ALBERT

Beschreibung: 〈span〉〈div〉Summary〈/div〉Magnitudes of differential stress in the lithosphere, especially in the crust, are still disputed. Earthquake-based stress drop estimates indicate median values 〈 10 MPa whereas the lateral variation of gravitational potential energy per unit area (〈span〉GPE〈/span〉) across significant relief indicates stress magnitudes of ca. 100 MPa in average across a 100 km thick lithosphere between the Indian lowland and the Tibetan plateau. These standard 〈span〉GPE〈/span〉-based stress estimates correspond to membrane stresses, because they are associated with a deformation that is uniform with depth. We show here with new analytical results that lateral variations in 〈span〉GPE〈/span〉 can also cause bending moments and related bending stresses of several hundreds of MPa. Furthermore, we perform two-dimensional thermo-mechanical numerical simulations (1) to evaluate estimates for membrane and bending stresses based on 〈span〉GPE〈/span〉 variations, (2) to quantify minimum crustal stress magnitudes that are required to maintain the topographic relief between Indian lowland and Tibetan plateau for ca. 10 Ma and (3) to quantify the corresponding relative contribution of crustal strength to the total lithospheric strength. The numerical model includes viscoelastoplastic deformation, gravity and heat transfer. The model configuration is based on density fields from the CRUST1.0 data set and from a geophysically and petrologically constrained density model based on 〈span〉in situ〈/span〉 field campaigns. The numerical results indicate that values of differential stress in the upper crust must be 〉 ca. 180 MPa, corresponding to a friction angle of ca. 10°, to maintain the topographic relief between lowland and plateau for 〉 10 Ma. The relative contribution of crustal strength to total lithospheric strength varies considerably laterally. In the region between lowland and plateau and inside the plateau the depth-integrated crustal strength is approximately equal to the depth-integrated strength of the mantle lithosphere. Simple analytical formulae predicting the lateral variation of depth-integrated stresses agree with numerically calculated stress fields, which show both the accuracy of the numerical results and the applicability of simple, rheology-independent, analytical predictions to highly variable, rheology-dependent, stress fields. Our results indicate that (1) crustal strength can be locally equal to mantle lithosphere strength and that (2) crustal stresses must be at least one order of magnitude larger than median stress drops in order to support the plateau relief over a duration of ca. 10 Ma.〈/span〉

Beschreibung: According to general relativity, a clock experiencing a shift in the gravitational potential U will measure a frequency change given by f / f    U / c 2 . The best clocks are optical clocks. After about 7 hr of integration they reach stabilities of f / f  ~ 10 –18 and can be used to detect changes in the gravitational potential that correspond to vertical displacements of the centimetre level. At this level of performance, ground-based atomic clock networks emerge as a tool that is complementary to existing technology for monitoring a wide range of geophysical processes by directly measuring changes in the gravitational potential. Vertical changes of the clock's position due to magmatic, post-seismic or tidal deformations can result in measurable variations in the clock tick rate. We illustrate the geopotential change arising due to an inflating magma chamber using the Mogi model and apply it to the Etna volcano. Its effect on an observer on the Earth's surface can be divided into two different terms: one purely due to uplift (free-air gradient) and one due to the redistribution of matter. Thus, with the centimetre-level precision of current clocks it is already possible to monitor volcanoes. The matter redistribution term is estimated to be 3 orders of magnitude smaller than the uplift term. Additionally, clocks can be compared over distances of thousands of kilometres over short periods of time, which improves our ability to monitor periodic effects with long wavelength like the solid Earth tide.

Beschreibung: We infer seismic azimuthal anisotropy from ambient-noise-derived Rayleigh waves in the wider Vienna Basin region. Cross-correlations of the ambient seismic field are computed for 1953 station pairs and periods from 5 to 25? s to measure the directional dependence of interstation Rayleigh-wave group velocities. We perform the analysis for each period on the whole data set, as well as in overlapping 2°-cells to regionalize the measurements, to study expected effects from isotropic structure, and isotropic–anisotropic trade-offs. To extract azimuthal anisotropy that relates to the anisotropic structure of the Earth, we analyse the group velocity residuals after isotropic inversion. The periods discussed in this study (5–20? s) are sensitive to crustal structure, and they allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths, fast orientations in the Eastern Alps are S/N to SSW/NNE, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress-field. At greater depths, fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the alignment of crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.

Beschreibung: Although crustal and sub-crustal structures in the Alps are some of the best studied of any orogen in the world,different hypotheses still exist regarding plate architecture and the nature of the subduction system. Additionally,rheological configurations of the different crustal units and of the lithospheric mantle, isostasy in the orogen-foreland system, and variations of flexural rigidity along and across the mountain belt are, at the present-day, poorlyconstrained with relation to spatial patterns of seismicity and deformation. The primary goal of INTEGRATE,a project in the DFG priority program Mountain Building in 4 Dimensions, a part of the AlpArray initiative,is to provide insights into these questions by integrating different 3D modelling techniques. Here we present agravity constrained, 3D, density differentiated, structural model of the Alps and their respective forelands derivedfrom integrating numerous existing geological and geophysical datasets. Results indicate the existence of lateralheterogeneities within the crust of the studied area, particularly in regards to the difference in thickness and densityof the European and Adriatic crusts. Within the plates, some density heterogeneities correspond to well-studiedtectonic features such as the Vosges, Black Forest and Bohemian massifs, along with the Ivrea geophysical body.However, in keeping with similar modelling works, the location of these density contrasts do not always correspondto present day tectonic structures, instead indicating older, inherited crustal features. A positive correlation betweenthese inherited crustal density contrasts and present day deformation maps of the region was identified, a trendnoted here for the first time. Additionally, we used the 3D density model together with information on seismicvelocities to derive lithologies for the different crustal units and calculate the 3D conductive field of the system. Astemperature is a key controlling factor for rock strength, we also assess the correlation of temperature variationsand deformation within the region.

Beschreibung: The Pannonian Basin of Central Europe is one of the key examples of Miocene continental extension that is easily accessible to surface seismological investigation. It comprises two major crustal blocks: AlCaPa and Tisza which abut along a poorly understood structure referred to as the Mid-Hungarian Zone (MHZ), the whole being surrounded by the arc of the Carpathian Mountains, the Alps and the Dinarides. Using data from the CBP (Carpathian Basins Project) temporary broadband seismic array of 46 stations deployed across the western Pannonian Basin in 2006–2007, we calculated receiver functions that constrain the variation of crustal thickness across the basin and derive a map of Moho depth across a NW–SE oriented swath about 450 km long and 75 km wide. The measured Moho depths show no significant change in crustal thickness between AlCaPa and Tisza terrains, but the Moho is not or very weakly imaged along a ca. 40 km wide strip centred on the MHZ. Moho depths within the Pannonian Basin are typically in the range 25–30 km, and increase toward the periphery of the basin. Our measurements are generally consistent with earlier VP models from controlled-source seismic surveys and recent VS models determined by tomographic analysis of ambient noise signals. The lack of a sharp Moho image beneath the MHZ suggests that the crust–mantle boundary in that zone may consist of a gradual increase in velocity with depth. The relatively constant crustal thickness across the two domains of the Pannonian Basin suggests that thinning to the same final state is controlled thermally. This structural characteristic seems to be governed by a large-scale balance of gravitational potential energy that is insensitive to the separate prior histories of the two regions.

Beschreibung: The Alpine orogen formed as a result of the collision between the Adriatic and European plates. Significant crustal heterogeneity exists within the region due to the long history of interplay between these plates, other continental and oceanic blocks in the region, and inherited crustal features from earlier orogenies. Deformation relating to the collision continues to the present day. Here, a seismically constrained, 3-D structural and density model of the lithosphere of the Alps and their respective forelands, derived from integrating numerous geoscientific datasets, was adjusted to match the observed gravity field. It is shown that the distribution of seismicity and deformation within the region correlates well to thickness and density changes within the crust, and that the present-day Adriatic crust is both thinner and denser (22.5 km, 2800 kg m−3) than the European crust (27.5 km, 2750 kg m−3). Alpine crust derived from each respective plate is found to show the same trend, with zones of Adriatic provenance (Austro-Alpine unit and Southern Alps) found to be denser and those of European provenance (Helvetic zone and Tauern Window) to be less dense. This suggests that the respective plates and related terranes had similar crustal properties to the present-day ones prior to orogenesis. The model generated here is available for open-access use to further discussions about the crust in the region.

Beschreibung: The Alps are one of the best studied mountain ranges in the world, yet significant unknowns remain regarding their crustal structure and density distribution at depth. Previous published interpretations of crustal features within the orogen have been primarily based upon 2D seismic sections, and those that do integrate multiple geo-scientific datasets in 3D, have either focused on smaller sub-sections of the Alps or included the Alps, in low resolution, as part of a much larger study area. Therefore the generation of a 3D, crustal scale, gravity constrained, structural model of the Alps and their forelands at an appropriate resolution has been created here to more accurately describe crustal heterogeneity in the region. The study area of this work focuses on a region of 660 km x 620 km covering the vast majority of the Alps and their forelands are included, with the Central and Eastern Alps and the northern foreland being the best covered regions.

Beschreibung: This work constitutes the preliminary results from the first phase of INTEGRATE, a project lying within the scope of the SPP: Mountain Building in 4-Dimensions (MB 4-D). Although the crustal and sub-crustal structures of the Alps are some of the best studied of any orogen in the world, different hypotheses still exist regarding plate architecture and the nature of the subduction system. Rheological configurations of the different crustal units and lithospheric mantle, isostasy in the orogen-foreland system, and variations of flexural rigidity along and across the mountain belt, at the present-day, poorly constrained with relation to spatial patterns of seismicity. The primary goal of INTEGRATE is to provide insights into these questions by generating a gravity constrained, 3D structural model of the Alps and their foreland basins, so that a lithospheric temperature field can be calculated and ultimately the distribution of deformation and seismicity derived across the whole region. Here we present a first 3D structural model of the entire Alpine orogen, constructed from an integration of all publicly available geoscientific observations on the study area. Our model will be constrained by gravity fields, and the results of previous models generated using similar techniques, in regions that overlap our study area, such as the Rhine Graben, the Molasse Basin and the Po Basin. Additionally, it will benefit from current efforts by the AlpArray Gravity research group to create high resolution terrestrial gravity fields of the region. A combined model such as this will provide estimates of flexural rigidity, loads, gravitational potential energy and stresses in the different crustal bodies and, additionally, allow testing of existing isostastic models of the lithosphere. These 3D models have the potential to be used as a reference for other types of data processing and are a crucial step forward in deciphering how deep-seated mass changes affect the evolution of an orogen.