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The Dolomites Indenter represents the eastern front segment of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Concomitant shortening is accommodated within a south-vergent thrust belt, situated between the Periadriatic Fault system and the South-Alpine front. A combination of low temperature thermochronological analyses, focussed mapping and analogue modelling is used for unravelling the Neoalpine history of the crustal and lithospheric scale tectonic processes during indentation.In our analogue models, extensional platform-basin geometries, formed at passive continental margins, are subject to subsequent shortening and orogenesis. Parallel to oblique (10 to 20 degrees with respect to the basin axes) contraction has been applied leading to the inversion of the pre-orogenic basins. The experiments show that the simple presence of an inherited platform-basin configuration controls the overall style of compressional deformation, no matter of including frictional or viscous basal décollements, of varying the rheology of the basin fill, or of changing platform-basin thickness ratios. Orientations of thrust faults change laterally across inherited platform-basin transitions throughout all experiments. New fault slip data and shortening directions from fold axes along the western segment of the Belluno thrust of the Valsugana fault system support variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along strike) along one single fault. Based on our modelling results, we infer that this variability of shortening directions depends on inherited structures and do not necessarily reflect different deformation phases.Our low-temperature thermochronological dataset (zircon and apatite (U-Th)/He, and apatite fission track analyses) focuses on the Dolomites Indenter and spans from the Periadriatic Fault System (Pustertal-Gailtal fault) in the north to the footwall of the Bassano thrust in the south. The results argue against an only in-sequence fault activity within the dominantly WSW – ENE striking thrust belt but indicate a more complex fault system development, including backstepping fault activity. This is locally supported by field observations of fault cross-cutting relationships (e.g., at the Moschesin fault near Agordo). Remarkable that west of the Transalp Corridor Mesozoic apatite fission track ages are preserved within fault delimited areas while further to east all apatite fission track data show Cenozoic ages indicating younger exhumation in the eastern part of the study area.

We present and demonstrate our new application of a geophysical seismic technique to acoustically characterise and image layers with different impedance contrast in analogue models. A high-powered pulsed laser in combination with a mirror galvanometer is used to generate a powerful acoustic shockwave at any point of the surface of the analogue model. Reflections, refractions, and diffractions of the acoustic source wave, induced by internal structures inside an analogue model, produce vibrations of the top surface of a model, which are measured by laser vibrometer.Using our setup, we acquire seismic receiver gathers in less than a minute. Interpretation of the gathers allowed to identify the presence of internal reflecting and refracting material interfaces. In a series of test models, we determined the speed of both P-waves and surface waves in a multitude of brittle analogue materials. In uniform layered models we performed 1D inversion using the gathered waveform data. The results are validated by simulating the test experiments in a finite-difference solver. The novel method will be developed further, aiming to determine stress build-up in the material prior to fault formation or activity.

Gas production from the Groningen gas field in the northeast of the Netherlands causes compaction and induced seismicity within the reservoir and overlying/underlying lithologies. Recent earthquake localization studies show that seismicity dominantly occurs on complex normal fault systems that juxtapose lithologies of contrasting mechanical properties. However, little is known about the effects of along-fault heterogeneity on the frictional behaviour of these faults. This study aims at understanding how material mixing and clay-smearing in fault gouges affects the mechanical strength and stability of faults that juxtapose contrasting lithologies (e.g. clay-rich and quartz-rich) by performing friction experiments.Velocity stepping tests are performed on homogeneously mixed and spatially segmented simulated fault gouges, within a rotary shear configuration. Experiments are performed under normal stresses ranging between 2.5 and 10 MPa and imposed velocities ranging between 10 and 1000 µm/s. The rotary shear configuration allows for the large shear-displacements (>145 mm in our experiments) required to study the effects of lithology mixing. Simulated gouges are saturated with DI-water and subsequently sheared under drained conditions. Because low-permeability clay-rich materials promote the build-up of local pressure transients, a specially designed piston with four installed pressure transducers is used to monitor fluid pressures in the vicinity of the simulated fault gouges.The mechanical data on segmented gouges show an evolution in frictional strength, characterized by a phase of strong displacement-weakening followed by displacement-strengthening. The frictional stability strongly increases with shear-displacement, comprising a transition from velocity-weakening to velocity-strengthening. Microstructural analysis of the sheared gouges provides evidence for the development of clay-smears and strain-localization within localized shear bands, explaining the evolution in frictional stability and the initial phase of shear-weakening. However, the dilatation observed at large displacements suggests that the quartz-rich gouge is incorporated within the clay smear. This incorporation is confirmed by microstructural analysis of the clay smear and provides a mechanism responsible for the observed strengthening at large shear-displacements. Monitoring of local pore fluid pressures shows that segmented gouges are more susceptible to pressure transients, depending on the initial distribution of high porosity sandstone gouges and low permeability claystone gouges.This study shows that the frictional strength and stability of spatially heterogeneous gouges highly depends on the amount of shear-displacement. The frictional strength is characterized by subsequent phases of displacement-weakening and strengthening, whereas the frictional stability only increases with shear-displacement. This eventually leads to relatively strong but also frictionally stable faults at large displacements. The results have important implications for modelling earthquake nucleation, propagation, and arrest and apply to faults in geological settings that exhibit induced seismicity, like the Groningen gas field, but are also relevant for tectonically active faults located elsewhere.

International audience; We present a new numerical modelling study of subduction initiation at (hyper-extended) magma-poor and magma-rich continental passive margins. In particular, we test how the structure and rheological stratification of these two end-member types control the formation and thermo-mechanical evolution of subduction zones. The serpentinization of mantle lithosphere in a magma-poor continental rifted margin leads to rheological decoupling at the base of the continental crust, which induces shear localization during subsequent shortening. Under these conditions, a shear zone propagates into the mantle lithosphere and leads to subduction initiation at the transition between ocean and passive margin. In contrast, a magma-rich rifted continental margin is rheologically coupled, which creates a buttressing effect and transfer of deformation into the oceanic domain during shortening, where the subduction zone initiates. These results are quantitatively compared and in agreement with the geological record of subduction initiation in the Alps and Dinarides – Hellenides, where the relics of end-member types of continental rifted margins of the same Adriatic micro-continent bordering the Alpine Tethys and Neotethys oceans, respectively, are observed.

Within the Northern Calcareous Alps fold-and-thrust belt of the Eastern Alps, multiple deformation phases have contributed to the structural grain that localised deformation at later stages. In particular, Jurassic rifting and opening of the Alpine Tethys led to the formation of extensional basins at the northern margin of the Apulian plate. Subsequent Cretaceous shortening within the Northern Calcareous Alps produced the enigmatic Achental structure, which forms a sigmoidal transition zone between two E-W striking major synclines. One of the major complexities of the Achental structure is that all structural elements are oblique to the Cretaceous direction of shortening. It was therefore proposed to be a result of forced folding at the boundaries of the Achental basin. This study analyses the structural evolution of the Achental structure through integrating field observations with crustal-scale physical analogue models, to elucidate the influence of pre-existing crustal heterogeneities on oblique basin inversion and the prerequisites for the formation of a sigmoidal hanging wall that outlines former basin margins. From brittle-ductile models, we infer that shortening oblique to pre-existing extensional faults can lead to the localisation of thrust faults at the existing structure within a single deformation phase. Prerequisites are 1) a weak basal décollement that is offset by an existing normal fault, 2) the presence of topography in the hinterland, 3) a thin-skinned deformation style. Consequently, the Achental low-angle thrust and corresponding folds was able to localise exactly at the basin margin, with a vergence opposite to the Jurassic normal fault, creating the characteristic sigmoidal morphology during a single phase of NW-directed shortening.

Polyphase deformation of continental crust is analysed through physical analogue models for settings where platform-basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle-ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength differences between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel to oblique (10 to 20 degrees) with respect to the basin axes has been applied leading to the inversion of earlier formed basins. The experiments show that the simple presence of an inherited platform-basin configuration controls the overall style of compressional deformation, no matter of including frictional or viscous basal décollements, of varying the rheology of the basin fill, or of changing platform-basin thickness ratios. Orientations of thrust faults change laterally across inherited platform-basin transitions throughout all experiments; higher obliquity of basin inversion leading to stronger curvature of thrusts with respect to the pre-existing rift axes. Variations in the strike of thrust fronts are accompanied by changes of the shortening direction along one single fault and time step. Furthermore, our models support localisation of deformation in areas of lateral strength contrasts, as platform-basin transitions represent. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform-basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Both parallel and oblique inversion experiments can be applied to polyphase deformed continental crust, as, e.g., the Dolomites Indenter of the eastern Southern Alps. Our models involving two phases of deformation, suggest that the whole tectonic evolution of the Dolomites Indenter is controlled by inherited features. Fault slip data and shortening directions from fold axes from our field case study along the western segment of the Belluno thrust of the Valsugana fault system support variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along strike) along one single fault. Based on our modelling results, we infer that this variability of shortening directions depends on inherited structures and do not necessarily reflect different deformation phases.

The Barents Shear Margin separates the Svalbard and Barents Sea from the North Atlantic. It includes one northern (Hornsund Fault Zone) and a southern (Senja Fracture Zone) margin segment in which structuring was dominated by dextral shear. These segments are separated by the Vestbakken Volcanic Province that rests in a releasing bend position between the two. During the break-up of the North Atlantic the plate tectonic configuration was characterized by sequential dextral shear, extension, contraction and inversion. This generated a complex zone of deformation that contain several structural families of over-lapping and reactivated structures Although the convolute structural pattern associated with the Barents Shear Margin has been noted, it has not yet been explained in this framework. A series of crustal-scale analogue experiments, utilizing a scaled stratified sand-silicon polymer sequence, serve to study the structural evolution of the shear margin in response to shear deformation along a pre-defined boundary representing the geometry of the Barents Shear Margin and variations in kinematic boundary conditions of subsequent deformation events, i.e. direction of extension and inversion. The observations that are of particular significance for interpretating the structural configuration of the Barents Shear Margin are: 1) The experiments reproduced the geometry and positions of the major basins and relations between structural elements (fault and fold systems) as observed along and adjacent to the Barents Shear Margin. This supports the present structural model for the shear margin. 2) Several of the structural features that were initiated during the early (dextral shear) stage became overprinted and obliterated in the subsequent stages. 3) Prominent early-stage positive structural elements (e.g. folds, push-ups) interacted with younger (e.g. inversion) structures and contributed to a complex final structural pattern. 4) All master faults, pull-part basins and extensional shear duplexes initiated during the shear stage quickly became linked in the extension stage, generating a connected basin system along the entire shear margin at the stage of maximum extension. 5) The fold pattern generated during the terminal stage (contraction/inversion became dominant in the basinal areas and was characterized by fold axes with traces striking parallel to the basin margins. These folds, however, most strongly affected the shallow intra-basinal layers. This is in general agreement with observations in previous and new reflection seismic data from the Barents Shear Margin.

In the evolution of the Alps, the Adriatic plate is traditionally considered as rigid indenter and research on collision and extrusion tectonics mainly focused on the areas north of it. However, the structure of the northernmost part of the Adriatic microplate in the eastern Southern Alps of Italy and Slovenia, referred to as Dolomites Indenter (DI), demonstrates significant internal deformation of a continental indenter that contains the structural memory of Jurassic extension leading to the formation of the Alpine Tethys. Here we argue that these pre-existing NNE-SSW trending normal faults are of paramount importance for understanding and explaining Paleogene to Neogene crustal deformation of the DI. In particular, we demonstrate through physical analogue modelling that lateral changes of thrust fault orientations are controlled by the inherited fault bound basin and platform configuration (e.g., in the Cadore area, where the Trento platform merges into the Belluno basin). In our brittle and brittle-ductile analogue experiments, shortening is orthogonal or oblique to platform and basin configuration, which is represented by either (i) pre-scribed strength contrasts between platforms/basins or (ii) graben structures modelled by an initial extensional phase. This approach allows us to test various deformational wavelengths as well as timing and localisation of uplift of the DI’s upper to middle crust. Modelling results indicate that the localisation of deformation is controlled by lateral strength contrasts, as transitions from platforms to basins represent. Analyses of surface displacement vectors show that these areas are associated with changes in shortening directions, resulting in, curved faults. All models emphasise that the overall style of deformation is less dependent on the material of the basal décollement, but is ruled by the inherited platform and basin configuration, independent of orthogonal or oblique inversion. To compare analogue modelling results with deformation in the DI, structural fieldwork accompanied by thermochronological sampling was carried out. Examined cross-cutting criteria covering the entire DI comprise evidence for four distinguishable deformation phases during Paleogene (Dinaric) shortening and subsequent Neogene (Alpine) continental indentation: Top SW, Top (S)SE, Top S and Top E(SE). However, shortening directions along several of the studied faults, e.g. the overall SSE-vergent Belluno thrust (Valsugana fault system), change locally from top SSW to top SSE along strike. Based on our modelling results, we infer that the variability of shortening directions along these thrust faults may depend on inherited structures and do not necessarily reflect different deformation phases. As such the number of deformation phases in the Southern Alps may have been overestimated so far.

Since the 19th century pioneering work of Sir James Hall, physical analogue modelling has been proven a valuable method for the study of geological phenomena and has significantly contributed to understanding fundamental mechanisms of crust and lithosphere deformation. Traditionally, in such analogue scale models, structural deformation is monitored and quantified using top-view images or cross-sections, where the latter allow for portraying the final state of internal deformation of the model in great detail. Monitoring the evolution of internal deformation while the experiment is running is however a major challenge, and currently is possible only with X-ray scanning using medical-type CT scanners. These, however, put stringent limitations on size of the model and, thus, the possible geometric configurations related to different modelling setups.To tackle these limitations, we are developing a novel method to image the evolving interior of analogue scale models using ultrasonic techniques. Similar to reflection seismology used in field studies, the internal structure of the analogue model can be imaged using sound waves. We employ a completely non-contact and non-invasive method, utilizing a laser Doppler vibrometer to detect the arrivals of seismic body waves at the model surface. A laser pulse from a powerful pulsed laser acts as a point source and is used to introduce acoustic waves in the model. By moving the detector and source, acoustic data is recorded for a number of source-recorder combinations, allowing the reconstruction of the internal layering and structure along cross sections, as will be illustrated by the results of several tests with analogue models and other samples. By developing this technique, we provide novel tools to characterize the acoustic behaviour of subsurface structures under well-controlled laboratory conditions with the aim of improving our understanding of waveforms and wave propagation in analogue models and earth materials in general.

The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. In this contribution, we focus on the internal deformation of the DI and its eastern continuation towards the Dinarides. Using a series of crustal scale analogue models, we investigate the effect of Jurassic E-W extension on the NW-SE directed shortening of the DI during Alpine orogeny.The brittle and brittle-ductile analogue experiments can be grouped in two sub-series. In sub-series A, the platform-basin topography has been created by pre-scribing an initial strength contrast between platforms and basins followed by one stage of indentation. In sub-series B, graben structures were developed through an initial extensional phase, subsequently followed by compression. The evolving grabens were syn-kinematically filled up to different thicknesses depending on the material used; either with quartz sand up to a platform/basin thickness ratio of 0,75 or with feldspar sand or glass beads up to the initial, non-stretched crustal thickness. The brittle upper crust of platforms was simulated with quartz sand, the brittle to ductile middle crust by either glass beads or by a mixture of polydimethylsiloxane (PDMS) silicon putty and quartz sand. In both sub-series, variations in the orientation of rheological boundaries with respect to the convergence direction have been modelled. This (oblique) basin inversion allows us to test various deformational wavelengths as well as timing and localisation of uplift of the DI’s upper to middle crust.Modelling results of (oblique) basin inversion confirm the localisation of deformation in areas of lateral strength contrasts (Brun and Nalpas, 1996), as transitions from platforms to basins represent. Spacing of in-sequence thrusts is larger on platforms and smaller in basins, visible in both, models of sub-series A (inversion of strength difference only) and B (inversion of strength difference and actual normal faults). The vergence of in-sequence structures varies from mostly foreland directed using glass beads as basal detachment, to pop-up structures using putty, to a combination of both using quartz sand only. Based on our modelling observations, we propose that the overall style of deformation is less dependent on the material of the basal décollement, but is ruled by the inherited platform/basin configuration.To compare analogue modelling results with deformation in the DI, structural fieldwork accompanied by thermochronological sampling was carried out (for details on structural and thermochronological data see the contribution of Klotz et al. in session GD8.4). According to field observations, the shortening direction along several of the studied faults, e.g. the overall SSE-vergent Belluno thrust (Valsugana fault system, external eastern Southern Alps, Italy), changes locally from top SSW to top SSE along strike. We infer that the variability of shortening directions along these thrust faults depends on inherited geometries and is not the result of different deformation phases. One possible conclusion from this observation is that the number of deformation phases in the Southern Alps may have been overestimated so far.References:Brun, J.-P., Nalpas, T. (1996). Graben inversion in nature and experiments. Tectonics. v. 15, no. 3, p. 677-687.

Although many different mechanisms for subduction initiation have been proposed, few of them are viable in terms of agreement with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra-oceanic subduction triggered by an upwelling mantle plume could contribute greatly to the onset and functioning of plate tectonics in the early Earth and, to a lesser extent, in the modern Earth. In contrast, the onset of intracontinental subduction is still underestimated. Here we review 1) observations demonstrating the upwelling of hot mantle material flanked by sinking proto-slabs of the continental mantle lithosphere, and 2) previously published and new numerical models of plume-induced subduction initiation. Numerical modelling shows that under the condition of a sufficiently thick (> 100 km) continental plate, incipient down thrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies with moderate temperatures and without significant strain and/or melt-induced weakening of the overlying rocks. This finding is in contrast to the requirements for plume-induced subduction initiation in oceanic or thin continental lithosphere. Consequently, plume-lithosphere interactions in the continental interior of Paleozoic-Proterozoic (Archean) platforms are the least demanding (and therefore potentially very common) mechanism for triggering subduction-like foundering in Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in a variety of intracontinental settings. A better understanding of the role of intracontinental mantle downthrusting and foundering in global plate tectonics and, in particular, in triggering "classic" oceanic-continental subduction will benefit from further detailed follow-up studies.

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). The transcurrent plate boundary system lengthens with time and is referred to as the STEP Fault. Lithospheric tearing was taken to start at the trench in the classical STEP model of Govers and Wortel (2005). They show that active STEPs and STEP Faults can be found alongside many subduction zones. However, recent seismicity studies show results near the active STEPs that are difficult to reconcile with the classical STEP model: there is significant and deep seismicity along the STEP Fault near to the west of Trinidad in the southeast Caribbean; a Wadati-Benioff zone perpendicular to the Pliny-Strabo trenches (the STEP Fault) in the eastern Mediterranean reaches 180 km depth; STEP Fault perpendicular earthquake slip vectors are observed along the northern termination of the South Sandwich trench. We seek to understand these discrepancies by studying the tearing process. We show results of new physical analog lab models that aim to elucidate what controls lithospheric tearing and the resulting geometry of the lithospheric STEP. We study the ductile tearing in the process of STEP evolution, which is 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. We find that complete tearing of the lithosphere typically occurs later than in the classical model, at 100-150 km depth. The slab is consequently highly curved near the lateral end of the trench. However, not all STEPs show evidence for such delay, e.g., the north end of the Tonga trench. In our model experiments we therefore investigate 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. Furthermore, we relate the tearing at depth to deformation at the surface along and across the STEP fault and we discuss potential consequences for STEP evolution for a number of subduction zones worldwide. Delayed lithospheric tearing explains the observations qualitatively.

The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Deformation of the DI is well studied along its rim, documented by important fault zones as, e.g., the Periadriatic fault system (PFS), the Giudicarie belt, and the Valsugana and Montello fault systems. With this study, we aim to investigate the internal deformation of the DI and its eastern continuation towards the Dinarides including the interference of Dinaric SW-directed and Alpine SE-directed folds and thrusts. What also remains unsolved at present is the relationship between deep-seated mantle dynamics and their control on the geometry and internal deformation of the DI. Our approach to unravel this tectonic history is a combination of (i) compilation and acquisition of detailed structural field data within the DI, (ii) collection of a new and comprehensive low-temperature thermochronological dataset covering the entire DI, and (iii) crustal- to lithospheric scale physical analogue experiments.The existing but limited thermochronological dataset already indicates the presence of relative vertical motions within the DI after the onset of indentation, including mostly Miocene Apatite fission track (AFT) ages along the PFS and the Valsugana fault and two age clusters of Triassic to Jurassic AFT data. One cluster represents the Monti Lessini east of Riva del Garda, the second is located SE of Bozen, in the footwall of the Truden line. Are these Mesozoic AFT age clusters resulting from tectonic vertical movements and/or are they linked to inhomogeneities within the DI, like the Mesozoic platform-basin geometries or the Permian Athesian Volcanic Complex? Ongoing thermochronological investigations aim to clarify these issues.By using crustal-scale (as a first step) physical analogue models, we aim to study (i) the impact of Jurassic E-W extension and (ii) the effect of crustal strengthening on the NW-SE directed deformation of the DI since Neogene times. Jurassic NNE-SSW trending normal faults led to a platform-basin-topography resulting, from west to east, in the Lombardian basin, Trento platform, Belluno basin, and Friuli platform (Winterer & Bosellini, 1981) but were inverted during Alpine orogeny. Moreover, the Trento platform approximately coincides with the extent of the up to ~2 km thick (Avanzini et al., 2013) Permian Athesian Volcanic Group. We simulate rigid Permian magmatic rocks, which could have led to a critical strengthening of the crust, in our analogue experiments by incorporating an additional strong domain to the lower upper crust. This, together with studying the influence of structural inheritance on the geometry and kinematics of Dinaric and Alpine deformation in the Southern Alps, allows us to model various deformational styles and -wavelengths of the DI during Neogene indentation.This study will contribute substantially to the understanding of internal deformation and thus enable conclusions to be drawn about the processes at lithospheric scale also addressed by AlpArray.References:Avanzini, M. et al. (2013): Note illustrative della carta geologica d'Italia, foglio 026 Appiano. Roma, Servizio Geologico d'Italia, 324 pp.Winterer, E. L., & Bosellini, A. (1981): Subsidence and Sedimentation on Jurassic Passive Continental Margin, Southern Alps, Italy. AAPG Bulletin, 65(3), 394-421.

The Carpatho-Balkanides of south-eastern Europe is a double 180° curved orogenic system. It is comprised of a foreland-convex orocline, situated in the north and east and a backarc-convex orocline situated in the south and west. The southern orocline of the Carpatho-Balkanides orogen formed during the Cretaceous closure of the Alpine Tethys Ocean and collision of the Dacia mega-unit with the Moesian Platform. Following the main orogen-building processes, the Carpathians subduction and Miocene slab retreat in the West and East Carpathians have driven the formation of the backarc-convex oroclinal bending in the south and west. The orocline formed during clockwise rotation of the Dacia mega-unit and coeval docking against the Moesian indenter. This oroclinal bending was associated with a Paleocene-Eocene orogen-parallel extension that exhumed the Danubian nappes of the South Carpathians and with a large late Oligocene – middle Miocene Circum-Moesian fault system that affected the orogenic system surrounding the Moesian Platform along its southern, western and northern margins. This fault system is composed of various segments that have different and contrasting types of kinematics, which often formed coevally, indicating a large degree of strain partitioning during oroclinal bending. It includes the curved Cerna and Timok faults that cumulate up to 100 km of dextral offset, the lower offset Sokobanja-Zvonce and Rtanj-Pirot dextral strike-slip faults, associated with orogen parallel extension that controls numerous intra-montane basins and thrusting of the western Balkans units over the Moesian Platform. We have performed a field structural study in order to understand the mechanisms of deformation transfer and strain partitioning around the Moesian indenter during oroclinal bending by focusing on kinematics and geometry of large-scale faults within the Circum-Moesian fault system.Our structural analysis shows that the major strike-slip faults are composed of multi-strand geometries associated with significant strain partitioning within tens to hundreds of metres wide deformation zones. Kinematics of the Circum-Moesian fault system changes from transtensional in the north, where the formation of numerous basins is controlled by the Cerna or Timok faults, to strike-slip and transpression in the south, where transcurrent offsets are gradually transferred to thrusting in the Balkanides. The characteristic feature of the whole system is splaying of major faults to facilitate movements around the Moesian indenter. Splaying towards the east connects the Circum-Moesian fault system with deformation observed in the Getic Depression in front of the South Carpathians, while in the south-west the Sokobanja-Zvonce and Rtanj-Pirot faults splay off the Timok Fault. These two faults are connected by coeval E-W oriented normal faults that control several intra-montane basins and accommodate orogen-parallel extension. We infer that all these deformations are driven by the roll-back of the Carpathians slab that exerts a northward pull on the upper Dacia plate in the Serbian Carpathians. However, the variability in deformation styles is controlled by geometry of the Moesian indenter and the distance to Moesia, as the rotation and northward displacements increase gradually to the north and west.

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We are interested in reconstructing the time evolution of 2D plane deformation of analogue models of tectonic processes. Under relevant forcings, these models develop internal deformation, such as faults, and broader zones of deformation. We use Particle Image Velocimetry (PIV) to derive incremental displacements from top-view images that we use in subsequent steps to calculate the shape changes that come with large deformation. Because PIV describes displacement in a spatial reference, and material moves through the area in view, displacements at any given time refer to fixed locations in space, and not to specific material points. By reconstructing the path of material, we can follow small regions of material while they translate, rotate and change shape.To aid the qualitative interpretation of this deformation, we have developed a novel method that can qualitatively describe shape changes coming from extensional, shortening and horizontal shearing (strike-slip) deformation or combinations of these. This method is based on a logarithmic measure of stretch and results agree well with the visual interpretation of structures that we observe in our models. Thus, we provide tools with which the evolution of 2D tectonic deformation can be interpreted in a physically meaningful manner, but our method may be useful outside the realm of tectonics. Our software to compute deformation is freely available and can be used to post-process incremental displacements from PIV or similar autocorrelation methods.

М30 М34

The dataset consists of the original top-view and cross-section digital photographs of the model presented in Broerse et al. (2020) (Mappingand classifying large deformation from digital imagery: application to analogue models of lithosphere deformation.). The top-view images have been taken at a regular interval of 45 seconds during the entire experiment. Cross-section images have been taken after the experiment was finished. The purpose of the top-view images is to track incremental displacement fields of the model surface in time, estimated by Particle Image Velocimetry (PIV) based on the digital images. The cross-section images serve to inspect the final internal deformation along vertical planes. The images are located in two folders; top-views contains the full range of top-view images as used in the PIV analysis, starting at the original undeformed state until the final deformed state at the end of the experiment. A sub-folder not used in PIV contains images from before the start and after the stop of the experiment, and that are not used in the accompanying manuscript. The folder cross-sections contains an overview image of the locations of all cross-sections taken (IMG 7243) and images of vertical model cross-sections. In total 13 cross-sections have been made, where the cross-section numbering starts from the north, and continues southward (see image IMG 7243). The cross-sections a-a’, b-b’ and c-c’ that are used in Broerse et al. (2020) correspond to numbers 2, 6 and 13, respectively. The cross- section images have been taken with a southward view, while in Broerse et al. (2020) we have mirrored the images to present a northward view on the cross-sections. All photographs are in .jpg format and their names are original generic names created by the camera software at the moment of acquisition. Contact person is Taco Broerse - D.B.T.Broerse@uu.nl

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. 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. 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 beside STEP fault zones along the Hellenic slab, the Lesser Antilles slab, and the New Hebrides slab.

The Sierra de los Filabres mountain range in the Betics system of SE Spain is one of the best natural laboratory to investigate processes associated with nappe stacking and subsequent exhumation of metamorphic rocks during the orogenic evolution. Existing research separates the Iberia-derived high-pressure, amphibolite facies Nevado-Fillabrides complex in a lower tectonic plate position from the lower grade ALCAPECA microcontinent-derived Alpujárride complex in an upper tectonic plate position. Their nappe-stack contact is also defined as an extensional detachment that controls the exhumation of the higher grade Nevado-Fillabrides complex. We have tested this model with a detailed (micro-)structural and lithological analysis complemented by 40Ar/39Ar white mica dating of key shear zones. We aim to define key shear zones that separate different tectonic units, to determine the kinematics and timing of main deformation phases, and to understand the interplay between burial and exhumation structures. The results show that shearing related to the subduction burial up to the amphibolite facies is ~ top-NW in present-day coordinates. Three amphibolite facies nappe units are distinguished, which may correspond to proximal and more distal parts of the former hyper-extended Iberian margin. The bottom and top nappes consist of continental material, while the middle nappe is largely made of mafic and ultramafic rocks. Top-NW shearing was coeval with the isoclinal and tight asymmetric folding of the formations. These structures were overprinted by upright folds and greenschist facies shear zones that still developed under compression. These contractional structures are cross-cut by ~ top-W shear zones associated with exhumation that show evidences of gradually decreasing P-T conditions during extension from ductile shearing to normal faulting. We show that the same protolith can be followed in amphibolite grade below and in low greenschists grade above the main extensional detachment. This demonstrates that the extensional detachment did not follow and reactivate exactly the former nappe contact between the Nevado-Fillabrides and Alpujárride complexes. Our single grain fusion 40Ar/39Ar ages on white micas show a range of 10 to 20 Ma in case of nappe contacts or extensional shear zones, while yield a significantly older, 25-40 Ma age cluster in case of a sample far away from the main shear zones in the core of the Nevado-Fillabrides dome. This age cluster could either represent excess Ar in the sample, or resetting due to a distinct tectono-metamorphic event that occurred prior to the Early Miocene subduction of the Nevado-Fillabrides complex. The latter case would require the reconsideration of recent tectonic reconstructions of the region.

Pull-apart basins are structural features closely linked to the interactions between strike-slip and extensional tectonics. Their morphology and structural evolution are determined by factors such as extension rate, width/length ratio, or changes in the extension direction. In this work, we focus on changes in extension direction during the formation of a pull-apart basin as a basis to further understand the evolution of the northern Gulf of California through a series of physical analogue modelling experiments.We investigate the effect of a variation in the basin extension direction, using a two-layer ductile-brittle configuration to simulate continental crust rheology. Pull-apart basin development is accomplished by displacing a plastic sheet at the bottom of the experiment, with pre-cut geometry resembling interconnected rift and strike-slip segments, orthogonal to the evolving rift axes. Subsequently, we change the relative motion of the base plate by 7o in accordance with the reconstructed plate vector from the Gulf of California. Oblique extension continues on this new plate motion vector to the end of the experiment.To analyse the results, we inserted the model cross-sections in a seismic interpretation software generating 3D interpretations for faulting and sedimentary thickness. Preliminary results show that the shift in the direction of plate motion produces sigmoidal oblique slip faults that become normal when deformation adjusts to the new plate motion vector. Furthermore, it appears that sediment distribution is controlled heavily by the relative plate rotation.Finally, we compare our observations with seismic reflection images, sedimentary package thicknesses and fault interpretations from the pull-apart structure in the Northern Gulf of California transtensional margin, where we find good agreement between model and nature.

This chapter describes the behaviour of faults in the laboratory. The chapter is organised from small scale to large scale experiments, introducing the reader to general and less general observations of faulting and friction, and showing how these observations are linked to faulting processes occurring in nature. The first section introduces cm-scale friction experiments on gouge materials including the concept of rate-and-state friction, i.e., how velocity affects friction in the quasi-static regime. The following section is devoted to dynamic friction, i.e., observations of friction at high velocity as well as observations of dynamic rupture. The third section discusses the evolution of discrete faults and fault zones in up to meter-scale physical analogue experiments, their dependence on material properties and their significance for the study of large-scale tectonic structures. Finally, the various microstructural features and their possible link to fault stability obtained in the quasi-static regime will be discussed.

In order to show the spatial and temporal evolution of deformation and associated surface topography, we present our results in terms of surface scans and final cross sections. The latter are pictures recorded at the end of the experiment. They are obtained at two-third of their width. Experiments are monitored using digital photography and laser scans of the top surface. Digital elevation models, DEM's, are used to retrieve the surface data (grid xyz) and evaluate the extent of deformation between consecutive time-steps of the experiments expressed in term of bulk shortening (BS). Vertical movements through time are monitored along topographic profiles to further delineate where deformation takes place. The data is provided in 6 subfolders for 6 experiments. Detailed information about the files in these subfolders as well as information on how the data is processed is given in the explanatory file Auzemery-et-al-2020-Data-Documentation.docx. Contact person is Antoine Auzemery - Researcher - a.auzemery@uu.nl

This dataset contains original top-view and cross-section photographs of 12 crustal-scale analogue models. Top-view photographs were taken in regular time intervals from the beginning until the end of each experiment (for details see below). Cross-section photographs were taken at the end of each experiment. Therefore, top-view photographs provide means to track and analyse surface deformation through time and space and cross-sections allow to demonstrate overall vertical deformation of each model. The data are grouped in 12 folders that contain the data for the individual models (model1 to model12). The numbering of the folders corresponds to the model numbers as described in Krstekanić et al. (2020, in prep.). Each folder contains two sub-folders named m#-cross-sections and m#-top-views where m stands for the model and # for the number of the model (i.e., the same number as the parent folder). The m#-cross-sections sub-folder contains one top-view photograph indicating the locations of the cross-sections as well as the pertinent cross-section photographs. A number in each cross-section photograph corresponds to the number next to the cross-section location in the top-view. The m#-top-views sub-folder contains all top-view photographs of that particular model, from its initial, undeformed state to the end of the experiment. All photographs are in .jpg format and their names are original generic names created by the camera software at the moment of acquisition. The data is provided in 12 subfolders for 12 models. Detailed information about the files in these subfolders as well as information on how the data is processed is given in the explanatory file krstekanic-et-al-2020-data-documentation.docx. Contact person is Nemanja Krstekanić - PhD Candidate - n.krstekanic@uu.nl - https://www.uu.nl/staff/nkrstekanic

We use lithospheric-scale analog models to study the reactivation of pre-existing heterogeneities under oblique shortening and its relation to the origin of arcuate orogens. Reactivation of inherited rheological heterogeneities is an important mechanism for localization of deformation in compressional settings and consequent initiation of contractional structures during orogenesis. However, the presence of an inherited heterogeneity in the lithosphere is in itself not sufficient for its reactivation once the continental lithosphere is shortened. The heterogeneity orientation is important in determining if reactivation occurs and to which extent. This study aims at giving insights on this process by means of analog experiments in which a linear lithospheric heterogeneity trends with various angles to the shortening direction. In particular, the key parameter investigated is the orientation (angle α) of a strong domain (SD) with respect to the shortening direction. Experimental results show that angles α ≥ 75° (high obliquity) allow for reactivation along the entire SD and the development of a linear orogen. For α ≤ 60° (low obliquity) the models are characterized by the development of an arcuate orogen, with the SD remaining partially non-reactivated. These results provide a new mechanism for the origin of some arcuate orogens, in which orocline formation was not driven by indentation or subduction processes, but by oblique shortening of inherited heterogeneities, as exemplified by the Ouachita orogen of the southern U.S.

The present-day topography in Iberia is related to geodynamic processes dealing with lithospheric-scale deformation. However, little attention has been paid to the role of inherited crustal- or lithospheric-scale structures involved in the recent observed large-scale topographic patterns. Whereas the analysis of brittle structures focuses on the evolution of Mesozoic sedimentary basins and their subsequent response to tectonic inversion, their contribution to mountain building has been underestimated. Large numbers of structures, from ductile to brittle, which affected the whole lithosphere, were developed during the evolution of the Cantabrian orocline (ca. 310-300 Ma). The contribution of these Paleozoic post-Variscan structures, together with lithospheric mantle evolution and replacement during orocline development in the Mesozoic and Cenozoic geological evolution of Iberia, remains unexplored. To explore the role of these inherited structures on the final configuration of topography during N-S Pyrenean shortening, we carried out a series of analogue experiments complemented by surface velocity field analyses. Our experiments indicate that strain was concentrated along preexisting crustal- to lithospheric-scale discontinuities, and they show several reactivation events marked by differences in the velocity vector field. Differences in fault displacement were also observed in the models depending upon preexisting fault trends. The obtained results may explain the different amount of displacement observed during the reactivation of some of the post-orocline structures in Iberia during the Cenozoic, indicating the key role of unveiled structures, which probably have accommodated most of the Alpine shortening., This work was funded by the Spanish Ministry of Science, Innovation and Universities under the project IBERCRUST (PGC2018–096534-B-100) and by Происхождение, металлогения, климатические эффекты и цикличность Крупных Изверженных Провинций (КИП; Origin, Metallogeny, Climatic Effects, and Cyclical Large Igneous Provinces; 14.Y26.31.0012; Russian Federation) to Gutiérrez-Alonso. Fruitful field discussions with M.I. Benito and P. SuárezGonzález are greatly appreciated. We are indebted to Daniel Pastor-Galán, the editor, and three anonym

A series of analogue experiments utilizing sequences of sand with interlayered silicone polymer have been performed to investigate the effects of multistage extension on rock sequences of different strength, with particular reference to the Hoop Fault Complex of the Barents Sea. It was found that the width and style of the graben systems as seen in map view depend strongly on the extension velocity. Wide areas of graben formation are promoted by fast extension, whereas narrowly constrained deep-graben structures are typical of slow extension rates. Furthermore, the decoupling strata are likely to be characterized by flow rather than by distinct detachment faults. The scaled experiments produced units of contrasting fault frequencies and styles in individual sand layers positioned between layers of silicone polymer. It was also found that the fault segments developed from different levels were related to varying extents (hard-linked, soft-linked, firm-linked, unlinked). The fault configurations and the general fault pattern obtained in the experiments is similar to that observed in natural faults where salt or unconsolidated mudstone separate sequences of sand.

International audience; It is widely accepted that subduction initiation at modern Earth passive margin systems critically depends on the buoyancy and strength of the oceanic lithosphere and requires failure of the load-bearing crustal and mantle layers. As such, subduction initiation upon orthogonal convergence is controlled by the age of the oceanic lithosphere and thus the strength contrast at the margin. However, it is still unclear where along the margin and how subduction initiates. In particular, rheologically-controlled mechanisms are poorly understood and require further investigation. Therefore, this combined analogue and numerical modelling study aims at exploring the effects of first order rheological and kinematic conditions on subduction initiation at passive margins. Our results highlight the sensitivity of early stages of subduction initiation to the initial rheological setup as well as evolving thermo-mechanical feedback mechanisms. Additionally, they provide more insights on the conditions responsible for the locus of subduction initiation. We infer that the locus of subduction is controlled by the rheology of both the crust and the mantle lithosphere at the margin, which is in strong correlation with the thermal age of the oceanic lithosphere. Suitable conditions for subduction initiation at passive margins correspond to an intermediate age (ca. 80 Myr) of the oceanic lithosphere. In all other cases deformation localizes within the oceanic lithosphere or affects the entire continent. We advocate the significance of crust-mantle decoupling at the passive margin for its inversion and possible evolution towards a subduction zone. Additionally, the development of a self-sustaining subduction zone will fail in the absence of weakening mechanisms taking place in the mantle lithosphere such as thermal softening (shear heating) and low temperature plasticity (Peierls mechanism).

International audience; We investigate the development of margin geometries during extension of a continental lithosphere containing lateral strength variations. These strength variations may originate from the amalgamation of continents with different mechanical properties as was probably the case when Pangea was assembled. Our aim is to infer if localization of deformation is controlled by the boundary between two lithospheres with different mechanical properties (e.g., "weak" and "strong") or not. We ran a series of lithosphere-scale physical analog models in which we vary the strength contrast across equally sized lithospheric domains. The models show that deformation always localizes in the relatively weaker compartment, not at the contact between the two domains because the contact is unfavorably oriented for the applied stress and does not behave as a weak, inherited discontinuity. Wide-rifts develop under coupled conditions when the weak lithosphere consists of a brittle crust, ductile crust and ductile mantle. When a brittle upper mantle layer is included in the weak segment, the rift system develops in two phases. First, a wide rift forms until the mechanically strong upper mantle develops a necking instability after which the weak lower crust and weak upper mantle become a coupled, narrow rift system. The margin geometries that result from this two-phase evolution show asymmetry in terms of crustal thickness and basin distribution. This depends heavily on the locus of failure of the strong part of the upper mantle. The models can explain asymmetric conjugate margin geometries without using weak zones to guide deformation localization.

The thrust sheets of the Northern Calcareous Alps were emplaced during Late Cretaceous thrustdominated transpression expressed by thrust sheets segmented by closely spaced tear faults. Thrust sheet-top sediments were deposited during thrusting and associated fold growth and were controlled by active folding and tearing. We observe two types of angular unconformities: (1) Angular unconformities above folds between tear faults conform with the model of progressive unconformities. Across these unconformities dip decreases upsection. (2) Here, we define progressive unconformities that are related to tear faults and are controlled by both folding and tearing. Across these unconformities both strike and dip change. In growth strata overlying folds dissected by high-angle faults, such unconformities are expected to be common. We used analogue modelling to define the geometry of the tear faults and related unconformities. Within the syn-tectonic sediments, a steep, upward flattening thrust within a broader, roughly tulip-shaped drag zone develops. The thrust roots in the tear fault in pre-tectonic deposits and is curved upward toward the downthrown block. Vertical offset on the thrust is related to differential vertical uplift caused by, for example, growth of folds with different wavelength and amplitude on either side of the tear fault. Formation of progressive unconformities is governed by the relationship between the rates of deposition and vertical growth of a structure. Fault-related progressive unconformities are additionally controlled by the growth of the vertical step across the tear fault. When the rates of vertical growth of two neighbouring folds separated by a tear fault are similar, the rate of growth across the tear fault is small; if the first differ, the latter is high. Episodic tear fault activity may create several angular unconformities attached to a tear fault or allow the generation of angular unconformities near tear faults in sedimentary systems that have a rate of deposition too high to generate classical progressive unconformities between the tear faults.

The lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intraplate mountain belts. A series of three-layer lithospheric-scale analog models, with a strong domain (SD) embedded at various depths, are presented to investigate the development of topography and deformation patterns by having lateral heterogeneities within a weak continental lithosphere. The experiments, performed at a constant velocity and under normal gravity, indicate that the presence or absence of the SD controls whether deformation is localized or distributed at a lithospheric scale. Deformation and topography localize above the edges of the SD, while the SD region itself is characterized by minor amounts of surficial deformation and topography. The depth of the SD (within the ductile crust, ductile mantle lithosphere, or both) controls the pattern of deformation and thus the topography. The presence of a SD in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the Moho topography. Strong Moho deflection occurs when the SD is in the ductile crust, while the Moho remains almost flat when the SD is in the mantle. When the SD occupies the ductile lithosphere, the SD is tilted. These analog experiments provide insights into intraplate strain localization and could in particular explain the topography around the Tarim Basin, a lithospheric-scale heterogeneity north of the India-Asia collision zone.

Reactivation of inherited nappe contacts is a common process in orogenic areas affected by back-arc extension. The amount of back-arc extension is often variable along the orogenic strike, owing to the evolution of arcuated mountain chains during stages of rapid slab retreat. This evolution creates low rates of extension near rotation poles, where kinematics and interplay with the pre-existing orogenic structure are less understood. The amount of Miocene extension recorded by the Pannonian Basin of Central Europe decreases SE-wards along the inherited Cretaceous – Paleogene contact between the Dinarides and Carpathian Mountains. Our study combines kinematic data obtained from field and micro-structural observations assisted with fission track thermochronological analysis and U-Pb zircon dating to demonstrate a complex poly-phase evolution in the key area of the Jastrebac Mountains of Serbia. A first event of Late Cretaceous exhumation was followed by latest Cretaceous – Eocene thrusting and magmatism related to a continental collision that sutured the accretionary wedge containing contractional trench turbidites. The suture zone was subsequently reactivated and exhumed by a newly observed Miocene extensional detachment that lasted longer in the Jastrebac Mountains when compared with similar structures situated elsewhere in the same structural position. Such extensional zones situated near the pole of extensional-driven rotation favour late stage truncations and migration of extension in a hanging-wall direction, while directions of tectonic transport show significant differences in short distances across the strike of major structures.

Southern Ethiopia is a key region to understand the evolution of the East African rift system, since it is the area of interaction between the main Ethiopian rift (MER) and the Kenyan rift. However, geological data constraining rift evolution in this remote area are still relatively sparse. In this study the timing, distribution, and style of rifting in southern Ethiopia are constrained by new structural, geochronological, and geomorphological data. The border faults in the area are roughly parallel to preexisting basement fabrics and are progressively more oblique with respect to the regional Nubia–Somalia motion proceeding southward. Kinematic indicators along these faults are mainly dip slip, pointing to a progressive rotation of the computed direction of extension toward the south. Radiocarbon data indicate post 30 ka faulting at both western and eastern margins of the MER with limited axial deformation. Similarly, geomorphological data suggest recent fault activity along the western margins of the basins composing the Gofa Province and in the Chew Bahir basin. This supports that interaction between the MER and the Kenyan rift in southern Ethiopia occurs in a 200 km wide zone of ongoing deformation. Fault-related exhumation at ~10–12 Ma in the Gofa Province, as constrained by new apatite fission track data, occurred later than the ~20 Ma basement exhumation of the Chew Bahir basin, thus pointing to a northward propagation of the Kenyan rift-related extension in the area.

Physical analogue experiments are used to investigate the effect of plate and intra-lithospheric coupling on the efficiency of continental lithosphere subduction and the style of collision. Key parameters investigated in this study are: the degree of plate coupling, regulated by the viscosity ratio of the decoupling zone and the surrounding crust and/or mantle lithosphere; and the depth of decoupling. The experimental results show that subduction of the slab is deepest in cases with strong decoupling at the plate interface and at the level of the lower crust of the downgoing plate, with upper-plate deformation restricted to the area close to the plate contact. In these cases, the strongly asymmetric orogenic wedge is widest, consists of a series of imbricated upper-crustal slices derived from the lower plate, and lacks a retro-wedge. In contrast, a reduced strength contrast across the plate interface, at the depth of either the lithospheric mantle or the ductile crust, leads to a combination of subduction and thickening of the mantle lithosphere in both the upper and the lower plates. The degree of plate coupling determines the efficiency of subduction of continental lithosphere under conditions of collision of neutrally buoyant lithospheres, whereas the vertical position of decoupling horizons within the subducting plate controls the amount of subducted lower crust. Transfer of strain to the upper plate depends critically on (1) the degree of plate coupling, with stronger coupling leading to more deformation, and (2) the presence of decoupling horizons within the upper plate, which act as strain guides to propagate deformation into the upper plate. The experimental results explain the geometry and the sequence of deformation in subduction dominated orogens, such as the Carpathians or the Dinarides, and provide a mechanical basis for the transfer of strain to the upper plate.

The role of the plate boundary in a continental collisional setting is investigated by lithospheric-scale analogue models. Key variables in this study are the degree of coupling at the plate interface and along the Moho of the lower plate as well as the geometry of the plate contact. They control the onset of intra plate deformation, orogenic architecture, amount of mantle lithosphere subduction and basin development. In all experiments, deformation initiates at the plate interface by the formation of a pop-up structure. A vertical plate boundary with respect to the shortening direction results in buckling of the lithosphere, whereas experiments with an inclined plate boundary show underthrusting and foreland basin development without orogenic wedge formation. Continental collision and coinciding mantle lithosphere subduction may occur only if the lower crust of the foreland plate is weak enough promoting crust-mantle decoupling. During decoupling the weak lower crust beneath the orogen thickens significantly by ductile flow as it detaches from the down going mantle lithosphere. This lower crustal thickening effects the distribution of upper crustal deformation and topography. Subduction of weak lower crust is favored when the weak plate interface has a significant thickness (~ 15 km in nature) and a high amount of shortening is applied. Increasing coupling at the plate interface through time leads to intra plate deformation by thickening and gentle folding and influences surface uplift and subsidence above the plate interface. The transition from a mechanically decoupled plate boundary with a significant amount of mantle lithosphere subduction towards stronger plate coupling resulting in intra plate deformation and topography development can be recorded in for instance the Caucasus, the Colombian Cordillera, the Pyrenees and the Alps. Thickening of the lower crust as portrayed for the Western Alps does not demand a strong, frictional-type behavior of the lower crust, but can also be the consequence of ductile processes. © 2009 Elsevier B.V. All rights reserved.

Unlike for subaerial settings, the impact of subaqueous relay ramps on sediment dispersal is still poorly understood. A combination of analogue laboratory experiments in a sandbox with numerical flow calculations is used to simulate relay ramp topographies on rifting continental margins and to analyse the resulting turbidity current pathways and their deposits. Various scenarios are investigated, including inflow perpendicular and oblique to the relay ramp axis as well as flow constrained by an incised channel on the ramp and by a landward-directed tilt of the ramp. Without channelling, most sedimentation takes place on the basin floor because the bulk of the flow follows the steepest gradient down the fault and into the rift basin. With a channel along the relay ramp, significant flow occurs initially down the ramp axis, but channel spillover and basinward ramp tilting combine to redirect much of the sediment down the fault slope into the basin. When the relay ramp has a landward-oriented tilt, most of the current flows down the ramp and deposits its sediment load there and at the foot of the ramp. However, also here a considerable amount of the flow is shed over the hanging wall fault and into the basin, forming a secondary depocentre, while ponding redistributes thin deposits over a wider area of the basin. The quantitative dependence of these results on the specific ramp geometries remains to be investigated further but may bear great importance for refined sedimentary models in subaqueous rifted settings as well as for hydrocarbon exploration therein. © 2009 The Authors. Journal compilation © 2009 International Association of Sedimentologists.

We present a database of geochronological data documenting the post-collisional cooling history of the Eastern Alps. This data is presented as (a) georeferenced isochrone maps based on Rb/Sr, K/Ar (biotite) and fission track (apatite, zircon) dating portraying cooling from upper greenschist/amphibolite facies metamorphism (500-600 °C) to 110 °C, and (b) as temperature maps documenting key times (25, 20, 15, 10 Ma) in the cooling history of the Eastern Alps. These cooling maps facilitate detecting of cooling patterns and cooling rates which give insight into the underlying processes governing rock exhumation and cooling on a regional scale. The compilation of available cooling-age data shows that the bulk of the Austroalpine units already cooled below 230 °C before the Paleocene. The onset of cooling of the Tauern Window (TW) was in the Oligocene-Early Miocene and was confined to the Penninic units, while in the Middle- to Late Miocene the surrounding Austroalpine units cooled together with the TW towards near surface conditions. High cooling rates (50 °C/Ma) within the TW are recorded for the temperature interval of 375-230 °C and occurred from Early Miocene in the east to Middle Miocene in the west. Fast cooling post-dates rapid, isothermal exhumation of the TW but was coeval with the climax of lateral extrusion tectonics. The cooling maps also portray the diachronous character of cooling within the TW (earlier in the east by ca. 5 Ma), which is recognized within all isotope systems considered in this study. Cooling in the western TW was controlled by activity along the Brenner normal fault as shown by gradually decreasing ages towards the Brenner Line. Cooling ages also decrease towards the E-W striking structural axis of the TW, indicating a thermal dome geometry. Both cooling trends and the timing of the highest cooling rates reveal a strong interplay between E-W extension and N-S orientated shortening during exhumation of the TW. © Birkhaueser 2008.

Lithospheric-scale analogue experiments have been conducted to investigate the influence of strength heterogeneities on the distribution and mode of crustal-scale deformation, on the resulting geometry of the deformed area, and on its topographic expression. Strength heterogeneities were incorporated by varying the strength of the crust and upper mantle analogue layers and by implementing a weak plate or part-of-a-plate between two stronger ones. Three (brittle crust/ viscous crust/strong viscous upper mantle) and four (brittle crust/ viscous crust/brittle upper mantle/strong viscous upper mantle) layer models were confined by a weak silicone layer on one side in order to contain but not oppose lateral extrusion. Experimental results show that relative strength contrasts between converging plates and intervening weak plates control the location and the shape of deformation sites taken as 'collision orogens'. If the contrast is small, internal deformation of the strong plates through fore- and backthrusting occurs early in the deformation history. However, the bulk system is dominated by buckling that nucleates on the weak plate whose antiformal topography is highest; model Moho of the bordering stronger plates is deepest under these conditions. If the contrast is large, deformation remains localized within the weak plate for a larger amount of shortening and develops a root zone below a narrow deformation belt, which coincides with the locus of maximum topography. Implementing a buoyant, low-viscosity layer above the model Moho of the weak plate favours the development of asymmetric model orogens notwithstanding the initial symmetric setup. Once the asymmetry is established strain remains localized in thrust faults and ductile shear zones documenting foreland directed displacement of the model orogen. Such laterally and vertically irregular configurations have applications in continent-continent collision settings such as the Eastern Alps. First-order mechanical boundary conditions recognized from modelling to be favourable to the post early Oligocene tectonics of the Eastern Alps include: (1) subtle rather than high-strength contrasts between the Adriatic indentor and the strongly deformed region comprising Penninic and Austroalpine units to the north of it; (2) decoupling of Penninic continental upper crust from its substratum to allow for crustal-scale buckling of the Tauern Window; (3) weak mechanical behaviour of the European lower crust during collision to account for its constant thickness along the TRANSALP deep seismic transect; and (4) the direct continuation of the basal detachment underlying the fold and thrust belt in the hangingwall of the European plate with a wide ductile shear zone in the core of the orogen, which separates the European from the Adriatic plate. © The Geological Society of London 2005.

Conditions for indentation and channelised flow are investigated with two-dimensional thermomechanical models of Alpine-type continental collision. The models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide, coherent with several geological reconstructions. We study in particular the role of lower crustal strength in developing peculiar geometries after 20 Ma of shortening at 1 cm/year. Crustal layers produce geometries of imbricate layers, which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid-like indentation: – Channelised lateral flow develops when the lateral lower crust has a viscosity less than 1021 Pa s, exhibiting velocities opposite to the direction of convergence. This mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle. It is also associated with a topographic plateau that equilibrates with a wide (about 200 km) but quasi-constant crustal root about 50 km deep. – In contrast, indentation occurs with lateral lower crust layers that have a viscosity greater than about 1023 Pa s, producing significant shortening and thickening of the central crust. In this case topography develops steep and narrow (around 100 km wide), associated with a thickened crust exceeding 60 km depth. A crustal-scale pop-up forms bounded by subvertical shear zones that root into the mantle lithosphere.

The tectonic evolution of a collisional hinterland sourcing the Ha?eg Basin, a Late Cretaceous syn-orogenic sedimentary basin in the South Carpathians (Romania), is revealed through fission track thermochronology of detrital apatite and zircon grains. This basin formed on the upper plate (Getic unit) in response to Late Cretaceous collision with the lower plate (Danubian unit), an allochtonous continental block of the Moesian Platform, upon closure of a narrow oceanic basin (Severin Basin). The fission track results suggest that Turonian to lower Maastrichtian sediments of the Ha?eg Basin have been dominantly derived from pre-Late Cretaceous sources. The age components they contain relate to pre-Cretaceous tectonothermal events such as the Variscan orogenic cycle, Jurassic rifting and Severin Basin formation, and to Early Cretaceous compressional tectonics. These results are compatible with the tectonic evolution of the upper plate that is identified as the primary source. From the onset of sedimentation (late Albian) until the early Campanian the Ha?eg Basin resembles a piggy-back basin formed on the upper plate concomitant with underthrusting and internal stacking of the lower plate. In contrast, important tectonic subsidence during the late Campanian and early Maastrichtian reflects a shift to extensional tectonics causing the unroofing of the collision zone and the exhumation of lower plate rocks back to the surface. Our fission track data place important constraints on the timing of lower plate erosion that must have commenced during the late Maastrichtian, as documented by the completely reset Late Cretaceous age component within upper Maastrichtian sediments (Sinpetru Formation). Late Maastrichtian uplift of the basin and the formation of positive relief at the site of the collision zone is an expression of continuous convergence. The mismatch between the amount of denudation and the amount of sediments trapped in the Ha?eg Basin underlines the importance of concomitant extensional unroofing.