Tectonostratigraphic Levels Research Articles

Complementarity of Hf isotopes from detrital and igneous zircon: an example from the Central Gneiss Belt, Grenville Province, Ontario

Combined U–Pb and Hf isotopic analysis of detrital and igneous zircon provides information about variations in crustal addition versus reworking through time. However, whereas detrital zircon data from only a few samples can provide information about extended periods of orogenic activity, this approach lacks the geological context that comes with studying igneous zircon and may not record mid- and deep-crustal sources or sources isolated from sites of deposition. Conversely, igneous zircon is likely to miss sources that were removed by later tectonic activity, but which may be preserved in the detrital record. The Central Gneiss Belt of the southwest Grenville Province in Ontario has a relatively well-understood orogenic architecture and records active-margin growth and reworking between ca. 1900 and 1200 Ma. We present U–Pb and Hf isotope data of zircon from 30 orthogneiss samples ranging in age from ca. 1750 to 1250 Ma, covering all tectonostratigraphic levels of the Central Gneiss Belt. As expected, the detrital and igneous datasets display many of the same overall features, such as crustal growth along the southwest Laurentian active margin; however, there are also some distinct differences. In particular, sedimentation appears to have taken place near the active margin, with little or no input from distal inboard sources, thus missing this significant component of reworked crust. This study highlights the need for a combination of detrital and igneous samples to provide the most complete picture of long-lived accretionary orogenic systems.

Oxygen and carbon isotope compositions of carbonates in a prominent lithologically mixed unit in the central South Norwegian Caledonides

A prominent pre-Scandian lithologically mixed unit in the central South Norwegian Caledonides contains more than 100 partly carbonated and hydrated metaperidotite bodies and locally fossiliferous detrital serpentinites. The lateral consistency of this mixed unit was not fully appreciated in the past. Therefore, parts of the mixed unit along strike were interpreted to belong to several different tectonostratigraphic levels. Here, we present new carbonate stable isotope data that suggest that the carbonates of the mixed unit between Bergen and Otta (re-)equilibrated at unit-wide similar peak metamorphic conditions. The isotope compositions are characteristic for this unit and indicate that it represented one single tectonic unit during the Scandian Orogeny. The carbonates in the melange are characterized by a narrow range of δ18O (SMOW) values between + 11 and + 15.5‰ and three groups of δ13C (PDB) values: (I) + 1.6 to + 0.3‰, (II) − 1.8 to − 3.9‰, and (III) − 6 to − 8.6‰. Carbonates of group III probably were affected by decarbonation or by a fluid containing organic carbon, whereas carbonates of group I and II overlap with δ13C values typical for Ediacaran–Silurian marine carbonates and may have retained their initial δ13C imprint. We suggest that the δ18O values (re-)equilibrated with unit-wide released metamorphic fluids during Scandian metamorphism. An outcrop-scale homogenisation of the δ13C values reflects the local carbon isotope signature of the released metamorphic fluids that circulated channelized through the melange unit.

Geochemical, isotopic and geochronological characterization of listvenite from the Upper Unit on Tinos, Cyclades, Greece

We describe a largely unknown listvenite deposit from Tinos, Cyclades, Greece and combine field observations with petrographic, bulk-rock geochemical, isotope (Sr, O, C), and Rb–Sr geochronological data. The volumetrically small listvenite occurrences are associated with metabasic phyllites, talc schists, meta-gabbros, ophicalcites and serpentinites of the Upper Unit. Geochemical characteristics (high Mg#, Cr, Ni), as well as preserved relic Cr-spinel and the typical mesh-texture of serpentinized Mg-silicates, document derivation from ultramafic precursors. Judging from field and textural observations it is very likely that carbonation affected serpentinite and not unaltered meta-peridotite. The direct contact or transition zones to ultramafic rocks are not preserved, but serpentinites that escaped carbonation are closely associated. The listvenites occur near a low-angle normal fault that probably focused fluid infiltration and distribution. The carbonation is associated with the influx of CO2-rich, K-bearing fluids that led to the formation of ferroan magnesite, quartz and Cr-bearing white mica (fuchsite), but otherwise the transformation of serpentinized peridotite into listvenite had been a largely isochemical process. The studied rocks do not contain elevated concentrations of precious metals (Au, Pt, Pd). Field relationships suggest that the listvenite-bearing occurrences most likely represent the same tectonostratigraphic level as Upper Unit rocks that had been thermally overprinted in the contact aureole of Miocene granitoids at ca. 15Ma. Accordingly the intrusion depth provides a minimum pressure constraint for the somewhat older carbonation. Pressure estimates for thermally overprinted rocks and the granitoids suggest an intrusion depth of ca. 7–10km that corresponds to a pressure of ca. 2–3kbar. Chlorite thermometry applied to the Tinos listvenites mostly indicates temperatures of ca. 250°C during carbonation. Internal Rb–Sr mineral isochrons (different grain-size fractions of fuchsite and magnesite) yielded apparent ages of ca. 16Ma and ca. 19Ma, respectively, which are interpreted to date carbonation and associated fuchsite formation. The new ages indicate that listvenite formation is considerably younger than the presumed Late Cretaceous or Jurassic protolith age of the ultramafic precursors and also post-dates tectonic juxtaposition of the Upper Unit onto the Lower Unit at ca. 21Ma. Although not the dominant process, a contribution of contact metamorphic decarbonation cannot completely be ruled out. The Sr isotope characteristics of magnesite and whole rocks correspond very well to the seawater curve for the formation age indicated by Rb–Sr dating. Carbonate carbon and oxygen isotopes measured for the listvenites suggest that magnesite formed following the deep circulation of fluids and their interaction with other carbonate rocks (possibly the marble units present on Tinos) including a possible contribution from magmatic CO2. The similarity in δ13C and δ18O between listvenite and some of the ophicalcite occurrences could indicate a common origin from the same circulating fluids, but remains elusive at present. Combined, P–T constraints and Sr isotope data imply infiltration of seawater-dominated fluids to a depth of several kilometers. This conclusion is supported by oxygen and carbon isotope data.

Detrital zircon signatures of the Baltoscandian margin along the Arctic Circle Caledonides in Sweden: The Sveconorwegian connection

New evidence is presented here that the Sveconorwegian Orogen continued northwards from type areas in southwestern Scandinavia along the Baltoscandian outer margin into the high Arctic. The Silver Road (Silvervägen) profile through the Scandinavian Caledonides, located in Sweden along the Arctic Circle at 66–67° N, provides a full section through the tectonostratigraphy of the Baltoscandian margin from the Autochthon, via the Lower Allochthon to the upperment parts of the Middle Allochthon. Metamorphic grade increases upwards through the nappes, being low greenschist facies at lowest levels and increasing to eclogite grade in the highest parts of the Seve Nappe Complex, the latter being related to early Ordovician subduction of the Baltoscandian outermost margin. The sedimentary rocks range in age from Neoproterozoic to Ordovician and provide evidence of the changes of environment from the Baltoscandian platform, westwards out over the Cryogenian rifted margin to the continent-ocean transition zone; also the Ordovician foreland basin. Twelve samples of psammites from the different tectonostratigraphic levels have yielded U/Pb detrital zircon age-signatures that reflect the changing character of their provenance. Autochthonous sandstones are derived from late Paleoproterozoic (1800–1950Ma) crystalline rocks in the vicinity to the east of the thrust front. Ediacaran-early Cambrian quartzites of the Lower Allochthon also yield mainly late Paleoproterozoic zircon signatures, but with subordinate Mesoproterozoic and late Archaean populations, whilst mid Ordovician, W-derived foreland basin turbidites are dominated by Sveconorwegian (950–1100Ma) signatures, with subordinate older Mesoproterozoic to latest Paleoproterozoic populations. All samples from the lower parts of the Middle Allochthon (lacking dolerite dykes) have signatures that are dominated by latest Paleoproterozoic and early Mesoproterozoic ages, with subordinate populations down to Sveconorwegian ages; the latter dominate the overlying Särv nappes and also the Seve Nappe Complex, where c. 945Ma rhyodacites have been previously reported. This evidence of Sveconorwegian source rocks in the hinterland, taken together with previously published detrital zircon data farther south and north of the Arctic Circle, clearly favours the interpretation that the Sveconorwegian Orogen, during the Neoproterozoic, extended along the entire Baltoscandian outer margin into the high Arctic.

The Bratten–Landegode gneiss complex: a fragment of Laurentian continental crust in the Uppermost Allochthon of the Scandinavian Caledonides

Abstract This report presents new field observations and geochronology (isotope-dilution thermal ionization mass spectrometry/ID-TIMS and secondary-ion mass spectrometry/SIMS) on an igneous complex and its country rocks in the Bodø area, northern Norway, traditionally interpreted to represent the (par)autochthonous crust of Baltica ( c. 1.8 Ga). Field observations however indicate that the rocks are allochthonous and comprise the uppermost tectonostratigraphic level in the area. The presence of a migmatitic megacrystic granite with an emplacement age of 946 Ma strongly supports such an interpretation and indicates that the Bratten–Landegode gneiss complex is exotic with respect to Baltica. The 946 Ma granite intrudes metasedimentary rocks. The rocks were metamorphosed in the Late Ordovician and intruded by granitic pegmatites and diorites at 430 and 427 Ma, respectively. The Bratten–Landegode gneiss complex shows a close correlation with Mesoproterozoic–Neoproterozoic rock complexes in the East Greenland Caledonides and we interpret it to be a Laurentian pre-Caledonian continental fragment. The discovery of Laurentian pre-Caledonian continental crust in the Uppermost Allochthon calls for a revision of the tectonostratigraphy of this part of the Caledonides and provides important constraints on the sequence of events on the Laurentian margin prior to continent–continent collision between Baltica and Laurentia as well as on intercontinental interactions during the Caledonian orogeny.

THE GEOLOGY AND GEOCHEMISTRY OF THE EAST AFRICAN OROGEN IN NORTHEASTERN MOZAMBIQUE

The geology of northeastern Mozambique has been remapped at 1:250 000 scale. Proterozoic rocks, which make up the bulk of the area, form a number of gneiss complexes defined on the basis of their lithologies, metamorphic grade, structures, tectonic relationships and ages. The gneiss complexes, which contain both ortho- and paragneisses, range from Palaeo- to Neoproterozoic in age, and were juxtaposed along tectonic contacts during the late Neoproterozoic to Cambrian Pan-African Orogeny. In this paper we describe the geological evolution of the terranes north of the Lurio Belt, a major tectonic boundary which separates the complexes described in this paper from the Nampula Complex to the south. The Marrupa, Nairoto and Meluco Complexes are dominated by orthogneisses of felsic to intermediate compositions. Granulitic rocks, including charnockites, are present in the Unango, M’Sawize, Xixano and Ocua Complexes (the last forms the centre of the Lurio Belt). The Neoproterozoic Geci and Txitonga Groups are dominated by metasupracrustal rocks at low metamorphic grades and have been tectonically juxtaposed with the Unango Complex. Geochemical data integrate and support a model of terrain assembly in northeast Mozambique, which is largely published and mainly derived from our new geochronological, lithostratigraphic and structural work. This model shows the contrast between the mainly felsic lower tectonostratigraphic levels (Unango, Marrupa, Nairoto and Meluco Complexes) and the significantly more juvenile overlying complexes (Xixano, Muaquia, M’Sawize, Lalamo and Montepuez Complexes), which were all assembled during the Cambrian Pan-African orogeny. The juxtaposed terranes were stitched by several suites of Cambrian late- to post-tectonic granitoids.

Exhuming Norwegian ultrahigh‐pressure rocks: Overprinting extensional structures and the role of the Nordfjord‐Sogn Detachment Zone

The Nordfjord‐Sogn Detachment Zone (NSDZ) is widely cited as one of the primary structures responsible for the exhumation of Norwegian (ultra)high‐pressure (UHP) rocks. Here we review data from the considerable volume of research describing this shear zone, and compile a strike‐parallel cross section along the NSDZ from the Solund Basin in the south to the Sørøyane UHP domain in the north. This cross section highlights several previously unrecognized patterns, revealing a shear zone with top‐to‐the‐west asymmetric fabrics that (1) initiated at amphibolite facies, (2) overprints metamorphic breaks and tectonostratigraphic contacts, and (3) has a gradational continuum of muscovite cooling ages. These patterns constrain the kinematic evolution of the NSDZ and suggest a new three‐step model for the exhumation of Norwegian (U)HP rocks. The initial stages of exhumation were characterized by the rise of crustal rocks from (U)HP depths to the base of the crust by buoyancy‐driven mechanisms not specified in this paper. Mantle exhumation was followed by top‐to‐the‐west, normal‐sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation. In the final stages of crustal exhumation, top‐W brittle‐ductile detachments soled into and partially excised this ductile shear zone, dropping the Devonian basins into contact with rocks of varying tectonostratigraphic levels. This new interpretation of the NSDZ is significant as it accounts for the extreme crustal excision observed in western Norway using three sequentially overprinting structures active at different stages of UHP rock exhumation.

The Continental Crust as a Source of Sand (Southern Alps Cross Section, Northern Italy)

All tectonostratigraphic levels of the continental crust, from carbonate platforms in the Dolomites to granulites in the Ivrea Zone, are exposed along strike in the world‐famous Southern Alps cross section. In this extraordinary natural laboratory, we studied the composition of modern stream sands shed by each distinct type of source rock in order to improve on existing models of sediment provenance and specifically to reconstruct the step‐by‐step changes of detrital signatures ideally produced during unroofing of a continental block. Detritus from progressively deeper crustal levels of the Southern Alps changes systematically from lithic sedimentaclastic (unmetamorphosed cover sequences) to lithoquartzose, quartzolithic, and quartzofeldspathic metamorphiclastic (greenschist facies to amphibolite facies basement units) and finally to feldspathoquartzose and feldspathic composition (granulite facies basement units). Six progressively richer types of heavy mineral assemblages (ZTR‐brookite, garnet‐chloritoid, garnet‐staurolite, blue/green hornblende–kyanite, green/brown hornblende–fibrolitic sillimanite, and garnet–brown hornblende–prismatic sillimanite) faithfully reflect the increasing grade of metamorphic source rocks. Hypersthene–brown hornblende and olivine‐enstatite assemblages characterize detritus from lower crustal gabbros and mantle peridotites. In alpine settings undergoing rapid erosion and minor chemical weathering, the mineralogy of detrital sediments faithfully mirrors the mineralogy of their parent rocks. Therefore, our database can be used to integrate direct measurements of physical and chemical properties of exposed bedrock and to reconstruct an ideally complete mineralogy and density profile from the base to the top of the South Alpine crust. This shows a progressive upward increase in quartz and muscovite at the expense of plagioclase and heavy minerals and an effective density stratification from lower crustal (3.10–3.00 g/cm3) to middle crustal (∼2.85 g/cm3) and upper crustal levels (2.75–2.70 g/cm3). Sharp jumps in average rock density observed across major structural discontinuities (e.g., Cossato‐Mergozzo‐Brissago Line) mirror the fact that the South Alpine basement is the extensional stack of several slivers that followed distinct metamorphic and exhumation paths during the multistage tectonic evolution linking the Carboniferous Variscan Orogeny with the Jurassic opening of the Alpine Tethys.

Three-dimensional geometry and tectonostratigraphy of the Pennine zone, Central Alps, Switzerland and Northern Italy

Continental collision during Alpine orogenesis entailed a polyphase deformation history (D 1–D 5) in the Pennine zone of the Central Alps. The regional tectonostratigraphy was basically developed during D 1 and D 2, characterised by isoclinal, typically north-closing recumbent anticlines, separated by pinched-in synclines, on the scale of tens of kilometres. Later deformation phases (D 3 and D 4) warped the stack into wavy to open folds. Exhumation of this zone resulted locally in later vertical shortening and folding of already steep fabrics (D 5). Three-dimensional models of the nappe pile were constructed, based on geostatistical assessment of the regional foliation field and considering the abundant structural field data. These models indicate the existence of five principal tectonostratigraphic levels developed during D 1 and thus equivalent to nappe units s. str.: the Gotthard, the Leventina–Antigorio, the Maggia–Simano (and probably the Monte Leone as well as the Composite Lepontine Series), Lebendun–Soja and Adula–Cima Lunga levels. All these tectonic units formed part of the passive continental margin of Europe prior to the onset of the Alpine orogenesis. Individual isoclinal post-nappe folds reflect relative displacements on the order of 40 km or more. The most prominent D 2 post-nappe structure is the Wandfluhhorn Fold, structurally equivalent to the northern closure of the Leventina–Lucomagno Antiform. The Lebendun and Monte Leone folds are of similar magnitudes and also affect the whole nappe pile, whereas the smaller Mogno and Molare synforms only refold the Maggia–Simano nappe internally. Principal D 3 and D 4 structures are the tight Mergoscia Synform directly north of the Insubric Fault between Bellinzona and Locarno (Southern Steep Belt), the Maggia Steep Zone, forming the steep western limb of the Campo Tencia Synform and subdividing the Lepontine dome into the Simplon and Ticino subdomes, the Chiéra Synform steepening the dominant foliation in the north (Northern Steep Belt) and the Vanzone and Claro Antiforms steepening the dominant foliation in the south (Southern Steep Belt). The current geometry of the Northern and Southern Steep Belts reflects an interplay between D 4 and D 3, involving both fold interference and reactivation/tightening.

Structure and kinematics of the northern Simano Nappe, Central Alps, Switzerland

New structural data allow the internal structure and kinematics of the Lower Penninic Simano Nappe to be established, together with the relationship of this unit to the underlying Leventina Gneisses and the also underlying Maggia Nappe. Three clearly distinguishable Alpine deformation phases (D1, D2, D3) are recognized within the Simano Nappe. D1 developed in mid-crustal levels under metamorphic conditions of ca. 1 GPa and 500°C and produced kilometre-scale, recumbent, north-closing anticlinal folds, called nappes. During D2 these nappes were intensely refolded on the same scale and with identical style to form the present day recumbent fold pattern. The axial planar foliation S2 is the dominant planar fabric throughout most of the Central Alps. The subsequent phase D3 overprints the entire tectonostratigraphy of the Central Alps, with broader and more upright D3 folds trending oblique to the orogen. Regionally, these broad D3 folds have a major influence on the overall foliation and nappe outcrop pattern of the Central Alps. During D2 and D3, peak metamorphic conditions reached temperatures of up to 650°C, under pressures of 0.7–0.8 GPa. An important consequence of this deformation history is that lithological units separated only by D2-synforms formed part of the same tectonostratigraphic level after D1 nappe-stacking and therefore should not be interpreted as different nappes. This principle can be directly applied to the relationship between the Simano and the two adjacent units. The underlying Leventina Gneisses are separated from the Simano rocks by an originally intrusive contact that has been strongly reactivated during both D1 and D2. Therefore, it is taken to represent a true nappe contact, although locally it still retains some original intrusive relationships and has also been strongly reactivated during D2. The Maggia and Simano units represent alternate limbs of a D2 synform-antiform pair, the Mogno synform and the Larecc antiform, or more generally the Verzasca-Larecc-Ganna antiform. If the Maggia unit is simply the continuation of the Simano unit around the D2 Mogno synform, then post-D1 they represented a single Simano-Maggia Nappe. The Maggia “Nappe” is therefore also part of the European margin and cannot be separately assigned to the Brianconnais paleogeographic domain, as has been proposed.

Palaeoproterozoic U–Pb ages of autochthonous and allochthonous granites from the northern Swedish Caledonides—regional and palaeogeographic implications

In the Silurian, Baltica and Laurentia collided to form the Caledonian orogenic belt. During this process, nappes derived from the pre-collisional Baltic margin, from outboard terranes of Baltica, and from exotic terranes of probable Laurentian origin, respectively, were thrust upon the indenting Baltica. The Middle Allochthon is a tectonostratigraphic level inferred to represent the rifted Neoproterozoic margin of Baltica and is heterogeneously composed of telescoped fault-controlled sedimentary basins and subjacent basement plinths. In the Akkajaure area of northern Sweden, the Middle Allochthon consists mainly of granites (Akkajaure Nappe Complex). The Ritsem Granite, a part of the (para-) authochthonous basement below the nappe complexes, yields a U–Pb zircon and titanite age of 1871±11 Ma. Granite and felsic dykes in the lowermost thrust-sheet in the Middle Allochthon yield ages of 1800±2 Ma and 1795±4 Ma, respectively. The granite of the overlying thrust-sheet is 1781±2 Ma old and crosscutting dykes are 1779±7 Ma. These ages constrain the pre-Caledonian position of the nappe complex to the general region west of Lofoten, northwestern Norway, substantiating a link with the Transscandinavian Igneous Belt (TIB) of the Fennoscandian Shield.

High-pressure/temperature metamorphism in the St. Leonhard Granulite Massif, Austria: evidence from intermediate pyroxene-bearing granulites

The St. Leonhard Granulite Massif, Lower Austria, is one of the small occurrences of high-pressure granulite found in the Gfohl unit at the highest tectono-stratigraphic level of the Moldanubian zone. Although predominantly composed of extremely deformed acidic, garnet+kyanite-bearing rocks, thin conformable layers of intermediate garnet+clinopyroxene-bearing granulites are seen. Pressure–temperature estimates for the peak metamorphic assemblage of garnet+clinopyroxene+ternary feldspar+quartz in these rocks are 15–19 kbar, 950–1050°C. A close coherence between results obtained from a combination of independent geothermobarometers and those derived from an internally consistent thermobarometric method indicate the retention of high-pressure/temperature equilibrium mineral compositions, even though there is a wealth of petrographic evidence for significant post-peak metamorphic decompression. Pressure–temperature estimates for the orthopyroxene-bearing, intermediate-pressure decompression stage, obtained from discrete reaction textures, are 8–12 kbar and 800–900°C. Post-decompressive cooling from 800 to 500°C, at ca. 5–8 kbar, is recorded by the final amphibolite-facies, biotite-bearing assemblage, together with petrological constraints from the enclosing acid granulites.

Duplex structures and their tectonic implication for the Southern Uplands accretionary complex

ABSTRACTVarious order duplex structures are described from oceanic sequences of basaltic and associated pelagic–hemipelagic sedimentary rocks in the Ordovician (northern) part of the Southern Uplands accretionary complex. The general structure of the terrane as a whole strikes ENE, but each component lithological tract strikes NE or more northerly, oblique to the regional trend, making an en echelon outcrop pattern. Further oblique relationships between structures and lithologies can be mapped at larger scales, up to 1 km scale or more. These duplex structures are thought to be originally SE-verging, now partly overturned to the NW. Differences in the en echelon geometry, either sinistral or dextral, are explained by variable plunge of the original structures.Peach & Home's first regional map of the Southern Uplands suggests an en echelon pattern of lithologies, implying large-scale duplex structures across the whole terrane. Here, the duplex structures are regarded as ubiquitous at both regional and smaller scales, suggesting considerable horizontal shortening. This was accommodated by such structures during underplating and out-of-sequence thrusting, in all parts of the accretionary prism, but particularly in the deeper tectonostratigraphic levels. The duplex structures are characteristic of ancient décollement zones.

40Ar/ 39Ar laserprobe direct dating of discrete deformational events: a continuous record of early Alpine tectonics in the Pelagonian Zone, NW Aegean area, Greece

The Pelagonian Zone of the Hellenides, which occurs in the northwestern Aegean region of the Eastern Mediterranean, contains remnants of a high-pressure metamorphic belt which subsequently underwent localized reworking during low-temperature deformational events. Formation of Alpine deformational structures in the Ossa Massif (northern Pelagonian Zone) occurred at metamorphic temperatures which failed to reset most pre-Alpine 40Ar/ 39Ar cooling ages in muscovite porphyroclasts. The low metamorphic temperatures at Alpine times have preserved the argon isotopic signature in white mica bearing deformational structures, which allows direct dating of these structures and refinement of the Alpine tectonic history in the region. After separation of carefully selected and characterized mica generations, application of sensitive 40Ar/ 39Ar laserprobe dating has resulted in a continuous record of the early Alpine tectonic history of the Ossa Massif from the Early Cretaceous well into the Eocene. The early Alpine history of the Ossa Massif records cooling of basement thrust slices, which are mostly Hercynian granites and metamorphics, below ca. 350°C by 100 Ma, while lower tectonostratigraphic levels of the basement had already cooled below these temperatures at around ca. 285 Ma. Blueschist facies mylonite fabrics, related to a top-to-ENE direction of tectonic transport, yield ages as old as 84.5±3.3 Ma. Exhumation of the blueschist facies sequences was initiated at ca. 54 Ma and involved tectonic activity along blueschist facies and greenschist facies mylonites zones. Final WSW-ward transport of the metamorphic sequence across the structurally lowest, and supposed, autochthonous, series occurred at around 45 Ma, and resulted in termination of the ductile deformation history in the studied area. This study confines the onset of high-pressure metamorphism in the Pelagonian Zone to an interval between 100 Ma and 85 Ma and has shown that high-pressure metamorphism had terminated by ca. 54 Ma. The recognition of an early to middle Alpine cycle, lasting over 30 million years and involving crustal shortening, high-pressure metamorphism, exhumation and subsequent crustal shortening is a key contribution to a better understanding of the early Alpine tectonic history of the Aegean region and eastern Mediterranean.

Geochemistry of the Sunnfjord Melange: sediment mixing from different sources during obduction of the Solund–Stavfjord Ophiolite Complex, Norwegian Caledonides

AbstractThe Sunnfjord Melange is a strongly tectonized sedimentary assemblage occurring tectonostratigraphically below the Solund–Stavfjord Ophiolite Complex of Ashgill age, and upon continental margin deposits of Wenlock or older age. The Sunnfjord Melange, which probably started to form within an oceanic transform fault, developed further during obduction of the Solund–Stavfjord Ophiolite Complex. The Sunnfjord Melange is divided into two units, the lower Øyravatn Unit of predominantly fine grained calcareous quartz-bearing chlorite–muscovite schists, and the upper Markavatn Unit, mainly metagreywackes. Both units contain blocks of metabasalts, marble and meta-arkose. Serpentinite blocks are restricted to the Markavatn Unit. The appearance of serpentinite detritus is marked by much higher contents of Cr and Ni in the metasediments of the Markavatn Unit than in those of the Øyravatn Unit. The geochemistry of the metasediments from the Øyravatn Unit and the Markavatn Unit is compatible with mixing of detritus from continental and ophiolitic sources. Despite the strong and pervasive deformation of the Sunnfjord Melange, its geochemical composition from the lowest to the highest tectonostratigraphic levels reflects that of the inverse pseudostratigraphy of the Solund–Stavfjord Ophiolite Complex. Thus, only metabasalts of the Solund–Stavfjord Complex and continental meta-arkoses yielded detritus to the schists of the Øyravatn Unit. The compositions of the schists and metagreywackes of the Markavatn Unit, on the other hand, show that a multicomponent detritus derived from metabasalt and serpentinite of the Solund–Stavfjord Ophiolite Complex, mixed with the detritus from continental meta-arkoses.

Large-Scale Tectonostratigraphic Controls on Source Rock Development in Extensional Basins

A study of some 180 of the world's source systems indicates the dominant control to be the prevailing tectonic regime. A limited number of favorable tectonostratigraphic settings can be identified in which source rocks most frequently occur. Source facies development is thus usually a regional phenomenon, with source rocks discontinually deposited and preserved over large-scale tectonically controlled 'fairways.' The majority of significant source rocks occur within post-rift megasequences in extensional basins. Favorable tectonostratigraphic levels in the post-rift include those tied to the passive infill stage following rifting, and to later stages of thermal sagging. These usually correspond to surfaces of transgression. Examples are given and reasons for the apparent favorability of these levels speculated upon. Good examples of large-scale tectonic control are shown within post-rift megasequences in passive margin basins. Oceanic circulation is controlled by the influence of plate movements on the geometry of the developing ocean. Oceanic anoxia often controls source rock development in the early stages of spreading while upwelling is of greater relative importance in the later stages of ocean development. The post-rift conditions favorable for source rock development are, on a global scale, somewhat concentrated in the late Paleozoic and middle Mesozoic, following the Wilson Cyclemore » of Plate Tectonics. The observed concentrations of source rocks in these two episodes is thus easily explained.« less

The Scandinavian Caledonides: a complexity of collisions

Thrust sheets dominate the structural framework of the Scandinavian Caledonides. Sheets at lower tectonostratigraphic levels comprise the shortened margin of the continent Baltica and, at higher levels, terranes derived outboard from this continent in oceanic or foreign continental environments. Amalgamation of these terranes with the margin of Baltica occurred during closure of the Iapetus Ocean in the early Palaeozoic. Closure involved subduction of oceanic crust, extensional tectonics and continent‐arc collisions during the late Cambrian and early Ordovician, and ultimate continent‐continent collision during the Silurian and Devonian.

Tectonostratigraphy and structure of the Kjukkelen area, Børgefjellet, north-central Norway

Abstract The Kjukkelen area in the B⊘rgefjellet National Park occupies a key position concerning the tectonostratigraphical relationship between the K⊘li Nappes north and south of the B⊘rgefjellet Window (BW). The thickness of the K⊘li Nappes between the rock units of the BW and the overlying Helgeland Nappe Complex (HNC) is locally reduced to 75 m in the Kjukkelen area. Several K⊘li Nappes (Stikke, Gelvenakko and Leipikvattnet N.) consisting of R⊘yrvik Group lithologies wedge out along the southwestern margin of the BW. Only the Gjersvik Nappe consisting of Limingen Group lithologies enters the narrow passage between the BW and the HNC. North of the passage, K⊘li lithologies belong to the R⊘yrvik Group. It is uncertain, which nappe unit they can be correlated with. Rocks belonging to the Limingen Group reappear further north in the Jofjallet Nappe which attenuates southward beneath the HNC. Investigations in the Kjukkelen area show that the Jofjallet Nappe appears at the tectonostratigraphic level of the...

A conspectus of Scandinavian Caledonian ophiolites

Summary Dismembered and fragmented ophiolite assemblages constitute important elements of the metamorphic allochthon of the Scandinavian Caledonides. Ophiolites recognized to date range in age from probable Vendian to Middle Ordovician and occur at two principal tectonostratigraphic levels. Field criteria, with support from geochemical data, permit a classification of many of the ophiolite complexes into two fundamental groups. Group I is characterized by a generally well-developed pseudostratigraphy, MORB as well as IAT and WPB petrochemistry, plagiogranites at higher levels, and a conformable cap of oceanic sediments with local ocean or immature arc volcanism. The group is representative of either a major ocean or evolved marginal-basin regime. Evidence points to their obduction, and initial internal deformation, in the Finnmarkian orogenic event in pre-Middle Arenig time. Group II complexes are younger, of late Arenig to Llanvirn and possibly Llandeilo age, and show a poorer-defined pseudostratigraphy. They are laterally and vertically intercalated with either siliciclastic sediments from a contemporary magmatic arc or arcs, and are considered as having been generated in a restricted marginal-basin setting. Their prime tectonic deformation, and translation within nappes, is essentially of Silurian age. Group I ophiolites were also further dissected and deformed during this main Caledonian event.

Occurrence and significance of inverted metamorphic gradients in the western Scandinavian Caledonides

In the Scandinavian Caledonides, the basement with a thin veneer of cover sediments can be traced from the frontal autochthon far westwards towards the centre of the orogen. The changing metamorphic grade can be recorded and compared with that of the overlying nappes. The metamorphic nature of these cover sediments is examined in the deepest structural levels of two windows, 200 km to the W of the Caledonian front. The emplacement of nappes above the cover caused a progressive recrystallization under greenschist facies conditions, with development of quartz + margarite + muscovite, quartz + chlorite + muscovite and quartz + white mica + graphite among parageneses defining the penetrative foliation related to the nappe displacement. The final overburden pressure is estimated to be 6–8 kb. Temperatures reached peak values around 550°C first after nappe emplacement. Despite the westerly situation and deep tectono-stratigraphic level of the metasediments, their metamorphic history is incompatible with a location close to an orogenic interior. The overlying cover sequence displays at least 2 inversions of metamorphic zonation. These developed as a result of the emplacement of high-grade nappes above low-grade or unmetamorphosed units, with incomplete subsequent restoration of the thermal profile, and in response to the catalytic effect of concentrated strain during thrusting. The imprinted saw-tooth pattern of the thermal profile in the western windows requires that tectonic discontinuities exist in this part of the western Scandes, considered by some geologists the axial zone of the orogen. It also suggests that the uplift and the formation of late regional syn- and antiforms followed soon after, or were simultaneous with, the late translation of the Caledonian front allochthon.