Alpine Tectonics Research Articles

Shaping the crustal structure of the SW-Alpine Foreland: Insights from 3D Geological modeling

Reactivation processes play a significative role in the localization of deformation but still remain hard to establish at the lithospheric scale. In this work, we built a 3D structural model, which enables to bridge the gap between the main tectonic structures observed at the surface and the geometry of the major interfaces (the Mohorovičić-discontinuity (hereafter Moho) and top of the basement) inferred from geophysical data acquired in the external Western Alps and their foreland. The geometry of these tectonic structures is interpreted in relation to their geodynamic evolution. The main results of this study highlight: (1) a strong contribution of thick-skinned Pyrenean-Provence and Alpine tectonics, (2) a lithospheric rooting of Variscan shear zones and related faults, and (3) the regional-scale influence of these inherited structures on the post-Paleozoic strain localization in the study area. Our 3D model shows that the pattern of Variscan shear zones that were developed at the end of the Paleozoic involved the whole crust, as emphasized by the Moho offsets. These shear zones were reactivated and localized Meso-Cenozoic deformation. The Variscan deformation pattern controlled the geometry of extensional basins, the propagation of Pyrenean-Provence deformation, and finally the Alpine deformation at crustal scale. Our 3D model shows minor crustal thickening (ca. 40 km) located below the Pelvoux External Crystalline Massif, which probably resulted from both Pyrenean and Alpine tectonic phases. In contrast, the southern part of the Alpine front shows a thinned crust (ca. 18 km) resulting from extensional Meso-Cenozoic phases between the Cévennes margin and the Durance basin.

Subsurface interconnection beneath the Mio-Plio-Quaternary volcanoes of the Ain Leuh causse (Middle Atlas, Morocco): Structural framework and emplacement mechanisms

In the Ain Leuh causse, located southwest of the Middle Atlas Volcanic Province (MAVP), a combined study using aeromagnetic data, fieldwork, and remote sensing is carried out to improve our understanding of the subsurface structure and its correlation with surface structural and volcanic features. The tectonically controlled emplacement mechanisms of the MAVP, which continue to hold undisclosed aspects, were investigated as well. Numerous techniques and mathematical filters for edge detection and 2D modelling were used. The main observations show that the positive magnetic anomalies in the study area are mainly related to the magmatic activity, which was organized into two main episodes: i) Triassic, with a tholeiitic affinity, well recorded along the NE-SW inherited Variscan faults; and ii) Mio-Plio-Quaternary, with an alkaline affinity, forming scoria cones, maars, and lava flows. They are also expressed as hidden domes, laccoliths, and interconnected magma bodies revealed by 2D modelling of the aeromagnetic data. Uncommon coarse-grained undersaturated alkaline rocks crop out in Talzast dome (Talzastite, Fasinites, and Ankaratrite). They are unique in the MAVP and restricted to this part of the Middle Atlas. 2D aeromagnetic modelling has revealed the presence of shallow magmatic rocks bodies beneath Talzast dome. The extracted structural lineaments using aeromagnetic data and remote sensing reveal that the structural framework of the study area is controlled by NE-trending faults reaching depths of up to 3000 m, which is the depth of the magnetic signature of the faults. Additionally, NNE, NNW and N-trending short-length lineaments are defined and rooted at depths up to 3000 m. The predominant NE-SW structural trend may be linked to the reactivation of Hercynian faults during Alpine tectonics. These faults likely controlled the emplacement of the Mio-Plio-Quaternary volcanism in the area. The magma reservoirs that would have fed the various volcanic edifices are located at a shallow depth of less than 500–2000 m.

Reconstructing a Super-Eruption From the Upper Ordovician Period in the Eastern Pyrenees, Spain

Abstract The Pyrenean basement rocks, NE of the Iberian Peninsula, southwestern Europe, include evidence of several pre-Variscan magmatic episodes which indicate the complex geodynamic history of this segment of the northern Gondwana margin from late Neoproterozoic to Early-Palaeozoic times. One of the most significant magmatic episodes was late Mid-early Upper Ordovician (Darriwilian-Katian) age that produced several granitic bodies and volcanic rocks interbedded with Sandbian-Katian sediments. This magmatism is well represented in the Ribes de Freser area (Freser valley, Bruguera and Campelles localities, eastern Pyrenees), where these Ordovician magmatic rocks were affected by an irregularly distributed Variscan deformation and mainly by severe Alpine tectonics, which originated the superposition of several structural units. We present a palinspatic reconstruction of this Alpine deformation (80-20 Ma), that permitted us to infer the geometry, facies distribution, original position, thickness, and significance of these volcanic rocks. This reconstruction allows us to interpret the volcanic rocks cropping out at the Freser valley, Bruguera, and Campelles areas as intra-caldera deposits representing a minimum preserved volume of the order of 100 km3. This may confirm the existence of super-eruptions of Upper-Ordovician age in that sector of the eastern Pyrenees and emphasizes the extent of the Upper-Ordovician felsic volcanism in this sector of the northern Gondwana margin, probably developed in an extensional scenario linked to the development of the Rheic Ocean during Gondwana margin breakup.

Model of Pleistocene geomorphological evolution in active Alpine neotectonics controlled margins in the western Mediterranean area: The case of SE Iberian Peninsula

At the northern tip of the Betic realm (SE Iberian Peninsula), some troughs (synclines) and elevations (anticlines) alternate, marking the present-day coastal lobed morphology of cape-bounded bays, where subsidence and uplift conditions prevailed, respectively. In this study, we were able to establish a clear coastal evolution. To this end, we considered the sedimentological and palaeoenvironmental conditions, the palaeogeographical reconstruction, and recent tectonics until Middle Pleistocene times (MIS 5) through the interpretation of onshore cores, raised marine deposits and geophysical profiles. In this regard, as reflected by both onshore and offshore information, there seems to be a stratigraphical gap from the end of the Pliocene to MIS 15 (Middle Pleistocene). In areas under uplift conditions, linked to tardive Alpine tectonics, the deposits of ancient shorelines and raised beaches were located at different post-depositional elevations, which were dated from odd MIS 15 to MIS 5 using amino acid racemization. Only deposits aged MIS 7 and MIS 5 are roughly at the present-day sea level or some meters above. In the troughs, which remain mostly as lagoons and salt marshes, subsidence did not allow the sedimentary record to be discerned. However, many borehole cores were recovered, attesting lagoonal, marsh, sabkha, or alluvial environmental conditions, which were usually unconnected from the sea. Micropaleontological and amino acid racemization dating revealed these cores to be of MIS15 to MIS5 age. Offshore seismic research revealed five erosive-bounded deposits that are stacked accretionary prisms corresponding to highstands between odd MIS 15 and MIS 5. In contrast, even MISs can be correlated to the erosive horizons that separated the seismic units, reflecting lowstands. In this regard, some bars, at a range of distances from the present-day coastline, protected wetlands from marine influence, allowing the development of diverse sub-environments under changing paleogeographical and paleoclimatological conditions.

Geochronology and geochemistry of the Gréixer rhyolitic caldera complex: Implications on Permo-Carboniferous magmatism (South-Central Pyrenees, NE Spain)

The Gréixer Rhyolitic Complex is one of the most important manifestations of the post-Variscan Permo-Carboniferous volcanism of the Catalan Pyrenees. This volcanic complex is represented by a > 800 m thick, > 12 km wide, intra-caldera succession, mostly composed of strongly-welded, lava-like rheomorphic rhyolitic pyroclastic rocks. Previous studies have attributed a Lower Permian age to this caldera complex and interpreted it as having resulted from the gravitational collapse of a central volcanic edifice after a large-volume explosive event, mostly represented by widespread extra-caldera pyroclastic deposits (the Castellar de n'Hug Ignimbritic Member), which was radially emplaced from the caldera structure. However, new stratigraphic, structural, geochemical, and geochronological data reveal that this co-genetic relationship between the inferred intra- and extra-caldera deposits does not exist. On the contrary, each of these two successions of pyroclastic deposits resulted from a different volcanic event, which had occurred within a time span of 15 Ma. Moreover, the geochemical signature of these volcanic rocks shows a different magma source for each case, with a clear crustal origin for the Gréixer Rhyolitic Complex and a mixed mantle-crustal origin for the Castellar de n'Hug Ignimbrite Member. The apparent stratigraphic correlation between both successions is the result of the Alpine tectonics, which put together different late and post-Variscan Permo-Carboniferous rocks that had originated in different areas and at different times.

Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore southwestern Iberia

Abstract. The structural styles and magnitudes of Alpine tectonic inversion are reviewed for the Atlantic margin of southwestern (SW) Iberia, a region known for its historical earthquakes, tsunamis and associated geohazards. Reprocessed, high-quality 2D seismic data provide new images of tectonic faults, which were mapped to a depth exceeding 10 km for the first time. A total of 26 of these faults comprise syn-rift structures accommodating vertical uplift and horizontal advection (shortening) during Alpine tectonics. At the regional scale, tectonic reactivation has been marked by (a) the exhumation of parts of the present-day continental shelf, (b) local folding and thrusting of strata at the foot of the continental slope, and (c) oversteepening of syn- and post-rift sequences near reactivated faults (e.g. “passive uplift”). This work proves, for the first time, that geometric coherence dominated the growth and linkage of the 26 offshore faults mapped in SW Iberia; therefore, they are prone to reactivate as a kinematically coherent fault network. They form 100–250 km long structures, the longest of which may generate earthquakes with a momentum magnitude (Mw) of 8.0. Tectonic inversion started in the Late Cretaceous, and its magnitude is greater close to where magmatic intrusions are identified. In contrast to previous models, this work postulates that regions in which Late Mesozoic magmatism was more intense comprise thickened, harder crust and form lateral buttresses to northwest–southeast compression. It shows these structural buttresses to have promoted the development of early stage fold-and-thrust belts – typical of convergent margins – in two distinct sectors.

Alpine tectono‐metamorphic evolution of the Corsica basement

AbstractThe Alpine orogenic edifice of Corsica (northern Tyrrhenian Sea) offers the possibility to investigate the mode through which continental crust responds to the propagation of regional shortening at convergent plate margins. The geology of Corsica has been traditionally described as separating domains affected by the Alpine tectonism (Alpine Corsica) from those that did not experience the Alpine tectono‐metamorphic overprint (Hercynian Corsica), but recent studies show that most of Hercynian Corsica was thermally reset in post‐Eocene times, questioning this paradigm. The continental units formed at the expenses of the stretched continental margin of the European plate and consist of Hercynian granitoid basement rocks and cover sequences (Permian volcaniclastics and Mesozoic sedimentary successions). By integrating meso‐ and microstructural investigations with metamorphic thermobarometry and 40Ar–39Ar geochronology along three E–W trending structural transects running across the basement section exposed below the Alpine orogenic wedge, we document middle‐late Eocene (c. 50–33 Ma) westward‐verging syn‐metamorphic (low‐grade blueschist facies) thick‐skinned, basement‐involved thrusting. Significantly, crustal shortening in the continental basement predated of c. 15–10 Ma the subduction zone metamorphism in the oceanic‐derived Schistes Lustrés Complex. When the P–T–t–deformation history as reconstructed from the Corsica basement is integrated with the regional scenario of the Alpine–Apennine orogeny, a tectonic reconstruction is proposed, which frames the Alpine orogeny in Corsica within the Apennine–Maghrebian subduction system in the retroside (retrowedge) of the Apennine orogenic wedge.

Orogenic reworking and reactivation in Central Iberia: A record of Variscan, Permian and Alpine tectonics

Interference between orogenic systems and deformation phases within them may lead to reworking and reactivation of previous structures. The eastern sector of the Spanish-Portuguese Central System holds evidence of two orogenic systems, Variscan and Alpine, plus a stage of Permian extension. We perform an integrated structural analysis to identify reworking and reactivation processes throughout the geological record. The Variscan record starts with crustal thickening (D1; E-verging overturned folds). A second phase features the intra-orogenic collapse of an overthickened crust (D2; top-to-the-SE ductile extensional shear zone), which produced intense structural reworking at the core of the shear zone and moderate reworking at its hanging wall. During subsequent strike-slip tectonics, crustal thickening parted transpressional deformation into a dextral shear zone and upright folds (D3). Variscan deformation did not reactivate previous structures, but exploited a weak rheological boundary defined by contrasted lithologies (sedimentary versus igneous rocks) to accommodate D2 shearing. Reactivation played a role afterwards: Variscan strike-slip shear zone acted as a transfer fault to accommodate Permian extension (post-orogenic collapse), and then Alpine contraction. The Permian extension record is blurred by Alpine inversion, although the trend of Alpine structures in Central Iberia, and the Spanish-Portuguese Central System, may result from Permian structural inheritance.

Detrital zircon ages of Oligocene to Miocene sandstone suites of the Southern Apennines foreland basin system, Italy

U–Pb spot ages on detrital zircons from turbidite quartzarenites, quartz-feldspathic and volcaniclastic sandstones of the Tufiti di Tusa (TTF), Numidian Flysch (NF), Bifurto (BF), Gorgoglione (GF) and Serra Palazzo (SPF) formations, exposed in southern Italy, were detected to define their provenance signatures aiming to contribute to reconstructing the complex geodynamic evolution of the southern Apennine chain. The deposition ages of these sandstones range from Rupelian (TTF), Burdigalian p.p. -Early Langhian (NF and BF) and Langhian-Serravallian -Early Tortonian (GF and SPF). The detrital zircon ages in each deposit show different clusters. In TTF, the zircon age clusters are: 610 ± 6 Ma; 390 ± 4 Ma; 287 ± 3 Ma; 157 ± 2 Ma; 78 ± 1 Ma and 67 ± 1 Ma; 33 ± 1 Ma. In NF and BF formations, the prevalent zircon ages are clustered at 661 ± 10 Ma, 607 ± 8 Ma and 547 ± 8 Ma. In GF sandstones, the ages of detrital zircons are clustered around 672 ± 28 Ma, 458 ± 9 Ma, 297 ± 8 Ma and 24 ± 1 Ma. The sandstones of SPF contain zircons showing age clusters at 649 ± 10 Ma, 486 ± 7 Ma and 315 ± 5 Ma. Zircons and lithic fragments from TTF display a dominant provenance from Oligocene calc-alkaline effusive products located on western domains of the Mesomediterranean Microplate, and minor detritus from granitoids and metamorphic rocks of Variscan ages. Zircons of NF and BF indicate a provenance of detritus from the African Craton without evidence of Variscan or Alpine signatures. Zircons from GF and SPF show a provenance from both Variscan crystalline basement rocks forming the Alpine chain (Calabria-Peloritani Terranes) and Variscan magmatic and metamorphic rocks unaffected by Alpine tectonics, similar to Sardinia–Corsica (pro-parte) block. In addition, the Oligocene magmatic component dated at 24 Ma in GF sandstones is evidence for the maximum age of sedimentation for these deposits. In light of the recent and preliminary geochronological data acquired, a revised paleogeographic restoration could be inferred for the location of sedimentary basins during the Apennines tectonic evolution. The Rupelian sedimentation age of TTF suggests that the deposition domains of the Numidian sequences and the successive deposits of GF and SPF, were located in a more complex paleogeographic scenario along the orogenic system.

What can high‐P sheared orthogneisses tell us? An example from the Curinga–Girifalco Line (Calabria, southern Italy)

AbstractHigh‐P (HP) mineral parageneses are usually poorly developed within metagranitoids, as these rocks are commonly affected by fluid‐deficient conditions when experiencing a metamorphic cycle. However, since ductile shear zones can act as preferential pathways for fluids in the Earth's crust, if metagranitoids are involved in ductile shear under HP conditions, the presence of fluids during deformation can induce recrystallization and equilibration in these rocks. With this in mind, we investigate the formation and evolution of mineral assemblages in the orthogneisses of the Castagna Unit from the northern Serre Massif (Calabria, southern Italy). During Alpine tectonics, the thrusting along the Curinga–Girifalco Line juxtaposed these rocks, representative of the Hercynian intermediate crust, below the lithologies of the Hercynian lower crust. A detailed microstructural study of the orthogneisses, sampled along a progressively increasing ductile deformation gradient, revealed a variation in the mineral assemblage between the weakly deformed orthogneisses and those in the shear zone. Phase diagram calculations in the MnNCKFMASHTO system indicate that the progressive replacement of relict minerals by new, Alpine minerals in the shear zone, was related to the presence of fluids during deformation. This allowed equilibration of the sheared orthogneisses up to metamorphic peak conditions of ~0.9–1.0 GPa and ~560–590°C. Our integrated study highlights that both weakly deformed and mylonitic orthogneiss share the same peak metamorphic conditions, and that the new equilibrium mineral assemblage was stabilized in the mylonitic orthogneisses along a fluid‐conservative prograde path, where no fluid was added or lost. After metamorphic peak, the fluid was channelled towards the inner part of the shear zone, with fluid‐present conditions that were restricted to the mylonitic orthogneisses close to the tectonic contact. These mylonitic orthogneisses record cooling and exhumation to 0.6–0.7 GPa and 360–400°C, showing an overall anticlockwise P–T path. By comparing our findings with existing structural studies, we highlight that the Castagna Unit was under‐thrusted to lower‐crustal depths during the Alpine orogeny, before the re‐activation of the Curinga–Girifalco Line during the Oligocene to Miocene extensional tectonic phase, that enabled the exhumation of this unit.

Deformed displacive halite crystals: Diagenetic or tectonic origin?

ABSTRACT Displacive halite crystals in fine-grained sediments are a common feature of evaporitic sequences worldwide. The crystal habit of halite precipitated from pure aqueous solution under laboratory conditions is a cube: equal side lengths (a = b = c) and rectangular angles (ab = bc = ac = 90°). Crystals with deformed-looking shape appear in mudstone of Permian evaporites of the Eastern Alps. They expose platy, rod-like, parallelepiped-like or rhomb-like shapes. The aim of the study was to explain the origin of these shapes. Samples were taken in the Altaussee, Berchtesgaden, and Hallstatt mines. Representative mudstone samples were scanned by high-resolution X-ray CT, and halite shapes were restored by 3D software. Additionally, 327 euhedral halite crystals were dissected from mudstone and subjected to individual shape analysis. Their sizes range from 3 to 30 mm in thirteen specimens. Crystals are compact, but a slightly platy preference prevails in all samples. Approximately 80% of all objects expose angles > 1° deviating from the right angle, but only ∼ 15% contain angles > 20°. The maximum deviation from the right angle was ca. 33°. Most objects combine a platy shape (a ≈ b ≠ c) with only one angle deviating from the right angle (2× 90°). The acute angle is preferably oriented parallel to the layering. We interpret a primary rectangular crystal growth to platy shapes in the mudstone (reduced fluid circulation). The displacive halite crystals subsequently deformed during compaction of the mudstone. Halite in deformed shapes was replaced by datable polyhalite ca. 15 My after deposition, which rules out deformation during Alpine tectonics.

Hooked on salt: Rethinking Alpine tectonics in Hallstatt (Eastern Alps, Austria)

AbstractPermian salt in the Hallstatt and neighboring salt structures of the Eastern Alps (Austria) crops out along with Triassic deep-water deposits that are at odds with the surrounding Triassic platform carbonates. The traditional interpretation of this juxtaposition is that the salt bodies were emplaced in the Late Jurassic as gravity nappes onto the carbonate platforms, in what has been considered to be the earliest orogenic event in the Eastern Alps. Here we describe for the first time a world-class outcrop of halokinetic sequences in Triassic platform carbonates flanking the Hallstatt diapir. Combining this with other outcrop evidence, we prove that the Hallstatt diapir grew passively during the Triassic, surrounded by carbonate platforms, and extruded to the seabed during the Jurassic. The development of the Hallstatt diapir in a platform setting disproves its emplacement as a gravity-driven nappe, proves the relevance of salt tectonics in the Mesozoic development of the Eastern Alps, and challenges the existence of a Late Jurassic Alpine orogenic event.

Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of the European margin (Mont Fort nappe, Pennine Alps)

The Mont Fort nappe, former uppermost subunit of the Grand St-Bernard nappe system, is an independent tectonic unit with specific structural and stratigraphic characteristics (Middle Penninic, NW Italy and SW Switzerland). It consists in a Paleozoic basement, overlain by a thin, discontinuous cover of Triassic-Jurassic metasediments, mainly breccias, called the Evolène Series. The contact of this Series over the Mont Fort basement is debated: stratigraphic or tectonic? We present new observations that support the stratigraphic interpretation and consequently imply that the Evolène Series belongs to the Mont Fort nappe. We moreover show that the Mont Fort nappe was strongly affected by normal faulting during Jurassic. These faults went long unnoticed because Alpine orogenic deformation blurred the record. Alpine strain erased their original obliquity, causing confusion with an Alpine low-angle thrust. These Jurassic faults have been passively deformed during Alpine tectonics, without inversion or any other kind of reactivation. They behaved like passive markers of the Alpine strain. Detailed field observations reveal the link between observed faults and specific breccia accumulations. Areas where the Evolène Series is missing correspond to sectors where the fault scarps were exposed on the bottom of the sea but were too steep to keep the syn- to post-faulting sediments. The Mont Fort nappe thus represents an example of a distal rifted margin. The succession of synsedimentary extensional movements followed by orogenic shortening generated a situation where passively deformed normal faults mimic an orogenic thrust.

Lithostratigraphy of a Cenozoic intra continental basin (Guadiana Basin, SW Spain) by using clay mineral assemblages and geophysical properties

In the Iberian Peninsula, Alpine tectonics built several Cenozoic basins that were mainly filled with continental deposits. The Guadiana Basin is located halfway between two of the most important Cenozoic basins of the SW of Spain, the Tajo and the Guadalquivir basins. In this paper, both geophysical and mineralogical studies have been carried out in order to establish the lithostratigraphy of the Guadiana basin. A combination of electric resistivity properties and variations in clay mineral assemblage content of the basin infill was used to identify several depositional sequences within the Cenozoic deposits of the basin. Three depositional sequences, DS1, DS2 and DS3 characterized by differentiated clay content and mineral assemblages were distinguished. The lower depositional sequence, DS1, consists of reddish siltstones and clays that scarcely crop out in the basin. Clay content of the <2 μm fraction is dominated by illite averaging 60% of the total clay content, with kaolinite and a few amount of smectite. DS2 deposits lie unconformably over DS1 and the Variscan basement and consist of a siliciclastic succession where carbonate units occur at bottom and at the top of the depositional sequence. Smectite dominates throughout the entire succession except in the basal carbonate unit where up to 70% of palygorskite is found. In the younger sequence DS3, kaolinite is the dominant clay mineral. Electrical resistivity imaging helped in the physical characterization and horizontal and vertical distribution of each unit and provided good insight on the structure of the Guadiana Basin, allowing to infer the basin structure due to high resistivity contrast between the basement and the sedimentary rocks. In addition, in areas where the lower sedimentary units do not crop out, and borehole information is not available, the geophysical response of the resistivity units imaged can be used to infer the corresponding depositional sequence.The joint interpretation of the clay mineral assemblages and geophysical properties of the sedimentary units showed to be a useful tool to gather a reliable identification of depositional sequences in basins characterized by scarcity of good outcrops, lack of borehole information and similarity of sedimentary rocks. This is very important to properly correlate the different sedimentary units mapped as well as to infer the basin structure and its evolution through time.

Quantitative microstructural analysis of mylonites formed during Alpine tectonics in the western Mediterranean realm

Quantitative microstructural investigations involving new GIS-based tools are applied to three different types of mylonitic rocks. These techniques enable mineral shape distribution maps to be produced, thereby allowing the nature and relative percentage of the weakening vs. hardening layers, as well as their kinematics, to be obtained within a statistically meaningful dataset. Selected rock-types are derived from shearing associated with the meso-Alpine strike-slip tectonics of the western Mediterranean realm, which since the Paleocene has involved, skarn, migmatitic paragneiss and tonalites. These rocks belong to a deep-seated portion of the original southern European Variscan chain known as the Calabrian Pelortani Orogen. Microstructural maps reveal porphyroclastic domains, represented alternatively by feldspar, amphibole, pyroxene or scapolite, that can be distinguished from weakening phase areas, characterised by quartz, biotite plus quartz, or alternatively by calcite when the weakening layer is controlled predominantly by skarns. Image elaboration of porphyroclastic domains was employed to estimate the dominant shear-type, with Rigid Grain Analysis, indicating a pure shear component of 66–68% for the mylonitic tonalites; 62–66% for the mylonitic paragneisses; and 58–62% for the mylonitic skarn. Grain boundary mapping of the quartz-rich domains also permits an estimate of shear strain rate, which ranges from 1.14*10−12 (1/s) for mylonitic paragneiss to 5.91*10−12 (1/s), for mylonitic tonalite, and is in accord with high strain zones in natural settings. Finally, our results opened up new perspectives in the interpretation of the kinematics and rheology of this exhumed sector of the southern European Variscan chain reflecting the deep-seated meso-Alpine strike-slip tectonics of the western Mediterranean realm.

Sedimentation in a synclinal shallow‐marine embayment: Coniacian of the North Sudetic Synclinorium, SW Poland

AbstractThis study is a reconstruction of the Coniacian palaeogeographic and palaeoenvironmental development in the North Sudetic Basin, a synclinal trough within the Late Cretaceous Central European seaway linking the Boreal and Tethyan marine provinces. The basin formed as an early side effect of the Alpine orogeny combined with the mid‐Cretaceous eustasy, and crucial stages of its evolution occurred during the Coniacian. The basin in the early Coniacian was a long and narrow shallow‐marine embayment with a hypothetical (non‐preserved) bayhead strait funnelling tidal currents. Coalescing tidal sand ridges formed a littoral platform that prograded from the bayhead zone along the basin axis, impinged on laterally by the basin‐margin shoreface and local river deltas. A mid‐Coniacian forced marine regression and closure of the bayhead strait, attributed to the Alpine tectonism combined with eustasy, brought about a dramatic change in the basin, whereby the basin‐wide littoral sand platform emerged and turned briefly into a denudated coastal plain. The late Coniacian eustatic marine transgression formed an in‐place growing coastal sand barrier at the outer edge of the former littoral platform, sheltering a paralic limno‐lagoonal plain with peat‐forming mires. The coastal barrier was eventually drowned by the sea and maximum marine flooding occurred, followed by a normal regression recorded as a rapidly upwards‐shallowing succession of offshore‐transition to fluvio‐deltaic deposits. This case study of the sedimentation pattern in an evolving, tectonically controlled marine embayment contributes to the existing facies models for estuarine embayments formed by a passive marine drowning of large fluvial or glacial valleys.

Pre-Alpine and Alpine deformation at San Pellegrino pass (Dolomites, Italy)

ABSTRACTIn this work, we present the geological map of the San Pellegrino pass, inserted in the spectacular scenario of the Dolomiti region (Southern Alps, Italy), at a scale of 1:10.000 and accompanied by geological cross-sections. The detailed distinction of lithological thin units allowed to achieve a consistent interpretation of the local structural setting by drawing brittle and ductile Alpine tectonic deformations. The differential deformation and structural styles within the geological map are the result of the different rheological nature of volcanic and sedimentary rocks, as well as of the superimposition of compressional Alpine tectonics over Permo-Mesozoic extensional tectonic phases, and consequent reactivation of inherited structures.

Constraining long‐term fault activity in the brittle domain through in situ dating of hydrothermal monazite

AbstractSIMS Th–Pb dating of hydrothermal fissure‐vein monazite‐(Ce) has the unique potential to date multiple tectonic events under low‐grade metamorphic brittle/ductile conditions over large time frames. Monazites‐(Ce) from brittle fault systems in the Eastern Alps allow us to constrain their Cretaceous activity over 20 Ma within single crystals, recording all major tectonic events. Eo‐Alpine formation of the fluid‐filled fissure‐veins occurred 90 Ma ago at 352 ± 19°C and 342 ± 42 MPa. This corresponds to peak conditions during regional metamorphism of the Cretaceous collisional nappe stacking. Several stages of dissolution–reprecipitation/recrystallization record fault activity between 84 and 70 Ma. Corresponding fluid inclusions indicate conditions of 229 ± 10°C and 143 ± 20 MPa. This correlates with the formation of sedimentary basins during post‐orogenic extension associated with strike‐slip movements. The results strengthen the hypothesis that many large fault systems in the Eastern Alps developed during the Cretaceous orogeny and became reactivated during Neogene Alpine tectonics.

Continuity and Episodicity in the Early Alpine Tectonic Evolution of the Western Carpathians: How Large‐Scale Processes Are Expressed by the Orogenic Architecture and Rock Record Data

Key Points Limited Cenozoic shortening of the Western Carpathians facilitates a comprehensive tectonic model of their pre‐Late Eocene evolution Western Carpathian orogeny was driven by pull of the Meliata oceanic slab and by the northward drift of Adria Multiple rock record data provide valuable constraints for evolutionary tectonic models of collisional orogens

Intrusion of shoshonitic magmas at shallow crustal depth: T–P path, H2O estimates, and AFC modeling of the Middle Triassic Predazzo Intrusive Complex (Southern Alps, Italy)

The multi-pulse shoshonitic Predazzo intrusive complex represents an ideal igneous laboratory for investigating the chemical and physical conditions of magma emplacement in a crustal context, since numerical models can be constrained by field evidence. It constitutes the most intriguing remnant of the Middle Triassic magmatic systems of the Dolomitic Area (Southern Alps), preserved by the Alpine tectonics. Predazzo Intrusive Complex comprises silica saturated (pyroxenites/gabbros to syenites), silica undersaturated (gabbros to syenites), and silica oversaturated (granites and syenogranites) rock suites. In this paper, we modeled its emplacement and evolution with a multiple thermo-/oxy-barometric, hygrometric, and EC-AFC approach. At odds with what proposed in literature but according to the field evidence, the emplacement of the Predazzo Intrusive Complex occurred at shallow depth (< 6 km). In this context, the different pulses differed slightly in bulk water content, but shared a common thermal regime, with temperatures between 1000 and 1100 °C and ~ 600 °C at low-to-moderate oxidizing conditions (− 0.1 to + 0.7 ΔFMQ). The interaction between the intrusion and the shallow crustal rocks was minimal, with Sr and Nd isotopic compositions indicating an average of 5–6% assimilation of crust. A thermo- and oxy-barometric comparison with the nearby Mt. Monzoni also enabled to speculate about the solidification time of the intrusion, which we infer took place over about 700 ka.