Seismic Anisotropy and Mantle Flow in the Izu-Bonin-Mariana Subduction System

Transcription

1 Seismic Anisotropy and Mantle Flow in the Izu-Bonin-Mariana Subduction System Matthew J. Fouch (Department of Geological Sciences, Arizona State University, Tempe, AZ 85287, INTRODUCTION Imaging mantle flow patterns is a fundamental component in understanding the dynamics of subduction systems, including slab-mantle coupling, interaction between overriding plate and mantle, and flow in the mantle wedge. One tool for examining these elements is the detailed evaluation of seismic anisotropy patterns, which can provide key constraints on the geometry of fabric and flow. In particular, the distribution of seismicity in subduction zones provides an ideal opportunity to evaluate both lateral and depth variations in anisotropy, which can in turn help constrain the history and distribution of strain and deformation in the crust and mantle as well as evaluate slab-mantle coupling. The Izu-Bonin-Mariana (IBM) subduction system is an ideal target in which to study seismic anisotropy variations, as it is a significantly seismogenic region with a broad range of earthquake hypocentral depths (Figure 1). Additionally, the geometry of the IBM system is somewhat simpler than many other subduction zones, providing an important laboratory in which to test fundamental hypotheses of mantle dynamics in convergent margins. SEISMIC ANISOTROPY AND SHEAR WAVE SPLITTING Seismic anisotropy measured via shear wave splitting has been examined in subduction zones worldwide. Shear wave splitting in phases such as S and SKS are generally determined using simple analysis techniques such as those described by [Silver and Chan, 1991] or other similar methods. Splitting measurements provide constraints on both the azimuth of propagation of the faster seismic wave (the fast direction) and the delay between the fast and slow components of the wave (the splitting time). Splitting patterns can be widely varied from region to region; in many cases, fast directions are generally Figure 1: Map view of bathymetry and seismicity in the Izu-Bonin-Mariana subduction system using the earthquake catalog of [Engdahl et al., 1998]. Circles denote epicentral locations; shading represents source depth of events. From [Stern et al., 2002]. Page 1

2 parallel to the convergence direction (e.g., [Fischer and Wiens, 1996; Fouch and Fischer, 1996; Fouch and Fischer, 1998]), while in other areas fast directions are parallel to the arc (e.g., [Russo and Silver, 1994; Yang et al., 1995]). The complexity of splitting patterns has also grown with the onset of higher resolution studies (e.g., [Peyton et al., 2001; Smith et al., 2001; Bernot and Fouch, 2002]). The interpretation of shear wave splitting results is currently under debate. For several years, the assumption has been that olivine, the principal component of the upper mantle, is responsible for the splitting via lattice-preferred orientation (LPO). The magnitude of the splitting time corresponds to a combination of both the alignment strength and the thickness of the anisotropic layer. The standard connection between anisotropy and mineralogy is that the fast direction (a-axis of olivine) aligns with either the flow direction or the direction of maximum extension (e.g., [Ribe, 1989; Zhang and Karato, 1995]) in areas where dislocation creep is likely the dominant deformation mechanism (the upper km of the mantle [Karato and Wu, 1993]). However, [Jung and Karato, 2001] recently examined water-rich peridotite samples, which may be more appropriate for mantle wedge mineralogy in subduction systems, and found that at modest stress levels LPO development is fundamentally different than for dry samples. In these samples, the c-axis of olivine aligns with the shear direction, indicating that our simple view of fast splitting directions mimicking flow or extension may not hold in all cases. As these results are currently under debate, in the following discussion of current splitting results for the IBM region I restrict discussion to the standard link between anisotropy and strain discussed above. SEISMIC ANISOTROPY IN THE IBM SYSTEM Past work on seismic anisotropy in the Izu-Bonin-Mariana (IBM) system has provided important initial constraints on the distribution of strain in the mantle wedge and subducting slab in the region [Xie, 1992; Fouch and Fischer, 1996; Fouch and Fischer, 1998]. In Izu-Bonin, [Fouch and Fischer, 1996] evaluated shear wave splitting from several local events and found fast directions of NW-SE to WNW-ESE (i.e., roughly convergence-parallel) and splitting times of ~0.5 s. Most of the ray paths sample mantle wedge structure. Modeling of the splitting data indicated that anisotropy may exist to depths no greater than ~410 km. The combination of observations and modeling implies a relatively small strength of anisotropy, and is consistent with a model in which subducting slab-entrained flow in the back-arc wedge is the primary cause of the anisotropy beneath Izu-Bonin (Figure 2). Figure 2: Shear wave splitting results for Izu-Bonin and central Japan [Fouch and Fischer, 1996]. Fast directions are denoted by the azimuth of the vectors and are roughly convergence parallel in the Izu-Bonin region; vector length is scaled to splitting time. Measurements are plotted at the map view midpoint between the source and receiver. Page 2

3 Figure 3: Shear wave splitting results for the Mariana region [Fouch and Fischer, 1998]. Fast directions are denoted by the azimuth of the vectors; vector length is scaled to splitting time. Measurements are plotted at the map view midpoint between the source and receiver. In the Mariana subduction zone, studies by [Xie, 1992] and [Fouch and Fischer, 1998] have provided initial estimates of anisotropy in the region. Using short-period waveforms and a qualitative analysis technique, [Xie, 1992] found ~E-W fast directions and splitting times of 0.35 s beneath Guam. [Fouch and Fischer, 1998] utilized broadband waveforms and a more accurate method of waveform analysis, and found ~NW-SE fast directions and splitting times ranging from 0.1 s to 0.4 s (Figure 3). The steep dip of the Mariana slab creates difficulties in uniquely identifying differences in anisotropy within slab and/or mantle wedge, but the use of broadband data by [Fouch and Fischer, 1998] facilitated the analysis of frequency dependence in fast directions. Modeling of these results suggested that a combination of fossil anisotropy in the subducting slab, slab-induced flow in the back-arc wedge, and anisotropy possibly due to back-arc extension in the overriding plate contribute to the shear wave splitting observations. OUTSTANDING ISSUES/FUTURE WORK The limited seismic data currently available for the IBM subduction system have provided a tantalizing initial estimate of mantle flow in the region. Recent results from the mineral physics community will likely force us to reevaluate our interpretations of existing seismic anisotropy results. Several outstanding issues therefore arise before we may place precise estimates on mantle dynamics in the region: Mineral physics: What is the role of water in deformation of upper mantle minerals? What is the role of transition zone phases, such as β-spinel and γ-spinel, in seismic anisotropy? [Sharp et al., 1994] determined that β-phase is inherently anisotropic, but little is known about the relationship between strain and orientation of these mineralogies. This work is ongoing, and will be critical to our interpretation of seismic results. Page 3

4 Dynamics: What are the differences in mantle flow between the Izu-Bonin and Mariana regions? What is the detailed picture of strain partitioning in the mantle beneath IBM? Are the slab and wedge strongly or weakly coupled? What is the regional mantle flow? Is it related to slab-driven flow processes or other regional effects, such as slab rollback? What effect does melt migration have on seismic anisotropy in subduction systems? How deep does anisotropy exist in subduction systems? Future seismic deployments, such as the MARGINS experiment currently planned for the Mariana region by Taylor, Klemperer, Wiens, Hildebrand, Suyehiro, and others, will help answer many of these fundamental questions. Additional broad-scale deployments in the Izu-Bonin and northern Mariana regions using a combination of ocean-bottom and land-based seismometers will also provide crucial data to elucidate the full mantle dynamics of IBM. Page 4

5 REFERENCES CITED Bernot, C.M., and M.J. Fouch, Seismic anisotropy and mantle flow beneath Japan, in prep., Fischer, K.M., and D.A. Wiens, The depth distribution of mantle anisotropy beneath the Tonga subduction zone, Earth Planet. Sci. Lett., 142, , Fouch, M.J., and K.M. Fischer, Mantle anisotropy beneath northwest Pacific subduction zones, J. Geophys. Res., 101, 15,987-16,002, Fouch, M.J., and K.M. Fischer, Shear wave anisotropy in the Mariana subduction zone, Geophys. Res. Lett., 25, , Jung, H.Y., and S. Karato, Water-induced fabric transitions in olivine, Science, 293, , Karato, S.-I., and P. Wu, Rheology of the upper mantle: A synthesis, Science, 260, , Peyton, V., V. Levin, J. Park, M. Brandon, J. Lees, E. Gordeev, and A. Ozerov, Mantle flow at a slab edge: Seismic anisotropy in the Kamchatka region, Geophys. Res. Lett., 28, , Ribe, N.M., Seismic anisotropy and mantle flow, J. Geophys. Res., 94, , Russo, R.M., and P.G. Silver, Trench-parallel flow beneath the Nazca plate from seismic anisotropy, Science, 263, , Sharp, T.G., G.Y.A. Bussod, and T. Katsura, Microstructures in β-mg 1.8 Fe 0.2 SiO 4 experimentally deformed at transition-zone conditions, Phys. Earth Planet. Inter., 86, 69-83, Silver, P.G., and W.W. Chan, Shear wave splitting and subcontinental mantle deformation, J. Geophys. Res., 96, 16,429-16,454, Smith, G.P., D.A. Wiens, K.M. Fischer, L.M. Dorman, S.C. Webb, and J.A. Hildebrand, A complex pattern of mantle flow in the Lau backarc, Science, 292, , Stern, R.J., M.J. Fouch, and S.L. Klemperer, An overview of the Izu-Bonin-Mariana subduction factory, in press, The Subduction Factory, AGU Monograph, Xie, J., Shear-wave splitting near Guam, Phys. Earth Planet. Int., 72, , Yang, X., K.M. Fischer, and G.A. Abers, Seismic anisotropy beneath the Shumagin Islands segment of the Aleutian-Alaska subduction zone, J. Geophys. Res., 100, 18,165-18,177, Zhang, S., and S. Karato, Lattice preferred orientation of olivine aggregates deformed in simple shear, Nature, 375, , Page 5