Structural analysis of the Subbriançonnais and Valais units in the area of Moûtiers (Savoy, Western Alps): paleogeographic and tectonic consequences

Valais and Subbriançonnais units of the Western Alps of Savoie underwent a common structural evolution, postdating peak pressure conditions associated with high-pressure metamorphism of internal parts of the Valais units. The first two phases, due to roughly north/south-directed shortening, are interpreted to be related to a NNE/SSW-striking corridor of sinistral transpression between the internal Western Alps and the European foreland. Both phases led to nappe formation, isoclinal folding and north–south elongation. Only the third phase of deformation is related to WNW-directed orogen-perpendicular shortening, thus far regarded as the predominant thrusting direction in the Western Alps. Late (post 5?Ma) normal faulting, evidenced by fission-track dating, reactivated the Houiller Front in the north and the Penninic Front in the south. Kinematics of movement, observed along the present-day Houiller Front and Penninic Front, change from north to south. In the north the Houiller Front indicates post-D3 normal faulting while the Penninic Front preserved WNW-directed thrusting (D3). In the south the Houiller Front preserves syn-D2 north-directed thrusting, whereas the Penninic Front is partly reactivated by post-D3 normal faulting. Our observations clearly favor tectonic reasons for the disappearance of the Valais units south of Moûtiers in present-day map view.     

Cretaceous transformation from passive to active continental margin in the Western Carpathians as indicated by the sedimentary record in the Infratatric unit

The Cretaceous orogen of the Western Carpathians comprises fragments of the destructed northern Centrocarpathian domain, which is defined as Infratatric unit and formed a continental margin facing the Penninic Ocean in Jurassic and Cretaceous times. The breakup event and opening of the Penninic Ocean occurred in the Early Jurassic (Pliensbachian), which is recorded by an abrupt deepening event from shallow-water sediments to deep-water nodular limestone in the Infratatric sediment succession. The transformation of the passive into an active continental margin by the onset of subduction of the Penninic oceanic crust occurred in Santonian times and is reflected by the beginning of flysch deposition in the Infratatric Belice domain, which took the position of a forearc basin in the convergent margin setting. The forearc basin was supplied by clastic material from the more internal part of the Infratatric unit, which experienced nappe stacking, metamorphism, and subsequent exhumation in Late Cretaceous times. In the frontal part of the forearc basin an accretionary wedge was built up, which formed an outer-arc ridge and delivered detrital material into the forearc basin in Maastrichtian time. Final collision between the European and the Adriatic plate occurred in the Eocene period and is responsible for weak metamorphism in the Infratatric unit.     

Tectonic progradation and plate tectonic evolution of the Alps

Rifting and spreading, trench formation, flysch deposition, subduction and nappe formation prograde from internal to external parts of the Alpine orogen. The progradation is a characteristic feature of the evolution of the Alps. A plate tectonics model based on this cognition is presented and an attempt is made to integrate the plate movements of the Alpine region during the Mesozoic and Cenozoic into the plate pattern of the Western Mediterranean.

Important events in the evolution of the Alps are the successive opening and closing of the Piedmont (South Penninic) and Valais (North Penninic) oceans, and the two continental collisions related to this. The southward drift of the Briançonian plate in the Cretaceous closes the Piedmont and opens the Valais ocean. The evolution of these oceans is related to the plate movements in the North Atlantic. The second continental collision is followed by the formation of an exogeosyncline, the molasse foredeep.

Prograding orogens like the Alps are most likely to evolve in an originally continental environment by rifting. Retrograding orogens, however, indicate an originally oceanic environment with well-developed magmatic arcs and back-arc basins.     

Cretaceous flysch and pelagic sequences of the Eastern Alps: correlations, heavy minerals, and palaeogeographic implications

Flysch and pelagic sedimentation of the Penninic and Austroalpine tectonic units of the Eastern Alps are results of the closure of the Tethyan-Vardar and the Ligurian-Piemontais Oceans as well as of the progressive deformation of the Austroalpine continental margin. The Austroalpine sequences are characterized by Lower Cretaceous pelagic limestones or minor carbonate flysch and various siliciclastic mid- and Upper Cretaceous flysch formations. Chrome spinel is the most characteristic heavy mineral delivered by the southern Vardar suture, the northern obduction belt at the South Penninic-Austroalpine margin and its continuation into the Klippen belt sensu lato of the Carpathians. The South Penninic sequences, e.g. the Arosa zone, the Ybbsitz Klippen zone and some flysch nappes also contain chrome spinel, whereas the sediments of the North Penninic Rhenodanubian flysch zone are characterized by stable minerals and garnet.     

The Eocene unconformity of the Briançonnais domain in the French—Italian Alps,revisited (Marguareis massif,Cuneo); a hint for a Late Cretaceous—Middle Eocene frontal bulge setting

Abstract

The tectonic significance of the Eocene unconformity in the Briançonnais domain, classically regarded as recording a compressional event, is re-evaluated, based on field studies in the Marguareis massif, Maritime-Ligurian Alps. In this external, weakly metamorphic Briançonnais unit, we describe N-trending, folded paleo-normal faults. These paleofaults operated during the Late Cretaceous-Late Eocene, and control both the thickness of the Senonian-Paleocene calcschists and the distribution of the disconformable Middle Eocene-Early Priabonian formations, i.e. the channelised, resedimented Nummulitic limestones, associated with sandy turbidites, and the sandy- calcareous Lower Flysch Noir. The chaotic Upper Flysch Noir (Priabonian), which includes olistoliths from the Helminthoid Flysch nappes, disconformably overlies the Late Cretaceous-Middle/Late Eocene levels. At the scale of the whole Briançonnais domain of the French-Italian Alps, the superimposed Senonian-Eocene disconformities would indicate extensional faulting and block tilting, associated with a regional uplift which caused emersion of part of the domain (most internal Briançonnais, Corsica). Extension and coeval uplift would record the crossing of the frontal flexure (external bulge) of the European/Briançonnais lower plate situated west of the Alpine subduction zone between 80–70 Ma and ～40 Ma, i.e. before the subduction of the Briançonnais plateau around 38–35 Ma. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.     

Cretaceous–Tertiary Rhenodanubian flysch wedge (Eastern Alps): clues to sediment supply and basin configuration from zircon fission-track data

The provenance of Cenomanian to Eocene flysch deposits accreted along the northern margin of the Eastern Alps has been investigated by means of zircon fission-track (FT) geochronology and zircon morphology. The Rhenodanubian flysch and Ybbsitz klippen zone comprise several nappes representing the Main flysch and Laab basins. The Laab basin received sediments of stable European provenance, indicated by pre-Variscan, Variscan, and Permian–Triassic zircon FT ages, and was thus located in the immediate south of the European margin. The Main flysch basin was supplied mainly from the evolving Eastern Alps and was therefore situated south of the Laab basin. Zircon populations with Permian to Jurassic cooling ages in the Main flysch basin are related to increased heat fluxes during the break-up of Pangaea and are probably derived from the northwestern part of the Eastern Alps. The dominant Cretaceous zircon FT cooling ages reflect Eoalpine metamorphism in the Austroalpine realm.     

Alpine geodynamic evolution of passive and active continental margin sequences in the Tauern Window (eastern Alps, Austria, Italy): a review

The Penninic oceanic sequence of the Glockner nappe and the foot-wall Penninic continental margin sequences exposed within the Tauern Window (eastern Alps) have been investigated in detail. Field data as well as structural and petrological data have been combined with data from the literature in order to constrain the geodynamic evolution of these units. Volcanic and sedimentary sequences document the evolution from a stable continent that was formed subsequent to the Variscan orogeny, to its disintegration associated with subsidence and rifting in the Triassic and Jurassic, the formation of the Glockner oceanic basin and its consumption during the Upper Cretaceous and the Paleogene. These units are incorporated into a nappe stack that was formed during the collision between a Penninic Zentralgneis block in the north and a southern Austroalpine block. The Venediger nappe and the Storz nappe are characterized by metamorphic Jurassic shelf deposits (Hochstegen group) and Cretaceous flysch sediments (Kaserer and Murtörl groups), the Eclogite Zone and the Rote Wand–Modereck nappe comprise Permian to Triassic clastic sequences (Wustkogel quartzite) and remnants of platform carbonates (Seidlwinkl group) as well as Jurassic volcanoclastic material and rift sediments (Brennkogel facies), covered by Cretaceous flyschoid sequences. Nappe stacking was contemporaneous to and postdated subduction-related (high-pressure) eclogite and blueschist facies metamorphism. Emplacement of the eclogite-bearing units of the Eclogite zone and the Glockner nappe onto Penninic continental units (Zentralgneis block) occurred subsequent to eclogite facies metamorphism. The Eclogite zone, a former extended continental margin, was subsequently overridden by a pile of basement-cover nappes (Rote Wand–Modereck nappe) along a ductile out-of-sequence thrust. Low-angle normal faults that have developed during the Jurassic extensional phase might have been inverted during nappe emplacement.     

Passive and active margin history of the northern Tatricum (Western Carpathians,Slovakia)

The Tatricum, an upper crustal thrust sheet of the Central Western Carpathians, comprises pre-Alpine crystalline basement and a Late Paleozoic-Mesozoic sedimentary cover. The sedimentary record indicates gradual subsidence during the Triassic, Early Jurassic initial rifting, a Jurassic-Early Cretaceous extensional tectonic regime with episodic rifting events and thermal subsidence periods, and Middle Cretaceous overall flexural subsidence in front of the orogenic wedge prograding from the hinterland. Passive rifting led to the separation of the Central Carpathian realm from the North European Platform. A passive margin, rimmed by peripheral half-graben, was formed along the northern Tatric edge, facing the Vahic (South Penninic) oceanic domain. The passive versus active margin inversion occurred during the Senonian, when the Vahic ocean began to be consumed southwards below the Tatricum. It is argued that passive to active margin conversion is an integral part of the general shortening polarity of the Western Carpathians during the Mesozoic that lacks features of an independent Wilson cycle. An attempt is presented to explain all the crustal deformation by one principal driving force - the south-eastward slab pull generated by the subduction of the Meliatic (Triassic-Jurassic Tethys) oceanic lithosphere followed by the subcrustal subduction of the continental mantle lithosphere.     

青藏高原中段古近纪早期古构造演化

青藏高原古近纪早期发育大量区域逆冲推覆构造系统, 典型实例如冈底斯逆冲断裂系、纳木错西逆冲推覆构造、伦坡拉逆冲推覆构造、唐古拉山北逆冲推覆构造、东昆仑南部左旋斜冲断裂系。古近纪逆冲推覆构造对古新世—始新世沉积盆地具有重要控制和改造作用。冈底斯古新世—始新世早期发育大量中酸性岩浆侵入和多期中酸性火山喷发, 岩石Sr/Y-Y地球化学显示为岛弧岩浆岩, 推断与古近纪早期新特提斯残留古大洋板块俯冲存在成因联系。古近纪早期新特提斯残留大洋板块俯冲向印度大陆板块俯冲的转换时代约为46-45 Ma, 转换期前逆冲推覆构造运动与新特提斯残留古大洋板块俯冲存在密切关系; 转换期后印度大陆板块俯冲导致更为强烈的逆冲推覆构造运动和挤压缩短变形, 不仅使早期很多逆冲推覆构造继续发生构造运动, 还在喜马拉雅、冈底斯、风火山、东昆仑南部形成大量新的逆冲推覆构造系统。     

Tectonics of the western part of the Polish Outer Carpathians

The western part of the Polish Outer Carpathians is built up from the thrust, imbricated Upper Jurassic-Neogene flysch deposits. The following Outer Carpathian nappes have been distinguished: Magura Nappe, Fore-Magura group of nappes, Silesian, Subsilesian and Skole nappes. Interpretation of seismic and magnetotelluric survey from the region South of Wadowice, allows observation of relationship between basement and flysch nappes in the Outer Carpathians. It also allows identification of dislocation cutting both flysch nappes and their basement. All the Outer Carpathian nappes are thrust over the southern part of the North European Platform. The platform basement is composed of older Precambrian metamorphic rocks belonging to the Bruno-Vistulicum terrane. Sedimentary cover consists of Paleozoic, Mesozoic and Neogene sequences. The characteristic features of this boundary are horsts and troughs of general direction NW-SE, turning W-E. Faults cutting only the consolidated basement and the Paleozoic cover were formed during the Hercynian Orogeny in the Carboniferous and the Early Permian. Most of the older normal faults were covered by allochtonous flysch nappes forming thus the blind faults. During the last stage of the geodynamic development the Carpathians thrust sheets moved towards their present position. Displacement of the Carpathians northwards is related to development of dextral strike-slip faults of N—S direction. The orientation of this strike-slip fault zones zone more or less coincides with the surface position of the major faults perpendicular to the strike of the Outer Carpathian thrustsheets. The huge fault cuts formations from the Paleozoic basement through the flysch allochton between the boreholes in Sucha Beskidzka area. The displacement of nappes of the Carpathian overthrust and diapiric extrusion of plastic formations of the lower flysch units occurred along this fault.     

A crustal‐scale cross‐section of the south‐western Alps combining geophysical and geological imagery

A geotransect across the south‐western Alps from the Pelvoux Massif (external French Alps) to the Dora‐Maira massif (internal Italian Alps), through the Monviso ophiolitic complex, was investigated in the framework of the ‘Géo‐France 3D Alpes’ programme. A new interpretative crustal‐scale section across the south‐western Alps is proposed, combining geological and geophysical 2D/3D data. The Apulian mantle (i.e. the Ivrea body) might be divided into two rigid pieces separated by the downward prolongation of the Penninic frontal thrust. These mantle indenters drive the decoupling of the European crust. Beneath the high to ultra‐high pressure metamorphic nappes, the deep structure results from the stacking of crustal slices extracted from the European lithosphere. The proposed structural model provides a basis for discussing the evidence of the crustal‐scale partitioning of the current strain pattern as well as the location of the seismicity.     

Late Oligocene-Miocene syn-/-late-orogenic successions in Western and Central Mediterranean Chains from the Betic Cordillera to the Southern Apennines*

The identification of syn- and late-orogenic flysch deposits, extending from the Betic Cordillera to the Southern Apennines, assists in the reconstruction of the tectonic-sedimentary evolution of the perimediterranean chains. A microplate was located between the European and African Plates during the Late Jurassic–Early Cretaceous, bordered northwards by the Piemontese Ocean and southwards by another (North Africa ‘Flysch’ Basin or Maghrebian) Ocean. The Piemontese Ocean and the northern margin of the microplate were structured from the Late Cretaceous to the Eocene to create an Eo-alpine Chain. The southern margin of the microplate was deformed in the Aquitanian, when the internal areas of the Maghrebian Ocean were characterized by syn-orogenic flysch deposits. This episode culminated with metamorphism (25–22 Ma) and nappe emplacement, which destroyed the former palaeogeography and created an orogenic belt (AlKaPeCa). Afterwards, a lower Burdigalian late-orogenic cycle started in the deformed area, which as a result of the opening of the Algero-Provençal Basin, caused the fragmentation of the AlKaPeCa, its thrusting on the ‘Flysch’ Basin and the collision with the North Africa and South Iberia Margins. These latter were folded and thrusted, the ‘Flysch’ Units pushed over the External Domain and also back-thrusted. Langhian late-orogenic deposits suture the new tectonic features. Finally, the whole orogen was thrust onto the foredeep during the Middle–Late Miocene.     

The erosion history of the Central Alps: evidence from zircon fission track data of the foreland basin sediments

Fission track dating on detrital zircons of Alpine debris in the Swiss molasse basin provides information about the erosion history of the Central Alps and the thermal evolution of source terrains. During Oligocene times, only sedimentary cover nappes, and Austroalpine basement units were eroded. Incision into Austroalpine basement units is indicated by increasing importance of Cretaceous cooling ages in granite pebbles upsection. Erosion of Penninic basement units started between 25 and 20 Ma. Early Oligocene zircon FT ages show that Penninic basement units were exposed at ∼20 Ma. Deeper Penninic units of the Lepontine Dome became exposed first at ∼14 Ma, contemporaneously with the opening of the Tauern window in the Eastern Alps. A middle Miocene cooling rate of 40 °C Myr⁻¹ is deduced for the Lower Penninic units of the Lepontine Dome.     

Sedimentological response to late Mesozoic subduction in the Penninic windows of the Eastern Alps

Late Mesozoic subduction of Penninic oceanic lithosphere finds its response in the sedimentary record. The corresponding sediments are deposited in a deep-sea trench environment and are developed as distal, partly proximal flysches, containing breccias and olistolites, which are up to kilometer-sized (wildflysch). In the Tauern window this facies is represented by the Nordrahmen zone, which is the equivalent to the Matrei zone. It is proposed to apply the term “Matrei zone” to the entire zone. It forms the high parts of the Bündner Schiefer and Tauernflysch formation. The olistolites derive from the unstable Austroalpine continental margin (Lower Austroalpine). In the Unterengadin window the wildflysch faciès is found in North, Middle and South Penninic position. In that there are kilometer-sized blocks of clearly Lower Austroalpine provenance in a North Penninic position, the Middle Penninic Tasna zone must already have been subducted at the time of emplacement of these olistolites. The Tasna zone itself contains a number of olistolites and disintegrates towards the northeast into a wildflysch zone. Its nappe character is discussed. After earlier fossil findings it is likely that the lower flysch zones of the Unterengadin window contain younger members than the higher ones. Thus, a mechanism of offscraping of the trench sediments and piling up in an accreting wedge above a subduction zone is proposed.     

Variscan geodynamic evolution of the Carnic Alps (Austria/Italy)

The South-Alpine Carnic Alps are part of the southern flank of the European Variscides and display a continuous sedimentary record from Late Ordovician to Devonian times followed by Carboniferous S-directed nappe stacking and Late Carboniferous to Early Permian post-collisional collapse. The tectonometamorphic and sedimentary evolution of the Carnic Alps resembles a continuous process where pre- and syn-orogenic volcanism, syn-orogenic flysch sedimentation, deformation including nappe stacking, metamorphism and tectonic collapse shift in age from internal zones in the N towards external zones in the S. New structural, petrological and sedimentological data are presented concerning the tectonometamorphic history of the Carnic Alps. We distinguish three thrust sheets or tectonic nappes differing in their stratigraphic, sedimentological, deformational and metamorphic histories which were thrust over each other in Carboniferous times. Our data lead to a new geodynamic model showing an evolution from rifting or back-arc spreading in the Late Ordovician to the establishment of a mature passive continental margin in the Late Devonian/Early Carboniferous, flysch sedimentation in an active continental margin setting during the Visean/Namurian and finally collision during the Late Carboniferous between the northern margin of Gondwana and a microcontinent to the N.     

The Ligurian Units of Western Tuscany (Northern Apennines): insight on the influence of pre-existing weakness zones during ocean closure

Most of the tectonic units cropping out in Western Tuscany are fragments of the Jurassic oceanic crust, ophiolitic successions, overlaid diachronously by Upper Cretaceous-middle Eocene carbonate and siliciclastic flysch successions with their Cenomanian-lower Eocene shalycalcareous basal complexes. These units, so called Ligurian, have been emplaced during the closure of the Ligurian-Piedmont Ocean. Ophiolite bearing debris flows are common in the flysch basins and their relationship with ophiolitic tectonic slices points to a strong relation between tectonics and sedimentation from the early compressive events of the Late Cretaceous. The tectonic activity reflects in a rough morphology of the ocean floor. It progressively influences the distribution and sedimentology of the turbidites. During middle Eocene this relationship begun very important and a paleogeographic reconstruction with prominent linear ophiolitic reliefs that bounded some turbiditic basins can be done. In our reconstruction the sedimentary and structural evolution can be framed in the context of strain partitioning, developed during the ocean closure, between subduction processes and ancient weakness zones crosscutting both the ocean and the Adria continental margin and reactivated in compressive regime. These weakness zones can be interpreted as transform faults of the Ligurian-Piedmont Ocean with prolongations in the Adria passive margin.

The weakness zones crosscut the oceanic lithosphere and the Adria continental margin and interfered with the subduction processes. The activity of the weakness zones is reflected in the Ligurian Units architecture where two main structural strike trends of thrusts and folds axial planes occur. The first trend is WSW-ENE oriented and it is connected with the reactivation of the weaknesses zones. This first orientation developed progressively from Late Cretaceous to Pliocene, from oceanic to ensialic convergence (D1, D2, and D4 deformation phases). The second trend is NNE-SSW oriented and is related to the late Eocene continental collision and the subsequent translation to the NE of the oceanic units onto the Adria continental margin (D3 deformation phase).     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.