Rediscovery of a major alpine thrust : the Helvetic Basal Decollement

<p>Since two centuries the European Alps are a natural laboratory to study continental lithosphere deformation during mountain building. Since the early studies, a constant question has been to evaluate the importance of vertical versus horizontal displacements in the building of reliefs. Whilst the occurrence of large thrust sheets, as initially proposed from field observations, are now well explained in the frame of plate tectonics, controversies still arise on the precise geometry, amount, and timing of major thrusting during the orogeny.</p><p>We present a new detailed 3D structural study of the cover/basement relationships in the Chamonix synclinorium in between the Mont-Blanc (MB) and Aiguilles Rouges (AR) ranges. These massifs are two of the main external basement ranges of the western Alps.  The study allows deciphering the area structural history: the Mesozoic sedimentary cover has been thrust at least 10km NW above the Helvetic Basal Décollement (HBD) before to be offset by late steep thrusts during exhumation in the Miocene.</p><p>Such interpretation fundamentally diverges from the classical view of the sedimentary cover of the Chamonix synclinorium being expulsed from a former graben during a single deformation phase and implies that a major thrust phase lasting ~10 Ma has been overlooked. Our observations show that the HBD was a major thrust system active between ~30 and ~20 Ma, possibly until 15 Ma, with a shortening of more than 10km in the south to 20km in the north. It extends below most of the subalpine ranges and emerges in front of the Bauges and within the Chartreuse and Vercors massifs, and was rooted east of the External Cristalline Massifs (Mont-Blanc and Belledonne). During the Miocene, the HBD was cut by steep reverse faults and uplifted above the basement culmination of the External Cristalline Massifs obscuring its continuity and precluding its recognition as a major structure even if it was previously described at several localities.</p>

The story of post-Variscan lamprophyres of the Bohemian Massif: from ultramafic (Upper Cretaceous–Palaeocene) to alkaline (Eocene–Oligocene) types

Post-Variscan lamprophyres of the Bohemian Massif hold the potential for the understanding of deep-mantle processes beneath the Bohemian Massif in association with mantle metasomatism as a consequence of Variscan subduction and Late Palaeozoic extension in Central Europe and tectonic processes between Variscan blocks. Two principal types of post-Variscan lamprophyres occur in the Bohemian Massif, contrasting in their age and composition: ultramafic lamprophyres of Late Cretaceous to Palaeocene age and alkaline lamprophyres of Mid Eocene to Late Oligocene age. Combination of published and new whole-rock, isotope (Sr-Nd-Pb) and radiometric (K/Ar) data on lamprophyres of both types (including new data from samples from the deep boreholes) significantly contributes to the understanding of the changing tectonomagmatic position of post-Variscan volcanism in the Bohemian Massif. The striking shift in lamprophyre geochemistry is paralleled by a change in their structural position from the initial pre-rift period of volcanism to the developed syn-rift period and the related change in their mantle sources beneath the Bohemian Massif. The Late Cretaceous and Cenozoic volcanism is explained as related to lithospheric flexuring during the Alpine orogeny, resulting in an asthenospheric upwelling, or associated with the lithosphere deformation and perturbation of the thermal boundary layer.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5277861

SKS splitting observations across the Iranian plateau and Zagros: the role of lithosphere deformation and mantle flow

<p>We used more than one decade of core-refracted teleseismic shear (SKS) waveforms recorded at more than 160 broadband seismic stations across the Iranian plateau and Zagros to investigate seismic anisotropy beneath the region. Splitting analysis of SKS waveforms provides two main parameters, i.e., fast polarization direction and split delay time, which serve as proxies for the trend and strength of seismic anisotropy beneath the stations. Our observation revealed a complex pattern of splitting parameters with variations in the trend and strength of anisotropy across the tectonic boundaries. We also verified the presence of multiple layers of anisotropy in conjunction with the lithosphere deformation and mantle flow field. Our observation and modeling imply that a combined system of lithosphere deformation and asthenospheric flow is likely responsible for the observed pattern of anisotropy across the Iranian Plateau and Zagros. The rotational pattern of the fast polarization directions observed locally in Central Zagros may indicate the diversion of mantle flow around a continental keel beneath the Zagros. The correlation between the variation in lithosphere thickness and the trend of anisotropy in the study area implies that the topography of the base of lithosphere is also a determining factor for the pattern of mantle flow inferred from the observations.</p>

Lithosphere deformation due to tearing at STEPs: an analogue model approach

<p>Tearing of the lithosphere at the lateral end of a subduction zone is a consequence of ongoing subduction. The location of active lithospheric tearing is known as a Subduction-Transform-Edge-Propagator (STEP), and the tearing decouples the down going plate and the part of the plate that stays at the surface. STEPs can be found alongside many subduction zones, such as at the south Caribbean or the northern end of the Tonga trench. Here we investigate what controls the evolution and geometry of the lithospheric STEP. Furthermore we study the type of lithosphere deformation in the vicinity of STEPs.</p><p> </p><p>We study the ductile tearing in the process of STEP evolution by physical analogue models, which are dynamically driven by the buoyancy of the subducting slab. In our experiments, the lithosphere as well as asthenosphere are viscoelastic media in a free subduction setup. A stress-dependent rheology plays a major role in localization of strain in tearing processes of lithosphere such as slab break-off. Therefore we developed and tested analogue materials that can serve as mechanical analogues for the stress-dependent lithosphere rheology, such as has been inferred by rock laboratory test for dislocation creep of olivine.</p><p> </p><p>We show the influence of age and integrated strength of the lithosphere and its contrasts across the passive margin, on the timing, depth, and the degree of localization of the tearing process. When tearing of the lithosphere is dominated by ductile deformation, we find that gradual necking of the passive margin precedes tearing. In many of our models we find that tearing at the lateral ends of the subduction zones is resisted by the lithospheric strength, such that tearing is delayed with respect to rollback of the slab. This has consequences for the shape of the subduction zone, and for the separation between the subducted slab and the surface lithosphere. We study the type of deformation in the vicinity of the STEP of the lithosphere that stays at the surface, and relate this to deformation observed <span>beside STEP fault zones along the Hellenic slab, the Lesser Antilles slab, and the New Hebrides slab. </span></p>

Traction and strain-rate at the base of the lithosphere: an insight into cratonic survival

SUMMARY Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain-rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model’s free-slip condition imposed at the Earth’s surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time.