Anna Maria Marotta

An integrated study of the NE German Basin

Abstract The NE German Basin is part of the Southern Permian Basin south of the TransEuropean Suture Zone (TESZ). Here we report an attempt to integrate a variety of geological and geophysical data in order to reveal the present day deep crustal structure of the NE German Basin. Special focus is taken on detailed geological information, available reflection seismic data, wide angle refraction seismic data and gravity data. Based on this integrative approach, a concise crustal model is developed which can be subject to further evaluation. Furthermore, it is shown that the NE German Basin is an outstanding feature quite different from classical basin types. As the Moho is flat below the basin, neither a simple-shear-, nor a pure-shear-extension model can be applied. Instead, a thick high-velocity lower crustal layer is present below the basin centre. This high-velocity lower crust is also observed in the western part of the basin and can be taken as characteristic for East Avalonia. Results of gravity modelling indicate the presence of a high-density lower crust, thus supporting the wide-angle data. Furthermore, changes in crustal structure can be derived from changes in reflectivity pattern and from the gravimetric signature along the Elbe Fault System, indicating the presence of different crustal domains north and south of the fault system. An elastic plate model, where the forces applied are the sediment load, a vertical load at the southern margin accounting for the Harz Moutains and a NNE–SSW-directed compression, indicates a buckling of the crust during Late Cretaceous to Tertiary inversion. However, the correlation between the observed crustal structure and the tectonic events that have affected the area remains a subject of discussion. A Permian thermal event affected the upper mantle and the lower crust and, therefore, may have modified the ‘crustal memory’ with regard to Caledonian and Variscan events. Additional tectonic events affected the area during the Mesozoic. In conclusion, it remains an open question whether the high-velocity and high density lower crustal structure represents a remnant of East Avalonia, whether it is related to the formation of the Permo-Triassic basin, or is the cumulative result of a series of events during the Palaeozoic and Mesozoic.

Numerical simulations of an ocean‐continent convergent system: Influence of subduction geometry and mantle wedge hydration on crustal recycling

The effects of the hydration mechanism on continental crust recycling are analyzed through a 2D finite element thermo-mechanical model. Oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method. Hydration is accomplished by lawsonite and serpentine breakdown; topography is treated as a free surface. Subduction rates of 1, 3, 5, 7.5 and 10 cm/y, slab angles of 30o, 45o and 60o and a mantle rheology represented by dry dunite and dry olivine flow laws, have been taken into account during successive numerical experiments. Model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material: the larger the viscosity contrast between hydrated and dry mantle, the larger the percentage of recycled material into the mantle wedge. Slab dip variation has a moderate impact on the recycling. Metamorphic evolution of recycled material is influenced by subduction style. TPmax, generally representative of eclogite facies conditions, is sensitive to changes in slab dip. A direct relationship between subduction rate and exhumation rate results for different slab dips that does not depend on the used mantle flow law. Thermal regimes predicted by different numerical models are compared to PT paths followed by continental crustal slices involved in ancient and recent subduction zones, making ablative subduction a suitable pre-collisional mechanism for burial and exhumation of continental crust.

Mantle unrooting in collisional settings

Abstract We present a two-dimensional numerical model to study the thermo-mechanical evolution of the lithosphere under a convergence regime in order to define the conditions that lead to lithospheric mantle break-up and consequent unrooting. A Newtonian rheology with a temperature-dependent viscosity is considered. The system is not closed and horizontal flow through lateral boundaries is permitted. A horizontal velocity is imposed at the top of the model to simulate compression, whereas velocity vanishes at the bottom of the model. The initial conditions correspond to a homogeneous lithosphere with a constant heat production in the crust. The analysis of variations of maximum shear stress, strain rate, and total kinetic energy allowed us to define four major stages during the mantle unrooting process: orogenic growth, initiation of gravitational instability until lithospheric failure, sinking of the detached lithosphere, and relaxation of the system. Numerical results also show that the conditions for lithospheric unrooting strongly depend on the convergence velocity, the wideness of the deformation zone, and the imposed rheology.

Simultaneous inversion for the Earth's mantle viscosity and ice mass imbalance in Antarctica and Greenland

Redistribution of mass in the Earth due to Pleistocene deglaciation and to present-day glacial melting induces secular changes in the Earths gravitational field. The Earth is affected today by the former mechanism because of the viscous memory of the mantle and by the latter because of ongoing surface mass redistribution and related elastic response. A self-consistent procedure allows us to invert simultaneously for the lower and upper mantle viscosity and for the present-day mass imbalance in Antarctica and Greenland using the observed time variations of the long-wavelength gravity field from satellite laser ranging (SLR) analyses. The procedure is based on our normal mode relaxation theory for the forward modeling and a newly developed inversion scheme based on the Levenberg-Marquardt method. We obtain a large viscosity increase across the 670-km depth transition zone separating the upper and the lower mantle, with the lower mantle viscosity varying over the range 5 × 1021 to 1022 Pa s and the less resolved upper mantle viscosity of the order of 1020 Pa s. When Antarctica is the only present-day source, its rate of melting is ?240 Gt yr?1, corresponding to a sea level rise of 0.7 mm yr?1; when Greenland is added as a source of ice loss, the rates of melting are ?280 Gt yr?1 for Antarctica and ?60 Gt yr?1 for Greenland, corresponding to sea level rises of 0.8 and 0.2 mm yr?1. SLR data indicate that ice melting in the polar regions of the Earth is ongoing.

Numerical models of tectonic deformation at the Baltica–Avalonia transition zone during the Paleocene phase of inversion

Abstract Predictions from dynamic modelling of the lithospheric deformation are presented for Northern Europe, where several basins underwent inversion during the Late Cretaceous and Early Cenozoic and contemporary uplift and erosion of sediments occurred. In order to analyse the evolution of the continental lithosphere, the equations for the deformation of a continuum are solved numerically under thin sheet assumption for the lithosphere. The most important stress sources are assumed to be the Late Cretaceous Alpine tectonics; localized rheological heterogeneities can also affect local deformation and stress patterns. Present-day observations available in the studied region and coming from seismic structural interpretations and stress measurements have been used to constrain the model. Our modelling results show that lateral variation in lithospheric strength below the basin systems in Central Europe strongly controls the regional deformation and the stress regime. Furthermore, we have demonstrated that the geometry of the boundary between Baltica and Avalonia, together with different rheological characteristics of the two plates, had a crucial role on local crustal deformation and faulting regime resulting in the Baltica–Avalonia transition zone from the S–N Alpine convergence.

Origin of the regional stress in the North German basin: results from numerical modelling

Abstract We use a thin sheet approach to investigate the effects induced by the Alpine collision on the deformation and regional stress in northern Europe, with special emphasis on the NE German Basin. Here new seismic crustal studies indicate a flexural-type basin, which may have been induced by compressive forces transmitted from the south, due to the Alpine orogeny. Finite-element techniques are used to solve the equations for the deformation of a continuum described by a linear creep rheology and a spatial resolution of about 0.5°. The model has been constrained by stress and seismic data. We show that a relatively strong lithosphere below the northern margin of the German Basin, at the transition with the Baltic Shield, may explain the characteristic regional stress field, in particular the fan-like pattern which is observed within the region. Furthermore, the predicted strain rate pattern resembles the seismically recognizable undeformed area of the North German Basin.

Integration of natural data within a numerical model of ablative subduction: a possible interpretation for the Alpine dynamics of the Austroalpine crust

A numerical modelling approach is used to validate the physical and geological reliability of the ablative subduction mechanism during Alpine convergence in order to interpret the tectonic and metamorphic evolution of an inner portion of the Alpine belt: the Austroalpine Domain. The model predictions and the natural data for the Austroalpine of the Western Alps agree very well in terms of P-T peak conditions, relative chronology of peak and exhumation events, P-T-t paths, thermal gradients and the tectonic evolution of the continental rocks. These findings suggest that a pre-collisional evolution of this domain, with the burial of the continental rocks (induced by ablative subduction of the overriding Adria plate) and their exhumation (driven by an upwelling flow generated in a hydrated mantle wedge) could be a valid mechanism that reproduces the actual tectono-metamorphic configuration of this part of the Alps. There is less agreement between the model predictions and the natural data for the Austroalpine of the Central-Eastern Alps. Based on the natural data available in the literature, a critical discussion of the other proposed mechanisms is presented, and additional geological factors that should be considered within the numerical model are suggested to improve the fitting to the numerical results; these factors include variations in the continental and/or oceanic thickness, variation of the subduction rate and/or slab dip, the initial thermal state of the passive margin, the occurrence of continental collision and an oblique convergence.

Combined effects of tectonics and glacial isostatic adjustment on intraplate deformation in central and northern Europe: Applications to geodetic baseline analyses

6 [1] We use a suite of spherical, thin sheet, finite element model calculations to investigate 7 the pattern of horizontal tectonic deformation within Europe. The calculations incorporate 8 the effects of Africa-Eurasia convergence, Atlantic Ridge push forces, and changes 9 in the lithospheric strength of the East European and Mediterranean subdomains. These 10 predictions are compared to the deformation computed for the same region using a 11 spherically symmetric, self-gravitating, viscoelastic Earth model of glacial isostatic 12 adjustment. The radial viscosity profile and ice history input into the GIA model are taken 13 from a model that ‘‘best fits’’ three-dimensional crustal velocities estimated from the 14 BIFROST Fennoscandian GPS network. The comparison of the tectonic and GIA signals 15 includes predictions of both crustal velocity maps and baseline length changes associated 16 with sites within the permanent ITRF2000 and BIFROST GPS networks. Our baseline 17 analysis includes reference sites in northern and central Europe that are representative of 18 sites at the center, edge, and periphery of the GIA-induced deformation. Baseline length 19 change predictions associated with all three reference sites are significantly impacted 20 by both tectonic and GIA effects, albeit with distinct geometric sensitivities. In this regard, 21 several of our tectonic models yield baseline rates from Vaas, Onsala, and Potsdam to sites 22 below 55� N which are consistent with observed trends. We find that a best fit to the 23 ITRF2000 data set is obtained by simultaneously considering the effects of GIA plus 24 tectonics, where the latter is modeled with a relatively weak Mediterranean subdomain. In 25 this case, the tectonic model contributes to the observed shortening between Onsala/ 26 Potsdam and sites to the south, without corrupting the extension observed for baselines 27 extending from these reference sites and sites to the north; this extension is well reconciled 28 by the GIA process alone. INDEX TERMS: 1208 Geodesy and Gravity: Crustal movements— 29 intraplate (8110); 3210 Mathematical Geophysics: Modeling; 8110 Tectonophysics: Continental tectonics— 30 general (0905); 9335 Information Related to Geographic Region: Europe; KEYWORDS: tectonics, GIA, 31 intraplate deformation

The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions

Abstract Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.

The style of the Tyrrhenian subduction

Momentum and energy equations are solved simultaneously for an incompressible viscous fluid in order to model the changes in the shape of a subducted slab when active convergence between the subducting and overriding plates comes to the end and slab pull becomes the dominant tectonic mechanism. This model can be applied to the Tyrrhenian domain, where it has been suggested that active convergence terminated about 7–9 Myr ago. During the active phase the angle of immersion of the slab at intermediate depths between 100 and 270 km is small, about 45°–50°, and large, about 80°–90°, at depths greater than 300 km. The phase of passive gravitational sinking is characterized by a substantial modification in the shape of the slab, with a large angle of immersion of 70° now at intermediate depths, decreasing to 50° in proximity of the tip of the slab. When the shape of the modelled slab is compared with the seismogenic portion of the subducted Ionian lithosphere in the Tyrrhenian, our results are consistent with subduction driven by slab pull and with cessation of active convergence between 7–9 Myr before present.