Seismic Anisotropy in the Asthenosphere Beneath the Eifel Region, Western Germany

Seismic Anisotropy in the Asthenosphere Beneath the Eifel Region, Western Germany Kristoffer T. Walker1, Götz H.R. Bokelmann2, Simon L. Klemperer3, Gün- ter Bock4 and The Eifel Plume Team5 1Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego 2Laboratoire de Tectonophysique, Université Montpellier II, Montpellier, France 3Department of Geophysics, Stanford University 4GeoForschungsZentrum Potsdam 5U. Achauer, M. Budweg, T. Camelbeeck, U. Christensen, F. Collin, N. d'Oreye, I. Eschghi, H. Gaensicke, K.-G. Hinzen, M. Jordan, T. Kaspar, M. Keyser, R. Pelzing, J.R.R. Ritter, F. Scherbaum, Y.-F. Temme, M. Weber, and U. Wüllner Abstract We provide evidence for a plume-like upwelling beneath the Eifel hotspot, Western Germany, by using teleseismic shear-wave splitting to resolve the anisotropy associated with upwelling flow that is spreading laterally into the asthenosphere. The variation in fast–polarization azimuth we find across the Eifel hotspot is explained by a model of slowly upwelling material that is horizontally being deflected or sheared in a parabolic astheno- spheric flow (PAF) pattern toward west-southwest, a direction that corre- lates with Eurasian absolute plate motion. We suggest that the lack of an age progression for Eifel volcanism, which is expected for a fixed- upwelling model, is a result of (1) sporadic volcanism due to a low excess plume temperature and/or varying crustal stresses that periodically relax and facilitate eruption, and (2) complex upwelling flow pathways and/or Late Tertiary changes in the slow Eurasian plate motion. The success of the PAF model in fitting the data is remarkable given the small number of parameters (four) and the consistency with the plate motion direction de- termined from geology and/or geodesy. This suggests that a predictable mantle-anisotropy pattern may exist also for other hotspots driven by plume-like upwellings, and that splitting can be a useful diagnostic to dif- ferentiate between plume-like and alternative sources for mantle hotspots. 440 Walker KT, Bokelmann GHR, Klemperer SL, Bock G, Eifel Plume Team 1 Introduction The plate-tectonic hypothesis, that rigid plates move over the Earth’s surface above a fluid-like mantle, is used to explain most of Earth’s geologi- cal and geophysical features (Dietz 1961, Hess 1962). The plume hypothesis, that cylindrical conduits of hot upwelling mantle material are responsible for up to 10 % of Earth's heat flow, has been used to explain “hotspots”, anomalous volcanic regions far from plate boundaries (e.g., Hawaii and Yellowstone) that are often associated with some combination of rapid and voluminous eruptions of basaltic lava, broad topographic swells and geoid highs, and linear age progressions of volcanic centres (Morgan 1971, Sleep 1990, Davies and Richards 1992). Recently the plume hypothesis has been challenged, and complex plate-tectonic models have been proposed instead to explain such regions (e.g., Anderson 1994, King and Anderson 1998, Smith and Lewis 1999). Plumes are elusive targets of seismological investigation due to their hypothesised geometry and depth. Regional tomographic inversions of teleseismic body-wave data, which suffer from the inherent trade-off between misfit reduction and model complexity, tend to smear anomalies vertically in regions penetrated by mostly subvertical rays. Consequently applications of this technique around hotspots (e.g., Bijwaard and Spak- man 1999, Foulger et al. 2001, Wolfe et al. 2002) have led to intriguing, but sometimes controversial image interpretations. In addition, resolution constraints and choice of reference velocity in past global and regional inversions have limited reliable detailed imaging of the deep lower mantle beneath hotspots (Ritsema and Allen 2003). Although there has been recent success in detecting some plumes with surface-wave anisotropy tomography (Montagner 2002), finite-frequency body-wave tomography (Montelli et al. 2004), and SS precursor modelling and receiver functions (Li et al. 2003), and the debate for some may be shifting from "whether plumes exist" to "where and at what depth they exist," new approaches for detecting plumes will help resolve the contro- versy. For example, seismic velocity anisotropy in the upper mantle that is detected by teleseismic shear-wave splitting (separation of the shear-wave in an anisotropic rock into two polarised waves travelling at different speeds) can be explained by a lattice preferred orientation (LPO) of olivine fast axes due to either past or present mantle deformation. This interpreta- tion has been made to explain correlations of fast polarisation directions with absolute plate motion (Vinnik et al. 1992, Bormann et al. 1996, Schutt and Humphreys 2001), although many other interpretations have been proposed to explain other correlations (e.g., Savage 1999). An essen- Seismic Anisotropy in the Asthenosphere Beneath the Eifel Region 441 tial part of the plume hypothesis, the deflection of plume material by a moving plate, can be tested by analysing shear-wave splitting of shear phases generated from teleseismic earthquakes (Walker 2004). Although, because mantle anisotropy from a plume would be mostly confined to the upper 300 km of the Earth’s mantle, a positive result only suggests a “plume-like” upwelling - the depth of origin not being constrained. Fig. 1. Shear-wave splitting example for station WLF (for location, see Figure 6). We use the method of Silver and Chan (1991) as modified by Walker (2004) to find the optimum fast polarisation azimuth (φ) and delay time (dt) by performing a grid search over φ and dt to minimise coherent energy on the IPAC-90° (trans- verse) component, energy that is a diagnostic signal of anisotropy. In the upper left we show the observed horizontal-component waveforms parallel and perpendicular to the initial polarisation azimuth (IPAO/IPAO-90°). After correcting for the splitting using φ and dt, the energies on the corresponding waveforms IPAC and IPAC-90° are maximised and minimised, respectively. The box in the upper right indicates the date and UTC time of the analysed event (year:julday:hour), the measured φ and dt, and the energy signal-to-noise ratio (SNR) 442 Walker KT, Bokelmann GHR, Klemperer SL, Bock G, Eifel Plume Team A vertically oriented low P- and S-seismic velocity anomaly was im- aged in the mantle (Ritter et al. 2001, Keyser et al. 2002, Ritter this volume) down to the 400 km depth limit of resolution beneath the 1997-98 Eifel Plume Project seismic network, a dense network of 158 mobile seismic stations centred on the Eifel hotspot in west-central Europe (Ritter et al. 2000). Receiver functions also resolve a depression of the 410 km dis- continuity and thinning of the transition zone beneath the Eifel (Grunewald et al. 2001, M. Budweg personal communication 2003, Weber et al. this volume). We test an upwelling model for the Eifel hotspot by analysing shear-wave splitting in seismograms recorded during the seven-month op- eration of the Eifel network, and comparing the fast directions to those predicted for a simple upwelling. 2 New Data During the seven-month deployment of the Eifel network, several core- refracted teleseismic shear phases (SKS) were recorded. However, only two of these events are analysed because they were of high quality, with energy signal-to-noise ratio (SNR) greater than 20 (Table A1 of Appen- dix). We used the method of Silver and Chan (1991) as modified by Walker et al. (2004a) to measure the apparent fast polarisation azimuth (φ) and delay time between the fast and slow waves (dt) and associated errors (Table A2 of Appendix). We also analysed up to 15 years of recorded data on permanent seismic stations throughout western and central Europe. Figure 1 shows an example of splitting at station WLF (see location in Figure 2). These permanent stations afforded us the possibility of testing between different models of mantle anisotropy: single-layer with a horizontal fast axis, single-layer with a dipping fast axis, and two-layer with horizontal fast axes. In general, we found that the best-fit models for both the dipping-layer and two-layer case fit the data at each station marginally better than the single-layer models, but the minor improvement in fit does not justify the inclusion of extra model parameters (see Walker 2004 for details of the statistics). In addition, these best-fit models for stations that are relatively close together are very different, and the misfit functions used to find them have many local minima. Because the single-layer model only has one local minimum, because these models are very similar for stations that are relatively close together, and because there are only two seismic events for which significant splitting was measured across the Eifel network, we cautiously prefer the single-layer model to estimate the Seismic Anisotropy in the Asthenosphere Beneath the Eifel Region 443 first-order anisotropy signal beneath the study area (Table A3 of Appen- dix). Fig. 2 shows the single-layer splitting φ in the Eifel region of west- central Europe (white bars). Numerous stations did not demonstrate any splitting (white crosses), which suggests that no anisotropy exists beneath them or that the fast axis of anisotropy (assuming single layer) is parallel or perpendicular to the backazimuth. The backazimuths of the two Eifel SKS events (~70° and ~250°) are ~180° apart. Because some of these “null stations” occur between nearby constrained stations that have φ subparallel to the backazimuths, we interpret the lack of splitting to indicate there is anisotropy beneath those stations with a φ ≈70°. Fig. 2. (next page) Shear-wave splitting measurements for study area and Eifel region. Short lines indicate the station splitting estimates, with orientation parallel to fast polarisation azimuth (φ) and length proportional to delay time (dt).

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