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Radiogenic isotopes of igneous and detrital minerals from various clastic rocks of mountain belts are used to reveal tectonic and sedimentary processes, which are otherwise difficult to detect. Here, we discuss the results of U–Pb and Lu–Hf zircon systems, and 40Ar/39Ar on detrital white mica in Eastern Alps. Zircon and white mica are chemically and mechanically stable and occur in magmatic, metamorphic and sedimentary rocks. During subsequent metamorphism, zircon is resistant against high temperature, >650 °C (U–Pb) and 900 °C (Lu–Hf). The Lu–Hf zircon system is used as a tracer of initial magma separation from the mantle, and the U–Pb zircon system records magmatic crystallization. The 40Ar/39Ar white mica system is stable up to 400–450 °C dating either formation or cooling after high-grade metamorphism. Detrital U–Pb zircon ages on two major rivers draining the Eastern Alps do not record any sign of Alpine orogeny or metamorphism. Consequently, U–Pb zircon studies can entirely miss the record of collisional orogeny in cool, magma-poor collision orogens. In contrast, 40Ar/39Ar white mica ages record Early and Late Alpine metamorphism but are limited to revealing the pre-orogenic history. U–Pb zircon and 40Ar/39Ar white mica yield different information in provenance studies. In the Eastern Alps, U–Pb zircon dating of magmatic and clastic rocks indicates intense formation of magmatic rocks between 630 and 230 Ma. Felsic rocks dominate the older age groups, and increasingly young mafic rocks were dated, specifically between 265 and 230 Ma. Hf isotopes record increasing juvenile input since ∼630 Ma. Two different groups with respect to Mesoproterozoic depleted mantle ages are shown: (1) one group with a Mesoproterozoic age gap typical for Gondwana-derived units, and (2) a rare group with Mesoproterozoic ages recording a new tectonic element in the Austroalpine basement in Alps. [ABSTRACT FROM AUTHOR]

Along-strike segmentation of orogens raises questions because its causes may predate orogeny in relation to structural inheritance. Here we focus on rift/margin linkage domains and their inversion by using analogue models with image analysis to extract the 3D strain field. Extensional models document, depending on the strike-perpendicular offset and the brittle-crust thickness, three types of rift linkage modes: (1) oblique linkage with early T-fault, (2) strongly-oblique linkage with R-fault network and, (3) transfer-linkage with late Y-strike-slip fault. Analogue model of inverted rift basins is used to analyse the misunderstood tectonic evolution of transition zones in the segmentedWestern Alpine belt. [ABSTRACT FROM AUTHOR]

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization. [ABSTRACT FROM AUTHOR]

Results of a combined petrological, geochemical and geochronological study suggest that metasedimentary rock units in the Krndija region of the Slavonian Mountains, Croatia, were affected by at least three major tectonometamorphic imprints: during the Middle Ordovician (Sardic event), the early Carboniferous (Variscan event), and the Cretaceous (Alpine event). All three metamorphic phases are established by electron microprobe-based in-situ U–Th–Pb dating of monazite grains. The Sardic metamorphic event is additionally confirmed by a precise Lu–Hf garnet-whole-rock isochron age of 466.0 ± 2.3 Ma. Taken together, the data unveil a relatively large and well-preserved piece of the cryptic Sardic orogen in central Krndija, that we name the Kutjevo Zone. A Sardic subduction-related metamorphic event (ca. 540-580 ℃, 8–11 kbar) at ca. 466 Ma is manifested in the mineral paragenesis Ca-rich garnet plus rutile. A low degree of retrograde reequilibration suggests a subsequent fast exhumation. Low-Ca cores in some garnets and staurolite relics record a pre-HP metamorphic event that involves isobaric heating from 570 to 610 ℃ at ~ 7 kbar. We attribute this (so far undated) event to mid-crustal contact metamorphism caused by early Sardic magmatism. Southern parts of Krndija (the Gradište Zone) experienced an (additional?) clockwise PT evolution in Variscan times at ca. 350 Ma. Garnet formed with ilmenite during a PT increase from 580 ℃/5 kbar to 600 ℃/6 kbar and underwent later strong retrograde resorption. Slow Variscan exhumation resulted in andalusite formation at < 550 ℃/ < 3.8 kbar. Penetrative Alpine metamorphism was observed in low-grade phyllites in the north. The lithology and metamorphic history of the Kutjevo Zone is similar to what has been reported from the Sardic Strona-Ceneri Zone in the western Alps. Both areas expose metapelitic (metagreywacke) rocks with a pre-middle Ordovician formation age. These metasedimentary rocks are inter-layered with numerous small amphibolitic units as well as metagranitoids and were likely deposited along the active Gondwana margin, perhaps in a fore-arc position, prior to their subduction during the middle Ordovician. According to recent palaeogeographic reconstructions, both the Kutjevo Zone and the Strona-Ceneri Zone have once resided in an eastern sector of the northern Gondwana margin (i.e., in E-Armorica). We conclude that in the Middle Ordovician, important subduction activities took place in this E-Armorican segment of north Gondwana, which is today exposed in the Alps. The W-Armorican segment of north Gondwana (now exposed in the French, German, and Czech Variscides) had probably already mutated from a (Cadomian) subduction setting to an extensional (transtensional–transpressional) setting by the late Cambrian. [ABSTRACT FROM AUTHOR]

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The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt. [ABSTRACT FROM AUTHOR]

The Late Cretaceous orogeny followed by the Eocene collision of the Adriatic with the European plate dissected the Northern Calcareous Alps (NCA) by a number of well-studied strike-slip fault systems accommodating N-S shortening and E-W stretching. However, the post-Miocene fault activity is poorly constrained due to lack of Neogene faulted sediments, and glacial erosion of geomorphic indicators. Using the protected environment of caves, we fill the knowledge gap in the post-Miocene evolution of the NCA by paleostress analysis of 172 reactivated faults that offset passages in 28 caves near major faults. Constrained maximum age of caves, our results indicate that the NCA have been subjected to N to NE trending compression since Pliocene. Faulted speleothems dated with 230Th/U method, indicate that the recorded present-day stress state did not significantly change during the last 0.5 Ma. In contrast to the previously proposed post-Miocene N-S extension of NCA, but in agreement with what was observed in Vienna and Pannonian basins, we conclude that the eastward extrusion resulting from N-S convergence has continued despite a distinct slowdown of plate tectonic velocities in the late Miocene. The N-S extension affected only the Alpine front during Pliocene Molasse basin inversion, while at the scale of the Alpine orogen the NCA underwent successive N-S shortening and E-W stretching. [ABSTRACT FROM AUTHOR]

Recent studies revealed that the calc-alkaline metagranitoids of the Seckau Complex comprise both (1) a Late Cambrian to Early Ordovician and (2) a Late Devonian to Early Carboniferous (early Variscan) intrusive complex. The older rocks of the Hochreichart Plutonic Suite reflect I to S-type affinity and are peraluminous and characterized by a general decrease in TiO2, Al2O3, MgO, CaO, P2O5, FeOt and MnO with increasing SiO2. Chondrite-normalized rare earth element (REE) plots display a slight enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) as well as negative Eu anomalies ((Eu/Eu*)N = 0.15- 0.77). The whole-rock initial 87Sr/86Sr ratios calculated back to the time of emplacement (~ 496 Ma) vary between 0.7056 to 0.7061. The early Variscan rocks of the Hintertal Plutonic Suite can be subdivided into (a) the meta- to peraluminous granodioritic suite of the Pletzen Pluton and (b) the peraluminous granitic suite of the Griessstein Pluton. The Pletzen Pluton shows typical magmatic fractionation trends for most of the major oxides and trace elements plotted against SiO2. On a chondrite-normalized diagram, metagranitoids are strongly enriched in LREE and show no significant negative Eu anomaly. Metagranitoids of the Griessstein Pluton have a more peraluminous character and similar major and trace element fractionation trends compared to the Pletzen Pluton. However, the contents in SiO2, major and trace elements clearly point towards a more evolved melt with generally lower TiO2, Al2O3, MgO and CaO values and higher K2O content. Metagranitoids of the Griessstein Pluton are additionally characterized by a slight negative Eu anomaly of about 0.81 on a chondrite-normalized REE plot. Initial 87Sr/86Sr values calculated back to the time of emplacement (~ 353 Ma) of the Pletzen Pluton and the Griessstein Pluton vary between 0.7051—0.7061 and 0.7054—0.7063, respectively, and suggest the same magmatic source for both units. Application of rhyolite-MELTS modelling to the Hintertal Plutonic Suite revealed that the Griessstein Pluton formed by fractional crystallization (~ 30%) from the more primitive Pletzen Pluton. Our geochemical data from the Hochreichart Suite granitoids suggest the existence of a Late Cambrian to Early Ordovician active margin with its remnants now exposed in the Seckau Complex. The early Variscan grainitoids of the Seckau Complex are inferred as part of a magmatic arc along the southern Bohemian active continental margin that was related to the subduction of differently termed oceanic domains (Galicia-Moldanubian Ocean or Paleotethys), prior to the final collision of Gondwana and Laurussia. The general paleogeographic position of the Seckau Complex during the Variscan orogeny is considered to be south to southeast of the Bohemian Massif, adjacent to the eastern Hohe Tauern, the Schladming Tauern, and the Western Carpathians. [ABSTRACT FROM AUTHOR]

High‐ and ultrahigh‐pressure rocks occur in the Austroalpine Nappes in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east. Garnet growth during pressure increase was dated using Lu‐Hf chronometry. The results range between c. 100 and 90 Ma, indicating a short‐lived period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from the northwest to the southeast, where the rocks experienced ultrahigh‐pressure metamorphism. The P‐T conditions of the eclogites generally lie on the 'warm' side of the global range of subduction‐zone metamorphic conditions. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe‐Koralpe area derived from widespread gabbros formed during Permian to Triassic rifting. In the Texel Complex garnets showing two growth phases yielded a Variscan‐Eoalpine mixed age indicating re‐subduction of Variscan eclogite‐bearing continental crust during the Eoalpine orogeny. Jurassic blueschist‐facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite‐facies metamorphism in the Austroalpine are separated by a time gap of c. 50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events. Thus, we propose that subduction initiation was intracontinental at the site of a Permian rift. [ABSTRACT FROM AUTHOR]

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction. [ABSTRACT FROM AUTHOR]

The Eocene represents the last greenhouse interval before Present. The maximum warming during the early Eocene was followed by a long‐term cooling trend culminating in the Antarctica glaciation at the base of the Oligocene. Superimposed on this long‐term cooling trend there is a prominent transient warming event known as the middle Eocene climatic optimum (MECO) occurring during the early Bartonian. The carbonate ramp succession cropping out in the Argentina Valley (Maritime Alps) offers new insights on the evolution of shallow water realms during this time interval. This ramp displays two main facies belts, middle and outer ramp. The middle ramp is recorded by larger benthic foraminifer floatstone to rudstone, passing to Solenomeris branches and nodule floatstone to rudstone evolving to branching coralline algal floatstone. The outer ramp is dominated by bioturbated marly wackestone to packstone alternating with larger benthic foraminifer floatstone with a silty matrix. The investigated ramp was affected by continuous dispersion and reworking of the skeletal components as other Tethyan Eocene ramps. During the Eocene, the Alpine foreland was influenced by fine terrigenous input controlling the trophic conditions and promoting seawater stratification and the development of a strong pycnocline, for which many perturbations could propagate as internal waves. The reworking of skeletal components of the ramp has been ascribed to the action of internal waves. The switch of carbonate production from a carbonate factory dominated by larger benthic foraminifera to a factory in which the encrusting foraminifer Solenomeris was the main carbonate producer biota, is indicative of a radical change in palaeoenvironmental conditions affecting the early Bartonian. The acme of Solenomeris often coincided with the crisis of carbonate producers during intervals of an evident deterioration of environmental conditions. In this case the acme is probably related to the adverse conditions linked to the (MECO) warming event. Finally, the drowning of the nummulitic ramp has been caused by light reduction for the photo‐dependent biota due to progressively increasing depth linked to flexural subsidence of the foreland plate, and minor efficiency of the aphotic carbonate factory. [ABSTRACT FROM AUTHOR]

The age and P-T conditions of Variscan high-P(HP) metamorphism in the Palaeozoic basement of the western Alps remain poorly constrained, but is nevertheless crucial to build a consistent tectonic scenario for the southeastern domain of the Variscan Belt. We report here the results of a structural, petrological, thermobarometric, and geochronological investigation of an eclogite-bearing unit exposed in the northeastern part of the Belledonne Massif (France). This unit is mostly composed of metasediments that are locally migmatized and contain decametre-to hectometre-scale lenses of orthogneiss and amphibolites. SIMS U-Pb dating of magmatic zircon cores in two retrogressed ecogites yields ages at 456 ± 4 Ma and 448 ± 6 Ma, which are interpreted to date the emplacement of the magmatic protoliths. The peak pressure stage in the retrogressed eclogites is estimated to be >1.4 GPa at 690-740° C, and was followed by decompression from 1.4 to ca. 1.0 GPa at 700-800° C. By contrast, the investigated migmatitic metasediment does not present any trace of HP metamorphism, but instead preserves prograde evolution from sub-solidus conditions (ca. 0.8-1.1 GPa and 600-700° C) to supra-solidus conditions (1.1-1.4 GPa and 700-780° C). A later stage of retrogression below ca. 0.5-0.8 GPa and 570-610° C is recorded in both lithologies, and is taken to indicate cooling and exhumation to upper crustal levels. Metamorphism was roughly coeval in the retrogressed eclogites and in the migmatitic metasediment. Metamorphic zircon rims yield U-Pb dates scattering between 340 and 310 Ma in both lithologies. In the migmatitic metasediment, a distinct younger age at 306 ± 3 Ma is interpreted to represent late stages of melt crystallization. In the retrogressed eclogites, zircon zoning and chemical composition (Th/U and REE) indicate initial crystallization of the rims during the HP stage followed by protracted growth during decompression to granulitic/amphibolitic conditions. Rutile U-Pb dating in one eclogite sample yields an age of 340 ± 11 Ma similar to the oldest zircon ages and is interpreted to approximate the age of the peak pressure metamorphism. Retrogression in the amphibolite facies is correlated with the development of a penetrative, N30° subvertical mylonitic S2 foliation. Regionally, this deformation occurs in a dextral transpressive corridor interpreted to represent a crustal-scale shear zone active during the mid-late Carboniferous. We therefore suggest that this structure has driven the exhumation of eclogites from the lower crust and their mixing with mid-crustal felsic lithologies devoid of HP assemblages. [ABSTRACT FROM AUTHOR]

Mesoscopic structural measurements near the top and bottom of the Pennine Zone in the Central Alps of eastern Switzerland indicate multiple, spatially heterogeneous directions of Tertiary movement relative to the Austroalpine allochthon above and the Helvetic zone below. At the top of the Pennine Zone in the Oberhalbstein Valley, motion varies mainly from top-E to top-SSE. At the bottom of the Pennine Zone in the Val Lumnezia area, Ultrahelvetic units exhibit distributed top-NW and top-N shear overprinted by relatively brittle top-NE shear localized just beneath the contact with Penninic units in the Peidener shear zone, which we interpret largely to postdate juxtaposition of Penninic and Helvetic units. Where observed in the Chur Rhine Valley, just 35 km ENE of Val Lumnezia, movement within the basal Pennine units is exclusively top-N. The contrast in movement directions, from top-N to top-NW at the base, to top-E to -SSE at the top, supports the interpretation, drawn from thermochronological data, that the Pennine Zone was tectonically interposed between Adria and Europe as a 20-km-thick "piston" or "mega-pip" from ca. 29 to 18 Ma, driven by its buoyancy contrast with surrounding deep crust and mantle. Emplacement occurred after "docking" of Adria with cratonic Europe at ca. 35 Ma (i.e., continent-continent collision), raising the question of whether the formation of Alpine nappe structure, high Alpine topography, and the peripheral Molasse and Lombardy basins require significant coeval plate convergence. [ABSTRACT FROM AUTHOR]

New whole-rock geochemical and coupled U–Pb and Lu–Hf LA-ICP-MS zircon data of metasedimentary rocks of the Austroalpine, South Alpine and Penninic basement domains are presented, to disentangle the pre-Variscan tectonic evolution of the proto-Alps. The studied units seem to record distinct stages of protracted Late Ediacaran to Carboniferous tectonosedimentary processes prior to the Variscan collision. In the case of Austroalpine and South Alpine units, nevertheless, no major differences in terms of provenance are observed, since most detrital zircon samples are characterized by a major Pan-African peak. Their detrital zircon spectra record a provenance from the northeastern Saharan Metacraton and the Sinai basement at the northern Arabian-Nubian Shield, being thus located along the eastern Early Paleozoic northern Gondwana margin, whereas sources located further west are inferred for the Penninic Unit, which might have been placed close to the Moldanubian Unit of the Bohemian Massif. In any case, it is thus clear that the Alpine basement remained in a close position to the Gondwana mainland at least during the Early Paleozoic. The Late Ediacaran to Silurian tectonic evolution, which includes Cadomian and Cenerian tectonometamorphic and magmatic processes, seem thus to record a continuum related to a retreating-mode accretionary orogen, with diachronous back-arc basin opening and possibly discrete compressional/transpressional pulses linked to changes in subduction zone dynamics. On the other hand, it is inferred that the Alpine basement essentially comprises Pan-African metasedimentary and subordinate metaigneous rocks, possibly with very few Early Neoproterozoic relics. This basement was significantly reworked during the protracted Paleozoic orogenic evolution, due to anatexis and/or assimilation by mantle-derived juvenile magmatism. [ABSTRACT FROM AUTHOR]

The Simplon normal fault in the Western Alps caused tens of kilometers of orogen-parallel extension during convergence of the European and Adriatic plates, but the slip rate of the fault and the time when normal faulting ended are still debated. Here, we constrain the slip history of the Simplon fault with low-T thermochronology and thermo-kinematic modeling. Closely spaced samples from an elevation profile in the center of the fault yield zircon (U–Th)/He ages (ZHe) that are nearly invariant over an altitude of 1.4 km and cluster around ∼6 Ma. In contrast, apatite (U–Th)/He ages (AHe) increase with altitude from 3.4 ± 0.3 to 4.6 ± 0.7 Ma, while the AFT ages range from 4.4 ± 0.7 to 5.8 ± 1.5 Ma. In addition, recently published 40Ar/39Ar ages constrain that our samples moved through the brittle-ductile transition (i.e., ∼300 °C) at 8–10 Ma. Our thermo-kinematic inverse modeling shows that these age data can be explained by a single phase of normal faulting, which lasted from 19.8 ± 1.8 to 5.3 ± 0.3 Ma and caused 45 ± 10 km of extension. The slip rate of the 30°-dipping model fault is 3.5 ± 0.3 km/Myr and equivalent to an exhumation rate of ∼1.8 km/Myr. Our modeling reveals that the altitude-dependent difference between ZHe and AHe ages reflects the thermal relaxation after faulting stopped at ∼5.3 Ma. Since then, exhumation by erosion continued at a rate of ∼0.5 km/Myr. Remarkably, the end of slip on the Simplon fault coincides with the cessation of reverse faulting at 6 ± 2 Ma in the external crystalline massifs of the Alps (Aar, Mont Blanc, Aiguilles Rouges) and with a decrease in strain rate by one order of magnitude at 5-4 Ma in the Swiss molasse basin and the Jura mountains. This temporal coincidence suggests that normal faulting in the internal part of the Alps ceased when plate convergence waned and the under-thrusting of European continental lithosphere beneath the Adriatic plate came to an end. • We constrain the slip on the Simplon normal fault with thermochronology and modeling. • The fault accommodated 45 ± 10 km of orogen-parallel extension in the Western Alps. • The low-angle fault was active between 20 ± 2 and 5.3 ± 0.3 Ma at a rate of ∼3.5 km/Myr. • Waning plate convergence between Europe and Adria at ∼5 Ma caused the end of faulting. [ABSTRACT FROM AUTHOR]

Buoyancy forces associated with subducting lithosphere control the dynamics of convergent margins. In the postcollisional stage these forces are significantly reduced, yet mountain building and seismicity are ongoing, albeit at lower rates. We leverage advances of a newly developed seismo‐thermo‐mechanical modeling approach to simulate tectonic and seismicity processes in a self‐driven subduction and continental collision setting. We demonstrate that the rearrangement of forces due to slab breakoff, in the postcollisional stage, causes bending and rollback of the residual slab, suction forces, and mantle traction at the base of the upper plate, while stress coupling transfers to the shallow crust. Our results provide an explanation for the postcollisional evolution of the Central Alps, where the so‐called Slab Rollback Orogeny model explains the slow yet persistent upper plate advance, the height of the mountain range, and a seismicity pattern consistent with the different tectonic regimes throughout the orogen. Plain Language Summary: A long‐standing debate in tectonophysics evolves around whether vertical or horizontal forces are the primary drivers of mountain building processes. Here we explore this problem using 2‐D numerical models in a generic subduction and continental collision setting. Our results show how the postcollisional evolution of the orogen is controlled by a slow, but persistent, sinking and bending of the postbreakoff residual slab. The resulting seismicity pattern shows a broad pattern of different style of faulting, which are consistent with the local tectonic regimes. We find good correlations between our numerical results and the previously conflicting tectonic observations in the Central Alps and the adjacent foreland basin. Our results thus support the hypothesis that the postbreakoff remaining slab exerts a first‐order control on the motions and deformations of collisional orogens. Key Points: Two‐dimensional seismo‐thermo‐mechanical modeling is performed to investigate the driving forces during the postcollisional stagesPost‐breakoff residual slab proves sufficient to sustain slab rollback and retreat of the entire orogenThe proposed scenario is consistent with the postcollisional seismotectonic evolution of the central European Alps [ABSTRACT FROM AUTHOR]

The problem of the observed very rapid advection of heat into metamorphic thrust stacks is reviewed. Conductive models relying on the thermal relaxation of a thickened crust will not produce the observed Barrovian (medium temperature, medium pressure) assemblages within some short-lived orogens (e.g., western Ireland and Timor). Studies of the rate and timing of metamorphic mineral growth suggest that this is commonly faster than predicted by thermal relaxation. Barrovian assemblages are localised in some orogens (e.g., the Alps) but extensive in others (e.g., the Himalayas). Metamorphic mineral growth brackets deformation; consequently, slow growth is inconsistent with the rapid uplift of many orogens. Thus, no single mechanism can account for the development of Barrovian assemblages during collisional orogeny. The only mechanisms that can supply large amounts of heat for regional metamorphism quickly (<10 Myr) are: rapidly thinning the lithosphere without stretching it (e.g., by plume thermal erosion, slab drop-off, or delamination); by emplacing magma into the crust (modest deep mafic underplate and (or) very large amounts of mafic and silicic magma emplaced into the middle and upper crust); or obducting hot nappes of arc with a thin ophiolite forearc ("hot iron" mechanism). Frictional and viscous heating produces local rapid heating but not fast regional heating. Back-arc or any kind of lithospheric extension increases the geothermal gradient and heat flow but does not heat rocks up. We suggest that magmatic advection of heat-associated lithospheric thinning or "hot iron" overthrusting of an arc/ophiolite are the primary sources of heat in short-lived orogens. [ABSTRACT FROM AUTHOR]

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

The Alpine Orogen formed as a result of the collision between the Adriatic and European plates. Significant crustal heterogeneity exists within the region due to the long history of interplay between these plates, other continental and oceanic blocks in the region, and inherited crustal features from earlier orogenys. Deformation relating to the collision continues to the present day. Here, a seismically constrained, 3D, structural and density model of the lithosphere of the Alps and their respective forelands, derived from integrating numerous geoscientific datasets, was adjusted to match the observed gravity field. It is shown that the distribution of seismicity and deformation within the region correlates strongly to thickness and density changes within the crust, and that the present day Adriatic crust is both thinner and denser (22.5 km, 2800 kg/m³) than the European crust (27.5 km, 2750 kg/m³). Alpine crust derived from each respective plate is found to show the same trend with zones of Adriatic provenance (Austro-Alpine and Southern Alps) found to be denser and those of European provenance (Helvetic Zone and Tauern Window) to be less dense suggesting the respective plates and related terrains had similar crustal properties to the present day prior to orogenesis. The model generated here is available for open access use to further discussions about the crust within the region. [ABSTRACT FROM AUTHOR]

The Eo-Alpine high-pressure belt in the Austroalpine nappes consists of pre-Mesozoic basement rocks overprinted by eclogite-facies metamorphism during the Late Cretaceous. Parts of this basement were already eclogitized during the Variscan orogeny. Lu–Hf geochronology allowed to identify two high-pressure events in an eclogite body in the Schobergruppe, an Austroalpine basement complex south of the Tauern Window. Two samples from closely neighboring outcrops were studied. Both contain two garnet generations. In one sample, PRI3, garnet belongs almost exclusively to the younger (Alpine) generation with only rare preservation of relic cores. In the other sample, PRI4, Variscan garnet is merely coated by a thin rim of the second, Alpine generation, which is in equilibrium with the high-pressure matrix assemblage. In PRI3, two-point garnet-whole rock ages scatter between ~ 97 and ~ 104 Ma, reflecting Alpine garnet growth with minor contamination by a Variscan component. In PRI4, two-point isochrons yield ages between ~ 300 and ~ 313 Ma. The limited spread in these ages suggests minor contamination by Alpine garnet rims. We propose 97 Ma as the maximum age for Alpine metamorphism, which is close to previously determined ages from other parts of the high-pressure belt, and 313 Ma as a minimum age for Variscan metamorphism. Thermodynamic modelling infers that eclogite-facies conditions were reached in both events; during the Late Cretaceous, these were ca. 1.9 GPa/650 °C. Variscan high-pressure conditions in PRI4 are inferred from the amount of garnet in the sample, which indicates at least 1.6 GPa. We propose that the occurrence of Alpine versus Variscan garnet in eclogites depends on the intensity of Variscan and post-Variscan retrogression: the more prograde, Variscan garnet was removed during retrogression, the more garnet grew during the Alpine cycle. [ABSTRACT FROM AUTHOR]

Thermodynamic modeling of compositionally mapped microdomains and whole-rock compositions is used to constrain the pressure–temperature (P–T) evolution of sapphirine granulites and migmatitic paragneisses from the Gruf Complex of the Central Alps. The P–T paths and conditions estimated from granulite microdomains and whole-rock compositions are consistent with one another, indicating that the estimates from both types of compositions are accurate. The sapphirine granulites were heated to ultra-high temperature conditions of 900–1000 °C and 7.0–9.5 kbar as they were decompressed from ca. 800 °C and 9–12 kbar, resulting in garnet breakdown. In a subsequent step, nearly isothermal decompression led to the development of cordierite-bearing coronae and symplectites. By ca. 27 Ma, the sapphirine granulites had been exhumed to the midcrustal level of the migmatitic paragneisses, which were undergoing peak metamorphism at ca. 675–750 °C and 5–7 kbar. These results are consistent with a geodynamic model that invokes heat advection to the lower crust closely following the continental-subduction (ultra-high pressure) stage of the Alpine orogeny. The most plausible geodynamic model consistent with the results of this study is breakoff of a southward subducting lithospheric slab, resulting in asthenospheric mantle flow. [ABSTRACT FROM AUTHOR]

The pre-Alpine metamorphic history of basement rocks of the Alpine Betic-Rif orogen is still uncertain as a result of intense Miocene reworking. Reaction sequences and textures recorded in pre-Mesozoic quartz-K-feldspar-plagioclasebiotite-muscovite anatectic gneisses of the Upper Alpujarride units underwent a complex polymetamorphic history. U-Pb SHRIMP dating of magmatic zircons from coarse-grained porphyritic gneisses indicates an anatectic event at 286±11 Ma. Refractory pelitic enclaves (kyanite-rutile-andalusite-ilmenite-biotite-muscovite±staurolite, devoid of quartz and feldspars) included in the gneisses formed after partial melting of pre-Permian metasediments at relatively high pressure (P). This was followed by late Variscan decompression, producing andalusite at low P in the enclaves and the granitic gneiss. During the Alpine Orogeny, the enclaves were affected by diverse local reactions: (1) andalusite was transformed to Alpine kyanite from reactions catalyzed bymuscovite and biotite; (2) garnet coronas formed close to the gneiss, together with phengitic muscovite; and (3) Variscan biotite in contact with garnet reequilibrated under high/ medium-P and medium-temperature metamorphic conditions. This reaction sequence occurring in polymetamorphic rocks allows the unraveling of their polyorogenic history and the assignment of relative ages to minerals and mineral assemblages through integrated field, mineralogical, petrological, and geochronological analyses. [ABSTRACT FROM AUTHOR]

The Zafarghand porphyry copper exploration district is located in the NE of Isfahan (Central Iran), about 15 km SW of the city of Ardestan. This area consists of Eocene extrusive and Miocene intrusive rocks. The Eocene ( ca. 56 Ma) units include andesites, dacites and rhyodacites spatially associated with pyroclastic materials (ignimbrites and pyroclastic tuff breccias), which are intruded by a Miocene (26-19 Ma) microgranular I-type granodiorite. The entire igneous complex is intersected by NW-SE to E-W-trending andesitic and dacitic dykes and N-S-trending andesitic basalt dykes. Both the Miocene granodiorite and Eocene extrusives have undergone several hydrothermal alteration stages: (i) potassic, (ii) phyllic, (iii) argillic and (iv) propylitic alteration. The geochemical data of these formations are consistent with (i) the modal mineralogy derived from petrographic analysis of the samples and (ii) the regional tectonic setting of the samples having been emplaced in an active subduction-related continental volcanic arc setting during the Zagros Orogeny. Copper mineralization in the Zafarghand area shows an average grade of 0.4% Cu, 200 ppm Mo, 0.2 ppm Ag and 0.01 g/t Au and a maximum age of 21 Ma, which implies that copper mineralization was synorogenic, coeval with the Alpine-Himalayan orogeny in this region. Copper mineralization occurs as chalcopyrite and bornite minerals as disseminated and stockwork mineralization associated with remobilized late epithermal vein mineralization. Chlorite geothermometry for the potassic and propylitic alteration zones yields a temperature range of 361-471 and 196-266 °C, respectively, which implies copper mineralization in a cooling synorogenic magmatic arc environment. Chlorite grains in these alteration zones indicate a classification of chlorites mainly as ripidolite, pycnochlorite, brunsvigite and diabantite. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In complex plate-boundary settings, a reliable 3-D geophysical characterization of the deep tectonic structure is a fundamental starting point for a breakthrough in the analysis of processes controlling plate subduction and (U)HP rock exhumation. The Western Alps host one of the best-studied fossil subduction zones worldwide, with a well-defined deep structure in 2-D based on recent geophysical experiments. However, a full 3-D characterization of its deep tectonic structure is still lacking. Here we present a series of new receiver function cross-sections across the northern and southern Western Alps, validated and complemented by a S-wave velocity model from ambient-noise tomography that provides additional constraints between the profiles. We document a marked change in Moho attitude from the northern Western Alps, where the eastward-dipping European Moho reaches ∼45 km depth beneath the Gran Paradiso dome, to the southern Western Alps, where the European Moho reaches ∼70 km depth beneath the equivalent Dora-Maira dome. This change in Moho attitude takes place over a few tens of kilometers and was likely emphasized by deformation of the slab during subduction. The Western Alps subduction wedge is much thicker in the south than in the north, and the mantle-wedge rocks are deeply involved in orogeny exclusively in the south, where coesite is found in continental (U)HP rocks at several locations. Our detailed information on the 3-D structure of the subduction wedge provides first-order constraints for the next-generation of thermo-mechanical numerical models and may help explain the lateral variations in exhumation style revealed by the geologic record. • New receiver function sections from a recent seismic experiment across the NW Alps. • 2D receiver functions profiles complemented by a 3D Vs model from ambient noise. • Strong change in Moho attitude between NW and SW Alps. • The Western Alps subduction wedge is much thicker in the south than in the north. • A steeper Moho beneath locations where coesite is found in continental UHP rocks. [ABSTRACT FROM AUTHOR]

An important part of the pre-Mesozoic basement of the Alps is the product of orogenic processes occurred in Ordovician times. Most of the geological constrains for this reconstruction have been obtained by the study of the Strona-Ceneri zone in the Southern Alps, where Alpine overprint was weak. The erosion of late Pan-African belts delivered large amounts of greywackes and pelites into subduction zones along Gondwana. The sediments were subducted and accreted to form wide complexes. As a result of the large sediment input, subduction retreated and the mantle-derived magmas intruded the base of the fertile 'mud pile.' This initiated substantial anatexis to produce peraluminous magmas, which intruded and extruded syntectonically. Thereby, the predominantly steeply structured subduction-accretion complex provided ideal pathways for the uprising magmas, and down-thrusting host rocks to result in an isostatically stable crust. The SE Australian Lachlan fold belt is interpreted as an upper crustal analog of the Strona-Ceneri zone. Based on the combination of their geology, a crustal profile through an Alaskan type of orogen is drawn. [ABSTRACT FROM AUTHOR]

The eclogitic Adula nappe of the Central Alps (cantons Graubünden and Ticino, Switzerland) displays an exceptionally complex internal structure with the particularity of enclosing numerous slices of Mesozoic cover rocks (Internal Mesozoic) within the Palaeozoic gneiss basement. This study is principally based on detailed lithological and structural mapping of selected areas of the northern Adula nappe. Specific focus was placed on the Mesozoic slivers embedded in pre-Mesozoic basement (Internal Mesozoic). The most pervasive structures are related to the Zapport deformation phase that is responsible for the development of a fold-nappe and ubiquitous north-directed shear. Locally, the structures in the upper and frontal part of the nappe can be assigned to the older ductile Ursprung phase. These earlier structures are only compatible with top-to-S shear movement. The superposition of the Ursprung and Zapport phases is responsible for the north-dipping internal duplex-like structure and the sliced aspect of the Northern and Central Adula nappe. We conclude that the Adula nappe represents a major shear zone involving the entire nappe and responsible for the emplacement of the Lower Penninic sediments and the Middle Penninic nappes in the eastern part of the Lepontine Dome. [ABSTRACT FROM AUTHOR]

Necking zones, located between the proximal and distal domains of passive margins, are characterized by important crustal thinning and the occurrence of extensional detachment faults. Here we provide a petrographic, microstructural and geochemical study of an extensional detachment fault discovered in the Mont-Blanc massif, which was located in the necking zone of the European margin during Early Jurassic rifting. The footwall of the fault consists of basement gneisses, while its hanging wall presents pre-to syn-rift deposits. A careful observation of the basement/cover contact also reveals the presence of a strain gradient affecting the Variscan gneisses, which are progressively deformed to cataclasite and to black gouges at the top basement, and sealed by undeformed Jurassic syn-rift sandstones, the Grès Singuliers, which rework fault rock pebbles. Microscale observations also indicate fluid-assisted deformation, characterized by the total breakdown of feldspars and the coeval formation of white mica, leading to strain softening along the fault plane. The overall observations point toward a rift-related detachment fault responsible for the exhumation of the internal Mont-Blanc basement, an interpretation which challenges previous models of crustal exhumation in the External massifs of the Alps. Finally, the preservation of an undeformed syn-rift sedimentary cover sealing a tectonized top basement makes this area a fossil analogue for detachment faults in necking domains. It demonstrates that preservation of pre-Alpine, rift related structures is possible even in highly deformed areas, and emphasizes the role played by pre-Alpine inheritance on the building of the Alpine Mountain belt. • Description of a Jurassic rift-related normal fault in the internal Mont-Blanc massif. • Syn-rift deposits lying conformably above the exhumed fault plane indicate that the fault was only slightly dipping at the surface, consistent with a detachment fault. • Deformation occurred in the sub-greenschist facies, and led to major crustal exhumation up to the surface during the lower Jurassic. • This pre-alpine structure was transported passively during alpine orogenesis, and thus preserved from significant alpine deformation. • The studied fault provides one of the first fossil analogue of the necking domain of a rifted margin, where both the syn-rift deposits and the basement are preserved. [ABSTRACT FROM AUTHOR]

The Oisans–Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460–550 °C and 0.4–1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ± plagioclase ± amphibole ± biotite and yield peak-P conditions at 650–730 °C and 1.5–1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800–870 °C and 0.6–0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ± 5 Ma. Timing of the prograde to HP evolution is estimated around 345–330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans–Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation. • Prograde evolution is recorded by Variscan high-P granulites in the western Alps. • High-P conditions of ca. 1.5–1.7 GPa and 650–730 °C were reached around 345–330 Ma. • High-T overprint at ca. 800–870 °C and 0.6–0.9 GPa occurred during decompression. • The prograde apparent geotherm is inconsistent with a cold subduction setting. [ABSTRACT FROM AUTHOR]

Eduard Suess recognized the unusual size and internal tectonic configuration of the mountainous terranes of Central Asia that he termed Altaids and which differ considerably from curvi-linear foldbelts such as the Alps and Urals. However, he did not realize that this orogenic domain or province evolved over a period of some 750 Ma from the late Mesoproterozoic (ca. 1000 Ma) to the late Permian (ca. 260 Ma), contains large volumes of pre-orogenic crust as well as many synorgenic metamorphic complexes, and exhibits many structures similar to modern thrust-and-fold belts. We therefore do not favour the name Altaids for this large orogenic domain because it is associated with the concept of an essentially Palaeozoic evolution and certain features that have since been shown to be incorrect. The non-genetic term Central Asian Orogenic Belt (CAOB) seems to be more appropriate and characterizes one of the largest accretionary terranes on Earth whose evolution has many similarities with the tectonics of the Indonesian Archipelago. In contrast to popular thinking, crustal growth in the CAOB during accretion was not anomalously high but was similar to that in the Palaeozoic Tasmanides of Australia and the Jurassic to Present evolution in southeastern Asia. [ABSTRACT FROM AUTHOR]

In his treatise "Die Entstehung der Alpen", Eduard Suess presents a wealth of geological observations and a fascinating conception of the orogeny of the Alps and mountain chains in general. It is an inspiring task to review the state of the Physics of the Earth at the time when Suess worked on this treatise and to estimate its impact on Suess' work. We find that only seismology, a discipline to which Suess himself contributed a substantial amount of research, supported his geological work and tectonic reasoning to some extent. Since the publication of "Die Entstehung der Alpen", Physics of the Earth has made tremendous progress and geophysical data and models have become of fundamental importance for tectonic theories. We review this evolution as well as modern geophysical data closely related to the orogeny of the Eastern Alps. We consider fundamental observations and findings of Suess and relate them to modern geophysical data on the structure and kinematics of the lithosphere. Issues we wish to highlight are the significant asymmetry and arcuate shape of mountain ranges and in particular the Alpine system. Suess related these observations to absolute movements of crustal blocks. In the case of the Alps, an Adriatic crustal block moves from south toward the axes of the mountain range. For the Himalayas, the direction is opposite and the Asian crust moves from north towards India. Suess' conception is consistent within the frame of the contraction theory, generally accepted at that time. We interpret the moving crustal block sensu Suess as the upper plate of a continent-continent collision. We show that this transformation of Suess' conception to a platetectonic frame is consistent with modern geophysical data and models of the lithosphere and upper mantle. Suess' ideas would require a uniform subduction of European continental lower lithosphere under the Adratic plate to be realizable. Suess points out that the architecture of the Southern Alps does not follow the general scheme of one-sided northward movement. We show that also a modern model of the lithospheric structure and kinematics addresses this observation and needs a minor modification of the general plate tectonic principles, i.e. some north-south oriented compression of the Adriatic plate. Finally we emphasize Suess' considerations regarding the tectonic situation south-east of the Bohemian promontory. Here the axis of the mountain range bends to the northeast and, according to Suess, the moving crustal masses are deflected to the east, thus introducing tension and forming the Vienna Basin. We value this interpretation as an ancestor of the extrusion model. We address the same issue on the basis of the modern geophysical data and present a tectonic model of the lithospheric mantle, which explains extrusion as a plate tectonic process. [ABSTRACT FROM AUTHOR]

Based on scattered earlier observations of other researchers and his own fieldwork, Eduard Suess analyzed the tectonic origin of the Alps and developed a number of basic concepts in his influential book "Die Entstehung der Alpen ("The Origin of the Alps"; published in 1875) and some concepts are still applied today. Suess based his ideas on the tectonic development of the Alps largely on the interplay between field observations and comparison with other mountain ranges. He therefore introduced and used a specific comparative methodology in the field of tectonics. The tectonic structure of the Alps, which he finally compiled in a map in year 1909, is quite similar to the present-day tectonic concept. The further development of ideas on the structure and origin of the Eastern Alps can be characterized by refinement of stratigraphic and structural observations, application of new tectonic concepts like plate tectonics since the early Seventies of the last century, by application of specific newly-developed geochronological and petrological methods mainly on metamorphic rocks, and then by increasingly detailed geophysical observations on the deep structure, and finally by application of experiments and numerical modeling. All these latter methods, specifically when validated by careful field observations and integrated into current knowledge, resulted in increasingly deeper understanding of the crustal and lithosphericscale structure and the tectonic development of the Eastern Alpine orogen. However, the present knowledge is still far from a full three-dimensional understanding of the overall structure and of the mountain building processes behind. The main reasons for this, aside from the lack of observations, are complications resulting from the oblique convergence and plate collision as well as by the lateral retreat of the subduction acting on a remnant land-locked basin in the eastern lateral extension of the Eastern Alps. [ABSTRACT FROM AUTHOR]

In all reconstructions published during the last two decades, the Austroalpine and the correlative Southalpine basement units of the Eastern Alps were considered to represent a uniform continental block that split off from the northern Gondwana margin during Early Paleozoic times and collided with microcontinental blocks during the Variscan orogeny in the early Late Carboniferous. Afterwards, these units formed finally the outboard part of the European Variscides adjacent to the Paleotethys Ocean. The combined Austroalpine/Southalpine basement extends to the Western Carpathians, contains Ediacaran and Early Paleozoic ophiolites and magmatic arcs, Devonian passive margin successions and represents a key region for resolving the Late Neoproterozoic to Late Paleozoic tectonic evolution of the basement in the Alpine-Mediterranean Mountain belts. The Austroalpine and Southalpine basement contains well-known fossil-rich Ordovician and Silurian rift and mainly Devonian passive margin successions summarized as the Noric and Carnic domains, which were juxtaposed to amphibolite-grade metamorphic complexes during Early Carboniferous plate collision. In the metamorphic units the following main stages of tectonic evolution are: Two distinct Ediacaran to Cambrian arc systems were recognized, correlating with subduction of the Prototethys (Ran) Ocean. The continental Wechsel Arc stopped its activity during Late Cambrian times, whereas the Silvretta-Gleinalpe Arc was reactivated at the Devonian/Carboniferous boundary during subduction of the Devonian Balkan-Carpathian Ocean. The Prototethyan oceanic crust is preserved in the ophiolitic Upper Neoproterozoic to Middle Ordovician Speik Complex, that was obducted onto the Silvretta-Gleinalpe Arc during Late Ordovician to Early Silurian times. On the other hand, the Noric domain was initially part of the northern Gondwana margin and includes a virtually continuous sedimentary section ranging from the Early Ordovician to earliest Pennsylvanian. It started with an Early to Late Ordovician rift succession with mafic and acidic volcanic rocks related to rifting of parts of the Noric domain from the northern Gondwana margin forming an oceanic basin (Rheic Ocean of previous interpretations) in between and back-arc rifting is the likely setting. In both Noric and Carnic domains, Silurian strata were deposited during a tectonically quiet period followed by onset of a second rifting period during Late Silurian times, which resulted in deposition of thick Devonian carbonates heralding the opening of the Balkan-Carpathian Ocean and separation of the Paleo-Adria microcontinent from Gondwana. Late Devonian–Carboniferous plate convergence led to subduction of this oceanic rift followed by subduction of the Paleo-Adria margin underneath the accreted Variscan convergence belt, collision and Late Carboniferous intramontane molasse deposition. However, new data argues that a third ophiolitic belt, the Plankogel ophiolitic mélange, which formed as part of the Paleotethys Ocean during the Devonian and was reactivated as trench during initial consumption of the Paleotethys Ocean during Late Permian–Triassic times. The Middle-Late Triassic plutonic and volcanic rocks of the Southern Alps are considered, in this preliminary model, to represent the magmatic arc associated with Paleotethys subduction. [ABSTRACT FROM AUTHOR]

Inherited and syn-compressional salt-controlled structures in orogens are difficult to distinguish from compressional features because classical salt features (halokinetic growth strata, salt welds) can be misinterpreted as shortening-related structures (syn-kinematic growth strata, tectonic unconformities). In the Sivas basin in Turkey, syn-kinematic salt diapirism entirely shaped the architecture of the foreland and allowed the development of primary and secondary minibasins. One of the most impressive, the Tuzlagözü minibasin, formed by sinking into a salt feeder while being upturned due to compression. This minibasin has been nicknamed the "Turkish Vélodrome" because of its similarity with the well-known Vélodrome structure in the French Alps which shows growth strata and successive unconformities, commonly interpreted as foreland growth syncline features. Based on structural field descriptions and geometric analogies with the Tuzlagözü minibasin, we revisit the Vélodrome as a salt-related minibasin. We argue that the Vélodrome developed as a secondary minibasin above an Oligocene aerial salt glacier: sheet of Upper Triassic evaporites extruded during Alpine orogeny. This new interpretation enables to propose a revised evolution of the sub-Alpine fold-and-thrust belt. In line with other recent works, this study emphasises once again the important role of salt tectonics in the evolution of the sub-Alpine fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

In the Western Alps, the Piemont-Ligurian oceanic domain records blueschist to eclogite metamorphic conditions during the Alpine orogeny. This domain is classically divided into two 'zones' (Combin and Zermatt-Saas), with contrasting metamorphic evolution, and separated tectonically by the Combin fault. This study presents new metamorphic and temperature (RSCM thermometry) data obtained in Piemont-Ligurian metasediments and proposes a reevaluation of the P-T evolution of this domain. In the upper unit (or 'Combin zone') temperatures are in the range of 420-530 °C, with an increase of temperature from upper to lower structural levels. Petrological evidences show that these temperatures are related to the retrograde path and to deformation at greenschist metamorphic conditions. This highlights heating during exhumation of HP metamorphic rocks. In the lower unit (or 'Zermatt-Saas zone'), temperatures are very homogeneous in the range of 500-540 °C. This shows almost continuous downward temperature increase in the Piemont-Ligurian domain. The observed thermal structure is interpreted as the result of the upper and lower unit juxtaposition along shear zones at a temperature of ~500 °C during the Middle Eocene. This juxtaposition probably occurred at shallow crustal levels (~15-20 km) within a subduction channel. We finally propose that the Piemont-Ligurian Domain should not be viewed as two distinct 'zones', but rather as a stack of several tectonic slices. [ABSTRACT FROM AUTHOR]

Abstract: A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution, from sea-floor spreading through subduction to collision in the Alps. Unlike previous models, which relate the fate of Alpine Tethys solely to relative motions of Africa, Iberia and Europe during opening of the Atlantic, our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record. The motions of these microplates (Adria, Iberia, Alcapia, Alkapecia, and Tiszia) relative to both Africa and Europe during Late Cretaceous to Cenozoic time involved the subduction of remnant Tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces: (1) 131–84Ma intra-oceanic subduction of the Ligurian part of Alpine Tethys attached to Iberia coincided with Eo-alpine orogenesis in the Alcapia microplate, north of Africa. These events were triggered primarily by foundering of the older (170–131Ma) Neotethyan subduction slab along the NE margin of the composite African–Adriatic plate; subduction was linked by a sinistral transform system to E–W opening of the Valais part of Alpine Tethys; (2) 84–35Ma subduction of primarily the Piemont and Valais parts of Alpine Tethys which were then attached to the European plate beneath the overriding African and later Adriatic plates. NW translation of Adria with respect to Africa was accommodated primarily by slow widening of the Ionian Sea; (3) 35Ma–Recent rollback subduction of the Ligurian part of Alpine Tethys coincided with Western Alpine orogenesis and involved the formation of the Gibraltar and Calabrian arcs. Rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable, retreating Adriatic and African slabs during slow convergence of Africa and Europe. The upper European–Iberian plate stretched to accommodate this slab retreat in a very mobile fashion, while the continental core of the Adriatic microplate acted as a rigid indenter within the Alpine collisional zone. The subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the Alps and Apennines, as well as beneath parts of the Pannonian Basin, the Adriatic Sea, the Ligurian Sea, and the Western Mediterranean. The predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10–30%, indicating that parts of slabs may be superposed within the mantle transition zone and/or that some of this subducted lithosphere became seismically transparent. [Copyright &y& Elsevier]

As with many mountain belts globally, the Alps have seen a large increase in sediment yield in the late Neogene. The hypothesis that this increase results from climate-change impacts on erosion rate over the past ~5 Ma is testable, given the extensive work completed in the Alps. Sediment budgets, thermochronology-based cooling rates, and estimates of modern rock uplift and erosion in the Western Alps are reviewed to search for correlation in space and time that might fingerprint climatic events. Major increases in sediment yield are apparent starting in the late Messinian (5.5 Mya) and accelerating toward the present day; this suggests the occurrence of a series of climatic changes that increased erosion rates, even as tectonic processes appear to have slowed or stopped. At present, tight correlations in time are difficult to establish and there remain open questions about the role of tectonics in the coupled system. [ABSTRACT FROM AUTHOR]

Voluminous Eocene gabbros to granites of the Kal-e-Kafi backarc composite intrusion were emplaced prior to the Alpine-Himalayan collision in the central Iranian backarc, but the reasons for precollisional high arc and backarc magmatic productivity (60-53 Ma) are unclear. Diagnostic geochemical signatures are high K2O-Sr contents and successively depleted middle rare earth element (REE) patterns, reflecting a highly metasomatized source and an increasing role for amphibole and garnet (0%-10%) in the relatively younger granites. Release of concealed K-St-rich fluids from oceanic fractures and faults during buckling and bulging of a precollisional choking oceanic slab, and melting of phlogopite-bearing lithosphere with subsequent interaction of the melt with lower crustal garnet-amphibolite of a ~40-km-thick crust, can explain the Kal-e-Kafi geochemical and isotopic signatures. Gravimetric data indicating a ∼39 km present-day backarc crustal thickness are consistent with geochemical results but also imply little if any collisional crustal thickening of the central Iranian Plateau. High Eocene arc-backarc melt flux prior to collision in this region reflects vigorous thermal convection, which may in fact be diagnostic of collisional magmatism, explaining the presence of postcollisional shoshonitic melts in this and other collisional orogenic settings. [ABSTRACT FROM AUTHOR]

Abstract: In the general discussion on the Variscan evolution of central Europe the pre-Mesozoic basement of the Alps is, in many cases, only included with hesitation. Relatively well-preserved from Alpine metamorphism, the Alpine External massifs can serve as an excellent example of evolution of the Variscan basement, including the earliest Gondwana-derived microcontinents with Cadomian relics. Testifying to the evolution at the Gondwana margin, at least since the Cambrian, such pieces took part in the birth of the Rheic Ocean. After the separation of Avalonia, the remaining Gondwana border was continuously transformed through crustal extension with contemporaneous separation of continental blocks composing future Pangea, but the opening of Palaeotethys had only a reduced significance since the Devonian. The Variscan evolution in the External domain is characterised by an early HP-evolution with subsequent granulitic decompression melts. During Visean crustal shortening, the areas of future formation of migmatites and intrusion of monzodioritic magmas in a general strike–slip regime, were probably in a lower plate situation, whereas the so called monometamorphic areas may have been in an upper plate position of the nappe pile. During the Latest Carboniferous, the emplacement of the youngest granites was associated with the strike–slip faulting and crustal extension at lower crustal levels, whereas, at the surface, detrital sediments accumulated in intramontaneous transtensional basins on a strongly eroded surface. [Copyright &y& Elsevier]

The Alps, as part of the Alpine-Mediterranean Mountain chain, are one of the classical localities for orogenic studies, where the Mesozoic-Cenozoic tectonic evolution is well known, and many classical models have been proposed to explain the tectonic evolution from Mesozoic rifting and breakup to Late Mesozoic-Cenozoic subduction, plate collision, and exhumation. However, the pre-Mesozoic tectonic evolution of the pre-Alpine basement remains poorly known because of the lack of sufficient age data due to complex polyphase deformation and multiple metamorphic overprints. New data from the mainly amphibolite-facies pre-Alpine basement of the Austroalpine mega-unit indicates that this basement is composed of a heterogeneous series of continental units, island arcs, ophiolites, subduction mélanges, accretionary wedges, and seamounts affected by different metamorphic grades. This study presents new results of LA-ICP-MS U-Pb dating and MC-ICP-MS Lu-Hf isotopic tracing of zircons from three key areas of Austroalpine basement, including the: i) Wechsel Gneiss and Waldbach Complexes, and Wechsel Phyllite Unit, (ii) Saualpe-Koralpe-Pohorje, and (iii) Schladming-Seckau areas. We determine the Wechsel Gneiss Complex to be a continental magmatic arc formed during 500–560 Ma in the proximity to a continental block with a 'memory' of Late Archean to Early Proterozoic continental crust. The Wechsel Gneiss Complex has U-Pb ages of 2.1 to 2.2 Ga and 2.5 to 2.8 Ga that indicate a close relationship to northern Gondwana, with depleted mantle Hf model ages as old as 3.5 Ga. The Wechsel Phyllite Unit structurally overlying the Wechsel Gneiss Complex has partly different sources, including juvenile crust formed at ca. 530 Ma. In contrast, the Waldbach Complex constantly added new crustal material during the 490–470 Ma period and bears considerably more positive ε Hf (t) values than the underlying Wechsel Gneiss Complex and gives relatively young, depleted mantle model ages of 700 to 500 Ma. The Waldbach Complex is, therefore, interpreted to be part of a magmatic arc that formed during closure of the Prototethys and was metamorphosed during Variscan orogenic events at ca. 350–330 Ma. The Schladming-Seckau and Wechsel Complexes represent a Cambro-Ordovician magmatic arc system formed by Prototethys subduction processes with the associated Late Neoproterozoic to Early Ordovician ophiolitic Speik complex having formed in its back-arc basin or as Prototethyan lithosphere. The Plankogel Complex and structurally overlying micaschist and amphibolite units represent accreted ocean, ocean island, and continent-derived materials, interpreted to be an accretionary complex formed during the Permo-Triassic closure of the Paleotethys. Many granites of the Austroalpine mega-unit with Permian ages (e.g., porphyric granite called Grobgneiss and other granite gneisses and associated pegmatites) were likely formed in an extensional environment that culminated in the opening of the Middle-Late Triassic Meliata oceanic rift. These granites formed by partial remelting of crust with mainly Middle Proterozoic Hf model ages. Taken all these data together, we find that the Austroalpine basement is heterogeneously composed and includes complexes of different ages, different tectonic evolutionary histories and different remolten sources representing different locations before final accretion. The composite of pre-Alpine complexes in the Austroalpine mega-unit likely assembled not earlier than Late Permian or Early Triassic. [ABSTRACT FROM AUTHOR]

A lithospheric model scaled for density and viscosity was shortened obliquely in front of an indenter whose shape and orientation match those of the South Alpine indenter. Deformation was initially localized into subparallel sets of conjugate transpressive thrusts. These structures evolved into folds, which were strongly amplified and converged closer together during ongoing shortening, finally forming a series of tight antiforms. At the same time, this tightly folded structure locally accommodated significant orogen-parallel extension, attaining 100% in front of the leading edge of the indenter, but only 15% on the scale of the entire model. The internal architecture and fault pattern of the model in map view are very similar to those of the Tauern Window in the Eastern Alps, emphasizing the importance of localization of folding and shortening rather than orogen-parallel extension for its formation. [ABSTRACT FROM AUTHOR]

Ophiolites of the Alpine belt derive from the closure of the Mesozoic Tethys Ocean that was interposed between the palaeo-Europe and palaeo-Adria continental plates. The Alpine orogeny has intensely reworked the oceanic rocks into metaophiolites with various metamorphic imprints. In the Western Alps, metaophiolites and continental-derived units are distributed within two paired bands: An inner band where Alpine subduction-related high-pressure (HP) metamorphism is preserved, and an outer band where blueschist to greenschist facies recrystallisation due to the decompression path prevails. The metaophiolites of the inner band are hugely important not just because they provide records of the prograde tectonic and metamorphic evolution of the Western Alps, but also because they retain the signature of the intra-oceanic tectono-sedimentary evolution. Lithostratigraphic and petrographic criteria applied to metasediments associated with HP metaophiolites reveal the occurrence of distinct tectono-stratigraphic successions including quartzites with marbles, chaotic rock units, and layered calc schists. These successions, although sliced, deformed, and superposed in complex ways during the orogenic stage, preserve remnants of their primary depositional setting constraining the pre-orogenic evolution of the Jurassic Tethys Ocean. [ABSTRACT FROM AUTHOR]

In 2004, first absolute gravity (AG) measurements were performed on the top of Mt. Zugspitze (2 sites) and at the foot (1 site) and top (1 site) of Mt. Wank. Mt. Wank (summit height 1780 m) and Mt. Zugspitze (2960 m) are about 15 km apart from each other and belong geologically to different parts of the Northern Limestone Alps. Bridging a time span of 15 years, the deduced gravity variations for Zugspitze are in the order of −0.30 μm/s2 with a standard uncertainty of 0.04 μm/s2. The Wank stations (foot and top) show no significant gravity variation. The vertical stability of Wank summit is also confirmed by results of continuous GNSS recordings. Because an Alpine mountain uplift of 1 or 2 mm/yr cannot explain the obtained gravity decline at Zugspitze, the dominating geophysical contributions are assumed to be due to the diminishing glaciers in the vicinity. The modelled gravity trend caused by glacier retreat between epochs 1999 and 2018 amounts to −0.012 μm/s2/yr at both Zugspitze AG sites. This explains more than half of the observed gravity decrease. Long-term variations on inter-annual and climate-relevant decadal scale will be investigated in the future using as supplement superconducting gravimetry (installed in 2019) and GNSS equipment (since 2018). [ABSTRACT FROM AUTHOR]

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals. The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab. Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle. • A mafic dyke swarm intruded the Corsica batholith during Permian times. • Permian gabbroic complexes were emplaced from western to eastern Alps. • U-Pb geochronological analyses on zircons yielded ages of about 280 Ma. • Melting of a heterogeneous mantle produced Permian post-collisional magmatism. [ABSTRACT FROM AUTHOR]

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D 1 , D 1 , and D 2. The pre-D 1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D 1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D 2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D 1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D 1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

The Alpine front's External Crystalline Massifs represent exhumation of mid-crustal basement during late-stage continent-continent collision. We unravel the geodynamic evolution of the Northalpine front (Aar Massif) by combining structural and low-T thermochronometric data. A crustal-scale tectonic restoration (Neogene-present) and estimation of associated changes in exhumation rates become possible. The European continental margin experienced multiple switches from horizontal- to vertical- and again to horizontal-dominated tectonics. Early thrust-related inversion of rift basins is followed by a switch to rapid vertical exhumation, with deformation along steep reverse/normal faults. This vertical tectonics is related to a positive buoyancy force of the European crust induced by lower crustal delamination of the Aar Massif during an orogenic rollback process of the European lithosphere (∼22-20 Ma). Differential rock uplift rates of up to 1.3–1.6 km/Myr resulted, which decreased northwards to 0.1 km/Myr. Reducing buoyancy forces (isostatic compensation) led to a horizontal tectonic style again. Persisting compressional forces induced an orogen-scale strain partitioning with dextral transpressive faults in the south and simultaneous crustal-scale northwest-directed thrusting in the north. 'En-bloc' exhumation of the entire Aar Massif in the thrust's hanging wall occurred between 13-5 Ma and 5-0 Ma at rates of 0.5-0.9 km/Myr and 0.6-0.8 km/Myr, respectively. It is this late-stage thrusting in combination with surface erosion, which shaped today's prominent Northalpine front. Switches between horizontal and vertical tectonics might generally be relevant for late continent-continent collisional stages. Here, buoyancy forces of non-thinned continental crust entering the subduction zone cause a strong vertical tectonic deformation style instead of classical thrust tectonics. [ABSTRACT FROM AUTHOR]

The Western Alps show persistent seismicity and deformation despite the end of the orogenesis. The first stations of the RENAG permanent GNSS network were thus installed in the French Alps since the end of the 1990s. Moreover, three temporary surveys took place in 1996, 2006 and 2011 in the Briançon area, ~50x50 km² wide, in the South-Western French Alps, where a steady moderate seismic activity along the Briançon seismic arc is observed. The horizontal velocity field computed from these first three surveys showed an east-west extension in the network, compatible in amplitude and in orientation with the seismic deformation of the area. A fourth campaign was led in 2016, yielding 20 years of observation span. The measures may now reach a sufficient accuracy to assess if the extension found within the Briançon network is localized on any particular tectonic feature. Several faults in this area indeed showed recent activity in normal faulting. Assessing the localization of the deformation may lead to better understand the origin and processes of the present-day deformation of the Alpine belt. To address this issue, the present study aimed at increasing the resolution of both the velocity and the strain field over the network, by exploiting the long measurement span and the exceptional redundancy of our campaign measurements : 1) A robust velocity field was computed from the combination of the different campaign and permanent data. A single strain tensor was then derived, revealing 20±2 nanostrain/yr, which is consistent with previous studies, and also more accurate. 2) Two inversion methods were then compared to assess the distribution of the deformation over the area. Despite their coherence on the orientation of the deformation, the size of the network compared to the small amplitude of the signal did not enable us to derive significant values of strain over the width of the grid cells. 3) We then tried to localize the deformation by fitting a model to a GPS velocity profile perpendicular to the faults. Despite the high uncertainties on the GPS velocities, a two fault model seems to be the best fit to the data. The localization of the potential faults being the best constrained parameter in this forward model, we used it as apriori information in a block modeling in order to compute fault slip rates as the velocities at the interface of the blocks. The velocities on the interfaces are found to be strongly dependent on the model parameters but remain below 1 mm/yr. Thanks to the unusual density of the network and long time span of its instrumentation, we were able to increase the accuracy of the deformation pattern observed through the Briançon area. Our results suggest that the extension is localized on two nearby tectonic features, even if the fault parameters retrieved from our models differ slightly from geological and seismotectonics clues. These results encourage us to believe that the next survey will add sufficient signal-to-noise ratio to achieve the data convergence towards the regional tectonic signal. [ABSTRACT FROM AUTHOR]

The process of continent–continent collision in shaping mountain ranges and adjacent foreland basins has been discussed for many decades. The orogeny driving forces — whether vertical or horizontal — are central for our understanding and it seemed settled in favor of horizontal forces. For the central European Alps, however, this traditional concept fails to explain first-order observations of the mountain belt. Recent stratigraphic, palaeo-altimetry and lithosphere structural evidence suggest that a Slab Rollback Orogeny model is capable of explaining the construction of thick nappe successions and the large-scale evolution of the Swiss Alps (Kissling and Schlunegger, 2018). This new concept questions the traditionally assumed bulldozing effect of Adria indenter as a necessary component of the dynamic system and proposes the buoyancy forces of the postulated post-collisional rollback is sufficient to drive the evolution and to shape the Central Alps as we know them today. Here we explore these challenging questions using a high-resolution, rheologically consistent, two-dimensional visco-elasto-plastic thermo-mechanical numerical model (Dal Zilio et al., 2018), which simulates both tectonic and seismic processes in a subduction and subsequent continental collisional setting. Our numerical experiments reproduce the self-driven stages of oceanic subduction, continent-continent collision, and spontaneous oceanic lithosphere slab breakoff. The post-collisional evolution of the orogen shows how slow — but persistent — flexural bending of the post-breakoff residual slab in combination with the crustal delamination control the most recent stage of the orogen. Bending-related slab suction and coeval development of crust-mantle and intracrustal delamination and nappe stacking through the slab bending zone, leads to the appearance of crustal shortening on the foreland basin, and widening of the orogen. We find good correlations of our numerical results with the previously conflicting tectonic observations in the Central Alps and the foreland (Molasse) basin. In particular, we discuss how the current crustal seismicity pattern implies the occurrence of extensional forces at work beneath the Molasse Basin and within the Alps. Our results thus support the emerging hypothesis that the remaining slab exerts a first-order control on the motions and deformations of the orogen. In light of these results, we propose that the tectonic history of the Central Alps does require an alternative view.Dal Zilio, L., van Dinther, Y., Gerya, T. V., & Pranger, C. C. (2018). Seismic behaviour of mountain belts controlled by plate convergence rate. Earth and Planetary Science Letters, 482, 81-92.Kissling, E., & Schlunegger, F. (2018). Rollback orogeny model for the evolution of the Swiss Alps. Tectonics, 37(4), 1097-1115. [ABSTRACT FROM AUTHOR]

The Eaux-Chaudes massif is a complex south-verging fold and fault structure located in the western Axial Zone of the Pyrenees. The stratigraphic succession consists of Upper Cretaceous limestones and shales lying unconformably on Paleozoic basement or Triassic pods of Buntsandstein, and featuring inliers of Keuper facies and ophites. The structure has been interpreted as a duplex in Cretaceous rocks with a roof thrust carrying allochtonous Paleozoic rocks, called the Eaux-Chaudes thrust (e.g. Ternet, 1965).Our work has consisted on a revision of the structural style by field analysis and mapping, sample collection, microstructural analysis by Electron Backscattered Diffraction, and paleotemperature analysis by Raman Spectroscopy of Carbonaceous Material. Our preliminary results lead to a modification of the previous interpretation of the massif. Most significantly, a large allochtonous Upper Cretaceous unit has been identified in overturned bedding polarity, featuring high-strain shearing microstructures. The general structure is thus defined as a large basement-involved recumbent anticline with Paleozoic rocks in the core, much in the style of the Helvetic nappes of the Alps, locally affected by early diapirism testified by Keuper and ophites injected among Cretaceous rocks. This fold nappe overrides a right-way-up thin undeformed Upper Cretaceous panel that lies on the Axial Zone Variscan basement.Kinematic indicators as stretching lineation, angular relationships between schistosity and bedding and S-C structures evidence a south-directed tectonic transport. As strain intensity increases in the overturned fold limb, recrystallization and strong grain-shape and lattice-preferred orientation of calcite grains appears. The recrystallized matrix grain size ranges between 10 and 35 µm. Fifteen samples have been analysed by RSCM and fitted by Beyssac method (Beyssac, et al. 2002), which yielded paleotemperatures from 325°C to 375°C in Upper Cretaceous rocks (i.e. lower greenschist facies) that are consistent with the ductile microstructures observed.These unexpected results lead us to a reassessment of the thermicity and the Pyrenean deformation conditions in the northern Axial Zone, characterized by high ductility with a subordinate role of thrust faulting. Finally, we discuss the thermal and tectonic implications of these observations for the current models for the Pyrenean orogeny.Ternet, Y. 1965. Etude du synclinal complexe des Eaux-Chaudes (Basses-Pyrénées). PhD Thesis, Univ. of Toulouse, 152 p.Beyssac, O., Goffé, B., Chopin, C. and Rouzaud, J. N. 2002. Raman spectra of carbonaceous material in metasediments: new geothermometer. J. Metamorph. Geol. 20, 859-871. [ABSTRACT FROM AUTHOR]

Despite being one of the most-studied orogens on Earth, recent deep-seismic data found profound heterogeneities in the deep mantle along-strike the Alpine orogen: in the Eastern Alps the subducting slab is steeply inclined, whereas in the Western Alps it is shallower and consistent with southward directed subduction after slab break-off possibly during Plio- to Pleistocene times. A goal of this study is to explore the long-term effects on the upper crustal structural and exhumation history by contrasting the western NFP-20E, central TRANSALP and eastern EASI seismic profiles utilising thermochronological and structural analyses. We achieve this in three steps: (1) by obtaining new low-temperature thermochronology data along the three profiles (apatite and zircon [U-Th/He]), (2) by modelling structural kinematics based on newly created balanced cross-sections for each profile, and (3) using these kinematics as input for an integrated thermo-kinematic forward model to predict cooling ages that can be compared to those measured at the surface. This approach will enable us to constrain rock uplift potentially associated with slab break-off in the Western Alps, and whether this mechanism is able to explain the overall higher topography in surface elevation compared to the Eastern Alps. Here we present new thermochronological ages along the NFP-20E and TRANSALP profiles together with preliminary thermo-kinematic forward models. These data are in accordance with recent exhumation focussed in the central part of the sections. The cooling ages show a slight north-to-south increase from ~5 Ma to ~10-15 Ma in apatite fission track ages and from ~10-15 Ma to ~20-25 Ma in zircon (U-Th/He) ages within the Tauern Window, and a sharp increase in cooling ages north of the Periadriatic Line. Apatite (U-Th/He) ages remain within ~5-10 Ma. North and South of the Tauern Window cooling ages tend to successively increase. We propose that the cooling age distribution measured at the surface is consistent with an in-sequence activation of mid-crustal ramps coeval with fault activity in the immediate hanging wall of the Periadriatic Line since the Oligocene. The unique setting, comparing the structural and exhumational history along sections above contrasting deep-seismic profiles, enables us to better understand, and possibly link, mantle processes and their influence on the crust and the surface during Palaeogene to Neogene Alpine orogenesis, while providing new insights into the formation of collisional mountain belts in general. [ABSTRACT FROM AUTHOR]