Low-grade Metamorphic Rocks Research Articles

Foliation relationships and structural facing vs. vergence determinations in refolded low-grade metamorphic rocks: an example from the Tuscan Metamorphic ‘Basement’ (Northern Apennines, Italy)

Three tectonic foliations and their angular relation with bedding allow the determination of facing vs. vergence and fold interference pattern in polyphase low-grade metamorphic terrains, composed of Palaeozoic and Triassic turbiditic rocks cropping out in the inner part of the Northern Apennines (Mt Leoni area). The D 1 deformational event (Late Oligocene–Early Miocene) is typified by southeast-verging folds (F 1) and related tectonic foliation (S 1) with a HP–LT mineralogical assemblage, developed during the emplacement of the Northern Apennines tectonic units. The D 2 deformational event (Early–Middle Miocene) caused F 2 east-verging folds and related tectonic foliation (S 2) during greenschist facies metamorphism. The D 3 deformational event (Middle Miocene–?) formed F 3 gentle upright folds with a foliation (S 3) developed only in the fold closures, and never accompanied by blastesis. Foliation angular relationships, as well as their intersections with the bedding, allowed us to define the facing, vergence and interference pattern of map-scale folds. Where a penetrative S 2 crenulation-cleavage affected the metapelitic tops of turbiditic strata, S 1 structural facing could be misinterpreted at the outcrop scale, simulating a northwest-vergence of F 1 folds. This is due to a false dihedral angle between S 0 and S 1 occurring in the metapelites due to the rotation of S 1 in the hinges of the overprinting crenulation-cleavage domains (S 2).

Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece

We present new Raman spectra data of carbonaceous material (CM) to extend the range of the Raman spectra of CM thermometer (RSCM) to temperatures as low as 100 8C. Previous work has demonstrated that Raman spectroscopy is an excellent tool to describe the degree of graphitization of CM, a process that is independent of pressure but strongly dependent on metamorphic temperature. A linear relationship between temperature and the Raman parameter R2 (derived from the area of the defect band relative to the ordered graphite band) forms the basis of a previous thermometer. Because R2 shows little variability in low-temperature samples, 330 8C serves as a lower limit on the existing thermometer. Herein, we present Raman spectra from a suite of low-temperature (100 to 300 8C) samples from the Olympics Mountains and describe other aspects of the Raman spectra of CM that vary over this range. In particular, the Raman parameter R1 (the ratio of heights of the disordered peak to ordered peak) varies regularly between 100 and 350 8C. These data, together with published results from higher-temperature rocks, are used to calibrate a modified RSCM thermometer, applicable from 100 to 700 8C. Application to low-grade metasediments in the Otago region in the South Island of New Zealand gives temperatures consistent with previous estimates, demonstrating the reliability of the modified RSCM thermometer. We apply the modified RSCM thermometer to 53 samples from Crete to evaluate the role of the Cretan detachment fault in exhuming Miocene high pressure/low-temperature metamorphic rocks exposed there. The metamorphic rocks below the detachment (the Plattenkalk and Phyllite–Quartzite units) give metamorphic temperatures that range from 250 to 400 8C, consistent with previous petrologic estimates. We also demonstrate that the Tripolitza unit, which lies directly above the detachment, gives an average metamorphic temperature of about 260 8C. The modest break in metamorphic temperature in central Crete indicates that the Cretan detachment accounts for only 5 to 7 km of exhumation of the underlying HP–LT metamorphic rocks, which were initially accreted at ~35 km. We argue that the bulk of the exhumation (~28 km out of 35 km total) occurred by pervasive brittle stretching and erosion of structural units above the detachment. D 2005 Elsevier B.V. All rights reserved.

4: Transpressional tectonics, lower crust decoupling and intrusion of deep mafic sills: A model for the unusual metallogenesis of SW Iberia

SW Iberia is interpreted as an accretionary magmatic belt resulting from the collision between the South Portuguese Zone and the autochthonous Iberian terrane in Variscan times (350 to 330 Ma). In the South Portuguese Zone, pull-apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the Iberian Pyrite Belt. Massive sulphides precipitated in highly efficient geochemical traps where metal-rich but sulphur-depleted fluids of dominant basinal derivation mixed with sulphide-rich modified seawater. Massive sulphides formed either in porous/reactive volcanic rocks by sub-seafloor replacement, or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity. Crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks, while magmatic fluids probably had a minor influence on the observed geochemical signatures. The Ossa Morena Zone was a coeval calc-alkaline magmatic arc. It was the site for unusual mineralization, particularly magmatic Ni–(Cu) and hydrothermal Fe-oxide–Cu–Au ores (IOCG). Most magmatism and mineralization took place at local extensional zones along first-order strike-slip faults and thrusts. The source of magmas and IOCG and Ni–(Cu) deposits probably lay in a large mafic–ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust. Here, juvenile melts extensively interacted with low-grade metamorphic rocks, inducing widespread anatexis, magma contamination and further exsolution of hydrothermal fluids. Hypersaline fluids ( δ 18O fluid > 5.4‰ to 12‰) were focused upward into thrusts and faults, leading to early magnetite mineralization associated with a high-temperature (> 500 °C) albite–actinolite–salite alteration and subsequent copper–gold-bearing vein mineralization at somewhat lower temperatures. Assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex. Further injection of both basic and sulphide-rich magmas into the upper crust led to the formation of Ni–(Cu)-rich breccia pipes. Younger (330 to 280 Ma?) peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis. These granitoids have W–(Sn)- and Pb–Zn-related mineralization that also shows geochemical evidence of major mantle–crust interaction. Late epithermal Hg–(Cu–Sb) and Pb–Zn–(Ag) mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex. In all cases, most of the sulphur seems to have been derived from leaching of the host sedimentary rocks ( δ 34S = 7‰ to 20‰) with only limited mixing with sulphur of magmatic derivation. The metallogenic characteristics of the two terranes are quite different. In the Ossa Morena Zone, juvenile magmatism played a major role as the source of metals, and controlled the styles of mineralization. In the South Portuguese Zone, magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids, which are dominated by crustal signatures. Most of the magmatic and tectonic features related to the Variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion, which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs.

Late Neogene low-angle thrusting on the northwestern margin of the South Carpathians (Poiana Rusca, West Romania)

Mineral exploration drillholes and geoelectric prospecting provide for the first time evidence for thrusting of the South Carpathian Paleozoic basement over northerly adjacent Middle Miocene sediments. Investigations were carried out in two locations, 30 km apart, along the northern margin of the Poiana Rusca Mountains, Romania, southwestern Carpathians. Drill holes in both locations encountered weakly consolidated Middle Miocene clay, sand, and fine gravel below Paleozoic low-grade metamorphic rocks. Intersections from various drill holes demonstrate the presence of low-angle thrusting. Kinematic indicators are so far lacking, but with a thrust direction oriented roughly normal to strike of the Poiana Rusca Mountains, minimum displacement is 1–1.4 km in northwestern or northern direction, respectively. Thrusting occurred most likely during the Late Miocene–Pliocene, whereafter Quaternary regional uplift dissected the thrust plane. In the tectonic framework of Neogene dextral translation of the Tisza–Dacia Block against the southerly adjacent Moesian Platform, transtension appears responsible for Middle Miocene basin formation along the northern margin of the Poiana Rusca region. Proceeding collision of the Tisza–Dacia Block with the East European Craton introduced stronger impingement of the Tisza–Dacia Block against the Moesian Platform, leading to a Late Miocene–Pliocene transpressional regime, in which the northern Poiana Rusca basement was thrust over its adjacent Middle Miocene sediments.

Low-Grade Metamorphic Rocks in the Dabie-Sulu Orogenic Belt: A Passive-Margin Accretionary Wedge Deformed during Continent Subduction

Greenschist-facies metasedimentary and meta-igneous rocks occur continuously along the northern margin, and sporadically in the interior, of the Dabie-Sulu orogenic belt in east-central China. An integrated study of geochronological, petrological, and paleontological observations demonstrates that precursors of flysch-facies metasedimentary rocks were deposited along the northern, passive continental margin of the Yangtze plate prior to the Triassic, and that protoliths of the meta-igneous rocks are a product of Middle Neoproterozoic bimodal magmatism along the northern margin of this plate. Except for the striking contrast in metamorphic grade, these low-grade rocks generally can be correlated in protolith origin and age with ultrahigh-pressure metamorphic rocks within the orogenic belt. Relationships in time and space between these rocks of contrasting grades can be reasonably interpreted through an accretionary wedge model that links their evolution with continent subduction. The low-grade metamorphic rocks of the subducting accretionary wedge consist of two parts: (1) large masses of metasedimentary rocks (including slates, schists, phyllites, metasandstones, and marble) along the northern margin of the Dabie-Sulu orogenic belt and deformed igneous rocks of Middle Neoproterozoic age; (2) sporadic outcrops in the interior of the belt of metavolcanics, metaclastics, phyllite, and marbles. During Triassic subduction of the Yangtze plate, the sedimentary cover and its underlying basement were partly scraped off by the overthrusted North China plate. The scraped-off materials accumulated in front of the overriding plate, forming an accretionary wedge that underwent deformation and metamorphism under greenschist-facies conditions. The present study also provides a constraint on the location of the Triassic suture zone between the North China and Yangtze plates. It is located below, or north of, the accretionary wedge (i.e., the Beihuaiyang zone in the Dabie region and the Wulian-Penglai zone in the Sulu region) rather than along the northern margin of the ultrahigh-pressure metamorphic zones.

Diferenças Entre os Depósitos Auríferos do Grupo Aguapei (Estado de Mato Grosso) e os Clássicos Exemplos de Ouro em Conglomerados

Gold prospects in the Aguapeí Folded Belt are situated in the southwestern portion of the Amazon Craton, within the Alto Guaporé Province, southwestern state of Mato Grosso. The mineralizations are mainly concentrated in the Pau-a-Pique Deposit (south), Lavrinha Region (central), and São Vicente Mine (north). Gold ore comprises quartz veins and disseminations in the low-grade metamorphic metassedimentary rocks from the Aguapeí Group. This stratigraphic unit was strongly deformed by the Sunsás-Aguapeí Orogeny (1.28-0.95 Ga). The Aguapeí gold deposits differ in their petrography, chemistry, structural aspects and age from world classic gold deposits associated with coarse clastic sedimentary sequences (Witwatersrand, Jacobina, Tarkwa, etc.). This study discusses the main differences between Aguapeí gold deposits and the classic worldwide deposits hosted by conglomerates.

The b-spacing values of white micas and their metamorphic implications in the Lesser Himalaya, central Nepal

The Lesser Himalaya in central Nepal exposes a very wide section (>80 km) of very low- to low-grade metamorphic rocks. Metamorphic data from these rocks are relatively rare because of fine-grained nature of the rocks and lack of suitable assemblage for themobarometric calculations. In the present study, b-spacing measurements and compositional analyses were carried out on white micas in very low- to low-grade metapelites collected along the Kali Gandaki river valley and Tansen–Pokhara road sections to determine evidence of polymetamorphism and P–T conditions of metamorphism. The study showed that the Lesser Himalaya experienced at least two metamorphic events, one prior to the Upper Main Central Thrust (Upper MCT) activity (pre-Himalayan? or M 0) and the other during the Upper MCT activity (Neohimalayan or M 2). The M 0 produced celadonite-rich white micas defining the S 1(=S 0) foliation which pre-dates the Upper MCT event. The M 0 white micas were partially or completely re-equilibrated during M 2. The white micas recrystallized during M 2 are relatively celadonite-poor and define S 2 foliation. The M 2 was of an intermediate-P type (Barrovian-type) in the south of the Phalebas Thrust while it was of relatively high-T type in the north near the Upper MCT. The geothermal gradient varied from 22 °C km −1 in the south to 26 °C km −1 in the north. The northward increase in geothermal gradient is thought to be due to the heat transfer from hot Higher Himalaya or shear heating along the Upper MCT as suggested by Le Fort [Le Fort, P., 1975. Himalaya, the collided range: present knowledge of the continental arc. American Journal of Science 275A, 1–44. Arita, K., 1983. Origin of the inverted metamorphism of the Lower Himalayas, central Nepal. Tectonophysics 95, 43–60].

Age resetting of hanging wall rocks above a low-angle detachment fault: Tinos Island (Aegean Sea)

The central Aegean region has been affected by extensional tectonics since at least the Oligo-Miocene. On Tinos Island (Cyclades, Greece), a low-angle normal fault places low-grade metamorphic rocks (the Upper Unit) above high-pressure rocks (the Cycladic Blueschist Unit). K-Ar and 40Ar/ 39Ar ages were obtained by us on white-micas, amphiboles, and whole-rock samples separated from various metamorphic rocks representing different structural levels of the hanging wall. The hanging wall Upper Unit is commonly thought to have resided at shallow crustal levels since Late Cretaceous times, but our geochronological data, supported by previously published Rb/Sr dating [Bröcker, M., Franz, L., 1998. Rb-Sr isotope studies on Tinos island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity, Geol. Mag., 135 (3), 369–382], indicate severe age resetting of the base of the hanging wall during Tertiary times. Amphiboles from the top of the hanging wall section yielded Late Cretaceous ages which correlate well with other Cretaceous Upper Unit rocks in the Cyclades. Their preservation implies that Tertiary age resetting was mainly confined to the lower part of the Upper Unit, while upper parts retained older ages. The base of the hanging wall section, where age resetting and ductile deformation are most pronounced, is taken to represent a part of a thick extensional shear zone that operated in low-greenschist-facies conditions during Tertiary time. The results of one-dimensional thermal modeling suggest that warm footwall rocks, emerging from great depth subsequently to HP/LT metamorphism, may have supplied the heat required for the metamorphism and ductile deformation at the base of the hanging wall. The model also indicates that only minor cooling of the footwall during decompression is allowed if the footwall is to warm up the hanging wall. This is a priori consistent with the isothermal decompression path of footwall blueschists and eclogites deduced from previous petrological studies on Tinos.

Clay mineralogy of the soils in the south Ecuadorian páramo region

The páramo soils of the mountainous upper Andean region (>3300 m a. s. l.) of the Rio Paute basin in central Ecuador are characterized by a thick, dark, highly organic epipedon and are classified as Andosols and Histosols. Their high water retention and buffering capacity play a key role in the hydrology of the region, which is subject to land use changes and increased cultivation. In the west (Western Cordillera), the soils are largely formed in the late Miocene and Pliocene volcanoclastic Tarqui formation, while in the east (Central Cordillera) they are formed in an older, mostly intermediate low-grade metamorphic rocks. Ten soil profiles were sampled and studied, using extraction techniques (oxalate and pyrophosphate) and XRD-techniques. Major differences in composition of the clay fractions were found that allow for distinction of three main groups of páramo soils. A first group consists of soils influenced by recent volcanic ashes and dominated by organometallic complexes and with minor but distinct amounts of degraded mica, most probably formed by weathering of primary mica, present in these ashes. The second group comprises soils formed in volcanoclastic material of various Tertiary and earlier formations, containing residual primary and secondary crystalline clay-size minerals, as well as organometallic complexes whose genesis can be linked to the abundant presence of easily weatherable materials in these formations. A third group consists of soils in relicts of Tertiary, highly weathered regolith, formed under humid tropical conditions before the Andean uplift and occurring in the Central Cordillera. These soils contain kaolinite and gibbsite and develop into Histosols in the absence of significant organometallic complexation.

Historical building stones in the province of Limburg (NE Belgium): role of petrography in provenance and durability assessment

Over 35 different stone species have been inventoried in historical monuments of Limburg (N-Belgium) and described in great detail. The resulting petrographical atlas provides a powerful identification and decision tool in restoration. Most indigenous building stones are no longer available: the historical monuments witness therefore of the former richness in geological materials. The atlas deals with macroscopic and microscopic features of the natural stones and provides suggestions for their replacement. The geographical distribution of the stones in the monuments reflects local geology and delimits geographical provinces. Most inventoried stones are sedimentary in origin. Rare volcanic and low-grade metamorphic rocks have been imported from adjacent areas. Petrography is very helpful, especially in discriminating different varieties of ferruginous, quartzarenitic, litharenitic and subarkosic sandstones. Moreover, thin-section analysis helps in assessing provenance, weathering and bioreceptivity potential. Finally, recommendations are made for replacement stones, whose intrinsic characteristics often differ from those of the original stones.

Is the Cambrian basin of the Western High Atlas (Morocco) related either to a subduction zone or a major shear zone?

By its geological and structural position, the Palaeozoic massif of the western High Atlas constitutes a hinge between the Anti-Atlasic domain in the South and the domain of Meseta in the North. The upper Proterozoic and lower Palaeozoic formations of the Anti-Atlas and the Cambrian successions of coastal Meseta are well known, while a synthetic vision of the western High Atlas coeval sequences is lacking. Recent field and analytical work performed on low-grade metamorphic rocks of the Moroccan “High Atlas old Massif” allows us to distinguish several sedimentary sequences and interbedded (essentially effusive) magmatic rocks, the geochemical characterization of which forms the main aim of this paper. By comparison with adjacent areas, the age of the metasedimentary rocks is attributed to the late Neoproterozoic and lower Palaeozoic. The volcanic rocks have calc-alkaline (late Neoproterozoic and lower Cambrian), alkaline and/or tholeiitic (Middle Cambrian) affinities. The results of this study impose some constraints on understanding the palaeogeographic links between the Anti-Atlas and the Moroccan Meseta during the Neoproterozoic–Cambrian transition.

SHEAR ZONE-HOSTED POLYMETALLIC SULFIDES IN THE SOUTH LIMOUSIN AREA, MASSIF CENTRAL, FRANCE: REMOBILIZED SULFIDE DEPOSITS RELATED TO VARISCAN COLLISIONAL TECTONICS AND AMPHIBOLITE FACIES METAMORPHISM

This paper investigates the relationship between deformation and mineralization developed in amphibolite facies shear zones in the south Limousin area using structural, microstructural, and textural analyses. The investigated shear zone is a ductile thrust fault that stacked low-grade metamorphic rocks onto a highly metamorphosed basement, with both being folded during a late orogenic tectonic phase. The shear zone hosts sulfide-bearing mylonitic quartzites composed of dynamically recrystallized quartz, syntectonic pyrite, pyrrhotite, sphalerite, chalcopyrite, and late covellite. Mylonites display northwest-southeast–trending lineation and asymmetric microstructures, including sigmoidal quartz aggregates and shear bands, that indicate a northwestward movement along the shear zone. Sulfides occur mainly along shear bands and exhibit primary, undeformed textures, attesting to their syntectonic emplacement. Based on petrographic analyses of the metamorphic host rocks, mineralization formed at P = 5.0 to 5.5 kbars, and T = ~300° to 600°C, consistent with P-T conditions for recrystallization of metapelites and metabasites involved in the shear zone. All data indicate that ductile deformation and emplacement of mineralization were coeval. The occurrence of similar pretectonic sulfide deposits in the hanging wall of the mineralized shear zone suggests that the sulfides in the thrust fault were remobilized from preexisting mineralization by fluid that migrated toward the thrust. This is the first evidence for thrust zone-hosted mineralization within the Massif Central and attests to the role of crustal-scale thrust faults as occasional pathways for mineralized fluids during the upper Paleozoic continental collision. These results suggest that ductile thrust faults may be important sites of ore deposition in the Variscan belt of western Europe and in similar collisional belts elsewhere.

Metamorfismo de contato no Complexo Bossoroca, porção SW do Complexo Granítico São Sepé, RS

The Sao Sepe Granitic Complex (SSGC) is an NE-SW elongate body that intruded low-grade metamorphic rocks of the Bossoroca Complex. The emplacement of the granite in high crustal levels resulted on an aureole of contact metamorphism with 1,5 to 5 kilometers of extension. The thermal effects are registered mainly in metapelitics and metavolcanoclastic rocks of the Bossoroca Complex, and the biotite, garnet, andaluzite and staurolite isograds were mapped. The contact metamorphism extended until the sediments of the Marica Group of the Camaqua Basin. The cooling of the magma of SSGC is a continuous process characterized by two different stages: the transfer of heat for conduction, resulting on the contact metamorphism, followed by the infiltration of tardi-magmatics fluids with generation of the hydrothermal assemblage. Petrographic and thermo barometric data indicates that the emplacement and the cooling of SSGC happened in little depth, with pressures of the order of the 2,5 to 4,5 kbares and maximum temperatures in the interval among 570 o to 600 o C. The effects of the contact metamorphism also recorded in the sediments of the Marica Group suggesting that the intrusion of the granite follows the sedimentation of the basal units of the Camaqua Basin

Low-grade metamorphic rocks from the Pulur complex, NE Turkey: implications for the pre-Liassic evolution of the Eastern Pontides

In the Pulur complex (Sakarya Zone, Eastern Pontides, Turkey) a low-grade tectonometamorphic unit (Dogankavak) is exposed in three tectonic windows beneath a complex medium-pressure high-temperature metamorphic unit of late Carboniferous age. The thrust plane between both units is transgressively covered by Liassic conglomerates. The Dogankavak unit comprises a sequence of metabasites with MORB-type chemical compositions and phyllites, with subordinate calcareous phyllites, marbles, quarzofeldspathic schists and metacherts. This sequence is interpreted as a former accretionary complex related to the consumption of the Palaeotethys. Mineral parageneses in the metabasites allow for the distinction of two domains with slightly different peak metamorphic conditions, i.e. 375–425 °C/0.5–0.8 GPa (greenschist facies) and 400–470 °C/0.6–1.1 GPa (albite-epidote amphibolite facies). The age of metamorphism is constrained at ~ 260 Ma (early Late Permian) by two Rb-Sr mineral-whole rock ages (hornblende, phengite) and one 40Ar/39Ar single step total fusion age (phengite). In conjunction with previous data on other accretionary complexes in the Sakarya zone in Northern Turkey, the data presented in this study suggest a continuous subduction of the Palaeotethys at least from Early/Late Permian to Late Triassic and a discontinuous preservation of accretion complexes in both space and time.

Fluid Inclusions in Epidote Minerals and Fluid Development in Epidote-Bearing Rocks

The widespread occurrence of epidote minerals in metamorphic and igneous rocks as well as in many ore deposit types makes it a promising candidate for fluid inclusion studies. Apart from high- to very high-temperature and low- to intermediate-pressure conditions, epidote minerals are stable over a wide range of pressure and temperature in the continental and oceanic crust (e.g., Poli and Schmidt 1998). Yet fluid inclusion studies on epidote minerals are surprisingly scarce, even in fluid-saturated environments like certain vein-type deposits or hydrothermal-volcanic vugs and druses. For example, epidote minerals are not mentioned in the subject index of Roedder’s (1984) outstanding summary and review of fluid inclusion studies and occurrences, which lists more than sixty different host minerals for fluid inclusions. Nonetheless, more recent studies showed fluid inclusions in epidote minerals to be the only direct witness of the physiochemical and compositional fluid evolution during certain geodynamic processes mainly found in fossil geothermal systems, ore deposits and high-pressure to ultra-high pressure rocks. The aim of this review is to outline and summarize some aspects and interpretations of geodynamic processes, which are based on temperature ( T ), pressure ( P ), molar volume ( V ) and composition ( X ) data from fluid inclusions in epidote minerals as well as associated host minerals from various geological environments. The review starts with a chapter on some typical mixed volatile solid-fluid equilibria involving epidote minerals, which are relevant for the here discussed environments. This is followed by a short introduction into the basic concepts of fluid inclusion research and the role of epidote minerals. The next section covers fluid inclusion studies on epidote minerals from active and fossil geothermal systems as well as low-grade metamorphic rocks and constraints on the P - T - X properties of the fluids present in these systems. This is followed by a short introduction …

Neoproterozoic high-grade metamorphism of the Kanin Peninsula, Timanide Orogen, northern Russia

Abstract Throughout most of the exposed Timanide Orogen, the grade of regional metamorphism does not exceed greenschist facies. Only on the Kanin Peninsula and in the northernmost Timan and in a few of the drillcores that sample basement beneath the Pechora Basin, are amphibolite facies rocks present. This study presents new P-T estimates by garnet-biotite geothermometry and GASP geobarometry from the high-grade rocks of the Kanin Peninsula, together with a new structural interpretation of the SE-Kanin area. Thick metaturbiditic successions show an increase of metamorphic grade downwards from low greenschist to high amphibolite facies. On the SE Kanin Peninsula, peak metamorphic conditions are estimated to have reached c. 0.72 GPa and c. 610 °C. These inferred Late Proterozoic successions were thickened by thrust-stacking and emplaced southwestwards onto low-grade metamorphic pericratonic sedimentary rocks of the East European Craton during the Timanian Orogeny. Timing of metamorphism is inferred to be late Neoproterozoic, but radiometric ages are not well constrained and more work is needed. The new field observations and analytical results suggest that high-grade regional metamorphism is not confined to the Timan-Kanin area. This is of importance when considering the geophysical data over the Pechora Basin and Barents Sea, particularly the interpretation of crustal velocity structure. Higher velocities, often inferred to represent pre-Riphean basement, may well be related to Neoproterozoic and Mesoproterozoic complexes.

Cambro‐Ordovician trace fossils from the SW‐Norwegian Caledonides

AbstractA diverse trace fossil association is described for the first time from low‐grade metamorphic rocks of the SW‐Norwegian Caledonides. The investigated cliff sections with autochthonous to parautochthonous metasediments comprise a coarsening‐ and thickening‐upward succession interpreted as prograding delta deposits. Sedimentary features indicate a tide‐influenced environment. Twenty‐one ichnospecies have been identified and assigned to the Cruziana and Skolithos ichnofacies, including the oldest record of Beaconites capronus and Macaronichnus segregatis. Cruziana ichnostratigraphy (sensu lato), previously rarely used in other palaeocontinents than Gondwana, allows an age determination for these metasediments of Middle Cambrian to Lower Ordovician as indicated by the presence of Cruziana barbata, C. furcifera, C. rugosa, C. semiplicata, Didymaulichnus rouaulti and Rusophycus ramellensis. Baltica was geographically the most isolated from the other three large continents (Gondwana, Laurentia and Siberia) during Cambro‐Ordovician time, and provinciality of faunal assemblages (e.g. brachiopods, conodonts) has been proved and is also supposed for trilobites by some authors. However, although the Cruziana ichnospecies result from a high specialization of their tracemakers, and therefore only a small group of trilobite species is eligible for its origin, the ichnospecies reported from Baltica occur also on other palaeocontinents and do not support the assumption of trilobite provincialism. Copyright © 2004 John Wiley & Sons, Ltd.

Ductile–brittle transition along the Alaşehir detachment fault and its structural relationship with the Simav detachment fault, Menderes massif, western Turkey

Western Turkey has experienced extensional deformation since the Tertiary. An early phase of extensional deformation resulted in the formation of detachment faults in the northern part of the Menderes massif, the Alaşehir and Simav detachment faults. The footwalls of these two detachment faults are mid-crustal level rocks. The Alaşehir detachment fault developed in metamorphic rocks and in a syn-extensional intrusion, the Salihli granodiorite. The Salihli granodiorite shows a gradual change from undeformed isotropic granodiorite to protomylonite, mylonite and ultramylonite towards its upper structural levels. The deformed granodiorite, in turn, grades into a cataclastic zone whose uppermost part is the Alaşehir detachment surface. This gradual upward change from the undeformed granodiorite to a brittlely deformed detachment surface suggests that Tertiary extension resulted in a ductile deformation at depth and the ductilely deformed granitoids were brought to shallower depths where they were brittlely deformed. The metamorphic wall rocks of the intrusion are also mylonitized structurally upward. A similar transition has also been found in the footwall of the Simav detachment fault. The upper part of which is also a cataclastic zone leading to the Simav detachment fault separating the low-grade metamorphic rocks and/or non-metamorphic rocks in its hanging wall from mainly high-grade metamorphic rocks and syn-tectonic granitoids in its footwall. During ductile extension, greenschist facies-grade mylonitic deformation developed in the metamorphic rocks and the granitoids. Shear sense indicators along the two shear zones show top to N-NE sense shear, consistent with the regional Cenozoic extension direction in western Turkey. Available radiometric age data from the granitoids in the footwall of the two detachment faults suggest that the Cenozoic extension in western Turkey was initiated in Oligocene to Early Miocene and the Simav detachment fault is older than the Alaşehir detachment fault. Extensional features of these detachment faults suggest that the Simav detachment fault characterise the earlier stage of the Tertiary extensional tectonics of the Menderes massif, and later stage of this deformation resulted in the development of the Alaşehir detachment fault.

Alluvial deposits from the strike-slip fault Lo River Basin (Oligocene/Miocene), Red River Fault Zone, north-western Vietnam

The Lo River Basin (LRB) is one of several narrow sedimentary basins associated with the main faults of the Red River Fault Zone separating the South China and Indochina microplates. The basin is located on the NE boundary of the high-grade metamorphic Con Voi Massif and the sedimentary and metasedimentary Viet Bac fold zone in north-eastern Vietnam. The LRB is filled with over 6000 m of Oligocene/Miocene alluvial deposits. The source area was probably located on the NE margin of the basin and was composed mostly of low-grade metamorphic rocks with a minor component of sedimentary rocks. Three alluvial systems are recognised. The oldest system was a proximal braided river system, with the minor occurrence of alluvial fans. The younger systems record changes in clast composition and lithofacies, which suggests a transition from a distal braided river to a distal braidplain system. The LRB fill shows a range of features characteristic of strike-slip fault basins. The origin of the LRB is correlated with the left-lateral transtensional regime. The present shape of the basin is a result of post-sedimentation tectonic activity.

Post-collisional extensional and thrust-nappe structures in northern part of Dabie Mountains

Thrust-nappe structures and extensional structures simultaneously occur in the northern part of the Dabie Mountains. The systematic structural study reveals that extensional structures along the Mozitan-Xiaotian fault and thrust-nappe structures that take the Jinzhai-Shucheng fault as their frontal thrust share the same shear sense, and display a transitional relationship from the ductile extensional structure of deep level in the south to the ductile-brittle and brittle thrust-nappe structure of shallow level in the north. The extensional and thrust-nappe structures in the region are explained to result from post-collisional processes by the continuous subduction of the Yangtze continental block and the extension induced by the uplifting of the core part of the Dabie Mountains, which are components of the extensional structures produced in the exhumation process of the ultrahigh pressure metamorphic rocks in the Dabie Mountains. Because of the frontal blocking in the process of the north-westward spreading, the extension and detachment of the low-grade metamorphic rocks along the Mozitan-Xiaotian fault was transformed into the northwestward thrusting, resulting in the thrust-nappe structures. They developed in the period of 200–170 Ma, maybe last till the late Jurassic.