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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.

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.

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.

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.

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.

A key feature of Late Cretaceous tectonics throughout the Alpine-Carpathian-Pannonian (ALCAPA) region is the synchronous formation of sedimentary basins (Gosau basins) and exhumation of metamorphic domes. Initial subsidence, spatially varying in time (Cenomanian-Santonian), within Gosau-type basins is associated with the development of a fluvial-lacustrine to shallow marine environment and the deposition of conglomerates, sandstones, coal-bearing marls and rudist limestones were deposited. The progressive deepening of the marine basins is documented by a second subsidence pulse during the Campanian-Early Maastrichtian leading to the establishment of an open marine environment. Gosau basins on top of the Northern Calcareous Alps and equivalent nappes of the Western Carpathians which were located at or close to the northern to northwestern active margin of the Austroalpine realm (external basins) locally subsided below the CCD. In contrast a distinctly shallower water environment prevailed in Gosau basins in central areas of the actively evolving Alpine-Carpathian mountain chain (internal basins). Deposition of flysch-type deposits is common for the deep-water facies. Subsidence analysis of internal Gosau basins were performed and their mutual relationship to coevally exhuming metamorphic domes, documented by a number of geochronologic data, is emphasised. A compilation of these data revealed a diachronic evolution of the ALCAPA region. Major vertical movements post-dating nappe imbrication and metamorphism started first in the Tisza-Dacia related orogenic compartments (East-, South Carpathians and Apuseni Mts.) as early as Late Aptian, whereas exhumation and subsequent cooling of metamorphic crust in the East Alpine-West Carpathian domain occurs from the Cenomanian onward. This characteristic basement-basin relationship suggests a strong coupling between lithospheric and surface processes, largely controlled by the rheology of the orogenic system. Formation of internal Gosau basins is seen in context with collapse of thickened and gravitationally unstable continental crust.

[1] We present an analysis of the distribution, timing, and characteristics of the volcano-tectonic activity on the western margin of the Southern Main Ethiopian Rift in the Soddo area (latitudes between ~7°10'N and ~6°30'N). The margin is characterized by the presence of numerous normal faults, with limited vertical offset and often sigmoidal in shape, which accommodate a gentle transition from the rift floor to the Ethiopian plateau. New radiocarbon dating indicates post-30 ka fault activity, pointing to a significant Late Pleistocene-Holocene tectonic activity of the Soddo margin. Comparison of the fault architecture with analog models suggests that deformation has been controlled by a sub-E-W (roughly N100°E) extension direction, resulting in an oblique extension with respect to the roughly NE-SW-trending rift. This well accords with inversion of fault slip data collected on faults with Pleistocene-Holocene activity and is also in good agreement with recent GPS data from the Southern Main Ethiopian Rift. Our data support a close correlation between the recent volcanic activity and deformation in the study area, with eruptive vents located along the recent border faults; the axial tectono-magmatic activity is subordinate in the area. These findings support a transition from axial tectono-magmatic deformation in the Northern Main Ethiopian Rift to marginal deformation in the Central and Southern Main Ethiopian Rift, in turn indicating an along-axis, north to south decrease in rift maturity.

The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the zone separating the domains, and the lithosphere geometry and rheology. The model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates, which have been separated by a narrow transition zone. A wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone. In addition, interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography. The lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains, the rheology of the mantle lithosphere and the amount of bulk shortening. The modelling results have major implications on the interpretation of seismic and tomographic data from the European Alps in terms of the crust and lithosphere geometries. It appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build-up between the Western/Central and Eastern Alps.

Crustal deformation in front of an indenter is often affected by the indenter’s geometry, rheology, and motion path. In this context, the kinematics of the Jaufen- and Passeier faults have been studied by carrying out paleostress analysis in combination with crustal-scale analogue modelling to infer (1) their relationship during indentation of the Adriatic plate and (2) their sensitivity in terms of fault kinematics to the geometry and motion path of Adria. The field study reveals mylonites along the Jaufen fault, which formed under lower greenschist facies conditions and is associated with top-to-the-west/northwest shear with a northern block down component. In addition, a brittle reactivation of the Jaufen shear zone under NNW–SSE to NW–SE compressional and ENE–WSW tensional stress conditions was deduced from paleostress analysis. The inferred shortening direction is consistent with fission track ages portraying Neogene exhumation of the Meran-Mauls basement south of the fault. Along the Passeier fault, deformation was only brittle to semi-ductile and paleostress tensors record that the fault was subjected to E–W extension along its northern segment varying into NW–SE compression and sinistral transpression along its southern segment. In the performed analogue experiments, a rigid, triangular shaped indenter was pushed into a sand pile resulting in the formation of a Passeier-like fault sprouting from the indenter’s tip. These kinds of north-trending tip faults formed in all experiments with shortening directions towards the NW, N, or NE. Consequently, we argue that the formation of the Passeier fault strongly corresponds to the outline of the Adriatic indenter and was only little affected by the indenter’s motion path due to induced strain partitioning in front of the different indenter segments. The associated fault kinematics along the Passeier fault including both E–W extension and NNW to NW shortening, however, is most consistent with a northward advancing Adriatic indenter.

In the western part of the Iberian microplate the main topographic highs trend E-W to NE-SW and are periodically spaced with wavelengths of 250 km. Conversely, in the northeastern part, the region of the Iberian Chain, topography is more irregular and strike directions vary from NW-SE to E-W and NE-SW. We relate this phenomenon to shortening of a continental lithosphere, which contains two different, well-defined domains of lithospheric strength. Our hypothesis is supported by physical analog models. A new processing method has been developed to assist the interpretation of the model results. It utilizes spectral analysis of gravity and topography data derived from the experiments. Folding of the crust and mantle lithosphere yields periodic gravity fluctuations, while thickening processes lead to localized gravity lows. In this way gravity data can be used to distinguish between the two forms of lithosphere deformation and to correlate areas that underwent the same type of deformation. Gravity modeling has been performed under full in-depth control of the experimental lithosphere structure. As such, gravity signals from the models may be compared to field gravity data for better understanding the underlying deformation mechanism.

[1] Using analog modeling aided by digital image analysis (DPIV), we constrained the long-term kinematic evolution of strain partitioning in transpressional brittle wedges as a function of convergence angle. We ran a series of dry quartz sand experiments representing highly oblique continent-continent collision (convergence angles of 4° to 30°). The digital image analysis provided high-resolution constraints on the long-term kinematic evolution of these wedges, which could be subdivided in distinct kinematic stages, comprising (1) an initial “distributed strain” stage and (2) an “oblique wedge” stage before (3) the stage of strain partitioning is reached. Thus, we document the evolution of different deformation stages from a single plate tectonic boundary condition. In addition, the relationship between convergence angle, kinematic stages, and wedge geometry (including fault dips and fault hierarchy) was established. The modeling results show that smaller convergence angles lead to steeper faults. Besides, for a constant convergence angle, the proshears that evolved during the strain partitioning stage were less steep than those formed during the oblique wedge stage. The fault slip vector on individual fault segments was derived from the DPIV data set for each time increment, quantifying the magnitude and orientation of slip on fault segments during the different kinematic stages. In addition, in the 7.5° and 15° models, rotation of the slip vector by up to 40° was observed on a single proshear during the strain partitioning stage. These observations allow to some degree a validation of existing analytical models of strain partitioning, in particular the assumption of steady state.

SUMMARY In this experimental study the influence of relay ramps on submarine fan deposition is investigated by combining physical laboratory experiments with fluid flow computations. The laboratory experiments comprised a two-layer sandbox model with sand and silicone put under extensional stresses to produce a rift basin with relay ramps. Its laser-scanned topography served as digital elevation model for the FanBuilder program, in which turbidity currents were modelled under different inflow directions to the rift axis. The relay ramps were found to behave as pathway for the flow if the angle of incident was small compared to the ramp incline. Wide and antithetic relay ramps with small dips can thus function as depocentres. Narrow ramps are by-passed, and the turbidity currents flow down the main gradient of the rift edge to form basin floor deposits. In order to simulate syn-tectonic sedimentation the deposits obtained from the numerical calculation were physically added to the sandbox model before and during resumed extension. Serial cuts in the analogue model after the experiments exhibited the internal structure of the modelled rift basin.

International audience; The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle–ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust–mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust–mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees.

Two-dimensional thermal modeling of the subduction and exhumation of the ultrahigh-pressure (UHP) Maksyutov Complex in the south Ural Mountains tests factors influencing the low modern heat flow in the Urals and the feeble preservation of UHP index minerals. Best-fit models are obtained with initial surface heat flow of 60 mW m 2 indicating that low modern heat flow in the Urals is a recent anomaly. Ultrahigh-pressure metamorphism was modeled at 388 Ma based on new U–Pb SHRIMP dating of zircon; the average modeled exhumation rate for the UHP eclogites to the mid-crust is ~5.0 mm/year, much slower than other UHP complexes. The model predicts that cooling during exhumation is strongly dependent on concomitant subduction/underthrusting in the footwall of the UHP unit and normal faulting in the hanging wall. Low radiogenic heat production in the crust and a relatively thin UHP slab (3–10 km) also favor cooling. For Maksyutov, the modeling shows that cooling was controlled by low radiogenic heat production and underthrusting during decompression to lower crustal levels; these cooling mechanisms accompany exhumation despite low exhumation rates (~5 mm/year) thereby denying the call for fast exhumation in order to preserve UHP index minerals. D 2004 Elsevier B.V. All rights reserved.

[1] We calculate the present-day lithospheric strength of the Eastern Alps along the new reflection seismic profile TRANSALP to examine vertical and lateral strength variations and their implications on neotectonic activity of the Eastern Alps. The large-scale geometry of the Eastern Alps and the spatial distribution of upper, and lower crustal layers, and the lithospheric mantle is constrained by the deep seismic line. Two rheological models, coupled to a kinematic thermal model that accounts for the thermal evolution of the Eastern Alps for the last 30 Myr, are investigated for the present-day lithospheric configuration in the Eastern Alps. Models with strong (Model A) and weak (Model B) crustal rheologies predict the European and the Adriatic plates to be stronger than the central zone of the orogen comprising the region between the Inntal Fault and the Periadriatic Fault. Model A is characterized by a brittle-ductile boundary between 14 and 9 km depth and strong coupling of the mechanically strong lower crust to the upper mantle, whereas Model B suggests the presence of a thick decoupling zone between the upper crust and the upper mantle and a shallower brittle-ductile boundary (7–10 km). Of these end-member scenarios, Model A is in better agreement with neotectonic data including seismicity down to the upper-lower crust boundary within the Adriatic plate, uplift of the central zone of the Eastern Alps and the Southern Alps, and eastward escape of fault-bound blocks. Such deformation pattern is best explained by lateral extrusion upon north-south compression supporting a strong-weak-strong configuration of tectonic units along the TRANSALP line.

The East European Craton (EEC) consists of Archaean nucleii and accreted Proterozoic terranes. Pressure–Temperature–time (P–T–t) paths for the West Lithuanian Granulite Domain (WLG), the East Lithuanian Domain (ELD) and the Mid-Lithuanian Suture Zone (MLSZ) in the southwestern part of the EEC constrain the sequence of tectonic events and their deformational and metamorphic conditions. A kinematic and thermal model has been developed for the evolution of these terranes, given the accretion of the ELD to the Sarmatian and the WLG to the Fennoscandian continents, respectively, based on their structural and compositional differences. For initial surface heat flow values ranging from 80–90 mWm−2, the numerical model predicts a thermal relaxation phase of at least 30–50 m.y., following very slow accretion (0.54 cm/yr) of the WLG in a subduction-type environment, prior to granulite facies metamorphism at 1.80 Ga. Only the use of very high initial surface heat flow values (c. 120 mWm−2) allows the direct...

The Scheiblingkirchen window, a Lower Austroalpine tectonic window at the eastern margin of the Eastern Alps (Austria) was formed during Late Cretaceous continent–continent collision. Structural investigations including structural mapping, microstructural studies and texture analysis revealed a decompression-related three-stage tectonic history of Lower Austroalpine units during the formation of the Scheiblingkirchen window. (1) Intra-Lower Austroalpine nappe stacking was by top-to-the-N out-of-sequence thrusting of the Kirchberg fold nappe over the Wechsel nappe under lower greenschist facies metamorphic conditions. The structural expression of the stacking event (D1) comprises a penetrative foliation containing a N–S trending stretching lineation, isoclinal recumbent folds trending subparallel to the stretching lineation and ultramylonites. Quartz and calcite microstructures indicate that dynamic recrystallization processes accompanied deformation. Their commonly moderately developed lattice preferred orientation record dominant slip on the prism and rhomb planes parallel to 〈a〉. (2) Subsequent exhumation of previously stacked rocks is related to the formation of foliation-parallel mylonitic shear zones within an E–W extensional regime (D2). Microstructures and textures suggest similar deformation temperatures during thrusting and extension. (3) A superimposed phase of NW–SE oriented horizontal shortening (D3) was accommodated by large- and small-scale upright folding of the area around NE–SW trending axes and by backthrusting leading to the antiformal doming of the Scheiblingkirchen Window. Subsequent subvertical flattening resulting from the shortening phase led to the formation of NE–SW trending, outcrop-scale open recumbent folds. Low temperature deformation conditions as inferred from the low degree of recrystallization of quartz and calcite aggregates and the dominance of glide on the basal planes point to a cooling-related deformation event.

The following changes made by the authors during the “uncorrected proof” state of the manuscript have not been taken into account bythe editorial staff of EPSL:1. The research highlights related to this publication are:● Analogue models were employed for studying the continental lithosphere response to shortening under laterally varying strength andcoupling conditions.● Decoupling along plate boundaries favours subduction-type processes and the formation of triangular shaped mountain topography.● Coupling favours deformation by buckling regardless of the initial geometry of weak zones in the lithosphere.● Plateaus and land-locked sedimentary basins at high elevations may form upon the presence of weak zones at mid-crustal levels.● Collision zones evolve from decoupled to coupled systems as the degree of orogenic wedge-foreland plate coupling increases throughtime.2. The in text citation to Cloetingh et al. (2005) relates to:Cloetingh, S. A. P. L., Ziegler, P.A., Beekman, F., Andriessen, P. A. M., Matenco, L., Bada, G., Garcia-Castellanos, D., Hardebol, N., Dezes, P.,Sokoutis, D. (2005). “Lithospheric memory, state of stress and rheology: neotectonic controls on Europe's intraplate continental topography.”Quat. Sci. Rev.3. The in text citation to Iaffaldano and Bunge (2008) relates to:Iaffaldano G., Bunge, H. P. (2008).“Strong plate coupling along the Nazca-South America convergent margin.” Geology 36(6): 443–446.In the above quoted publication the arrowheads in Fig. 6 had disappeared. The correct version of Fig. 6 is shown below.

Continental escape or lateral extrusion often results from late-stage contraction within continental collision zones when convergence is partitioned into orthogonal contraction, crustal thickening, surface uplift, and sideward motion of fault-bounded blocks. Geometrical arguments suggest that each individual fault-bounded block suffers a specific sequence of deformation. The style of deformation also depends on the location within the block. This includes: (1) initial shortening at the continental couple (future zone of maximum shortening: ZMS); (2) formation of a conjugate shear fracture system and initiation of orogen-parallel displacement of the decoupled extruding block away from the ZMS; (3) because of the changing width of the escaping block away from the ZMS the style of internal deformation changes within the extruding block: (i) shortening (thrusting, folding), surface uplift at the ZMS; (ii) strike-slip faulting along confining wrench corridors and formation of pull-apart basins at oversteps of en echelon shear fractures; (iii) extension parallel and perpendicular to the displacement vector far away from the ZMS. (4) Finally, the extruding block is gradually overprinted by general, laterally expanding contraction that starts to develop from the ZMS. This inferred sequence of deformation is tested by the Oligocene to Recent development of the Alpine-Pannonian system where late stage formation and extrusion of an orogen-parallel block started during the Oligocene. Stages 2 and 3 developed during Early to Middle Miocene, and final general contraction occurred during Late Miocene to Recent.

The balance between extension and contraction in back-arc basins is very sensitive to a number of parameters related to on-going subduction and collision processes. This leads to complex back-arc geometries, where a lateral transition between crustal blocks with contrasting rheologies is often recorded. One good example is the back-arc region of the Balkanides–Pontides orogens, where lateral variations in rheologies are observed between the Balkanides–Moesian block and the Pontides–Western Black Sea Basin. The latter opened during Cretaceous–Eocene, and has been inverted together with the former starting during late Middle Eocene. The inversion generated contrasting geometries along the orogenic strike, with a narrow zone of high deformation in the Balkanides–Moesia region, wide areas of thrusting with low offsets in the Pontides– Western Black Sea Basin and a transitional zone characterized by highly curved geometries. This overall type of inversion is investigated here by the means of analogue modelling testing the role of inherited crustal geometries during inversion. Our modelling suggests that the contrasting architecture of inverted structures observed in the Balkanides–Pontides domain are the result of pre-existing crustal stretching geometries of various blocks inherited from the Cretaceous–Eocene extension. The stretched and weak back-arc basins can transfer contraction deformation at large distances, explaining structures derived by observational studies. The collisional deformation recorded in the Pontides was transmitted at large distances that are in the range of the contraction structures observed in the centre and northern part of the Western Black Sea. In the light of analogue modelling results we argue that the Western Black Sea was a rheologically weaker domain when compared with the adjacent western onshore at the beginning of the inversion, in contrast with previous results derived from numerical modelling studies that argued for a strong West Black Sea domain at the beginning of inversion.

We use two-dimensional numerical modeling techniques to investigate the thermomechanical consequences of closure of the Meliata-Hallstatt ocean and consequent Cretaceous continent-continent collision in the eastern Alps (Austria). In the modeling a lower plate position of the Austro-Alpine (AA) continental block is adopted during collision with the Upper Juvavic-Silice block. The thermal structure of the lithosphere was calculated for major AA tectonic units (Upper, Middle, and Lower Austro-Alpine) by integration of the transient heat flow equation along an approximately NW–SE cross section east of the Tauern Window. Indications of the rheological evolution of the AA were determined by calculating strength profiles at key stages of the Cretaceous orogeny, making use of the thermal modeling predictions combined with rock mechanics data. Cooling in the upper plate and lower greenschist facies metamorphism within footwall parts of the lower Upper Austro-Alpine (UA) plate, related to SE directed underthrusting of the UA beneath the Upper Juvavic-Silice block at circa 100 Ma, were predicted by the numerical model. The observed pressure-temperature path for deeply buried Middle Austro-Alpine (MA) upper crustal units and their subsequent isothermal exhumation are best reproduced assuming a pressure peak at 95 Ma and exhumation rates ranging between 4 and 7.5 mm yr−1. From the modeling results, we deduce that the temperature evolution during eclogite exhumation is primarily dependent on rates of tectonic movements and largely independent of the mode of exhumation (thrusting versus erosion). Furthermore, very rapid postmetamorphic exhumation of southern Lower Austro-Alpine (LA) units is predicted in order to account for subsequent cooling. This is constrained by 40Ar/39Ar data. The cooling paths of MA and LA rocks appear to be primarily controlled by their near-surface positions at the end of the Cretaceous rather than by other processes such as concurrent underthrusting. Upward advection of heat by rapid exhumation induced thermal weakening of the thickened crust. The inferred weakness of the central parts of the orogenic system may play an important role during detachment-related tectonic unroofing, orogenic collapse, and concomitant basin formation (central Alpine Gosau basins).