The crustal scale seismic image of the Southern Uralide orogen from geology and the URSEIS CMP-reflection profiling show a doubly vergent fabric with a significant crustal root in a low-relief and little eroded and unextended mountain belt preserved since Early Mesozoic times. The structures are the result of two major, partly super-imposed collisional events: (1) the development of an Upper Devonian W-facing accretionary complex with an exhumed subduction-related hP-belt resulting from an continent-arc collision and (2) a Permo-Triassic collision with an eastward accretion of the Trans-uralian terranes and general intracontinental shortening. While the E-vergent accretionary prism involves the entire crust with the Moho as the basal detachment, the latter thick-skinned W-vergent backthrusts only include the upper part of the crust, emphasizing different rheologies of the plate fragments involved. The rather low thermal influence across the entire Urals emphasizes that the compositional features of the crustal units take the main role: mafic to intermediate rocks dominate in the accreted Siberian island arc and oceanic collage, while the former passive margin of the Archean East European craton is controlled by a quartz-feldspar rheology. The orogenic root zone as defined by wide-angle data is poorly imaged in the near vertical reflection data. Gravity and petrophysical modelling suggest that the crustal root is composed of mafic eclogitic rocks which inhibits Moho reflectivity and together with the supra-crustal oceanic fragments the buoyancy potential of the Uralian crust. The old, cold and low-density Archean lithospheric mantle below is not delaminated and screens the orogenic thickened crust from thermal overprint. In combination with permanent compressive boundary forces the Urals thus evolved near isostasy and never departed far from equilibrium.In contrast, the broad and dismembered Variscides suffered a major late- to post- collisional high-thermal overprint combined with processes of substantial exhumation, tectonic denudation, crustal melting and magmatic underplating. The continent-continent collisional systems involved Early Paleozoic (e.g.Cadomian-Panafrican) fragments. High buoyancy-potential and delamination of the juvenile lithospheric fragments and astenospheric upwelling cause the high crustal mobility associated with a substantial reorganisation and re-equilibration during basically Upper Carboniferous times.
Riphean - Early Paleozoic geological history of the Urals, is mainly the history of riftogenesis. During more than 1,1 Ga in the course of long but interrupted riftogenesis and extension it took place a transition from platform environment to rift ones, and then - to the generation of Urals paleoocean. During rift development the thicknesses of formations have been increasing with rapid decreasing of the share of sedimentary rocks and increasing of volcanic ones, while their alkalinity has been gradually lowering. The time of the beginning of oceanic spreading of Urals paleoocean is Late Arenigian. At the same time between Russian platform and Urals paleoocean it has being formed passive continental slope and rise, that is fixed by the beginning of cherty sedimentation, caused by the deepening of paleobasin. On the base of numerous new finds of conodonts collected mainly in the formations, which age was not properly determined, the view about stratigraphy and the history of the development of practically all Urals zones have been radically revised. For the first time many of the continental-slope formations, ophiolites and island-arc complexes were reliably dated and correlated. It has been studied the development and structural evolution of the Main Uralian Fault (MUF), which separates the western paleocontinental sector of the Urals from the eastern paleoisland-arc sector of the region. New geodynamic models of formation of the Urals eclogite-glaucophan belt and Platiniferous belt (Ivanov, Shmelev, 1996) were proposed. In middle Paleozoic MUF was a subduction zone, dipping to the east and consumed the crust of early Paleozoic Urals ocean. In the process there were generated coeval (in the South Urals - 380±10 Ma) eclogite-glaucophane complexes (western part of MUF zone) and andesite island-arc association which have been forming over them. Platiniferous Urals belt situated to east from MUF is a magmatic trace of subduction zone. The belts massifs, composed with dunites, clinopyroxenites, olivine gabbro, gabbro-norites and granitoids are island-arc complexes - the products of different depth crystallization of melts, which have been generating over subduction zone with the age about 420 Ma. Subduction consumed the crust of early Paleozoic ocean by the middle of Late Devonian. The direction of the collision of Urals terrains with Russian platform was oblique (north-western). So the Urals folded belt undergone all stages of complete cycle of geodynamic development: pre-rifting (Riphean-Vendian), continental rifting (Cambrian-Lower Ordovician), oceanic spreading (Middle-Upper Ordovician), island-arc (Upper Ordovician - Devonian), collision (Upper Devonian-Permian), limited post-orogenic extension (Triassic)and platform (from Jurassic up to present days).
Ivanov KS & Shmelev VR, J. Reports RAS, 347, 649-652, (1996).
Garnet amphibole pyroxenites were exhumed within the Mindyak ophiolite, Southern Urals, during the Palaeozoic Uralian orogeny. The plagioclase/spinel lherzolite ophiolite massif, situated along the Main Uralian Fault suture of the orogen, contains metamorphic garnet-bearing rocks in a tectonic breccia in contact with its mantle and transition zone sequences.
The garnetiferous blocks are compositionally varied but may be divided into two main types. Type 1 comprises predominantly grossular-rich garnet and diopside, and has high-MgO, slight LREE/HREE depletion (LaN/YbN of 0.3-1.3), positive Eu anomalies (Eu/Eu* = 1.33-1.51), and LILE enrichments. In contrast, Type 2 has more Fe-rich garnet and diopside, pargasite, and abundant accessory minerals including rutile, titanite, apatite, and zircon, lower-MgO, and LREE/HREE enrichment (LaN/YbN of 2-5.5) and LILE depletions. Both types have low 87Sr/86Sr400 0.7040-0.7044 and high (sum)Nd400 5.0-7.5 indicating that they had a depleted mantle source. We suggest that these rocks are metamorphosed, metasomatised, gabbros. An inferred depletion in Si, alkalis, and Al, and an enrichment in Ca and Fe relative to igneous compositions prior to metamorphism is attributed to rodingitisation of igneous protoliths. The metasomatised protoliths were taken to depth and metamorphosed during initiation of subduction of ocean lithosphere. Subsequent exhumation, during the Uralian orogeny, is evidenced by retrograde re-equilibration to amphibolite facies, and, later, second stage, rodingitisation.
Preliminary geochronological results include: a Pb-Pb single zircon age of 411 ± 4 Ma for crystal rims (in good agreement with a U-Pb population zircon age of 410 ± 5 Ma obtained by Saveliev et al., in press), and 467 Ma for crystal cores: and a Sm-Nd age of 414 ± 4 Ma for a whole rock-garnet pair. We suggest that the variously determined age of 410-415 Ma at the Silurian-Devonian boundary dates a metasomatic/metamorphic event in the history of the garnet amphibole pyroxenite blocks; and, tentatively, conclude that the older core age of 467 Ma is a magmatic zircon crystallisation age associated with ocean crust formation.
So, the petrogenetic history of the garnet amphibole pyroxenites traces a change from plate divergence and ocean formation, to plate convergence and, ultimately, to the Uralian orogeny.
http://www.dgt.uniud.it/petrology/
Saveliev A.A., Bibikova E.V., Savelieva G.N., Spadea P., Scarrow J.H., Pertsev A.N., Kirnozova T.I., Petrologia, (in press).
In the Polar Urals, eclogite-facies rocks crop out in the Marun-Keu complex (67°N, 66°E). This N-S striking complex covers an area of several 100 km2. It is situated along the western edge of the main suture zone of the Uralian orogen, and is bordered to the east by ophiolitic successions and island arc rocks. In order to understand the process of burial and exhumation of the HP rocks in the context of a continental collision, a detailed geochronological and petrological study of the eclogite complex was carried out. It was found that in the study area, major parts of the total eclogite volume resemble former hybrid, gabbroic to granitic magma bodies which originally intruded in a middle- to upper-crustal environment. Single zircon Pb evaporation data define crystallisation ages grouped around 540 and 490 Ma. Minor amounts of metasediments, with zircon ages exceeding 600 Ma are also present. During eclogite facies conditions, transformation of preexisting rocks to eclogites was incomplete and triggered by local presence of fluids. Fluid flow was partly channelised. Fluid pathways are represented by quartz-phengite-rutile veins. These veins generally show aureoles with rocks being transformed to eclogite-facies assemblages. Syn-metamorphic deformation is focussed in shear zones. In the main volume, however, magmatic, pre-eclogite facies macrotextural features are preserved. Internal Rb/Sr isochrons, based on eclogite facies minerals from a variety of lithologies, precisely define an age of 358 ± 3 Ma for all syn- and post- eclogite-facies processes studied. Within limits of error, the same age was obtained for amphibolite facies assemblages, with the exception of two samples showing slightly higher ages. The age of 358 ± 3 Ma also applies for minerals out of a fluid vein which caused local reamphibolitisation of eclogites. Rb/Sr biotite ages, interpreted as cooling ages, are indistinguishable from the age value given above. We conclude that transformation of the rocks to eclogites (600°C, 14 kbar; Udovkina, 1971) and subsequent uplift-related cooling was a rapid process, which took place in a narrow time interval of 6 Ma or less. In the Rai-Iz massif, a nearby ophiolite complex, phlogopite-amphibole-bearing fracture fills with serpentinisation aureoles were found. We interpret these veins as being formed by subduction-related dehydration fluids, passing the overlying mantle wedge. A provisional age for these veins is 381 ± 10 Ma. Implications of the data for the Uralian orogeny will be discussed.
Udovkina NG, Eclogites of the Polar Urals [in Russian], Nauka, Moscow, (1971).
The hinterland of the Middle Urals is dominated by island-arc volcanic rocks, separated by high grade, locally granulite facies, gneisses and major intrusive complexes. The high-grade rocks have previously been considered to be Precambrian complexes of (micro) continental affinities. One of the most well preserved granulite terranes in the Urals is the Early Devonian to Middle Carboniferous Salda Metamorphic Complex (SMC). The latter is partly covered by amphibolite and greenschist facies ophiolitic melange (Istok) of Silurian age that inturn is overlain by lower grade (sub-greenschist to greenschist grade) island-arc volcanics of mainly Silurian and Devonian age. Recent microprobe studies and pressure-temperature calculations combined with single zircon analyses isotopic age studies have allowed us to reconstruct the tectonic history of SMC. The oldest rocks found in the SMC are granulite-facies tonalite gneisses (Brodovo Suite) in the Brodovo-Maligino antiform. Zircons from these yield Early Devonian ages (387±12-405±6 Ma) and the minerals mainly display a prograde zonation. The second magmatic event was the emplacement of the Teliana gabbro-tonalite suite (354±3-359±3 Ma), these are mainly granulite-facies rocks but locally with an amphibolite to greenschist overprint. The well preserved granulites still show primary magmatic features and minor mineral zonations. The two magmatic suites are separated by a granulite- to upper amphibolite-facies shear zone that is well imaged by the ESRU96 seismic profile (Juhlin et. al 1998). Rocks of similar composition to the Teliana Suite inside the shear zone are dated to 341±8 Ma and display prograde metamorphism. The high-grade gneisses from the SMC are cut by subalkaline gabbros and monzonites dated to 337±5 and 333±5 Ma respectively. These intrusions only display a minor greenschist facies overprint. Earlier work by Grevtsova et. al (1967) and Bea et. al (1997) has shown the existence of Permian batholiths in the SMC as well as in the rest of the hinterland terranes. These intrusions show little Uralian tectonic overprint. We will propose the following tectonic history for the SMC: 1. An oceanic/back-arc environment in the Silurian (Istok Melange). 2. Early island-arc(?) setting in the Devonian (Brodovo Suite). 3. Late Devonian to Early Carboniferous evolved island-arc with intrusions in a granulite facies environment (Teliana) combined with eastward thrusting. 4. Early uplift in the Late Carboniferous (gabbro-monzonite intrusions). 5. Continued uplift in the Permian related to the batholith intrusions and 6. Widespread crustal extension and formation of Triassic West Siberian Basin and smaller graben structures across the Middle Urals.
Bea F, Fershtater G, Montero P, Smirnov V & Zin'kova E, Tectonophysics, 276, 103-116, (1997).
Grevtsova AP, Zukozurnikova GA & Dolgal AS, USSR Minestry of Geology (Report), (1970).
Juhlin C, Friberg M, Echtler H, Green AG, Ansorge J, Hismatulin T & Rybalka A, Tectonics, 17, 710-725, (1998).
The Bashkirian Anticlinorium of the southwestern Urals shows a much more complex structural architecture and tectonic evolution than previously known. Pre-Uralian Proterozoic extensional and compressional structures controlled significantly the Uralian tectonic convergence. A long-lasting Proterozoic rift process created extensional basement structures and a Riphean basin topography which influenced the formation of the western fold-and-thrust-belt with inversion structures during the Uralian deformation. A complete orogenic cycle during Cadomian times including terrane accretion at the eastern margin of the East European Platform resulted in a high-level Cadomian basement complex, which controlled the onset of Uralian deformation, and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the Main Uralian Fault.
The Uralian orogenic evolution can be subdivided into three deformation stages with different oriented stress-regimes. Tectonic convergence started in the Late Devonian with the formation of an accretionary complex which prograded from SE to the NW. Continuous NW-SE directed convergence resulted finally in the formation of an early orogenic wedge thrusting the Cadomian basement complex onto the East European Platform. The main tectonic shortening was connected with these two stages and, although not well constrained, appears to be of Late Devonian to Carboniferous age. In the Permian a final stage of E-W compression can be observed throughout the SW Urals. In the West the fold-and-thrust-belt prograded to the W with reactivation of former extensional structures and minor shortening. In the East this phase was related to intense back thrusting.
The thick and old crust of the East European Platform was extremely cold when it collided with the Magnitogorsk magmatic arc during the Late Palaeozoic. This resulted in a rather narrow zone of intense crustal shortening, tectonic stacking and high strain at its eastern margin (Ural-Tau zone). While the first orogenic wedge is of thick-skinned type with the involvement of crystalline basement, even the later W-directed wedge is not typically thin-skinned as the depth of the basal detachment appears below 15 km and the involvement of Archean basement can be assumed. The structural architecture can be correlated with deep seismic images of the URSEIS '95 experiment. The western Uralian fold-and-thrust belt appears a typical example which developed on rifted, cold and rigid continental crust.
Among Palaeozoic orogenic belts, the Caledonide-Appalachian belt is notable for its subdivision into several 'orogenies'. While the Appalachians are treated in terms of six orogenies, two major episodes have been established within the Scandinavian segment of the same belt: an Early Ordovician (Finnmarkian) and a Mid-Silurian to Early Devonian (Scandian) event. However, recent concepts of mountain building processes make the relevance of subdividing an orogen into distinct episodes less obvious. The increasing evidence of syn-convergence tectonic erosion and diachroneity due to oblique collision make important criteria as unconformities and molasse ambiguous, and delimitation of an 'orogeny' in the traditional sense problematic. Amalgamation of terranes prior to accretion to the continental margin results in mountain belts composed of numerous segments with complex magmatic and metamorphic histories, rather than a few large segments, reworked during single orogenic episodes. Moreover, what is the critical mass of an orogen(y)? A single unconformity? A dated pegmatite dyke cutting a fold?
With regard to the Scandinavian Caledonides, increasing radiometric dating tends to erode the statistical basis for a subdivision into two distinct events. This is particularly true for the imbricated continental margin (Baltoscandian Margin, BM), which occurs as thrust sheets of strongly varying intensity of deformation, age and grade of metamorphism. Assessment of the extent of involvement of the BM during accretion of the mountain belt is fundamental to understanding thermal regimes of metamorphic processes and interpretations of isotopic signatures of marginal basin magmatism. Subduction and imbrication of the BM with eclogite facies metamorphism of rift facies dolerites at 505-475 Ma was accompanied by detachment of a sheeted-dyke complex from the continent-ocean transition. At 480 Ma, the detached dyke complex was penetrated by granitic dykes with inherited Precambrian ages, suggesting partial melting of the BM at depth at that time. Continuous imbrication and understacking of thrust sheets is indicated by rapid exhumation of eclogites (490-470 Ma) and ages of high-P metamorphism ranging between 470-415 Ma, and reflected in thrust zone fabrics cooling below 500°C at 450 Ma. Amphibolite to granulite facies metamorphism and partial melting of the BM occurred from 445 Ma and into the Scandian phase of ultimate continent-continent collision.
Thus, from a review of published and new radiometric data we conclude that the BM was continuously involved in tectonic processes at amphibolite facies and higher metamorphic grades throuhgout accretion of the Scandinavian Caledonides. This requires an active 'western' margin of Continent Baltica in Ordovician-Silurian plate reconstructions of the North Atlantic tract and modifies the alledged concept of an episodic orogenic process.
The Vendian to Cambrian evolution is interpreted as a stage of post-rift subsidence following the transition from continental rifting to ocean floor formation off western Baltica at c. 600 Ma ago. Although early Cambrian flooding events lead to temporarily higher sedimentation rates, post-rift subsidence generally decreased through time. Such a decrease is consistent with models of lithospheric stretching and thermal subsidence. The gradual decrease of thermal subsidence through Cambrian time shows that the Baltica lithosphere was essentially thermally re-equilibrated prior to earliest Caledonian, Finnmarkian tectonic activity in early Orodvician time. Finnmarkian activitiy lead to the deposition of Ordovician greywacke beds but not to any orogenic deformation at the western Baltica margin.
However, Silurian to Devonian, Scandian tectonic activity built up an orogenic wedge. This wedge can be divided into three mechanically distinct tiers: A: Lower Allochthon with exclusively Scandian structures and metamorphism, B: Middle Allochthon and Seve units of Upper Allochthon with traces of Finnmarkian events, and C: overlying exotic terranes (Köli of Upper Allochthon, Uppermost Allochthon). Detailed structural work documented shear criteria at the tier B-C (Seve-Köli) boundary, which consistently indicate a down-dip, top-to-the-west movement at a regional scale, immediately after peak PT conditions (c. late Wenlockian). Subsequently, the Seve-Köli boundary was deformed by regional folds, caused by the stacking of thrust systems in the underlying Lower Allochthon.
The overall shape of the orogenic wedge and its critical taper angle are dependent mainly on basal friction and wedge strength. Due to lithology and early cooling, the tier B of the Caledonian orogenic wedge is dominated by strong rocks, whereas the lower tier (A), containing horizons with low friction, is of relatively weaker strength. There, organic-rich black shales provide low-friction horizons, both at the basal detachment surface and within the wedge itself. As a result, the lower, external part of the wedge had a lower strength and a smaller critical taper angle than its internal part, so the orogenic load is upward concave.
Modelling the effect of such a load on the Baltica lithosphere shows a very small depression in front of the load (~2 km). The flexural depression produced by the main part of the orogenic load is filled up by the thickening thrust-and-fold belt, so that there is little space left for a foreland-basin. These results imply that the extremely small size of the peripheral or pro-foreland basin in front of the Scandinavian Caledonides is not due to subsequent erosion, but is a primary feature.
Global plate tectonics in the Paleozoic Era were broadly dominated by plate convergence, as various elements coalesced to form the short-lived Pangea supercontinent by Triassic time. Most of the plate motion related to Pangea assembly was driven by oceanic subduction, sometimes coupled with continental subduction/collision and genesis of very high-pressure rocks. In addition to this broad convergent setting, field evidence indicates that many regions were undergoing concomitant extension or crustal thinning, not necessarily related to 'exhumation' or 'gravitational collapse' scenarios. The importance of this type of Paleozoic crustal stretching and related finite strain patterns may go unaddressed in areas lacking extensive sedimentary or magmatic manifestations. We discuss the judicious application of structural, paleomagnetic and geochronologic analyses to the 'post-Caledonian' SW margin of Baltica where the paucity of late Paleozoic rocks has previously limited discussion of extensional tectonics during this time period.
First order field structural analysis has demonstrated that the current geometries of ductile extensional detachments in western Norway have been modified from their 'original', early to middle Devonian orientations; this was affected by imposition of post-early Devonian folding and/or several generations of younger, brittle faults. Paleomagnetic dating of brittle faults and dikes has indicated latest Permian ages for some of these brittle events; these ages are complemented by 40Ar/39Ar ages from the brittle fault rocks and dikes of latest Permian age (238 to 248 Ma for dikes and 250-260 Ma for some brittle faults). Further work with K-feldspar thermochronometry has demonstrated an Early Carboniferous unroofing event, possibly related to folding of one detachment. A natural concern in such studies is fluid flow in the faults (and dike margins) that may have disturbed the Ar- and/or magnetic signatures of the fault rocks. We have found that conducting profiles through fault zones, in areas with well-controlled tectonostratigraphy, is a satisfactory means by which to support the accuracy of the given age data. Examples from the Nordfjord-Sogn Detachment, the Oslo Rift, the Lærdal-Gjedde fault and the Møre-Trøndelag Fault Complex will be discussed.
The Seve-Kalak Superterrane (SKS) in the Scandinavian Caledonides contains the fragmented Late Precambrian continent-ocean transition between Baltica and the Iapetus Ocean. This passive margin was thrust eastwards over the Baltic Shield during Caledonian orogenesis (c. 520-390 Ma). The individual thrust sheets in the SKS went through different PTt-evolutions, resulting in dramatic metamorphic contrasts: Eclogite-bearing nappes are juxtaposed with nappes showing no evidence of Caledonian deformation or metamorphism in their interiors. A pronounced strain localisation to the marginal parts of the thrust sheets often left the interiors exceptionally well preserved. Excellent records of both pre-orogenic (rift) and early-orogenic (subduction and subsequent uplift) processes are thus preserved in the thrust sheets of the SKS.
Even though it has been transported several hundred kilometres, only the margins of the eastern part of the Sarektjåkkå Nappe (SN) are affected by penetrative Caledonian deformation. This part of the SN is dominated by pristine tholeiitic dykes and cross-bedded sandstones. The dykes are 608 Ma old and make up 70-80% of the nappe. Widely spaced (several km's), west-vergent thin shear zones of the Ruopsok fault system (RFS) are the only visible signs of Caledonian penetrative deformation in the interior of the nappe. Previously published Ar-Ar-datings indicate cooling below the closure temperature of hornblende at c. 470 Ma, but aberrant mica ages of up to 500 Ma have been recorded.
Ar dating of biotite and muscovite from a cross-laminated metapsammite in the SN gave well-defined ages of 428.5 and 432.4 Ma, respectively. Muscovite from a shear zone in the RFS gave 428.2 Ma, whereas hornblende from the same locality did not yield interpretable data. All data sets indicate that the rocks were completely degassed at some unknown earlier event, presumably the emplacement of the dyke swarm. No subsequent contamination with excess argon can be detected. These Ar ages date an event which is not recorded in any other way; there are no other indications of an event later than the emplacement of the dykes at 608 Ma.
The interior of the nappe - and thus the entire nappe complex, since the interiors should be the last to become affected by conductive heating - cooled below c. 350°C at around 430 Ma. Simple linear cooling from the c. 500°C at 470 Ma to 350°C at 430 Ma suggest a cooling rate of ~4°C/Ma. This is a low figure, but not unreasonable. It suggests a prolonged period of slow cooling (=exhumation?), following the initial, rapid uplift of the eclogite-bearing nappes and Early Ordovician amalgamation of the SKS. However, mica ages indicating ages older than 430 Ma are difficult to reconcile with this model; their geological significance should be critically reevaluated.
The Ordovician-Silurian accretionary wedge of the Southern Uplands and the structurally associated Ballantrae Ophiolite Complex formed part of a Caledonian convergent plate margin in SW Scotland. Provenance analysis of sediments from both tectonic units and the unconformably overlying Devonian Old Red sandstones were carried out to constrain source environments and elucidate processes of sediment recycling in a fossil convergent margin setting.
Detrital framework modes of sand size fractions in greywacke and sandstone samples were determined using the Gazzi-Dickinson method. The sedimentary cover of the Ballantrae Ophiolite Complex has a recycled orogenic provenance. The Ordovician units, with a distinctly magmatic source rock signature, show increasing compositional maturity with decreasing age. This may reflect either increasing distance to the source area with time, or continuous recycling and resedimentation. Samples from the Old Red sandstone unit show a clear cratonic provenance.
Our data from Southern Uplands show that the greywackes there were derived from a volcanic island arc. This interpretation is confirmed by earlier studies and by our results of XRF-analyses of detrital clinopyroxenes found in the greywackes. Here, a mixed calc- alkaline and tholeiitic signature is evident for mid-Ordovician to Silurian coarse clastics. In the oldest samples analysed (Tappins Group of Llanvirn age) clinopyroxenes are found, however, that are derived from alkaline basalts pointing to an intraplate volcanic source.
Geodynamic models for the Scottish Grampian orogeny envolve collision between a peri-Laurentian island arc and the passive margin of Laurentia. The collision produced large scale knappe stuctures, regional Barrovian metamorphism, syn- and post-metamorphic acid and basic magmas and deposition of flysch into neighbouring terranes. In Scotland, this is traditionally regarded as having lasted over a period of ~200 Ma. To testif the Grampian orogeny was actually a catastrophic Himalayan scale ~20 Maorogeny or not, both syn-metamorphic and post-metamorphic granites have been sampled. Prismatic, brown to light brown, transparent zircon grains with a c-axes length of up to 200 µm have been picked for dating by the single grain evaporation method of Kober (1986). Cathoden lominicense has shown that all grains are without cores and inclusions. These new data indicate that the Grampian orogeny in Scotland occurred between ~480 and~465 Ma. A study of the mineral composition of Ordovician and Silurian flysch from the neighbouring terranes reveals characteristics of a metamorphic/plutonic source very similar in type and age to the Grampian terrane. The depositional age of the earliest flysch is Middle Llanvirn. The emplacement of post-orogenic granites during the later Ordovician and though the Silurian into the Lower Devonian caused the Grampian terrane to be bouantly uplifted resulting in the continuum of erosion and flysh deposition.Comparison of these data from the west of Ireland (Friedrichs et al. 1997) suggest that the Grampian Orogeny was short catastrophic Himalayan styleorogeny between ~480 and ~465 Ma.
Friedrich et al, Terra Nova, 9, 31, (1997).
Kober, Mineral. Petrol, 93, 482-490, (1986).
Understanding mountain building evolution is based upon a good knowledge the pre-orogenic location and the collision period of the different accreted terranes. The traditional paleogeographic tools, paleomagnetism and paleontology, are not always able to resolve these problems. Espescially when remagnetization effects appears during orogenic events and when two areas contain same faunas which were separated by a narrow oceanic domain. All these limits lead to different paleogeographical models.
For example, in the northern Appalachian belt, several orogenic events occurred during the Paleozoic. The origin and the timing of the collision of the two exotic terranes (Avalon and Meguma) are discussed. There is no consensus about their collision time which can be occurred between middle Ordovician (450 Ma) and Late Devonian (370 Ma) after the different models. The gondwanian origin of the two exotic zones is well demonstrated but the precise location around this continent is not clear. Before the collision, they may came from the north of South America or from West Africa. Moreover, it is not clear if their accretion consisted to an arc-continent or to a continent-continent collision.
In order to resolve these problems, we use the Sm/Nd isotopic system in shales and we use published data of detrital zircons ages to constrain the location of the exotic zones during the Neoproterozoic. Assuming that the Nd isotopic initial composition in shales characterize a similar tectonic domain, we compare Sm-Nd isotopic data from the Humber zone, which corresponds to the sedimentary record of the Laurentian domain, with data from the exotic zones (Avalon and Meguma). All these data permit us to propose a model in which during the Neoproterozoic (670 Ma), the Avalon zone was located near the north of South America and the Meguma zone was situated near West Africa. We propose that the Avalon zone collided with north America in middle Ordovician (450 Ma) and the Meguma zone collided during early Carboniferous (between 370 and 330 Ma).
The Dalradian rocks of northwestern Britain and Ireland provide a classic example of collisional orogeny involving Barrovian metamorphism and clockwise PTt paths. This Grampian Orogeny was a thick-skinned event involving both basement and cover rocks. Controversy has surrounded the identification of basement elements as well as their affinity with Laurentia. The Dalradian was deposited on the rifted passive margin of Laurentia, during the Neoproterozoic to early Cambrian. Convergence leading to orogeny in the Early Ordovician cannot be the result of a simple Wilson cycle because the conjugate ocean margin is missing. Consequently models for the Grampian Orogeny have appealed to ophiolite obduction and collision with a magmatic arc.
Field observations and geochronology at a basement-Dalradian contact in northwestern Ireland lead us to suggest a new tectonic model involving indenter tectonics.
The metasedimentary Slishwood Division records pre-Grampian c. 600 Ma high-pressure granulite- and earlier eclogite-facies metamorphic events not seen within the Dalradian. Tectonic juxtaposition of these units occurred at c. 470 Ma during Grampian orogenesis, followed by peak Barrovian metamorphism at c. 465 Ma. Orogenic collapse is recorded by southeasterly-directed extensional deformation and pegmatite intrusion at c. 455 Ma.
At 600 Ma Laurentia is believed to have been in extension, a condition not conducive to high-pressure metamorphism. Although decompression from eclogite facies into the granulite field might be a lower crustal response to extension, it is our view that the early metamorphic history recorded in the Slishwood Division is unrelated to events on the Laurentian margin.
We propose that the Slishwood Division was the basement to a magmatic arc that collided with the Laurentian margin with the motion initially accommodated by subduction under its leading edge. Fluid infiltration into the Slishwood Division before collision is manifest by static growth of hydrous minerals, possibly in a supra-subduction zone setting. Hornblende geochronology dates this event at c. 480 Ma, 10 Ma before collision of the Slishwood Division with Laurentia.
Magnetic, gravity and seismic data suggest that the Slishwood Division does not extend laterally much beyond the present outcrop (c. 2000 km2). Transpressional structures either side of the Slishwood indenter have an opposite shear sense - dextral on northeast side, sinistral to the southwest - consistent with escape tectonics. The original geometry and extent of the indenter is unknown but sub-arc basement may be regionally extensive and could include the Tyrone Central Inlier and the enigmatic Midland Valley Terrane of Scotland.
Field and geochronological data suggest synchronous deformation, metamorphism and granite emplacement in transpressive orogens. Clockwise P-T paths are characteristic, and result from the effects of heat conduction to the surface during syntectonic erosion of the thickening orogen, dissipation of mechanical energy generated during deformation, and syntectonic mass transfer that advects hot material to shallow depths. In the Northern Appalachians, oblique translation during Devonian (Acadian) dextral transpression thickened the Silurian stratigraphic sequence of the Central Maine belt (CMB) and displaced isotherms toward the surface, creating a near-isothermal corridor. The typical high-T metamorphism of the CMB, however, reflects the additional effects of high heat production in the sequence, a consequence of high U and Th contents fixed in strongly reduced sediments of the precursor anoxic basin. Furthermore, basin subsidence likely prestressed the lithosphere, making this belt a weak link in responding to Devonian transpression.
Strain was accommodated heterogeneously, being localized into rheologically weaker strata. It was partitioned between steeply-dipping 'straight' zones of enhanced deformation that accommodated more displacement and record higher strain (higher strain zones, HSZs), and intervening zones, composed of rheologically stronger strata, that record lower strain (lower strain zones, LSZs). Perturbations in ductile flow caused folding and thrusting, but different rheological behavior between stratigraphic units resulted in enhanced fold tightening, overturning and limb shear strain in HSZs that was not recorded in LSZs. In HSZ rocks, penetrative continuous mica and quartz-ribbon foliation and bladed muscovite and biotite mineral elongation lineation define the tectonic fabric, recording apparent flattening-to-plane strain. In contrast, in LSZ rocks, foliation is weakly developed or absent and bladed muscovite that forms the prominent penetrative mineral elongation lineation defines the tectonic fabric, recording apparent constrictional strain. In both types of zone, deformation and mineral growth were coeval, because the same minerals define fabrics at the same grade.
At metamorphic grades above the contemporary solidus, stromatic migmatite and concordant or weakly discordant irregular 'sheet-like' bodies of granite occur in HSZs, suggesting percolative flow of melt along the flattening fabric and viscous flow of magma in planar conduits. Inhomogeneous migmatite and irregular 'rod-like' bodies of schlieric granite occur in intervening LSZs, suggesting transport of partially-molten material through these zones en masse by melt-assisted granular flow and in 'pipe-like' conduits. This relationship between fabric shape and the form of melt escape structures suggests flow was deformation-controlled and governed by strain partitioning. Thus, the coupled mechanical and thermal evolution of transpressive orogens enables transport of melt to progressively shallower crustal levels by differential stress-induced processes as the near-isothermal corridor is propagated upward, advecting heat to drive upper crustal high-T metamorphism. One consequence of this feedback relation is the syntectonic nature of the metamorphism, and granite ascent and emplacement in transpressive orogens.
The paper presents a detailed description of the interpretation of deep seismic data along profiles in the Palaeozoic Platform in Poland close to the Variscan Front. Total length of the seismic refraction and wide angle reflection profiles in this area is about 2000 km and several short (20-30 km) near vertical reflection profiles located in the Polish Basin. The purpose of the interpretation is to determine the thickness of the sedimentary cover, to investigate low velocities in the upper crust indicate a possible mid to late Palaeozoic basin to a depth of 20 km, and to study the nature of the lower crust and the transition zone between the crust and upper mantle in this area. Seismic models of the crust has implication for Caledonian and Variscan collision tectonics and for subsequent basin formation in the area.
In the Izera-Karkonosze Block, West Sudetes, the 580-540 Ma late- to post-Cadomian granodiorites and Neoproterozoic metasediments were intruded by the 515-480 Ma Izera granite that became gneissified in Palaeozoic times. The granite contains xenoliths of high-P metapelites (17 kbar, 680°C)and possesses a diatexitic envelope with HT-LP mineralogy, all obviously pre-Ordovician in age. Besides, the pre-515 Ma country rocks are also preserved in narrow schist belts occurring within the Izera granite. The mica schist belts differ from one another in their lithologic contents, structure and P-T-t-d histories and cannot represent any continuous sheet of metasediments. Metamorphic grade of these belts decreases discretely northward and along the S-N section across the Izera massif, the schistose remnants are represented by (1) rocks metamorphosed in the Kfs-Crd zone (T > 750°C, 4 kbar), (2) upper amphibolite facies Grt-Sil-Crd metapelites, (3) lower amphibolite facies mica schists (3-5 kbar, 500°C) and (4) greenschist facies rocks (3 kbar, max. 350-400°C ), all testifying to c. 400°C difference in PT conditions between the belts. The widest belt consists of rocks metamorphosed at temperature conditions different by 250°C, which, because of tectonic shuffling, are now set side by side in a 0.5-2 km wide belt. These schistose remnants are interpreted as derived from a c.15 km thick crustal section embracing upper amphibolite through greenschist facies rocks. The Neoproterozoic crust which was partially molten to produce the Izera granite was already strongly deformed and metamorphosed in a typically orogenic way, with HP and LP rocks derived from different portions of the orogen and tectonically intermingled prior to the 515 Ma onset of the rift-related granite magmatism of the next tectogenic cycle. Before the Izera granite intruded its schistose country rocks, they had already been stacked by the Cadomian N-vergent thrusting and folding, which brought into contact rocks from different crustal levels and produced the northerly tilt of the Cadomian metamorphic isograds. During the Palaeozoic the overlying country rocks were incised, in normal to oblique regime, into the Izera granite and further deformed. Accordingly, elements of the Cadomian crust are identifiable despite the Variscan overprint. These are also recognizable elsewhere in the Sudetes and their presence is commonly proved by zircon (grain cores) inheritance ages.
Contribution to 'Europrobe' TESZ and 'Orogene Prozesse...' projects.
The Sudetes on the NE margin of the Bohemian Massif expose a mosaic of various pre-Permian complexes, traditionally included in the Variscides. An idea of a significant influence of the Caledonian orogeny in that area has developed since 1920's, was subsequently rejected as a result of accumulating new data and developing new methods and concepts in geology. This idea was recently revived in models invoking Early Palaeozoic to, possibly, Early-Mid Devonian subduction and continental collision following the final closure of the Tornquist Sea and introducing a hypothetical Caledonian suture zone in the Sudetes. We reassess the evidence for the timing of orogenesis, and suggest that a regional pre-Upper Devonian unconformity traditionally regarded as the conclusive evidence in favour of the Caledonian orogeny, cannot be recognized in the Sudetes. The widespread Ordovician bimodal volcanism cannot be explained as subduction-related, but is, most probably, the result of continental break-up. No structural data from the Sudetes confirm the idea of a Caledonian orogeny. The palaeontological and palaeomagnetic data used as evidence of Caledonian orogenesis is inconclusive and its interpretation disregards large-scale Late Palaeozoic strike-slip displacements in SW Poland and the collage structure of its pre-Permian basement.
The only Early Palaeozoic ("Caledonian") orogenic record is confined to the gneisses of the Góry Sowie Massif, which occupies an exotic, allochthonous position within the Variscan tectonic framework of the Sudetes. By contrast, the evidence for a leading role of the Variscan orogeny in the Sudetes, includes Late Devonian blueschist metamorphism followed by Early Carboniferous regional high temperature event, widespread Late Devonian/Early Carboniferous flysch/molasse sedimentation and abundant granite intrusion in the Carboniferous to Early Permian. We conclude that the dominant tectonic imprint on the Polish Sudetes is Variscan.
Independently, we suggest that on the grounds of plate tectonic principles it is undesirable to extend the regionally localized term 'Caledonides' over other orogenic belts affected by any kind of early to mid-Palaeozoic tectonism. It seems more logical to term early events in the tectonic development of the Variscan belt rather as 'eo-Variscan' than to recourse to the broadly coeval, but little related, processes within the Caledonian orogenic domain.
Mafic meta-volcanic rocks of Upper Cambrian to Lower Devonian age in the Bohemian Massif may be correlated across much of the European Variscides from the Polish Sudetes, through the Czech Republic (West Sudetes, Mariánské-Lázne, Kladská) and Germany (KTB, ZEV, Münchberg). This magmatism was generated during fragmentation of the Gondwanan margin and subsequent basin formation.
Two separate categories of mafic-volcanics are recognised. Alkali basalts and enriched tholeiites (<epsilon>Nd +2.8 to +6.3) were formed by small degrees of partial melting of an upwelling asthenosphere (primitive plume mantle). Minor crustal contamination of these mafic volcanics involving continental crust and pelagic sediment occurred. Advection of heat from mantle-derived melts resulted in crustal melting; most associated silicic magmas contain a major continental crust component (<epsilon>Nd -3.6 to -4.6). MORB-like volcanic rocks (<epsilon>Nd +7.2 to +8.7) were developed in areas that underwent more extensive lithospheric thinning and decompressional melting. True oceanic crust was developed only where extensive, prolonged rifting and magmatism occurred.
Early Palaeozoic rift-related magmatism is also known from other parts of the European Variscides, notably Iberia and the Massif Central in France. This Cambro-Ordovician event is coeval with that preceding the separation of Avalonia from the Gondwanan margin. The exact timing of the migration of these micro-continents is enigmatic but it appears to have occurred throughout much of the early Palaeozoic. This fragmentation could have resulted from (1) back-arc spreading, (2) the continental margin over-riding an oceanic spreading centre or (3) plume activity. With the possible exception of the western Alps, compelling evidence for the existence of island arcs in Lower Palaeozoic rift zones of the European Variscides is lacking and the volcanism is unlikely to have been in a back-arc basin. The association of enriched alkali basalts and MORB may be best explained by plume activity in an extensional environment, as trace element ratios (e.g. Th/Nb, Ce/Nb) of the meta-basalts are comparable to that of modern plume-generated basalts. Compositional variability of the mafic volcanics attests to the utilisation of sources of different composition and depth in the sub-continental mantle. These heterogeneities in the source area may ultimately be explained by the presence of an upwelling mantle plume experiencing episodic periods of activity. However, the plume could have been situated close to a pre-existing oceanic spreading centre that was being over-ridden by the active Gondwanan continental margin.
We propose that the break-up of the Gondwanan margin generated an archipelago of microcontinents, of which Avalonia was the first to rift away. This break-up was produced by continental margin migration across an oceanic spreading centre with associated episodic plume activity. Such a model provides the simplest mechanism for repeatedly deriving relatively small microcontinents from the Gondwanan margin during early Palaeozoic time.
We have determined the U-Pb ages of single detrital zircons from Cambrian sandstones across the Teisseyre-Tornquist Line, in order to constrain possible source areas and the docking of terranes against the East European Platform (Baltica). In addition, Cambrian faunas and their biogeographic affinities were taken into consideration when assessing the clastic provenance.The Cambrian trilobite faunas from the margin of the East European Platform (EEP) are characteristic of the Baltic province. Preliminary data from Middle Cambrian sandstones (Okuniew IG-1 borehole; c. 15 km E. of Warsaw) show the presence of sources with Mid-Early Proterozoic and Late Archean zircon ages. Additionally, subconcordant/slightly discordant analyses unequivocally indicate provenance from a source with 650-550 Ma zircons, potentially the Late Vendian volcanic rocks of the EEP (Compston et al., 1995). In the same borehole the crystalline basement has been dated at 1800+/-3 Ma. In the Lysogory Unit (NE part of the Holy Cross Mtns.), which now adjoins the EEP, the Middle to Late Cambrian trilobite and brachiopod faunas suggest affinities with Armorica and Avalonia, respectively. The Upper Cambrian sandstones contain zircons with concordant/subconcordant 207/206Pb ages of 608, 1798 and 2668 Ma. Discordant data points suggest the presence of Late and Early Proterozoic and Late Archean zircon detritus.These new data show that provenance of detrital zircon across the Teisseyre-Tornquist Line (Trans-European Suture Zone) is more complex than expected since: (1) c. 600 Ma zircon ages are not restricted to Cadomian (peri-Gondwanan) sources; (2) Terranes with Avalonian faunas, like the Lysogory, may contain detrital zircons whose ages can be matched with sources in Baltica.
Acknowledgements: This project is carried out within the PACE network of EUROPROBE
Compston et al, J. Geol. Soc. Lon, 152, 599-611, (1995).
The palaeocontinent of Avalonia rifted from the northern margin of Gondwana in the Early Ordovician and started to collide with Baltica probably in the Ashgill, subsequently creating the so-called North German-Polish Caledonides at the Trans-European Suture Zone (Cocks et al., 1997; Giese et al., 1997; McCann, 1998). The northeast margin of Avalonia is a matter of debate, but based on detailed petrographic and geochemical evidence it seems likely that it lies close to the present Caledonian Deformation Front. The island of Rügen lies just south of this front in the southern Baltic Sea and sedimentary rocks underlying the island have been penetrated by extensive drilling. The Rügen rocks range in age from the late Tremadoc probably up to the early Caradoc and they underlie undeformed sediments of a Devonian and Carboniferous age. The microfossil group Chitinozoa, discovered by Alfred Eisenack in 1930, is found in great abundances in the Lower Palaeozoic of Rügen. The usefulness of Chitinozoa for Lower Palaeozoic biostratigraphy is well-known (Miller, 1996). Like other planktonic organisms, chitinozoans appear to exhibit a degree of palaeobiogeographic differentiation into latitudinal bands, principally controlled by climate. By analyzing chitinozoan assemblages from more than 38 samples deriving from six different wells in the vicinity of Rügen (i.e. Rügen 5/66, Binz 1/73, Lohme 2/70, G14 1/86, K5 1/88 and H2 1/90), we are able to complement earlier observations concerning the age of the youngest sediments below the unconformity, the general stratigraphy and the geographic affinities of the Lower Palaeozoic rocks underlying the island. An example is the presence of Siphonochitina formosa in the Llanvirn of Lohme 2/70 and Binz 1/73 that indicates high latitude and colder water conditions for this period of Rügen sediment deposition.
The concealed Lower Palaeozoic rocks of Rügen were interpreted as deposited in the Tornquist Ocean, but hitherto, all the fossils recovered (mainly graptolites, chitinozoans and acritarchs) are planktonic and doubts have been raised wheather these organisms are useful for attributing an origin to any particular continent (e.g. Cocks et al., 1997; Verniers & Cocks, 1998). In order to circumvent this problem, we have compared the newly obtained chitinozoan data with independent provenance studies of the Rügen lower Palaeozoic sediments (see papers by Giese and co-workers). In addition, our results will also be compared with studies of the benthic faunas (Zagora, 1997).
Cocks LRM, McKerrow WS & van Staal CR, Geol. Mag, 134(5), 627-636, (1997).
Giese U, Katzung G, Walter R & Weber J, Geol. Mag, 134(5), 637-652, (1997).
McCann, T, Geol. Mag, 135(1), 129-142, (1998).
Miller, MA, Palynology: principles and applications, AASP, 1, 307-336, (1996).
Verniers, J & Cocks LRM, Schr. Stat. Mus. Min. Geol. Dres, 9, 195, (1998).
Zagora, I, NJb. Geol. Paläont. Abh., 203(3), 351-368, (1997).
Three rock associations critical for reconstructing Variscan crustal evolution are: 1) widespread HT-LP (cord-sill-K-spar, 700-800oC, ca. 4 kbar) paragneisses cut by syn- to posttectonic, mostly S-type granites (330-290 Ma); 2) felsic HP-HT (Ky-mesoperthite, 1000oC, >15 kbar) granulites, yielding consistent zircon ages 5-10 Ma older than the granites and enclosing HP-HT garnet peridotite/eclogite/pyroxenite (30-35 kbar!, 1000-1200oC) and 3) an early (pre-380 Ma and possibly as old as 480 Ma) subduction-type eclogite facies metamorphism.
The Carboniferous evolution requires a heat source at mantle depths before 340 Ma (producing HP granulite) followed by high temperatures at mid/lower crustal levels, starting around 330 Ma (to produce the granites). Detachment of lithospheric mantle, with concomitant invasion of the asthenosphere, would provide heat but would also lead to significant basalt production and underplating unless the residual lithosphere was at least 50 km thick. No seismic evidence for a thick basalt layer exists and granite geochemistry reveals only a minor mantle signature. However, if detachment occurred following collision and major lithospheric thickening, juxtaposition of >50 km thick crust and asthenosphere is a realistic possibility. In addition, the thickened crust would itself heat up due to disturbance of the distribution of internal heat sources. This scenario allows for fast heating at granulite-forming depths and slower heating in the upper/middle crust. However, as granulite-bearing units were already at shallow crustal levels to be intruded by the granites, and if the common ca. 340 Ma zircon ages record the HP event in the granulites, minimum exhumation rates of 3 mm/a are required, over an extremely large area, to juxtapose granulites and HT-LP rocks. In fact, 340 Ma ages both for spherical, metamorphic zircons in HP minerals and prismatic, magmatic zircons in LP cordierite-bearing melt pods cross-cutting the granulites points to extremely fast extraction of these rocks from deep to shallow levels. Overturn of a hot, thickened, crustal root could explain this fast exhumation and would also, in itself, produce an extra advected heat source at mid crustal levels at the right time for major granite production. Is this catastrophic crustal collapse the reason for the great width of the Variscan high grade core?
A pyropic garnetite from the Podolsko Complex, Bohemian Massif, contains relic mineral assemblage of pyrope-rich garnet, Al-poor orthopyroxene and quartz, reflecting pressures of 26/28±3 kbar at 830±30°C (Kotková et al., 1997). It represents the first definitive record of a crustal material subjected to very-high pressures and exhumed on the surface in the Moldanubian Zone. An important event on the exhumation path is marked by extensively developed MP/HT reaction textures involving Spr, Cd, Spi, OpxII and Pl, overprinting the VHP mineral assemblage. The garnetite contains two distinct zircon populations: equant, spherical grains, similar to those from granulites, and longer prismatic grains, differing from the typical granitic or rhyolitic ones by rounded edges and terminations. Single-grain U-Pb dating yielded ages of 433+27/-17 Ma, interpreted as the protolith age.
Largely euhedral monazite displays distinct zoning with low-Th cores and high-Th rims. Total-Pb electron-probe dating of the Th-rich parts yielded a weighted average age of 341±15 Ma, probably reflecting a HP/MP metamorphic event. Similar ages were recorded by monazite enclosed in garnet as well as that within the late symplectite.
The Podolsko Complex comprising mainly orthogneisses and leucocratic migmatites with relics of granulites and mantle rocks has been correlated with the Gföhl Unit. By its location close to the N limit of the South Bohemian Moldanubicum towards the Tepla-Barrandian zone, it represents the northermost extension of this unit. Present juxtaposition of the Podolsko Complex and the Monotonous Unit results from a combination of the compressional and extensional tectonic movements. Petrological and geochronological evidence imply rapid exhumation of the garnetite from depths of ca 90 km, requiring an active tectonic unroofing prior to the final collisional stages of the Variscan orogeny. MP reaction textures could result from a short-lived thermal pulse resulting from processes such as a fast extensional uplift or nappe stacking (cf. Kotková et al., 1997).
Kotková J, Harley SL, Fiera M, Eur J Mineral, 9, 1017-1033, (1997).
In the present study, Rb-Sr age determinations were carried out on biotites (n = 7) and muscovites (n = 8) from lowgrade metapelites of the northern Bavarian Forest (Moldanubian unit around Lam, Germany) in order to elucidate the Variscan cooling history of the studied rocks. The Moldanubian unit is characterized by a multistage metamorphic history. The youngest and best preserved metamorphic imprint is of the LP-HT type. The age of this stage is well documented at around 320-330 Ma by different geochronological methods. Age data corresponding to the Devonian MP metamorphism (~ 380 Ma) of the adjacent units (i.e. ZEV, Münchberg Massif, Teplá-Barrandian) are completely lacking. The isotopic record of this event, if ever present, could have been totally erased by the younger LP-HT stage. Therefore, the lowgrade metapelites from the Lam area, where the LP-HT metamorphism reaches a minimum, are the most probable rocks in the Moldanubian unit to preserve Devonian age data.From our Rb-Sr age determinations we can say: (1) The biotite ages cluster around 318 Ma, in the range of cooling ages of the Variscan LP metamorphism. (2) Compared to the biotite ages, the muscovite ages exhibit a larger scatter and a tendency to older values (327-374 Ma). (3) The oldest muscovite ages lie around 373 Ma within the range of cooling ages of the Variscan MP metamorphism. These ages provide the first geochronological evidence of a Devonian metamorphic event in the Moldanubian unit. (4) Even closely spaced samples yield variable muscovite ages.We interprete these results in the following way: All samples were affected by an Early Devonian metamorphic event. During subsequent cooling the Rb-Sr system of muscovite was closed at around 373 Ma. Following this episode, the rocks were overprinted by the Carboniferous LP metamorphism reaching temperatures in the range between the closure temperatures of muscovite (500 ± 50°C) and biotite (320 ± 40°C). Final cooling to the closure temperature of biotite took place around 318 Ma. The considerable scatter of the muscovite ages which contrasts the uniformity of the biotite ages is thought to reflect variable degrees of isotopic resetting during shear deformation below the muscovite and above the biotite closure temperature. According to this interpretation, only the oldest muscovite ages are assumed to date the original, purely thermostatic cooling through the muscovite closure temperature. Comparing our results with geochronological data from the adjacent MP units, we suggest that the Moldanubian and the MP units experienced a parallel metamorphic evolution during the Variscan orogeny. The different age patterns preserved in the Moldanubian and the MP units are purely due to the fact that the latter were sitting at a shallower crustal level in the course of the Carboniferous LP metamorphic event.
Metaperidotites from a part of narrow continental rifted domain of Cambro-Ordovician age at the NE margin of the Bohemian Massif were investigated. Here, the mafic lower crust of Cambro-Ordovician age and upper mantle were exhumed during the Variscan convergence. At the present day, the boudins of spinel metaperidotites mark a major interplate thrust boundary between the Moldanubian/Lugian orogenic root domain and the Brunian Pan-African microcontinent.
Metamorphic evolution of spinel peridotites is marked by early hydration and serpentinization followed by prograde metamorphism resulting in development of a mineral assemblage consisting of orthopyroxene - magnesiohornblende - spinel - chlorite - olivine. Increase in temperature is documented by overgrowths of higher-grade minerals over lower-grade minerals, and by prograde chemical zoning of spinel, amphibole and orthopyroxene. The maximum temperature conditions estimated on the basis of calculated reactions and conventional thermometry correspond to 700-800°C at pressure conditions below ~10 kbar.
Proposed scenario of thermal evolution and tectonic significance of studied peridotites is based on their metamorphic evolution, metamorphism, structure and zircon geochronology of surrounding rocks and thermal and rheological modelling. Upwelling of upper mantle and thinning of lower crust occured during Cambro-Ordovician rifting, and consequently both lithologies were exhumed to rather shalow level. After ~ 150 Ma of cooling at the beginning of Variscan collision, both lithologies were "cold" and very "strong". Such previously rifted domain cannot be thickened because it is stronger than the adjacent continental lithosphere. Therefore, the prograde metamorphism of partly serpentinized mantle rocks can be seen as a result of thermal effect of underplated magma, indicated at the present erosion surface by close granodioritic sill. The heat from magma was sufficient to considerably weaken thin layer of the uppermost mantle and to allow initiation of a ductile thrust in peridotites which exhumed cold and brittle part of the lower mafic crust.
The Erzgebirge is situated at the northern margin of the Bohemian Massif. It is composed mainly of crystalline units (Erzgebirge Crystalline Complex:ECC) and of low-grade Palaeozoic sequences at its margins. This region has been traditionally interpreted as a NE-SW-trending anticlinal zone with an axis plunging to the SW. The eclogite-bearing units within the ECC and the low-grade Palaeozoic sequences have been interpreted as part of the para-autochthonous Saxothuringian zone.
This study focuses on the timing of the high pressure metamorphism and the protolith age in the ECC using the Sm-Nd, the conventional and Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) zircon technique. In addition, LA-ICP-MS was used for the trace and REE study on zircon crystals as well as on glass pellets.
Zircons were extracted from an eclogite at Wolkenstein and Voigtdorf (HP unit 2, Schmädicke et al.1994), from Eppendorf and Forchheim (both HP unit 1, Schmädicke et al.(1994). Oscillatory zoning as detected by cathodoluminescence (CL) using a scanning electron microscope, demonstrate the magmatic origin of zircon from Wolkenstein. All analyses of the magmatic domains yield a 206Pb/ 238U mean SHRIMP age of 490 ± 8 Ma (95%c.l.). LA-ICP-MS analyses of the same zircons yield a 206Pb/ 238U mean age of 495 ± 8 Ma. These ages are interpreted to reflect the emplacement age of the protolith of this eclogite of Wolkenstein. Conventional U-Pb single zircon analyses for the eclogite Forchheim yield concordant data points with an age of 339 ± 1.6 Ma; the LA-ICP-MS method detected for the zircons (eclogite Forchheim and Eppendorf) concordant 206Pb/ 238U mean ages of 338 ± 5 Ma as well as 344 ± 6 Ma. All Carboniferous ages are interpreted to reflect the time of the HP-metamorphism. There is no information on the protolith age.
Sm-Nd whole rock, garnet and cpx analyses display for the eclogite Wolkenstein an age of 343 ± 1.8 Ma, for the eclogite Voigtsdorf 335 ± 4 Ma and for the eclogite Eppendorf 341 ± 2 Ma. These ages reflect the time of the Carboniferous metamorphic overprint. This is similar to Carboniferous HP-metamorphism as detected in garnet-peridotites of the Czech part of the Erzgebirge (SHRIMP U-Pb zircon), the Moldanubian Zone or at the southern rim of the Bohemian Massif in Austria.
At the present stage of investigation the existence of Cambro-Ordovician protoliths - eclogitic and granulitic rocks - at the northern margin of the Bohemian Massif seems quite common.
The Saxo-Thuringian belt on the N flank of the European Variscides resulted from SE-ward subduction of a Cambro-Ordovician rift basin under the Teplá-Barrandian (Bohemian) margin. The resulting HP rocks are now exposed in different tectonic settings, and were exhumed in different modes, developed in different thermal regimes. Eclogites contained in the tectonic klippen of Münchberg, Wildenfels, and Frankenberg originated from early Devonian (c. 400 Ma) subduction to ~70 km depth. Cooling ages in the klippen, combined with clast spectra and mineral ages in the foreland flysch record exhumation in Famennan time (Schäfer et al., 1997). The HP rocks rose in a narrow corridor along the suture zone, were retrogressed under amphibolite facies conditions, and rapidly recycled into the foreland flysch. Rocks exposed in the cores of the Saxonian Granulites and the Erzgebirge record high-grade metamorphism at c. 340 Ma, although peak pressure might be older (Kröner et al., 1998). Eclogites were subducted to 150 km depth (Massonne, 1998); HP granulites were heated to 1.050°C (Hagen et al., 1997). These rocks were emplaced immediately after peak metamorphism, under the floor of the Saxothuringian foreland basin. This is most clearly documented in the core complex of the Saxonian Granulites, in which HP granulites are juxtaposed against LP/HT rocks of the hanging wall. The interface is an extensional zone of HT simple shear, which cuts out ~30 km of crustal thickness (Kroner, 1995). It appears that granulite emplacement under the foreland crust was accomodated by extension, which permitted continued sedimentation on the hanging wall. The 340 Ma rocks do not appear in the clastic record of the flysch.Emplacement of hot, low viscosity rocks was probably driven by buoyancy and the hydraulic gradient between the crustal root to the SE and the lower crust of the foreland. Unlike the earlier HP rocks exposed in the klippen, the younger HP rocks were thermally softened, and - instead of piercing their cover - intruded into the foreland. In more internal parts of the belt (Erzgebirge), the injected high grade rocks were subsequently affected by thrusting and polyphase refolding (Nega, 1998).
Schäfer J, Neuroth H, Ahrendt H, Dörr W & Franke W, Geol Rundsch, 86, 599-611, (1997).
Kröner A, Jaeckel P, Reischmann T & Kroner U, Geol Rundsch, 86, 751-766, (1998).
Massonne H-J, Ext Abs 7th Int Kimberlite Conf , Cape Town, 552-554, (1998).
Hagen B, Rötzler J & Hoernes S, Terra Nostra, 97/5, 58-60, (1997).
Kroner U, Freiberger Forschungshefte, C 457, 1-114, (1995).
Nega M, Dissertation TU München, 1-162, (1998).
Strength profiles through the continental lithosphere show the lower crust as a particular zone of low strength and low viscosity, respectively. It has been suggested that the lower crust can behave like a fluid on a geological time scale and can flow according to pressure gradients resulting from lateral differences in lithostatic head. Such pressure gradients exist, for example, between an orogen and its foreland. In addition, the increased radiogenic heat production within the thickened crust as well as heat from external sources (e.g., delamination) will further decrease the strength of the crust in the orogenic realm with respect to the surrounding. Consequently, there usually is a spatial correlation between lower crustal flow and thickened crust as the cooler and stronger foreland crust prevents any outward movement of lower crustal rocks. Thus, in order to allow substantial foreland-directed flow of lower crustal rocks an abnormal high temperature field in the foreland reducing the strength of the crust is required. Possible geodynamic scenarios which could generate such a hot foreland crust are active mantle plumes or a tandem of parallel subduction zones in which one back-arc mantle plume heats the foreland of the adjacent collision zone.
In order to gain a quantitative insight into the processes of lower crustal flow in orogenic settings a two-dimensional thermo-mechanical finite element model was developed and applied specifically to the rapid exhumation of the Saxonian Granulites in East Germany. Modeling results show that if a favourable temperature field exists in the foreland the lower crust is indeed capable to flow over substantial horizontal distances (> 100 km). The flow pattern of the lower crust can be described by a plug flow showing pronounced high-strain zones at the boundaries to the upper crust and mantle lithosphere. This process of foreland-directed lower crustal flow could be an effective mechanism to exhume HT/HP rocks like the Saxonian Granulites in areas which never experienced much crustal thickening.
The scope of this contribution is to present the occurrence of coesite-bearing eclogites in the Eastern French Massif Central (Mt. du Lyonnais unit) and to discuss the exhumation of these very high-pressure rocks. Detailed mineralogical investigations and geochronological data allow construction of both P-T and T-time paths; the combination of both datasets suggests a depth-time path and related exhumation rates, where the change in geotherm has been taken into account during the calculation. High-pressure metamorphism (minimum 28 kbar or ca. 90 km) is constrained between 380-400 Ma. The rocks cooled below 500°C at pressures of 8-9 kbar at 360 Ma and below 350 °C at 340 Ma. Thus at 360 Ma, the rocks had been exhumed to ca. 30 km depth. These kinetic results conform with the geological constraints extracted from the tectonic and sedimentary record of the Eastern French Massif Central.
These multidisciplinary approaches provide new information on Palaeozoic orogeny and allow us to discuss the relative roles of subduction and collision in exhumation of very high pressure rocks. We suggest that a significant amount of exhumation of these rocks occurred during subduction in a back-arc environment, prior to continental collision; significantly, continental collision itself was responsible only for the final stages of exhumation of these rocks under a transpressive regime.
The Hoher-Bogen shear zone is situated in the SW-Tepla-Barrandian at the border to the Moldanubian unit (sensu strictu). Here the NNW-SSE striking West-Bohemian shear zone and the WSW-ENE striking Central Bohemian shear zone merge in a large bend. Neither ductile nor brittle faults associated with these shear zones continue to the south or west of the Hoher Bogen. The Hoher-Bogen shear zone is characterized by an up to 4 km wide zone of high temperature amphibolitic mylonites, a thin zone of ultrabasic mylonites, and rapid increase of metamorphic grade to the SW from greenschist to granulite grade. The bordering Moldanubian unit here with 450°C shows a minimum temperature of metamorphism, but calculated pressures are about 10 kb at both sides of the Hoher-Bogen shear zone. The early Variscan age of deformation in the shear zone is deduced from K/Ar hornblende cooling ages with 380 Ma on average and Rb/Sr white mica cooling ages of 375 Ma (see Ihlenfeld et al., this volume). Intrusion ages for the metabasites are Cambrian. The shear zone is mostly made up by amphibolitic mylonites with ubiquituous large plagioclases, that increasingly recrystallize into elliptic plagioclase domains. A transition from gabbroic to amphibolitic rocks, that we interprete as metagabbros, can be observed following a NE-SW profile. We measured the strain distribution in the shear zone and investigated the regional distribution of amphibole and plagioclase microstructures. High temperatures (750°-850°C) for the ductile deformation were deduced using amphibole/plagioclase equilibria and the Al-in sphene thermometer on syntectonic sphene. Strain generally increases to the Tepla-Barrandian/Moldanubian border, but with an inhomogeneous strain pattern. The increase in deformation towards the Tepla-Barrandian/Moldanubian border is contemporaneous with the regional increase in metamorphism, and increasing recrystallization of amphibole and plagioclase.The major ductile deformation occured during uplift from lower crustal depths of about 35 km during the Devonian, forming a steeply to the NNW to E dipping foliation and steep NE stretching lineation. The uplift is also shown by the regional distribution of shear sense criteria, that generally indicate uplift of the SW part. A reconstruction of a SW-NE profile through the Hoher Bogen shear zone using average strain values indicates vertical stretching of about 385%. These lower crustal ductile movements are interpreted to be the result of a Devonian SW-NE collision between the Tepla-Barrandian and Moldanubian blocks, where the relatively strong amphibolites formed an indenter at the SW-edge of the Tepla Barrandian. This indenter was pushed deep into the Moldanubian, that is folded around it, and was squeezed upwards to mid to high crustal depths together with the Moldanubian unit during the Devonian. The shape of the indenter results from the inherited geometry of the pre-Devonian plate margin.
Brittle microtectonics, the application of techniques of mesofracture analysis (Hancock 1985), is a well established approach to the solution of tectonic problems. This study will concentrate on the commonest of brittle mesofractures, joints, and attempt to characterise them in relation to their role in the tectonic evolution of the Irish Variscides. It is widely believed that joints are more reliable indicators of stress/strain trajectories in weakly deformed rocks than in fold-thrust belts and regions with higher and more complex strain histories. It is normally assumed that systematic jointing in the latter case post-dates the main compressional orogenic event. However, while systematic jointing in SW Ireland is a late stage episode in the Late Palaeozoic structural evolution of the area, it is linked in a very real way to the Variscan orogenic stress/strain regime.
A single joint set dominates the mesoscopic fracture pattern of Beara, with a modal range of 165°-180°. Photo-lineament data from the area indicate the presence of two trends, a dominant set with a modal range of 040°-070° and a minor set with a modal range of 000°-020°. The overall fracture pattern for the study area is surprisingly systematic for a fold/thrust belt setting, thereby prompting the need for a re-examination of the significance of these structures in such settings. It is argued that the dominant photolineament set represents Variscan tectonic fabric control on joint development during late stage uplift. The dominant mesofracture joint set is thought to be a consequence of tensile failure influenced by a waning Variscan stress regime, again during uplift. The minor macrofracture set post-dates the influence of the Variscan palaeostress regime and is tentatively postulated to be related to Tertiary opening of the North Atlantic
Hancock, P, Journal of Structural Geology, 7, 443-457, (1985).
The final convergence stage of the Variscan orogeny in Central Europe is characterized - among others - by HT/LP metamorphism and widespread granite intrusions. The low strength of the lithosphere provides an important constraint for the maximum crustal thickness in the Variscides and may have caused a widening of the deformation zone both perpendicular and parallel to the orogen rather than further crustal stacking. The resulting syn-convergent extension in combination with erosion can provide an effective mechanism for the rapid exhumation which is typical for several Variscan metamorphic complexes. In order to get a quantitative insight into the complex strain distribution in zones of continent-continent collision this study utilizes numerical modeling techniques and compares the results of the numerical simulations to field observations from the Variscides. The numerical simulations are based on a two-dimensional plane-strain finite element approach. The modeling concept and boundary conditions follow work of Beaumont & Quinlan (1994) who studied evolutionary models of doubly vergent compressional orogens. In addition to various parameter studies the numerical model is applied to two examples from the Variscan orogen. In each case the modeling results are compared to field data, in particular petrologic and geochronologic data as well as seismic sections. The first case study deals with the synconvergent evolution of the Black Forest (SW Germany). The second example concentrates on a NW-SE cross-section through the northern Variscan orogen and examines the effect of two collision zones and laterally varying lithospheric strength on the resulting deformation pattern. A special aspect of this model setup is the overlap of the retro- and pro-side shear zones in the area between the two collision zones.
Beaumont C & Quinlan G, Geophys. J. Int, 116, 754-783, (1994).
Southeastern sectors of the Bohemian Massif are locally transected by two generations of lamprophyric dykes which post-date internal Variscan deformation of the variably metamorphosed constituent nappe units. Dykes of the first generation trend ESE, are locally foliated and have been variably affected by low grade metamorphism. This generation is interpreted to have been emplaced during final WNW-ESE shortening of the Variscan nappe complex. A 322.5 ± 0.3 Ma (2 sigma internal error) 40Ar/39Ar biotite plateau age a representative, most external,unmetamorphosed dyke of the first generation is interpreted to reflect post-magma crystallization cooling. Most of the second generation dykes are unfoliated and unmetamorphosed. These follow a major NNE-trend, and were emplaced in all major tectonic units in central sectors of the orogen postdating regional Variscan deformation and metamorphism. Three biotite concentrates from the younger dykes record 40Ar/39Ar plateau ages of 316 - 306 Ma (2 sigma internal error). Chemical compositions of second generation dykes vary from gabbroic, potassic mafic to trachyandesitic compositions of a high-K series. They are interpreted to have originated from a crust-contaminated mantle source which likely resulted from post-collisional remelting of subducted lithosphere. Emplacement was related to late stage orogenic extension which allowed ascent of magmas generated by post-collisional remelting of subducted lithosphere or within the asthenosphere following lithospheric break-off. The new data constrain a c. 15-10 Ma interval between Variscan plate collision and subsequent extension. A similar, short duration between compression and subsequent lamprophyre emplacement has been observed in other orogens, e.g., Dabieshan. We suggest, therefore, that post-collisional lamprophyre emplecement result from similar geodynamic scenarios.
Sm-Nd isotope and geochemical data were obtained for clastic (meta)sediments from the Saxothuringian flysch basin and from possible source regions to determine the provenance of siliciclastic detritus deposited and to reconstruct aspects of the sedimentation history. Since Sm-Nd isotope systematics are in general preserved throughout diagenesis and metamorphism, Nd-model ages reflect the average crustal residence age of a sediment, e.g. the predominance of older or younger crustal materials. In combination with main and trace element geochemistry changes in source material are monitored.
Clast spectra and isotopic ages of detrital mica and zircon (Schäfer et al., 1997; Neuroth, 1997) are interpreted to document the dominant influx of debris from the southern border of the Saxothuringian basin, e.g. the Cadomian basement, from middle Cambrian until lower Carboniferous times. <epsilon>Nd for the same strata decreases from -7/-8 for the Upper Proterozoic/oldest Cambrian in the Barrandean basin to -9/-10 for the Ordovician/lower Silurian indicating the entry of higher amounts of older and highly reworked material. While the inset of sedimentation from Ordivician sources (Neuroth 1997) in both allochthonous and autochthonous sediments of lower Carboniferous age coincides with an increase of <epsilon>Nd to about -8, a further increase to about -4 in strata from the uppermost lower Carboniferous might coincides with the finding of early Variscan detrital muscovites possibly from the Bohemian Massif (Neuroth, 1997).
Episodical peak values for <epsilon>Nd of 0 to -3 in the Devonian are related to the local influx of volcaniclastic debris. Young Nd model ages (1.2-1.4 Ga) and high <epsilon>Nd (0 to -3) from the Upper Precambrian of the Tepla-Barrandean and the ZEV are to be seen in the context of other sedimentary strata in Europe of similar age and Nd systematics (Michard et al., 1985; Nägler et al., 1995; Ugidos et al., 1997) being still controversely discussed.
Michard A, Gurriet P, Soudant M & Albarède F, Geochim. Cosmochim. Acta, 49, 601-610, (1985).
Nägler TF, Schäfer H-J, & Gebauer, D, Chem. Geol, 121, 345-357, (1995).
Neuroth H, Göttinger Arb. zur Geologie und Paläontologie, 72, 143 pp, (1997).
Schäfer J, Neuroth H, Ahrendt H, Dörr W & Franke W, Geol. Rdsch, 86, 599-611, (1997).
Ugidos JM, Valladares MI, Recio C, Rogers G, Fallick AE & Stephens WE, Chem. Geol, 136, 55-70, (1997).
The Mid German Crystalline Rise (MGCR) is a crustal segment of crystalline rocks within the Saxothuringian zone of the Variscan orogenic belt. It consists predominantly of granites and a variety of metamorphic rocks. The chemical composition of the magmatic rocks is typical for a subduction-related origin. This implies, that the MGCR was a magmatic arc terrane within the Variscan orogenic belt. In this contribution we present constraints on the timing of magmatic activity and we try to quantify the arc addition and the crustal growth in the MGCR during the Variscan orogeny.
The arc activity in the MGCR was not continuous throughout the time of the Variscan orogenic cycle. Three distinct episodes of arc-related igneous activity can be identified. The first magmatic phase lasted from about 440-410 Ma. Rocks of this Silurian period are exposed in the northern part of the MGCR and in the Northern Phyllite zone. The second pulse of igneous activity lasted for about 10 Ma from 370-360 Ma. This late Devonian magmatic phase is only documented in the western part of the MGCR. The most important and wide spread magmatic episode took place in the lower Carboniferous from about 340 Ma to 320 Ma. The number of precise ages now provides a sufficient data base to identify these distinct magmatic pulses. This age distribution underlines that even in a small region such as the MGCR the Variscan orogeny was not a single event but is characterized by three distinct episodes.
The contribution of juvenile, subduction-related material is known from Cenozoic arcs to be 20-40 km3km-1Ma-1 (km3 material per km arc length and million years, Reymer & Schubert, 1984). We used the Nd and Sr isotope data of the igneous rocks to estimate the amount of reworked crust and the addition of mantle-derived material. These values and the geochronological data of the arc-related magmatic episodes were taken to calculate the arc addition rate for the MGCR. The resulting rates are 12-32 km3km-1Ma-1 for the Silurian, 32 km3km-1Ma-1 for the Devonian, and 56-80 km3km-1Ma-1 for the Carboniferous episode. The Silurian and Devonian rates are in the same range or slightly lower than the rates of modern arcs. The addition rate of the lower Carboniferous arc, however, is significantly higher than that of modern arcs. We suggest that this was triggered by a high convergence rate of the Variscan terranes during the Lower Carboniferous.
Reymer A & Schubert G, Tectonics, 3, 63-77, (1984).
Palaeontological criteria, for a longtime, favoured a Gondwanan origin for the metasedimentary units and acidic extrusive rocks of the Upper Austroalpine nappes in the Eastern Alps (Noric Terrane: Frisch and Neubauer, 1989). In the Central and Western Alps, Variscan amphibolite facies metamorphism and anatexis strongly transformed pre-existing units, but lithostratigraphic comparisons and increasing isotopic data show that a zone containing Ordovician migmatites and intrusive granitoids can be followed through the different Alpine realms. The striking resemblance of the granitoids is confirmed through geochemical data, and most carry the fingerprints of a former arc volcanic situation. A mainly twofold evolution must be envisaged for these rocks: i) Late Precambrian oceanic crust and Cambrian island arcs indicate different tectonic regimes at and off the Gondwana margin (Schaltegger et al 1997). Early Cambrian ultramafic, mafic and granitoid rocks document the general tendency of rifting from that time until the Ordovician, leading to the break-up into the so-called peri-Gondwanan microcontinents. An Ordovician, Gondwana-directed orogenic evolution is preserved in meta-eclogites, occurring together with ultramafic and metabasic rocks in the Alpine basement areas (Biino 1995, Zurbriggen et al 1997). The minimum age of such rocks is constrained by Ordovician migmatites and the intrusion of Late Ordovician granitoids. All these relics of pre-Variscan crust in the Alpine domain can be explained by a former peri-Gondwanan organization (Stampfli and Mosar, 1998).
ii) Besides traces of Devonian evolution, all domains testify the large-scale Variscan super-collision amalgamating different microcontinental blocks into Pangea. The linear distribution of Visean durbachites could be a relic of the Variscan suture, comparable to other Variscan areas (Matte 1998), and could help to find a Visean palinspastic reconstruction for the Alpine domains. Late- and post-Variscan events are characterised by regional anatexis and different pulses of magmatic rocks associated with renewed rifting.
Biino G, Eur. J. Mineral, 7, 57-70, (1995).
Frisch W & Neubauer F, Geol. Soc. Amer. Spec. Paper, 230, 91-100, (1989).
Matte P, GFF, 120, 209-222, (1998).
Schaltegger U, Nägler TF, Corfu F, Maggetti M, Galetti G & Stosch HG, Schweiz Mineral Petrogr. Mitt, 77, 337-350, (1997).
Stampfli G & Mosar J, www-sst. uni. ch, (1998).
Zurbriggen R, Franz L & Handy M, Schweiz Mineral Petrogr. Mitt, 77, 361-381, (1997).
Large-scale correlations of the metamorphic terranes across the Ibero-Armorican arc have been hampered by several difficulties. New structural and petrological data in some key areas in southern Brittany reveal striking similarities with the tectonic evolution described in the well-exposed cross-section observed in northern Galicia.
The key element is provided by the basal unit of the Champtoceaux nappe (Cellier), closely similar to the basal units of the Galician nappes (Malpica-Tuy - Santiago). Both units present the same lithology: various types of granites-rhyolites, of the same chemistry and age (about 480 Ma), basaltic to doleritic lava flows or dykes, and rare hornfelses. The metamorphic evolution is closely comparable, with an early eclogite-facies event followed by decompression at decreasing temperatures. Moreover, geochronological data show that the age of the high-pressure event is uppermost Devonian in both cases (about 360 Ma: U-Pb, Sm-Nd) and that cooling (Rb-Sr and Ar-Ar) occurred shortly after the peak P-T conditions.
The basal units are overthrusted by intermediate units consisting of mafic (amphibolites) and ultramafic rocks which could represent a former oceanic crust and are devoid of eclogite-facies relics. In the upper unit (i.e. in the Champtoceaux unit, which could be compared to the Agualada unit), a few poorly-preserved eclogites are observed in partially-melted orthogneisses. Thrusting of the upper unit is accompanied by intense ductile deformation at subsolidus conditions in the hangingwall. In the footwall, thrusting results in an inverted metamorphic gradient, where Grt-Cld assemblages are overprinted by Grt-St-Bt assemblages, with occasional Ky. Microstructural data show that the HT minerals partly overgrow the fabric associated to the thrusting. This inverted gradient is also known in Galicia. The highest unit in the nappe pile in Brittany consists of Precambrian micaschists (Mauges), and are intruded by gabbros (le Pallet). This unit could be correlated with the Ordenes schists and Monte Castelo gabbro in Galicia.
The proposed correlations permit an estimation of the amount of displacement along the South-Armorican shear zone (at least 200-300 km), and indicate a protracted history of crustal thickening up to the Devonian-Carboniferous boundary.
In SW Iberia the suture of the Variscides is exposed in the SW vergent branch of this bilateral orogen. Early obduction to N and NE, in Lower Devonian, is synchronous with subduction in the same sense and followed by SW directed continent collision between the Iberian and South Portuguese Terrane, SPT, since the Lower Carboniferous. Hot obduction emplaced the Beja-Acebuches ophiolite complex and cold obduction followed by collision emplaced ophiolite Klippe that travelled at least 50 km to NE of the suture and footwall duplexes of high pressure metamorphic rocks up to eclogite facies. Subduction and collision are oblique and induce transtension followed by transpression both in the SPT and in Ossa-Morena Zone, OMZ, the domain of the Iberian Terrane in the footwall to the suture; there is a combination of predominant orthogonal compression and subsidiary sinistral strike-slip in the NW-SE oriented structures. The most proeminent, located 60 to 120 Km NE of the curved suture, is the Tomar-Badajoz-Cordoba shear zone, reactivating an earlier Cadomian suture; it is a flower structure that separates the NE vergent structures that transport OMZ over the Centro-Iberian Zone, CIZ, and OMZ over SPT, respectively in the NE and SW branching of the Flower Structure. The SW vergent strucures of the OMZ and SPT represent a pro-wedge above the NE dipping subduction zone and the NE vergent structures of the OMZ and CIZ represent a retro-wedge in models of asymmetrical orogens modified to account for the transpressive tectonic regime. The amount of NE subduction is unknown and probably much greater than the amount of SW underthrusting of CIZ below OMZ; the last has minimum in the order of 50 Km. The deep structure of the suture and the possible thin-skinned floating character of OMZ remains to be investigated by seismic profiling, the main goal of the SW-Iberia project (EUROPROBE, ESF), with major implications in terms of Resources and Risks.
Variscan and pre-variscan orogens with a NW-SE trending are exposed in the Southwest branch of the Iberian Armorican Arc (Matte & Ribeiro 1975; Ribeiro et al. 1995). Three main tectonostratigraphic domains are exposed, from North to the South: the Central Iberian Zone (CIZ), the Ossa Morena Zone (OMZ) and the South Portuguese Zone (SPZ). Tectonic trenches parallel to the orogen trending have been originated during sinistral wrench transtensional pulses, controlling the frontiers between the tectonostratigraphic domains. These troughs correspond to half-graben structures bounded to the South by growth shear zones, with low angle dips to the North. In lower structural levels, this regional stretching tectonic style was linked to high-pressure rocks exhumation. Synorogenic flysch, diachronically filled these basins since the Precambrian times in the central parts of the orogen (CIZ), through the Lower Palaeozoic (in the OMZ), until the Upper Carboniferous (Upper Westphalian) in the forelands (SPZ).
Upper Palaeozoic basins decrease the age towards the SW according to a migration of the orogeny in the same sense. From the Middle Devonian until the Upper Carboniferous, the Iberian virgation has been induced by the counter-clockwise rotation of a kinematic indenter component (Brun & Burg 1982). The previous sinistral NE dipping accidents associated to a regional shear sense with top to the North, gradually turned the movement to the West and Southwest. Thus, the prior transtensional regime gives place to a transpressional one, favouring the orogenic shortening.
The early simple shear regime acting along the boundary between the OMZ and the SPZ, favoured the syntectonic intrusion of a mafic and ultramafic magmatic complex, the so called Beja-Acebuches Ophiolite (Quesada et al. 1994). Unless a MORB geochemical affinity has been described, this banded magmatic complex is intruded in the Lower Palaeozoic metasediments of OMZ. From the Upper Devonian onwards, this suture zone evolved to an intracontinental subduction geometry, associated to synorogenic Carboniferous flysch in the SPZ and to centrifugal vergences towards the SW, according to the thin-skinned model (Silva et al. 1990).
A sinistral transtensional-transpressional regime worked along an almost linear strike-slip orogen, probably since the Cadomian times. The diachronic succession of Palaeozoic basins controlled by a similar geometry, kinematics and dynamics, suggests the existence of a continuous process, according to an heritage of the opening and closure of the Iapetus Ocean, situated further West from the Iberia. The progressive deformation of a linear transcurrent orogen (Badham 1982), evolving to an arcuate structure coeval with foreland Carboniferous basins, is made with no turning points. Therefore, we do not recognize the existence of a major variscan ocean in SW Iberia.
Matte Ph & Ribeiro A, C. R. Ac. Sc. Paris, Sér. D, 280, 2825-2828, (1975).
Badham JP, Journ. Geol. Soc. London, 132, 493-504, (1982).
Brun J & Burg J, Earth Planet. Sci. Letters, 61, 319-332, (1982).
Silva JB, Oliveira J & Ribeiro A, Pre-Mesozoic Geology of Iberia, 348-362, (1990).
Quesada C, Fonseca P, Munhá J, Ribeiro A & Oliveira J, Bol. Geol. y Minero de España, 105-1, 3-49, (1994).
Ribeiro A, Dias R & Silva JB, Geodinamica Acta, 8-4, 173-184, (1995).
Within the Iberian Massif, the Central Iberian Zone (CIZ) and the Ossa-Morena Zone (OMZ) constitute two distinct tectonostratigraphic and metamorphic domains, along an WNW-ESE trending orogen. Different criteria can be used for the boundary characterisation between these two domains, depending upon the age of the geodynamic events.
High-grade rocks occur along the Coimbra-Cordoba Blastomylonite Zone, a major sinistral wrench shear zone (Burg 1981). The lack of consensual geochronological data (Schäfer 1990; Ordoñes-Casado 1998) supports divergent tectonic interpretations for the real significance of eclogites (Abalos et al. 1993; Azor et al. 1993) as suture zone testimonies, which have been found within high-grade gneisses. Nevertheless, this major shear zone is situated inside the OMZ in the northern parts, and it should not correspond to a Variscan boundary with the CIZ.
According to palaeogeographic criteria, the CIZ overlies the OMZ through a continuous unconformity of Ordovician (Arenigian) Armorican quartzites covering Upper Proterozoic units of the OMZ. This North dipping stratigraphic contact could be more acceptable for the boundary between OMZ and CIZ, but its pattern has been modified by the Variscan deformation.
During Upper Palaeozoic times, the transpressional mechanisms linked to the Iberian Armorican Arc virgation, generated a dissymmetric flower structure in the northern parts of OMZ. Due to a North directed vergence in the northern branch of this flower structure, some OMZ units, tectonically overlie the CIZ. Coalescent flower structures at various scales have been formed, bounded by scissors-type faults (Schreurs & Colleta 1998) with opposing vergence. Left-lateral strike-slip and thrust fault systems, controlled the horizontal and vertical growth of the flower structures, and such major planar structures show variation of geometric and kinematic features along the orogenic trending. The previous Ordovician unconformity works during the Variscan times, as a thrust contact with shear sense to the North.
A distinct Lower Palaeozoic stratigraphic sequence and a late Proterozoic basement with OMZ affinities, record a polycyclic geodynamic history. Upper Palaeozoic geodynamic events developed under low to high-grade metamorphic conditions transpose the former Cadomian evolution. The exposure of metamorphic rocks of different grades along different shear zones is related to a variable amount of movement along major fault boundaries in different times. Thus, the OMZ / CIZ boundary worked as a continental transpressional zone with major flower structures transposing a continuous history, from the Proterozoic through the Upper Palaeozoic times.
Abalos B, Ibarguchi G & Eguiluz L, Tectonophysics, 217, 347-353, (1993).
Azor A, Lodeiro FG & Simancas JF, Tectonophysics, 217, 343-346, (1993).
Burg J, Iglesias M, Laurent P, Matte Ph & Ribeiro A, Tectonophysics, 76, 161-177, (1981).
Ordoñes-Casado B, Ph D, ETH, Zurich, 12940, (1998).
Schafer HJ, Ph D, ETH, Zurich, 9246, (1990).
Schreus G, Colletta B, Continental Transpressional & Transtensional Tectonics, Geol. Soc. Special Publ., 135, 56-79, (1998).
The southern border of the Ossa-Morena Zone displays a relatively complex fracture network that affects all geologic and lithostratigraphic units belonging to three main domains: the Autochthonous Iberian Terrane, the oceanic exotic terrane known as the Beja-Acebuches Ophiolite Complex and a calc-alkaline intrusive complex - the Beja Igneous Complex. The fracture network comprises mainly four different systems: 1) NNE-SSW to NE-SW left-lateral strike-slip faults, represented by several major accidents (e.g. the Messejana Fault), whose development is usually ascribable to the late-variscan stress-field; 2) N-S to NW-SE right-lateral faults, generally of minor extension and commonly interpreted as the conjugate system of the prevailing structures referred to in the previous item; 3) ENE-WSW left-lateral, brittle shear zones (e.g. the Ficalho Fault) that form complex structural arrays with the structures mentioned below; and 4) major E-W to WNW-ESE shear corridors of predominant sinistral kinematics, generated during the variscan collisional events (under a left-lateral transpressional deformation regime) between the Iberian and the South Portuguese Terranes (Quesada et al., 1994; Fonseca, 1995) and object of late, polyphasic reactivation.The Ferreira-Ficalho Thrust, which presently forms the northern border of the South Portuguese Terranes, is one of the most important tectonic accidents that belong to the last mentioned fracture system. In fact, there are many subparallel, E-W to WNW-ESE shear corridors that can be followed for several tens of kilometres and whose geological importance is noteworthy, since they often mark the main contacts between all the domains, destroying the initial geometric relationships, and being also responsible for the strong tectonic dismembering of the ophiolite sequence. Structural and geological mapping together with the available magnetic and gravimetric surveys confirm the lateral continuity of the shear zones and their importance in the regional structural arrangement of the OMZ southern border.According to the proposed geodynamic models (Quesada et al., 1994, Fonseca, 1995), the structural/metamorphic evolution of the OMZ southern border can be generally envisaged as a continuous succession of phenomena developed during three main variscan deformation phases; the genesis of the major shears took place during the late stages of continental collision.
(*) Contribution from MIZOMOR Project (PBICT/P/CTA/2112/95), JNICT-Portugal
Fonseca PE, PhD Thesis University of Lisbon, (1995).
Quesada C, Fonseca PE, Munhá J, Oliveira JT & Ribeiro A, Bol. Geológico y Minero, 105-1, 4-39, (1994).
In SW Iberia (Ossa Morena Zone) the Variscan tectonics of Alvito-Viana do Alentejo's critical sector is dominated by two main deformation events afecting several tectonostratigraphic units: late Proterozoic Série Negra's metassediments; (supposed) early Cambrian marbles; Silurian (?) Xisto de Moura's metassediments and imbricated felsites and metabasites (including eclogites) of unknown age. The early (D1) deformation event is characterised by a pervasive planar and linear fabric imprinted at mesoscopic and microscopic scales. Sin-D1 cleavage (S1) shows a variable geometry on top of which a roughly N-S trending stretching lineation is recognisable. At mesoscopic scale sigmoidal and delta shaped rigid mafic 'boudins', embedded in a more ductile marble matrix, indicate a top to the North (NNW, sometimes NNE) sense of shear. In Série Negra's metassediments (e. g. garnet mica schists) thin sections show helicitic inclusions in rotated garnet porphyroclasts confirming, among other kinematic shear criteria, a northwards sense of shear for the early, D1, Variscan structures. At a regional scale the second deformation event (D2) overprints D1 structures by folding S1 according to a N-S trending antiform macrostructure. Its geometry is characterised by an eastern normal long limb (N-S, East deeping) and by a western, locally inverted, short limb (N-S, subvertical), thus geometrically defining a West verging structure. At chosen outcrops it is also possible to recognise development of sin-D2 axial plane cleavage (S2) cutting S1.Mesoscopic structures related with D1 and D2 geometric interference are well observed in the Marbles' unit near Viana do Alentejo. The D2 refolding of sin-D1 structures originates a geometric pattern, mainly characterised by a complex set of folds with vertical axis. Some of these structures are interpreted as a consequence of possible diachronic actuation of the two main Variscan deformation events (D1 and D2). D1 and D2 are also responsible for the final geometry of the possible (originally sin-D1) westwards thrusting of the Série Negra's late Proterozoic metassediments over the Cambrian (?) marbles. Although true nature of this contact is still understudy, N-S sinform structures comprising Série Negra lithologies on top of the marbles allow us to speculate about the existence of klippen associated to this possible thrust.
Acknowledgements F. Rosas benefits of a PhD scholarship (PRAXIS XXI/BD/9220/96) granted by FCT (Fundação para a Ciência e Tecnologia).