Lateral and vertical heterogeneity in the lithospheric mantle at the northern margin of the Pannonian Basin reconstructed from peridotite xenolith microstructures

International audienceThis study analyzes the microstructures and deformational characteristics of spinel peridotite xenoliths from the Nógrád‐Gömör Volcanic Field (NGVF), located on the northern margin of a young extensional basin presently affected by compression. The xenoliths show a wide range of microstructures, bearing the imprints of heterogeneous deformation and variable degrees of subsequent annealing. Olivine crystal preferred orientations (CPOs) have dominantly [010]‐fiber and orthorhombic patterns. Orthopyroxene CPOs indicate coeval deformation with olivine. Olivine J indices correlate positively with equilibration temperatures, suggesting that the CPO strength increases with depth. In contrast, the intensity of intragranular deformation in olivine varies as a function of the sampling locality. We interpret the microstructures and CPO patterns as recording deformation by dislocation creep in a transpressional regime, which is consistent with recent tectonic evolution in the Carpathian‐Pannonian region due to the convergence between the Adria microplate and the European platform. Postkinematic annealing is probably linked to percolation of metasomatism by mafic melts through the upper mantle of the NGVF prior to the eruption of the host alkali basalt. Elevated equilibration temperatures in xenoliths from the central part of the volcanic field are interpreted to be associated with the last metasomatic event, which only shortly preceded the ascent of the host magma. Despite well‐developed olivine CPOs in the xenoliths, which imply a strong seismic anisotropy, the lithospheric mantle alone cannot account for the shear wave splitting delay times measured in the NGVF, indicating that deformation in both the lithosphere and the asthenosphere contributes to the observed shear wave splitting

Tectonically-determined distribution of monogenetic volcanoes in a compressive tectonic regime: An example from the Pannonian continental back-arc system (Central Europe)

This paper presents the results from a geographic information systems (GIS) workflow, which was used to analyze the spatial distribution and temporal evolution of volcanoes in the Mio-Pleistocene monogenetic Bakony-Balaton Highland Volcanic Field (BBHVF), located in the Pannonian Basin, Hungary. Volcanism occurred during the tectonic inversion in a back-arc setting and a compressive/transpressive tectonic regime on the hottest and thinnest lithosphere of continental Europe. The main goal of this study is to clarify the effect of the pre-existing structure of the upper lithosphere in the distribution of the volcanic centers across the volcanic field using an innovative GIS methodology. Orientation of the volcanic field was compared to the orientation of the faults in the BBHVF, and in its larger vicinity, which resulted in correspondence, suggesting the dominance of the SW-NE direction. The directions of the volcanic lineaments fit well to the two main fault directions. The fault-volcano proximity analysis suggests that the fault plane of a thrust fault was an important structural feature during the lifespan of the volcanism. All results suggest that the fault plane of a regionally significant Cretaceous thrust fault (Litér Fault) might have served as a temporary pathway for the ascending magma, whereby (similarly to other, smaller faults) redirecting the magmas causing clustering of the volcanoes. This highlights the importance of major upper crustal structural heterogeneities for magma transport in a compressive tectonic system, especially in the case of active, monogenetic volcanic fields from a volcanic hazard perspective. The present GIS workflow can be effective in analyzing the spatial patterns of the volcanism and its connection with crustal structures at monogenetic volcanic fields worldwide

Seismic anisotropy in the mantle of a tectonically inverted extensional basin: A shear-wave splitting and mantle xenolith study on the western Carpathian-Pannonian region

Information on seismic anisotropy in the Earth's mantle can be obtained from (1) shear-wave splitting analyses which allow to distinguish single or multi-layered anisotropy and delay time of the fast and slow polarized wave can indicate its thickness, and (2) studying mantle peridotites where seismic properties can be inferred from lattice preferred orientation of deformed minerals. We provide a detailed shear-wave splitting map of the western part of the Carpathian-Pannonian region (CPR), an extensional basin recently experiencing tectonic inversion, using splitting data. We then compare the results with seismic properties reported from mantle xenoliths to characterize the depth, thickness, and regional differences of the anisotropic layer in the mantle. Mantle anisotropy is different in the northern and the central/southern part of the western CPR. In the northern part, the lack of azimuthal dependence of the fast split S-wave indicates a single anisotropic layer, which agrees with xenolith data from the Nógrád-Gömör volcanic field. Systematic azimuthal variations in several stations in the central areas point to multiple anisotropic layers, which may be explained by two distinct xenolith subgroups described in the Bakony-Balaton Highland. The shallower layer probably has a ‘fossilized’ lithospheric structure, representing former asthenospheric flow, whereas the deeper one reflects structures attributed to present-day convergent tectonics, also observed in the regional NW-SE fast S-wave orientations. In the Styrian Basin at the western rim of the CPR, results are ambiguous as shear-wave splitting data hint at the presence of multiple anisotropic layers. Spatial coherency analysis of the splitting parameters places the center of the anisotropic layer at ~140–150 km depth under the Western Carpathians, which implies a total thickness of ~220–240 km. Thicknesses estimated from seismic properties of xenoliths give lower values, pointing to heterogeneously distributed anisotropy or different orientation of the mineral deformation structures

The link between lithospheric scale deformations and deep fluid emanations: Inferences from the Southeastern Carpathians, Romania

Understanding the formation, migration and emanation of deep CO2, H2O and noble gases (He–Ne) in deep-seated deformation settings is crucial to understand the complex relationship between deep-originated fluids and lithospheric deformation. To gain a better insight into these phenomena, we studied the origin of H2O, CO2 and noble gases of gas-rich springs found in the Târgu Secuiesc Basin located in the southeasternmost part of the Carpathian-Pannonian region of Europe. This study area is one of the best natural examples to understand the connection between the deep sources of gas emanations and deep-seated deformation zones, providing an excellent analogue for regions worldwide with similar tectonic settings and fluid emanation properties. We studied the δ2H and δ18O stable isotopic ratios of the spring waters, and the δ13C, He and Ne stable isotopic ratio of the emanating CO2-rich gases dissolved in the mineral spring waters in Covasna town and its vicinity. Based on the δ2H, δ13C, δ18O stable isotopic ratios, the spring waters and the majority of the gases are released through two consecutive fluid infiltration events. The preservation of the metamorphic signal of the upwelling H2O is linked to the local groundwater flow and fault abundancy. Furthermore, the noble gas isotopic ratios show a high degree of atmospheric contamination in the dissolved water gasses that is most likely related to the local hydrogeology. Nevertheless, the elevated corrected helium stable isotopic ratios (Rc/Ra) of our filtered data suggest that part of the emanating gases have a potential upper mantle source component. Beneath the Southeastern Carpathians, mantle fluids can have multiple origin including the dehydration of the sinking slab hosting the Vrancea seismogenic zone, the local asthenospheric upwelling and the lithospheric mantle itself. The flux of the mantle fluids is enhanced by lithospheric scale deformation zones that also support the fluid inflow from the upper mantle into the lower crust. The upwelling CO2–H2O mantle fluids may induce the release of crustal fluids by shifting the pore fluid composition (X(CO2)) and, consequently, initiating decarbonisation and devolatilization metamorphic reactions as a result of carbonate and hydrous mineral destabilisation in the crust. Based on the p-T-X(CO2) conditions of calc-silicates and the local low geotherm, we emphasise the importance of the upwelling fluids in the release and upward migration of further H2O and CO2 in the shallower lower and upper crust. Our observations in the Southeastern Carpathians show a strong similarity to other deep-seated deformation zones worldwide (e.g., Himalayas, Alps, San Andreas Fault). We infer that migration of deep fluids may also play an important role in addition to temperature control on the generation of crustal fluids in deep-seated deformation zones

Controls by rheological structure of the lithosphere on the temporal evolution of continental magmatism: Inferences from the Pannonian Basin system

Physical-chemical controls of continental magmatism evolution remain enigmatic. Of prime and controversial nature is the temporal transition from calc-alkaline magmas to alkali basalts, correlated with a switch in tectonic regime from extension to compression. We perform 1D thermo-kinematic modelling to analyze the evolution of the thermo-rheological structure of the lithosphere in such settings using the Northwestern Pannonian Basin as a test-bed. Given well-known evidence for major reduction of brittle deformation parameters due to melt-related softening, we use a relatively low internal angle of friction. We demonstrate that at the termination of extension, the presence of intra-crustal low-viscosity layers in the lithosphere provides optimal condition for emplacement and differentiation of intermediate crustal magmatic chambers along the pathway of deep-sourced basaltic melts. In contrast, subsequent lithosphere cooling after the end of extension combined with tectonic and magmatic thickening lead to a disappearance of the low-viscosity layers and formation of lithospheric-scale faults. The latter serve as conduits for rapid ascent of uncontaminated alkali basaltic melts from the mantle to the surface. These findings shed new light on the geodynamic controls of magmatism in extensional settings

Controls by rheological structure of the lithosphere on the temporal evolution of continental magmatism: Inferences from the Pannonian Basin system

Physical-chemical controls of continental magmatism evolution remain enigmatic. Of prime and controversial nature is the temporal transition from calc-alkaline magmas to alkali basalts, correlated with a switch in tectonic regime from extension to compression. We perform 1D thermo-kinematic modelling to analyze the evolution of the thermo-rheological structure of the lithosphere in such settings using the Northwestern Pannonian Basin as a test-bed. Given well-known evidence for major reduction of brittle deformation parameters due to melt-related softening, we use a relatively low internal angle of friction. We demonstrate that at the termination of extension, the presence of intra-crustal low-viscosity layers in the lithosphere provides optimal condition for emplacement and differentiation of intermediate crustal magmatic chambers along the pathway of deep-sourced basaltic melts. In contrast, subsequent lithosphere cooling after the end of extension combined with tectonic and magmatic thickening lead to a disappearance of the low-viscosity layers and formation of lithospheric-scale faults. The latter serve as conduits for rapid ascent of uncontaminated alkali basaltic melts from the mantle to the surface. These findings shed new light on the geodynamic controls of magmatism in extensional settings

Seismic anisotropy in the mantle of a tectonically inverted extensional basin: A shear-wave splitting and mantle xenolith study on the western Carpathian-Pannonian region

Information on seismic anisotropy in the Earth's mantle can be obtained from (1) shear-wave splitting analyses which allow to distinguish single or multi-layered anisotropy and delay time of the fast and slow polarized wave can indicate its thickness, and (2) studying mantle peridotites where seismic properties can be inferred from lattice preferred orientation of deformed minerals. We provide a detailed shear-wave splitting map of the western part of the Carpathian-Pannonian region (CPR), an extensional basin recently experiencing tectonic inversion, using splitting data. We then compare the results with seismic properties reported from mantle xenoliths to characterize the depth, thickness, and regional differences of the anisotropic layer in the mantle. Mantle anisotropy is different in the northern and the central/southern part of the western CPR. In the northern part, the lack of azimuthal dependence of the fast split S-wave indicates a single anisotropic layer, which agrees with xenolith data from the Nógrád-Gömör volcanic field. Systematic azimuthal variations in several stations in the central areas point to multiple anisotropic layers, which may be explained by two distinct xenolith subgroups described in the Bakony-Balaton Highland. The shallower layer probably has a ‘fossilized’ lithospheric structure, representing former asthenospheric flow, whereas the deeper one reflects structures attributed to present-day convergent tectonics, also observed in the regional NW-SE fast S-wave orientations. In the Styrian Basin at the western rim of the CPR, results are ambiguous as shear-wave splitting data hint at the presence of multiple anisotropic layers. Spatial coherency analysis of the splitting parameters places the center of the anisotropic layer at ~140–150 km depth under the Western Carpathians, which implies a total thickness of ~220–240 km. Thicknesses estimated from seismic properties of xenoliths give lower values, pointing to heterogeneously distributed anisotropy or different orientation of the mineral deformation structures

The ‘pargasosphere’ hypothesis: Looking at global plate tectonics from a new perspective

Apart from the lithosphere-asthenosphere boundary (LAB), mid-lithospheric discontinuities (MLDs) in thick and old continental lithospheres appear to play an important role in global plate tectonics. Initiation of intra-continental subduction, delamination of the lower continental lithospheric mantle and removal of cratonic roots are likely to occur along MLDs. Here we introduce the ‘pargasosphere’ hypothesis which could account for the origin of both boundaries. The observation that pargasitic amphibole is stable even at very low bulk ‘water’ concentration (~a few hundreds ppm wt.) implies that the solidus of the shallow upper mantle (<3 GPa) is usually the pargasite dehydration solidus at ~1100 °C. In young continental and oceanic lithosphere (<70 Ma) this solidus defines the LAB. The LAB separates the deeper, partial melt bearing asthenosphere from the shallower melt barren lithosphere, explaining their contrasting rheology. In old continents pargasite breaks down at the sub-solidus pargasite dehydration boundary at ~3 GPa and liberates ‘water’-rich fluids. This latter process may be responsible for the formation of MLDs. The occurrence of partial melts or fluids beyond the pargasite stability field can explain commonly observed geophysical anomalies associated with the LAB and MLDs. We present forward modelled variations of shear wave velocity and resistivity at the LAB and MLDs for idealised lithospheric columns. These columns are constructed based on the ‘pargasosphere’ hypothesis and geotherms corresponding to continental lithospheres with various tectono-thermal ages. The ‘pargasosphere’ hypothesis offers a number of other empirically testable implications. For instance, cooling asthenosphere beneath young extensional continental and oceanic lithosphere settings can be the source of surface CO2 emanations even at locations distant from areas with active volcanoes. The Vrancea zone (Eastern Europe) appears to be a suitable site for testing the ‘pargasosphere’ hypothesis for elucidating the origin of intermediate-depth earthquakes (70–300 km) and to explain the delamination of the lower continental lithospheric mantle

The ‘pargasosphere’ hypothesis: Looking at global plate tectonics from a new perspective

Apart from the lithosphere-asthenosphere boundary (LAB), mid-lithospheric discontinuities (MLDs) in thick and old continental lithospheres appear to play an important role in global plate tectonics. Initiation of intra-continental subduction, delamination of the lower continental lithospheric mantle and removal of cratonic roots are likely to occur along MLDs. Here we introduce the ‘pargasosphere’ hypothesis which could account for the origin of both boundaries. The observation that pargasitic amphibole is stable even at very low bulk ‘water’ concentration (~a few hundreds ppm wt.) implies that the solidus of the shallow upper mantle (<3 GPa) is usually the pargasite dehydration solidus at ~1100 °C. In young continental and oceanic lithosphere (<70 Ma) this solidus defines the LAB. The LAB separates the deeper, partial melt bearing asthenosphere from the shallower melt barren lithosphere, explaining their contrasting rheology. In old continents pargasite breaks down at the sub-solidus pargasite dehydration boundary at ~3 GPa and liberates ‘water’-rich fluids. This latter process may be responsible for the formation of MLDs. The occurrence of partial melts or fluids beyond the pargasite stability field can explain commonly observed geophysical anomalies associated with the LAB and MLDs. We present forward modelled variations of shear wave velocity and resistivity at the LAB and MLDs for idealised lithospheric columns. These columns are constructed based on the ‘pargasosphere’ hypothesis and geotherms corresponding to continental lithospheres with various tectono-thermal ages. The ‘pargasosphere’ hypothesis offers a number of other empirically testable implications. For instance, cooling asthenosphere beneath young extensional continental and oceanic lithosphere settings can be the source of surface CO2 emanations even at locations distant from areas with active volcanoes. The Vrancea zone (Eastern Europe) appears to be a suitable site for testing the ‘pargasosphere’ hypothesis for elucidating the origin of intermediate-depth earthquakes (70–300 km) and to explain the delamination of the lower continental lithospheric mantle