Lower Nappe Research Articles

Hydraulic investigation of flows at high-head overflow spillway with multiple aerators: a physical and numerical study of Mohmand Dam, Pakistan

ABSTRACT A spillway is the essential part of the dam body, which releases surplus flows. At higher floods, the spillway operates at high heads, which results in high flow velocities along the chute and may cause negative pressures and cavitation. Therefore, to minimize such issues, aerators are provided along the spillway's chutes. This study aims to analyze the performance of the high-head overflow spillway of Mohmand Dam, Pakistan, having a steep chute of 32° with multiple aerators. Based on Froude's law of similitude, the physical model study was carried out at Irrigation Research Institute, Nandipur, on a scale of 1:60, while FLOW-3D numerical models were used to compare different hydraulic parameters, i.e., flow depth, velocity and pressure. The numerical models were validated with the results of a physical model, which were found in an acceptable range (i.e., 4.93%), and the hydraulic performance of two aerators was evaluated at different discharges. The models indicated negative pressures inside the aerator cavity, which allowed the suction of air to the lower nappe. The maximum air entrainment at the first aerator was about 8.5%. The results also showed that air entrainment to the lower nappe decreased when discharge was increased, whereas the maximum air detrainment reached 11.3% downstream of the second aerator.

Mechanisms and durations of metamorphic garnet crystallization in the lower nappes of the Caledonian Kalak Nappe Complex, Arctic Norway

AbstractThe 3D microstructure and compositional zoning of garnet populations in micaschists from the Kolvik and Bekkarfjord nappes indicate the quasi‐equilibration of their major components across the rock matrices during interface‐controlled, size‐independent garnet growth. There is microstructural evidence for foliation‐parallel, small‐scale resorption of garnet rims in the Kolvik Nappe, influencing the metamorphic peak conditions obtained from thermodynamic modelling. The local chemical compositions of rims less affected by resorption indicate a peak temperature of ~630°C, which is ~40°C higher than the temperature obtained from resorbed rims of the largest garnet crystal. Using a diffusion geospeedometry approach that considers the geometry of the compositional zoning of the garnet population, as well as the higher, more realistic peak temperature, a duration of 1 to 4.9 Myr is obtained for garnet growth in the Kolvik Nappe. This is approximately 1 order of magnitude faster than duration estimates obtained when using the apparent, lower temperature estimated from the resorbed garnet rims. Concomitantly to garnet growth in the Kolvik Nappe, garnet overgrowths formed in the Bekkarfjord Nappe for circa 2.5 Myr at metamorphic peak temperatures of ~560°C. The garnet growth durations obtained here are comparable with the uncertainty on the Lu–Hf garnet–whole rock isochron ages of 419.9 ± 2.4 Ma and 423.0 ± 1.9 Ma, previously obtained for these rocks. These results provide new insight into the timescales of repeated Barrovian‐type metamorphic events experienced by the lower nappes of the Kalak Nappe Complex during the Caledonian Orogeny in Arctic Norway. This study emphasizes the importance of microstructural and chemical characterization of garnet populations in assessing metamorphic crystallization mechanisms and the extent of equilibration of garnet‐forming components during prograde metamorphism. Moreover, our results provide means for reducing the uncertainty on metamorphic durations obtained via diffusion geospeedometry and, so, contributing to our understanding of geological timescales and processes.

Analysis of upper and lower nappe profiles of large orifice for the design of bottom and roof profiles of high head orifice spillway

Abstract Large orifices are constructed for dams to release water and sediments from reservoirs. Such structures are called submerged spillways. Numerous studies have investigated discharge coefficient, velocity coefficient, and head loss coefficient of large orifices; however, the literature lacks data on the upper and lower nappes of the jets from these orifices. In the present experimental study, the upper and lower nappes are investigated up to 80 m head at different gate openings. The observed minor deviation between the lower nappe profile and trajectory profile equation suggests sensitivity to different factors. The significant role of the coefficient of velocity, averaging at 0.926, highlights its impact on minor deviation. Subsequently, the impact of the solid bottom profile on the discharge coefficient and upper nappe profile are also examined. The results show improvement in discharge coefficient of a sharp-edged large orifice, which increased from 0.69 to 0.74. The results also indicate that the upper nappe profiles and United States Bureau of Reclamation (USBR) profiles are similar. The improvement in the upper nappe profile indicates the significant role of the solid bottom profile, which consequently was found to be helpful in defining the roof profile of an orifice spillway. .

Monazite–xenotime thermometry: a review of best practices and an example from the Caledonides of northern Scotland

Abstract Monazite–xenotime thermometry is a potentially powerful technique for understanding the evolution of Earth systems. While a rich set of experimental and empirical datasets are available for monazite–xenotime equilibria, five different thermometric calibrations yield significantly different results, making this technique difficult to apply in practice. To clarify best practices for monazite–xenotime thermometry, a compilation of published compositional data for monazite and xenotime with independently determined pressure–temperature conditions is evaluated. For each existing thermometer, we examine how closely estimated temperatures match independent empirical temperatures and consider how best to calculate monazite end-members for each thermometer. Monazite–xenotime thermometry is applied to samples from the Northern Highlands Terrane of northern Scotland, which experienced amphibolite–upper greenschist facies metamorphism and penetrative deformation during the Scandian orogeny. Thermometry data in conjunction with U–Pb dating define relatively slow regional cooling across the Scandian thrust nappes. Thermometry data closely match quartz c -axis fabric-based deformation thermometry across the structurally lower nappes, suggesting that monazite and xenotime record the timing and temperature of penetrative deformation and shearing. The data suggest that ductile deformation in the hinterland nappes of the Scandian orogen in Scotland occurred as late as 415–410 Ma.

TECTONOTHERMAL EVOLUTION OF THE ZAGAN METAMORPHIC CORE COMPLEX IN TRANSBAIKALIA AS A RESULT OF THE CRETACEOUS – PALEOCENE MONGOL-OKHOTSK POST-COLLISIONAL OROGEN DESTRUCTION

Thermochronological reconstructions of the Zagan metamorphic core complex were carried out using samples from the central part of the core, mylonite zone detachment and lower nappe with U/Pb zircon dating, 40Ar/39Ar amphibole and mica dating, and apatite fission-track dating. In the tectonothermal evolution of the metamorphic core, there was distinguished an active phase (tectonic denudation) of the dome structure formation during the Early Cretaceous (131–114 Ma), which continued in the Late Cretaceous – Paleocene (111–54 Ma) in passive phase (erosive denudation). During an active phase, there was initiated a large-amplitude gently dipping normal fault (detachment), which was accompanied by tilting (sliding of rocks along subparallel listric faults). As a result, about 7 km thick rock strata underwent denudation over 17 Ma at a rate of about 0.4 mm/year. In passive phase, about 6 km thick rock strata were eroded over 57 Ma, with a denudation rate of about 0.1 mm/year. Thus, the Zagan metamorphic core complex was tectonically exposed from the mid-crust to depths of about 9 km in the Early Cretaceous as a result of post-collisional collapse of the Mongol-Okhotsk orogen. Further cooling of the rocks in the metamorphic core to depths of about 3 km occurred in the Late Cretaceous – Pliocene as a result of destruction of more than 6 km high mountains.

Apatite and biotite thermochronometers help explain an Arctic Caledonide inverted metamorphic gradient

Biotite generates significant radiogenic Sr and is thus suitable for Rb–Sr dating, which can be achieved directly in thin section via laser ablation and provides a means to directly date growth of this important fabric-forming mineral or its cooling. In contrast, apatite has an extremely low Rb/Sr ratio and hence offers a useful tool to estimate source 87Sr/86Sri. Plagioclase also contains significant Sr and may further assist in understanding the rocks Sr isotopic system. However, the fluid source of growing or recrystallizing apatite, plagioclase, and biotite need not be the same. Compounding the complexity, apatite may gain radiogenic Sr from biotite during recrystallization or dissolution–reprecipitation that accompanies metamorphism. In this contribution, we present a case study from the Kalak Nappe Complex in the Scandinavian Caledonides, Arctic Norway, where a combined geochronology program, critically supported by biotite Rb–Sr dating, helps to resolve the conspicuous inverted metamorphic field gradient found upwards through the nappe stack in this orogen. Apatite U–Pb ages of 417–413 Ma in the upper nappes are interpreted as cooling through the Pb retention zone. Such ages contrast with the suggestion of older, partially reset apatite ages in the lower nappes. Biotite Rb–Sr ages of c. 413 Ma in the upper nappes are similar to U–Pb ages of coexisting apatite but conspicuously different from c. 433 Ma biotite Rb–Sr ages in the lower nappes. Ages of c. 433 Ma are comparable to muscovite 40Ar/39Ar and titanite and monazite U–Pb ages throughout the complex and may directly date mineral growth at peak thermal conditions during the Scandian orogenic event. Apatite 87Sr/86Sr ratios also track the influence of metamorphism and imply more radiogenic signatures relative to relic plagioclase in samples which have greater overprinting in the upper nappes of the complex. Phase equilibrium modelling establishes Palaeozoic peak P–T conditions of 610–750 °C at <12 kbar in the upper nappes, comparable to peak conditions previously established for the lower nappes. Combined thermal modelling of the post-peak Scandian orogenic history based on published 40Ar/39Ar, paired with new Rb–Sr, and U–Pb data, implies a slower cooling pathway in the upper nappes relative to the lower nappes. We reconcile this thermal history with a geodynamic model of obduction of a hot back-arc, as represented by the Silurian Magerøy Nappe, onto the upper nappes of the Kalak Nappe Complex at c. 430 Ma. These results demonstrate that in-situ laser ablation Rb–Sr dating of biotite provides a powerful tool for resolving orogenic tectonothermal histories in orogens flushed with excess argon.

Air–Water Properties in Rectangular Free-Falling Jets

This study analyzes the air–water flow properties in overflow nappe jets. Data were measured in several cross-sections of rectangular free-falling jets downstream of a sharp-crested weir, with a maximum fall distance of 2.0 m. The flow properties were obtained using a conductivity phase-detection probe. Furthermore, a back-flushing Pitot-Prandtl probe was used in order to obtain the velocity profiles. Five specific flows rates were analyzed, from 0.024 to 0.096 m3/s/m. The measurements of the air–water flow allowed us to characterize the increment of the air entrainment during the fall, affecting the flow characteristic distributions, reducing the non-aerated water inner core, and increasing the lateral spread, thereby leading to changes in the jet thickness. The results showed slight differences between the upper and lower nappe trajectories. The experimental data of the jet thickness related to a local void fraction of 50% seemed to be similar to the jet thickness due only to gravitational effects until the break-up length was reached. The amount of energy tended to remain constant until the falling distance was over 15 times greater than the total energy head over the weir crest, a distance at which the entrained air affected the entire cross-section, and the non-aerated core tended to disappear. The new experiments related with air–water properties in free-falling jets allow us to improve the current knowledge of turbulent rectangular jets.

Revised tectonostratigraphy and structural evolution of the Köli Nappe Complex, Central Caledonides in Nordland, Norway

The nappe stack in the Røssvatnet–Hattfjelldal region in the Central Norwegian Caledonides consists of seven nappes formed at the boundary between tectonostratigraphically upper and uppermost Caledonian levels. The rocks of all nappes share a polyphase tectonometamorphic evolution that is younger than the 491 ± 10 Ma depositional (volcanic) age of parts of the succession. Early stages of deformation characterized by centimetre- to kilometre-scale folding and intense shearing accompanied by greenschist to amphibolite facies peak metamorphism are correlated with the Early Ordovician Taconian accretionary orogeny along the Laurentian margin. The Taconian structures are cut by the Krutfjellet gabbro and diorite, which yield U–Pb zircon ages of 446 ± 5 and 444 ± 4 Ma, respectively. Large-scale nappe stacking and folding post-dating the emplacement of the gabbro is related to the collision of Laurentia with Baltica (Scandian orogeny) and was followed by late- to post-orogenic extension. The revised tectonostratigraphy assigns the structurally higher nappes to the Uppermost Allochthon, whereas the lower nappes are correlated with the Middle Köli Nappe Complex (Upper Allochthon). The boundary between these nappes is marked by an imbricate zone. Taconian deformation was probably much more penetrative and widespread than hitherto thought and therefore parts of the nappe stack were probably assembled before Scandian collision. Supplementary material: Electronic Supplement 1: U–Pb zircon data and Electronic Supplement 2: Methods (laser ablation inductively coupled plasma mass spectrometry U–Pb zircon geochronology) are available at https://doi.org/10.6084/m9.figshare.c.5357255

The fast exhumation pattern of a Neoproterozoic nappe system built during West Gondwana amalgamation: Insights from thermochronology

In this contribution we investigate the exhumation T-t path of a nappe system located in the southernmost edge of the Brasília Orogen, southeast Brazil, developed during the West Gondwana assembly. The allochthons represent an inverted metamorphic pile of nappes and were deformed during the collision between the Paranapanema (active margin) and São Francisco (passive margin) paleocontinents. The nappe system comprises UHT rocks of a magmatic arc root (upper Socorro-Guaxupé Nappe), the accretionary wedge – foreland units metamorphosed under high-pressure conditions (intermediate Andrelândia Nappe System) and the lower passive margin metasedimentary sequences (the high-pressure Carrancas Nappes System and the Lima Duarte Nappe). New U-Th-PbT in monazite and 40Ar/39Ar in hornblende, biotite and muscovite ages combined with previously published data indicate different patterns of cooling for each allochthon. The upper nappes register a collision to exhumation/cooling path, from 630–625 Ma to 590–580 Ma, which indicates that the geological process active during the West Gondwana amalgamation involved fast exhumation tectonics. Cross-sections along the main transport direction of the nappes indicate a progressive decrease of metamorphic age peaks (630–625 Ma to 590–570 Ma) and 40Ar/39Ar cooling ages (600 Ma to 540 Ma) from the internal regions (SW) to the front (NE) of the nappe system, which indicate that the propagation of the nappe pile advanced progressively from the upper to the lower nappes.

Provenance of passive-margin and syn-collisional units: Implications for the geodynamic evolution of the Southern Brasília Orogen, West Gondwana

Collisional orogens are one of the most conspicuous features of plate tectonics on Earth. They record the dynamics of continent-continent amalgamation, often confined to linear deformational belts. The Southern Brasília Orogen is one of the most well-studied and complete orogens of West Gondwana. Newly obtained isotopic data, coupled with previously published analyses, expand the occurrence of metamorphosed passive margin sequences (maximum depositional age of ~900 Ma), with a main source in the São Francisco paleocontinent, to large sections of the orogen previously interpreted, by some investigators, as related to younger active margin deposits. This has implications for the location of the suture zone: as most of the upper and lower nappes are dominated by metamorphosed passive margin deposits, the suture is probably located at the base of the uppermost nappe, related to the active margin (Paranapanema paleocontinent). Zircon (U-Pb and Lu-Hf) and whole-rock (Sm-Nd and geochemistry) data obtained for syn-collisional meta-graywackes (~650–630 Ma), that uncomformably overlie passive-margin-related successions, point to a bimodal provenance from both the passive (São Francisco paleocontinent) and active (Paranapanema paleocontinent) margins, depending on the structural level from which the samples were collected. Despite the conspicuous zircon age differences, the maximum depositional age is the same for samples from the upper and the lower nappes. Furthermore, the meta-graywackes from the lower and upper nappes are petrographically and geochemically similar. This suggests concomitant deposition in a common collisional setting. Two metamorphic ages were obtained by U-Pb monazite dating at ~633 and 606 Ma, indicating that protracted medium-grade metamorphic conditions affected the Southern Brasília Orogen.

The Caledonian Structures of Shetland – Funzie and Norwick Geological Conservation Review sites

Locations at Funzie on the island of Fetlar, and at Norwick on the island of Unst contain excellent examples of structures relating to the Caledonian age disruption and emplacement of the Shetland Ophiolite Complex. The Caledonian age Shetland Ophiolite Complex contains exceptionally well developed and exposed sections of the lower parts of the characteristic ophiolite sequence, including layered gabbro, cumulates and mantle rock. Composed of two tectonic units (the Lower Nappe and Upper Nappe) each underlain by an imbricate zone, it is among the finest and most accessible examples of ophiolitic rocks in Europe. The sites at Funzie and Norwick have been selected to be part of the Caledonian Structures of Great Britain Geological Conservation Review (GCR) network, as the best examples in Britain of structures relating to Caledonian ophiolite disruption and emplacement. As the Funzie GCR site and the Norwick GCR site they form the Caledonian Structures of Shetland GCR block.Accounts of all other sites in the Caledonian Structures of Great Britain Geological Conservation Review (GCR) network are already published (Treagus, 1992). Accounts of the Caledonian Structures of Shetland GCR block sites are, therefore, presented in this paper to complete publication of the Caledonian Structures of Great Britain Geological Conservation Review (GCR) network.

Metamorphic Response to Alpine Thrusting of a Crustal-scale Basement Nappe in Southern Calabria (Italy)

Abstract Structural, geophysical and age data indicate that the tilted cross-section of the Variscan continental crust exposed in the Serre of southern Calabria forms the uppermost Alpine nappe (‘Serre nappe’) of three Variscan basement slices derived from the southern European margin. This Alpine nappe stack is a fragment of the western Mediterranean Alps and rests now, after Miocene emplacement, on top of the Apennine carbonate platform. We report for the first time a P–T path for prograde Alpine metamorphism, which is restricted to the two lower nappes (Castagna and Bagni nappes) that are squeezed between cooler tectonic units, the Serre nappe above and the Apennine platform below. Therefore, we attribute their metamorphism to tectonic loading and concomitant shear heating during Eocene south-directed overthrusting of the crustal-scale Serre nappe. In the underlying Castagna nappe, Alpine metamorphism is only locally recorded, mainly by new growth of garnet, forming at the expense of retrogressed Variscan biotite dated at 43 Ma. The local existence of Alpine besides relict Variscan mineral assemblages in the strongly but heterogeneously overprinted rocks allows for characterization of metamorphic evolutions during both the Alpine and Variscan orogeneses in the former intermediate level of the Variscan crust of Calabria. The metamorphic evolutions have been reconstructed through P–T pseudosection modelling for Al-rich metasediments. In the Castagna nappe, rarely preserved Variscan garnet–sillimanite–biotite–ilmenite–plagioclase–quartz (± K-feldspar ± Si-poor white mica) assemblages formed under amphibolite-facies subsolidus conditions (650 ± 60 °C, 4·0 ± 0·5 kbar). During subsequent decompression and cooling to greenschist-facies conditions garnet was replaced by biotite–sillimanite and later by white mica–chlorite intergrowths. Retrogression of Variscan biotite is evidenced by the exsolution of ilmenite along grain boundaries and cleavages, textures that were subsequently overgrown by Alpine garnet coexisting with Si-rich white mica, rare chloritoid (in metapelites), and hornblende (in metagreywackes). Alpine garnet shows prograde zoning and is Ca-rich, and thus is distinct from unzoned and Ca-poor Variscan garnet porphyroblasts. Estimated conditions (520 ± 40 °C, 8·0 ± 1·0 kbar) record elevated pressures during Alpine lower amphibolite-facies metamorphism. In the lowermost Bagni nappe, rare prograde-zoned, Ca-rich garnet in strongly retrogressed mylonitic quartz-phyllites allows isopleth thermobarometry, which indicates lower amphibolite-facies conditions (555 ± 10 °C, 7·4 ± 0·3 kbar) resembling those for Alpine garnet growth in the Castagna nappe. The similar clockwise P–T paths for prograde Alpine metamorphism and the consistent peak pressures of 7–9 kbar in the Castagna and Bagni nappes point to a joint short-lived metamorphism during overthrusting of the crustal-scale Serre nappe within the south European margin during the north-directed subduction of the Alpine Tethys. South-directed overthrusting of the now tilted Variscan crustal section of the Serre along the up to 500 m thick mylonite horizon of the Curinga–Girifalco Line is in agreement with seismic data indicating an extended, few kilometres thick low-velocity zone (Bagni and Castagna nappes and mylonites of the Curinga–Girifalco Line) below the exposed lower crustal section of the Serre nappe. Alpine tectonic transport direction, timing and metamorphic conditions described here are consistent with those reported from the Aspromonte area in southernmost Calabria, suggesting a coeval Alpine history characterized by metamorphism owing to nappe loading and concomitant shear heating. The Alpine subduction–erosion–accretion processes inferred here for the Calabrian basement nappes resemble those proposed for the Dent Blanche nappe system in the Western Alps.

Alanya Birliği Metamorfitlerinin Stratigrafik ve Petrografik Karekteristikleri ve Sugözü Karışığında (Orta Nap) Porfiroblast - Foliasyon İlişkileri, Anamur-Mersin-Türkiye

The study area is located in the southern part of the Central Taurus in Anamur (Mersin) vicinity. The Alanya Unit consisting of completely metamorphites consists of threeimbricate metamorphic nappes. Lower-Middle Cambrian aged the Mahmutlar group (lower nappe) is composed of widespread mica schists containing crystallized limestone, cherty dolomite, quartzite interlevels and greenschist lenses. Precambrian aged Sugözü complex (middle nappe) contains kyanite-sillimanite-staurolite mica schists and widespread garnet-mica schists, which contain greenschist-amphibolite-amphibol schist bands-lenses and metagabro-metadiabase dykes-interlevels. In the middle nappe, Cambrian and Permian blocks belonging to Alanya Unit are observed. The Yumrudağ group (upper nappe) begins with Lower-Middle Cambrian aged mica schists containing cherty dolomite, crystallized limestone, quartzite interlevels and greenschist lenses. Higher up with an unconformity pass into the Upper Permian aged alternation of quartz schist - calc schist - crystallized limestone - phyllite and thick bituminous crystallized limestone containing greenschist lenses. Barite, copper and galenite formations take place in upper nappe lithologies (Ağzıkara formation). Upper Paleocene - Middle Eocene aged Kötekler formation unconformably overlies the Alanya Unit. Alanya Unit has been subjected to multi-stage deformations in connection with intense tectonic movements. Especially, biotite (brown, green) + muscovite + garnet (prop-almandine-grossular) + quartz ± chlorite (ripidolite-picnochlorite, pennin-clinochlore) ± kyanite ± staurolite ± sillimanite + plagioclase (albite-oligoclase-andesine) ± epidote ± zoisite/clinozoisite ± orthoclase + tourmaline (green, brown) ± graphite ± sphene ± rutile ± apatite mineral paragenesis are observed in metapelitic and metasemipelitic rocks belonging to Sugözü complex (middle nappe). Amphibolites and amphibole schists of this complex contain hornblende (tchermacite, ferro-tchermacite, magnesio-hornblende and edenite) + garnet + plagioclase (albite-oligoclase-andezine) ± relic clinopyroxene (augite-pigeonite) + chlorite (ripidolite, clinochlore) ± orthoclase ± epidote ± zoisite/clinozoisite ± biotite (green, brown) ± actinolite ± muscovite ± quartz ± calcite ± sphene ± apatite ± rutile mineral assemblage. Hornblende (tchermacite, ferro-tchermacite) + plagioclase (albite-oligoclase-andesine) + relic clinopyroxene (augite) ± garnet + quartz + chlorite (ripidolite, pennin-clinochlore) ± actinolite ± zoisite/clinozoisite ± epidote ± calcite ± sphene ± apatite is observed in metagabbros and metadiabases. The greenschists are characterized by tremolite + actinolite + chlorite (ripidolite-picnochlorite) + plagioclase (albite) ± garnet + zoisite / clinozoisite + epidote ± quartz ± microcline ± relic clinopyroxene (augite) ± calcite + sphene ± apatite mineral assemblage. Especially when the index minerals, different mineral paragenesis and deformation-metamorphism relations within the Sugözü complex take into account, it has been seen that Alanya unity underwent three phases of metamorphism which follow one another.

Back to Normal: Direct Evidence of the Cretan Detachment as a North‐Directed Normal Fault During the Miocene

AbstractThe Cretan Detachment represents a major tectonic contact between an Alpine high‐pressure Lower Nappe System in the footwall and an Alpine unmetamorphosed Upper Nappe System in the hanging wall. Interestingly, the kinematics and the tectonic nature of this contact is still highly controversial and has been interpreted either as a top‐to‐the S thrust or as a normal fault with top‐to‐the N and/or top‐to‐the S displacement sense. Whereas some models suggest exhumation of the high‐pressure rocks synchronous with out‐of‐sequence thrusting or Himalayan‐type extrusion tectonics, other models propose that Crete represents a metamorphic core complex that exhumed below the extensional Cretan Detachment. In this work, we show that the Cretan Detachment in eastern Crete is an up to 100‐m‐thick cataclastic low‐angle fault zone, which localized below the Tripolitza Unit (Upper Nappe System) within the upper part of the Upper Violet Slates (top of Lower Nappe System) cutting downsection to lower structural levels. The unequivocal kinematic indicators in this fault zone clearly suggest a top‐to‐the N displacement of the hanging wall. New low‐temperature geochronological data and numerical modeling suggest rapid cooling of the rocks in the footwall of the Cretan Detachment at circa 15 Ma. Top‐to‐the S ductile shear sense toward lower structural level in the Lower Nappe System support the idea that the Cretan Detachment facilitated the exhumation of the high‐pressure rocks in an extrusion tectonic setting. Therefore, in the early to middle Miocene, subduction‐related extrusion in Crete was simultaneously active with back‐arc extension‐related metamorphic core formation in the Cyclades.

A Late Carboniferous olistostrome at the front of the Southern Tian Shan nappes (Kadamzhai and Khaidarkan deposits, Kyrgyzstan)

A Late Carboniferous (Early Moscovian) olistostrome developed in the Kadamzhai and Khaidarkan gold-antimony-mercury deposits on the Alay Ridge northern slope (Kyrgyzstan), at the front of the Late Paleozoic Southern Tian Shan nappes, is characterized. It comprises a sub-nappe olistostrome in a collisional tectonic setting. The olistostrome contains olistoliths and olistoplaques containing parts of the mid-Paleozoic sedimentary successions belonging to the parautochthon and lower nappes of the northern Bukantau–Kokshaal branch of the Southern Tian Shan nappe belt. The olistostrome accumulated ahead the advancing nappes in the foredeep basin that was filled with turbidities and debris-flow deposits (Tolubai Formathion). The parautochthon was partly dismembered into thrust limestone sheets which disintegrated and slid into unconsolidated sediments of the foredeep basin, forming large limestone olistoliths and olistoplaques. Olistoliths containing shales and bedded cherts were slid from the lower nappes. Tectonic breccias up to melange scale are present in some olistoliths, suggesting tectonic disintegration within the nappe pile and the subsequent sliding of the tectonized blocks into the olistostrome basin. Ore-bearing silicified rocks (so-called “jasperoids”) with antimony-mercury and gold mineralization are located predominantly along the contacts of the limestone olistoliths/olistoplaques with a terrigenous matrix.

Petrology of ultramafic to mafic cumulate rocks from the Göksun (Kahramanmaraş) ophiolite, southeast Turkey

The Göksun (Kahramanmaraş) ophiolite (GKO), cropping out in a tectonic window bounded by the Malatya metamorphic unit on both the north and south, is located in the EW-trending lower nappe zone of the southeast Anatolian orogenic belt (Turkey). It exhibits a complete oceanic lithospheric section and overlies the Middle Eocene Maden Group/Complex with a tectonic contact at its base. The ophiolitic rocks and the tectonically overlying Malatya metamorphic (continental) unit were intruded by I-type calc-alkaline Late Cretaceous granitoid (∼81–84 Ma). The ultramafic to cumulates in the GKO are represented by wehrlite, plagioclase wehrlite, olivine gabbro and gabbro. The crystallization order for the cumulate rocks is as follows: olivine ± chromian spinel→clinopyroxene→plagioclase. The major and trace element geochemistry as well as the mineral chemistry of the ultramafic to mafic cumulate rocks suggest that the primary magma generating the GKO is compositionally similar to that observed in the modern island-arc tholeiitic sequences. The mineral chemistry of the ultramafic to mafic cumulates indicates that they were derived from a mantle source that was previously depleted by earlier partial melting events. The highly magnesian olivine (Fo77–83), clinopyroxene (Mg# of 82–90) and the highly Ca-plagioclase (An81–89) exhibit a close similarity to those, which formed in a supra-subduction zone (SSZ) setting. The field and the geochemical evidence suggest that the GKO formed as part of a much larger sheet of oceanic lithosphere, which accreted to the base of the Tauride active continental margin, including the İspendere, Kömürhan and the Guleman ophiolites. The latter were contemporaneous and genetically/tectonically related within the same SSZ setting during the closure of the Neotethyan oceanic basin (Berit Ocean) between the Taurides to the north and the Bitlis-Pütürge massif to the south during the Late Cretaceous.

Nappe units along the Caledonian margin in central Scandinavia (Grong–Olden to Nasafjället): definition, distinction criteria and tectonic evolution

This study reviews lithological and stratigraphic data, new geological mapping results and microscopic work from the lower and middle Caledonian nappes in central Scandinavia, and suggests a model of their tectonic evolution. The Jämtlandian and Blaik Nappes represent the lower nappes, and are overlain, successively, by the Stalon, Danabergen and Slussfors Nappes. The latter occupies the level immediately beneath the Seve Nappe Complex. Farther north, around Lake Gautojaure, the Åkerberget Nappe represents the level between the Stalon Nappe and the Ammarnäs Complex. Their hangingwall is the Sadenåive Nappe and the overlying Seve Nappe Complex. The Danabergen, Åkerberget, Sadenåive and Slussfors Nappes are formally defined here. In the Blaik Nappe, brittle deformation gave rise to composite thrust systems. The overlying nappes show a more ductile deformation. The arkose sequence of the Stalon Nappe shows fine-grained recrystallization of muscovite. In contrast, finely recrystallized mica minerals form a phyllitic foliation in the Åkerberget Nappe. The Danabergen Nappe contains small garnet porphyroblasts. The Ammarnäs Complex and the Sadenåive Nappes show garnet with a straight internal foliation (Si), while the external foliation is refolded. This fabric suggests a time interval after early foliation development, in which garnet grew but no deformation occurred, and subsequent deformation. This may represent an episode of arrest of deformational activity and thrust movement. Spiral-shaped Si in garnets of the Slussfors Nappe indicate a contrasting and more complex metamorphic and structural evolution, perhaps similar to that of the Seve Nappe Complex, with a well-known, polyphase tectonic evolution.

Air concentration and velocity downstream of an expanding chute aerator

ABSTRACTThe mechanisms of air entrainment and velocity distribution at the upper, lower and side air–water interfaces downstream of chute aerators remain unclear. This study reports a series of experiments involving various approach-flow features and geometric parameters of aerators. Analytical solutions are developed for the air concentration profiles in the air–water layers, and it is shown that the solutions agree well with the experiments. Empirical equations are established for the turbulent diffusion of bubbles and the diffusion rate of the aeration layer. The turbulent diffusion coefficient of the lower nappe is found to be approximately 10 times and 1.2–1.8 times larger than those of the upper and side nappes, respectively. The diffusion rate of the aeration layer is the highest at the lower nappe and lowest at the upper nappe, gradually decreasing along the cavity. The velocity distribution indices are found to be different for the upper, lower and side nappes.

The tectonometamorphic and magmatic evolution of the Uppermost Unit in central Crete (Melambes area): constraints on a Late Cretaceous magmatic arc in the Internal Hellenides (Greece)

Crete consists of a nappe pile that formed during Alpine subduction and collision. The lower nappes belong to the External Hellenides, whereas the uppermost nappe is ascribed to the Pelagonian Zone of the Internal Hellenides. The Uppermost Unit consists of several subunits including the Asterousia Crystalline Complex (ACC), which comprises metasedimentary rocks, (meta)granitoids and serpentinite, the protolith age and the tectonometamorphic evolution of which are largely unknown. In the present study, we present new structural, microfabric and geochronological data from the Uppermost Unit in the Melambes area (central Crete). 206Pb/238U zircon ages (LA-ICP-MS and ID-TIMS) indicate granitic and dioritic intrusions between 71.9±0.6 and 76.9±0.3Ma. Identical ages have previously been obtained from comparable intrusions in eastern Crete and on Anafi. The composition and geochemical signature suggest an extended magmatic arc along the southern active margin of the Pelagonian-Lycian Block. Post-intrusive shearing transformed granite into orthogneiss, whereas diorite remained free from foliation, because of the lower amount of mechanically weak phases. Deformation microfabrics suggest top-to-the SE shearing under amphibolite facies conditions of the ACC and at greenschist facies conditions of rocks at the base of the ACC referred to as Akoumianos Greenschist. The Akoumianos Greenschist is considered as the northern part of the Pindos realm that was subducted underneath the Pelagonian-Lycian active margin. Based on our new and on published data, the following orogenic stages are suggested to have contributed to the evolution of the Hellenides during the Late Cretaceous to Eocene: (1) pre-middle Campanian collision and subduction of the Pindos lithosphere underneath the southern margin of the Pelagonian-Lycian terrane led to obduction and offscraping of serpentinized ocean floor and stacking of the ACC during amphibolite facies top-to-the SE thrusting, (2) formation of a Campanian magmatic arc along the Pelagonian-Lycian active margin; (3) Maastrichtian collision and stacking of the magmatic arc during top-to-the SE mylonitic shearing; (4) Palaeocene top-to-the SE greenschist-facies shearing of the ACC on top of the Akoumianos Greenschist; (5) Late Eocene thrusting of the Uppermost Unit on top of the Arvi and Pindos units. Thus, top-to the SE was the dominant shear sense in the southern Aegean from at least the mid-Late Cretaceous until the Eocene.

Comparison the hydraulic characteristics of finite crest length weir with quarter-circular crested weir

Weirs are used for flow measurement, flow diversion and water level control in open channels. In this study, experiments were conducted on finite crest length weir and quarter-circular crested weir and hydraulic characteristics of two weirs were compared together. Depending on the ratio of total upstream head H to weir crest length L, flow over the finite crest length weir was undular, parallel to the crest surface or curvilinear. Also, for H/L>1.82, lower nappe separated from the upstream corner of crest and did not reattach to the crest surface. In the range of conducted tests (H/L<2.20), flow over the quarter-circular crested weir was curvilinear and lower nappe did not separate from the crest surface. Due to the elimination of separation zone, which is the primary source of energy loss, discharge coefficient for quarter-circular crested weir is significantly larger than for finite crest length weir. Furthermore, negative (sub-atmospheric) pressure on the crest surface of finite crest length weir and quarter-circular crested weir occurred for H/L>1.52 and H/L>2.2, respectively. This result indicates that quarter-circular crested weir is less sensitive to cavitation danger.