Pre-Mesozoic Geology in the Alps

Relics of the pre-Mesozoic geology are preserved in the five crystalline basement complexes of the External Massifs. Precambrian metamorphic units and Precambrian to Palaeozoic sediments, inter-layered with volcanics and ultramafic rocks, suffered in most areas a polymetamorphic history. After formation of eclogitic mineral assemblages (possible Ordovician), the geotectonic evolution followed a collisional type of metamorphism (Ky-Sill-And) accompanied by crustal thickening. Rising geothermal gradients since the Devonian may be the expression of updoming and erosion and/or post-orogenic collapse. Strike-slip since the Early Carboniferous was accompanied by anatexis, guided the formation of transtensional sedimentary troughs, the nappe stacking of metamorphic units that had not been subjected to the main metamorphic event, and the intrusion of Late-Variscan granite bodies.The External Massifs, during the Variscan evolution, were part of the internal zone of the Variscides corresponding to the Moldanubian zone of the extra-Alpine Variscan domain.

This chapter deals with the geodynamic significance of the pre-Westphalian mafic magmatic events which are recorded in the basement of the Western and Central Alps. It is based on the available petrological, chemical and geochronological data concerning the meta-igneous rocks. This review leads to the following results: (1) The mafic and bimodal magmatisms show evidence of two successive periods of pre-orogenic lithospheric extension in the Cambrian-Ordovician and Devonian-Dinantian times. Their chemical characteristics testify to attenuated continental crust environment, sometimes reaching to narrow oceanic basins. The diversity of their post-magmatic orogenic evolution implies a Late Variscan tectonic “collage”. (2) Most of the external crystalline massifs can be correlated with the westernmost segment of the European Variscan fold belt, when the Aar-Gotthard Massifs display striking affinities with the basement of the Eastern and Southern Alps.

The pre-Triassic formations of the Ligurian Alps build up: (1) the pre-Namurian basement; (2) the Permian-Carboniferous tegument; (3) the Upper Permian Verrucano.1. The pre-Namurian basement is made up of two main rock sequences: I, which shows a pre-Alpine polymetamorphic evolution and II, which is affected by only one Variscan foliation. The basement of the lower Briançonnais units is only represented by orthogneisses II, which are stratigraphically covered by Permian-Carboniferous formations.The upper Briançonnais rootless CalizzanoSavona nappe is mainly composed of older polymetamorphic, tightly associated rocks of the I sequence (paragneisses, orthogneisses I and amphibolites) and by the younger orthogneisses II.Rarer rock types occur; they belong either in the older I sequence (more or less retrograded eclogites, granulites) or in the younger II sequence (anatexites, metagabbros). Paragneisses and amphibolites are believed to derive from pre-Ordovician, originally interfingered basalts and fine detrital sediments; most of the orthogneisses I might represent nearly contemporaneous acidic intrusions.The first tectonometamorphic event (between Ordovician and Silurian times?) produced re-equilibration of nearly all pre-existing rocks under amphibolite facies conditions (with low to intermediate thermal gradient). Only a minor part of these rocks might have been driven into lower crust levels, where migmatites (from sediments), eclogites and granulites (from basalts) were generated.During the Silurian — (?) Early Carboniferous interval, fine, probably very thick, sediments were deposited, while at deeper levels monzogranitic masses (actually orthogneisses II) were emplaced.The second tectonometamorphic event took place before Namurian times; it overprinted both older metamorphites and later magmatites under amphibolite facies conditions (with intermediate to high thermal gradient); the post-Ordovician sedimentary cover was correspondingly transformed under upward decreasing metamorphic conditions.2. Late Variscan progressive basement exhumation was accompanied by conspicuous erosion, which removed nearly all the Siluro-Devonian cover; Permian-Carboniferous tegument began to be laid down, under tectonic conditions that appear to have been — at least in the uppermost crust — mainly extensional.Late Variscan formations in the Briançonnais belt consist of sediments and volcanics, which filled up continental graben and half-graben, bordered by two systems of active faults, presently trending N 30° and N 110°. The volcanic sequence, whose petrochemical characters are principally calc-alkaline although sometimes potassic sub-alkaline with shoshonitic affinity, was built up during three main, mostly explosive, events. Rhyolitic, then andesitic, finally again rhyolitic products were erupted, for a total volume of at least 1000 km3.3. The last, Late Permian faulting phase, devoid of volcanic activity, produced erosion of inner sectors and deposition of disconformable Verrucano sediments. The continental Verrucano arenaceous to conglomeratic deposits grade into the Scythian marine quartzites.

In the Briançon-Bernhard zone the pre-Alpine units, from the Ruitor Massif to the Acceglio Zone, show a variety of lithological, metamorphic and mineralogical characteristics. Younger and ancient basements are distinguished. The younger basement sequences, which include the Mt. Pourri-Bellecôte Massif, the Zona interna, the Ambin formation and the Acceglio basement, are defined on the basis of their monocyclic metamorphic imprints: only Alpine phases are recorded. A recently acquired U-Pb zircon result indicates a Late Cambrian age for the protoliths. The ancient basement sequences are characterized by remnants of pre-Alpine amphibolite and eclogite assemblages, the minimum age of which is Upper Proterozoic-(Lower Cambrian?), i.e. Upper Pan-African. The ancient basement is further subdivided into a more external Sapey-Ruitor type and a more internal Chasseforêt-Clarea type where the Alpine imprint has been pervasive.In the Zone Houillère, the Upper Carboniferous (Silesian) basin had a S-to-N drainage pattern. The series includes a lower, coal-bearing, part consisting of Namurian to lower Stephanian sandstones and pelites, unconformably overlain by an upper part containing various conglomerates and sandstones of an assumed middle Stephanian to Upper Permian age, which grades into quartzites assigned to Scythian.A clastic series lithologically similar to the uppermost, Permian to Scythian, part of the Zone Houillère overlies some basement units, especially the younger type.It is proposed to correlate the younger basement with the Métailler-Mt. Fort unit of the Swiss Bernhard zone, and both types of the ancient basement units with the Pontis and Siviez-Mischabel units. Emphasis is laid on the Briançon-Bernhard units pertaining to Gondwana until the beginning of the Alpine times.

The basement lithostratigraphy within the Middle Penninic domain in Wallis, Switzerland (formerly the Grand St. Bernard nappe), is described in order to try to produce reconstruction of the pre-Alpine history. The combined effect of pervasive Alpine deformation and metamorphism (mainly Tertiary greenschist facies overprint) as well as the lack of biostratigraphic and radiometric data make this attempt difficult.Polymetamorphic basements, affected successively by pre-Alpine metamorphism(s) and by Alpine metamorphism, are distinguished from monometamorphic basements, that apparently were only subject to this latter disturbance. The Pontis and Siviez-Mischabel nappes comprise both poly- and mono-metamorphic units while the rocks of the “Zone Houillère” and the Mont Fort nappe do not seem to have suffered Pre-Alpine metamorphism.Upper Proterozoic to lower Palaeozoic(?) siliciclastic meta-sediments of the polymetamorphic basement form a thick coarse-grained series overlaid by a dominantly pelitic series. Carbonate rocks (skarns, marbles) do not exist. A wide variety of magmatites (calc-alkaline granitoids, pyroxenites, felsic and mafic volcanites) are presumed to have been generated in different geodynamic environments from upper Proterozoic to lower Palaeozoic time.Two metamorphic preserved windows allow a partial reconstitution of the pre-Alpine metamorphic history: (1) outcrops of eclogites and retroeclogites (Siviez-Mischabel nappe), usually associated with banded amphibolites, show HP mineral assemblages (pyrope-rich almandine and omphacite) which are overprinted by an amphibolite-facies event (syn-Variscan?); (2) the most internal part of the Ruitor zone (Pontis nappe) is essentially composed of garnet-staurolite micaschists and aluminum silicate-bearing schists (andalusite-sillimanite). Textures and mineral chemistry support a possible evolution in staurolite-almandine subfacies and P-T estimates lead to a polyphase metamorphic trajectory, retrograde in pressure (10→2.5 kb) and thermally prograde (470→580 °C). Correlation with Variscan climax is likely, even if radiometric data is still lacking.Post-metamorphic late Carboniferous to Early Triassic fluviatil deposits form a thick sequence within the “Zone Houillère” and other parallel grabens(?). The most southern pull-apart(?) basin that is filled up with a thick volcano-sedimentary series (Carboniferous(?) Early Triassic) is largely incorporated into the Mont Fort nappe. Permian gabbros and subalkaline granitoids form sills and laccoliths intruding both poly- and mono-metamorphic basements.The major uncertainties still concern: (1) the precise age of all rocks and series, except those belonging to the “Zone Houillère”; (2) the initial nature of the polymetamorphic magmatites and the geodynamic context during their ascent; (3) the age and the geodynamic meaning of the different pre-Alpine metamorphic events (HP, HT), especially the age of Variscan(?) paroxysm (amphibolite-facies).

Shaped as an “ellipsoid” of about 70 × 25 km, the Dora-Maira Massif is the crystalline basement of the Cottian Alps. It consists of an upper complex formed of pre-Carboniferous metasediments and metabasites, and a lower complex composed of (Permo)-Carboniferous metasediments. Both complexes contain meta-intrusives of granitic to dioritic composition, which are mainly regarded as Variscan. The sedimentary protoliths in the pre-Carboniferous basement are represented mostly by pelites with subordinate, more or less dolomitic limestones, whereas coarser clastic facies predominate in the (Permo)-Carboniferous cover. The Mesozoic sequence overlying the massif is similar to that of the other Penninic units.The Alpine, tectono-metamorphic overprint is very strong throughout the massif: disregarding the “Polymetamorphic Unit” with its very high-pressure assemblages, the conditions of the early-Alpine metamorphic event in the massif are comparable with those of the other internal units of the Western Alps. The eclogite and blueschist facies stages were followed — and in some cases almost completely obliterated — by recrystallization into the greenschist facies. The Alpine metamorphism was accompanied by tectonic events including at least three phases of ductile deformation.

The oldest protoliths of the Austro-Alpine nappe system from the Western Alps consist of the intimate association of metapelites, tholeiitic metabasites and marbles which closely recalls the lithology and metamorphic evolution of the lower crustal section of the South-Alpine Ivrea-Verbano kinzigitic complex. This association may tentatively be interpreted as an Early Palaeozoic accretionary wedge, later deformed, thermally reworked and consolidated during the collisional Variscan orogeny; it was further characterized by Permian igneous underplating, low-P/high-T re-equilibration and production of anatectic melts. Similar high-T paragneisses are also the oldest protoliths in the Penninic Monte Rosa and Gran Paradiso nappes, although lesser amounts of mafic rocks and especially of marbles are recorded here; likewise the Penninic basement displays a late evolution characterized by cordierite-rich anatectic melts.Lithospheric extension, uplift and thermal perturbance were demonstrably active during the Late Palaeozoic, when some basement sections operated as wall/roof rocks for huge intrusions of Late Carboniferous granites (Monte Rosa and Gran Paradiso) or Permian granitoids and ensialic cumulus gabbros (lower Austro-Alpine units), or when they were covered by Permian-Early Triassic clastic deposits. Late Palaeozoic bimodal magmatic activity and graben/ pull-apart opening are also extensively recorded in the surrounding Grand St. Bernard, Canavese and South-Alpine domains and elsewhere in the whole Alpine chain. These Late Palaeozoic processes have traditionally been referred to the Late Variscan orogeny as evidence of an ensialic mobile belt or a trench-arc system. Both these models are rejected here and most of the “Late Hercynian orogenic events” are re-interpreted as post-Variscan metamorphic and magmatic signatures of Late Palaeozoic onset of lithosphere extension and thermal perturbance through simple shear mechanisms along deep low-angle detachment zones, evolving to asymmetric continental rifting. This may account for mantle melting by adiabatic decompression, underplating of gabbros, fast transfer of heat and fluids into the lower crust allowing production and removal of anatectic melts, and low-P/high-T re-equilibration of uplifting lower crustal segments.

Within the subcontinental mantle of the Lepontine area evidence for a 1725-Ma-old magmatism is still preserved. The formation of oceanic crust and island arcs started about 1000 Ma ago. Orthogneisses constitute the dominating rock type of the area. The majority of them are of Variscan age, but some are of Caledonian origin. The depositional age of the pre-Permian metasediments is not well contrained. Zircon U-Pb ages of metapsammites and metapsephites indicate that some of the pre-Variscan metasediments were deposited after the emplacement of the Caledonian granites. It seems reasonable to assume that also pre-Orodovician metasediments are present.

Penninic basement is exposed in the Venediger nappe system of the Tauern window. Three lithostratigraphic units are distinguished: (1) the ophiolitic Stubach group which consists of splinters derived from back-arc oceanic crust, (2) the Habach-Storz group which is dominated by volcanic protoliths and represents remnants of an island arc probably formed on top of the back-arc oceanic crust, and (3) the Zentralgneis group which is composed of large volumes of Variscan granitoids. The island arc experienced maturation during its evolution in the Late Proterozoic and Early Palaeozoic. Carboniferous high-grade metamorphism and granitoid formation reflect a collisional event and collision- and subduction-related magma generation. S-type, originally cordieritebearing, high-K granitoids, I-type high-K granitoids, and I-type medium-K granitoids form a sequence of decreasing age. Permian magmatism with a tendency to the A-type indicates an intraplate setting and signalizes the end of the Variscan orogenic cycle. The evolution of the basement in the Tauern window shows the progressive buildup of continental crust by long-lived subduction activity, starting in an intra-oceanic environment. The overall picture is reminiscent of granite-greenstone belts.

The pre-Permian Habach formation is a complicated metamorphosed sequence of magmatic and sedimentary rocks within the Tauern window. It can be separated into three subunits: 1.Ophiolites including the Basisamphibolit;2.An island arc volcanic sequence;3.The Eiser sequence (Biotitporphyroblastenschiefer).The ophiolites consist of metamorphosed ultramafic and mafic plutonics as well as volcanics. Their chemistry resembles that of basalts from a marginal basin with some MORB characteristics. The island arc sequence consists of basaltic to rhyolitic rocks overlain by sediments. The volcanics display a geochemical pattern consistent with an island arc volcanic series formed on continental crust. The Eiser sequence comprises pelitic and psammitic metasediments interlayered by basic to acidic metavolcanics. The few analyses available indicate a possible oceanic island arc sequence.Age data from all subunits of the Habach formation based on radiometric data and scarce fossil findings range from the Upper Proterozoic to the Carboniferous. The evolution of the Habach formation despite the uncertainty of the age dating fits a model of a long-lasting active continental margin on which different terranes were accreted during the Upper Proterozoic to the Palaeozoic.

Geological, petrographic and geochemical data of the “Zentralgneise” (central gneisses) of the Tauern window (Eastern Alps) are presented. The rocks are interpreted as metamorphosed formation of late Palaeozoic granitoids, which once formed a batholitic terrain on the southern flank of the Variscan orogen. The primary structures of this batholitic terrain and its plutonic evolution are investigated. Field evidence suggests that the plutonic activity set in with widespread anatexis of pre-existing continental crust in the late Viseau to early Namurian, during which a series of K₂O-rich magmas with dominantly granitic, but also syenitic and monzonitic compositions were produced. Subsequently, large volumes of calc-alkaline I-type granitoids intruded (dominantly granodiorites and tonalites). Most of the Zentralgneise represent such calc-alkaline rocks. A late generation of leucocratic granites approaches within-plate (A-type) granites in composition. The change of magma-types with time is accompanied by a rapid uplift of the batholitic terrain. Plate tectonic implications for the evolution of the Variscan orogen are discussed.

In this work we review the present knowledge about stratigraphy, development of facies and tectonic evolution of the Variscan sequences of the Eastern and Southern Alps. In the Eastern Alps outcrops of fossiliferous rocks of Lower Palaeozoic age are irregularly distributed. They form a mosaic-like pattern of dismembered units incorporated into the Alpine nappe system (see Fig. 1 of Schönlaub, p.65 this Vol.). Such areas include the Gurktal Nappe of Middle Carinthia and parts of Styria, the surroundings of Graz, a small area in southern Burgenland and the Greywacke Zone of Styria, Salzburg and Tyrol. South of the Periadriatic Line Variscan sequences are represented in the Carnic and Karawanken Alps where they form the basement of the Southern Alps. As regards the regions occupied by quartzphyllitic rocks of presumably Palaeozoic age the reader is referred to the contribution by Neubauer and Sassi (this Vol.).Based on a comprehensive set of data a distinct geological history on either side of the Periadriatic Line is inferred. Main differences concern the distribution of fossils, the development of facies, rates of subsidence, supply area, amount of volcanism and the spatial and temporal relationship of climate sensitive rocks from north and south of the Periadriatic Line (Schönlaub this Vol.).

The Austro-Alpine quartzphyllites and related clastic- and volcanic-rich sequences are interpreted as the infilling of an Early Ordovician to Early Carboniferous basin. The basin may have been initially formed as a back-arc basin on a Late Cadomian/Pan-African metamorphic crust during the Ordovician. A pulse of renewed rifting during Silurian and Early Devonian probably led to the opening of a nearby oceanic basin and formation of a passive margin which is mainly represented by a Middle to Late Devonian carbonate platform. Contraction and basin closure occurred during Visean to Early Westphalian. It was accompanied by: (i) rapid subsidence due to lithospheric loading by thrust sheets, (ii) formation of a foredeep with orogenic flysch deposits in front of incoming thrust sheets, and (iii) weak metamorphic processes.

The rock record of Palaeozoic volcanic rocks of the Austro-Alpine realm reveals that within-plate alkalibasalts erupted during the Ordovician (Magdalensberg-Series), the Silurian (Gurktaler Alpen, Murau, eastern Greywacke zone, Saualpe), the Devonian (western Greywacke zone, Palaeozoic of Graz) and probably also in the Lower Carboniferous (Eisenkappel area, Karawanken Mountains). The eruption of these within-plate alkalibasalts and the contemporaneous continuous sedimentation indicate a long-lasting extensional process which is responsible for a general thinning of the continental crust and subsidence of marine basins. The sediments of the basins show a widespread facies differentiation depending on the source area and water depth.In addition to these within-plate alkalibasalts a second group of volcanic rocks erupted during the Upper Ordovician which are represented by pyroclastic rocks and lavas of intermediate to acidic composition (“porphyroids” of the Greywacke zone, volcanic rocks of the Nock-Series, Gurktaler Alpen). These rocks have a calc-alkaline composition. Their plate tectonic setting is difficult to establish. Since the eruption of calcalkaline volcanic rocks is usually connected with convergent plate boundaries a late or post-collisional volcanic event is the most likely interpretation.

Pre-Alpine basement rocks in the Bernina Massif occur in the cores of large recumbent anticlines which define Eo-Alpine nappe structures. Polyphase Alpine overprinting led to a strong recrystallization of the lower parts of the nappe pile. Basement rocks with a pre-Alpine tectonic and metamorphic overprinting were the country rocks of late Variscan intrusive rocks, which show exclusively Alpine overprints. Namurian intrusives (326 Ma) display a transition from calc-alkaline to alkaline igneous activity. Uplift and erosion led to pre-Permian exhumation of calcalkaline and alkaline intrusive rocks. In the lowermost Permian (285 Ma), rhyolitic extrusive activity was followed by continental clastic sedimentation marking the onset of the Alpine sedimentation cycle. Incipient rifting is indicated by basic igneous activity which preceded the opening of the oceanic Piemonte basin and the formation of a passive continental margin in Jurassic time.

The Silvretta basement nappe had a complex Precambrian and Palaeozoic history. During the Proterozoic, sedimentation of mostly greywackes occurred in a back-arc situation, probably contemporaneously with voluminous intrusions/extrusions of MOR basaltic magmas. A first metamorphic overprint in amphibolite facies conditions preceded a high pressure event (“eclogitic” peak conditions 550–650 °C, min. 14–16 kb). In Late Proterozoic time, intrusions of basic to intermediate magmas (older orthogneisses) occurred. A Rb-Sr isochron of 895 + 130/−140 Ma is interpreted as crystallization age. The geochemical composition indicates, as for the eclogitic event, a collisional regime for the generation of these magmas. A second HP event occurred before the intrusion/extrusion of acid magmas (younger orthogneisses). Their Rb-Sr isochron of 451 ± 2 Ma is interpreted as an Ordovician crystallization age. This magmatism occurred probably during an Early Palaeozoic continental rifting causing their generation and emplacement. The Variscan deformation and metamorphism was a polyphasic amphibolite facies event. The metamorphic peak (600–650 °C, 5.5–7.5 kb) was dated with Rb-Sr small scale isochron at 370 ± 17 Ma. An uplift caused the crystallization of andalusite in the rocks and in veins. The following compressional event (assumed 450–550 °C, ~4 kb) led to the formation of kilometre-long folds. Rapid rise of the Silvretta crystalline complex and extensional tectonics in the Pangea resulted in the intrusion of Late Carboniferous (310–280 Ma?) rift tholeiites, which are preserved as dykes. Continuous extensional tectonics resulted in the formation of horst and graben structures, filled with Permian acid volcanic detritic material and ignimbritic flows. The Silvretta nappe then began to subside and terrigeneous material as well as dolomites were deposited in Triassic-Jurassic time. The Alpine metamorphism (110–90 Ma) reached anchizonal facies conditions in the west and greenschist facies conditions in the east of the Silvretta. Then the Silvretta was uplifted differentially and detached from the basement. Pseudotachylites (~75 Ma old) formed at this time and document fossil hypocentres of earthquakes. The overthrust of the Silvretta nappe onto the Pennine foreland occurred about 60–55 Ma ago.

The Austro-Alpine basement, between the Tauern window to the E and the Western Alps to the W, treated in this contribution may be divided into three subunits due to differing lithology and metamorphic history. These three subunits, the Ötztal-Stubai-, the Scarl-Campo- and the Ulten basement, are divided by two major tectonic lines, the Schlinig line (ÖtztalStubai/Scarl-Campo) and the Peio line (Scarl-Campo/Ulten). Although each of the three units consists of quartzo-feldspatic and pelitic metasediments, acid to intermediate metamagmatites, in addition to metabasites and metacarbonates, the dominant rock types of the Ötztal-Stubai-, the Campo- and the Ulten-unit are metasediments. The Scarl unit however is mainly composed of acid metamagmatites. Ages of the acid magmatic protoliths are most probably Variscan in the Scarl unit and Caledonian in the Ötztal-Stubai unit.Additional characteristic features of the three subunits are: pre-Variscan eclogites in the Ötztal-Stubai-, Late-Variscan pegmatites in the Campo and Variscan metaperidotites in the Ulten-units. In terms of pre-Alpine metamorphic evolution the Ötztal-Stubaiand Scarl-Campo-units suffered from common, medium to high grade, metamorphic conditions (with relics of high-pressure conditions), whereas the Ulten unit shows indications of very high grade (granulite facies) conditions.In Alpine times the Ötztal-Stubai and Scarl-Campo basement units were both overprinted with increasing intensity from northwest to southeast, whereas the Ulten zone shows almost no Alpine metamorphism.

The pre-Upper Ordovician units of the Altkristallin and the Ordovician to Upper Devonian sedimentary and volcanic rocks of the phyllitic/fossiliferous Palaeozoic sequences show similar and parallel post-Upper Ordovician deformational structures but different P-T evolutions. In the Altkristallin, early prograde high pressure — low temperature metamorphism followed by a high pressure eclogitic stage and later decompression, evolved synchronous to a foliation-forming shearing deformation. Temperature-dominated amphibolite-facies overprinting with local formation of first melts post-dates this deformation. Greenschist-facies low pressure metamorphism, locally reaching the amphibolite facies during early shearing, is observed in the phyllitic sequences. A pre-Alpine age of medium-grade metamorphism of fossiliferous Palaeozoic sequences is not definitely proved. Folding of the foliation and later formation of shear bands are related to a common late Variscan uplift/cooling history and overprinted the foliation-parallel lithological contacts of the units. A Variscan process of progressive crustal stacking which first reworked the Altkristallin to the NW and then affected the phyllitic/fossiliferous Palaeozoic sequences to the SE can be inferred. An increasing degree of Alpine greenschist-to amphibolite-facies ductile overprinting from south to north is observed in the northern parts of the basement. In the southern parts, pre-Alpine structures and mineral assemblages remained well preserved during late Alpine brittle deformations.

The Austro-Alpine metamorphic basement east of the Tauern window contains various tectono-stratigraphic units which are highly diverse with respect to protolith and metamorphic ages and also geodynamic environment of formation. Early Precambrian elements are represented in certain units, by detrital minerals or possible magmatic rocks. The continuous history started with Late Proterozoic to Cambrian island arc and ophiolite sequences, which were accreted during Late Cambrian and Ordovician tectono-thermal events. These are overlain by fossiliferous Silurian-Devonian shelf sequences. The shelf sequence is contrasted by the Silurian to Devonian active continental margin with partly S-type granite magmatism. Final collision of all units occurred during the Early Carboniferous, involving deep-crustal thrusting, metamorphism, collisional-type magmatism, and subsequent uplift. Sedimentary overstep sequences were deposited at different times from the Early Carboniferous to the Permian, and indicate the progradation of the deformation front within the internal Variscan belt.

In the Southern and Eastern Alps late- to post-Variscan sedimentation processes started during the Late Carboniferous (Late Moscovian/Cantabrian). The sediments of the Carboniferous sequence of Nötsch and Veitsch, which are also briefly discussed, are interpreted as “synorogenic sediments”.The late to post-Variscan Molasse sediments of the Southern and Eastern Alps were deposited in NE-SW and E-W to ESE-WNW oriented intramontane basins which probably formed as a result of crustal thinning and transtensional and transpressional tectonics along megashear zones due to the eastward drift to the Eurasian plate and westward drift of the African plate.Tectonic processes and climate, including climatic changes, were the most important controlling factors of sedimentation processes. Based on major tectonic and climatic events, as well as on fossil assemblages, a close correlation of the late to post-Variscan sequences of the Southern and Eastern Alps is possible.The late to post-Variscan sequence of the Southern and Eastern Alps is divided into two evolutionary cycles, which are separated by a major intra-Permian tectonic event.The lower cycle (Late Carboniferous/Early Permian) is characterized by the formation of intramontane basins which were filled with sediments of different environments and with volcanic rocks. Transgressive-regressive clastic-carbonate cycles, within the shallow marine sequence of the Carnic Alps (Southern Alps), are related to eustatic sea-level fluctuations caused by the Gondwana glaciation. The climatic shift from humid to semiarid conditions near the Carboniferous/Permian boundary caused a significant change in sedimentary processes in the continental sequences of the Eastern Alps.During the upper cycle (?Middle — Late Permian to Early Triassic) sedimentation patterns were more uniform and similar sequences developed in the Southern and Eastern Alps. At this time, sedimentation was characterized by the transgression of the Tethys Sea from SE to NW and interfingering of shallow marine sediments with continental deposits. Sedimentation processes were also influenced by a climatic shift near the Permian/Triassic boundary and sedimentary cycles of the Scythian are probably caused by different spreading rates of mid-oceanic ridges. The question where to draw the boundary between the “late to post-Variscan” Molasse sediments and the following Alpidic sedimentation cycle still remains open.

The available literature is critically assessed and existing controversies are emphasized. Based on a synthesis of published ideas the extended discussion proposes the following evolutionary scheme (compare sketches in Fig. 3): (1) an impressive volume of Proterozoic metasediments in the Ivrea zone and Serie dei Laghi has first been juxtaposed during accretionary wedge formation and underplating above a subducted Ivrea oceanic crust during a Late Proterozoic to Early Palaeozoic event (“Cadomian” or Caledonian). (2) A high geotherm was established by Ordovician times (460–480 Ma) as evidenced by anatexis in paragneisses of the Ivrea zone and granitic intrusion associated with granitization of adjacent metasediments in the Serie dei Laghi. This time marks the onset of a long-lasting history of deep burial under elevated temperatures for both Ivrea zone and Serie dei Laghi. (3) Variscan orogeny at around 320 Ma led to crustal thickening and decoupling between lower crust (Ivrea zone) and intermediate crustal levels (Serie dei Laghi). Large parts of the Serie dei Laghi were exhumed during Variscan orogeny, thereby preserving pressure-dominated Variscan mineral assemblages while the Ivrea zone remained in a lower crustal environment resulting in widespread re-equilibration of mineral assembleges to moderate pressures. (4) An important Late Palaeozoic magmatic event of Permian age is unrelated to Variscan orogeny since it post-dates re-establishment of normal crustal thickness. Synchronous magmatic activity is documented at all crustal levels from the still granulite facies metamorphic crust-mantle transition to the sedimentary cover and is best explained in the context of intracontinental wrench tectonics. (5) Crustal thinning associated with passive continental margin formation in the Early Mesozoic leads to considerable uplift of the Ivrea zone and cooling to temperatures below 300 °C. (6) Late Oligocene to Early Miocene deformation is responsible for the final exposure of a vertical section through attenuated continental crust.

Petrological and structural data on the Orobic South-Alpine basement are reviewed in order to reconstruct its thermomechanical history. P-T-t paths are discussed for Val Vedello — Passo S. Marco and Lario areas, i.e. for sectors traditionally attributed to different metamorphic grades (greenschist and amphibolite facies respectively). Structural investigations evidenced that, in these zones, the first recognizable phase of deformation (D₁) developed in all the Oro-bic basement under amphibolite facies conditions during the Variscan period (ca. 330 Ma). Although some metagranitoids, emplaced in the Orobic basement, are considered to be Ordovician, no relics testify to the real pre-Variscan basement during the intrusion of the granites. After the D₁ phase in the central-eastern areas (Val Vedello — Passo S. Marco) during D₂ phase, a greenschist retrogression is observed. In the westernmost area (Lario basement) the D₂ phase is characterized by an increase in temperature under low pressure regime. During the D₂ phase of deformation, considering the available geochronological data, a late Variscan post-thickening uplift is suggested for the Val Vedello-Passo S. Marco areas, and a Permo-Mesozoic uplift related to an extensional tectonic regime for the Lario area. These contrasted evolutions result from the tectonic piling up of various crustal segments of the South-Alpine basement, each representative of different tectonic histories. The polyphased character of the two tectono-metamorphic histories appears to be in contradiction with the metamorphic zoneography traditionally accepted for the South-Alpine domain.

A model of lithostratigraphic sequence is presented, ranging from Cambrian to Devonian; its chronological frame is, however, mainly hypothetical due to the lack of fossils. The interposition of a volcano-sedimentary complex (VSC) makes the distinction between an upper (UPC) and a lower complex (LPC) possible, within this rather monotonous pelitic-psammitic sequence. The LPC is assumed to be preCaradocian: it includes the level in which acritarch data demonstrated the occurrence of Cambrian metasediments in the Alps for the first time. The VSC, assumed to be Late Ordovician due to interregional correlations, records an important volcanic activity, mainly acidic and locally basic in composition. The metamorphism developed under low pressure, greenschist facies conditions during two main stages, one synkinematic (S₂ foliation) and one post-kinematic, the latter recording the thermal climax. Its Variscan age is demonstrated by isotopic data. The value of 350 Ma has been related to the early regional heating, 320 Ma to the thermal climax, and 316–300 Ma to the cooling. The whole set of the Variscan age values in the Eastern Alps indicates that Variscan metamorphism was synchronous over large areas. The regional trend of the main foliation (S₂) and crenulation axis is reported. E-W strikes and verticality are typical along the Periadriatic lineament.An extensional regime during Ordovician could explain the widespread Upper Ordovician volcanism in the Southern Alps; it also represents a suitable frame to understand both the whole “Upper Ordovician granite-rhyolite association” and the “Caledonian” metamorphism occurring in the Austro-Alpine, for which Pan-African pertinence has been proposed.

The Alpi Apuane is a well-known tectonic window in the Northern Apennines (Tuscany), in which a pre-Alpine lithostratigraphic sequence crops out widely and which appears to have been deeply affected, together with the Alpine cycle cover, by the Tertiary tectonometamorphic evolution of the Apenninic chain.The authors illustrate the main lithological and structural features of these formations and discuss the relationships between the Apuan Variscan basement and the regional Variscan system of southern Europe.