Western Extension Research Articles

Suprasubduction zone ophiolite from Agali hill: Petrology, zircon SHRIMP U–Pb geochronology, geochemistry and implications for Neoarchean plate tectonics in southern India

The southern margin of the Dharwar Craton in Peninsular India preserves the records of an active convergent margin during the Neoarchean. Here we report the discovery of a relatively well-preserved suprasubduction zone ophiolite suite from the Agali hill in Attappadi, along the western extension of the Bhavani Shear Zone. In the Agali hill, from base to top, the rock sequence includes altered ultramafics with vestiges of dunite, thin layer of cumulate pyroxenite, a thick unit of metagabbro with the upper part grading into anorthositic gabbro and carrying thin layers of hornblendite, capped by metavolcanics (amphibolites) carrying veins and pools of trondhjemite. Fragments of metabasite (dolerite) dykes occur within the gabbroic horizon. Elongate bands of metamorphosed banded iron formation in association with amphibolite occur proximally. The lithological distribution in the area represents a typical ‘Ocean Plate Stratigraphy’ sequence with arc and exhumed sub-arc mantle material toward the north-west, followed by accreted remnants of suprasubduction zone ophiolites, large tracts of TTG gneisses, and amphibolites in association with BIF bands. The central domain is occupied by a granite batholith. Toward the south-east, the dominant lithology grades to a continental shelf sequence represented by metamorphosed psammitic and pelitic rocks (trench) and metacarbonates. The common occurrence of magnesite in association with ultramafic units in the area suggests CO2-induced metasomatism of peridotites in the mantle wedge through fluids released within the subduction zone.We present major, trace and REE data on the Agali Ophiolite Complex which clearly suggest magma derivation in a suprasubduction setting in the absence of any significant crustal contamination. The internal structure as imaged from CL images of zircons separated from metagabbro, trondhjemite and metagranite show features typical of magmatic crystallization, with the grains mantled by bright structureless thin rims developed during a dominantly dry metamorphic event. The U–Pb concordia ages of 2547±17Ma (MSWD 0.84) and 2547±7.4Ma (MSWD 2.4) obtained from zircons in the metagabbro and trondhjemite are indistinguishable; the zircons in the metagranite also show comparable magmatic age of 2532±8.6Ma (MSWD 2.5) with metamorphic overgrowth at ca. 2470Ma. These ages correlate well with similar age data reported recently from suprasubduction zone and arc-related rocks elsewhere along the southern margin of the Dharwar Craton. We propose a tectonic model that envisages accretion of oceanic arcs and micro-continents onto the margin of the Dharwar Craton during Neoarchean, marking an important event of continental growth, and broadly coinciding with the global crustal growth event at this time.

Deep-seated gravitational slope deformations as possible suitable locations for prehistoric human settlements: An example from the Italian Western Alps

The Plan di Modzon is a mountain area (2300 m) located in the Verrogne Valley, NW of Aosta (Western Alps). It occurs along the contact between the Middle Penninic (micaschist and gneiss from the Gran San Bernardo Nappe) and the overlying upper units of the Piedmont Zone (carbonate calcschist alternating with marble).This area, largely shaped by Pleistocene glaciers, was involved in a wide deep-seated gravitational slope deformation (Pointe Leysser DSGSD) on the western extension of the Becca France doubled ridges. Several ridges that were affected by glacial erosion, discontinuously covered by glacial sediments, are present throughout the area. Extremely fractured rocks and various gravitational forms (minor scarps and trenches) mark the DSGSD.Several archaeological sites (MF1–MF9) have recently been found between 2242 and 2292 m asl. They have revealed artifacts of rock crystal (hyaline quartz) referred to the Sauveterrian stage of the Mesolithic. An ensemble of other archaeological evidence is referred to the Copper Age. The investigation in progress specifically concerns the systematic excavation of sites MF1 and MF3.Some concomitant morphological factors have created very favorable conditions for prehistoric settlements in the Plan di Modzon area, including the exceptionally wide valley floor directly perched on the main Dora Baltea Valley, in consequence of the Verrogne Glacier diversion promoted by the P. Leysser DSGSD. Easy and direct accessibility to this area is provided by the gently-dipping slope of the Dora Baltea Valley affected by the DSGSD. The ridged and grooved morphology as the result of the glacial and gravitational interaction, offered wide surfaces free from geological hazards (debris flow and avalanche processes). The DSGSD is, therefore, one of the primary causes of the archaeological settlement of this area, contributing to create a morphology adapted to prehistoric settlements.

Zircon U–Pb ages and Lu–Hf isotopes of Paleoproterozoic metasedimentary rocks in the Korla Complex, NW China: Implications for metamorphic zircon formation and geological evolution of the Tarim Craton

Widespread Paleoproterozoic supracrustal rocks in the northern Tarim Craton contain important information about its geological evolution and correlation with adjacent blocks. We present new in situ LA-(MC-)ICP-MS zircon U–Pb and Lu–Hf isotopic data for six mica schist samples from the Korla Complex. Field and petrological studies indicate a pelitic to semi-pelitic protolith and a high pressure upper amphibolite-facies peak metamorphic condition (T=690±50̊C and P=11±2kbar) for these samples. CL-images reveal that zircons in these samples are dominantly metamorphic origin and only a few detrital zircons occur as relics in sample T1, the ages of which suggest a maximum deposition age of ca. 2.0Ga and a sedimentary provenance from the Tarim Craton itself. All metamorphic zircons consistently record a metamorphic age of ca. 1.85Ga, despite of various degrees of discordance probably due to later Pb-loss. Both recrystallization and new zircon growth are recognized for the genesis of these metamorphic zircons. The metamorphic zircon domains in sample T1 show a relatively large range of initial 176Hf/177Hf ratios similar to the detrital cores, whereas those in the other samples show similar initial 176Hf/177Hf ratios (ca. 0.28140±0.00010, 2σ) regardless of their internal structures and degrees of discordance. The former is interpreted as a result of complete U–Pb resetting through fluid-mediated recrystallization, whereas the later probably implies a large-scale Hf isotopic homogenization during new zircon growth. Petrological and zircon isotopic evidence supports that the new zircon growth and Hf isotopic homogenization probably resulted from the mixing of Hf–Zr derived from dissolution of tiny detrital zircons and decomposition of garnet to chlorite in hydrothermal fluids during retrograde metamorphism. Accordingly, the ages of these new zircon growths may postdate the peak metamorphism, which was probably related to a late Paleoproterozoic collisional orogenic event in the northern Tarim Craton. A compilation of available geological and geochronological data enables us to identify two Late Paleoproterozoic orogenic belts: the ca. 1.9–1.8Ga North Tarim Orogen and the ca. 2.0–1.9Ga South Tarim Orogen. It is suggested that the Tarim Craton, including the Dunhuang and Quanji Blocks, was correlative with the Alxa–Yinshan Block of the North China Craton, and they probably formed a coherent massif during the Neoarchean–early Paleoproterozoic, which collided with the Ordos Block and its western extension along the ca. 1.95Ga Khondalite Belt–South Tarim Orogen to form a larger landmass in the Columbia Supercontinent.

The Carboniferous ophiolite in the middle of the Qiangtang terrane, Northern Tibet: SHRIMP U–Pb dating, geochemical and Sr–Nd–Hf isotopic characteristics

Ophiolite plays a key role in identifying paleo-ocean and paleo-plate and rebuilding the evolutionary history of ancient orogen. Mafic–ultramafic rocks are distributed in a broadly E–W direction in the middle of the Qiangtang terrane, northern Tibetan plateau. However, interpretation of these rocks as ophiolite and subsequent tectonic implications have been much disputed, and they were regarded by some to have an origin in a continental rift setting. A detailed zircon dating and geochemical and Sr–Nd–Hf isotopic study of this suite of rocks has been undertaken in order to clarify this important issue. The mafic–ultramafic suite was collected from the Gangma Co and Guoganjianian localities and is composed of cumulate and isotropic gabbro, basalt, actinolitite and plagiogranite. All basaltic rocks are tholeiitic and have low rare earth element (REE) abundances with variable REE patterns and slight negative Nb and Ti anomalies. These features are comparable with those of normal mid-ocean ridge basalts (N-MORB) and/or enriched mid-ocean ridge basalts (E-MORB). The positive whole-rock εNd(t) and zircon εHf(t) values indicate that these rocks were derived from a long-term depleted mantle source. Zircon U–Pb dating using a sensitive high-resolution ion microprobe (SHRIMP) on two cumulate gabbros and two plagiogranites samples yielded Carboniferous ages of 357±2.5Ma, 356.1±3.0Ma, 354.7±4.7Ma and 345.4±4.6Ma. The ophiolite marks a Paleo-Tethys Ocean basin in middle of the Qiangtang terrane, and it is interpreted as the western extension of the Changning–Menglian Paleo-Tethys ophiolite in the eastern margin of the Tibetan plateau.

Significant range extensions forDelma labialisand a review of its biology

We report southern, western and northern extensions of known range for the vulnerable legless lizard, Delma labialis: respectively, north of Moranbah from a sandstone ridge near a river; east of Charters Towers from grassy open woodland; and Cape York Peninsula from sandy heath. These extensions increase the species' extent of occurrence by more than 66,000 km2, a nearly nine-fold increase in area. We suggest that D. labialis is widespread within its extent, but that it is patchily distributed and at low population densities. We review current knowledge of the species, with particular reference to known habitats, and make comments on its conservation status and potential threats.

Repeating deep tremors on the plate interface beneath Kyushu, southwest Japan

In the subduction zone south of Kyushu Island, at the western extension of the Nankai subduction zone, southwest Japan, the age of the oceanic crust increases toward the south across the subducting Kyushu-Palau ridge. While tremor activity is very high in Nankai, tectonic tremors have only recently been discovered in Kyushu. In this study, we examined tremors beneath Kyushu using an improved version of the envelope correlation method. In doing so, we distinguished tremors from normal earthquakes and background noise using the criteria of source duration and the spectrum ratio between low and high frequencies. Accurate measurement of S-P times, using cross-correlation between vertical and horizontal seismograms, constrains the tremor depth precisely. Tremor activity is low and within a small region in southern Kyushu, where thick crust of the Kyushu-Palau ridge is being subducted, at depths between 35 and 45 km (i.e., shallower than intra-slab earthquakes by about 20 km), which is consistent with the location of the plate interface within uncertainties proposed in previous studies. Establishing precise depth estimates for tectonic tremors beneath Kyushu, which results from shear slip along the plate interface, is useful in defining the plate interface within the Nankai subduction zone.

Western U.S.: Deep‐band placement of fertilizer increases corn grain yield in eroded soils

Crops & SoilsVolume 46, Issue 1 p. 18-19 Regional Roundup Western U.S.: Deep-band placement of fertilizer increases corn grain yield in eroded soils D.D. Tarkalson, D.D. Tarkalson USDA-ARS, Northwest Irrigation and Soils Research Lab, Kimberly, IDSearch for more papers by this authorD.D. Bjorneberg, D.D. Bjorneberg USDA-ARS, Northwest Irrigation and Soils Research Lab, Kimberly, IDSearch for more papers by this author D.D. Tarkalson, D.D. Tarkalson USDA-ARS, Northwest Irrigation and Soils Research Lab, Kimberly, IDSearch for more papers by this authorD.D. Bjorneberg, D.D. Bjorneberg USDA-ARS, Northwest Irrigation and Soils Research Lab, Kimberly, IDSearch for more papers by this author First published: 18 February 2013 https://doi.org/10.2134/cs2013-46-1-6 This article was prepared by members of the Western Extension and Research Activities (WERA 103) Committee on Nutrient Management and Water Quality. For more information about WERA 103, see: https://community.ipni.net/site/wera.nsf/home.xsp. Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume46, Issue1January‐February 2013Pages 18-19 RelatedInformation

EXTENSION AND OBFUSCATION: TWO CONTRASTING ATTITUDES TO THE MORAL BOUNDARY

EXTENSION AND OBFUSCATION: TWO CONTRASTING ATTITUDES TO THE MORAL BOUNDARY

Irrigation Outreach in Afghanistan: Exposure to Afghan Water Security Challenges

Abstract:The authors, from Colorado State University were invited by the United Sates Department of Agriculture's Foreign Agriculture Service and Afghanistan's Ministry for Agriculture Irrigation and Livestock to lead a “train‐the‐trainer” workshop with Afghanistan's best and brightest irrigation outreach professionals. The six day workshop on the outskirts of Kabul helped clarify that prolonged conflict has damaged agriculture's access to what should be a plentiful supply of irrigation water. The violence of two wars still lingers today continuing to inhibit foreign aid's ability to rebuild Afghanistan's water resources infrastructure. In spite of these challenges participants in the workshop demonstrated remarkable resourcefulness and courage helping producers throughout Afghanistan take advantage of improvements as they came online. With continued assistance from Western researchers and extension professionals that is sensitive to the traditional methods of water administration, there is reason to be hopeful for the future success of Afghanistan's agricultural sector.

Precambrian geology of China

Precambrian geology of China

Evolution of the Bababurnu Basin and shelf of the Biga Peninsula: Western extension of the middle strand of the North Anatolian Fault Zone, Northeast Aegean Sea, Turkey

The generally transpressional character of the North Anatolian Fault Zone in western Turkey implies a transtensional character in the Northern Aegean Sea. The geological evolution of the basins lying above and immediately adjacent to this fault zone is still poorly understood. Similarly little information exists regarding the tectonic configurations created by the westerly and southerly extension of the middle and southern strands of the North Anatolian Fault Zone into the Northern Aegean Sea, other than what has been inferred using somewhat speculative maps onland. This study combines ∼1600km of single-channel seismic reflection profiles and piston-core data with detailed land mapping to develop a comprehensive evolutionary model. We suggest that the tectonic system of the Biga Peninsula and the northeastern Aegean Sea are characterized by transpressional broken-slat geometry onland and transtensional broken-slat geometry at sea. The system includes a set of prominent NE–SW-striking faults with considerable strike-slip component in the east (northwestern Turkey and eastern Aegean Sea) and a second set of major NW–SE-striking extensional faults in the west (eastern Greece). We further suggest that rotational wedges form the link between the NE–SW- and NW–SE-striking faults. Detailed mapping of the seismic data and fault linkages with the onland geology demonstrate that the middle strand of the North Anatolian Fault Zone exits into the Aegean Sea near the town of Behramkale and that it extends into the Bababurnu pull-apart basin in the northeastern Aegean Sea as a right-releasing step-over active during the Quaternary.

Mid-twenty-first century climate change in the Central United States. Part II: Climate change processes

Ensemble regional model simulations over the central US with 30-km resolution are analyzed to investigate the physical processes of projected precipitation changes in the mid-twenty-first century under greenhouse gas forcing. An atmospheric moisture balance is constructed, and changes in the diurnal cycle are evaluated. Wetter conditions over the central US in April and May occur most strongly in the afternoon and evening, supported primarily by moisture convergence by transient eddy activity, indicating enhanced daytime convection. In June, increased rainfall over the northern Great Plains is strongest from 0000 to 0600 LT. It is supported by positive changes in stationary meridional moisture convergence related to a strengthening of the GPLLJ accompanied by an intensification of the western extension of the North Atlantic subtropical high. In the Midwest, decreased rainfall is strongest at 1500 LT and 0000 LT. Both a suppression of daytime convection as well as changes in the zonal flow in the GPLLJ exit region are important. Future drying over the northern Great Plains in summer is triggered by weakened daytime convection, and persists throughout August and September when a deficit in soil moisture develops and land–atmosphere feedbacks become increasingly important.

Constraining the Jurassic extent of Greater India: Tectonic evolution of the West Australian margin

Alternative reconstructions of the Jurassic northern extent of Greater India differ by up to several thousand kilometers. We present a new model that is constrained by revised seafloor spreading anomalies, fracture zones and crustal ages based on drillsites/dredges from all the abyssal plains along the West Australian margin and the Wharton Basin, where an unexpected sliver of Jurassic seafloor (153 Ma) has been found embedded in Cretaceous (95 My old) seafloor. Based on fracture zone trajectories, this NeoTethyan sliver must have originally formed along a western extension of the spreading center that formed the Argo Abyssal Plain, separating a western extension of West Argoland/West Burma from Greater India as a ribbon terrane. The NeoTethyan sliver, Zenith and Wallaby plateaus moved as part of Greater India until westward ridge jumps isolated them. Following another spreading reorganization, the Jurassic crust resumed migrating with Greater India until it was re‐attached to the Australian plate ∼95 Ma. The new Wharton Basin data and kinematic model place strong constraints on the disputed northern Jurassic extent of Greater India. Late Jurassic seafloor spreading must have reached south to the Cuvier Abyssal Plain on the West Australian margin, connected to a spreading ridge wrapping around northern Greater India, but this Jurassic crust is no longer preserved there, having been entirely transferred to the conjugate plate by ridge propagations. This discovery constrains the major portion of Greater India to have been located south of the large‐offset Wallaby‐Zenith Fracture Zone, excluding much larger previously proposed shapes of Greater India.

First Record of the CicadaTibicen pruinosusin Colorado, with a Key to Colorado Species ofTibicen(Hemiptera: Cicadidae)

Tibicen pruinosus (Say)—a large, arboreal cicada typically found in the central United States—is reported for the first time from Colorado. This new state record represents a significant western extension of this cicada's known range. Also reported are new records from Kansas that link the Colorado population to the previously known distribution of T. pruinosus. These results suggest that T. pruinosus has expanded its range westward, likely facilitated by human-induced changes in vegetation on the Great Plains. They also indicate the effectiveness of acoustic surveys as a tool for studying cicada distributions. Finally, an updated key including all 7 species of Tibicen now known from Colorado is provided.

Triassic Latemar cycle tops — Subaerial exposure of platform carbonates under tropical arid climate

The Triassic Latemar platform in the Dolomites, Italy, is the site of several ongoing controversies. Perhaps the most interesting debate focuses on apparent cyclic deposition within the Latemar platform, whose nature and duration are still open to debate. Further disagreement concerns the lack of meteoric diagenesis-related isotope shifts at cycle tops that bear circumstantial petrographic evidence for subaerial emergence. Here, an evaluation of the nature of Latemar cycle tops is presented combining evidence from previous work and new field, petrographic and geochemical data. Cycle tops are ranked according to increasing exposure duration and spatial extent: type I surfaces lacking unequivocal evidence of prolonged supratidal conditions; type II dolomite caps formed in warm, evaporitic, intertidal lagoonal waters followed by exposure of perhaps intermediate duration; type III clastic-rich, red calcareous horizons with some showing platform-wide extent, representing prolonged supratidal conditions, and type IV discontinuities in tepee belts, genetically related to type II and III surfaces, but likely representing shorter-lived exposure stages. Petrographic and geochemical criteria indicate that most diagenesis occurred in the shallow marine and burial domain whilst an extensive meteoric overprint of cycle tops is lacking. This is underlined by the scarcity of meteoric diagenetic fabrics such as gravitational cements that, where present, are here interpreted as marine-vadose in origin. The scarcity of carbon and oxygen isotope signatures commonly assigned to subaerial exposure stages is best explained in the context of mid-Triassic climate. The low latitude, tropical but arid setting of the Latemar, situated in the western extension of the Tethys ocean, its isolation from nearby continental areas and overall short-term emergence episodes are in agreement with a limited degree of meteoric alteration of most cycle tops. High amounts of aeolian clastic material beneath some cycle tops, along with high Fe and Mn elemental abundances argue for intermittent subaerial conditions. This study proposes an enhancement of the classical Allan and Matthews (1982) isotope model for subaerial exposure under strongly arid climates. As the subaerial exposure nature of Latemar cycle tops, and therefore eustasy as the cause for cyclicity, have been previously challenged due to the lack of meteoric-induced isotopic signatures, the outcome of this study is of significance for the ongoing Latemar stratigraphic controversy.

Trace elements, U–Pb ages and Hf isotopes of zircons from Mesozoic granites in the western Nanling Range, South China: Implications for petrogenesis and W–Sn mineralization

The E-W trending Nanling Range, located in the southern part of the South China Block (SCB), is characterized by widespread intra-continental Mesozoic granitoids and accompanying W–Sn polymetallic ore deposits. Detailed studies are presented on trace elements, U–Pb ages and Lu–Hf isotopes in zircons from the Mesozoic A2-type like granitoids (including the Qianlishan, Qitianling, Xianghualing, Huashan, Guposhan, Niumiao, Tong'an, Jinziling and Jinjiling Plutons) from the western Nanling Range. LA–ICP–MS U–Pb analyses of zircon indicate that these granites intruded mainly from ca 160–150Ma. Two types of zircon (magmatic and late-magmatic hydrothermal) with similar ages can be distinguished based on their zircon trace element compositions. The late-magmatic hydrothermal zircons have high contents of metallogenic elements (e.g., W and Sn) and may be closely related with the ore deposits. The εHf(t) values of zircons from the Mesozoic granitoids are mildly negative to positive, indicating the reworking of both old and young crustal components. The significant involvement of Mesozoic juvenile mantle-derived materials in the source can be excluded because of the rare occurrences of mafic rocks with radiogenic isotopic signatures in the area. The average crustal residence age patterns of the Mesozoic granites in the western Nanling Range are similar to those of the Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen, which may represent the Neoproterozoic collisional belt between the Yangtze and Cathaysia blocks. Considering the specific isotopic and metallogenic features of the Mesozoic granitoids in the study area, we propose that the western Nanling Range may represent a western extension of Neoproterozoic arc–continent collisional belt in South China. The Mesozoic granitoids of this area may have been generated mainly by reworking of deeply-buried Neoproterozoic arc-related volcanic-sedimentary materials due to thermal perturbations triggered by the subduction of the Paleo-Pacific Plate in Mesozoic time.

Paleoproterozoic evolution of the eastern Alxa Block, westernmost North China: Evidence from in situ zircon U–Pb dating and Hf–O isotopes

The Alxa Block is the westernmost part of the North China Craton (NCC). In the past, it has been considered to be part of the Archean NCC. However, formation and evolution of this block remain poorly understood, and this has hampered a broader understanding of the NCC. In this study we analyzed the in situ zircon U–Pb age and Hf–O isotopes for the two oldest rock units in the eastern Alxa Block, namely the Bayanwulashan and Diebusige Complexes. The Bayanwulashan Complex consists mainly of metamorphic rocks with mafic and felsic protoliths. SIMS U–Pb zircon dating results indicate that the primary magmatic ages of the mafic and felsic igneous rocks are ca. 2.34Ga and ca. 2.32–2.30Ga, respectively, and both sets of rocks were overprinted by two metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Geochemical and zircon Hf–O isotopic data suggest that the ca. 2.34Ga amphibolites within the Bayanwulashan Complex are characteristically high in TiO2 (up to 3.2%), Zr (up to 394ppm) and Ti/V (>30), resembling the basaltic rocks formed within continental rifts. The ca. 2.32–2.30Ga felsic rocks were generated by remelting of dominant meta-igneous rocks that have zircon Hf model ages of 2.92–2.81Ga. The Diebusige Complex is composed of amphibolites, mafic gneisses and paragneisses. It was intruded by granites dated at ca. 1.97–1.98Ga, and subjected to high grade metamorphic events at ca. 1.89Ga and ca. 1.79Ga. Re-examination of U–Pb ages for detrital and metamorphic zircons indicate that the depositional ages of protoliths of the Diebusige paragneisses may be considered to be between ca. 2.45 and 2.0Ga. These U–Pb zircon age data do not support the existence of exposed Archean rocks in the eastern Alxa Block, although the possibility of Archean rocks at deeper crustal levels cannot be ruled out entirely.The eastern Alxa Block contrasts to the neighboring Yinshan Block and Khondlite Belt, as well as the Trans-North China Orogen, in terms of the timing and evolution of magmatism and metamorphism. Thus, we conclude that the Alxa Block is a separated Paleoproterozoic terrane from the Western Block of the NCC, rather than the western extension of the Yinshan Block or part of the Khondalite Belt, as previously proposed.

Triassic Subduction of the Paleo-Tethys in northern Tibet, China: Evidence from the geochemical and isotopic characteristics of eclogites and blueschists of the Qiangtang Block

A ∼500 km-long high-pressure metamorphic belt occurs in the Qiangtang Block of northern Tibet. The occurrence of high-pressure metamorphic rocks (eclogite and blueschist) has a significant bearing on the Paleo-Tethys subduction and plate suturing processes. A few studies have been conducted on petrology, mineralogy and geochronology of high-pressure rocks. In this paper we present the results of a geochemical and isotopic investigation on eclogite and blueschist from this belt, and discuss the probable geodynamic implications. Geochemical analyses show that the two rock types, eclogite and blueschist, have grossly similar bulk compositions, but they can be subdivided into two groups based on their TiO 2 contents. The low TiO 2 (<2.75 wt.%) rocks exhibit REE distribution patterns and trace element abundances similar to enriched mid-ocean ridge basalts (E-MORB), whereas the high TiO 2 (>2.75 wt.%) rocks are more akin to ocean island basalts (OIB). The positive ε Nd( T) values (ranging from 0.4 to 6.3) for all samples suggest that the basaltic protoliths of the eclogites and blueschists were derived from a long-term depleted mantle source. However, their geochemical characteristics require that the source be metasomatized and enriched in large-ion-lithophile and light rare-earth elements shortly before the formation of these basaltic protoliths, like for most OIB and E-MORB of the world. The recognition of oceanic basalts suggests that a Paleozoic ocean should be present in the central Qiangtang Block. This ocean was hypothesized to be the western extension of the Changning–Menglian Paleo-Tethys Ocean in the eastern margin of the Tibetan plateau. The eclogite and blueschist were formed by northward subduction of this Paleozoic ocean in the Triassic.

Topographic control on the nascent Mediterranean outflow

Data collected during a 12-day cruise in July 2009 served to examine the structure of the nascent Mediterranean Outflow Water (MOW) immediately west of the Espartel Sill, the westernmost sill in the Strait of Gibraltar. The MOW is characterized by high salinities (>37.0 and reaching 38.3) and high velocities (exceeding 1 m s−1 at 100 m above the seafloor), and follows a submerged valley along a 30 km stretch, the natural western extension of the strait. It is approx. 150 m thick and 10 km wide, and experiences a substantial drop from 420 to 530 m over a distance of some 3 km between two relatively flat regions. Measurements indicate that the nascent MOW behaves as a gravity current with nearly maximal traveling speed; if this condition is maintained, then the maximum MOW velocity would decrease slowly with distance from the Espartel Sill, remaining significantly high until the gravity current excess density is only a small fraction of its original value. The sharp pycnocline between the Mediterranean and the overlying North Atlantic Central waters is dynamically unstable, particularly where the flow interacts with the 100 m decrease in bottom depth. Here, subcritical gradient Richardson numbers coincide with the development of large interfacial undulations and billows. The very energetic downslope flow is likely responsible for the development of a narrow V-shaped channel downstream of the seafloor drop along the axis of the submerged valley, this probably being the very first erosional scour produced by the nascent MOW. The coincidence of subcritical gradient Richardson numbers with relatively high turbidity values above the channel flanks suggests it may be undergoing upstream erosion.

Mean-flow and topographic control on surface eddy-mixing in the Southern Ocean

Surface cross-stream eddy diffusion in the Southern Ocean is estimated by monitoring dispersion of particles numerically advected with observed satellite altimetry velocity fields. To gain statistical significance and accuracy in the resolution of the jets, more than 1,5 million particles are released every 6 months over 16 years and advected for one year. Results are analyzed in a dynamic height coordinate system. Cross-stream eddy diffusion is highly inhomogenous. Diffusivity is larger on the equatorward flank of the Antarctic Circumpolar Current (ACC) along eddy stagnation bands, where eddy displacement speed approaches zero. Along such bands, diffusivities reach typical values of 3500 m2 s-1. Local maxima of about 8-12.103 m2 s-1 occur in the energetic western boundary current systems. In contrast, diffusivity is lower in the core of the Antarctic Circumpolar Current with values of 1500-3000 m2 s-1, and continues to decrease south of the main ACC system. The distribution of eddy diffusion is set at three scales: at circumpolar scale, the mean flow reduces diffusion in the ACC and enhances it on the equatorward side of the current; at basin scale, diffusion is enhanced in the energetic western boundary current extension regions; at regional scale, diffusion is enhanced in the wake of large topographic obstacles. We find that the zonally average structure of eddy diffusion can be explained by theory which takes mean flow into account; however, local values depend on eddy propagation, not simply described by a single wave speed, and topography.