Maureen D. Long

The Subduction Zone Flow Field from Seismic Anisotropy: A Global View

Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity |Vt|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |Vt|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity Vc; their relative influence is governed by the relative magnitude of |Vt| and Vc.

Rotation and plate locking at the Southern Cascadia Subduction Zone

Global Positioning System vectors and surface tiltratesareinvertedsimultaneouslyfortherotationofwest- ernOregonandplate lockingonthesouthernCascadia sub- duction thrust fault. Plate locking appears to be largely oshore, consistent with earlier studies, and is sucient to allow occasional great earthquakes inferred from geology. Clockwise rotation ofmostofOregonaboutanearbypoleis likely driven by collapse of the Basin and Range and results in shortening in NW Washington State. The rotation pole liesalongtheOlympic-Wallowalineamentandexplainsthe predominanceofextensionsouthofthepoleandcontraction north of it.

Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains

Received 24 March 2011; revised 25 May 2011; accepted 31 May 2011; published 8 July 2011. [1] We present three‐dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the Northwest U.S. under the High Lava Plains (HLP). We show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast‐axis, style of subduction, and time evolving flow are important. Due to increased horizontal shear, systems with a component of rollback subduction have simple trench‐normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex. In the HLP, splitting observations are consistent with rollback‐driven laboratory results. Citation: Druken, K. A., M. D. Long, and C. Kincaid (2011), Patterns in seismic anisotropy driven by rollback subduction beneath the High Lava Plains, Geophys. Res. Lett., 38, L13310, doi:10.1029/2011GL047541.

A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States—A relic of craton formation

The mid-lithospheric discontinuity (MLD) is a seemingly sharp decrease in seismic velocity at depths internal to the lithosphere and appears to be a pervasive feature beneath continental interiors. Its presence within cratons, which have remained relatively stable since formation, suggests that the MLD may result from processes associated with continent formation. We use P- to S-wave receiver functions to interrogate seismic anisotropy across the MLD within the ca. 1.35– 1.55 Ga Granite-Rhyolite Province of the central United States. Our analysis reveals strong evidence for sharp changes in the orientation of anisotropy across multiple MLDs, with an approximately north to northwest fast orientation of anisotropy in the upper lithosphere. The consistency of this signature over a large region suggests that the observed anisotropy is a relic of North American craton formation. In addition, the presence of several distinct anisotropic layers within the cratonic lithosphere supports models for craton formation via stacked subducted slabs or a series of underthrusting events.

Lowermost mantle anisotropy and deformation along the boundary of the African LLSVP

Shear wave splitting of SK(K)S phases is often used to examine upper mantle anisotropy. In specific cases, however, splitting of these phases may reflect anisotropy in the lowermost mantle. Here we present SKS and SKKS splitting measurements for 233 event-station pairs at 34 seismic stations that sample D″ beneath Africa. Of these, 36 pairs show significantly different splitting between the two phases, which likely reflects a contribution from lowermost mantle anisotropy. The vast majority of discrepant pairs sample the boundary of the African large low shear velocity province (LLSVP), which dominates the lower mantle structure beneath this region. In general, we observe little or no splitting of phases that have passed through the LLSVP itself and significant splitting for phases that have sampled the boundary of the LLSVP. We infer that the D″ region just outside the LLSVP boundary is strongly deformed, while its interior remains undeformed (or weakly deformed).

Sub‐slab anisotropy beneath the Sumatra and circum‐Pacific subduction zones from source‐side shear wave splitting observations

Understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid Earth system. Observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs, providing a tool for studying subduction dynamics. While many observations of seismic anisotropy have been made in subduction systems, our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub-slab region. Recently, the source-side shear wave splitting technique has been refined and applied to several subduction systems worldwide, making accurate and direct measurements of sub-slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub-slab mantle. Here we present source-side shear wave splitting measurements for the Central America, Alaska-Aleutians, Sumatra, Ryukyu, and Izu-Bonin-Japan-Kurile subduction systems. We find that measured fast splitting directions in these regions generally fall into two broad categories, aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate. Trench parallel fast splitting directions dominate beneath the Izu-Bonin, Japan, and southern Kurile slabs and part of the Sumatra system, while fast directions that parallel the motion of the downgoing plate dominate in the Ryukyu, Central America, northern Kurile, western Sumatra, and Alaska-Aleutian regions. We find that plate motion parallel fast splitting directions in the sub-slab mantle are more common than previously thought. We observe a correlation between fast direction and age of the subducting lithosphere; older lithosphere (>95 Ma) is associated with trench parallel splitting while younger lithosphere (<95 Ma) is associated with plate motion parallel fast splitting directions. Finally, we observe source-side splitting for deep earthquakes (transition zone depths) beneath Japan and Sumatra, suggesting the presence of anisotropy at midmantle depths beneath these regions.

Estimating shear-wave splitting parameters from broadband recordings in Japan : A comparison of three methods

The goal of this study was to evaluate the performance of different splitting measurement techniques in the particularly complicated tectonic setting of subduction beneath Japan. We use data from the broadband Japanese F-net array and consider the methods of Silver and Chan (1991), Levin et al. (1999), and Chevrot (2000). We find that the results generally agree well, although discrepancies arise if the anisotropy beneath the station is more complex than the simple single-layer an- isotropic model often assumed in splitting studies. A combination of multichannel and single-record methods may serve as a powerful tool for recognizing complexities and for characterizing upper-mantle anisotropy beneath a station.

SKS splitting beneath Transportable Array stations in eastern North America and the signature of past lithospheric deformation

Seismic anisotropy in the upper mantle beneath continental interiors is generally complicated, with contributions from both the lithosphere and the asthenosphere. Previous studies of SKS splitting beneath the eastern United States have yielded evidence for complex and laterally variable anisotropy, but until the recent arrival of the USArray Transportable Array (TA) the station coverage has been sparse. Here we present SKS splitting measurements at TA stations in eastern North America and compare the measured fast directions with indicators such as absolute plate motion, surface geology, and magnetic lineations. We find few correlations between fast directions and absolute plate motion, except in the northeastern U.S. and southern Canada, where some stations exhibit variations in apparent splitting with backazimuth that would suggest multiple layers of anisotropy. A region of the southeastern U.S. is dominated by null SKS arrivals over a range of backazimuths, consistent with previous work. We document a pattern of fast directions parallel to the Appalachian mountain chain, suggesting a contribution from lithospheric deformation associated with Appalachian orogenesis. Overall, our measurements suggest that upper mantle anisotropy beneath the eastern United States is complex, with likely contributions from both asthenospheric and lithospheric anisotropy in many regions.

Upper mantle anisotropy and transition zone thickness beneath southeastern North America and implications for mantle dynamics

A variety of models for mantle flow beneath southeastern North America have been proposed, including those that invoke westward driven return flow from the sinking Farallon slab, small-scale convective downwelling at the edge of the continental root, or the upward advective transport of volatiles from the deep slab through the upper mantle. We use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern United States to test several of these proposed mantle flow geometries. Near the coast, stations exhibit well-resolved null (no splitting) behavior for SKS phases over a range of back azimuths, consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry. Farther inland we identify splitting with mainly NE–SW fast directions, consistent with asthenospheric shear due to absolute plate motion (APM), lithospheric anisotropy aligned with Appalachian tectonic structure, or a combination of these. Phase-weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations. At most stations we find transition zone thicknesses that are consistent with the global average (∼240 km), with two stations showing evidence for a slightly thickened transition zone (∼250 km). Our results are relevant for testing different models for mantle dynamics beneath the southeastern United States, but due to the sparse station coverage, we are unable to uniquely constrain the pattern of mantle flow beneath the region. Our SKS splitting observations support a model in which mantle flow is primarily vertical (either upwelling or downwelling) beneath the southeastern edge of the North American continent, in contrast to the likely horizontal, APM-driven flow beneath the continental interior. However, our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material. We expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks, including the Transportable Array and Flexible Array components of USArray.

Response of the Mantle to Flat Slab Evolution: Insights from Local S Splitting beneath Peru

The dynamics of flat subduction, particularly the interaction between a flat slab and the overriding plate, are poorly understood. Here we study the (seismically) anisotropic properties and deformational regime of the mantle directly above the Peruvian flat slab. We analyze shear wave splitting from 370 local S events at 49 stations across southern Peru. We find that the mantle above the flat slab appears to be anisotropic, with modest average delay times (~0.28 s) that are consistent with ~4% anisotropy in a ~30 km thick mantle layer. The most likely mechanism is the lattice-preferred orientation of olivine, which suggests that the observed splitting pattern preserves information about the mantle deformation. We observe a pronounced change in anisotropy along strike, with predominately trench-parallel fast directions in the north and more variable orientations in the south, which we attribute to the ongoing migration of the Nazca Ridge through the flat slab system.