Potassium-argon ages from the Ceneri Zone,Southern Swiss Alps

The Ceneri Zone is a unit of the crystalline basement of the Southern Alps. Its northern boundary is the Tonale Line segment of the Periadriatic Line, an important tectonic lineament separating the Oligocene and younger features of the Central Alps from the older metamorphic and structural trends of the Southern Alps. Unmetamorphosed Permian and younger sedimentary units lap onto the Southern Alpine basement from the south.Potassium-argon results from the Ceneri Zone define a Hercynian age pattern typical for the basement of continental Europe. This pattern extends to within at least 100 meters of the Tonale Line. Thus, amphibolite facies metamorphism in this region occurred around 325 m.y. ago. The geochronologic similarity of the Southern Alps to many other European regions must be taken into account in megatectonic theories.In detail, the Hercynian age pattern of the Ceneri Zone is complicated. Some hornblendes have apparent ages between the Hercynian and a Caledonian value (430 m.y.). They probably retained some radiogenic argon during the Hercynian upper amphibolite facies metamorphism. In addition, mica results between 200 and 300 m.y. have a strong geographic correlation. Apparently, the northwestern portion of the Ceneri Zone was reheated or mildly metamorphosed during the Upper Triassic to Lower Jurassic. A relationship between these ages and 170–180 m.y. ages from the neighboring Ivrea-Verbano Zone seems likely. No geologic evidence for any post-Hercynian event has been noted as yet in the Ceneri Zone.     

On the role of the Hercynian and Alpine thrusts in the Upper Paleozoic rocks of the Central and Eastern Pyrenees

Abstract

The structure of the Pyrenean pre-Hercynian rocks involved in the “Axial Zone” antiformal stack, results from the association of Hercynian cleavage-related folds and Hercynian and Alpine thrusts. Some of these Alpine and Hercynian thrusts separate thrust sheets in which Upper Paleozoic rocks, Devonian and pre-Hercynian Carboniferous, exhibit different lithostratigraphy and internal structure.

In order to know both, the original Devonian facies distribution and the structural characteristics, the effects of the Alpine and the Hercynian thrusts must be considered. If a conceptual restored cross-section is constructed taking into account both the Alpine and Hercynian thrusts, a different Devonian facies distribution is achieved. Devonian carbonatic successions were originally located in a northernmost position, whereas sequences made by alternations of slates and limestones lie in southernmost areas. Moreover, a N-S variation of the Hercynian structural style appears. In the northern units thrusts are synchronous to folding development and they are the most conspicuous structures. In the intermediate units, thrust postdate cleavage-related folds, and in the southernmost units several folding episodes, previous to the thrusts, are well developed.

We present some examples which enable us to discuss the importance of the Hercynian and Alpine thrusts in the reconstruction of the Pyrenean pre-Alpine geology.     

Facies and geochemical correlations in the Upper Hauptdolomit (Norian) of the eastern Lechtaler Alps

An unusually thick (over 1000 m) inter- to infratidal, calcareous, facies development in the Upper Hauptdolomit (Norian) of the eastern Lechtaler Alps, northern Tyrol, was investigated. The special litho- and bio-facies are described in two sections (A and B) and are compared with some geochemical parameters.With reference to Fischer's (1965) original concept, these special facies are shown to correspond, in many details, to the restricted environments of the Trucial Coast, Persian Gulf. Based on previously published ideas (Fischer, 1965; Zankl, 1967, 1971), the special facies development of the Upper Hauptdolomit (Hd) of the eastern Lechtaler Alps can be schematically correlated with the Norian Dachsteinkalk facies: the investigated sections appear to represent the transitional facies between wide supratidal flats (typical Hd) and the less restricted facies of the shallow-marine Dachsteinkalk platform.The paper shows a statistical comparison of the interrelationships of Ca/Mg, Mn, Fe, Zn and insoluble residue, based on correlation coefficients. This data is related to facies.     

Dating synmagmatic folds: a case study of Schlingen structures in the Strona-Ceneri Zone (Southern Alps, northern Italy)

The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3  Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298  Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294  Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm.     

Stratigraphic record of thrust propagation, Carboniferous foreland basin, Pyrenees, with emphasis on Pays-de-Sault (France/Spain)

The Carboniferous culm of the Pays-de-Sault is divided into two diachronous and synshortening series. These series are dated Late Visean (Pic d'Ourtiset series in a northern overthrust unit) and Early Namurian E2 (La Fajolle series in a southern underthrust unit) from an association of foraminifers, algae, and microproblematica identified in clasts of conglomerates. According to structural positions and facies criteria, these two series are interpreted as two turbiditic depocenters which were generated by southward thrust propagation during Late Visean and Early Namurian. At the scale of the Pyrenean Hercynian range, this evolution is consistent with a thrust and depocenter sequence propagating on the wedge-top depozone of a foreland basin system from the northeast (Mouthoumet subpyrenean massif) to the southwest (end of the High Primary Range) during Late Visean to Westphalian C time interval.     

Sedimentary facies of a glacially influenced continental succession in the Pennsylvanian Jericho Formation, Galilee Basin, Australia

Recent work on the Late Palaeozoic Ice Age in eastern Australia has shown the Joe Joe Group in the eastern Galilee Basin, Queensland, to be of critical importance as it is one of few records of Pennsylvanian glacial activity outside South America. This paper presents detailed sedimentological data, from which the Late Palaeozoic environment of the region is reconstructed and which, consequently, allows for robust comment on the broader Gondwanan glaciation. The Jericho Formation, in the lower Joe Joe Group, was deposited in an active extensional basin in lacustrine to fluvial environments, during the mid‐Namurian to early Stephanian. The region experienced a cool climate during this time, and polythermal mountain or valley‐type glaciers periodically advanced into the area from highlands to the north‐east. The Jericho Formation preserves a suite of proglacial to terminal glacial facies that is characterized by massive and stratified diamictites deposited from debris flows, massive and horizontally laminated conglomerates and sandstones deposited from hyperconcentrated density flows, laminated siltstones with outsized clasts and interlaminated siltstone/conglomerate deposited through ice‐rafting into lakes, and sedimentary dykes and breccias deposited through overpressurization of groundwater beneath permafrost. Non‐glacial facies are dominated by fluvial sandstones and lacustrine/overbank siltstones. The glacigenic rocks of the Jericho Formation are confined to discrete packages, recording three separate glacial advances during the latest Namurian to late Westphalian. This arrangement is consistent with the temporal distribution of glacigenic rocks from around the remainder of Australia and Gondwana, which supports the theory that glacial deposits occurred in discrete intervals. The Joe Joe Group is a key succession in the world in this context as, at this time, eastern Australia provides the only unequivocal evidence of a Namurian/Westphalian glaciation outside South America. The continuous record of sedimentation through the Pennsylvanian and Early Permian is indicative of significant warming between glacial intervals, which is difficult to reconcile with the development of long‐lived, cold‐based ice sheets across the supercontinent.     

Granitoïdes de la zone houillère briançonnaise en Savoie et en Val d'Aoste (Alpes occidentales): géologie et géochronologie U-Pb sur zircon

The “Zone Houillère Briançonnaise” (ZHB) is the westernmost continuous unit of the Pennine domain that overlies the Pennine Front. The ZHB comprises Namurian and Westphalian formations locally overlain by Permian and Triassic cover rocks. As with other basements of the Pennine domain (Briançonnais in France. Grand Saint Bernard in Italy and Switzerland), the ZHB comprises gneissic units, whose age, origin and tectonicsignificance have been discussed for a long time. This study deals with the Sapey gneisses, defined near Modane (Savoie), and the Costa Citrin metagranites (Valle d'Aosta. Italy). The alpine tectonic evolution of the ZHB involves an early thrusting event that produced a non-penetrative foliation in the Carboniferous formations. This foliation occurs only in the vicinity of tectonic contacts between individual slices. The thrusting event is overprinted by a regional large-scale, eastward-verging, deformation event and then by a partly extensional event. Pre-alpine relict amphibolilcfacics mineral assemblages found in the Sapey gneisses confirm that they belong to a poly-metamorphic basement. Ages from U-Pb geochronology on zircons (three samples of different lithology and from different regions) confirm this interpretation. Concordia diagrams evidence that zircons have a Proterozoic inheritance and that minimum ages are older than 360 Ma (lower intercepts with the concordia). By contrast, two samples of the Costa Citrin metagranite yielded well-defined Visean-Namurian ages at 324±17 MA and 323±8 Ma. This old basement age (early Hercynian or older) associated with evidences for a Visean-Namurian magmatic event in the ZHB are consistent with the recent reappraisal of the age of the sedimentary formations of the ZHB (now dated of Namurian to Westphalian). These data support the possible “exotism” of a “Briançonnais” terrane with respect to external domains of the Alps. The ages of the basement (pre-Carboniferous), of the sediments (Visean-Namurian) and of the recorded magmatic event (Visean-Namurian) of the ZHB cannot be related to the late orogenic basin formation and magmatic evolution of the Eastern Massif Central or of the External Basement Massifs during Stephanian times. The significance of the Visean-Namurian plutonism of the Costa Citrin is discussed with respect to other parts of the european hercynian belt. It is interpreted either to be a consequence of a late Hercynian “eastern” subduction or rather, to relate to an early extensional stage during the orogeny.     

Palaeobotanical and geochronological evidence for the Alpine age of the metamorphism in the Sesia-Zone

For a long time the age of the last metamorphism of the Sesia-zone was considered to be Hercynian or older. Basement inclusions in basic volcanics were the main argument for this interpretation. The Trachyandesites — Andesites of the Sesia-zone were regarded as Permian, analogoues to the widespread Permian volcanics of the Southern Alps. Recently, plant remnants have been found in tuffitic interlayers of the Sesia volcanics. These fossils have been described as palaeozoic plants, in contradiction to structual observations and to the numerous radiometric data acquired in the region. To check the age of the fossils a detailed palaeobotanical study was carried out. Our samples contained a very modern flora of definitely Tertiary age. Not one palaeozoic fossil was detected. The Tertiary age found on palaeobotanical evidence proved to be between 29 and 33 m. y. on the basis of total rock K-Ar ages. The last high pressure metamorphism of the Sesia-zone occurred between 90 and 60 m. y. as has been shown by radiometric ages on micas. The external part of the zone was overprinted by the Lepontine phase of metamorphism in greenschist facies 38 m. y. ago.     

Facies and genesis of Carboniferous coal seams of Northwest Germany

In order to get detailed information about the facies and genesis of Upper Carboniferous coal seams of Northwest Germany, maceral analyses of complete seam profiles (Westphalian B-D, mainly Westphalian C) were carried out. Four main facies and twelve subfacies could be distinguished. The main facies are:

1. (1) The sapropelic-coal facies, consisting of fine-grained inertinite and liptinite, which forms from organic sediments deposited at the bottom of moor lakes.

2. (2) The densosporinite facies which is high in inertinite and liptinite and low in vitrinite. Syngenetic pyrites, clastic layers, thick vitrains and fusains do not occur. This facies originates from peats of ‘open mires’ with higher groundwater table and herbaceous vegetation. The ‘open mire’ was situated in the centre of extensive swamps. Consequently, clastic sedimentation did not affect this swamp type and nutrient supply and pH values were low.

3. (3) The vitrinite-fusinite facies, which is high in vitrinite. This is the result of abundant vitrains. Under the microscope, fusains were mostly identified as fusinite. The vitrinite-fusinite facies originates from a forest mire. More or less abundant seam splits and clastic layers show that rivers flowed in the neighbourhood of this area.

4. (4) The shaly-coal facies, which represents the most marginal part of the former swamp frequently affected by clastic sedimentation.

Within the Carboniferous of the Ruhr Region it seems unlikely that the thin coal seams of the Namurian C and Westphalian A1 contain a densosporinite facies. The swamps were situated in the lower delta plain where they were often affected by marine influences. Consequently, coals are high in minerals and sulfur and they are thin and discontinous. The best conditions for the formation of extensive swamps, with open mires (densosporinite facies) in their central parts, prevailed during Westphalian A2 and B1 times. Low contents of sulfur and minerals and high content of inertinite are typical for these coals. Sedimentation mainly took place in the transitional zone from the lower to the upper delta plain. During the Westphalian B2 and C fluvial sedimentation dominated. Within the coal seams minerals, sulfur and pseudovitrinite increase while inertinite decreases. This is the consequence of coal of the densosporinite facies occurring with increased rarity. The coal seams of the Westphalian C2 contain no densosporinite facies because peat formation was restricted by increasing fluvial sedimentation and by a better drainage. As a consequence, extensive swamps with ‘open mires’ in the centre were no longer formed after the formation of the “Odin” seams. Above the “Odin” seams coal of the vitrinite-fusinite facies contains thick-walled torisporinites. Variations and lowering of the groundwater table caused mild oxidative influences during peat formation. This is documented by an increase in pseudovitrinite, the occurrence of torisporinites and the absence of spheroidal sideritic concretions. Sulfur content increases in the absence of the low-ash and low-sulfur coal of the densosporinite facies.In Upper Carboniferous coal seams of the Ibbenbüren Region the inertinite and telocollinite contents are higher than in those of the Ruhr Region. Therefore, variations of the groundwater table have been more pronounced and resulting oxidative influences must have been more severe. Seldom occurring marine and brackish horizons and a higher fusinite (fusain) content indicate a slight elevation of this area. From Early Westphalian D times onward, peat formation was no longer possible because of the better drainage. This resulted in severe oxidative conditions which excluded peat formation.     

A map-view restoration of the Alpine-Carpathian-Dinaridic system for the Early Miocene

A map-view palinspastic restoration of tectonic units in the Alps, Carpathians and Dinarides reveals the plate tectonic configuration before the onset of Miocene to recent deformations. Estimates of shortening and extension from the entire orogenic system allow for a semi-quantitative restoration of translations and rotations of tectonic units during the last 20 Ma. Our restoration yielded the following results: (1) The Balaton Fault and its eastern extension along the northern margin of the Mid-Hungarian Fault Zone align with the Periadriatic Fault, a geometry that allows for the eastward lateral extrusion of the Alpine-Carpathian-Pannonian (ALCAPA) Mega-Unit. The Mid-Hungarian Fault Zone accommodated simultaneous strike-perpendicular shortening and strike-slip movements, concomitant with strike-parallel extension. (2) The Mid-Hungarian Fault Zone is also the locus of a former plate boundary transforming opposed subduction polarities between Alps (including Western Carpathians) and Dinarides. (3) The ALCAPA Mega-Unit was affected by 290 km extension and fits into an area W of present-day Budapest in its restored position, while the Tisza-Dacia Mega-Unit was affected by up to 180 km extension during its emplacement into the Carpathian embayment. (4) The external Dinarides experienced Neogene shortening of over 200 km in the south, contemporaneous with dextral wrench movements in the internal Dinarides and the easterly adjacent Carpatho-Balkan orogen. (5) N–S convergence between the European and Adriatic plates amounts to some 200 km at a longitude of 14° E, in line with post-20 Ma subduction of Adriatic lithosphere underneath the Eastern Alps, corroborating the discussion of results based on high-resolution teleseismic tomography.The displacement of the Adriatic Plate indenter led to a change in subduction polarity along a transect through the easternmost Alps and to substantial Neogene shortening in the eastern Southern Alps and external Dinarides. While we confirm that slab-pull and rollback of oceanic lithosphere subducted beneath the Carpathians triggered back-arc extension in the Pannonian Basin and much of the concomitant folding and thrusting in the Carpathians, we propose that the rotational displacement of this indenter provided a second important driving force for the severe Neogene modifications of the Alpine-Carpathian-Dinaridic orogenic system.     

Crustal structure of the Southern Alps beneath the intersection with the European Geotraverse

Newly digitized and amplitude controlled record sections from the 1977 Southern Alps refraction campaign permitted a reinterpretation of the crustal structure in the area between western Lombardy and the Giudicaria fault. The resulting model exhibits considerable lateral heterogeneity: in the west, below 7.5 km of sediments of the Lombardy Basin, the crust reaches a depth of only 31 km, whereas it thickens towards the more mountainous area in the east, reaching a depth of 46 km below the Adamello Massif. Although the signal character of the corresponding reflections is somewhat erratic, the data are satisfied best by models with a low-velocity zone in the upper crust. An additional small velocity discontinuity from 6.2 to 6.4 km/s was found in the middle crust at around 20 km. Earlier interpretations, based on travel-times alone, included a layer with a velocity of about 7 km/s at this depth. This high-velocity layer was then interpreted as lower-crustal material of the Adriatic — African plate, which had been overthrust onto the European plate during the Alpine orogeny, thus explaining the uplift of the Southern Alps. However, this model of crustal doubling is questionable, because such a mid-crustal high-velocity layer is not in agreement with the amplitude data. The relatively thin crystalline part of the crust under the Lombardy Basin is interpreted, in accordance with geological evidence, as a relic of a Late Hercynian rifting event.     

On the Structure and Metamorphism of the Roc de Frausa Massif (Eastern Pyrenees)

Abstract

The Roc de Frausa Massif, located at the Eastern Pyrenees, is formed by a stratoid Pre-Hercynian deformed granite (orthogneiss) interbedded with metasedimentary series. Hercynian granitoids (St. Llorenç — La Jonquera pluton) surround the southern and eastern part of the massif and Hercynian basic igneous rocks (Ceret stock) occupy the central part of it. The Pre-Hercynian granite and the sedimentary series were involved, during the Hercynian orogeny, in complex polyphasic tectonics and metamorphism. As a result, an ubiquitous penetrative foliation was developed during the earlier stages. This foliation was subsequently folded into a complex antiformal structural formed by a double dome : Roc de Frausa dome and Mas Blanc dome. Main lithological boundaries (gneiss — metasediments and metasediments — granitoids) are broadly parallel to the regional foliation, and they all display the dome geometry. Interference fold pattern between two late phases, an ealier one with NE-SW trending folds and a younger one with NW-SE trending folds is responsible for the dome geometry. Mylonitic deformation, with W-E to NW-SE orientations has been attributed to the last folding phase. Regional metamorphic climax and contact metamorphism, the last one resulting from Hercynian granitoid emplacement, preceeded the above mentioned late folding event, which developed under retrograde metamorphic conditions. Regional peak metamorphism is recognized by the static crystallization of cordierite + potassium feldspar. This paragenesis indicates pressure — temperature conditions of about 3.1 Kbar and 660 °C maximum. Contact metamorphism overprints the earlier regional metamorphism. Parageneses and thermal gradient of contact metamorphism around La Jonquera pluton are very similar to those related to regional metamorphism, whereas parageneses produced around Ceret stock present garnet + potassium feldspar. Geothermometry indicates metamorphic conditions locally higher for this paragenesis (around 700 °C).     

Plate and intraplate processes of Hercynian Europe during the late paleozoic

It is speculated that until Late Carboniferous time the region of Hercynian Europe was occupied by an elongated island arc system underlain by a segment of continental crust. In the Upper Carboniferous, two subduction zones are assumed to have extended from the north and south beneath Hercynian Europe. An extensive zone of hot, partially molten upper mantle lay above and between these, and diapiric uprise of portions of this material led to separation of mafic magmas, widespread partial melting in the lower and middle crust, high temperature-low pressure metamorphism in crustal rocks, and regional uplift and extension of the crust, as indicated by intermontane troughs and their associated volcanic rocks.In Visean to Westphalian time Hercynian Europe collided with both the large neighbouring plates North America-Europe and Africa. During these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust, the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of Hercynian Europe. After the collision processes had been terminated, processes of upper mantle activity continued, causing further crustal uplift and even, enhanced crustal extension for several tens of million years into the Lower Permian. Decline of the upper mantle activity beneath Hercynian Europe is indicated by crustal subsidence and formation of a peneplain in Permian time followed by the Upper Permian transgression of both the Zechstein sea and the Tethys sea which mark the end of the Hercynian geodynamic cycle.     

Foreland folding

In the northern foreland of the Alps lithospheric subplate boundaries such as the Rheingraben may be distinguished from structures developed by deformation of the main plate boundary (foreland folding in the strict sense). The latter consists of a very gentle lithospheric bulge (foreland trough and welt) of regional dimensions, and superposed smaller-scale features which are sometimes compressive (Jura) and sometimes extensive (normal faults in the eastern Molasse basin). An explanation is sought in the distribution of weak and strong masses under the Alps and their foreland; a pronounced intracrustal low-velocity cushion under the Alps, and various incompetent sedimentary layers under the foreland. As the subducted lithosphere below and the competent crust above the intracrustal cushion are affected by different boundary displacements, separate stress systems are set up for the two and are superposed in the foreland. Under some circumstances the bending stresses of the lithospheric bulge may predominate and cause extensional (normal) faulting, whereas under other circumstances compression of the supra-cushion crust may be the dominant influence and cause focal mechanisms typical for horizontal compression or, where there is a suitable decollement horizon, even thrusting and folding.     

U–Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps

Permian granulites associated with noritic intrusions and websterites are a common feature of the post-Variscan European crust. Such granulites are common in the Southern Alps (e.g. Ivrea Zone), but occur only in the Gruf Complex in the Central Alps. To understand the geotectonic significance of these granulites, in particular in the context of Alpine migmatisation, zircons from 15 high-grade samples have been U–Pb dated by SHRIMP II analysis. Oscillatory zoned zircons from charnockite sheets, interpreted as melts generated through granulite facies fluid-absent biotite melting at 920–940°C, yield ages of 282–260 Ma. Some of these zircons contain inclusions of opx, unequivocally attributable to the granulite facies, thus confirming a Permian age for the charnockites and associated granulites. Two samples from an enclave-rich orthogneiss sheet yield Cambrian and Ordovician zircon cores. Two deformed leucogranites and six ortho- and augengneisses, which compose two-thirds of the Gruf Complex, give zircon ages of 290–260 Ma. Most zircons have milky rims with ages of 34–29 Ma. These rims date the Alpine amphibolite facies migmatisation, an interpretation confirmed by directly dating a leucosome pocket from upper amphibolite facies metapelites. The Gruf charnockites associated with metre-scale schlieren and boudins of opx–sapphirine–garnet–granulites, websterites and gabbronorites can thus be identified as part of the post-Variscan European lower crust. A geotectonic reconstruction reveals that this piece of lower crust stranded in the (European) North upon rifting of the Neotethys, such contrasting the widespread granulite units in the Southern Alps. Emplacement of the Gruf lower crust into its present-day position occurred during migmatisation and formation of the Bergell Pluton in the aftermath of the breakoff of the European slab.     

A cross section through the central-Pyrenees

Summary A structural analysis of the Pyrenees contains successively from young to old the Miocene morphogenic uplift, a Pyreneic and a Laramide folding phase each preceded by basin formation on the flanks of the central zone, and a rather complicated Hercynian folding period accompanied by magmatic hases. This Hercynian phase starts with a Devonian geosyncline and its folding movement is directed towards the centre, and proceeds later from the centre outwards. In the Hercynian structure one can discern a central anticlinorium and two or three anticlinoria in the flank.Summary of a lecture given in Köln on the 27^th Jan. 1956.     

Effects of CO2-H2O and oblique collision on orogenesis—the european hercynides as an example

The European Hercynides are considered the collisional result of Baltica and the microcontinents of Southern Europe, after subduction destroyed the intervening Rheic Ocean during the early Paleozoic. Their geotectonic development is assumed to consist of four overlapping stages:

1. (1) lithospheric thinning, upwelling of hot asthenospheric material, subsidence along listric faults, and basinal and geosynclinal development on the opposing shelves of the Rheic Ocean starting in pre-Devonian time;

2. (2) intermittent subduction of the Rheic Ocean interspersed with episodes of fracturing, volcanism, local uplift and subsidence, and basement reactivation as a result of compression with dextral megashear, particularly since the earliest Devonian;

3. (3) several phases of folding with a northward vergence, and thrusting and overthrusting along listric surfaces, the true orogenic stage;

4. (4) post-orogenic stage of massive granite intrusions and subsequent volcanism in the Permo-Triassic

.The high clastic content (as opposed to carbonates) of the sedimentary sequences involved in the subduction and folding processes and the consequent release of large amounts of meteoric water are held responsible for the synorogenic and post-orogenic magma rise, and for the wide zone of anatectic granites and migmatites. The dominant dextral megashear, the constant re-adjustment of the microcontinents of Southern Europe (oblique collision) and the scarcity of back-arc basins, oceanic plateaus and microcontinents led to the poor preservation of ophiolites and ultrabasic rocks, and to a wide (over 1500 km) Hercynian Foldbelt.During the Paleozoic, the depositional center of the Rhenish Massif shifted from south polar latitudes in the early Ordovician to tropical positions within a period of about 100 m.y. The sediment facies reflects this paleogeographic development.     

The arditurri Pb-Zn-F-Ba deposit (Cinco Villas massif,Basque Pyrenees): A deformed and metamorphosed stratiform deposit

Stratiform banded and massive Pb-Zn-F-Ba mineralizations in metasediments of the Westphalian Cinco Villas formation (Basque Pyrenees) show several features related to Hercynian deformation and low-grade regional metamorphism. However, some primary depositional textures and structures are preserved. Minerals like pyrite reveal preregional metamorphic textures (framboidal-colloform pyrite with automorphic overgrowths, dendritic and atoll textures), typical of rapid precipitation from supersaturated solutions during hydrothermal-sedimentary activity. Deformation and subsequent micro- and mesoscale remobilization have caused morphological and textural changes in the mineralizations producing folding, mineral foliation, transposition, boudinages, piercements, etc., as well as the development of veinlike orebodies. Pyrite shows a general tendency toward cataclasis whereas sphalerite, galena, and chalcopyrite exhibit a variety of microstructures (subgrains, twining, recrystallization, etc.) indicative of ductile deformation. Microscopy suggests the deformation of more plastic sulfide occurred mainly by a dislocation-type mechanism at low temperatures (350°C), which are compatible with the regional metamorphism recorded in the host rocks.     

Syndeformational fluid trapping in quartz: determining the pressure-temperature conditions of deformation from fluid inclusions and the formation of pure CO2 fluid inclusions during grain-boundary migration

Abstract Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain-boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO₂ fluid inclusions in metamorphic rocks are likely to be the residue of deformation-recrystallization process rather than representing a primary metamorphic fluid.
Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization-primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post-metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.     

The Alpine geodynamic evolution of Penninic nappes in the eastern Central Alps

The eastern Central Alps consist of several Pennine nappes with different tectonometamorphic histories. The tectonically uppermost units (oceanic Avers Bündnerschiefer, continental Suretta and Tambo nappes, oceanic Vals Bündnerschiefer) show Cretaceous/early Tertiary W-directed thrusting with associated blueschist facies metamorphism related to subduction of the Pennine units beneath the Austroalpine continental crust. This event caused eclogite facies metamorphism in the underlying continental Adula nappe. The gross effect was crustal thickening. The tectonically lower, continental Simano nappe is devoid of any imprint from this event. In the course of continent-continent collision, high- T metamorphism and N-directed movements occurred. Both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions. Crustal thinning commenced during the regional temperature peak. A final phase is related to differential uplift under retrograde P–T conditions. Further thinning of the crust was accommodated by E- to NE-directed extensional deformation.