Geophysical Journal International

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1 Geophysical Journal International Geophys. J. Int. (2012) 190, doi: /j X x Seismic anisotropy and heterogeneity in the Alaska subduction zone You Tian and Dapeng Zhao Department of Geophysics, Tohoku University, Sendai , Japan. (YT); (DZ) Accepted 2012 April 17. Received 2012 April 17; in original form 2011 September 12 SUMMARY We determined P- and S-wave tomography and P-wave anisotropic structure of the Alaska subduction zone using P- and S-wave arrival times from 7268 local shallow and intermediate-depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low-velocity anomalies are revealed in the mantle wedge with significant along-arc variations under the active volcanoes. In the mantle wedge, the low-velocity zone extends down to km depth under the backarc. The results indicate that H 2 O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench-parallel in the shallow part of the mantle wedge (<90 km depth) and in the subslab mantle, whereas the FVD is trench-normal within the subducting Pacific slab. The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by 3-D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVD under the slab. The trench-normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge. Key words: Mantle processes; Seismicity and tectonics; Seismic anisotropy; Seismic tomography; Subduction zone processes; Volcanic arc processes. 1 INTRODUCTION Southern-central Alaska is tectonically the most active region in the United States. It is situated at the receiving end of the Pacific Plate as it slides laterally past southeast Alaska and collides directly with the North American Plate across central-southern Alaska and along the length of Aleutian Island Chain. In Alaska, the plate boundary deformation is spread over a wide zone (Fig. 1) and is partitioned into regions defined by the direct subduction of the Pacific Plate, the uplift of the Alaskan range, strike-slip faulting on the Denali fault and the block rotation within Alaskan interior (Freeland & Dietz 1973). From the south the Pacific Plate is subducting at a rate of about 50 mm yr 1 from the Aleutian Trench (Sauber et al. 1997; Fournier & Freymueller 2008). Many active volcanoes exist in the Alaskan region, but the relationship between the subducting slab and arc volcanism varies in different portions. Most of the volcanoes are distributed along the Alaska Peninsula and the chain of arc volcanoes lies above the 100 km isobath of the subducted Pacific slab in the southwest of Mount Spurr (Fig. 1). There exists a volcano gap in central Alaska between Mount Spurr and the Wrangell volcano region where only a 3000-yr-old volcano is located at the Buzzard Creek Maars (Eberhart-Philips et al. 2006). Alaska is made up of many geological terranes, which resulted from the subduction of oceanic plates that carried several continental fragments (e.g. Jones et al. 1987), and the edges of these terranes are often major geological faults such as the Denali Fault and the Border Ranges Fault (Fig. 2b). Among these terranes, the accreting Yakutat terrane, or called microplate, is well studied, which occupies corner bound by right-lateral transform faults on the east and a subduction zone on the west (Fig. 2b; Plafker & Berg 1994; Eberhart-Philips et al. 2006). In southern Alaska, the Yakutat terrane is obliquely colliding with the North American Plate and it is the major process affecting most of the deformation and earthquake distribution in this region (Mazzotti & Hyndman 2002; Soofi & Wu 2008). Geodetic studies estimated that the convergence rate between the Yakutat terrane and the North American Plate is about 45 mm yr 1 (Sauber et al. 1997). The Yakutat terrane began to collide with Alaska since the Miocene and approximately more than half of the Yakutat terrane has subducted under Alaska (Plafker 1987; Plafker et al. 1994), which has significantly affected the corner structure and volcanism in this region. Arc magmatism and volcanism are generally considered to be caused by the dehydration of the subducting oceanic slab and corner flow in the mantle wedge (e.g. Zhao et al. 1995, 2011a). The flux of slab-released water into the overlying mantle wedge lowers the solidus of the peridotite and furthers development of partial melts (e.g. van Stiphout et al. 2009), which may penetrate upwards into the crust and form the arc magmatism. High-resolution 3-D GJI Seismology C 2012 The Authors 629

2 630 Y. Tian and D. Zhao Figure 1. Map showing the surface topography and major tectonic features in Alaska. The Pacific Plate subducts northwestwards beneath south-central Alaska at a rate of mm yr 1. The active faults are shown in solid black lines. The black box shows the present study area. Depth distribution of the upper boundary of the subducting Pacific Plate is shown in dashed contour lines. Edges of the subducted Yakutat microplate are drawn by white dashed line (modified from Eberhart-Philips et al. 2006). Red triangles denote the active arc volcanoes. TF, Transitional Fault; YAK, Yakutat. The blue sawtooth lines denote the Aleutian Trench. tomographic images of subduction zones provide important insights into the conditions for the generation of arc volcanism and subduction dynamics (e.g. Zhao et al. 2011a,b and the references therein). The Alaska subduction zone is an ideal location to study the structural heterogeneity in the crust and upper mantle and subduction dynamics, because intense seismicity exists throughout the crust and in the Wadati Benioff zone to a depth of approximately 200 km, which characterizes the slab shape and can be used in tomographic imaging. Several researchers have applied seismic tomography to study the 3-D seismic velocity structure of the crust and upper mantle under central-southern Alaska using local-earthquake arrival times (Kissling & Lahr 1991; Zhao et al. 1995; Eberhart- Philips et al. 2006; Qi et al. 2007a) and teleseismic data (Searcy 1996; Qi et al. 2007b). Recently van Stiphout et al. (2009) determined spatial distributions of P-wave velocity, V p /V s,andb value to discuss the magma-generation processes in a relatively small area of the Alaska subduction zone. The previous tomographic studies as mentioned earlier determined only the 3-D isotropic velocity images beneath centralsouthern Alaska. Seismic anisotropy, however, is important for understanding the dynamics of subduction zones (e.g. Long & Silver 2008; Wang & Zhao 2008; Huang et al. 2011a,b). Studying seismic anisotropy makes it possible to relate the surface geology, crustal deformation and plate tectonics to the underlying mantle convection processes. Shear wave splitting observations have become a common tool to detecting anisotropy in the Earth. In the mantle the anisotropy may be controlled by lattice-preferred orientation of the fast, α-axis of olivine, aligned parallel to flow through shear accommodated by dislocation creep (e.g. Mainprice et al. 1998; Christensen & Abers 2010), which is generally considered to be related to the mantle flow. Christensen & Abers (2010) studied

3 Anisotropy and heterogeneity in Alaska 631 Figure 2. (a) SKS splitting results in south-central Alaska (after Christensen & Abers 2010). (b) Tectonic setting of Alaska (after Eberhart-Philips et al. 2006). Active faults are TIF, Tintina; DF, Denali; TKF, Talkeetna; BRF, Border Ranges; CF, Contact. Terranes are YT, Yukon-Tanana; KH, Kahiltna; WR, Wrangellia composite; CG, Chugach; PW, Prince William. The other labelling is the same as that in Fig. 1. seismic anisotropy using shear wave splitting observations of SKS waves recorded by the Broadband Experiment Across the Alaska Range under central Alaska, and their splitting results show two patterns, that is, trench-parallel directions and convergence-parallel directions (Fig. 2a). Although the shear wave splitting provides important information on the dynamic process of the subduction zone, it has poor depth resolution and cannot clarify in what depth range the anisotropy exists. Recent P-wave anisotropy tomographic studies revealed complex anisotropy structures in the crust, mantle wedge and subducting slab beneath New Zealand and Japan (e.g. Eberhart-Phillips & Henderson 2004; Wang & Zhao 2008, 2009; Cheng et al. 2011; Huang et al. 2011a), indicating that P-wave anisotropy tomography is a powerful tool to reveal 3-D structure of seismic anisotropy. In this work, we have used a large number of high-quality arrivaltime data recorded by the dense Alaska seismic network to determine 3-D P- and S-wave velocity models and the first P-wave anisotropy tomography beneath central-southern Alaska. Our results provide new insight into the dynamics of mantle flow associated with the subducting Pacific slab and the Yakutat microplate in this region. 2 DATA AND METHOD We used P- and S-wave arrival times of local shallow and intermediate-depth earthquakes recorded by the permanent and temporary seismic stations operated by the Alaska Earthquake Information Center (AEIC) during a period from 1977 January to 2007 December. More than 400 seismic stations are used in this study (Fig. 3). We can see that the majority of stations exist in centralsouthern Alaska, while a few stations are located on the Alaska Peninsula. A uniform distribution of seismic rays is required in tomographic imaging, hence we selected our data set based on the event distribution and data quality. As a result, 7268 local earthquakes were selected (Fig. 4) which generated P- and S-wave arrival times. Most earthquakes occur in the crust and in the Wadati Benioff zone down to a depth of 200 km, though a few events occur below that depth. Each of the events was recorded by more than 10 seismic station and their hypocentre locations are accurate to 3 km in the horizontal direction and 5 km in depth. The accuracy of the arrival-time picks is estimated to be s for P-wave data and s for S-wave data. We used the tomographic method of Zhao et al. (1992, 2011b) to determine the 3-D V p and V s models under the study region. The starting velocity model for the tomographic inversion was derived from Zhao et al. (1995). Lateral depth variations of the upper boundary of the subducting Pacific slab were taken into account in the 3-D ray tracing and the tomographic inversion (Figs 5 and 6). To express the 3-D velocity structure, a 3-D grid net was set up with a lateral grid spacing of approximately 40 km. Meshes of grid nodes were set at 8, 25, 40, 65, 90, 115, 140, 165 and 200 km depths (Fig. 5). Within the subducting Pacific slab we set up another grid net. The vertical cross-section and map views of the 3-D slab model are shown in Figs 5(d) and 6, respectively. Hypocentre locations and velocity perturbations at the grid nodes were taken as unknown parameters. The velocity perturbation at any point in the model was calculated by linearly interpolating the velocity perturbations at the eight grid nodes surrounding that point. An efficient 3-D ray tracing technique (Zhao et al. 1992) was used to compute traveltimes and ray paths accurately and rapidly. The surface topography and station elevations were taken into account in the 3-D ray tracing and the tomographic inversion.

4 632 Y. Tian and D. Zhao Figure 3. Distribution of seismic stations (triangles) used in this study. The observation equations that relate the observed arrival times to the unknown velocity and hypocentre parameters were solved by using the damped least-squares method and the parameter-separation technique (Thurber 1983; Zhao et al. 1992). Smoothing and damping regularizations were adopted to suppress the dramatic shortscale variations of the velocity anomalies (Zhao et al. 2011b). The resolution and covariance matrices for the unknown velocity parameters are also calculated after the tomographic inversion. In this study 3-D P- ands-wave velocity models are determined simultaneously. We also determined the first 3-D P-wave anisotropic tomography under Alaska using the method of Wang & Zhao (2008) that is modified from Zhao et al. (1992). Two anisotropic parameters are added to each grid node to express P-wave azimuthal anisotropy with hexagonal symmetry axis, and the isotropic and anisotropic velocity parameters at every grid nodes are determined simultaneously by inverting the P-wave arrival times. The fast velocity direction (FVD) ψ and the anisotropic amplitude α of P-wave anisotropy are calculated as follows: π 2, A > 0 ( ) 1 2 tan 1 B A + 0, A < 0 ψ(a, B) = π, A = 0, B > 0 4 π 4, A = 0, B < 0, (1) A2 + B α(a, B) = per cent. (2) 1 (A 2 + B 2 ) For details of the anisotropic tomography method, see Wang & Zhao (2008, 2010). 3 ANALYSIS AND RESOLUTION TESTS The starting velocity model for the tomographic inversion was modified from Zhao et al. (1995; Fig. 5a). Several previous studies have shown that it is necessary to introduce the high-velocity (high-v) subducting slab into the model for the tomographic inversion to obtain a more accurate 3-D velocity model of the subduction zone (e.g. Zhao et al. 1992, 1995; Cheng et al. 2011; Huang et al. 2011a). There are at least three reasons for that. The first is that the subducting slab actually exists and the slab upper boundary is a sharp seismic discontinuity since it generates converted and reflected waves observed in the seismograms (see Zhao et al. 1992; Zhao 2012). The second is that ray paths and traveltimes can be calculated more accurately when the slab is considered in the model because the high-v and thick slab can deviate the ray paths significantly. The third is that the later-phase (converted and reflected waves) data, if observed, can also be used in the hypocentre location and velocity inversion (see Zhao et al for the actual use of later-phase data in the tomographic inversion). In the Alaska subduction zone, abundant seismicity in the Wadati Benioff seismic zone to a depth of 200 km can be used to characterize the slab geometry (Stephens et al. 1984). Zhao et al. (1995) constructed the geometry of the upper boundary of the subducting Pacific slab under Alaska from seismicity (Fig. 6), and they showed that the Pacific slab has a thickness of km and a P-wave velocity 3 6 per cent higher than that of the normal mantle. Inthis work we followed Zhaoet al. (1995) to introduce the high-v subducting slab into the model (Figs 5d and 6) (hereafter we call it the slab inversion ). By referring to the results of Zhao et al. (1995), the slab thickness is assigned to be 50 km and the initial slab velocity is 4 and 6 per cent higher than that of normal mantle for V p and V s, respectively. We conducted detailed checkerboard resolution tests (CRT) to examine the resolution scale of the tomographic images with our data set following the approach of Zhao et al. (1992, 2011b). We first assigned positive and negative velocity anomalies of 6 per cent to all the 3-D grid nodes, then calculated synthetic arrival times for the checkerboard model, and then inverted the synthetic data to obtain recovered images of the checkerboard model. The numbers of seismic stations, earthquakes and ray paths in the synthetic data are

5 Anisotropy and heterogeneity in Alaska 633 the same as those in the real data set. Random errors with a normal distribution having a standard deviation of s were added to the synthetic arrival times before the tomographic inversion. Figs 7 and 8 show the CRT results for Vp and Vs tomography in the crust and upper mantle. We also conducted the CRT for the P-wave anisotropic tomography. In addition to velocity anomalies of ±6 per cent assigned alternatively at the grid nodes, we assigned anomalies of ±3 per cent to two anisotropic parameters at each grid node for the anisotropic tomography, which represent the FVD of 22.5 and in azimuth with anisotropic amplitudes of 4.2 per cent (Wang & Zhao 2008, 2010). The anisotropic CRT results are shown in Fig. 9. Fig. S1 shows the CRT results for Vp and Vs tomography and Pwave anisotropy in the subducting Pacific slab. These results show that both the pattern and amplitude of the isotropic and anisotropic velocity anomalies are well recovered under the seismic network in the crust and mantle wedge down to 120 km depth (Figs 7 and 8). However, the resolution is not very good in the subslab mantle, especially for Vs tomography because of the small number of long rays C 2012 The Authors, GJI, 190, C 2012 RAS Geophysical Journal International that pass through and crisscross in the subslab mantle (Figs 7d i, 8d i and 9d f). In the subducting Pacific slab, the input anomalies are generally recovered for the Vp and Vs isotropic tomography in the area where many intermediate-depth events occur (Figs S1a, b, e and f). Both the checkerboard pattern and amplitudes of velocity anomalies are well recovered, indicating that our tomographic model has a resolution of km in the horizontal direction and km in depth. The resolution of P-wave anisotropic tomography is lower than that of the isotropic tomography because of the imperfect ray coverage and the increase in the unknown parameters (Figs S1c and d). 4 R E S U LT S Figs 10 and 11 show the map views of the obtained Vp and Vs tomographic images. Earthquakes within 5 10 km depth range of each layer are also plotted. Figs 12 and 13 show six vertical cross-sections of the Vp and Vs images along the profiles as shown in Fig. 12(g) Figure 4. Hypocentre distribution of 7268 earthquakes used in this study.

6 634 Y. Tian and D. Zhao Figure 5. (a) 1-D P-wave velocity model (solid line) used in this study. Hc, Hm, Hs1 and Hs2 are depths of the Conrad, Moho and the upper and lower boundaries of the subducting Pacific slab under the location shown with a star symbol in (b). (b) Map showing the Aleutian Trench (the sawtooth line) and the slab upper boundary at 65 km depth (the dashed line). (c) Vertical distribution of grid nodes adopted for the non-slab tomographic inversion with a homogeneous starting velocity model. (d) The inhomogeneous starting model adopted for the slab inversion, which includes the subducting Pacific slab (the grey zone), see text for details. together with the earthquakes that occurred within a 10 km width of each profile. The distributions of errors of the velocity anomalies estimated from the covariance matrix are shown in Figs S2 S5. The errors are generally smaller than 0.1 per cent under the volcanic front at shallow depths (<100 km), while they are per cent under the Alaska inland area and in the Pacific Ocean. The errors show the relative spatial variations in the reliability of the tomographic images obtained. We also conducted a tomographic inversion with a homogeneous 1-D starting model without the Pacific slab (hereafter we call it the non-slab inversion ). The configuration of the 3-D grid net set in the modelling space is shown in Fig. 5(c). Figs 16 and S10 S12 show the map views and vertical cross-sections of the V p and V s images obtained from the non-slab inversion. The results show that low-velocity (low-v) zones are distributed in the crust and upper mantle beneath the active volcanoes. Some inclined high-v anomalies are visible, which show the subducting Pacific slab. The slab image, however, is complicated and blurred throughout the entire depth range, in comparison with the images obtained by the slab inversion (Figs 10 15). There also exist some differences between the anisotropic results obtained by the slab inversion (Fig. 15) and the non-slab inversion (Fig. 16). The dominant FVD obtained by the slab inversion, on the whole, is trench normal within the subducting Pacific slab (Figs 14 and 15), but it is not in the non-slab inversion (Fig. 16). Within the mantle wedge, the two inversion results are similar to each other. The root-mean-square (rms) traveltime residuals after the non-slab anisotropic inversion are s for P wave and s for S wave, which are much larger than those for the anisotropic slab inversion (0.670 s for P wave and s for S wave). These results indicate that the obtained 3-D velocity models fit the data better when the Pacific slab is taken into account in the starting model, similar to the previous studies as mentioned earlier.

7 Anisotropy and heterogeneity in Alaska 635 Figure 6. Maps showing the locations of the upper and lower boundaries of the subducting Pacific slab at each depth. The grey dots show the intermediate-depth seismicity in each depth range. Hence we prefer the model obtained with the slab-inversion and take it as our final result. The obtained V p and V s images are similar to each other. The velocity variations in the upper crust (8 km depth, Figs 10a and 11a), on the whole, reflect the features of surface topography and geomorphology in central-southern Alaska. The larger Cenozoic basins show obvious low velocity, such as the Gulf of Alaska Basin, Cook Inlet Basin and Copper River Basin, because they are composed of thick sedimentary materials. In contrast, the mountain ranges that generally consist of plutonic rocks or granite rocks show high- V anomalies, such as Chugach Mountains. A prominent high-v anomaly exists in the western edge of the Cook Inlet Basin, correlating with the extensive Late Crataceous early Tertiary plutonism, which was also imaged by Eberhart-Philips et al. (2006) but their high-v anomaly was imaged at 15 and 24 km depths. Our results show significant low-v p and low-v s anomalies in the crust and middle of the mantle wedge beneath the active arc volcanoes. The low-v p anomalies up to 6 per cent are distributed continuously above the Pacific slab, but the low-v s anomalies are not very significant as compared with the V p anomalies (Figs 12a d and 13a d). The mantle wedge low-v zones are subparallel with the subducting Pacific slab and extend down to km depth under the backarc region (Figs 12a d and 13a d). These features are generally consistent with the previous tomographic results in this region (Kissling & Lahr 1991; Zhao et al. 1995; Eberhart- Philips et al. 2006; Qi et al. 2007a,b). Eberhart-Philips et al. (2006) showed that in the volcanogenic subduction zone, the mantle wedge is imaged as a broad low-v zone above the slab from 30 to 110 km depth, with V p from 6.8 km s 1 at 30 km depth to 7.8 km s 1 at 85 km depth. In the northern part of the subduction zone where volcanic activity is absent, the velocity structure is different from that in the volcanogenic parts. The crust and mantle wedge is also characterized by broad low-v anomalies but the anomalies are not subparallel with the subducting slab but only extending down to

8 636 Y. Tian and D. Zhao about km depth. Previous tomographic results also showed that the mantle wedge tends to exhibit moderately low Vp, but the low-v zone is not broad in this region (Zhao et al. 1995; EberhartPhilips et al. 2006). In the mantle below the slab, some low-vp anomalies exist in a depth range of km and they are also subparallel with the Pacific slab, and the anomalies are approximately 3 per cent lower than that of the normal mantle. S-wave velocity is not resolved in the mantle below the slab because of the poor S ray coverage there. Figs 14 and 15 show P-wave anisotropic tomography at nine depths and along six vertical cross-sections. The errors of the FVD and the anisotropic amplitude are shown in Figs S6 S9. Just as the isotropic tomographic results, the errors of FVD and anisotropic amplitude are smaller under the volcanic front than those under the oceanic areas and northern Alaska where few seismic stations exist (Figs S6 S9). The anisotropic velocity images, on the whole, are similar to that determined by the isotropic tomography (Figs 10 13), but there exist some differences between the two results. The subducting Pacific slab exhibits a high-vp anomaly in the anisotropic tomography beneath the Kenai Peninsula (Fig. 15d), while it shows up as a low-vp anomaly in the isotropic tomography (Fig. 12d). In the northern part of the subduction zone a low-vp anomaly is visible which corresponds to the low-v Yakutat terrane in the anisotropic tomography (Fig. 15f). The final rms traveltime residual for the C 2012 The Authors, GJI, 190, C 2012 RAS Geophysical Journal International Figure 7. Results of a checkerboard resolution test for P-wave tomography at nine depths in the crust and mantle wedge (a i). The depth to each layer is shown at the lower-right corner. The open and solid circles denote high and low velocities, respectively. The velocity perturbation scale (in per cent) is shown at the bottom. The solid sawtooth lines denote the trench. The dashed lines indicate the upper boundary of the subducting Pacific slab.

9 Anisotropy and heterogeneity in Alaska 637 P-wave anisotropic tomography (0.670 s) is smaller than that for the P-wave isotropic inversion (0.716 s). In the upper crust (8 km depth), the P-wave anisotropic structure is very complex (Fig. 14a). The FVD is subparallel to the strike of the Denali Fault, particularly at 25 km depth. In the central Alaska region (near the Denali fault), the anisotropic amplitude is smaller than that near the subducting slab. The amplitude of the anisotropy in the mantle, on the whole, is larger than that in the crust. There are three main features of P-wave anisotropy below the crust. (1) The predominant FVD is NE SW and subparallel to the strike of the subducting slab in the upper part of the mantle wedge (<90 km depth). (2) Within the subducting slab, the FVD is generally subparallel to the plate convergence direction. (3) The C 2012 The Authors, GJI, 190, C 2012 RAS Geophysical Journal International anisotropic structure is complex beneath the subducting slab, while the predominant FVD is subparallel to the strike of the slab, similar to the pattern in the mantle wedge (Figs 14c f and 15). Recent SKS splitting observations show that waves travelling through the thicker mantle wedge show fast directions parallel to the strike of the slab, whereas the waves travelling through the Pacific Plate and the nose of the mantle wedge show fast directions parallel with the direction of plate motion (Christensen & Abers 2010), which is roughly consistent with the present result. S-wave anisotropy studies show the trench-parallel anisotropic direction in the mantle wedge beneath south-central Alaska (Wiemer et al. 1999) and the Shumagin islands segment of the Alaska subduction zone (southeast of our study area; Yang et al. 1995). Figure 8. The same as Fig. 7 but for S-wave tomography.

10 638 Y. Tian and D. Zhao Figure 9. Results of a checkerboard resolution test for P-wave anisotropic tomography at six depths in the crust and upper mantle (a f). The thin bars show the input anisotropic model, while the thick bars denote the inverted results. The orientation and length of the bars represent the FVD and the anisotropic amplitude with the scale shown at the bottom. In the input model the anisotropic amplitude is 4.2 per cent (see text for details). The other labelling is the same as that in Fig. 7. C 2012 The Authors, GJI, 190, C 2012 RAS Geophysical Journal International

11 Anisotropy and heterogeneity in Alaska 639 Figure 10. Results of P-wave velocity perturbations at nine depths under Alaska obtained by the isotropic tomographic inversion. Red and blue colours denote slow and fast velocities, respectively. The velocity perturbation scale (in per cent) is shown at the bottom. Black triangles denote active arc volcanoes. The blue sawtooth lines denote the trench. The dashed lines indicate the upper boundary of the Pacific slab. The grey dots show the earthquakes occurring in the subducting Pacific slab. 5 DISCUSSION 5.1 Slab dehydration and arc magmatism Arc magmatism is generally considered to be caused by the convective circulation (corner flow) in the mantle wedge and dehydration reactions in the subduction slab (e.g. Zhao et al. 2011a,b and references therein). Magma is produced when the dehydration of the subducted sediments at the top of the slab reacts with the overlying mantle wedge peridotite (e.g. Shimoda et al. 1998). The dehydration reaction occurs at the upper boundary of the slab in some pressure and temperature conditions where the water is expelled from the hydrous minerals (e.g. Grove et al. 2006). The mantle wedge corner flow and the slab dehydration process have been investigated in many subduction zones, for example, Japan (Zhao et al. 1992, 2011a,b; Wang & Zhao 2005), South America (Rietbrock 2009), Tonga (Conder & Wiens 2006), Kamchatka (Jiang et al. 2009; Zhao 2012) and Alaska (e.g. Zhao et al. 1995; Eberhart-Philips et al. 2006; Qi et al. 2007a,b). The subducted oceanic crust atop the slab contains much aqueous fluids in its pore spaces and bounded in hydrous minerals, and sea water may also infiltrate into the interior of the oceanic plate

12 640 Y. Tian and D. Zhao Figure 11. ThesameasFig.10butforS-wave tomography. through the large normal faults near the trench at least soon after the occurrence of a large outer-rise earthquake (Zhao 2012). At shallow depths (<100 km), free water is expelled by the compaction of the subducted sediments and the collapse of porosity in the upper subducted oceanic crust. The fluids may be responsible for the low-v anomalies in the mantle wedge under the forearc region. At greater depths (<200 km), aqueous fluids are also produced due to the progressive metamorphic dehydration reactions in which the slab material loses its water content (e.g. Peacock 1990). The water released can be involved with the dry ultramafic rocks from sets of hydrous minerals in a serpentinization process in the forearc mantle (e.g. Rüpke et al. 2004; Abdelwahed & Zhao 2007; Xia et al. 2008; Tong et al. 2011, 2012). The forearc mantle is exceptionally cool ( 400 C) due to the cooling of the subducting slab (e.g. Hyndman & Peacock 2003; Wada et al. 2011; van Keken et al. 2011, and references therein). In contrast, the backarc mantle is too hot (>1000 C; e.g. Wada et al. 2011; van Keken et al. 2011, and references therein), so that the serpentine is not stable there. A number of hydrous minerals are stable at temperatures less than 700 C (e.g. Hyndman & Peacock 2003). The presence of the serpentine and other hydrous minerals has significant effects on the physical and mechanical properties of the forearc mantle, which may decrease the seismic velocities and rock density (e.g. Abdelwahed & Zhao 2007; Tong et al. 2011, 2012; Zhao et al. 2011a). The observed forearc P-wave velocities are commonly 7.8 km s 1 or less and the expected Pn velocity for anhydrous mantle is greater than 8.2 km s 1, so that the velocity reduction is about 5 per cent (e.g. Hyndman & Peacock 2003 and references therein). Density also decreases

13 Anisotropy and heterogeneity in Alaska 641 Figure 12. (a f) Vertical cross-sections of P-wave tomography along six profiles shown in (g). Red triangles denote active volcanoes within a 30 km width along each profile. Red and blue colours denote slow and fast velocities, respectively. The velocity perturbation scale (in per cent) is shown at the bottom. White dots denote the relocated events that occurred within a 10 km width of each profile. The four curved lines in each panel denote the Conrad and Moho discontinuities and the upper and lower boundaries of the Pacific slab. substantially with increasing degree of serpentinization, from about 3200 kg m 3 for unaltered ultramafic rocks to about 2500 kg m 3 for 100 per cent serpentinization (e.g. Hyndman & Peacock 2003 and references therein). Fluids released from the subduction slab are the main contributor to the formation of low-v anomalies in the mantle wedge. Fig. 17 shows vertical cross-sections of V p and V s tomography and schematic patterns of the FVDs along the volcanic front. The low-v p and low-v s anomalies are distributed continuously from the surface down to approximately 100 km depth in the mantle wedge (Figs 17a and b). In the cross-sections passing through the active volcanoes (Figs 15a d and 17), prominent low-v anomalies are clearly imaged above the Pacific slab in most parts of the mantle wedge extending from the surface down to km depth. The expelled water from the subducting slab contributes to melt generation. It lowers the melting temperature and seismic velocity of rocks, which causes melting even in a relatively cool environment (e.g. Zhao 2012). The melted material migrates upwards to feed the arc volcanoes, which is well reflected in the tomographic images. Both seismic tomography and imaging of b-value of earthquakes show the presence of fluids, that is, significantly higher V p /V s ratio rising in columns from the top of the Wadati Benioff zone at 100 km depth and high b-value below the active volcanoes in the Alaska subduction zone (van Stiphout et al. 2009). The region of higher b-value suggests that the process of dehydration is more dominant there. Seismic tomography also revealed that the mantle wedge exhibits low-v p and high V p /V s beneath some active volcanoes in this region (Eberhart-Philips et al. 2006). We consider that the low-v anomalies in the mantle wedge under those active volcanoes represent the process of slab dehydration and corner flow,

14 642 Y. Tian and D. Zhao Figure 13. ThesameasFig.12butforS-wave tomography. just as that revealed in the Japan subduction zone (e.g. Huang et al. 2011a; Zhao et al. 2011a,b). Grove et al. (2006) showed that although the water is released by slab dehydration, the magma production does not occur at the top of the slab, but rather in the hot core of the mantle wedge. Our anisotropic tomography shows that the P-wave anisotropy is less significant in the hot core of the mantle wedge, while it is larger and its FVD is mainly trench-normal at the top of the slab, which may indicate the flux of fresh materials into the melting zone. The volcanic front is located at 100 km above the subducting Pacific slab where most of arc magmatism occurs. This depth of arc-magma generation is the consequence of the interaction between fluids, released from the subducting slab, and the arc mantle wedge that is hot enough to generate hydrous melting. However, the northern part of the Alaska subduction zone where no modern volcanism exists shows a different velocity pattern in the subducted slab and mantle wedge (Figs 10, 11, 12e and f and 13e and f). Recent seismic studies show that the Yakutat micropalte is imaged as a low-v zone with a high V p /V s and its crustal thickness is km (e.g. Ferris et al. 2003; Eberhart-Philips et al. 2006). In this study, our tomography with the limited resolution could not resolve this thin low-v layer, and part of the slab beneath this corner region is also imaged as low velocity as compared with our initial model of the slab (+4 per cent high), particularly in the cross-section passing through the Yakutat microplate. Although the low-v anomalies are significant in the mantle wedge in this region, they extend only down to about 70 km depth (Figs 12e and f and 13e and f). The Wrangell volcanic region lies east of the subducted Yakutat microplate. Significant low-v anomalies are imaged beneath the active Wrangell volcanoes (Figs 10 and 11). With the few deep seismicity and coarse inversion grids, V p and V s structures are not resolved below 65 km depth there. 5.2 Varied anisotropy in the mantle wedge The features of P-wave anisotropy in Alaska revealed by this study are complex. The results show mainly three anisotropic domains, that is, trench-parallel FVDs in the upper part of the mantle wedge (<90 km depth) and subslab mantle, and trench-normal FVDs within the subducting slab. In the lower part of the mantle wedge (>90 km depth), the FVDs are also trench-normal. There are several processes that can cause the subduction-zone anisotropy, including corner flow in the mantle wedge, flow beneath the subducted slab and around the slab edge, the generation and migration of melt and anisotropic structure in the slab itself or in the overriding plate (e.g. Park & Levin 2002; Long & van der Hilst 2006). The complex anisotropy structures in the subduction system may be caused by two or more of these processes. In subduction zones, both trench-normal (e.g. Long & van der Hilst 2006; Wang & Zhao 2008; Huang et al. 2011a,b) and

15 Anisotropy and heterogeneity in Alaska 643 Figure 14. P-wave anisotropic tomography at nine depths under Alaska. The orientation and length of the short bars represent the fast velocity direction and the anisotropic amplitude with the scale shown at the bottom. The other labelling is the same as that in Fig. 10. trench-parallel (e.g. Christensen & Abers 2010) FVDs have been observed in the mantle wedge. The observations of trench-normal FVDs in the mantle wedge are more common and can be explained by the deformation and alignment of the A-type fabric olivine caused by the slab subduction and the induced mantle wedge convection (e.g. Fischer et al. 2000; Hall et al. 2000). In contrast, interpretation of the trench-parallel FVDs requires other mechanisms. Corner flow with B-type olivine fabric is commonly used to interpret the trench-parallel FVDs. Laboratory experiments by Jung & Karato (2001) showed that when olivine aggregates are deformed under high-stress, low-temperature and waterrich conditions, the fast axes of individual olivine crystals tend to align 90 from the flow direction. These B-type olivine fabrics, in conjunction with trench-normal flow in the mantle wedge, may explain the trench-parallel FVD in subduction zones (e.g. Kneller et al. 2005; Long & van der Hilst 2006). The B-type fabric conditions may occur in part of the mantle wedge, for example, the low-temperature wedge nose under the forearc region (Kneller et al. 2005). However, this mechanism cannot explain our results that trench-parallel FVDs exist in the high-temperature backarc region and beneath the active volcanoes. Strain-induced crystallographic preferred orientation (CPO; e.g. Holtzman et al. 2003; Karato et al. 2008) and shape-preferred orientation (SPO; e.g. Kendall 1994; Fischer et al. 2000) of melt

16 644 Y. Tian and D. Zhao Figure 15. (a f) Vertical cross-sections of P-wave anisotropic tomography along six profiles shown in (g), which is obtained by the slab inversion (see text for details). The labelling is the same as that in Fig. 12. Note that the orientations of short bars represent the azimuth of fast velocity direction (FVD), that is, the vertical bars represent N S FVD, while the horizontal bars express E W FVD. pockets could also result in trench-parallel FVD in the mantle wedge. Christensen & Abers (2010) suggested that fabric created by partial melt is less likely in central Alaska, because arc volcanism is virtually absent considering the low V p /V s in the mantle wedge there (e.g. Eberhart-Philips et al. 2006; Rossi et al. 2006). However, their study region is limited around central Alaska where the Yakutat microplate is subducting and recent volcanic activity is absent. In our result, significant low-v p and low-v s anomalies are visible in the crust and mantle wedge beneath the active arc volcanoes in southern Alaska and the predominant trench-parallel FVDs in the mantle wedge extend continuously to central Alaska (Fig. 14). Although the melt hypothesis can explain the anisotropic structure beneath southern Alaska, it also remains to demonstrate that it is unlikely for central Alaska (Christensen & Abers 2010). The 3-D flow hypothesis that has potential to explain the widespread trench-parallel shear wave splitting in the hot arc and backarc mantle of subduction system involves a significant deviation from simple corner flow and the development of trench-parallel stretching (e.g. Kneller & van Keken 2008). There are several 3-D flow mechanisms (Kneller & van Keken 2008), for example, smallscale convection (e.g. Honda et al. 2002; Behn et al. 2007), slabedge effects (e.g. Kincaid & Griffiths 2003) and variable slab geometry (e.g. Hall et al. 2000). Geodynamic modelling studies showed that a subduction system with large variations in slab geometry can give rise to strong trench-parallel stretching which can result in trench-parallel anisotropy (Hall et al. 2000; Kneller & van Keken 2007, 2008). The complex geometry of the subducting slab under south-central Alaska suggests that the trench-parallel flow in the mantle wedge may be caused by such a mechanism. The slab geometry in Alaska (Fig. 1) varies along the Alaska subduction zone, forming a nearly flat subducting slab beneath south-central Alaska and steepening close to the transform boundary. Therefore, the strong

17 Anisotropy and heterogeneity in Alaska 645 Figure 16. The same as Fig. 15 but obtained by the non-slab inversion, that is, the starting velocity model does not contain the subducting Pacific slab. curvature of the slab geometry and its transition to shallow slab dip in central-southern Alaska may lead to strong trench-parallel stretching in the mantle wedge and to abrupt rotations in stretching directions accompanied by the strong trench-parallel stretching, similar to the Andean subduction zone (e.g. Kneller & van Keken 2007). Jadamec & Billen (2010) presented 3-D numerical models of buoyancy-driven deformation with realistic slab geometry for the Alaska subduction-transform system and showed that the complex mantle flow field is consistent with the observations of SKS splitting (Christensen & Abers 2010). The anisotropic patterns in the mantle wedge revealed by this study (Fig. 14) are very similar to those of Jadamec & Billen (2010). In the lower part of the mantle wedge (Fig. 14f h), the FVDs are trench-normal near the slab and trench-parallel in the region far from slab (see fig. 4 of Jadamec & Billen (2010)). From the above analyses, we think that the most likely explanation of the dominant trench-parallel anisotropy in the upper part of the mantel wedge is the trench-parallel flow induced by the complex slab geometry in the Alaska subduction zone, while the trench-normal anisotropy in the lower part of the mantle wedge is affected by the cold subducting slab, because the trench-normal FVDs occur near the Pacific slab (Fig. 15). Although trench-normal anisotropy in the cold nose of the mantle wedge was suggested by the SKS splitting study (Christensen & Abers 2010) and geodynamic modelling (Jadamec & Billen 2010), our results show that trench-normal anisotropy does not exist in the cold nose of mantle wedge in Alaska, and the SKS splitting result there (Christensen & Abers 2010) may reflect, or at least be affected by, the anisotropy in the slab. 5.3 Trench-parallel anisotropy in the subslab mantle SKS splitting studies also revealed trench-parallel anisotropy in the mantle under the subducting oceanic slab in South America (e.g. Russo & Silver 1994; Anderson et al. 2004; Long & Silver 2008) and Kamchatka (e.g. Peyton et al. 2001). Russo & Silver (1994) showed that the trench-parallel anisotropy could be explained by horizontal trench-parallel flow in the mantle beneath the subducting Nazca Plate, which is attributable to retrograde motion of the slab, the decoupling of the slab and underlying mantle, and a partial barrier to flow at depth, resulting in lateral mantle flow beneath the slab along much of the Andean subduction zone. Peyton et al. (2001) and Anderson et al. (2004) used a similar model to explain the trenchparallel anisotropy in Kamchatka and Chile, respectively, whereas Anderson et al. (2004) imposed the local slab geometry on this flow model. Laboratory experiments showed that slab rollback can also produce strong trench-parallel a-axis alignment in the seaward-side mantle beneath the slab (Buttles & Olson 1998). Long & Silver (2008, 2009) suggested that trench migration may cause the 3-D

18 646 Y. Tian and D. Zhao Figure 17. Vertical cross-section of (a) P and (b) S wave tomography along the volcanic front (A A ) as shown on the map (c). The vertical exaggeration is 2.5. The black short bars in (a) show the general result of the obtained P-wave anisotropy along the volcanic front; the vertical and horizontal bars denote the N S and E W fast velocity directions, respectively. The labelling is the same as those in Figs 1 and 12. flow beneath the subducting slab, which may explain the worldwide trench-parallel anisotropic structure in the subslab mantle. The 3-D flow model can also explain the trench-parallel FVDs in the subslab mantle under Alaska, but we emphasize the role of complex geometry and strong curvature of the subduction slab in Alaska. The transpressional tectonic regime in southern Alaska is a common characteristic of regions undergoing oblique subduction of shallow slabs. The complex geometry and oblique subduction of the slab may result in complex mantle flow in the mantle wedge and subslab mantle. In Alaska there are two possibilities to cause the trench parallel or subparallel anisotropy in the subslab mantle. First, the mantle flowing northwestwards (trench normal) entrained by the Pacific Plate encounters resistance due to retrograde motion of the subducting slab and is diverted into a northeastward or northward

19 (trench parallel) oriented flow under the study area. Second, oblique subduction of the slab may cause the trench parallel or subparallel (north south) oriented flow. The first model alone cannot explain our results, because strong trench-parallel anisotropy is revealed in the mantle beneath the flat-slab region. If the slab-geometry hypothesis is the correct mechanism for the trench-parallel anisotropy observed in the subslab mantle, then our results suggest that the strong curvature and flat and oblique subduction of the Pacific slab can play an important role in causing the strong trench-parallel anisotropy in the subslab mantle. The asthenospheric deformation in a subduction zone can be largely guided by the geometry and subduction direction (normal or oblique) of the subducting lithosphere. This hypothesis should be further tested with numerical simulations to clarify the relationship between the subslab mantle dynamics and complex geometry and curvature of the Pacific slab in Alaska. 5.4 Trench-normal anisotropy in the subducting slab The dominant FVD is NW SE (trench normal) within the subducting Pacific slab (Figs 14 and 15). Previous studies showed that the anisotropy in the oceanic crust and sub-moho mantle is induced by fossil fabric formed at the spreading mid-ocean ridge, and so the FVD is parallel to the spreading direction while perpendicular to the magnetic isochrones (e.g. Hess 1964; Kawasaki 1986). Recent surface wave tomography showed a good correlation between the FVDs and palaeoridge spreading directions as recorded by seafloor magnetic anomalies at lithospheric depths of the Pacific Plate, and the FVDs are subparallel to the magnetic isochrons in the Pacific Ocean floor south of the Alaska Trench (Trampert & Woodhouse 2003). From this viewpoint, the NW SE FVD in the Pacific slab as revealed by this study is consistent with the anisotropy frozen in the oceanic crust and sub-moho mantle. Hence we suggest that the subducting Pacific slab under Alaska keeps the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge. In northeast Japan, trench-parallel FVDs were observed within the subducting Pacific slab by P-wave anisotropy tomography (Wang & Zhao 2008, 2010; Huang et al. 2011a) and shear wave splitting analyses (e.g. Huang et al. 2011b,c). In addition to the fossil-anisotropy hypothesis as mentioned earlier, another hypothesis was used by those researchers to explain the trench-parallel anisotropy in the slab under NE Japan. Considering that many trench-parallel normal faults are produced in the outer slope of the Pacific Plate to the east of Japan Trench due to the bending when the plate enters the mantle, Faccenda et al. (2008) suggested that both the trench-parallel CPO and SPO of the faults and cracks may develop in the upper part of the subducting slab where strong serpentinization takes place, and the CPO and SPO in the slab may result in the trench-parallel FVDs. Such a mechanism, however, seems not applicable to the Alaska subduction zone because our present results show a trench-normal anisotropy within the Pacific slab under Alaska which may represent the original fossil anisotropy when the Pacific Plate was produced. 6 CONCLUSIONS We determined 3-D V p and V s tomography and P-wave anisotropic structure of the Alaska subduction zone using P- and73817 S-wave arrival times from 7268 local earthquakes. Main findings of this study are summarized as follows. (1) Prominent low-v anomalies are imaged in the crust and uppermost mantle beneath the active arc volcanoes and in the central portion of the mantle wedge. The low-v zone in the mantle wedge Anisotropy and heterogeneity in Alaska 647 extends down km depth under the backarc region. This result indicates that H 2 O contained in the serpentine and chlorite is brought downwards by the subducting slab and then released to the mantle wedge under the volcanic front and backarc by slab dehydration. (2) P-wave anisotropic tomography shows three anisotropic domains in the Alaska subduction zone: (i) trench-parallel FVD in the mantle wedge, (ii) trench-normal FVD in the subducting Pacific slab and (iii) trench-parallel FVD in the subslab mantle. (3) The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by the trench-parallel flows, which are induced by the complex geometry and strong curvature of the Pacific slab in Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVDs under the slab. (4) The trench-normal FVD in the subducting Pacific slab under Alaska may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge. ACKNOWLEDGMENTS The data used in this study were supplied by the AEIC seismic network in Alaska jointly run by the Geophysical Institute of UAF and the US Geological Survey. We appreciate Drs Zhouchuan Huang, Jian Wang and Ping Tong for the thoughtful discussions. This work was partially supported by the Global-COE Program of Earth and Planetary Sciences of Tohoku University, and a grant (Kiban-S ) from Japan Society for the Promotion of Science to DZ, as well as a grant (No ) from National Natural Science Foundations of China to YT. Prof. J. Trampert (the Editor) and two anonymous referees provided thoughtful review comments that have improved the manuscript. The figures were made by using GMT (Wessel & Smith 1998). REFERENCES Abdelwahed, M. & Zhao, D., Deep structure of the Japan subduction zone, Phys. 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