Investigation of the Upper Mantle Anisotropy Beneath the Anatolian Plate and Surroundings by Shear Wave Splitting Analysis

<p>The Anatolia, one of the most actively deforming continental regions of the Earth, is considered to be a natural laboratory for studying tectonic structures, complex deformation patterns, and intense seismicity at various scales. Active tectonics of this plate has been shaped by complex interactions between the Arabian, African and Eurasian plates. In the region, there are several suture zones associated with the closure of Tethys Ocean, large-scale transform faults (e.g. North Anatolian Fault) and geological structures developed in relation to extensional and compressional tectonics. Seismic anisotropy studies are needed to better understand the relationship between surface deformation and mantle dynamics, and to establish a connection between the involved deformation models and anisotropic structures in the lithosphere and asthenosphere layers beneath Anatolia. To evaluate lateral and vertical variations in the upper mantle anisotropy and thus underlying geodynamic processes, we apply teleseismic shear wave splitting (e.g. SKS, PKS, SKKS) analyses using about 500 broad-band seismic stations located throughout Anatolia, which belong to AFAD, KOERI and NOA seismic networks. Splitting intensities (SI) were calculated for the entire data set to compare piercing parameters obtained from both SI and SWS techniques. Overall, the NE-SW fast directions were observed for the entire Anatolia. Local changes in FPDs and DTs should be interpreted with caution as they will give important clues about the correlation between existing tectonic forces and upper mantle deformation. In particular, complex anisotropy signature along the large-scale transform faults (NAF and EAF) was investigated by using multisplit approach (e.g., Eken and Tilmann, 2014) that uses a grid search over four splitting parameters of two-layer anisotropy. A bootstrap-based analysis was performed to statistically evaluate the possible variations in two-layer models. Preliminary results reveal that a two-layer anisotropy exists at the western part of the Anatolia along the NAF. The obtained two-layer anisotropy models imply that signatures of lithospheric deformation and of asthenospheric flow driven shearing remarkably differ in NW Anatolia. In this part of the Anatolian plate, we observed large time delays up to ~2.2 sec, and fast polarization directions: i) mainly consistent with the strike of NAF in the lithosphere, ii) N-S oriented in the asthenosphere that is likely attributed to the mantle flow regime under the influence of slab roll-back and trench retreat along the Hellenic subduction zone.</p>

Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations

SUMMARY Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic P-wave imaging remains largely ignored. Unmodelled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities. Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy. However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as synthetic testing has been restricted to the recovery of simplified block-like structures using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm). Here, we present a modified parametrization for imaging arbitrarily oriented hexagonal anisotropy and test the method by reconstructing geodynamic simulations of subduction. Our inversion approach allows for isotropic starting models and includes approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Synthetic seismic data are created by propagating teleseismic waves through an elastically anisotropic subduction zone model created via petrologic-thermomechanical modelling. Delay times across a synthetic seismic array are measured using conventional cross-correlation techniques. We find that our imaging algorithm is capable of resolving large-scale features in subduction zone anisotropic structure (e.g. toroidal flow pattern and dipping fabrics associated with the descending slab). Allowing for arbitrarily oriented anisotropy also results in a more accurate reconstruction of isotropic slab structure. In comparison, models created assuming isotropy or only azimuthal anisotropy contain significant isotropic and anisotropic imaging artefacts that may lead to spurious interpretations. We conclude that teleseismic P-wave traveltimes are a useful observable for probing the 3-D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies particularly in subduction settings.

Widespread Small-Scale Anisotropic Structure in the Lowermost Mantle beneath the North American Continent and Northeastern Pacific

We constrain D″ anisotropy beneath the North American continent and northeastern Pacific using two approaches: (1) joint splitting analysis of SKS and SKKS phase pair for a common event, in which we obtain 158 pairs exhibiting discrepant splitting results and 791 pairs nondiscrepant splitting results; and (2) group splitting analysis of SKS (or SKKS) phase from neighboring events recorded at a common station, in which we observe 109 2°×2° grids with consistent splitting parameters, and 164 grids with abrupt changes from splitting to no splitting within 30–100 km. The seismic data from both analyses indicate that small-scale variations of D″ anisotropy are widespread beneath the studied regions, with a lateral scale up to tens of kilometers. For portion of the data recorded at the stations of simple upper-mantle anisotropy, we correct for the effects of upper-mantle anisotropy and obtain the splitting parameters of D″ anisotropy. The inferred D″ anisotropy exhibits a changing geographic pattern and lateral transition of anisotropy to a lateral scale of tens of kilometers. Such a length scale of changing anisotropy is also confirmed by synthetics modeling of the seismic data. We suggest that the inferred small-scale anisotropies could be best explained by the shape preferred orientation of widespread small-scale partial melt pockets derived by a composition change produced early in the Earth’s history, a similar compositional origin that was invoked to explain the African anomaly in the lower mantle.

Mantle flow under the Central Alps: Constraints from non-vertical-ray SKS shear-wave splittting

<p>The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation <em>φ</em> and delay time <em>Δt</em>, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations <em>φ</em> show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters <em>φ</em> and <em>Δt</em>, and in the backazimuthal variation of <em>δφ</em> and <em>δΔt</em>, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.</p>