ALBERT

Description: Many vertical seismic velocity anomalies observed below different parts of the Eurasian plate are rooted in the transition zone between the upper and lower mantle (410–660 km), forming so-called secondary plumes. These anomalies are interpreted as the result of thermal effects of large-scale thermal upwelling (primary plume) in the lower mantle or deep dehydration of fluid-rich subducting oceanic plates. We present the results of thermo-mechanical numerical modelling to investigate the dynamics of such small-scale thermal and chemical (hydrous) anomalies rising from the lower part of the Earth's upper mantle. Our objective is to determine the conditions that allow thermo-chemical secondary plumes of moderate size (initial radius of 50 km) to penetrate the continental lithosphere, as often detected in seismo-tomographic studies. To this end, we examine the effect of the following parameters: (1) the compositional deficit of the plume density due to the presence of water and hydrous silicate melts, (2) the width of the weak zone in the overlying lithosphere formed because of plume-induced magmatic weakening and/or previous tectonic events, and (3) a tectonic regime varied from neutral to extensional. In our models, secondary plumes of purely thermal origin do not penetrate the overlying plate, but flatten at its base, forming “mushroom”-shaped structures at the level of the lithosphere-asthenosphere boundary. On the contrary, plumes with enhanced density contrast due to a chemical (hydrous) component are shown to be able to pass upwards through the lithospheric mantle to shallow depths near the Moho when (1) the compositional density contrast is ≥ 100 kg m−3 and (2) the width of the lithospheric weakness zone above the plume is ≥ 100 km. An extensional tectonic regime facilitates plume penetration into the lithosphere but is not mandatory. Our findings can explain observations that have long remained enigmatic, such as the “arrow”-shaped zone of low seismic velocities below the Tengchong volcano in south-western China and the columnar (“finger”-shaped) anomaly within the lithospheric mantle discovered more than two decades ago beneath the Eifel volcanic fields in north-western Germany. It appears that a chemical component is a characteristic feature not only of conventional hydrous plumes located over presently downgoing oceanic slabs, but also of upper mantle plumes in other tectonic settings.

Description: The Nógrád-Gömör Volcanic Field (NGVF) is one of the five mantle xenolith bearing alkaline basalt locations in the Carpathian Pannonian Region. This allows us to constrain the structure and properties (e.g. composition, current deformation state, seismic anisotropy, electrical conductivity) of the upper mantle, including the lithosphere-asthenosphere boundary (LAB) using not only geophysical, but also petrologic and geochemical methods. For this pilot study, eight upper mantle xenoliths have been chosen from Bárna-Nagykő, the southernmost location of the NGVF. The aim of this study is estimating the average seismic properties of the underlying mantle. Based on these estimations, the thickness of the anisotropic layer causing the observed average SKS delay time in the area was modelled considering five lineation and foliation end-member orientations. We conclude that a 142–333 km thick layer is required to explain the observed SKS anisotropy, assuming seismic properties calculated by averaging the properties of the eight xenoliths. It is larger than the thickness of the lithospheric mantle. Therefore, the majority of the delay time accumulates in the sublithospheric mantle. However, it is still in question whether a single anisotropic layer, represented by the studied xenoliths, is responsible for the observed SKS anisotropy, as it is assumed beneath the Bakony–Balaton Highland Volcanic Field (Kovács et al. 2012), or the sublithospheric mantle has different layers. In addition, the depths of the Moho and the LAB (25±5,65±10km, respectively) were estimated based on S receiver function analyses of data from three nearby permanent seismological stations.

Description: In the framework of the Pannon LitH〈sub〉2〈/sub〉Oscope Lendület project in Hungary, we investigated the lithosphere structure of the Pannonian Basin. Besides seismological and geochemical methods, a large-scale grid of MT stations was used for an in-depth investigation of the sedimentary basins. The aim of the group is to explain how the thinner-than-average continental lithosphere could have formed and what is the role of fluids. The theory that discusses the effect of fluids on the melting relations and rheology of the lithosphere and upper mantle is called the pargasosphere hypothesis. For the MT part of the study, aimed at imaging the well conductive layer at the base of the lithosphere, 38 soundings were implemented close to the broad-band seismological stations, achieving a nearly uniform MT coverage in the Pannonian Basin. Our goal was to achieve as accurate as possible mapping of the lithosphere-asthenosphere boundary (LAB) from the MT data. In the MT modeling and inversion, we have considered resistivity profiles calculated from geochemical data, providing an estimate for the rock compositions and fluid contents assumed to be present in the lower crust and upper mantle. Therefore, MT modeling provides a simple approach to test these geochemical assumptions. Furthermore, the new MT data were also suitable for mapping the resistivity distribution of the deep structure of the Pannonian Basin more accurately than ever before. In the 1D and 3D MT modeling, we also incorporated some available a priori datasets into the models. In this work, we present some selected preliminary MT results.