Original Article
On the provenance of mid-Cretaceous turbidites of the Pindos zone (Greece): implications from heavy mineral distribution, detrital zircon ages and chrome spinel chemistry
- P. FAUPL, A. PAVLOPOULOS, U. KLÖTZLI, K. PETRAKAKIS
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- Published online by Cambridge University Press:
- 06 April 2006, pp. 329-342
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Two heavy mineral populations characterize the siliciclastic material of the mid-Cretaceous turbidites of the Katafito Formation (‘First Flysch’) of the Pindos zone: a stable, zircon-rich group and an ophiolite-derived, chrome spinel-rich one. U/Pb and Pb/Pb dating on magmatic zircons from the stable heavy mineral group clearly illustrate the existence of Variscan magmatic complexes in the source terrain, but also provide evidence for magmatism as old as Precambrian. Based on microprobe analyses, the chrome spinel detritus was predominantly supplied from peridotites of mid-ocean ridge as well as suprasubduction zone origin. A small volcanic spinel population was mainly derived from MORB and back-arc basin basalts. The lithological variability of the mid-Cretaceous ophiolite bodies, based on spinel chemistry, is much broader than that of ophiolite complexes presently exposed in the Hellenides. The chrome spinel detritus compares closely with that from the Outer and Inner Dinarides. The source terrain of the ophiolite-derived heavy minerals was situated in a more internal palaeogeographic position than that of the Pindos zone. The zircon-rich heavy mineral group could have had either an external and/or an internal source, but the chrome spinel constantly accompanying the stable mineral detritus seems to be more indicative of an internal source terrain.
A new stratigraphy for the Latady Basin, Antarctic Peninsula: Part 2, Latady Group and basin evolution
- M. A. HUNTER, D. J. CANTRILL
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- Published online by Cambridge University Press:
- 28 September 2006, pp. 797-819
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Recent detailed mapping, section logging and an improved understanding of the geological evolution of the Antarctic Peninsula provide a robust framework for an improved lithostratigraphic subdivision of the Latady Basin, eastern Ellsworth Land. Within the Latady Basin we recognize two main groups: Ellsworth Land Volcanic Group and Latady Group. The focus of this paper is the Latady Group, which is formally subdivided into five formations: Anderson Formation, Witte Formation, Hauberg Mountains Formation, Cape Zumberge Formation and Nordsim Formation. Middle Jurassic, shallow marine deposits of the Anderson Formation are overlain by quiet anoxic deposits assigned to the Witte Formation. The start of the Late Jurassic is marked by the deposition of higher energy deposits of the Hauberg Mountains Formation, subdivided into three members (Long Ridge, Mount Hirman and Novocin members) that reflect varying lithological and environmental characteristics. Thermal subsidence during the latest Jurassic led to deposition of the basinal Cape Zumberge Formation, while uplift of an active continental arc along the Antarctic Peninsula led to deposition of the terrestrial Nordsim Formation in the latest Jurassic to earliest Cretaceous. The evolution of the Latady Basin reflects early extension during Gondwana break-up, from the Early Jurassic to earliest Cretaceous, and is consistent with a shift in the underlying forces driving extension in the Weddell Sea area from intracontinental rifting related to a mantle plume, to active margin forces in response to subduction.
Ammonoid stratigraphy and sedimentary evolution across the Permian–Triassic boundary in East Greenland
- MORTEN BJERAGER, LARS SEIDLER, LARS STEMMERIK, FINN SURLYK
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- 07 July 2006, pp. 635-656
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East Greenland is a classical area for the study of the Permian–Triassic transition and the succession is one of the most expanded in the world. New ammonoid data from the Wordie Creek Formation have allowed us to better reconstruct the history of the East Greenland basin from semi-isolated basins with an endemic fauna during latest Permian–earliest Triassic H. triviale–H. martini zones time to well-connected open marine shelf basins during the Early Triassic M. subdemissum, O. commune, W. decipiens and B. rosenkrantzi Zone times. The East Greenland zonation can be correlated with Boreal zonations in Arctic Canada, Svalbard and northeastern Asia. It allows precise relative dating and correlation of important events across the Permian–Triassic boundary. The new ammonoid data indicate that deposition was continuous across the Permian–Triassic boundary and developed as a marine mudstone–mudstone contact in basinal areas of Hold With Hope, northern and southern Jameson Land. Correlation of the ammonoid stratigraphy with the FAD of Hindeodus parvus, which defines the base of the Triassic in Global Stratotype Section and Point (GSSP) in Meishan, China, suggests that the Hypophiceras triviale Zone is to be referred to the uppermost Permian, whereas the H. martini Zone is lowermost Triassic. Accordingly, the end-Permian marine and terrestrial extinctions and associated isotope changes as well as the subsequent adaptive radiations in East Greenland took place in latest Permian time. New Boreal faunas and floras were well established and diversified in the Hypophiceras triviale Zone prior to the beginning of the Triassic, and the Permian–Triassic boundary, in its present definition, is no longer reflecting major changes in the Earth system. It would have been fortunate if a GSSP were defined in a protracted section at a point of major environmental perturbations, marked by isotope excursions, chemical anomalies and mass extinction, rather than in the strongly condensed section like Meishan at a point which post-dates all significant events.
The oldest rocks of Greece: first evidence for a Precambrian terrane within the Pelagonian Zone
- B. ANDERS, T. REISCHMANN, D. KOSTOPOULOS, U. POLLER
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- 20 October 2005, pp. 41-58
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The Pelagonian Zone in Greece represents the westernmost belt of the Hellenide hinterland (Internal Hellenides). Previous geochronological studies of basement rocks from the Pelagonian Zone have systematically yielded Permo-Carboniferous ages. In this study we demonstrate, for the first time, the existence of a Precambrian crustal unit within the crystalline basement of the Pelagonian Zone. The U–Pb single-zircon and SHRIMP ages of these orthogneisses vary from 699 ± 7 Ma to 713 ± 18 Ma, which identify them as the oldest rocks in Greece. These Late Proterozoic rocks, which today occupy an area of c. 20 × 100 km, are significantly different from the neighbouring rocks of the Pelagonian Zone. They are therefore interpreted as delineating a terrane, named here the Florina Terrane. During the Permo-Carboniferous, Florina was incorporated into an active continental margin, where it formed part of the basement for the Pelagonian magmatic arc. The activity of this arc was dated in this study by single-zircon Pb/Pb ages as having taken place at 292 ± 5 Ma and 298 ± 7 Ma. During the Alpine orogeny, Florina, together with the Pelagonian Zone, eventually became a constituent of the Hellenides. Geochemically, the Florina orthogneisses represent granites formed at an active continental margin. Because of the Late Proterozoic ages, this Late Proterozoic active continental margin can be correlated to a Pan-African or Cadomian arc. As the gneisses contain inherited zircons of Late to Middle Proterozoic age, the original location of Florina was probably at the northwestern margin of Gondwana. Similar to other Gondwana-derived terranes, such as East Avalonia, Florina approached the southern margin of Eurasia during Palaeozoic times, where it became part of an active continental margin above the subducting Palaeotethys. These interpretations further indicate that terrane accretion was already playing an important role in the early pre-alpine evolution of the Hellenides.
Structural evolution and tectonic setting of the Porongos belt, southern Brazil
- K. SAALMANN, M. V. D. REMUS, L. A. HARTMANN
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- 23 January 2006, pp. 59-88
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The SW–NE-striking Porongos belt, located between juvenile Neoproterozoic rocks in the west and the Dom Feliciano belt, characterized by intense reworking of older crust, in the east, comprises a greenschist to amphibolite-facies metavolcano-metasedimentary succession (Porongos sequence) of unknown age with some exposures of Palaeoproterozoic gneisses (Encantadas gneisses). High-temperature ductile deformation of the basement gneisses comprises at least two magmatic events followed by three deformational phases including folding and shearing (DT1–DT3) and can be attributed to the Palaeoproterozoic Trans-Amazonian orogeny. The deformation of the Porongos sequence occurred during the Neoproterozoic Brasiliano orogeny and comprises four ductile deformation phases (DB1–DB4), including two phases of isoclinal folding associated with shearing recorded in mylonitic layers, followed by closed NW-vergent folding and thrusting leading to formation of a thrust stack. Uplift of the basement and formation of late tectonic sedimentary basins occurred as a result of semi-ductile to brittle block faulting in a sinistral strike-slip regime. The Porongos sequence can be subdivided into a southeastern and a northwestern part. Trace element analyses as well as Sm–Nd and Rb–Sr geochemical data indicate partial melting and significant contamination by old continental crust for the metavolcanic rocks. The metavolcanic rocks show εNd(t=780 Ma) values of −20.64 and −21.72 (northwestern units) and −6.87 (southeastern sequence). The metasedimentary rocks were derived from late Palaeoproterozoic to Archaean sources, and the data indicate different sources for the northwestern and southeastern rock units of the Porongos sequence. εNd(t=780 Ma) are −6.25 and −6.85 in the southeastern units, with TDM model ages between 1734 and 1954 Ma, and vary between −14.72 and −17.96 in the northwestern parts, which have TDM model ages between 2346 and 2710 Ma. High 87Sr/86Sr(t) values between 0.7064 and 0.7286 confirm reworking of older crust. Isotopic signatures of the Porongos sequence do not show indications for a significant contribution from a Neoproterozoic juvenile source. A passive margin or continental rift environment is suggested for the tectonic setting of the Porongos belt, which is compatible with both deposition of shallow marine to deep marine sediments and stretching of continental crust leading to volcanism which is characterized by significant contamination by old continental crust.
The massive sulphide event in the Iberian Pyrite Belt: confirmatory evidence from the Sotiel-Coronada Mine
- FELIPE GONZÁLEZ, CARMEN MORENO, AMÉRICO SANTOS
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- 14 September 2006, pp. 821-827
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Well-preserved miospore and organic-walled microphytoplankton assemblages have been recovered from the black shaly series hosting the massive sulphide deposits of Sotiel-Coronada Mine (Iberian Pyrite Belt). The productive samples yielded miospore assemblages representing the uppermost Famennian Retispora lepidophyta–Verrucosisporites nitidus (LN) miospore Biozone of Western Europe. This palynological evidence has important implications for the local geology, constraining the commencement of the volcanic activity and corroborating the previously-defined local tectonic style. Moreover, at regional scale, the new data permit correlation of the black shaly series (which hosts the mineralization in all the sulphide deposits hitherto dated in the region), reinforcing the hypothesis of an anoxic event occurring in the Iberian Pyrite Belt in the latest Devonian times.
Environmental control on granular clinoforms of ancient carbonate shelves
- A. QUIQUEREZ, G. DROMART
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- 30 March 2006, pp. 343-365
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The purpose of this paper is to document the influence of depositional environments on shallow-water, low-relief clinoforms from the description of five ancient carbonate platforms: the Neoproterozoic (Namibia), Middle Jurassic (France), Lower Cretaceous (France), Upper Cretaceous (Oman) and Miocene (Turkey). These examples have been investigated on the basis of field observations. The clinoforms are described with reference to geometric and compositional attributes: declivity, shape, height, sedimentary structures, sediment fabric and components. The results show great variability in stratal geometry, declivity and facies distribution: (1) depositional profiles vary from exponential, to sigmoidal, to oblique; (2) maximal slope angles range from 3 to 25°, most of them being grouped between 10 and 18°; (3) facies differentiation identified from lateral facies successions along beds, and vertical facies successions through beds, is pronounced to subtle. This study documents linkages between depositional environments and clinoform attributes. Proximal/shallow clinoforms display round-edged exponential profiles. Sediment deposition has resulted from unidirectional currents in the upper convex section, and storm-generated oscillatory currents in the lower concave part. The sediment fabric changes gradually along this type of clinoform. There is little vertical facies differentiation through these clinobeds which have formed from a continuous amalgamation of deposits. By contrast, distal clinoforms (shelf break, distally steepened ramp settings) yield a much broader spectrum of profiles and are generally shorter and steeper. Sedimentary structures in gravel-sized deposits of the upper slope indicate pure traction by unidirectional currents. Conversely, marks of oscillatory flows (undular, wavy top bounding surfaces of clinobeds) are common in the lower slope. Intercalation of massive, fine-grained deposits suggests offshore transport of carbonate mud by suspension. Each distal clinobed represents a single flow event. Accordingly, facies differentiation is weak laterally but may be pronounced through the clinobeds. Our study suggests that low-relief forms of proximal/shallow environments, which contain coarse-grained and photo-independently produced debris, record hydrodynamic equilibrium profiles, whereas the higher-relief forms of this setting rather reflect a high differential production rate of carbonate sediment with water depth. The carbonate sediment of the distal clinobeds mainly derives from skeletal production by oligophotic and photo-independent biota of the middle shelf/ramp and upper portion of the clinoforms. The contribution by in situ skeletal biota only becomes significant on the lower slope, indicating that the distal, submerged slopes of carbonate platforms are not organically but hydrodynamically generated. Our compilation shows that the slope angles of shallow marine, low-relief clinoforms do not simply correlate to the sediment grain size and fabric, in contrast to what has been documented for the high, linear slope profiles. This difference stems from the depositional settings, namely the involved transport mechanisms. Low-relief clinoform accretion seems to be dominantly influenced by wave-induced sediment transport, in contrast to linear flanks of high-relief clinoforms that build to the angle of repose, and for which gravity is the primary transport process.
First record of the Hirnantian (Upper Ordovician) δ13C excursion in the North American Midcontinent and its regional implications
- STIG M. BERGSTRÖM, MATTHEW M. SALTZMAN, BIRGER SCHMITZ
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- 16 August 2006, pp. 657-678
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The most prominent of the two major global δ13C excursions in the Ordovician, the Hirnantian δ13C excursion (HICE), which is previously recorded from the uppermost Ordovician in a few sections in Nevada, Quebec, Arctic Canada, Baltoscandia, Scotland and China, is documented for the first time from the North American Midcontinent. Samples through the Girardeau Limestone and Leemon Formation in Missouri and Illinois show elevated δ13C values of +4‰ to +5‰. Although not determined precisely, the beginning of the HICE is likely to be in the upper part of the Orchard Creek Shale, and it ends in the upper Leemon Formation. Being extraordinarily useful chronostratigraphically, the presence of the HICE makes it possible to provide a firm dating of the study interval, whose age has long been controversial. Comparison between the study sections and coeval HICE sequences in North America and Europe show striking similarities, especially in sea-level history, indicating that major local lowstands reflect eustatic sea-level changes. A comparison with Hirnantian diamictite successions in North and South Africa and Argentina suggests that these lowstands correspond to two major Gondwanan glacial episodes.
Precise 40Ar–39Ar ages from the metamorphic sole rocks of the Tauride Belt Ophiolites, southern Turkey: implications for the rapid cooling history
- ÖMER FARUK ÇELİK, MICHEL DELALOYE, GILBERT FERAUD
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- 28 February 2006, pp. 213-227
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The Tauride Belt Ophiolites in southern Turkey are located on both sides of the E–W-trending, Mesozoic Tauride carbonate platform. They comprise the Lycian, Antalya, Beyşehir, Mersin, Alihoca and Pozantı-Karsantı ophiolites from west to east. Each ophiolite has a metamorphic rock unit either at the base of the peridotites or in the mélange units. The metamorphic sole rocks generally consist of amphibolite at the top and near the contact with the overlying tectonized harzburgite of the ophiolites, and mica schists mostly at the base, near the tectonic contact with the underlying ophiolitic mélange. 40Ar–39Ar measurements from the metamorphic sole rocks of the Lycian, Antalya and Beyşehir ophiolites are the first precise ages dating intra-oceanic thrusting and the cooling age history during the closure of the Neotethyan Ocean. Amphiboles and white micas from the metamorphic sole rocks of the ophiolites yielded 40Ar–39Ar ages between 90.7 ± 0.5 Ma and 93.8 ± 1.7 Ma and between 91.2 ± 2.3 Ma and 93.6 ± 0.8 Ma, respectively. Hornblende plateau ages from the amphibolites of the Lycian ophiolites (near Köyceǧiz) agree with those of Antalya, indicating that they were metamorphosed simultaneously in the Neotethyan Ocean. The white micas display plateau ages concordant with the amphiboles from the same units in Köyceǧiz and Yeşilova (Lycian ophiolites) and from the Pozantı-Karsantı ophiolite, suggesting that the metamorphic sole rocks were rapidly cooled after their generation.
Early African hyaenodontid mammals and their bearing on the origin of the Creodonta
- EMMANUEL GHEERBRANT, MOHAMED IAROCHENE, MBAREK AMAGHZAZ, BAÂDI BOUYA
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- 13 June 2006, pp. 475-489
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We report a new proviverrine hyaenodontid creodont mammal, Boualitomus marocanensis, n.g., n.sp., from the earliest Eocene of Morocco, and provide new comments on Tinerhodon from the late Paleocene of Morocco. Aside from the autapomorphic loss of P/1, Boualitomus is characterized by a primitive morphology (e.g. M/3 subequal to M/2, short molar trigonid, narrow talonid, metaconid comparable to paraconid) which resembles most closely the proviverrine Prototomus. Boualitomus is more primitive than Prototomus, especially in its small size and the talonid of P/4 not being fully simplified, bearing at least two accessory cusps including a bulbous protostylid. These primitive features are remarkably reminiscent of Tinerhodon. The morphological relationship of Boualitomus and Tinerhodon supports the proviverrine affinity of the latter. Significant basal hyaenodontid synapomorphies of Boualitomus and Tinerhodon are the paraconid and paracristid development in M/1–3, anterior premolar morphology and occurrence of diastemata. Boualitomus and Tinerhodon throw new light upon the question of the origin of the Creodonta. Tinerhodon further fills the structural gap between Hyaenodontidae and primitive insectivore-like eutherians, and it provides additional data for the hypothesis of a didelphodontan origin for the Creodonta. The presence of cimolestids (as the stem-group of hyaenodontids) in the late Paleocene of Morocco, and the identification of Boualitomus and Tinerhodon as the most primitive and earliest known Hyaenodontidae, support an African origin of the family and its order.
Ontogeny of Drevermannia and the origin of blindness in Late Devonian proetoid trilobites
- RUDY LEROSEY-AUBRIL
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- 23 January 2006, pp. 89-104
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Numerous silicified and calcareous sclerites of various sizes, recovered from the latest Famennian of Thuringia (Germany), allow the description of the first complete growth series of a blind proetoid trilobite: Drevermannia richteri. In addition, the partial ontogenetic development of Drevermannia antecurvata sp. nov. and undetermined species, Drevermannia sp. 1, are described. The proetoid anaprotaspides, associated with D. richteri, illustrate that a marked increase in larval size occurred prior to the terminal Devonian extinction event. Considering the homogeneity of larval size in older Devonian proetoids, it is interpreted as evidence that the developmental strategy of these trilobites was significantly modified. Though largely speculative, two alternative hypotheses are proposed to explain this modification. Finally, all three ontogenetic sequences show that ocular structures never develop externally in Drevermannia, but also illustrate that the development of optical nerves is not completely lost in this group. This suggests that blindness in the Drevermannia lineage followed a centripetal mode of eye reduction.
Multistage growth and reworking of the Palaeoproterozoic crust in the Bergslagen area, southern Sweden: evidence from U–Pb geochronology
- ULF B. ANDERSSON, KARIN HÖGDAHL, HÅKAN SJÖSTRÖM, STEFAN BERGMAN
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- Published online by Cambridge University Press:
- 16 August 2006, pp. 679-697
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The Svecofennian Domain of the Fennoscandian Shield constitutes a considerable volume of Palaeoproterozoic crustal growth, 2.1–1.86 Ga ago, in between the Archaean craton in the NE and the 1.85–1.65 Ga Transscandinavian Igneous Belt (TIB) in the south and west. The Bergslagen area is a classical ore province located in the southwestern part of the Svecofennian Domain of south-central Sweden. Its northern part is dominated by volcanic and plutonic rocks of a magmatic arc with continental affinity, while the SE part is made up by a sedimentary basin. The Bergslagen area shows a metamorphic zonation from lower to middle amphibolite facies in the north to upper amphibolite facies and locally granulite facies in the south; a small greenschist area exists in the west. Identifying the age spectra of inherited components, magmatic crystallization, as well as metamorphic episodes, provide important constraints on the geodynamic evolution of this centrally located piece of the Shield.
U–Pb zircon SIMS data presented in this paper complement the previous, regionally scattered TIMS data from this area. Magmatic zircons from two felsic metavolcanic rocks and two amphibolites (metagabbros) yield 1888±12, 1892±7 and 1887±5, 1895±5 Ma, respectively; i.e. within the 1.91–1.86 Ga range previously obtained for Early Svecofennian magmatism in Bergslagen. An augen gneiss from southern Bergslagen, assigned to the earliest TIB generation, yield an intrusive age of 1855±6 Ma. Metamorphic monazites from the same rock indicate that deformation and elevated thermal activity prevailed 1.83–1.82 Ga ago (TIMS). Metamorphic zircons in high-grade metasedimentary rocks from the south and west yield ages of 1793±5 and 1804±10 Ma, in accordance with ages for regional peak metamorphism and migmatite formation found elsewhere in the southern Svecofennian province of Sweden. More importantly, a few zircon crystals and overgrowths in rocks from the north indicate an early metamorphic episode at c. 1.87 Ga, indicating that Bergslagen has experienced two major metamorphic events. Detrital and inherited zircons span the range 2.78–1.90 Ga, with an apparent gap at 2.45–2.1 Ga, which further emphasize previous observations of a major juvenile (<2.1 Ga) and a minor Archaean provenance. This, and in particular the 1.94–1.91 Ga crystals present in the c. 1.89 Ga amphibolites, support the suggestion of a former Palaeoproterozoic pre-1.91 Ga crust in the Bergslagen area.
Neotectonics of the SW Marmara region, NW Anatolia, Turkey
- ÖMER FEYZI GÜRER, ERCAN SANGU, MUZAFFER ÖZBURAN
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- 13 February 2006, pp. 229-241
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This study reports on the geometric and structural characteristics of the North Anatolian Fault Zone in the southwest Marmara region. The geometric and kinematic features of the faults in the region are described, based on field observations. In addition, the Neogene and Quaternary basin fill which occupies large areas in the region has been determined, and the tectonic regimes controlling these basins are explained. The neotectonic regime is also explained considering different deformation phases affecting the region. The N–S extension and E–W strike-slip have affected the region possibly since the latest Pliocene–Quaternary. Field observations show that these extensional tectonics around the south Marmara region are related to right strike-slip on the E–W North Anatolian fault zone and the N–S Aegean extensional system. The faults in this zone trend approximately E–W in the eastern part of the region and NE–SW towards the west of the region, indicating that they accommodate rotation in addition to differential movement between adjacent blocks.
Variscan veins: record of fluid circulation and Variscan tectonothermal events in Upper Palaeozoic limestones of the Moravian Karst, Czech Republic
- M. SLOBODNÍK, PH. MUCHEZ, J. KRÁL, E. KEPPENS
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- 11 May 2006, pp. 491-508
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Numerous Variscan syntectonic calcite veins cross-cut Palaeozoic rocks in the Moravian Karst. A structural, petrographic and stable isotopic analysis of the calcite veins and a microthermometric study of fluid inclusions in these vein cements have been carried out to determine the origin of the Variscan fluids and their migration during burial and deformation. The isotopic parameters of white (older, more deformed) and rose (younger) calcites are: 87Sr/86Sr is between 0.7078 and 0.7082 (white) and 0.7086 (rose), δ18O is between +17.7 and +26.1 (white) and between +14.8 and +20.7 ‰ SMOW (rose), δ13C ranges from +0.1 to +2.5 (white) and from −0.3 to +1.6 ‰ V-PDB (rose). The isotopic signatures point to precipitation in an older fluid system buffered by the host rock (white calcites) and to an open, younger fluid-dominated system (rose calcites). Parent fluids (H2O–NaCl system) had salinities between 0.35 and 17.25 eq. wt % NaCl. The pressure-corrected and confined homogenization temperatures suggest formation of the calcite veins from a fluid with a temperature between 120 and 170 °C, a pressure of 300–880 bar at a depth between 2.1 and 3.2 km. The fluids were most likely confined to a particular sedimentary bed as a bed-scale fluid migration (white older calcite veins) or, later, to a pile of Palaeozoic sediments as a stratigraphically restricted fluid flow (rose younger calcite veins). The low temperatures and pressures during precipitation of calcites, which took place close to a peak of burial/deformation, confirm the distal position of the Moravian Karst region within the Variscan orogen.
Upper Devonian and Mississippian foraminiferal and rugose coral zonations of Belgium and northern France: a tool for Eurasian correlations
- EDOUARD POTY, FRANCOIS-XAVIER DEVUYST, LUC HANCE
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- 04 September 2006, pp. 829-857
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The radiation of early Carboniferous foraminifers and rugose corals following the Devonian–Carboniferous crisis offers the best tool for high-resolution correlations in the Mississippian, together with the conodonts in the Tournaisian, notably in the Namur–Dinant Basin. However, some of the guides are facies-controlled and an integrated approach combining biostratigraphy, sedimentology and sequence stratigraphy is critical to identify delayed entries, potential stratigraphic gaps and to avoid diachronous correlations. The main difficulty is in correlating shallow and deeper water facies at any given time. In existing zonations, the Viséan part of the scheme is always more detailed, reflecting the widespread development of shallow-water platforms in the early Viséan which created conditions more suitable for foraminifers and rugose corals over large areas. In contrast, the Tournaisian zones, less well documented, reflect unfavourable environmental conditions in the lower ramp (Dinant Sedimentation Area) and pervasive dolomitization of the inner ramp (Condroz and Namur Sedimentation Areas). Recent progress in understanding the Belgian early Carboniferous sequence stratigraphy and lithostratigraphy, and revision of the biostratigraphy of the key sections, strongly modify former biostratigraphic interpretations. Improvements mainly concern the latest Devonian, the late Tournaisian and the early Viséan. The late Devonian and the Tournaisian are equated with foraminifer zones DFZ1 to DFZ8 and MFZ1 to MFZ8 respectively. The Viséan correlates with zones MFZ9 to MFZ14. Zone MFZ15 straddles the Viséan–Namurian boundary and Zone MFZ16 is the youngest Mississippian zone. The rugose corals allow the recognition of ten zones, RC0 to RC9, covering the Strunian (late Famennian) to Serpukhovian interval. Discrepancies with former zonations are discussed. The Moliniacian Stage is emended to restore the coincidence between its base and that of the Viséan.
Provenance of north Gondwana Cambrian–Ordovician sandstone: U–Pb SHRIMP dating of detrital zircons from Israel and Jordan
- K. KOLODNER, D. AVIGAD, M. McWILLIAMS, J. L. WOODEN, T. WEISSBROD, S. FEINSTEIN
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- 31 March 2006, pp. 367-391
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A vast sequence of quartz-rich sandstone was deposited over North Africa and Arabia during Early Palaeozoic times, in the aftermath of Neoproterozoic Pan-African orogeny and the amalgamation of Gondwana. This rock sequence forms a relatively thin sheet (1–3 km thick) that was transported over a very gentle slope and deposited over a huge area. The sense of transport indicates unroofing of Gondwana terranes but the exact provenance of the siliciclastic deposit remains unclear. Detrital zircons from Cambrian arkoses that immediately overlie the Neoproterozoic Arabian–Nubian Shield in Israel and Jordan yielded Neoproterozoic U–Pb ages (900–530 Ma), suggesting derivation from a proximal source such as the Arabian–Nubian Shield. A minor fraction of earliest Neoproterozoic and older age zircons was also detected. Upward in the section, the proportion of old zircons increases and reaches a maximum (40%) in the Ordovician strata of Jordan. The major earliest Neoproterozoic and older age groups detected are 0.95–1.1, 1.8–1.9 and 2.65–2.7 Ga, among which the 0.95–1.1 Ga group is ubiquitous and makes up as much as 27% in the Ordovician of Jordan, indicating it is a prominent component of the detrital zircon age spectra of northeast Gondwana. The pattern of zircon ages obtained in the present work reflects progressive blanketing of the northern Arabian–Nubian Shield by Cambrian–Ordovician sediments and an increasing contribution from a more distal source, possibly south of the Arabian–Nubian Shield. The significant changes in the zircon age signal reflect many hundreds of kilometres of southward migration of the provenance.
Ordovician and Silurian chitinozoan biozones of western Gondwana
- YNGVE GRAHN
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- 06 June 2006, pp. 509-529
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A formal Ordovician–Silurian chitinozoan biozonation for western Gondwana is proposed. This palaeogeographic province includes South America, and was located in medium to high latitudes during Ordovician and Silurian times. Ordovician chitinozoans are known from northern Argentina, southern Bolivia, and Brazil. Silurian chitinozoans occur in Brazil, northern Argentina, southern Bolivia and southern Peru. No published information is available about Ordovician–Silurian chitinozoans from Ecuador, Colombia or Venezuela. Altogether more than 150 localities (including wells and outcrops) are included in this study, and 154 species have been encountered. A biozonation based on the first occurrence of critical chitinozoan species is introduced. Five biozones are defined in the Ordovician (zones of Desmochitina sp. gr. minor, Conochitina decipiens, Eremochitina brevis, Lagenochitina obeligis and Tanuchitina anticostiensis), and nine in the Silurian (zones of Belonechitina postrobusta, Spinachitina harringtoni, Pogonochitina djalmai, Margachitina margaritana–Salopochitina monterrosae, Angochitina echinata, Eisenackitina granulata, Fungochitina kosovensis and the subzones of Sphaerochitina solutidina and Desmochitina cf. D. densa). These biozones are compared with known graptolite, conodont, acritarch and spore zones from the same area, and chitinozoan zones on a global basis.
Exhumation of high-pressure rocks under continuous compression: a working hypothesis for the southern Hellenides (central Crete, Greece)
- V. CHATZARAS, P. XYPOLIAS, T. DOUTSOS
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- Published online by Cambridge University Press:
- 18 September 2006, pp. 859-876
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Combined kinematic, structural and palaeostress (calcite twinning, fault-slip data) analyses are used to study the exhumation mechanism of the high-pressure rocks exposed on the island of Crete (southern Aegean, Greece). Our study shows that the evolution of windows in central Crete was controlled by two main contractional phases of deformation. The first phase (D1) was related to the ductile-stage of exhumation. NNW–SSE compression during D1 caused layer- and transport-parallel shortening in the upper thrust sheets, resulting in nappe stacking via low-angle thrusting. Synchronously, intracontinental subduction led to high-pressure metamorphism which, however, did not affect the most external parts of the southern Hellenides. Subsequent upward ductile extrusion of high-pressure rocks was characterized by both down-section increase of strain and up-section increase of the pure shear component. The second phase (D2) was associated with the brittle-stage of exhumation. D2 was governed by NNE–SSW compression and involved conspicuous thrust-related folding, considerable tectonic imbrication and formation of a Middle Miocene basin. The major D2-related Psiloritis Thrust cross-cuts the entire nappe pile, and its trajectory partially follows and reworks the D1-related contact between upper and lower (high-pressure) tectonic units. Eduction and doming of the Talea Window was accompanied by gravity sliding of the upper thrust sheets and by out-of-the-syncline thrusting. Late-orogenic collapse also contributed to the exhumation process. Therefore, it seems that the high-pressure rocks of central Crete were exhumed under continuous compression and that the role of extension was previously overestimated.
Rapid Communication
Strain estimation from flattened parallel folds: application of the Wellman method and Mohr circle
- JYOTI SHAH, DEEPAK C. SRIVASTAVA
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- Published online by Cambridge University Press:
- 27 March 2006, pp. 243-247
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Parallel folds exhibit a characteristic orthogonal relationship between the tangent and the corresponding isogon drawn at any point on folded surface. Modification of parallel fold to flattened parallel fold by superimposition of homogeneous strain introduces an angular shear along the tangents at different points. The angular shears in different directions, obtained by measuring angles between the tangents and the corresponding isogons, can be used for estimation of flattening strain by a variety of geometrical and numerical methods. We show that several simple geometrical techniques, such as the Wellman method and the Mohr circle method, can rapidly decipher the strain from flattened parallel folds. These methods, in contrast to most of the existing methods of strain estimation, are independent of the assumption that one of the principal strain directions parallels the axial trace on the profile plane of fold.
Original Article
Temporal variations of the trace fossil Zoophycos in a 425 ka long sediment record from the South China Sea: implications for the ethology of the Zoophycos producer
- LUDVIG LÖWEMARK, HUI-LING LIN, MICHAEL SARNTHEIN
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- Published online by Cambridge University Press:
- 02 December 2005, pp. 105-114
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A 425 ka long record from piston core GIK17925-3 taken in the northeastern South China Sea was used to study the environmental conditions controlling the distribution of the Zoophycos trace fossil. The 12 m long core offers a unique opportunity to study the response of the Zoophycos-producing animal to environmental variations over four glacial–interglacial cycles. The trace fossils show a strong glacial to interglacial variation in their abundance with a special preference for intervals with low sedimentation rates. Additional X-ray radiograph studies of piston cores from the Southeast Asian Marginal Seas show that the trace fossil Zoophycos is widespread in slope and deep-marine sediments, with the highest abundances encountered in low sedimentation rate settings. The preference of the Zoophycos producer for low sedimentation rates in a setting with strong seasonal fluctuations in food supply due to the shift between winter and summer monsoons, is interpreted to be the result of a cache-model behaviour, where food is collected during rich times and squirrelled away for poor times. Core GIK17925-3 also offers an opportunity to assess the impact of Zoophycos bioturbation on various palaeoenvironmental proxies. In this core, more than 30 % of the measured data points were more or less strongly affected by Zoophycos bioturbation. Together with the widespread occurrence this percentage indicates that Zoophycos may pose a serious threat to palaeoclimatic reconstructions in cores from low to moderate sedimentation rate sites.