Journal of Geodynamics最新文献

Knowledge the initial extension in the Indian continent during the Jurassic is important for understanding the dynamics of its breakup from Eastern Gondwana. The absence of Jurassic magmatic activity in the eastern Tethyan Himalaya hinders the understanding of this process. We report a provenance and tectonic setting study on the Middle Jurassic Zhela and Late Jurassic Weimei Formations sandstone in Gyangze, eastern Tethyan Himalaya. Detrital zircons of Jurassic sediments indicate four major age peaks: ∼500 Ma, ∼820 Ma, ∼950 Ma, and ∼2450 Ma, which reflect the affinity between the Tethyan Himalaya and India. The differences between the crystallization age and depositional age of isolated detrital zircons from the Middle Jurassic Zhela and Late Jurassic Weimei Formations, indicate that they were formed in an extensional continental margin and deposited on the passive continental margin of India. Combined with regional geological information, our results show that extensional tectonics was dominant on the northern margin of the Indian continent during the Jurassic. The source-sink sedimentary system and the topography remained stable at this time. After the extension event in the Middle and Late Jurassic, the Kerguelen mantle plume commenced activity, indicating the transition from the lithospheric thinning process to active rifting. The mantle plume activity in the Early Cretaceous led to large-scale uplift in the southeast part of the Tethyan Himalaya, that ultimately led to the breakup of India from Eastern Gondwana.

The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Teplá-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle lithosphere domain beneath the Teplá-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Teplá-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (∼340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Teplá-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their granulite-amphibolite facies thermal overprint.

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.