Complex anisotropy beneath the Peruvian flat slab from frequency-dependent, multiple-phase shear wave splitting analysis

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 ? and delay time ?t) 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

Seismic anisotropy in Eastern Tibet from shear wave splitting reveals changes in lithospheric deformation

Knowledge about seismic anisotropy can provide important insight into the deformation of the crust and upper mantle beneath tectonically active regions. Here we focus on the southeastern part of the Tibetan plateau, in Sichuan and Yunnan provinces, SW China. We measured shear wave splitting of core-refracted phases (SKS and SKKS) at a temporary array of 25 IRIS-PASSCAL stations. We calculated splitting parameters using a multi-channel and a single-record cross-correlation method. Multiple layers of anisotropy cannot be ruled out but are not required by the data. A Fresnel zone analysis suggests that the shallow mantle (between 60 and 160 km depth) is the most likely source of anisotropy. The polarization directions reveal a pronounced transition from primarily north–south in the north (Sichuan) to mostly east–west orientations in the south (Yunnan). In the southern part of the study region, that is, south of ~26°N, the fast polarization directions do not correlate well with known surface features and geodetic estimates of the crustal displacement fields. Whereas GPS campaigns provide evidence suggesting north–south crustal flow across the Red River Fault, the pattern of anisotropy argues against such flow in the upper mantle. These observations support models that allow differential movement of upper crust relative to lithospheric mantle. In the northern part of the study region the relationships are more ambiguous and coherent deformation of the crust and mantle lithosphere cannot be excluded. The interpretation of the shear wave splitting results is non-unique, but we suggest that the observed N–S transition reflects a fundamental change in deformation regime across our study region. It may be related to lateral variations in lithospheric rheology, or may mark a transition from the direct impact of the continental collision to dominance of the far-field strain field associated with regional subduction processes. Understanding the nature of the lateral change in deformation regime may prove critical for our understanding the geotectonic evolution of (eastern) Tibet

Wavefield Migration Imaging of Moho Geometry and Upper Mantle Structure Beneath Southern New England

The crust and upper mantle beneath the New England Appalachians exhibit a large offset of the Moho across the boundary between Laurentia and accreted terranes and several dipping discontinuities, which reflect Paleozoic or younger tectonic movements. We apply scattered wavefield migration to the SEISConn array deployed across northern Connecticut and obtain insights not previously available from receiver function studies. We resolve a doubled Moho at a previously imaged Moho offset, which may reflect westward thrusting of rifted Grenville crust. The migration image suggests laterally variable velocity contrasts across the Moho, perhaps reflecting mafic underplating during continental rifting. A west-dipping feature in the lithospheric mantle is further constrained to have a slab-like geometry, representing a relict slab subducted during an Appalachian orogenic event. Localized low seismic velocities in the upper mantle beneath the eastern portion of the array may indicate that the Northern Appalachian Anomaly extends relatively far to the south.publishedVersio

ScS shear-wave splitting in the lowermost mantle: Practical challenges and new global measurements

Many regions of the Earth's mantle are seismically anisotropic, including portions of the lowermost mantle, which may indicate deformation due to convective flow. The splitting of ScS phases, which reflect once off the core-mantle boundary (CMB), is commonly measured to identify lowermost mantle anisotropy, although some challenges exist. Here, we use global wavefield simulations to evaluate commonly used approaches to inferring a lowermost mantle contribution to ScS splitting. We show that due to effects of the CMB reflection, only the epicentral distance range between 60° and 70° is appropriate for ScS splitting measurements. For this distance range, splitting is diagnostic of deep mantle anisotropy if no upper mantle anisotropy is present; however, if ScS is also split due to upper mantle anisotropy, the reliable diagnosis of deep mantle anisotropy is challenging. Moreover, even in the case of a homogeneously anisotropic deep mantle region sampled from a single azimuth by multiple ScS waves with different source polarizations (in absence of upper mantle anisotropy), different apparent fast directions are produced. We suggest that ScS splitting should only be measured at "null" stations and conduct such an analysis worldwide. Our results indicate that seismic anisotropy is globally widespread in the deep mantle

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

High-Resolution Ps Receiver Function Imaging of the Crust and Mantle Lithosphere Beneath Southern New England and Tectonic Implications

Southern New England exhibits diverse geologic features resulting from past tectonic events. These include Proterozoic and early Paleozoic Laurentian units in the west, several Gondwana-derived terranes that accreted during the Paleozoic in the east, and the Mesozoic Hartford Basin in the central part of the region. The Seismic Experiment for Imaging Structure beneath Connecticut (SEISConn) project involved the deployment of a dense array of 15 broadband seismometers across northern Connecticut to investigate the architecture of lithospheric structures beneath this region and interpret how they were created and modified by past tectonic events in the context of surface geology. We carried out P-to-S receiver function analysis on SEISConn data, including both single-station analysis and common conversion point (CCP) stacking. Our images show that the westernmost part of Connecticut has a much deeper Moho than central and eastern Connecticut. The lateral transition is a well-defined, ∼15 km step-like offset of the Moho over a ∼20 km horizontal distance. The Moho step appears near the surface boundary between the Laurentian margin and the Gondwana-derived Moretown terrane. Possible models for its formation include Ordovician underthrusting of Laurentia and/or modification by younger tectonic events. Other prominent features include a strong positive velocity gradient (PVG) beneath the Hartford basin corresponding to the bottom of the sedimentary units, several west-dipping PVGs in the crust and mantle lithosphere that may correspond to relict slabs or shear zones from past subduction episodes, and a negative velocity gradient (NVG) that may correspond to the base of the lithosphere.publishedVersio

Response of the mantle to flat slab evolution: Insights from local 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

Seismicity and state of stress in the central and southern Peruvian flat slab

We have determined the Wadati–Benioff Zone seismicity and state of stress of the subducting Nazca slab beneath central and southern Peru using data from three recently deployed local seismic networks. Our relocated hypocenters are consistent with a flat slab geometry that is shallowest near the Nazca Ridge, and changes from steep to normal without tearing to the south. These locations also indicate numerous abrupt along-strike changes in seismicity, most notably an absence of seismicity along the projected location of subducting Nazca Ridge. This stands in stark contrast to the very high seismicity observed along the Juan Fernandez ridge beneath central Chile where, a similar flat slab geometry is observed. We interpret this as indicative of an absence of water in the mantle beneath the overthickened crust of the Nazca Ridge. This may provide important new constraints on the conditions required to produce intermediate depth seismicity. Our focal mechanisms and stress tensor inversions indicate dominantly down-dip extension, consistent with slab pull, with minor variations that are likely due to the variable slab geometry and stress from adjacent regions. We observe significantly greater variability in the P-axis orientations and maximum compressive stress directions. The along strike change in the orientation of maximum compressive stress is likely related to slab bending and unbending south of the Nazca Ridge

Overriding plate, mantle wedge, slab, and subslab contributions to seismic anisotropy beneath the northern Central Andean Plateau

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

Constraining lowermost mantle anisotropy with body waves:A synthetic modeling study

International audienc