anisotropy is observed, more likely to be related to present-day deformation due to the northward motion of the Australian plate. Our current data set allows constraint of the anisotropic directions at different depths with an unprecedented lateral resolution. The observation of significant changes of anisotropic directions with depth in the Australian continental mantle suggests that care should be taken in the interpretation of anisotropy from SKS observations.

Anisotropy in the Australasian upper mantle from Love and Rayleigh waveform inversion

Abstract Records of both Rayleigh and Love waves have been analyzed to determine the pattern of anisotropy in the Australasian region. The approach is based on a two-stage inversion. Starting from a smooth PREM model with transverse isotropy about a vertical symmetry axis, the first step is an inversion of the waveforms of surface waves to produce path specific one-dimensional (1-D) upper mantle models. Under the assumption that the 1-D models represent averages along the paths, the results from 1584 Love and Rayleigh wave seismograms are combined in a tomographic inversion to provide a representation of three-dimensional structure for wavespeed heterogeneities and anisotropy. Polarization anisotropy with SH faster than SV is retrieved in the upper 200–250 km of the mantle for most of Precambrian Australia. In this depth interval, significant lateral variations in the level of polarization anisotropy are present. Locally, the anisotropy can be large, reaching an extreme value of 9% that is difficult to reconcile with current mineralogical models. However, the discrepancy may be explained in part by the presence of strong lateral heterogeneities along the path, or by effects introduced by the simplifying assumption of transverse isotropy for each path. The consistency between the location of polarization and azimuthal anisotropy in depth suggests that both observations share a common origin. The observation of polarization anisotropy down to at least 200 km supports a two-layered anisotropic model as constrained by the azimuthal anisotropy of SV waves. In the upper layer, 150 km thick, anisotropy would be related to past deformation frozen in the lithosphere while in the lower layer, anisotropy would reflect present day deformation due to plate motion.

Upper mantle heterogeneities in the Indian Ocean from waveform inversion

A waveform inversion method is applied to 156 Love and Rayleigh wave seismograms to build up a 3-D model of the shear velocity in the upper mantle beneath the Indian Ocean. The first step of the method consists in finding, for each path, a radially stratified upper mantle model compatible with the Love and Rayleigh wave seismograms relative to that path. The fundamental mode and few higher modes are modelled in the waveform inversion. In a second step, the models related to the different paths are used in a tomographic inversion to map the 3-D upper mantle structure. The 3-D velocity model has a lateral resolution of 1000 km. No significant velocity anomalies are found below 300 km, although the resolution is still good. Continental roots are confined to the upper 300 km and low velocities below mid-oceanic ridges to the upper 250 km, depending on their spreading rates. At shallow depths (<80 km), we find a positive velocity anomaly beneath the West Indian ridge near the Rodriguez triple junction, where gravimetric and bathymetric data indicate a reduced volcanic activity. At greater depth (around 200 km), a low velocity signature is found beneath the hot-spot of Reunion-Mauritius islands and the Central Indian ridge. It could reflect a real connection between the two structures.

A global shear velocity model of the upper mantle from fundamental and higher Rayleigh mode measurements

We present DR2012, a global SV-wave tomographic model of the upper mantle. We use an extension of the automated waveform inversion approach of Debayle (1999) which improves our mapping of the transition zone with extraction of fundamental and higher-mode information. The new approach is fully automated and has been successfully used to match approximately 375,000 Rayleigh waveforms. For each seismogram, we obtain a path average shear velocity and quality factor model, and a set of fundamental and higher-mode dispersion and attenuation curves. We incorporate the resulting set of path average shear velocity models into a tomographic inversion. In the uppermost 200 km of the mantle, SV wave heterogeneities correlate with surface tectonics. The high velocity signature of cratons is slightly shallower (approximate to 200 km) than in other seismic models. Thicker continental roots are not required by our data, but can be produced by imposing a priori a smoother model in the vertical direction. Regions deeper than 200 km show no velocity contrasts larger than +/- 1\% at large scale, except for high velocity slabs within the transition zone. Comparisons with other seismic models show that current surface wave datasets allow to build consistent models up to degrees 40 in the upper 200 km of the mantle. The agreement is poorer in the transition zone and confined to low harmonic degrees (<= 10).

Seismic evidence for deep low-velocity anomalies in the transition zone beneath West Antarctica

Abstract We present a three-dimensional (3D) SV-wave velocity model of the upper mantle beneath the Antarctic plate constrained by fundamental and higher mode Rayleigh waves recorded at regional distances. The good agreement between our results and previous surface wave studies in the uppermost 200 km of the mantle confirms that despite strong differences in data processing, modern surface wave tomographic techniques allow to produce consistent velocity models, even at regional scale. At greater depths the higher mode information present in our data set allows us to improve the resolution compared to previous regional surface wave studies in Antarctica that were all restricted to the analysis of the fundamental mode. This paper is therefore mostly devoted to the discussion of the deeper part of the model. Our seismic model displays broad domains of anomalously low seismic velocities in the asthenosphere. Moreover, we show that some of these broad, low-velocity regions can be more deeply rooted. The most remarkable new features of our model are vertical low-velocity structures extending from the asthenosphere down to the transition zone beneath the volcanic region of Marie Byrd Land, West Antarctica and a portion of the Pacific–Antarctic Ridge close to the Balleny Islands hotspot. A deep low-velocity anomaly may also exist beneath the Ross Sea hotspot. These vertical structures cannot be explained by vertical smearing of shallow seismic anomalies and synthetic tests show that they are compatible with a structure narrower than 200 km which would have been horizontally smoothed by the tomographic inversion. These deep low-velocity anomalies may favor the existence of several distinct mantle plumes, instead of a large single one, as the origin of volcanism in and around West Antarctica. These hypothetical deep plumes could feed large regions of low seismic velocities in the asthenosphere.

The mantle transition zone as seen by global Pds phases: No clear evidence for a thin transition zone beneath hotspots

We present a new global study of the transition zone from Pds converted waves at the 410- and 660-km discontinuities. Our observations extend previous global Pds studies with a larger data set, especially in oceanic regions where we have been able to measure Pds travel times, sampling the mantle transition zone (MTZ) beneath 26 hotspot locations. We find significant lateral variations of the MTZ thickness. Both the maximum variations (+/- 35 - 40 km) and the long-wavelength pattern are in overall agreement with previous SS precursors studies. The MTZ is generally thick beneath subduction zones, where the observed MTZ variations are consistent with thermal anomalies ranging between -100 degrees K and -300 degrees K. In Central and North America, we observe an NW - SE pattern of thick MTZ, which can be associated with the fossil Farallon subduction. We do not find clear evidence for a thin MTZ beneath hotspots. However, the 410- km discontinuity remains generally deepened after correcting our Pds travel times for the 3D heterogeneities located above the MTZ, and its topography variations can be explained by thermal anomalies between + 100 degrees K and +300 degrees K. The depth of the 660-km discontinuity may be less temperature sensitive in hot regions of the mantle, which is consistent with the effect of a phase transition from majorite garnet to perovskite at a depth of 660 km.

Geodynamic Context of the Taiwan Orogen

Four independent arguments suggest that the Ryukyu subduction zone extended from Japan to southwest Taiwan (118°E) from the late Cretaceous to early Miocene (17-18 Ma): i) An analysis of the structure and timing of rifting in the basins of the East Asia continental shelf and west of Taiwan shows that they are located within four belts parallel to the mainland Chinese shoreline, which becomes younger oceanward since early Tertiary. Ridges with volcanic products are present between these belts. We interpret these basins and associated ridges as relict backarc basins and arcs of the Ryukyu subduction system. ii) Subsidence curves across west Taiwan Basins show that rifting ceased 17-18 Ma. iii) A new shear wave velocity model suggests that the Ryukyu slab extended in the past southwest of Taiwan, beneath the northern China Sea margin. iv) A deep seismic line shot across the north-eastern South China Sea margin also suggests that this margin was active in the past. We conclude that about 15-20 Ma, the southwestern extremity of the Ryukyu subduction zone jumped from 118°E (southwest of the Tainan Basin) to 126°E (where the present-day trend of the Ryukyu subduction zone changes direction). Since that time, the southwestern extremity of the Ryukyu subduction zone continuously moved westwards to its present-day location at 122°E. Since the beginning of formation of proto-Taiwan during late Miocene (9 Ma), the subducting PH Sea plate moved continuously through time in a N307° direction at 5.6 cm/yr with respect to EU, tearing the EU plate.

An automatically updated S‐wave model of the upper mantle and the depth extent of azimuthal anisotropy

We present 3D2015_07Sv, an S wave model of the upper mantle based on the waveform modeling of 1,359,470 Rayleigh waves recorded since 1976. The use of approximate forward theory and modeling allows updating the model with new data on a regular basis. 3D2015_07Sv contains azimuthal anisotropy, achieves a lateral resolution of ∼600 km, and is consistent with other recent models up to degree 60 in the uppermost 200 km and degree 15 in the transition zone. Although radial anisotropy has been found to extend deeper beneath continents than beneath oceans, we find no such difference for azimuthal anisotropy, suggesting that beneath most continents, the alignment of olivine crystal is preferentially horizontal and azimuthally random at large scale. As most continents are located on slow moving plates, this supports the idea that azimuthal anisotropy aligns at large scale with the present plate motion only for plates faster than ∼4 cm yr−1.