Shear wave anisotropy in upper mantle beneath East Antarctica

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					         Shear wave anisotropy in upper mantle beneath East Antarctica
                          from broadband seismic data;
         - contribution to Antarctic Arrays Deployments / POLENET at
                                 IPY 2007-2008-

                          Yusuke USUI1)*, Masaki KANAO2), Atsuki KUBO3)

                1) Transdisciplinary Research Integration Center, Research Organization of
               Information and Systems, 1-9-10 Kaga, Itabashi-ku, Tokyo, 173-8515, Japan
                                 * Corresponding author: y.usui@nipr.ac.jp
                           2) National Institute of Polar Research, Tokyo, Japan
                                     3) Kochi University, Kochi, Japan

                                                   Abstract
         Shear wave splitting of SKS waves are recorded at 8 stations around Lützow-Holm Bay,
         East Antarctica. We calculate apparent splitting parameters φ (fast polarization direction)
         and δt (delay time between split waves) assuming anisotropic layer of hexagonal &
         horizontal symmetry. As compared with typical delay times of SKS waves having 1.2s, the
         result shows generally the same degree. Two layer modeling reveals anisotropy in the
         Pan-African terrains in the upper layer corresponds well to polarization of NE-SW paleo
         compressional stress in the region. Lower layer anisotropy is supposed to be created by
         recent asthenospheric flow, as compared with the absolute plate motion by HS2-NUVEL1
         model. These kinds of field broadband stations deployed at several outcrops & continental
         ice sheet around the region has significance on coming Antarctic Arrays program, as one
         of the contributions in the marginal part of East Antarctica at IPY.


1. Introduction                                                We investigate upper mantle anisotropy using
     The analysis of seismic anisotropy has                teleseismic SKS wave beneath LHB, and discuss
developed into a powerful tool to know                     the origin of the anisotropy associated with the
deformations in the Earth's interior. It has been          history of the Antarctic plate motion and the effects
known to the existence of seismic anisotropy at            of continental collision and/or break-up.
crust, upper mantle, D" layer and inner core. Upper
mantle anisotropy is mainly produced by lattice
                                                           2. Data set and measurements
                                                                We use broadband seismographs recorded at 8
preferred orientation of highly anisotropic mantle         seismic stations from 1996 to 2004 in LHB (Figure
peridotites during deformation processes [Nicolas          1). We selected the events located within the
and Christensen, 1987]. Seismological and                  epicentral distances of 85° ~ 130° from each
petrophysical studies suggest that most of                 station. For this distance range, SKS phases are
anisotropy is restricted to the olivine stability field,   clearly separated from direct S, Sdiff, and ScS. We
i.e., above 410km depth, in the lithosphere and/or         chose the data of the good signal to noise ratio, and
asthenosphere [Mainprice and Silver, 1993].                simple and impulsive source time function which
     The studies of the upper mantle anisotropy            we could easily found SKS arrivals. In this study,
have been performed using the broadband data               we use the data as follows: (1) magnitudes greater
recorded at major permanent seismic stations in            than 5.6 (2) the number of events is 115 (3) the
Antarctica. However the results in East Antarctica         analyzed waveforms are 205. The events and
are poor. In this study, we focus on around Syowa          stations location maps are shown by Figure 1.
station in the Lützow-Holm Bay (LHB), East                     We calculate the splitting parameter (φ, δt) for
Antarctica. The LHB has been considered the part           teleseismic SKS waves using Silver and Chan
of Pan-African orogen during assembly of                   [1991]. φ is fast direction of split shear wave and δt
Gondwana (500Ma). LHB is one of the most                   is the delay time of two split waves. The splitting
important regions to understand the history of the         parameters are determined by minimizing the
continental collision and break-up.                        energy of the transverse component by net grid
                                                           search technique with intervals of 1° and 0.1s,
respectively (Figure 2). The error estimate of each     consider that the anisotropy of upper layers is
combination of splitting parameters can be given        caused by lithospheric deformation during
by 95% confidence level of F test [Silver and Chan,     Pan-African orogen event (~500Ma).
1988].
    Additionally, we assume more complex models         4. Conclusions
as a two-layer model with four independent                  We observed upper mantle shear wave splitting
parameters [Silver and Savage, 1994]. The               using teleseismic SKS waves for 8 stations in LHB
apparent splitting parameters are fitted by the four    East Antarctica. We propose that the anisotropy is
parameters (φ1, δt1, φ2, δt2), with index 1             caused by lattice preferred orientation of mantle
corresponding to the lower layer and index 2 to the     minerals along the NE-SW paleo compressional
upper layer.                                            stress for upper layers and asthenospheric mantle
                                                        flow for lower layers. The mapping of anisotropic
3. Results and Discussions                              structures allows the continuation of geological
    The weighted average of the apparent splitting      structures into the Earth's upper mantle. The
parameter assuming single anisotropy layer are          anisotropy is explained as originating from the
shown by Figure 3, where the weights are                influence of tectonic events in Pre-Cambrian ages.
inversely proportional to the standard deviations           In past studies, these upper mantle structures
for each solution. Although the station STR is no       have been only performed at near coast in
clear evidence, we observed two null directions         Antarctica. However, in near future, it is thought
associated fast polarization direction. Note that the   that the Antarctic Array projects make it possible,
delay times at all stations are the same degree in      and we can study the structure of mantle including
comparison to the average value of the results in       the inland of Antarctica. This study result is able to
global continental studies, and the fast polarization   contribute to resolve the past tectonism and recent
directions are systematically parallel to near coast    and/or past mantle flows in Antarctic Plate.
line in the LHB. For 6 stations except PAD and
STR, azimuthal variations of the splitting              5. Figures and tables
parameters do exists. In this case, we modeled
two-layer model of azimuthal anisotropy. From a
geodynamic point of view, since two layers may
correspond to anisotropy in the lithosphere and
asthenosphere, such a model is reasonable.
    The results of four parameters are AKR (131°,
0.7s, 43°, 1.7s), LNG (124°, 0.9s, 35°, 1.5s), SKL
(6°, 0.6s, 39°, 0.8s), SKV (49°, 0.1s, 40°, 1.1s),
SYO (145°, 0.4s, 57°, 1.3s), and TOT (133°, 0.9s,
47°, 1.9s). Investigations of seismic anisotropy
may contribute to ideas about influence of recent
or fossil mantle flows and/or the tectonic evolution
of the study regions. Fast polarizations directions
of the lower layer are generally parallel to the
directions of Absolute Plate Motion (APM) based
on the HS2-NUVEL1 [Gripp and Gordon, 1990].
The directions are about N120°E and the velocity
is about 1cm/yr in this study region. The APM
velocities are slow for East Antarctica and the
delay time is small relative to upper layer. We
consider that it is reasonable that the structures of
lower layers anisotropy might have been produced
asthenospheric mantle flow.
    The upper layers don’t coincide with the APM
direction (the difference is about 70° ~ 90°). We
should consider the anisotropic structure which is
past tectonic events of East Antarctica. The
direction of Gondwana continent break up was            Fig. 1. (top) The global distributions of source and stations.
NW-SE [Nogi et al., 1995]. This is perpendicular        Gray circles are source and triangles are stations. Gray lines
to the observed direction. In general, the fast         are ray paths. (bottom) Local map with the seismic stations
polarization directions are consistent with NE-SW       (triangles) in this study. Gray lines indicate SKS wave path
paleo-compressional stress [Board et al., 2005]. We     and black line passing through above 410km.
                                                                  Acknowledgements: This research was partially
                                                                  supported by Function and Induction Research
                                                                  Project, Transdisciplinary Research Integration
                                                                  Center, Research Organization of Information and
                                                                  Systems.

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Fig. 3. Rose diagrams and weighted average values of the              Geophys. Res., 96, 16,429-16,454.
apparent splitting parameters on station locations (triangles).
Bold lines are average fast polarization directions and dash
lines are null directions.