Variations of shear-wave splitting in Greenland: Mantle anisotropy and possible impact of the Iceland plume
Introduction
Shear-wave splitting analysis is an important tool to characterize the strength and geometry of anisotropy beneath seismographs and thus deformation and flow if the anisotropy and its relationships between strain and tectonic processes are known (Silver, 1996, Park and Levin, 2002).
Anisotropy can be related to stress in the Earth's crust and past or present deformation in the mantle and therefore provides useful information about tectonic processes. Although the origin of anisotropy and its localization are enigmatic, the main source of anisotropy observed in vertically propagating shear waves is thought to be confined to the upper mantle. Azimuthal anisotropy is caused by the orientation of upper-mantle minerals (e.g. Nicolas and Christensen, 1987), mainly olivine, which is both highly anisotropic and develops strain-induced lattice-preferred orientation (LPO) (e.g., Hess, 1964, Vinnik et al., 1992, Silver, 1996 and references therein]. The a axis (fast velocity) of olivine aligns nearly parallel to the flow direction for large strains, but deviates from this for relatively small strains (Zhang and Karato, 1995) and aligns nearly parallel to the maximum, finite-strain direction (e.g., Christensen, 1984, Mainprice and Silver, 1993). These relationships between deformation and olivine alignment are complicated in the presence of significant amounts of water (Jung and Karato, 2001).
When a polarized shear-wave enters an anisotropic medium, it splits into two orthogonal quasi shear-waves (a fast and a slow shear wave). These phases travel with different wave speeds causing a time delay between them. Splitting parameters, the fast polarization, ϕ, and the time delay, δt, describe the polarization direction of the fast shear wave and the time difference between the fast and slow wave arrivals, respectively. The ϕ orientation is measured in the horizontal plane as azimuth (clockwise from north) and depends on the orientation of the anisotropic structure. The δt depends on both the path length and the strength of anisotropy in the medium (Plomerová et al., 1998). Core-refracted phases (e.g. SKS and SKKS) isolate receiver-side anisotropy due to P-to-S conversions at the core-mantle boundary (CMB). SKS and SKKS (hereafter SK⁎S) phases are radially (SV) polarized after the phase conversion at the CMB and therefore the energy on the transverse component and the elliptical particle motion are diagnostics of anisotropy or lateral heterogeneity beneath a receiver. These phases arrive nearly vertically with a steep incidence angle at the surface and thus provide good lateral resolution under the receiver. The splitting of teleseismic SK⁎S waves is, therefore, often used to study seismic anisotropy in the mantle beneath seismographs (Savage, 1999).
Splitting parameters can be determined from several methods developed in the last two decades. For instance, cross correlation methods have been used by Ando et al. (1983), Fukao (1984), Vinnik et al. (1984), Tong et al. (1994), Levin et al. (1999) and others. Inversion methods have been developed by Vinnik et al., 1988, Vinnik et al., 1989, Silver and Chan, 1988, Silver and Chan, 1991, Šílený and Plomerová (1996), Plomerová et al. (1996), Wolfe and Silver (1998), Rümpker and Silver (1998) and Chevrot (2000). Vinnik et al. (1984) were the first to use shear-wave splitting observations on the continents from teleseismic core-refracted phases (see also Kind et al., 1985).
Here we use the inversion methods of Silver and Chan (1991) and Wolfe and Silver (1998). Firstly, north and east components of the original seismograms have been rotated to radial and transverse components. The aim is to minimize the energy on the transverse component since there would be no energy on the transverse component if the medium was isotropic or transversely isotropic with a vertical symmetry axis (i.e. a special case of anisotropy) beneath a seismograph. The method searches over a grid of possible splitting parameters in order to find the best parameters that minimize the energy on the transverse component. The advantage of the methods of Silver and Chan (1991) and Wolfe and Silver (1998), which is based on the Silver and Chan (1991) method, is that they give information on the accuracy of the splitting parameters determined for each single measurement by using F-test statistics. Sandvol and Hearn (1994) introduced a bootstrap technique for estimating uncertainty in shear-wave splitting measurements instead of using the F-test statistics to estimate the 95% confidence region.
Our objective in this study is to constrain seismic anisotropy in the upper mantle beneath Greenland, investigate possible variations of splitting parameters in the region, examine implications of seismic anisotropy for flow and deformation processes and observe if there is shear-wave splitting evidence related to the impact of the Iceland plume.
Section snippets
Evolution and geology of Greenland
The Greenland continental block is mostly Precambrian in age (Fig. 1). The Archean core is bordered to the north by the Nagssugtoqidian and Ammassalik mobile belts on the west and east coasts, respectively, and to the south by the Ketilidian orogenic block, all of Proterozoic age. The East Greenland Tertiary Basalt Province lies to the north of the Ammassalik mobile belt, and was associated with the opening of the North Atlantic and the arrival or passage of the Iceland plume. North of the
Data and methods
We have measured shear-wave splitting at 19 temporary and permanent broadband seismographs in Greenland. Results of 7 of those, along the east coast, were published recently by Ucisik et al. (2005). Most of the remaining 12 seismographs were deployed temporarily by GEUS (Geological Survey of Denmark and Greenland) and GEOFON (Geoforschungszentrum Potsdam) as part of the GLATIS (Greenland Lithosphere Analyzed Teleseismically on the Ice Sheet) project. Two of those (TULE and UPN) are operated by
Results
Examples of data and single-record analyses are illustrated in Fig. 3, Fig. 4. Fig. 3 shows an example of an SKS phase arriving at station KAG in southernmost Greenland within the Archaean craton. The phase possesses a clearly elliptical particle motion. The residual energy on the transverse component has a clear minimum and the particle motion is clearly linear after minimizing the energy on the transverse component. Fig. 4 shows a similar example of an SKKS phase at station IS2 in central
Discussion
Time delays range from 0.4 to 1.4 s (Table 3). This is difficult to explain by crustal anisotropy alone except possibly where the δt is smallest (NUK, NRS, HJO and DAG). Lithospheric thickness in Greenland is about 100 km in the south east and thicker to the west (Darbyshire et al., 2004, Kumar et al., 2005). Darbyshire et al. (2004) find about 180 km thick lithosphere beneath central southern Greenland where Archaean craton lies. Time delays of up to 1.4 s indicate anisotropy of up to 6% in
Conclusions
- 1.
Time delays (0.4–1.4 s) cannot be explained by crustal anisotropy alone and correspond to ∼ 2–6% anisotropy if distributed throughout a 100 km thick lithosphere which is similar to what has been found in mantle xenoliths (e.g., Mainprice and Silver, 1993).
- 2.
The overall pattern of anisotropy mostly from SKS and SKKS phases is similar to that obtained with Rayleigh waves at 75 and 100 km depth (Pilidou et al., 2005), with notable deviations across central Greenland. Complications of this pattern
Acknowledgments
The GFZ instrument pool (GIPP) provided mobile stations for GLATIS and data were retrieved from the GIPP and data archive facilities at GFZ-Potsdam, SEIS-UK, GEUS and IRIS. We thank P. Voss and S. Pilidou for assistance of retrieving GEUS and SEIS-UK data. We are grateful to A. Higgins to check over the Greenland Geology section of the manuscript and his improvements. We thank R. Kind for his constructive comments. We also thank the editor J. Plomerová and three anonymous reviewers for their
References (60)
- et al.
A quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic SKS measurements
Phys. Earth Planet. Inter.
(1993) - et al.
Depth to Moho in Greenland: receiver-function analysis suggests two Proterozoic blocks in Greenland
Earth Planet. Sci. Lett.
(2003) - et al.
Alpha ridge and Iceland: products of the same plume?
J. Geodyn.
(1986) - et al.
SAC2000: signal processing and analysis tools for seismologists and engineers
- et al.
An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy
Tectonophysics
(1998) - et al.
The lithosphere-asthenosphere boundary in the North-West Atlantic region
Earth Planet. Sci. Lett.
(2005) - et al.
Interpretation of SKS waves using samples from the subcontinental lithosphere
Phys. Earth Planet. Inter.
(1993) - et al.
Contrasting rifted margin styles south of Greenland: implications for mantle plume dynamics
Earth Planet. Sci. Lett.
(2002) - et al.
Rayleigh wave tomography in the North Atlantic: high resolution images of the Iceland, Azores and Eifel mantle plumes
Lithos
(2005) - et al.
Joint interpretation of upper mantle anisotropy based on telesismic P travel-time delays and 3-D inversion of shear-wave splitting parameters
Phys. Earth Planet. Inter.
(1996)
Mantle deformation and tectonics: constraints from seismic anisotropy in the western United States
Phys. Earth Planet. Inter.
Inversion of shear-wave splitting parameters to retrieve three-dimensional orientation of anisotropy in continental lithosphere
Phys. Earth Planet. Inter.
Margin segmentation of Baffin Bay/Davis Strait, eastern Canada based on seismic reflection and potential field data
Mar. Pet. Geol.
Microstructure, texture and seismic anisotropy of the lithospheric mantle above a mantle plume: Insights from the Labait volcano xenoliths (Tanzania)
Earth Planet. Sci. Lett.
Asthenospheric channeling of the Icelandic upwelling: evidence from seismic anisotropy
Earth Planet. Sci. Lett.
Small-scale sublithospheric continental mantle deformation: constraints from SKS splitting observations
Geophys. J. Int.
Shear wave polarization anisotropy in the upper mantle beneath Honshu, Japan
J. Geophys. Res.
Spatial and temporal constraints on sources of seismic anisotropy: evidence from the Scottish highlands
Geophys. Res. Lett.
Shear wave splitting across the Iceland hot spot: results from the ICEMELT experiment
J. Geophys. Res.
Backazimuthal variations of splitting parameters of teleseismic SKS-phases observed at the broadband stations in Germany
Pure Appl. Geophys.
Multichannel analysis of shear wave splitting
J. Geophys. Res.
The magnitude, symmetry and origin of upper mantle anisotropy based on fabric analyses of ultrafamic tectonites
Geophys. J. R. Astron. Soc.
A first detailed look at the Greenland lithosphere and upper mantle, using Rayleigh wave tomography
Geophys. J. Int.
North Atlantic volcanic margins: dimensions and production rates
J. Geophys. Res.
Small-scale variations in seismic anisotropy near Kimberley, South Africa
Geophys. J. Int.
Evidence from core-reflected shear waves for anisotropy in the Earth's mantle
Nature
Seismic study of the transform-rifted margin in Davis Strait between Baffin Island (Canada) and Greenland: what happens when a plume meets a transform
J. Geophys. Res.
Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model
Geophys. Res. Lett.
On the effect of diffraction on traveltime measurements
Geophys. J. Int.
Shear-wave splitting variation over short spatial scales on continents
Geophys. J. Int.
Cited by (9)
Davis Strait Paleocene picrites: Products of a plume or plates?
2020, Earth-Science ReviewsLove-to-Rayleigh scattering across the eastern North American passive margin
2020, TectonophysicsCitation Excerpt :Observations of splitting beneath Greenland are relatively sparse, and mostly limited to coastal regions, but a few studies have explored upper mantle anisotropy using SKS measurements (Ucisik et al., 2005, 2008). In particular, Ucisik et al. (2008) documented a sharp transition in SKS fast splitting directions between the northern portion of Greenland and its southern portion. It is possible that this transition is contributing to the scattering observations shown in Fig. 9C and D, although our raypaths are effectively propagating nearly parallel to this boundary, rather than across it.
Effects of the Iceland plume on Greenland's lithosphere: New insights from ambient noise tomography
2018, Polar ScienceCitation Excerpt :The velocity reduction in central Greenland observed at intermediate periods (18–20 s) near SUMG station (Fig. 1) collocates with measured null anisotropy from SKS splitting (Ucisik et al., 2008) (Fig. 4b). Based on the proximity between the estimated Iceland plume track ∼60 Ma (Forsyth et al., 1986; Braun et al., 2007) and the detected null splitting observation, Ucisik et al., (2008) proposed that the anisotropy measurement represents frozen preferred crystal orientation from upwelling. The collocated observed slow anomaly (Figs. 3 and 4b) is consistent with the proposed plume track, with reduced velocities reflecting reduced rigidity of the lithosphere due to plume-related increased temperature.
Seismic anisotropy and geodynamics of the lithosphere-asthenosphere system
2008, TectonophysicsP-Wave Tomography for 3-D Radial and Azimuthal Anisotropy Beneath Greenland and Surrounding Regions
2021, Earth and Space ScienceMulti-Layer Seismic Anisotropy Beneath Greenland
2021, Geochemistry, Geophysics, Geosystems