Small Ocean Basin Research Articles

An evaluation of Mesozoic rift-related magmatism on the margins of the Labrador Sea: Implications for rifting and passive margin asymmetry

The Labrador Sea is a small (∼900 km wide) ocean basin separating southwest Greenland from Labrador, Canada. It opened following a series of rifting events that began as early as the Late Triassic or Jurassic, culminating in a brief period of seafloor spreading commencing by polarity chron 27 (C27; Danian) and ending by C13 (Eocene-Oligocene boundary). Rift-related magmatism has been documented on both conjugate margins of the Labrador Sea. In southwest Greenland this magmatism formed a major coast-parallel dike swarm as well as other smaller dikes and intrusions. Evidence for rift-related magmatism on the conjugate Labrador margin is limited to igneous lithologies found in deep offshore exploration wells, mostly belonging to the Alexis Formation, along with a postulated Early Cretaceous nephelinite dike swarm (ca. 142 Ma) that crops out onshore, near Makkovik, Labrador. Our field observations of this Early Cretaceous nephelinite suite lead us to conclude that the early rift-related magmatism exposed around Makkovik is volumetrically and spatially limited compared to the contemporaneous magmatism on the conjugate southwest Greenland margin. This asymmetry in the spatial extent of the exposed onshore magmatism is consistent with other observations of asymmetry between the conjugate margins of the Labrador Sea, including the total sediment thickness in offshore basins, the crustal structure, and the bathymetric profile of the shelf width. We propose that the magmatic and structural asymmetry observed between these two conjugate margins is consistent with an early rifting phase dominated by simple shear rather than pure shear deformation. In such a setting Labrador would be the lower plate margin to the southwest Greenland upper plate.

High-resolution seismic stratigraphy and morphology of the Scan Basin contourite fan, southern Scotia Sea, Antarctica

The Scan Basin is the easternmost of the southern Scotia Sea basins. It is located in a complex tectonic setting, north of the boundary between the Scotia and Antarctic plates. In addition, Scan Basin is directly to the north of Bruce Passage, which connects the Scotia and Weddell seas through the northward overflow of the Weddell Sea Deep Water (WSDW). Scan Basin has been described as an isolated small oceanic basin, where the fan-shaped abyssal plain is bounded by Bruce and Discovery banks. Sedimentary processes involved in the development of the Scan Basin sediment record during the late Pliocene-Quaternary are discussed here, through the examination of the distribution and morphological and acoustic characteristics of erosional, depositional, gravitational, fluid-escape, and volcanic features in correlation to the geological and oceanographic setting. The abyssal plain reflects the action of high-energy bottom-current circulation along its western side and lower-energy bottom currents along the eastern side. In contrast, Discovery Bank reflects a highly dynamic interplay between bottom-current activity, mass-wasting processes, and processes linked to the water mass interfaces. Variations in the abyssal plain stratigraphy are associated with events that may be related to climate changes since the late Miocene, involving increased bottom-current circulation and variations in the interaction between the WSDW and the Antarctic Circumpolar Current. The Scan Basin sector is one example of a contourite depositional system developed downstream of a deep gateway exit, and its major depositional characteristics allow its classification as a contourite fan. These new findings contribute to clarifying the development of contourite fans in regions of significant bottom-water interchange.

A Big Climate Driver in a Small Ocean Basin

Scientists review Atlantic Ocean circulation variability and its applications for predicting decadal climate variation.

Assessing the impact of orogenic inheritance on the architecture, timing and magmatic budget of the North Atlantic rift system: a mapping approach

To investigate the impact of orogenic inheritance on the characteristics of the North Atlantic rift system we develop new mapping methods that highlight the first-order architecture and timing of rifts, as well as the distribution of heterogeneities inherited from the Palaeozoic Caledonian and Variscan orogenies. These maps demonstrate major differences in the behaviour of the North Atlantic rift system relative to the two orogens, with the Variscan front appearing to be an important boundary: the rift cuts through the Caledonian orogen and is parallel to its structural grain to the north, whereas it circumvents the core of the Variscides to the south. In addition, rifting is protracted and polyphase with break-up being magma-rich north of the Variscan front, as opposed to the south, where a single, apparently continuous extensional event led to magma-poor break-up in less than 50 myr. Also, the North Atlantic rift system reactivates sutures corresponding to large (>2000 km) former oceans, whereas sutures of small (<500–1000 km) oceanic basins are little affected in both the northern and southern North Atlantic. These observations point to a major influence of orogenic inheritance on the characteristics of rift systems.

Tectonic and oceanographic control of sedimentary patterns in a small oceanic basin: Dove Basin (Scotia Sea, Antarctica)

AbstractDove Basin, a small oceanic domain located within the southern Scotia Sea, evidences a complex tectonic evolution linked to the development of the Scotia Arc. The basin also straddles the junction between the main Southern Ocean water masses: the Antarctic Circumpolar Current (ACC), the Southeast Pacific Deep Water (SPDW) and the Weddell Sea Deep Water (WSDW). Analysis of multichannel seismic reflection profiles, together with swath bathymetry data, reveals the main structure and sediment distribution of the basin, allowing a reconstruction of the tectonostratigraphic evolution of the basin and assessment of the main bottom water flows that influenced its depositional development. Sediment dispersed in the basin was largely influenced by gravity‐driven transport from adjacent continental margins, later modified by deep bottom currents. Sediments derived from melting icebergs and extensive ice sheets also contributed to a fraction of the basin deposits. We identify four stages in the basin evolution which – based on regional age assumptions – took place during the early Miocene, middle Miocene, late Miocene–early Pliocene and late Pliocene–Quaternary. The onsets of the ACC flow in Dove Basin during the early Miocene, the WSDW flow during the middle Miocene, and the SPDW during the late Miocene were influenced by tectonic events that facilitated the opening of new oceanic gateways in the region. The analysis of Dove Basin reveals that tectonics is a primary factor influencing its sedimentary stacking patterns, the structural development of new oceanic gateways permitting the inception of deep‐water flows that have since controlled the sedimentary processes.

U- and Th-series isotopes in deep Baffin Bay sediments: Tracers of detrital sources and of contrasted glacial/interglacial sedimentary processes

Abstract Uranium (U) and thorium (Th) series have been used to document the sedimentary sources, processes and fluxes of the last ~ 115 ka in central Baffin Bay, a major gateway between the Arctic Ocean and the North Atlantic Ocean, which has been fed throughout the last ice age by sediments and meltwaters from the northeastern Laurentide Ice Sheet, the Innuitian Ice Sheet and the western Greenland Ice Sheet. Measurements have been made with a ~ 1.6 ka time resolution in sedimentary core HU2008-029-016PC (70°27′N, 64°39′W; 2063 m water depth). The lack of correlation between U-concentration, 234 U/ 238 U activity ratio (( 234 U/ 238 U) AR ), organic carbon and redox sensitive elements suggests minimum early diagenetic U-uptake if any, likely due to very low organic matter sedimentary fluxes in such a subarctic environment. Throughout the last glaciation, U- and Th-concentrations and their mass ratio, as well as ( 234 U/ 238 U) AR , do not show either any unequivocal linkage with detrital dolomitic carbonate and/or detrital silicate fluxes. Data rather suggest sedimentary supplies with singular U- and Th-series signatures linked to specific glacial margin/ice streaming events and highlighting the diversity of sedimentary sources surrounding this small oceanic basin. Large surplus of 230 Th (( 230 Th xs ) 0 ), well above the production from dissolved-U decay in the overlying water column, are observed throughout the last glacial interval in response to sediment focusing and high-sedimentation rates in the deepest part of the Bay. During this interval, high-frequency fluctuations in ( 230 Th xs ) 0 indicate either a similar variability in sedimentation rates, not captured by the age model used, and/or grain size or mineralogical-dependent 230 Th xs -scavenging rates. In the much lower sedimentation rate Holocene core–top section, the ( 230 Th xs ) 0 -inventory falls below production in the overlying water column, despite evidence for some primary productivity revival as suggested by the higher organic carbon content. 230 Th xs -losses through lateral advection via Baffin Bay water-masses overflowing through Davis Strait might account for this pattern, pointing to ocean circulation drastically differing from that of the glacial interval.

Reprint of “Magmatism, structure and age of Dove Basin (Antarctica): A key to understanding South Scotia Arc development”

Dove Basin is situated in the south-central Scotia Sea, between Pirie and Bruce banks, and was formed during the development of the Scotia Arc. The basin has a roughly sigmoidal shape, with a prominent NNE–SSW elongated ridge located in its central part, the Dove Ridge, which is considered as an extinct spreading axis. A NE–SW elongated tectonic high, the Dove Seamount, is located in the north-eastern region of the basin, bounded by a normal fault dipping to the southeast. Dredged rocks and geophysical data were collected during the SCAN2004 and SCAN2008 cruises. Dredged samples were recovered from three positive features in the center of the basin, two from Dove Ridge, and one from Dove Seamount. Igneous rocks along the Dove Ridge are mainly tholeiitic basalts, derived from asthenospheric upper mantle within an extensional supra-subduction back-arc tectonic setting, which evolved over time from back-arc basin basalts (BABB) toward Mid Oceanic Ridge Basalts (MORBs). Altered olivine-bearing fine- and medium-grained basaltic rocks were also dredged from Dove Ridge and the seamount, together with minor oceanic island arc basalts and basaltic andesites. The mantle source was affected, up to early Miocene times, by a subducted oceanic slab related to an arc to the east, with Dove Basin forming in a back-arc position. Minor alkaline oceanic island basalts dredged at the seamount might represent a final extensional stage, genetically related with the dying Dove Ridge volcanism or, less probably, to a later, late Miocene–Pliocene extensional stage, producing incipient volcanism deriving from a deeper mantle source.40Ar/39Ar dating of MORB samples dredged from the Dove Ridge provided ages of 20.4±2.6 to 22.8±3.1Ma. These outcrops were later coated by Fe–Mn crusts with Co-Chronometer ages ranging from at least 12.6Ma and probably up to 18Ma. Analysis of magnetic anomaly profiles shows the best fit in the central profile, corresponding to chrons C6B (21.7Ma) to C7 (24.5Ma), although alternative ages may be proposed due to the short length. This interpretation supports the 40Ar/39Ar dating of a late Oligocene to early Miocene age. The spreading was asymmetrical, the asynchronous age of extinction of spreading in the basin being confirmed by the variable character of magnetic anomalies. The western part extended faster than the eastern part, suggesting an eastward location for a westward deepening subduction zone.The age obtained for the Dove Basin is older than the age previously proposed for the Protector Basin located to the west, thus suggesting an opposite polarity for the development of some small oceanic basins of the southern Scotia Sea with respect to others generated by an eastward migrating arc over subducted Atlantic oceanic floor. This apparent contradiction could be explained by considering the Dove Basin as originated from an Oligocene to early Miocene arc–back-arc spreading system, which would contribute to the eastward general development of the Scotia Arc, meanwhile Protector Basin was formed by a middle Miocene extensional phase, not genetically related with the main oceanic spreading along the Dove Basin.

Sabzevar Ophiolite, NE Iran: Progress from embryonic oceanic lithosphere into magmatic arc constrained by new isotopic and geochemical data

Abstract The poorly known Sabzevar–Torbat-e-Heydarieh ophiolite belt (STOB) covers a large region in NE Iran, over 400 km E-W and almost 200 km N-S. The Sabzevar mantle sequence includes harzburgite, lherzolite, dunite and chromitite. Spinel Cr# (100Cr/(Cr + Al)) in harzburgites and lherzolites ranges from 44 to 47 and 24 to 26 respectively. The crustal sequence of the Sabzevar ophiolite is dominated by supra-subduction zone (SSZ)-type volcanic as well as plutonic rocks with minor Oceanic Island Basalt (OIB)-like pillowed and massive lavas. The ophiolite is covered by Late Campanian to Early Maastrichtian (~ 75–68 Ma) pelagic sediments and four plagiogranites yield zircon U–Pb ages of 99.9, 98.4, 90.2 and 77.8 Ma, indicating that the sequence evolved over a considerable period of time. Most Sabzevar ophiolitic magmatic rocks are enriched in Large Ion Lithophile Elements (LILEs) and depleted in High Field Strength Elements (HFSEs), similar to SSZ-type magmatic rocks. They (except OIB-type lavas) have higher Th/Yb and plot far away from mantle array and are similar to arc-related rocks. Subordinate OIB-type lavas show Nb–Ta enrichment with high Light Rare Earth Elements (LREE)/Heavy Rare Earth Elements (HREE) ratio, suggesting a plume or subcontinental lithosphere signature in their source. The ophiolitic rocks have positive eNd (t) values (+ 5.4 to + 8.3) and most have high 207Pb/204Pb, indicating a significant contribution of subducted sediments to their mantle source. The geochemical and Sr–Nd–Pb isotope characteristics suggest that the Sabzevar magmatic rocks originated from a Mid-Ocean Ridge Basalt (MORB)-type mantle source metasomatized by fluids or melts from subducted sediments, implying an SSZ environment. We suggest that the Sabzevar ophiolites formed in an embryonic oceanic arc basin between the Lut Block to the south and east and the Binalud mountains (Turan block) to the north, and that this small oceanic arc basin existed from at least mid-Cretaceous times. Intraoceanic subduction began before the Albian (100–113 Ma) and was responsible for generating Sabzevar SSZ-related magmas, ultimately forming a magmatic arc between the Sabzevar ophiolites to the north and the Cheshmeshir and Torbat-e-Heydarieh ophiolites to the south-southeast.

Magmatism, structure and age of Dove Basin (Antarctica): A key to understanding South Scotia Arc development

Dove Basin is situated in the south-central Scotia Sea, between Pirie and Bruce banks, and was formed during the development of the Scotia Arc. The basin has a roughly sigmoidal shape, with a prominent NNE–SSW elongated ridge located in its central part, the Dove Ridge, which is considered as an extinct spreading axis. A NE–SW elongated tectonic high, the Dove Seamount, is located in the north-eastern region of the basin, bounded by a normal fault dipping to the southeast. Dredged rocks and geophysical data were collected during the SCAN2004 and SCAN2008 cruises. Dredged samples were recovered from three positive features in the center of the basin, two from Dove Ridge, and one from Dove Seamount. Igneous rocks along the Dove Ridge are mainly tholeiitic basalts, derived from asthenospheric upper mantle within an extensional supra-subduction back-arc tectonic setting, which evolved over time from back-arc basin basalts (BABB) toward Mid Oceanic Ridge Basalts (MORBs). Altered olivine-bearing fine- and medium-grained basaltic rocks were also dredged from Dove Ridge and the seamount, together with minor oceanic island arc basalts and basaltic andesites. The mantle source was affected, up to early Miocene times, by a subducted oceanic slab related to an arc to the east, with Dove Basin forming in a back-arc position. Minor alkaline oceanic island basalts dredged at the seamount might represent a final extensional stage, genetically related with the dying Dove Ridge volcanism or, less probably, to a later, late Miocene–Pliocene extensional stage, producing incipient volcanism deriving from a deeper mantle source.40Ar/39Ar dating of MORB samples dredged from the Dove Ridge provided ages of 20.4±2.6 to 22.8±3.1Ma. These outcrops were later coated by Fe–Mn crusts with Co-Chronometer ages ranging from at least 12.6Ma and probably up to 18Ma. Analysis of magnetic anomaly profiles shows the best fit in the central profile, corresponding to chrons C6B (21.7Ma) to C7 (24.5Ma), although alternative ages may be proposed due to the short length. This interpretation supports the 40Ar/39Ar dating of a late Oligocene to early Miocene age. The spreading was asymmetrical, the asynchronous age of extinction of spreading in the basin being confirmed by the variable character of magnetic anomalies. The western part extended faster than the eastern part, suggesting an eastward location for a westward deepening subduction zone.The age obtained for the Dove Basin is older than the age previously proposed for the Protector Basin located to the west, thus suggesting an opposite polarity for the development of some small oceanic basins of the southern Scotia Sea with respect to others generated by an eastward migrating arc over subducted Atlantic oceanic floor. This apparent contradiction could be explained by considering the Dove Basin as originated from an Oligocene to early Miocene arc–back-arc spreading system, which would contribute to the eastward general development of the Scotia Arc, meanwhile Protector Basin was formed by a middle Miocene extensional phase, not genetically related with the main oceanic spreading along the Dove Basin.

Shear-velocity structure of the Tyrrhenian Sea: Tectonics, volcanism and mantle (de)hydration of a back-arc basin

The Tyrrhenian Sea in the Mediterranean formed as the result of roll-back of the Adriatic and Ionian subducting plates. It is mostly underlain by strongly thinned continental lithosphere, but contains two small oceanic basins in the southern Tyrrhenian, the youngest one located just behind the active magmatic arc. Its regional setting with dense station coverage provides a unique opportunity to derive a high-resolution, 3-D shear-velocity model of this back-arc basin and surrounding onshore areas using interstation Rayleigh-wave dispersion measurements. Our tomographic model, extending to a depth of approximately 160 km, displays a pronounced, nearly ring-shaped low shear-velocity zone between 70 and 110 km depth which surrounds the older oceanic Vavilov Basin. The sharp velocity decrease at 70 km depth can be explained by the transition from a relatively dry lithospheric mantle to more hydrous asthenospheric mantle material. The tectonic evolution of the region and the correlation of the low-velocity anomaly with subduction-related volcanism indicate that the low-velocity anomaly reflects hydrous mantle material in (present or former) mantle wedge regions. We suggest that the absence of the low-velocity zone beneath the Vavilov Basin is due to mantle dehydration caused by the creation of MORB crust. Whereas temperature effects may dominate the asthenospheric shear-velocity differences between various back-arc basins, we find that the variations in shear-velocity structure within the Tyrrhenian area are best explained by variations in mantle water content.

Multi-stage crustal growth and cratonization of the North China Craton

The North China Craton (NCC) has a complicated evolutionary history with multi-stage crustal growth, recording nearly all important geological events in the early geotectonic history of the Earth. Our studies propose that the NCC can be divided into six micro-blocks with >∼3.0–3.8 Ga old continental nuclei that are surrounded by Neoarchean greenstone belts (GRB). The micro-blocks are also termed as high-grade regions (HGR) and are mainly composed of orthogneisses with minor gabbros and BIF-bearing supracrustal beds or lenses, all of which underwent strong deformation and metamorphism of granulite- to high-grade amphibolite-facies. The micro-blocks are, in turn, from east to west, the Jiaoliao (JL), Qianhuai (QH), Ordos (ODS), Ji'ning (JN) and Alashan (ALS) blocks, and Xuchang (XCH) in the south. Recent studies led to a consensus that the basement of the NCC was composed of different blocks/terranes that were finally amalgamated to form a coherent craton at the end of Neoarchean.Zircon U-Pb data show that TTG gneisses in the HGRs have two prominent age peaks at ca. 2.9–2.7 and 2.6–2.5 Ga which may correspond to the earliest events of major crustal growth in the NCC. Hafnium isotopic model ages range from ca. 3.8 to 2.5 Ga and mostly are in the range of 3.0–2.6 Ga with a peak at 2.82 Ga. Recent studies revealed a much larger volume of TTG gneisses in the NCC than previously considered, with a dominant ca. 2.7 Ga magmatic zircon ages. Most of the ca. 2.7 Ga TTG gneisses underwent metamorphism in 2.6–2.5 Ga as indicated by ubiquitous metamorphic rims around the cores of magmatic zircon in these rocks. Abundant ca. 2.6–2.5 Ga orthogneisses have Hf-in-zircon and Nd whole-rock model ages mostly around 2.9–2.7 Ga and some around 2.6–2.5 Ga, indicating the timing of protolith formation or extraction of the protolith magma was from the mantle. Therefore, it is suggested that the 2.6–2.5 Ga TTGs probably represent a coherent event of continental accretion and major reworking (crustal melting).As a distinct characteristic, nearly all GRBs in the NCC underwent amphibolite-facies metamorphism. Zircon U-Pb ages of metamorphosed GRB mafic rocks mainly show two peak ranges at ∼2.6–2.5 and 2.8–2.7 Ga. The mafic rocks are commonly believed to be derived from metabasalts, it is therefore possible that the ages represent the time of metamorphism. The tectonic settings of the GRBs are still a problem. Their geochemical characteristics are, respectively, similar to back-arc basins, rifts, island arcs or suggest imprints of mantle plumes. BIFs occur in all GRBs but also in the HGRs. This metallogenic specificity is quite different from all Phanerozoic geotectonic settings.The ∼2.5 Ga metamorphic-magmatic event is stronger than in most other cratons in the world. How to understand the geological significance of the 2.5 Ga event? The following points are emphasized: (1) nearly all old rocks >2.5 Ga underwent metamorphism at ∼2.52–2.5 Ga; (2) Archean basement rocks in the NCC experienced strong partial melting and migmatization; (3) granitoid rocks derived from partial melting include potassium granites, TTG granites and monzonites. These granitoids rocks intruded both the Archean greenstone belts and micro-blocks; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic-ultramafic dykes are also developed. Therefore, we suggest an assembly model that all micro-blocks in the NCC were welded together by late Archean greenstone belts at the end of the late Neoarchean. We also propose that the various micro-blocks were surrounded by small ocean basins, and the old continental crust and the oceanic crust were hotter than today. Subduction and collision were on much smaller scales as compared to the Phanerozoic plate tectonic regime, although the tectonic style and mechanisms were more or less similar. The formation of crustal melt granites is one of the processes of cratonization, inducing generation of stable upper and lower crustal layers. This process also generated an upper crust of more felsic composition and a lower crust of more mafic composition, due to molten residual materials and some underplated gabbros.

Relationship of Mediterranean type lamproites to large shoshonite volcanoes, Miocene of Lesbos, NE Aegean Sea

Shoshonites, which are high-K trachyandesitic rocks, are found in many orogenic belts and are commonly of post-collisional origin. The petrogenesis of shoshonites has been widely debated. Small lava flows and dykes of lamproite and related lamproitic rocks of early Miocene age in Lesbos are coeval with voluminous shoshonite volcanoes. Their distinctive petrology and isotope geochemistry provide an exceptional opportunity to assess the petrogenetic relationship between lamproites and shoshonites. The lamproitic rocks contain phenocrysts of forsteritic olivine (as high as Fo93) and clinopyroxene, both with inclusions of chrome spinel (Cr# ~0.9 or ~0.6) and carbonate melt inclusions, indicating the presence of carbonatite melts. Some complexly zoned clinopyroxene from lamproitic rocks have salite cores with chemical composition suggesting they formed in the upper mantle in a melt strongly enriched in LILE and LREE. Both lamproites and shoshonites show continuous trends of trace elements and their isotopic compositions overlap. Lack of variation in K with Mg# or SiO2 for particular temporal–spatial groups of shoshonites suggests derivation from particular inhomogeneous mantle rather than fractionation processes. In contrast to other peri-Mediterranean lamproites, the Lesbos lamproites and shoshonites have unusual Pb isotope composition that requires a common origin from subcontinental lithospheric mantle enriched in LILE in the Paleozoic. This enrichment process involved partial melting of subducted carbonate-bearing pelites. Triassic rift-related volcanism and formation of Jurassic small ocean basins produced extreme depletion of parts of the mantle. Lamproitic magma was derived from melting of enriched refractory harzburgite, whereas enriched lherzolite, wehrlite and pyroxenite partially melted to supply larger volumes of shoshonitic and related magmas. The NE Aegean Miocene shoshonite province is thus not directly related to contemporary subduction, but may have been triggered by related back-arc extension.

Late Palaeozoic–Early Cenozoic tectonic development of Southern Turkey and the easternmost Mediterranean region: evidence from the inter-relations of continental and oceanic units

Abstract Reconstructions of the Anatolian continent and adjacent areas assume the existence of one or more continental fragments during Mesozoic–Early Cenozoic time. These rifted from North Africa (Gondwana) during the Triassic, drifted across the Mesozoic Tethys and collided with Eurasia during latest Cretaceous–Paleocene time. Current reconstructions range from a regional-scale Tauride–Anatolide continent with oceanic basins to the north and south, to numerous rifted continental fragments separated by small oceanic basins. Field-based evidence for the inter-relations of the continental blocks and associated carbonate platforms is discussed and evaluated here, especially to distinguish between sutured oceans and intra-continental convergence zones. Several crustal units are restored as different parts of one large Tauride–Anatolide continent, whereas several smaller crustal units (e.g. Kırşehir massif; Bitlis/Pütürge and Alanya/Kyrenia units) are interpreted as continental fragments bordered by oceanic crust. We infer a relatively wide İzmir–Ankara–Erzincan ocean in the north and also a wide South Neotethyan ocean in the south. Several smaller oceanic strands (e.g. Inner Tauride ocean, Berit ocean and Alanya ocean) were separated by continental fragments. Our proposed reconstructions are shown on palaeotectonic maps for Late Permian to Mid-Miocene. The reconstructions have interesting implications for crustal processes, including ophiolite genesis and emplacement.

Heat-flow determinations of basement age in small oceanic basins of the southern central Scotia Sea

Abstract Results are reported from seven heat flow stations in small basins of the southern part of the central Scotia Sea (CSS), undertaken in order to determine basement ages. The basins are small, which makes magnetic anomaly-based ages ambiguous and preserves basin subsidence that may have been anomalous as a result of local factors. The fact that these small basins formed in a back-arc setting adds additional uncertainty to depth-based age estimates. The results confirm that basin extension commenced in the Eocene, and indirectly support a relatively young, back-arc origin for the northern CSS, but do not affect previously published suggestions of the age of onset of the Antarctic Circumpolar Current.

Editorial introduction to ‘Geological Development of Anatolia and the Easternmost Mediterranean Region’

The present set of 22 papers stems from the 7th International Symposium on Eastern Mediterranean Geology that was held in Adana, Turkey, 18–22 October 2010. After its initiation in l992, in Adana this international conference has been held successively in Jerusalem (Israel) in l995, Nicosia (Cyprus) in 1998, Isparta (Turkey) in 2001, Thessalonica (Greece) in 2004 and Amman (Jordan) in 2007. The Cyprus and Thessalonica conferences were followed by substantive publications, including one with a focus on Cyprus (Panayides et al. 2000) and another mainly concerned with the Balkan region (Robertson & Mountrakis 2006). A subset of the papers that were presented at the 7th Adana meeting, together with some others, have been prepared and edited for the present volume. Anatolia and the surrounding region provide an excellent opportunity for the study of fundamental geological processes, including rifting, seafloor spreading, ophiolite genesis and emplacement, collision, continental assembly and neotectonics. This volume should interest a wide cross-section of international researchers, including those concerned with hydrocarbons, mineral deposits and seismic risk, and also postgraduate students and advanced undergraduates. The papers highlight the role of fieldwork, the multidisciplinary nature of much of the current research in the region, the role of teamwork and the strong contribution being made by young scientists. Following an introductory chapter, the volume is divided into four sections covering different aspects of the region as a whole. The area discussed mainly lies within Turkey, Cyprus and Syria. Section 1 is made up of a small number of papers that are mainly concerned with the Pontide belt of northern Turkey. Section 2 is concerned with the geological development of the Tauride and Anatolide belts of central and southern Anatolia, especially the Triassic–Jurassic period of rifting and passive margin development and the Late Cretaceous period of ophiolite genesis and emplacement. Section 3 is mainly concerned with the formation of sedimentary basins during closure of several Mesozoic ocean basins and the related structural development during Late Cretaceous to Pliocene time. Finally, Section 4 is devoted to aspects of the structural development of the region, mainly during the Pliocene–Quaternary (i.e. neotectonics) when the plate configuration was essentially as it is today. The area covered by each paper is shown in Figure 1. The introduction by Robertson et al. covers the southern part of Anatolia and the adjacent easternmost Mediterranean region that was the main subject of the international conference. The main focus is on Late Permian–Recent time. A review of the Late Precambrian–Recent geological development of the easternmost Mediterranean region is published elsewhere (Robertson et al. 2012). The authors discuss alternative interpretations of the Mesozoic–Cenozoic inter-relations of the various crustal units that make up the region. In particular, they consider whether these should be interpreted as individual microcontinents separated by Mesozoic small ocean basins or as parts of larger continental units (i.e. microcontinents). The Anatolides in the north are generally interpreted as the metamorphosed equivalents of the Taurides, although different reconstructions exist. The Anatolides are commonly seen as the northern, leading edge of the Mesozoic Tauride–Anatolide continent that subducted and underwent high-pressure/lowtemperature (HP/LT) metamorphism during Late Cretaceous–Early Cenozoic time. The Anatolides are divided into two parts, namely the HP/LTmetamorphosed Afyon–Bolkar Dag zone in the south, which can be closely correlated with the Taurides, and the very HP/LT Tavsanli Zone further north, which also shows some affinities with the Taurides but is less well understood. The Kirsehir Massif is interpreted as a rifted continental block that was separated from a larger Tauride continent to the south by a Mesozoic oceanic basin known as the Inner-Tauride Ocean. However, uncertainties remain, including the reconstruction

Paleozoic multiple accretionary and collisional tectonics of the Chinese Tianshan orogenic collage

Subduction-related accretion in the Junggar–Balkash and South Tianshan Oceans (Paleo-Asian Ocean), mainly in the Paleozoic, gave rise to the present 2400km-long Tianshan orogenic collage that extends from the Aral Sea eastwards through Uzbekistan, Tajikistan, Kyrgyzstan, to Xinjiang in China. This paper provides an up-to-date along-strike synthesis of this orogenic collage and a new tectonic model to explain its accretionary evolution.The northern part of the orogenic collage developed by consumption of the Junggar–Balkash Ocean together with Paleozoic island arcs (Northern Ili, Issyk Kul, and Chatkal) located in the west, which may have amalgamated into a composite arc in the Paleozoic in the west and by addition of another two, roughly parallel, arcs (Dananhu and Central Tianshan) in the east. The western composite arc and the eastern Dananhu and Central Tianshan arcs formed a late Paleozoic archipelago with multiple subduction zones. The southern part of the orogenic collage developed by the consumption of the South Tianshan Ocean which gave rise to a continuous accretionary complex (Kokshaal–Kumishi), which separated the Central Tianshan in the east and other Paleozoic arcs in the west from cratons (Tarim and Karakum) to the south. Cross-border correlations of this accretionary complex indicate a general southward and oceanward accretion by northward subduction in the early Paleozoic to Permian as recorded by successive southward juxtaposition of ophiolites, slices of ophiolitic mélanges, cherts, island arcs, olistostromes, blueschists, and turbidites, which are mainly Paleozoic in age, with the youngest main phase being Late Carboniferous–Permian. The initial docking of the southerly Tarim and Karakum cratons to this complicated late Paleozoic archipelago and accretionary complexes occurred in the Late Carboniferous–Early Permian in the eastern part of the Tianshan and in the Late Permian in the western part, which might have terminated collisional deformation on this suture zone. The final stages of closure of the Junggar–Balkash Ocean resembled the small ocean basin scenario of the Mediterranean Sea in the Cenozoic. In summary, the history of the Altaids is characterized by complicated multiple accretionary and collisional tectonics.

The Arctic Ocean Estuary

Large freshwater contributions to the Arctic Ocean from a variety of sources combine in what is, by global standards, a remarkably small ocean basin. Indeed, the Arctic Ocean receives ∼11% of global river discharge while accounting for only ∼1% of global ocean volume. As a consequence, estuarine gradients are a defining feature not only near-shore, but throughout the Arctic Ocean. Sea-ice dynamics also play a pivotal role in the salinity regime, adding salt to the underlying water during ice formation and releasing fresh water during ice thaw. Our understanding of physical–chemical–biological interactions within this complex system is rapidly advancing. However, much of the estuarine research to date has focused on summer, open water conditions. Furthermore, our current conceptual model for Arctic estuaries is primarily based on studies of a few major river inflows. Future advancement of estuarine research in the Arctic requires concerted seasonal coverage as well as a commitment to working within a broader range of systems. With clear signals of climate change occurring in the Arctic and greater changes anticipated in the future, there is good reason to accelerate estuarine research efforts in the region. In particular, elucidating estuarine dynamics across the near-shore to ocean-wide domains is vital for understanding potential climate impacts on local ecosystems as well as broader climate feedbacks associated with storage and release of fresh water and carbon.

Lithospheric structure of the Gorringe Bank: Insights into its origin and tectonic evolution

[1] The Gorringe Bank is a 5000 m high seamount near the Atlantic coast of Iberia characterized by a 9 m high geoid anomaly and a ∼120 mGal Bouguer anomaly relative to the surrounding abyssal plains. It has been linked to a NW directed thrust carrying exhumed upper mantle rocks and transitional crust on top of flexed-down Eurasian oceanic crust along the Tagus Abyssal Plain. However, estimations of crustal shortening have yielded dissimilar results, and the deep structure of the ridge remains highly unknown. We present a restored cross section and a new model of the lithospheric structure based on gravity, geoid, elevation, and the presence of serpentinized peridotites. At least 20 km of shortening took place along a flat-ramp-flat thrust fault, and the density structure of the lithosphere is consistent with mantle serpentinization varying from 70% at the surface to 20% at 14 km depth and 0% at 40 km. The topographic relief and gravity anomalies are explained by assuming a flexural isostatic model with an elastic thickness Te of ∼30 km. The evolution of the Gorringe Bank since the Late Jurassic is interpreted in relation to Eurasia-Africa-North America plate motion in four stages: (1) transtension between Newfoundland-Iberia and Africa, which generated small oceanic basins and mantle exhumation; (2) opening of the North Atlantic and seafloor spreading at the NW side of the exhumed Gorringe, which produced gabbro intrusions and serpentinization; (3) a quiescent tectonic period dominated by subsidence and sediment accumulation; and (4) a transpressional plate boundary between Eurasia and Africa with NW directed subcrustal thrusting and generation of the present Gorringe relief.

Early Cambrian eclogites in SW Mongolia: evidence that the Palaeo‐Asian Ocean suture extends further east than expected

AbstractNewly discovered eclogites, Early Cambrian carbonates and chloritoid‐bearing metapelites form the Tsakhir Uul accretionary wedge, which was thrust during the Early Cambrian over the Mesoproterozoic Dzabkhan‐Baydrag continent. The rock association of the wedge forms a tectonic window emerging through the hangingwall Khantaishir ophiolite unit, which preserves a typical Tethyan‐type ophiolitic sequence. The eclogites correspond geochemically to T‐MORB modified by fluid circulation. They are composed of garnet, omphacite, amphibole, rutile ±muscovite ±quartz ±epidote and exhibit well‐equilibrated matrix textures. Jadeite content of the omphacite reaches up to 45 mol.%, the Si content of muscovite is between 3.40 and 3.45 p.f.u., amphibole is winchite to barroisite, but reaches tschermakitic composition at some rims, and garnet composition is grs0.24–0.36, alm0.43–0.56, py0.05–0.18, sps0.00–0.18, . The peak assemblage, together with the composition of garnet rims, omphacite, amphibole and muscovite, correspond in a pseudosection to 20−22.5 kbar and 590−610 °C. The tschermakitic rim of amphibole is interpreted as partial reequilibration on decompression below 16 kbar and ∼600−630 °C. Two muscovite separates from the eclogite yielded an Ar–Ar plateau age of 543.1 ± 3.9 Ma (1σ) and a mean age of 547.9 ± 2.6 Ma (1σ), whereas muscovite from an interbedded garnet‐chloritoid micaschist yielded an Ar–Ar plateau age of 536.9 ± 2.7 Ma (1σ); these ages are interpreted as cooling ages. The P–T data, geochemistry of eclogites and cooling ages suggest an affinity between the Tsakhir Uul wedge and the Gorny Altai and the north Mongolian blueschist belt, which are believed typical for subduction of warm oceanic lithosphere and closure of small oceanic basins. Thus, the discovery of the Tsakhir Uul eclogites represents an important finding suggesting extension of the Early Cambrian subduction system of the Central Asian Orogenic Belt far to the east in a region where it was not expected.

The Neoproterozoic Sergipano orogenic belt, NE Brazil: A complete plate tectonic cycle in western Gondwana

The Neoproterozoic Sergipano Belt formed by the collision of the Pernambuco-Alagoas Block in the north with the São Francisco Craton in the south, but the timing, duration and mechanics of this amalgamation are poorly understood. The belt is divided into the Canindé, Poço Redondo-Marancó, Macururé, Vaza Barris, and Estância lithostratigraphic domains; the first three are composed of plutonic, volcanic and sedimentary rocks and the last three of sedimentary rocks. Our new field, structural, and geochemical data, and Sm–Nd, Ar–Ar and U–Pb geochronology provide robust constraints for the following evolution. A Mesoproterozoic (∼980–960 Ma) continental arc (Poço Redondo tonalitic gneisses) developed on the margin of the Palaeoproterozoic Pernambuco-Alagoas Block. Extension of this continental block gave rise to (i) A-type crustal granites and associated sedimentary rocks on the stretched, rifted margin of the Poço Redondo-Marancó Domain, (ii) the Canindé rift sequence between the Pernambuco-Alagoas Block and the Poço Redondo/Marancó domain, (iii) a passive margin on the southern boundary of the Pernambuco-Alagoas Block on which sediments were deposited after 900 Ma, (iv) and a second passive margin on the São Francisco Craton. In the Canindé Domain, rifting continued until ca. 640 Ma and led to emplacement of a bimodal association of A-type granite (715 Ma) and continental mafic volcanic rocks, a continental-type layered gabbroic complex (ca. 700 Ma), magma-mingled gabbro/quartz–monzodiorite (688 Ma), and rapakivi granites (684 Ma and 641 Ma). Deformed pillow basalts and interleaved marble lenses are likely ocean floor relicts in the Canindé Domain. Closure of the Canindé oceanic basin began at ca. 630 Ma with the intrusion of arc-type granitic plutons. Convergence of the Pernambuco-Alagoas Block and the São Francisco Craton led to deformation on the passive margins and granite emplacement in the Macururé (628–625 Ma, and 590–570 Ma), Canindé (ca. 621 Ma) and Poço Redondo-Marancó (ca. 625 Ma) domains. A small oceanic basin was most likely subducted beneath the Poço Redondo-Marancó Domain to account for the presence of 602 Ma arc-type volcanic rocks. Shortly after, exhumation of the Pernambuco-Alagoas Block and Canindé, Poço Redondo-Marancó and Macururé domains in the north led to deposition of uppermost clastic sediments in the Estância and Vaza Barris domains in the south, possibly in a foreland basin, and to final thrusting of the continental margin sedimentary rocks onto the São Francisco Craton. Our results indicate that the construction of western Gondwana involved a ca. 300 million years long history of plate breakup and collision.