Journal of Geodynamics最新文献

The tomographic inversion of shear wave splitting data for upper mantle anisotropy has been a longstanding challenge. This is due to the ray-based approximation of classical approaches and the near-vertical incidence of the core-mantle converted phases such as SKS that are often used. Recent developments include the calculation of finite-frequency sensitivity kernels for SKS splitting intensity observations, which allows us to accurately take into account the sensitivity to anisotropic structure with depth. A requirement of this tomographic technique is a dense station spacing, which results in overlapping sensitivity kernels at depth and allows for the localization of anisotropic structure. This is satisfied by a growing number of temporary seismic deployments, which motivates the desire to image anisotropic complexities with depth. Here, we introduce and make available a toolbox for the MATLAB environment that facilitates the application of finite-frequency splitting intensity tomography to dense seismic arrays. Our implementation includes several key features, including: 1) A forward calculation of splitting intensities and sensitivity kernels for a complex anisotropic model space. 2) Consideration of the dominant period of the wave, allowing for multiple-frequency analysis, as well as the incoming wave’s non-vertical incidence. 3) The inversion can be based on a classical gradient descent, on a form of the conjugate gradient method known as the BFGS algorithm, or on a gradient-informed stochastic reversible jump algorithm, allowing for a data-driven parametrization of the model space. 4) Importing splitting intensity measurements from waveforms processed in SplitRacer allows for fast pre-processing of large data sets due to its fully automatic design. To illustrate our method, we present both synthetic tests and an application to real data. We apply our inversion procedure to data from the Swath-D network, which densely covers the transition of the Central to the Eastern Alps. Previous studies showed evidence for an abrupt lateral change of layered seismic anisotropy that had been attributed to an opening for channeled asthenospheric flow. Using an SKS splitting intensity tomography approach, we can confirm previous inferences while providing additional constraints on the distribution of anisotropy laterally and with depth.

Stress fields may exhibit variegated patterns, especially in volcanic areas where several processes superimpose their effects in space and time. The comprehension of such patterns may not be straightforward to investigate. This work investigates the pattern of the crustal stress in the area of Mt. Etna Volcano (Sicily, Italy). This has been possible through a collection of more than 800 stress indicators derived from seismological and volcanological/geological information. In particular, the type of collected data allows to consider, for the first time in this area, two different temporal steps in the evolution of Etna volcano: the present-day and the previous volcanic phase at 15 ka. Results indicate a transition between a background shallow NW-SE tensional regime and a deep SW-NE compressional one that occurs between 6 and 16 km depth and which well fits with the present-day geodynamic framework of the area. The occurrence of small-scale lateral variations is interpreted as the second-order effect of the structures of the active front buried beneath the volcano, to the volcano loading, and to the feeding system. The temporal variations in the area surrounding the volcano suggest a major rearrangement of the background stress field evidenced by the swap between minimum and maximum horizontal stress directions. Conversely, during the same period, the stress pattern in the exact correspondence of the volcanic edifice showed to be stable and with a radial arrangement. Such coherence would support the literature which suggests a long-term inflation process started at least 15 kyr ago.

Carbonaceous materials are widely present in the seismic fault zone. They play a crucial role in lubricating the fault slipping. To date, the formation mechanism of carbonaceous materials is still unclear. In this work, we have conducted a carbon dioxide hydrogenation reaction experiment in a homemade high temperature reactor for the purpose to insight the formation mechanism of carbonaceous materials, with fault gouge used as the catalyst. During the reaction process, carbonaceous materials are formed on the fault gouge, suggesting that the carbonaceous materials in the fault zone are possibly generated from carbon dioxide hydrogenation reaction. These results are important for understanding fault behavior and earthquake physics.

Northeastern Eurasia is one of the least explored regions in the world. Very little geophysical data is available for this inaccessible area. Even the exact location of the plate boundary between Eurasia and North America remains a subject of ongoing debate. The effective elastic thickness (EET) of the lithosphere is a proxy for lithospheric strength and can provide insight into the thermal regime and tectonic processes. We have computed a high-resolution map of the EET for northeastern Eurasia using the fan wavelet coherence technique applied to the Bouguer gravity anomalies and topography/bathymetry data, appropriately adjusted to account for the influence of density variations within sediments. The results obtained provide insights into different tectonic regimes within this predominantly understudied region. In particular, we identify the boundary between the Eurasian and North American plates in Siberia as a rheologically weak diffusive zone extending from the Verkhoyansk and Sette-Daban Ranges to the eastern boundary of the Chersky Range. Unlike the Sette-Daban and Verkhoyansk Ranges, which were formed by plate collision and have an EET of 30–50 km, other mountainous regions have much lower EET values, usually less than 15 km. These areas have recently experienced tectonic activity that has weakened the lithosphere.

Cocos intraslab earthquakes were used to make a 3-D tomographic inversion to define a crystallographic orientation model for the mantle wedge beneath southeastern Mexico. This model provided insights regarding the pattern of the mantle wedge flow and its relationship to the geometry of the subducting slab. The mantle wedge was parametrized as a 3-D block model of crystallographic orientations assuming the elastic constants of olivine and orthopyroxene with orthorhombic symmetry (hexagonal symmetry was also tested). A linearized, damped, and iterative least-squares approach was used to account for the nonlinear behavior of the shear-wave splitting, numerically recalculating partial derivatives after each iteration. The best-fitting model is consistent with two main flow regimes: (1) 2-D corner flow in a mantle wedge core made up of A-type olivine fabric northwest of the Tehuantepec Ridge extension, and (2) 3-D trench-parallel mantle flow in a mantle wedge core made up of A-, C-, or E-type olivine fabric southeast of this geological feature. Around the Tehuantepec Ridge extension, a partially serpentinized mantle wedge tip is inferred since olivine a-axis orientations are trench-parallel regardless of whether a 2-D corner flow or a 3-D trench-parallel flow prevails. Right above the Tehuantepec Ridge extension (beyond the 100 km isodepth contour of the subducting slab), a change of well-resolved olivine a-axis orientations from trench-normal to trench-parallel while going from northwest to southeast is observed. It signals an abrupt change in the mantle flow pattern possibly through a vertical tear in the Cocos slab. 3-D toroidal flow could be driving subslab mantle material into the mantle wedge around the deepest slab segment. Lastly, approximately trench-normal olivine a-axis orientations are observed in the mantle wedge tip near the Mexico and Guatemala border region, and they could be explained by assuming the presence of B-type olivine fabric.

A long-standing question in geodynamics is whether mantle flow is driven by the plate motion alone, or mantle upwelling makes a significant contribution to it. Subducting slabs and lateral variations of the continental lithosphere can further influence the asthenospheric flow and control its direction. The Middle East region (MER) is a complex continental setting where different processes such as rifting, break-up, plate collision, and tectonic escape kinematically interact with each other. In this context, the role that lithospheric structure, mantle flow, and active upwellings may play is debated. Tomographic images provide a snapshot of the current thermal conditions of a region and seismic anisotropy can also help resolve mantle convection. Here, we synthesize shear-wave splitting observations together with up-to-date tomography models of the mantle structure beneath the MER and other geophysical data. Low-velocity anomalies are seen at asthenospheric depths beneath W Arabia, NW Iran, and Anatolia, suggesting a spreading zone of warm mantle. Two deep low-velocity bodies in Afar and Levant –interpreted as hot mantle plumes– are the sources of this shallower mantle flow. Where low velocities are imaged, we observe predominantly NE–SW oriented anisotropy, anomalously high topography, and abundant basaltic volcanism. The integrated analysis suggests that a horizontal component associated with active upwelling is present in the upper-mantle flow field. The large-scale circulation flow fed by the Afar and Levant Plumes, aided by the subduction-induced forces, facilitates the lateral motion of the Anatolian microplate and affects the dynamic evolution of the Zagros orogen. The proposed scenario demonstrates that the interplay between plate-tectonic events and mantle dynamics controls the kinematics of the region and can explain the general patterns of deformation observed at the surface.

We examined the microstructures and crystal-fabrics of peridotites within a large area (6 ×5 km) of the Horoman peridotite pomplex in the Hidaka metamorphic belt of Hokkaido, Japan. Thirteen peridotite samples were analyzed for olivine and orthopyroxene grain sizes, fabric strength (J-index), and crystallographic preferred orientations (CPOs). Mean grain sizes of olivine and orthopyroxene were ranged in 295–497 µm and in 257–537 µm, respectively. The olivine fabric strength values decreased from the lower to the upper part of the complex, whereas the orthopyroxene fabric strength values showed no systematic trends. The peridotites contained three different olivine CPOs, previously known as A, E, and AG types. Combined with a previous study, we found that olivine CPOs showed a transitional distribution from E to A to AG type from south to north. E type peridotites occur at the basement of the complex in the south, suggesting that local water infiltration might occur at the basement of the complex. The A type peridotites occurred mainly in the middle of the studied area and subsequently the AG type peridotites occurred towards the north. Moreover, we calculated the seismic properties of peridotite as olivine 100% aggregates and mixed (olivine and orthopyroxene) aggregates. It showed that orthopyroxene CPOs reduce P-wave anisotropies of peridotite (0.2–2.2%) without modification of the P-wave propagation patterns.

We investigate the seismic structure of the mantle wedge of the Apennines subduction zone (Central Mediterranean) using teleseismic receiver function (RF). We inverted RF for both isotropic and anisotropic properties of the mantle wedge, from below the overriding Moho to the “plate boundary”, i.e. the interface that separate the slab from the mantle wedge. Given the distribution of the seismic network, we are able to map out the change in the elastic properties at the transition between southern apennines and the Calabrian arc, given by the change in the subduction style (i.e from the subduction of continental materials to oceanic plate). We found that the anisotropy in the mantle wedge is similar between all seismic stations, generally highly anisotropic (> 10%), with a direction of the symmetry axis that rotates clockwise from North to South, following the Calabrian arc geometry and likely indicating the mantle flow driven by the slab retreat. The elastic properties of the subducted crust are more heterogeneous. To the North, the subducted crust shows a highly anisotropic (> 10%) behavior, and it occurs at larger depth (around 70 km depth), where to the South anisotropy is less intense (around 7%) and the subducted crust is shallower (around 60 km depth). These results point out a change in the subduction style that can be given by either a change in the metamorphic phase (more evolved blueschist facies stage to the North, initial greenschist facies stage to the South) or a different origin for the subducted materials (continental to the North and oceanic to the South). The differences in the anisotropic behavior of the subducted crust are reflected in the topography of the plate boundary, which becomes shallower from North to South, suggesting the existence of either a step in the slab topography or a more gentle ramp.

Conventional seismic tomography studies consider the Earth’s interior as mechanically isotropic, despite seismic anisotropy being widely observed. This current standard approach to seismic imaging is likely to lead to significant artefacts in tomographic images with first-order effects on interpretations and hinders the quantitative integration of seismology with geodynamic flow models. Although a few methodologies have been proposed for carrying out anisotropic tomography, their ability in simultaneously recovering isotropic and anisotropic structures has not been rigorously tested. In this contribution we use geodynamic and seismological modeling to predict the elastic properties and synthetic teleseismic P- and S-wave travel-time datasets for three different tectonic settings: a plume rising in an intraplate setting, a divergent margin, and a subduction zone. Subsequently, we perform seismic anisotropy tomography testing a recently developed methodology that allows for the inversion of an arbitrarily oriented weakly anisotropic hexagonally symmetric medium using multiple body-wave datasets. The tomography experiments indicate that anisotropic inversions of separate and joint P- and S-wave travel-times are capable of recovering the first order isotropic velocity anomalies and anisotropic patterns. In particular, joint P- and S-wave anisotropic inversions show that by leveraging both phases it is possible to greatly mitigate issues related to imperfect data coverage common in seismology and reduce parameter trade-offs. In contrast, by neglecting seismic anisotropy, isotropic tomographic models provide no information on the mantle fabrics and in all cases are contaminated by strong velocity artifacts. In the inversions the magnitude of anisotropy (as well as that of seismic anomalies) is always underestimated owing to regularization procedures and smearing effects. It follows that the true seismic anisotropy of mantle rocks is likely higher than estimated from anisotropic tomographies, and more consistent with predictions from laboratory and numerical micro-mechanical experiments. Altogether, these results suggest that anisotropic body-wave tomography could provide unprecedented information about the Earth’s deep geological structure, and that the latter could be better recovered by complementing teleseismic body-wave travel-times with other geophysical datasets.