Search

The construction of five crustal-scale profiles across the Western Alps and the Ivrea mantle wedge integrates up-to-date geological and geophysical information and reveals important along strike changes in the overall structure of the crust of the Western Alpine arc. Tectonic analysis of the profiles, together with a review of the existing literature allows for proposing the following multistage evolution of the arc of the Western Alps: (1) exhumation of the mantle beneath the Ivrea Zone to shallow crustal depths during Mesozoic is a prerequisite for the formation of a strong Ivrea mantle wedge whose strength exceeds that of surrounding mostly quartz-bearing units, and consequently allows for indentation of the Ivrea mantle wedge and eastward back-thrusting of the western Alps during Alpine orogeny. (2) A first early stage (pre-35 Ma) of the West-Alpine orogenic evolution is characterized by top-NNW thrusting in sinistral transpression causing at least some 260 km displacement of internal Western Alps and E-W-striking Alps farther east, together with the Adria micro-plate, towards N to NNW with respect to stable Europe. (3) The second stage (35-25 Ma), further accentuating the arc, is associated with top-WNW thrusting in the external zones of the central portion of the arc and is related to the lateral indentation of the Ivrea mantle slice towards WNW by some 100-150 km. (4) The final stage of arc formation (25-0 Ma) is associated with orogeny in the Apennines leading to oroclinal bending in the southernmost Western Alps in connection with the 50° counterclockwise rotation of the Corsica-Sardinia block and the Ligurian Alps. Analysis of existing literature data on the Alps-Apennines transition zone reveals that substantial parts of the Northern Apennines formerly suffered Alpine-type shortening associated with an E-dipping Alpine subduction zone and were backthrusted to the NE during Apenninic orogeny that commences in the Oligocene. [ABSTRACT FROM AUTHOR]

Accommodation of collisional shortening in the Central Alps varies dramatically along strike, and this change is inferred to result from along-strike changes of rheology. In the western Central Alps, 90% of shortening is accommodated in the thickened lower plate. In the eastern Central Alps, 90% of shortening is accommodated in the upper plate. In the central Central Alps, shortening is almost equally partitioned between the two plates. The lower crust of the Adriatic plate forms a wedge that reaches a maximum north-south extension of almost 70 km in the Engadine section, progressively decreasing westward and disappearing along the Simplon section. This difference indicates an along-strike increase of intra-plate decoupling, limiting shortening of the Adriatic plate to the middle and upper parts of the crust. Whereas the upper plate indents into the thickened accreted lower plate in the Simplon section, it is the lower plate that indents an intensely deforming upper plate in the Engadine section. In the west, the Ivrea mantle body increases the strength of the Adriatic upper plate, and Barrovian metamorphism weakens the lower plate. Therefore, along-strike transfer of shortening from one plate to the other appears to be a manifestation of along-strike changes in rheology deep in the crust. [ABSTRACT FROM AUTHOR]

In geophysical inverse problems, the distribution of physical properties in an Earth model is inferred from a set of measured data. A necessary step is to select data that are best suited to the problem at hand. This step is performed ahead of solving the inverse problem, generally on the basis of expert knowledge. However, expert‐opinion can introduce bias based on pre‐conceptions. Here we apply a trans‐dimensional algorithm to automatically weigh data on the basis of how consistent they are with the fundamental hypotheses made to solve the inverse problem. We demonstrate this approach by inverting arrival times for the location of a seismic source in an elastic half‐space, assuming a point‐source and uniform weights in concentric shells. The key advantage is that the data do no longer need to be selected by an expert, but they are assigned varying weights during the inversion procedure. Plain Language Summary: In the Big data era, automated approaches to data evaluation are needed for two main reasons: to be able to process a large amount of data in a limited time, and to avoid bias introduced by data analysists. In this study we present a novel approach to data analysis, where the data themselves measure their consistency with our hypotheses. The approach is applied to earthquake location in mines, where millions of seismic events occur every year, and automatic processing of seismic data is mandatory. We demonstrate that our approach outperforms standard ones when almost nothing is known about the data and their measurement errors. Key Points: We develop a novel approach for automatic weighting of data in geophysical inverse problems, based on a trans‐dimensional algorithmWe apply the novel approach to seismic event location in mines, obtaining consistent results compared to a more standard methodOur approach outperforms standard seismic monitoring approaches, when limited information are available on local seismic structure [ABSTRACT FROM AUTHOR]

Seismic anisotropy provides a unique constraint on the past and present dynamics of the lithosphere and sublithospheric mantle. To contribute to studies of large-scale tectonic fabric, we have developed code AniTomo for regional anisotropic tomography. AniTomo allows us to invert simultaneously relative traveltime residuals of teleseismic P waves for 3-D distribution of isotropic-velocity perturbations and velocity anisotropy in the upper mantle. Weak hexagonal anisotropy with the symmetry axis oriented generally in 3-D is considered. The first application of novel code AniTomo to data from passive seismic experiment LAPNET results in a model of anisotropic velocities of the upper mantle beneath northern Fennoscandia. We have opted for northern Fennoscandia for the first application because it is a tectonically stable Precambrian region with a thick anisotropic mantle lithosphere without significant thermal heterogeneities. We carefully analyse the distribution of the rays to limit the fully anisotropic inversion only to the volume with the sufficient directional ray coverage. Capability of the given inversion setup to reveal large-scale anisotropic structures in the upper mantle is documented by a series of synthetic tests. The strongest anisotropy and the largest velocity perturbations concentrate at depths corresponding to the mantle lithosphere, while in deeper parts of the tomographic model, the lateral variations are insignificant. We delimit regions of laterally and vertically consistent anisotropy in the mantle–lithospheric part of the model. We attribute the retrieved domain-like anisotropic structure of the mantle lithosphere in northern Fennoscandia to preserved fossil fabrics of the Archean microplates, accreted during the Precambrian orogenic processes. [ABSTRACT FROM AUTHOR]

Considering only isotropic wave propagation and neglecting anisotropy in teleseismic tomography studies is a simplification obviously incongruous with current understanding of the mantle–lithosphere plate dynamics. Therefore, we have developed a code for anisotropic–teleseismic tomography (AniTomo), which allows to invert relative traveltime residuals of teleseismic P waves simultaneously for coupled anisotropic–isotropic P -wave velocity models of the upper mantle. Due to a more complex anisotropic propagation of S waves, the AniTomo is applicable only to P -wave data. Weak hexagonal anisotropy together with isotropic velocity heterogeneities are interpreted as a cause of the observed P -wave traveltime residuals. Moreover, the axis of the hexagonal symmetry can be oriented freely in all directions, which represents a unique approach among recent approaches that usually incorporate only azimuthal or radial anisotropy into the body-wave tomography. Apart from outlining the theoretical background of AniTomo, we examine various aspects coming along with anisotropic tomography such as choice of a set of initial anisotropic models and setup of parameters controlling the inversion. Synthetic testing furthermore allows investigation of the well-known trade-off between effects of P -wave anisotropy and lateral variations of isotropic velocity. The target synthetic models are designed to schematically represent different heterogeneous anisotropic structures of the upper mantle. Considering realistic distributions of stations and events at teleseismic distances, a separation of seismic anisotropy and isotropic velocity heterogeneities is plausible and a stable output model can be achieved within a few iterations. Careful testing of the new code on synthetics, concentrating on its functionality, strength and weaknesses, is a necessary step before AniTomo is applied to real data sets. [ABSTRACT FROM AUTHOR]

The POLENET/LAPNET (Polar Earth Observing Network) broadband seismic network was deployed in northern Fennoscandia (Finland, Sweden, Norway, and Russia) during the third International Polar Year 2007-2009. The array consisted of roughly 60 seismic stations. In our study, we estimate the 3-D architecture of the upper mantle beneath the northern Fennoscandian Shield using high-resolution teleseismic P wave tomography. The P wave tomography method can complement previous studies in the area by efficiently mapping lateral velocity variations in the mantle. For this purpose 111 clearly recorded teleseismic events were selected and the data from the stations handpicked and analysed. Our study reveals a highly heterogeneous lithospheric mantle beneath the northern Fennoscandian Shield though without any large high P wave velocity area that may indicate the presence of thick depleted lithospheric "keel". The most significant feature seen in the velocity model is a large elongated negative velocity anomaly (up to -3:5%) in depth range 100-150 km in the central part of our study area that can be followed down to a depth of 200 km in some local areas. This low-velocity area separates three high-velocity regions corresponding to the cratonic units forming the area. [ABSTRACT FROM AUTHOR]

Palinspastic map reconstructions and plate motion studies reveal that switches in subduction polarity and the opening of slab gaps beneath the Alps and Dinarides were triggered by slab tearing and involved widespread intracrustal and crust-mantle decoupling during Adria-Europe collision. In particular, the switch from south-directed European subduction to north-directed 'wrong-way' Adriatic subduction beneath the Eastern Alps was preconditioned by two slab-tearing events that were continuous in Cenozoic time: (1) late Eocene to early Oligocene rupturing of the oppositely dipping European and Adriatic slabs; these ruptures nucleated along a trench-trench transfer fault connecting the Alps and Dinarides; (2) Oligocene to Miocene steepening and tearing of the remaining European slab under the Eastern Alps and western Carpathians, while subduction of European lithosphere continued beneath the Western and Central Alps. Following the first event, post-late Eocene NW motion of the Adriatic Plate with respect to Europe opened a gap along the Alps-Dinarides transfer fault which was filled with upwelling asthenosphere. The resulting thermal erosion of the lithosphere led to the present slab gap beneath the northern Dinarides. This upwelling also weakened the upper plate of the easternmost part of the Alpine orogen and induced widespread crust-mantle decoupling, thus facilitating Pannonian extension and roll-back subduction of the Carpathian oceanic embayment. The second slab-tearing event triggered uplift and peneplainization in the Eastern Alps while opening a second slab gap, still present between the Eastern and Central Alps, that was partly filled by northward counterclockwise subduction of previously unsubducted Adriatic continental lithosphere. In Miocene time, Adriatic subduction thus jumped westward from the Dinarides into the heart of the Alpine orogen, where northward indentation and wedging of Adriatic crust led to rapid exhumation and orogen-parallel escape of decoupled Eastern Alpine crust toward the Pannonian Basin. The plate reconstructions presented here suggest that Miocene subduction and indentation of Adriatic lithosphere in the Eastern Alps were driven primarily by the northward push of the African Plate and possibly enhanced by neutral buoyancy of the slab itself, which included dense lower crust of the Adriatic continental margin. [ABSTRACT FROM AUTHOR]

Abstract: Distinct alpine landforms in most high to mid-latitude mountain chains highlight the importance of glacial erosion in shaping mountain topography. The modifications to the initially, fluvially shaped landscape are associated with a massive and sustainable change in the distribution and magnitude of denudation following deglaciation. In this paper we focus on the glacially induced modifications to the short-wavelength topography of the deglaciated European Central Alps in an attempt to characterize the degree of glacial erosion on mountain topography and to explore the potential impact on millennial scale catchment denudation. We propose that short-wavelength topography is characteristically obliterated by glacial action and a measure of this process is provided by drainage density, which can be obtained by measuring the topographic curvature extracted from a DEM. Drainage density is well correlated with catchment-wide denudation rates from cosmogenic nuclides (10Be), but in two separate domains, identified by the degree of glacial conditioning. At lower elevations, where fluvial erosion processes dominate at present, drainage density tends to increase with denudation rate and mean slope. At higher elevations drainage density tends to decrease with increasing denudation rate but is not sensitive to mean slope. The transition between these domains is approximately coincident with the equilibrium line altitude of the last glacial maximum. Our results indicate that the decreasing drainage density in the higher domain reflects the cumulative impact of glacial erosion. We speculate that the commensurate lengthening of hillslopes increases slope instability and mass flux, thereby resulting in higher denudation rates. Rock mass strength seems to have a further significant effect on these relationships. [Copyright &y& Elsevier]

• NOAM is subducting obliquely under Hispaniola with a dip angle of 11°. • Contact between Caribbean plate and Hispaniola produces a dip angle of 8°–9°. • Double subduction NOAM-Hispaniola-Caribbean provokes anomalous deep zone under EPGFZ. • Southern coast presents lateral velocity variations due to Beata Ridge indentation. • Muertos Trough and Beata Ridge characterize the crustal and upper mantle deformation. The contact between the Caribbean and North American plates is a tectonically complicated boundary where the deformation is accommodated in north and south of Hispaniola by the Enriquillo-Plantain Garden and Septentrional–Oriente Fault Zones (EPGFZ and SOFZ). We present a crustal and tectonic study of the Northeastern Caribbean Plate Boundary from wide-angle seismic data (WAS) acquired during the GEOPRICO-DO (2005) and CARIBE NORTE (2009) surveys, showing two transects crossing, from north to south, North American Plate (NOAM), Bahamas Platform, central Hispaniola and Caribbean Plate (CP). The results presented include two 2-D P wave velocity models of 425 km and 200 km long oriented NNE-SSW and ENE-SSW, respectively, obtained by the travel time forward and inverse modeling of the WAS data. Our study defines that the contact between Bahamas Platform, North American Plate and Hispaniola corresponds to oblique subduction with the Moho dipping 11° in the NNE-SSW direction. Furthermore, in the south, our results reveal the existence of an anomalous deep-reaching zone of lateral velocity variation in the mantle that could be associated with EPGFZ and a possible detached oceanic slab from NOAM that could explain the deep seismicity in the region. [ABSTRACT FROM AUTHOR]

Teleseismic body waves recorded during passive seismic experiments allow us to investigate isotropic velocities of the Earth's upper mantle in a great detail, on scales of tens of kilometres, by means of the standard high-resolution travel-time tomography. Nowadays, the quality of data enables exploration of anisotropic properties of the mantle by the means of travel-time tomography as well. Nevertheless, most of the tomography studies neglect the body-wave anisotropy completely or they are limited to either azimuthal or radial anisotropy.Therefore, we have developed a code called AniTomo for coupled anisotropic-isotropic travel-time tomography of the upper mantle (Munzarová et al., Geophys. J. Int. 2018 – Part I). The novel code allows inversion of relative travel-time residuals of teleseismic P waves simultaneously for 3D distribution of P-wave isotropic-velocity perturbations and anisotropy of the upper mantle. We assume weak anisotropy of hexagonal symmetry with either the 'high-velocity' axis or the 'low-velocity' axis that is oriented generally in 3D. Such an approach of searching for orientation of the symmetry axes freely in any direction is unique and more general in comparison with the published methods that usually assume only horizontal or vertical orientation of the high-velocity symmetry axis. The code represents a step further from modelling homogeneously anisotropic blocks of the mantle lithosphere (e.g., Vecsey et al., Tectonophysics 2007; Plomerová et al., Solid Earth 2011) towards modelling anisotropy arbitrarily varying in 3D.We tested the new code thoroughly, involving simple methodological tests to find out basic characteristics of the method and tests mimicking real tomographic inversions as to the target structures and the station-event distribution. Regarding the well-known trade-off between P-wave anisotropy and isotropic heterogeneities, the inversion with code AniTomo can successfully distinguish the isotropic and the anisotropic components of the velocity, depending, of course, on data quality.For a first application of the novel code (Munzarová et al., Geophys. J. Int. 2018 – Part II), we opted for data from international passive seismic experiment LAPNET (2007 – 2009) deployed in a tectonically stable region of northern Fennoscandia. The resulting tomographic model shows that the strongest anisotropy and the largest isotropic-velocity perturbations concentrate at the mantle-lithospheric depths while in the deeper parts their amplitudes decrease significantly. It is possible to delimit regions of laterally and vertically consistent anisotropy in the mantle-lithospheric part of the model. These regions are compatible with domains inferred from a joint interpretation of directional variations of P-wave travel-time residuals and SKS-wave splitting parameters. We associate the domain-like anisotropy with fossil fabrics of blocks of the Archean mantle lithosphere, preserved probably from the time of the lithosphere origin. [ABSTRACT FROM AUTHOR]

Abstract: A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution, from sea-floor spreading through subduction to collision in the Alps. Unlike previous models, which relate the fate of Alpine Tethys solely to relative motions of Africa, Iberia and Europe during opening of the Atlantic, our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record. The motions of these microplates (Adria, Iberia, Alcapia, Alkapecia, and Tiszia) relative to both Africa and Europe during Late Cretaceous to Cenozoic time involved the subduction of remnant Tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces: (1) 131–84Ma intra-oceanic subduction of the Ligurian part of Alpine Tethys attached to Iberia coincided with Eo-alpine orogenesis in the Alcapia microplate, north of Africa. These events were triggered primarily by foundering of the older (170–131Ma) Neotethyan subduction slab along the NE margin of the composite African–Adriatic plate; subduction was linked by a sinistral transform system to E–W opening of the Valais part of Alpine Tethys; (2) 84–35Ma subduction of primarily the Piemont and Valais parts of Alpine Tethys which were then attached to the European plate beneath the overriding African and later Adriatic plates. NW translation of Adria with respect to Africa was accommodated primarily by slow widening of the Ionian Sea; (3) 35Ma–Recent rollback subduction of the Ligurian part of Alpine Tethys coincided with Western Alpine orogenesis and involved the formation of the Gibraltar and Calabrian arcs. Rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable, retreating Adriatic and African slabs during slow convergence of Africa and Europe. The upper European–Iberian plate stretched to accommodate this slab retreat in a very mobile fashion, while the continental core of the Adriatic microplate acted as a rigid indenter within the Alpine collisional zone. The subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the Alps and Apennines, as well as beneath parts of the Pannonian Basin, the Adriatic Sea, the Ligurian Sea, and the Western Mediterranean. The predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10–30%, indicating that parts of slabs may be superposed within the mantle transition zone and/or that some of this subducted lithosphere became seismically transparent. [Copyright &y& Elsevier]

The AlpArray seismic experiment provides an unique opportunity to study mountain-building processes in the European Alps. By further constraining the 3D geometries of foreland basins, of the complex Europe-Adria plate boundary and of other important structures such as the Ivrea body in the western Alps and the Alps-Apennines and Alps-Dinarides transitions we aim to improve our understanding of past and ongoing tectonic processes in the region. Ambient-noise tomography, when applied to the high-quality AlpArray datab provides high-resolution information throughout the crust and over the whole region thus enabling to link near-surface geologic information with deep crustal structures and mantle processUsing two years of data collected by the AlpArray seismic network, we are able to obtain more than 150,000 station-station cross-correlations from which we extract phase- and group-velocity measurements. Compared to previous studies (e.g. 25,000 measurements, Kästle et al., 2018), this is an order of magnitude increase in measurement density, giving us the opportunity to study the crustal shear-velocity structure in the entire Alpine arc with unprecedented resolution. We present first results of our work-in-progress, including the technical aspects of handling such a large dataset and first results of the phase-velocity maps, for both Love- and Rayleigh waves. [ABSTRACT FROM AUTHOR]