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Mantle anisotropy beneath mid-ocean ridges and oceanic transforms is key to our understanding of seafloor spreading and underlying dynamics of divergent plate boundaries. Observations are sparse however, given the remoteness of the oceans and the difficulties of seismic instrumentation. To overcome this we utilise the global distribution of seismicity along transform faults to measure shear wave splitting of over 550 direct S phases recorded at 56 carefully selected seismic stations worldwide. Applying this source-side splitting technique allows for characterisation of the upper mantle seismic anisotropy, and therefore the pattern of mantle flow, directly beneath seismically active transform faults. The majority of the results (60%) return nulls (no splitting), while the non-null measurements display clear azimuthal dependency. This is best simply explained by anisotropy with a near vertical symmetry axis, consistent with mantle upwelling beneath oceanic transforms as suggested by numerical models. It appears therefore that the long-term stability of seafloor spreading may be associated with widespread mantle upwelling beneath the transforms creating warm and weak faults that localise strain to the plate boundary.

Trabajo presentado en el AGU (American Geophysical Union) Fall Meeting: Advancing Earth and Space Science, celebrado en San Francisco (Estados Unidos), del 12 al 16 de diciembre de 2016, Subduction zones show significant variability in shear wave splitting, both above and below the slab, and along-strike. For example, in the Peruvian flat slab region, where the slab has a 3D morphology, shear wave splitting shows complex patterns that indicate that the anisotropic structure varies both laterally and vertically. Above the flat-slab, fast directions measured on shear wave splitting of local S phases are dominantly trench-parallel north of the Nazca Ridge, but become more complicated towards the south as the slab steepens. For SKS phases that sample the whole subduction zone, there is an intriguing and pronounced region of nulls measurements, which could indicate either an apparent lack of mantle anisotropy or seismic anisotropy with a vertically aligned symmetry axis. Seismic anisotropy is often interpreted as the result of lattice preferred orientation (LPO) of olivine due to flow in the mantle. We provide some insight into the relationship between mantle flow and seismic anisotropy using 3D numerical modeling and infinite strain axis (ISA) calculations. Our models include a slab with along-strike variable geometry that changes rapidly with time. The changing geometry of the slab induces a complex mantle flow with both vertical and trench-parallel components. Trench-parallel flow is intense in broad regions in the mantle wedge (~1 cm/yr) and in the sub-slab mantle (>2 cm/yr). We obtain some regions of trench-parallel ISA in the supra-slab mantle, compatible with observations. However, below the slab, the ISA is mostly vertical, with trench-parallel ISA present only in narrow regions. Vertical ISA in broad regions could potentially offer an explanation for the null SKS splitting measurements found below the Peruvian flat-slab region.

The Central Andean Plateau, the second‐highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases (SKS, SKKS, PKS, and sSKS), local S, and source‐side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench‐parallel to trench‐oblique, contradicting predictions from the simplest two‐dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two‐dimensional mantle flow regime. Por pares

[1] Flat or shallow subduction is a relatively widespread global occurrence, but the dynamics remain poorly understood. In particular, the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail. Here we present measurements of seismic anisotropy to investigate mantle flow beneath the Peruvian flat‐slab segment, the largest present‐day region of flat subduction. We conduct a detailed shear wave splitting analysis at a long‐running seismic station (NNA) located near Lima, Peru. We present measurements of apparent splitting parameters (fast direction φ and delay time δt) for SKS, ScS, and local S phases from 80 events. We observe well‐defined frequency dependence and backazimuthal variability, indicating the likely presence of complex anisotropy. Forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench‐normal fast direction underlying a layer with a more trench‐oblique (to trench‐subparallel) fast direction. In order to further constrain the anisotropic geometry, we analyzed the source‐side splitting from events originating within the slab measured at distant stations. Beneath the flat‐slab segment, we found trench‐normal fast splitting directions in the subslab mantle, while within the dipping portion of the slab further to the east, likely trench‐subparallel anisotropy within the slab itself. This subslab pattern contradicts observations from elsewhere in South America for “normal” (i.e., more steeply dipping) slab conditions. It is similar, however, to inferences from other shallowly dipping subduction zones around the world. While there is an apparent link between slab dip and the surrounding mantle flow, at least beneath Peru, the precise nature of the relationship remains to be clarified.

The thermal and compositional structure of oceanic lithosphere, which exerts an important control on plate behavior, is still debated. Our set of 60,000 PP-P and SS-S traveltime differences with oceanic PP and SS bounce points provides a good constraint on both compressional- and shear-wave velocity. By calculating traveltimes for thermal models that are converted to seismic structures with a thermodynamic approach, we test whether lithospheric cooling can explain PP-P and SS-S traveltime variations with plate age. The PP-P and SS-S traveltimes have substantial scatter but, on average, decrease by 0.2 and 0.7 s/Myr½, respectively, when the PP and SS waves reflect off progressively older oceanic crust. Both a half-space and a plate cooling model with a mid-ocean ridge basalt-source mantle potential temperature (1315° ± 50°C) explain the average values of the PP-P and SS-S anomalies and their decrease with plate age. Residual PP-P and SS-S anomalies relative to a cooling model reveal large-scale patterns. Along a few paths (e.g., Tonga–Fiji to western North America), seismic heterogeneity in the deep mantle is responsible for a significant fraction of the PP-P and SS-S traveltime variation. Most anomalies probably correspond to broad temperature variations in the upper mantle, such as a very slow central–northern Pacific (which may require a 100°C excess temperature) and high- and low-velocity anomalies along the ridges that correlate with deep and shallow bathymetry, respectively.