Crustal evolution

New U-Pb geochronological data obtained via LA-ICP-MS from detrital zircons in quartzo-feldspathic schists and quartzites from the Jebba-Bode Saadu area, southwestern Nigeria, provide maximum depositional ages and insights into their provenance. The Jebba quartzo-feldspathic schist and quartzite were deposited after the Late Mesoproterozoic (1 199 Ma) and Early Neoproterozoic (938 Ma), respectively, while the units from Bode Saadu were likely deposited after the Late Palaeoproterozoic (1 701 to 1 729 Ma). This suggests that the metasedimentary units in Bode Saadu are probably older than those in Jebba to the north. The protoliths of these units were derived from sources spanning the Archaean to Early Neoproterozoic, with some linked to the local basement and others potentially connected to more distant sources, such as the Borborema Province and the São Francisco Craton in Brazil. The similarities in both age distributions and lithologies between the analysed rock samples and their counterparts from South America support the hypothesis of the Atlantica Continent in the Proterozoic, which included the major cratonic blocks of West Africa and South America.

The Archean Dharwar craton in south India is known for long time to be different from most other cratons. Specifically, at station Hyderabad (HYB) the Ps converted phases from the 410and 660-km mantle discontinuities arrive up to 2 s later than in other cratons of comparable age, which implies lower upper mantle velocities. To resolve the unique lithosphere-asthenosphere system of the Dharwar craton, we inverted jointly P and S receiver functions and teleseismic P and S traveltime residuals at 10 seismograph stations. This method operates in the same depth range as long-period surface waves but differs by much higher lateral and radial resolution. We observe striking differences in crustal structures between the eastern and western Dharwar craton (EDC and WDC, respectively): crustal thickness is of around 31 km, with predominantly felsic velocities, in the EDC and of around 55 km, with predominantly mafic velocities, in the WDC. In the mantle we observe significant variations in the P velocity with depth, practically without accompanying variations in the S velocity. In the mantle S velocity there are azimuthdependent indications of the Hales discontinuity at a depth of ∼100 km. The most conspicuous feature of our models is the lack of the high velocity mantle keel with the S velocity of ∼4.7 km s -1 , typical of other Archean cratons. The S velocity in our models is close to 4.5 km s -1 from the Moho to a depth of ∼250 km. There are indications of a similar upper mantle structure in the northeast of the Indian craton and of a partial recovery of the normal shield structure in the northwest. A division between the high S-velocity western Tibet and low S-velocity eastern Tibet may be related to a similar division between the northeastern and northwestern Indian craton.

The new geochronologic, petrologic and structural data acquired during the mapping project of the Programme de Gouvernance des Ressources Minerales de Madagascar, completed by academic works, lead to the re-definition of eight geological domains which make up the metamorphic Malagasy basement. The Center of the Island only consists of the Antananarivo and Antongil/Masora domain respectively. The Paleo- Mesoarchean Antongil/Masora domain (3.32-3.15 Ga) is interpreted as a fragment of the wider Dharwar craton of India. Its adjacent Neoarchean Antananarivo domain (2.7-2.5 Ga) is well know because it comprises a critical component of mafic-ultramafic gneisses which crop out as five N-striking belts interpreted as Archaean metamorphosed greenstone belts referred hereafter as “Tsaratanana Complex”. , In the conventional view, the Tsaratanana complex is interpreted as a single allochthonous unit thrust over granitoid gneisses of the Antananarivo block during the Panafrican orogeny. Two asp...

Several profound changes, including those involving formation of the continental crust, occurred on Earth during the Neoarchaean Era. However, the tectonic settings associated with Neoarchaean crustal growth are not well understood and vigorously debated. The Neoarchaean Veligallu greenstone belt, eastern Dharwar craton hosts a variety of ultramafic, mafic and felsic volcanic rocks. Whole-rock elemental and Nd isotope data along with zircon U-Pb dating on these rocks provide significant insights into the origin and tectonic setting of Neoarchaean crust formation. The volcanism in the Veligallu belt started with ~2.67 Ga tholeiitic basalts derived from shallow melting of a slightly depleted mantle (ε Ndt = +0.6 to +1.1). Moderate negative Nb anomalies, slightly elevated Th/Yb and LREE, and an absence of evidence for crustal contamination are consistent with extraction of these basalts from a mantle source weakly metasomatized by subducted slab-derived fluids in an incipient oceanic arc setting. As the arc matured, clastic sediments started forming with concurrent emplacement of komatiites, komatiitic basalts and ferropicrites showing strong signatures of contamination with continental crust (negative Nb and Ti anomalies, LREE enrichment and negative ε Ndt ). In the final stage (~2.58 Ga), a variety of felsic volcanic rocks (sodic trachyandesite, high Mg# andesite, rhyolite, calc-alkaline andesite) formed. The rock association and distinct geochemical signatures (enrichment of LILE, negative Nb and Ti anomalies, Mesoarchaean Nd model ages and inherited older zircons) suggest a continental margin arc environment which contained older crust. The evolutionary history of the Veligallu belt implies that both the arc-and plume-related processes, and their interplay contributed significantly to the growth of Neoarchaean crust.

O Superterreno Pernambuco Alagoas (PEAL), previamente designado de Maciço e/ou Terreno, é o segmento tectônico identificado imediatamente ao sul do lineamento Pernambuco. Apresenta uma área de cerca de 60.000km2, forma triangular, com a base entre o Recife e imediações de Maceió, e com vértice nas imediações de Santa Cruz, oeste de Pernambuco. Sua composição é extremamente variada, com terrenos de alto grau, arqueanos, paleoproterozoicos, eo-neoproterozoicos e tratos metavulcanossedimentares neoproterozoicos (faixas supracrustais). Estes terrenos são cortados por granitoides neoproterozoicos diversos, incluindo batólitos lineares de grandes dimensões, por stocks e batólitos menores relativamente isolados. Localmente, na sua parte centro sudoeste, foi recentemente reconhecido magmatismo granítico de arco (Ediacarano em idade). O comportamento geotectônico deste amplo superterreno- posicionado entre as faixas orogênicas da Zona Transversal e aquelas da periferia norte do craton do São...

U-Pb sensitive high resolution ion microprobe mass spectrometer (SHRIMP) ages of zircon, monazite and xenotime crystals from felsic intrusive rocks from the Rio Itapicuru greenstone belt show two development stages between 2,152 and 2,130 Ma, and between 2,130 and 2,080 Ma. The older intrusions yielded ages of 2,152±6 Ma in monazite crystals and 2,155±9 Ma in zircon crystals derived from the Trilhado granodiorite, and ages of 2,130± 7 Ma and 2,128±8 Ma in zircon crystals derived from the Teofilândia tonalite. The emplacement age of the syntectonic Ambrósio dome as indicated by a 2,080±2-Ma xenotime age for a granite dyke probably marks the end of the felsic magmatism. This age shows good agreement with the Ar-Ar plateau age of 2,080±5 Ma obtained in hornblendes from an amphibolite and with a U-Pb SHRIMP age of 2,076±10 Ma in detrital zircon crystals from a quartzite, interpreted as the age of the peak of the metamorphism. The predominance of inherited zircons in the syntectonic Ambrósio dome suggests that the basement of the supracrustal rocks was composed of Archaean continental crust with components of 2,937±16, 3,111±13 and 3,162±13 Ma. Ar-Ar plateau ages of 2,050±4 Ma and 2,054±2 Ma on hydrothermal muscovite samples from the Fazenda Brasileiro gold deposit are interpreted as minimum ages for gold mineralisation and close to the true age of gold deposition. The Ar-Ar data indicate that the mineralisation must have occurred less than 30 million years after the peak of the metamorphism, or episodically between 2,080 Ma and 2,050 Ma, during uplift and exhumation of the orogen.

Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1–2 Myr along the northern continent‐ocean boundary (COB). A southward ridge jump of ∼20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from ∼23.6 to ∼21.5 Ma. The terminal age of seafloor spreading is ∼15 Ma in the East Subbasin and ∼16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from ∼20 to ∼80 km/Myr, but mostly decreased with time except for the period between ∼26.0 Ma and the ridge jump (∼23.6 Ma), within which the rate was the fastest at ∼70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic a...

The generation of continental crust, its bulk composition and temporal evolution provide important records of plate tectonics and associated magma-generating processes. However, the long-term integrated effects of repeated magmatic events on crustal growth, composition and differentiation and, therefore, on crustal evolution are rarely considered. Here, we examine long-term (∼350 Myr) temporal compositional trends of granitic magmatism within a limited (∼200 km × 100 km) area in the Northern New England Orogen of Queensland, Australia to avoid lateral crustal variations in order to understand how temporal–compositional variations of silicic igneous rocks record crustal evolution. Long-term temporal compositional variations are tracked using whole-rock chemistry, zircon chronochemistry and zircon Hf isotopic compositions. We particularly focus on whole-rock U, Th and K abundances and calculated heat-production values as proxies for crustal evolution, and tracking crustal sources invo...

The High Arctic Large Igneous Province (HALIP) represents extensive Cretaceous magmatism throughout the circum-Arctic borderlands and within the Arctic Ocean (e.g., the Alpha-Mendeleev Ridge). Recent aeromagnetic data shows anomalies that extend from the Alpha Ridge onto the northern coast of Ellesmere Island, Nunavut, Canada. To test this linkage we present new bulk rock major and trace element geochemistry, and mineral compositions for clinopyroxene, plagioclase, and olivine of basaltic dykes and sheets and rhyolitic lavas for the stratotype section at Hansen Point, which coincides geographically with the magnetic anomaly at northern Ellesmere Island. New U-Pb chronology is also presented. The basaltic and basaltic-andesite dykes and sheets at Hansen Point are all evolved with 5.5-2.5 wt% MgO, 48.3-57.0 wt% SiO 2 , and have light rare-earth element enriched patterns. They classify as tholeiites and in Th/Yb vs. Nb/Yb space they define a trend extending from the mantle array toward upper continental crust. This trend, also including a rhyolite lava, can be modeled successfully by assimilation and fractional crystallization. The U-Pb data for a dacite sample, that is cut by basaltic dykes at Hansen Point, yields a crystallization age of 95.5 ± 1.0 Ma, and also shows crustal inheritance. The chronology and the geochemistry of the Hansen Point samples are correlative with the basaltic lavas, sills, and dykes of the Strand Fiord Formation on Axel Heiberg Island, Nunavut, Canada. In contrast, a new U-Pb age for an alkaline syenite at Audhild Bay is significantly younger at 79.5 ± 0.5 Ma, and correlative to alkaline basalts and rhyolites from other locations of northern Ellesmere Island (Audhild Bay, Philips Inlet, and Yelverton Bay West; 83-73 Ma). We propose these volcanic occurrences be referred to collectively as the Audhild Bay alkaline suite (ABAS). In this revised nomenclature, the rocks of Hansen Point stratotype and other tholeiitic rocks are ascribed to the Hansen Point tholeiitic suite (HPTS) that was emplaced at 97-93 Ma. We suggest this subdivision into suites replace the collective term Hansen Point volcanic complex. The few dredge samples of alkali basalt available from the top of the Alpha Ridge are akin to ABAS in terms of geochemistry. Our revised dates also suggest that the HPTS and Strand Fiord Formation volcanic rocks may be the hypothesized subaerial large igneous province eruption that drove the Cretaceous Ocean Anoxic Event 2.

Geomorphological and sedimentological indicators are often used to reconstruct not only coastal evolution, but also relative sea level changes. In this work, we studied the coastal sediments of Psatha bay (Alkyonides Gulf, Greece) and beachrock outcrops in order to reconstruct the coastal evolution of the area. The drillings analysis included stratigraphy, sediment texture and radiocarbon dating. Detailed mapping of the beachrocks was accomplished using DGPS-GNSS, as well as mineralogical analysis and OSL dating of beachrock samples. The new beachrock index points indicate a sea level that fell by 0.64 ± 0.13 m since 2200 ± 210 years BP and by 0.95 ± 0.13 m since 4160 ± 320 years BP, as a direct result of its location near the uplifting footwall of Psatha fault, suggesting further a rate of tectonic uplift of ~0.26 mm/yr for the late Holocene.

Geomorphological and sedimentological indicators are often used to reconstruct not only coastal evolution, but also relative sea level changes. In this work, we studied the coastal sediments of Psatha bay (Alkyonides Gulf, Greece) and beachrock outcrops in order to reconstruct the coastal evolution of the area. The drillings analysis included stratigraphy, sediment texture and radiocarbon dating. Detailed mapping of the beachrocks was accomplished using DGPS-GNSS, as well as mineralogical analysis and OSL dating of beachrock samples. The new beachrock index points indicate a sea level that fell by 0.64 ± 0.13 m since 2200 ± 210 years BP and by 0.95 ± 0.13 m since 4160 ± 320 years BP, as a direct result of its location near the uplifting footwall of Psatha fault, suggesting further a rate of tectonic uplift of ~0.26 mm/yr for the late Holocene.

The Neoproterozoic basement of Wadi Al-Baroudarea located at the northern Eastern Desert (ED) of Egypt, at the northernmost segment of the Arabian-Nubian Shield (ANS), is comprised of two different granite suites. A large batholith ascribed to the Older Granite suite, extends across the boundary between the northern and central ED, and is intruded by two isolated plutons of the Younger Granite suite. The Older Granite suite includes grey-colored, massive to gneissose, granodiorites to tonalites typically containing microgranular mafic enclaves. These are calc-alkaline, magensian, metaluminous I-type granitoids, with high Sr contents, and depleted in Rb, Nb, Y and REE. The Younger Granite suite plutons are pink to red, biotite and two-mica monzogranites. These are peraluminous A-type granites exhibiting a high-K calc-alkaline nature, and varying between ferroan and magnesian type granites. The A-type granites of the Younger Granite suite are enriched in Ga, Y, HFSE and REE elements, and depleted in the LILE elements Ba, Sr and Rb and transition metals Cr, Ni, Co, Sc and V. Magmatic saturation temperatures indicate early crystallization of apatite at high temperarture in the metaluminous I-type Older Granite suite, while in the peraluminous A-type Younger Granites its crystallization occurs later after separation of zircon and monazite. The plutons of the Younger Granite suite were generated during the post-collisional stage of the northern ANS, following collision between the juvenile ANS crust and the pre-Neoproterozoic continental blocks of west Gondwana. The emplacement of the Older Granite suite took place earlier, within a normally mature continental arc prior to the collision. These pre-collision granitoids evolved through assimilation-fractional crystallization processes from mantle-derived parental magmas, which have interacted with crustal materials during ascent and storage. The post-collisional Younger Granite suite seems to have been derived by high degree, partial melting of metasedimentary sources, particularly psammitic and pelitic metasediments.

The complex geodynamic evolution of Anatolia and the Caucasus region included the assembly of a mosaic of terranes of different tectonic origin during several pulses of collisional events associated with the closure of the Tethys oceans and the Eurasia-Arabia collision. To date, evolutionary tectonic models for the region, largely based on geological and paleomagnetic data on outcrops of magmatic rocks and ophiolites (Figure ), remain highly controversial (e.g.

The collision of oceanic arcs with continents is a common plate tectonic process in the Phanerozoic, but its recognition in the Precambrian is hampered by deformation and metamorphism. The Rio Capim volcanicplutonic-sedimentary belt lies in sharp tectonic contact with Archaean rocks of the Uauá block in the northern part of the São Francisco craton. Field relationships and high-precision geochronology indicated that the Rio Capim basalts, gabbros, diorites, and dacites were emplaced approximately at 2148-2143 Ma, and later intruded by 2128 Ma-old diorite to tonalite plutons. All rocks were metamorphosed under amphibolite to granulite facies conditions mainly between 2080 Ma and 2070 Ma, but deformation may have lasted until about 2040 Ma as estimated from syn-deformation zircon and titanite grains. The association of basalt, andesite, dacite, and their plutonic counterparts, combined with their positive ε Nd (t) values and incompatible trace element geochemical signatures similar to island arc magmas, support the proposition that the Rio Capim belt was a Palaeoproteorozoic intra-oceanic arc sequence that collided with a continent, of which the Mesoarchaean Uauá block is a remnant. The implications for the regional evolution and metallogenesis are also discussed.

Provenance studies on metasedimentary rocks of the Baixo Araguaia Supergroup of the Brasiliano Araguaia Belt, central Brazil, yield 207Pb/206Pb zircon evaporation ages for detrital zircons from quartzites concentrated around 1000–1200 Ma and 2800–2900 Ma; Sm–Nd TDM model ages of schists and phyllites scatter around 1600–1700 Ma. Facies analysis of low-grade metasedimentary rocks from drill cores suggests a sedimentary environment of basin floor and lower- to upper-slope turbidites. Nearby sources are indicated by the textural and mineralogical immaturity; together with structural geological data indicating tectonic transport of the supracrustal pile towards the NW, this suggests probable provenance from the southeastern portion of the Araguaia Belt and not from the Amazonian Craton as usually believed. The Goiás Massif, Goiás Magmatic Arc, São Francisco Craton and Paranapanema block are considered to be the best candidates. They may have formed a larger continental mass during West ...

This paper presents laser-ablation (MC)-ICP-MS single-zircon Hf isotopic data coupled with U-Pb geochronology from a unique rock inventory in southern Israel, recording ca. 300 m.y. of Neoproterozoic crustal evolution in the northern Arabian-Nubian Shield (ANS). The early, island-arc magmatic cycle in this area is recorded by detrital zircons from arc-derived metasediments, dated between 870 and 760 Ma, and by ca. 790 and 740 Ma orthogneisses. Zircon ε Hf(t) values from these rocks are all positive, supporting previous isotopic evidence for the overall juvenile nature of northern ANS crust. Nonetheless, detrital zircons from the metasedimentary units display a wide range of ε Hf(t) values between +1 and +13. The lower values in this range suggest some contribution from an older crustal component. The involvement of pre-Neoproterozoic crustal component is also indicated by the occurrence of

The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts a...

Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zone 50. All locations are quoted to at least the nearest 100 m. U-Pb measurements were conducted using the SHRIMP II ion microprobes at the John de Laeter Centre of Mass Spectrometry at

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The Archaean North Atlantic Craton of West Greenland collided at c. 1.9 Ga with a lesser-known Archaean craton to the north, to form the Nagssugtoqidian orogen. The Palaeoproterozoic metamorphic grade and strain intensity decrease northward through the orogen, allowing investigation of the reworked Archaean components in its northern part. Two Archaean supracrustal belts in this region – the Ikamiut and Kangilinaaq belts – are investigated here using field mapping, aeromagnetic data, zircon geochronology, and geochemistry. Both belts comprise quartzo-feldspathic and pelitic metasedimentary rocks, amphibolite, and minor calc-silicate rocks, anorthosite and ultramafic rocks. Pb-Pb and U-Pb dating of detrital zircons and host orthogneisses suggest deposition at c. 2800 Ma (Kangilinaaq belt) and after 2740 Ma (Ikamiut belt); both belts have zircons with Neoarchaean metamorphic rims. Metasedimentary rocks and orthogneisses at Ikamiut share similar steep REE signatures with strong LREE en...

In reconstructions of the Gondwana supercontinent, correlations of Archean domains between Madagascar and India remain debated. In this paper, we aim to establish correlations among these Archean domains using whole-rock geochemistry and U-Pb zircon geochronology of meta-granitoids from the Masora and the Antananarivo domains, central-eastern Madagascar. A meta-granitoid from the central part of Masora domain is dated at 3277 Ma and shows a typical Archean tonalite-trondhjemite-granodiorite composition, whereas a tonalitic gneiss from the southeastern part of the Antananarivo domain gives an age of 2744 Ma. The geochemical signature of this tonalitic gneiss differs from that of the~2500 Ma granitoids of the northwestern part of Antananarivo domain. In addition, the geochemical composition of the~760 Ma granitic gneisses is consistent with a volcanic-arc origin for the protolith. Based on the geochemical and geochronological results, along with existing data, we identified three episodes of granitic magmatism in central-eastern Madagascar at 3300, 2700, and 2500 Ma. These three magmatic events are consistent with those reported for the Dharwar Craton in India, suggesting that the Archean Masora and Antananarivo domains in Madagascar were part of the Greater Dharwar Craton during the period of 3300-2500 Ma. The 700-800 Ma volcanic arc granites identified in eastern Madagascar have not been reported in India. Therefore, the subduction of the oceanic plate that led to the formation of these granites likely took place at the western margin of the Greater Dharwar Craton, which included part of eastern Madagascar.

We study the link between deep geodynamic processes and their surface expression in the North Atlantic region which has an anomalous, complex structure compared to other oceans. We calculate a model of residual mantle gravity between the Charlie Gibbs Fracture Zone and Svalbard. The calculations are based on GOCE satellite data the regional crustal model EUNAseis (Artemieva and Thybo, 2013) ; for the crustal and topography effects, and the global totpgraphy and bathymetry model ETOPO1 from NOAA (Amante and Eakis, 2009). Results are complemented by sensitivity analysis of the various parameters' effects on the models. Our results identify strong heterogeneity in the upper mantle residual gravity, expressed as a sharp contrasts at the continent-ocean transition, positive mantle gravity below the continental blocks and negative -below oceanic blocks; the MOR has low-gravity anomaly. By introducing regional geochemical data and analysis of the tectonical history, we identify a strong correlation between residual mantle gravity anomalies and geochemical anomalies in Nd and Mg#. This analysis identifies three zones of North Atlantic mantle based on the correlation between upper mantle gravity and ocean floor age. In the area around Iceland, the residual mantle gravity is systematically lower than predicted from the half-space cooling model, and we estimate the thermal anomaly that could cause this shift.

We construct a density model of lithospheric mantle for the North China Craton based on analysis of gravity, seismic and thermal data. A new seismic crustal model is applied to remove the effect of the sedimentary cover and crystalline crust from observed gravity field. An updated thermal lithosphere thickness data is used to calculate density of lithospheric mantle by removing gravity effect of lithosphere depth variation. Density structure of the lithospheric mantle shows strong lateral heterogeneity (at standard P-T condition the value ranges from 3.32 g/cm 3 to 3.42 g/cm3) in different tectonic blocks of the North China Craton, and the lithospheric mantle is denser than typical Archean cratonic lithosphere mantle (3.31 ± 0.02 g/cm3). The Trans North China Orogen which is a Paleoproterozoic collision belt is characteristic by high lithospheric mantle density (3.38 -3.39 g/cm3 can up to 3.42 g/cm3). The lowest density values of 3.32 -3.36 g/cm3 are observed in the northern part of the Eastern Block, central part of the Trans North China Orogen and most of the Western Block. These low density values suggest the presence of depleted Archean -Proterozoic lithosphere mantle. The density of the lithospheric mantle in the southern part of the Eastern block is 3.36-3.38 g/cm3 with density values typical of Phanerozoic fertile mantle.

We present a new density model of the (1) European crust and (2) lithospheric mantle by integrating seismic, thermal and gravimetric data. (1) Crustal density: We obtain crustal segment thicknesses (i.e. sediments, upper crust, middle crust, lower crust and lowermost crust) and mean p-wave segment velocity (v p ) from the recent seismic crustal model EUNAseis (Artemieva and Thybo, 2013). By converting each mean crustal segment v p into mean crustal segment rock density using Brocher's empirical polynomial relationship (Brocher, 2005), we calculate mean crustal density for the entire crustal column as a weighted mean. (2) Lithospheric mantle density: Due to strong seismic anisotropy in the European lithospheric mantle, we limit the v p to rock density conversion to the crust. Instead, we estimate lithospheric mantle densities from residual mantle gravity. We use crustal segment densities to calculate the total crustal gravimetric contribution. We find a strong linear relationship between crustal thickness and crustal gravimetric contribution, and we exploit this relationship as our reference model. We remove the anomalous crustal gravimetric contribution by subtracting it from the Free-Air Anomaly in WGM2012 (Balmino et al., 2012). By implementing the thermal model TC1 (Artemieva, 2006) and by defining the base of the lithosphere as the depth where the local geotherm reaches a temperature of 1300 o C we effectively constrain the thickness of the thermal lithosphere. We remove the effect of undulating Moho and LAB (Lithosphere Asthenosphere Boundary) depth variations, and the effect of thermal expansion. From the resulting residual mantle gravity, we estimate lithospheric mantle density on a 1 x 1 degree's grid. Our results show that the thick crust in the East European Craton (East European Platform and shield and the Baltic shield) is significantly denser (2.83 ± 0.05 Mg m -3 ) than the thin crust in Western Europe (2.77 ± 0.04 Mg m -3 ). Contrastingly, the thick lithospheric mantle in Eastern Europe is slightly less dense (3.34 ± 0.03 Mg m -3 ) than the thin lithospheric mantle in Western Europe (3.36 ± 0.03 Mg m -3 ). The weighted mean density for the entire lithosphere of Eastern and Western Europe is remarkably similar with a mean density of 3.21 ± 0.03 Mg m -3 and 3.2 ± 0.04 Mg m -3 respectively. Additionally, we find very high densities (3.4 -3.46 Mg m -3 ) in the lithospheric mantle beneath the Peri-Caspian basin and the North German basin, suggesting that eclogitization processes could play a role in the formation of these basins.

The São Francisco Craton (SFC) is an Archaean-Proterozoic craton that hosts a significant number of mafic intrusions. Previous attempts to date the igneous emplacement of these rocks are limited to a few poor precision Rb-Sr and Sm-Nd isochrons. Here, dykes from the Curaçá belt, in the northeastern portion of the craton, as well as from the Chapada Diamantina region in the central part, were dated using U-Pb on baddeleyite (ID-TIMS). These dykes yielded similar baddeleyite U-Pb ages of 1506.7± 6.9 Ma (Curaçá) and 1501.0 ± 9.1 Ma (Chapada), with converging trends indicating a possible magmatic centre located near the western margin of the São Francisco Craton. The two dyke swarms are subalkaline, and have light rare earth enriched chondritenormalized patterns but the Curaçá dykes have lower Mg-number and higher abundances of most incompatible trace elements than the Chapada dykes. Geochemical modelling and trace element ratios suggest that the two dyke swarms had different petrogenesis, with the Chapada dykes being derived from more depleted mantle sources and the Curaçá dykes from enriched sources, however the mantle sources of both dyke swarms appear to have had contributions from an enriched plume. Current geochronological and palaeomagnetic data suggest a coherent São Francisco-Congo block from at least the late Mesoproterozoic until the opening of the South Atlantic (ca. 130 Ma ago). The discovery of ca. 1500 Ma intrusions in the São Francisco Craton is a major step forward to a more complete barcode record for the SFC-Congo Craton, which can be used in future palaeo-continental reconstructions.

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This paper presents laser-ablation (MC)-ICP-MS single-zircon Hf isotopic data coupled with U-Pb geochronology from a unique rock inventory in southern Israel, recording ca. 300 m.y. of Neoproterozoic crustal evolution in the northern Arabian-Nubian Shield (ANS). The early, island-arc magmatic cycle in this area is recorded by detrital zircons from arc-derived metasediments, dated between 870 and 760 Ma, and by ca. 790 and 740 Ma orthogneisses. Zircon ε Hf(t) values from these rocks are all positive, supporting previous isotopic evidence for the overall juvenile nature of northern ANS crust. Nonetheless, detrital zircons from the metasedimentary units display a wide range of ε Hf(t) values between +1 and +13. The lower values in this range suggest some contribution from an older crustal component. The involvement of pre-Neoproterozoic crustal component is also indicated by the occurrence of

Strange to say, we know much less about the world's deep ocean than we do about outer space. At the present time we have probably accessed no more than fi ve percent of it and still know even less about the estimated two million species that inhabit it. In this article I will discuss one of several voyages of exploration and scientifi c discovery in extreme undersea environments that have spanned a major part of my life, of which more than fi ve and a half years have been spent under water, completely divorced from the outside atmosphere. I was a nuclear submarine offi cer for 24 years and commander of a nuclear attack submarine for four of these years. To live for long periods in a confi ned space like that of a submarine, one must have a sense of humor and the ability to live within one's head in order to escape beyond the ships' physical restrictions. Certainly the height of my submarine career, and that of my crew, was during the summer of 1970, when USS Queenfi sh (SSN 651), under my command, surveyed the entire, uncharted, shallow, ice-covered Siberian continental shelf at the height of the Cold War. Accompanying us on this voyage as scientifi c advisors were Dr. Alan Beal and Richard Boyle of the Arctic Submarine Laboratory, San Diego. A historic feat, it has yet to be repeated. I have recently written a book about this survey entitled Unknown Waters (University of Alabama Press, 2008). In the following pages I will touch upon a few highlights of this perilous expedition, the route of which is depicted in Figure 1. BACKGROUND For centuries preceding the beginning of the 20 th , there existed a wide range of unfounded ideas as to how best to achieve the North Pole and what one might fi nd when one fi nally got there. In the late 17th century, for example, it was believed that the Arctic Ocean and North Pole were surrounded by as-yet-undiscovered, inhabited landmasses and a temperate climate. Later, it was thought that, no, massive ice ridges surrounded the Arctic Ocean and Pole. It was also believed that once these ridges were successfully traversed, a warm open sea would be found and one could literally sail or row to the North Pole. Numerous expeditions during the 19 th century did,

Central Anatolia, a transition zone between extensional regime in the west and compressional deformation in the east, is characterized as a plateu with several volcanic provinces. Within the Central Anatolian Plateu (CAP) two blocks exist as seperated by Central Anatolian Fault (CAF). These are Kırşehir Block in the north and Anatolide-Tauride Block in the south. Due to displacement along CAF, volcanic activity has been identified. Our main aim is to elucidate crustal attenuation properties linking to tectonic features in this region. Intrinsic and scattering attenuation variations provide present day properties of seismic attenuation in the crust that can hint at the type of deformation in the past/present. We utilized local earthquakes with focal depths smaller than 10 km. To estimate intrinsic and scattering attenuation parameters, we compute synthetic seismograms via an non-emprical approach that is based on acoustic radiative transfer theory to compute synthetic coda wave envelopes. A fit between observed and synthetic envelopes is performed for each individual earthquake in five different frequency bands (0.75 to 12 Hz) in order to estimate attenuation parameters. Our results suggest that, intrinsic attenuation appears to be slighty dominant over scattering attenuation in the study area. 2D spatial distributions of attenuation parameters clearly marks Kırşehir and Anatolide-Tauride blocks. Regions with high absorption values corresponds to volcanic provinces and we observe relatively low attenuation values in southeastern parts.

Miles wide and miles deep" e Exploring the depth and breadth of geoscience during the first ten years of Geoscience Frontiers When early explorers first crossed the Platte River in what is now Nebraska (USA), it was said the river was "a mile wide and an inch deep" (Mokler, 1923; Smith, 1971). This phrase was used to describe not only the difficulty in crossing the river but also in navigating its length. The trouble with a river being too wide is the risk that it won't provide the depth necessary to be useful. The same thing can be said of multidisciplinary scientific journals. While a journal can claim to be multidisciplinary, there is a risk of it being so broad that its articles can only engage the reader at a superficial level. Nothing could be further from the truth with Geoscience Frontiers. Over the past ten years, this journal has successfully navigated the wide breadth of geoscience while providing a level of depth and detail that rivals discipline-specific journals. From humble beginnings, Geoscience Frontiers quickly rose to prominence and has maintained an impact factor above 4 for the past three years. Under the direction of Prof. Xuanxue Mo and Prof. M. Santosh, and a multinational editorial board, the journal has attracted articles from a wide array of prominent scientists. Currently, Geoscience Frontiers is ranked in the top 10% of multidisciplinary geoscience journals worldwide by the Thomson Reuters InCites Journal Citation Reports. This Virtual Special Issue has been put together to celebrate the 10th Anniversary of Geoscience Frontiers and highlights the breadth and depth of the scope of the journal. As our understanding of the Earth grows, the fields of geoscience reveal a profound level of interconnection. The future of geoscience is consequently integrative and as geoscientists, we have an opportunity to reap the benefits of the synergy a multidisciplinary geoscience journal provides. The future of our planet also requires a responsible use of the Earth's resources and the protection of its environments that make up the biosphere, atmosphere, hydrosphere and geosphere. Geoscientists are key custodians in this role, not just in scientific research, but in dissemination of this science to the public and to the policy-makers. Peer-reviewed journals provide the major route for the publication of scientific results, and have a major role to play in scientific communication, dissemination and discussion. Geoscience Frontiers is a forward-thinking, free-toall, open-access journal that promotes the publication of research from all nations and is accessible around the world. With Geoscience Frontiers we can be confident that we are 'doing our part' for the progression of science. Articles in this issue have been selected from sub-disciplines spanning the spectrum from modern methodologies of machine

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Supercontinent Pangea was preceded by the formation of Gondwana, a “megacontinent” about half the size of Pangea. There is much debate, however, over what role the assembly of the precursor megacontinent played in the Pangean supercontinent cycle. Here we demonstrate that the past three cycles of supercontinent amalgamation were each preceded by ∼200 m.y. by the assembly of a megacontinent akin to Gondwana, and that the building of a megacontinent is a geodynamically important precursor to supercontinent amalgamation. The recent assembly of Eurasia is considered as a fourth megacontinent associated with future supercontinent Amasia. We use constraints from seismology of the deep mantle for Eurasia and paleogeography for Gondwana to develop a geodynamic model for megacontinent assembly and subsequent supercontinent amalgamation. As a supercontinent breaks up, a megacontinent assembles along the subduction girdle that encircled it, at a specific location where the downwelling is most ...

Gondwana margin recorded in zircons from the basal units of Galicia, NW Magmatism and early-Variscan continental subduction in the northern Email alerting services articles cite this article to receive free e-mail alerts when new www.gsapubs.org/cgi/alerts click Subscribe America Bulletin to subscribe to Geological Society of www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click official positions of the Society. citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect presentation of diverse opinions and positions by scientists worldwide, regardless of their race, includes a reference to the article's full citation. GSA provides this and other forums for the the abstracts only of their articles on their own or their organization's Web site providing the posting to further education and science. This file may not be posted to any Web site, but authors may post works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent their employment. Individual scientists are hereby granted permission, without fees or further Copyright not claimed on content prepared wholly by U.S. government employees within scope of Notes articles must include the digital object identifier (DOIs) and date of initial publication.

Subduction of tectonic plates into Earth's mantle occurs when one plate bends beneath another at convergent plate boundaries. The characteristic time of deformation at these convergent boundaries approximates the Maxwell relaxation time for olivine at lithospheric temperatures and pressures, it is therefore by definition a viscoelastic process. While this is widely acknowledged, the large‐scale features of subduction can, and have been, successfully reproduced assuming the plate deforms by a viscous mechanism alone. However, the energy rates and stress profile within convergent margins are influenced by viscoelastic deformation. In this study, viscoelastic stresses have been systematically introduced into numerical models of free subduction, using both the viscosity and shear modulus to control the Maxwell relaxation time. The introduction of an elastic deformation mechanism into subduction models produces deviations in both the stress profile and energy rates within the subduct...

A 2-D absolute shear wave velocity model has been resolved beneath a seismic profile across the northeastern margin of the Arabian Plate-Central Iran by simultaneously inverting data from P receiver functions and fundamental mode Rayleigh wave phase velocity. The data were gathered by a linear seismic array crossing the Zagros fold and thrust belt, Urmia-Dokhtar magmatic arc and Central Iran block assemblage as three major structural components of the Arabia-Eurasia collision. Our model shows a low-velocity tongue protruding from upper to lower crust which, north of the Zagros suture, indicates the signature of an intracontinent lowstrength shear zone between the underthrusting and overriding continents. The velocity model confirms the presence of a significant crustal root as well as a thick high-velocity lithosphere in footwall of the suture, continuing northwards beneath the overriding continent for at least 200 km. These features are interpreted as underthrusting of Arabia beneath Central Iran. Time to depth migration of P receiver functions reveals an intracrustal flat interface at ∼17 km depth south of the suture; we interpret it as a significant decoupling within the upper crust. All these crustal scale structural features coherently explain different styles and kinematics of deformation in northern Zagros (Lorestan zone) with respect to its southern part (Fars zone).

The geometry of intra-continental lithosphere boundaries along the Zagros orogenic belt in the Arabia-Eurasia collision is investigated by means of teleseismic data. The data are gathered over a seismic linear profile extending across south Zagros, the Sanandaj-Sirjan metamorphic zone, the Urumieh-Dokhtar magmatic arc, Central Iran, and the Kopeh Dagh-Binalud mountains. We exploit the P and S receiver functions leading to map the geometry of the crustal and subcrustal interfaces. The migrated depth sections reveal an abrupt crustal thickening and a gentle crustal thinning ~60 km north and ~30 km south of the Zagros suture, respectively. Associated to the buckled antiformal Moho south of the suture, a deeper synform in the lithospheric lid of the lower Arabia plate is shown by migrated depth sections affecting the lithospheric mantle of the Arabia plate beneath the suture zone. This geometry implies an unexpected intra-lid decoupling. These features imply that the Central Iran lithosphere acts as a relatively strong backstop producing significant internal deformation expressed by shortening and thickening at the edge of the Arabian lithosphere. The 410 km and 660 km transition zones are imaged by P to S converted phases and showed lateral continuity implying an originally low dip angle subduction of the oceanic Arabian plate beneath Central Iran.

The generation of continental crust, its bulk composition and temporal evolution provide important records of plate tectonics and associated magma-generating processes. However, the long-term integrated effects of repeated magmatic events on crustal growth, composition and differentiation and, therefore, on crustal evolution are rarely considered. Here, we examine long-term (∼350 Myr) temporal compositional trends of granitic magmatism within a limited (∼200 km × 100 km) area in the Northern New England Orogen of Queensland, Australia to avoid lateral crustal variations in order to understand how temporal–compositional variations of silicic igneous rocks record crustal evolution. Long-term temporal compositional variations are tracked using whole-rock chemistry, zircon chronochemistry and zircon Hf isotopic compositions. We particularly focus on whole-rock U, Th and K abundances and calculated heat-production values as proxies for crustal evolution, and tracking crustal sources invo...

Origin and tectonic setting of the late Neoproterozoic Dokhan Volcanics (ca. 610-560 Ma by whole rock Rb-Sr and ca. 600-590 Ma by SHRIMP U-Pb zircon) in the Egyptian Eastern Desert is debated. Debate concerns the tectonic setting they formed in, during transition between convergent to extensional, during or after collision of E and W Gondwana ∼600 Ma. In order to solve this problem, we studied the geology and analyzed major and trace elements of lavas from Wadi Um Sidra and Um Asmer in the northern Eastern Desert. These range from medium-to high-K basalt to rhyolite. They comprise voluminous medium-to high-K calc-alkaline lavas, subordinate adakitic lavas, and minor alkali basalt. Adakitic lavas contain very low Y (<18 ppm) and heavy REE (HREE) (Yb ≤ 1 ppm) contents, with high Sr (>767 ppm) and Sr/Y (>40). They also have high Cr and Ni and low Nb, Rb, and Zr contents compared to coexisting calc-alkaline lavas. The adakitic lavas exhibit a fractionated rare earth element (REE) patterns with HREE depletion (La N /Lu N = 11.1-14.4), while calc-alkaline rocks have less fractionated REE patterns (La N /Lu N = 6.3-9.4). REE patterns of the alkali basalt are also similar to that of adakitic lavas. These alkali basalt, calc-alkaline, and adakitic lavas all show enrichment in large ion lithophile elements (LILEs) and show negative Nb-Ta anomalies with high Th/Zr, which suggest involvement of subducted slab melts in their petrogenesis. Our proposal is subduction of hot oceanic ridge before collision and later melting of the hot oceanic slab beneath Dokhan even after collision event.

Temporal trends in granitoid chemistry and thermometry constrain major global changes in magmatism, tectonism or crustal thickness in the continents. Our study relies on zircon geochronology and trace element geochemistry on four new detrital rocks (two modern sediments and two Archean metasedimentary rocks) and a global compilation of published single zircon detrital chronology and trace chemistry data acquired on 5587 individual grains. Zircons of all ages from 4.4 Ga to present exist in this archive. Ti-in-zircon thermometry indicates that more than 98% of the grains with concordant U-Pb ages formed at temperatures exceeding 650 • C. The great majority of these zircons formed in the 650-850 • C range consistent with growth in intermediate to silicic magmas. Magmatic temperatures increased over time for the first 1.2 Ga of Earth's history after which they stayed constant before decreasing during the more recent past. U/Th < 5 values in the overwhelming majority of grains are consistent with a magmatic origin. La/Yb, Sm/Yb and Eu/Eu* values are relatively constant throughout the history of the Earth suggesting that most granitoids formed at, or evolved from magmatic reservoirs located at depths of 35-45 km in the presence of amphibole, garnet and limited plagioclase. Such reservoirs are common today in hot deep crustal environments beneath some of the thicker island arcs and all continental arcs along subduction zones. Processes other than modern day style subduction may have contributed to the formation of granitoids in the early Earth but temperatures, depths and the presence of water arbitrated by the presence of amphibole were similar. These results also suggest that the thickness of continental crust in areas that produced granitoids was similar to today's global average throughout the 4.4 Ga time period covered by the zircon archive. There is no correlation between zircon chemistry over time and the assembly of supercontinents.

The mid-Holocene sea-level highstand refers to the development of higher-than-present relative sea levels (RSLs) in far-field regions between 7,000 and 4,000 years ago because of equatorial ocean syphoning and continental levering. The timing, magnitude and spatial variability of the highstand are uncertain and the highstand parameterization in Glacial Isostatic Adjustment (GIA) modelling is understudied. Here, we use the RSL records of Southeast Asia to investigate the sensitivity of the mid-Holocene highstand properties to ice and Earth model parameters, including lithospheric thickness, mantle viscosity (both 1D and 3D), and deglaciation history of Antarctica and global ice sheets. We found that the Earth model variation only affects the magnitude of the mid-Holocene highstand unless low upper mantle viscosity is used. The timing of the highstand moves towards present and there is an absence of the highstand if upper mantle viscosity is &lt;4.0 ×1019 Pa s or ≤1.0 ×1019 Pa s, resp...

Understanding how the Earth's first continental land masses were generated is important because the processes responsible directly affected the evolution of the planet's primordial silicate interior, and also its atmosphere and hydrosphere. Archaean continental crust is dominated by rocks of the trondhjemite-tonalite-granodiorite (TTG) suite. These can be divided into (1) a mid-to late Archaean ($3Á5-2Á5 Ga) suite with low SiO 2 and high MgO, Sr and transition element contents, and (2) an Eoarchaean (>3Á5 Ga) suite with higher SiO 2 and lower MgO, Sr and transition element concentrations. Cenozoic adakites are considered to be compositionally similar to mid-to late Archaean ($3Á5-2Á5 Ga) TTGs, but not the oldest TTG rocks. Conversely, a suite of Early Eocene adakite-like rhyodacites (Jamaican-type adakites: JTA) from Jamaica are shown to be geochemically similar to the Eoarchaean TTGs. In contrast to newly discovered JTA-like rocks (Ryozen low Sr/Y) in Japan, new trace element and Nd-Hf radiogenic isotope data in this study confirm that the Jamaican JTA cannot be formed by complex mixing, assimilation and fractional crystallization processes. New partial melt models here explore several different source compositions (mid-ocean ridge basalt, ocean island basalt and oceanic plateau), mineral modes, melt modes and partition coefficients. The results of these models clearly demonstrate that the JTA and the Eoarchaean TTG can be generated by partial melting of plagioclase-and garnet-bearing amphibolite source regions with oceanic plateau-like compositions. Further modelling shows that the JTA and Eoarchaean TTG low MgO and transition element abundances can be derived from two dominant processes: (1) relatively shallow partial melting of subducting oceanic crust (compositionally similar to Mesozoic oceanic plateau basalt) whereby the slab melts ascend without interacting with a mantle wedge; (2) partial melting of oceanic plateau-like subducting oceanic crust followed by interaction of the slab melts with a thin and/or discontinuous (boudinage-like?) mantle wedge whereby the expected increase of MgO, Ni, and Cr in the slab melts is obliterated by fractional crystallization of ferromagnesian minerals (mostly amphibole). Consequently, using the JTA as a modern analogue for Eoarchaean TTG production, we propose the existence of subduction zones consuming oceanic plateau-like oceanic crust in Eoarchaean times.

at the Missouri Geological Survey, Rolla, Mo for their collaborative insights and making their knowledge and facilities available to this investigation. Thanks to R.M. Pillers for help with mineral separations, scanning electron microscope work, and sensitive high resolution ion microprobe analysis. Thanks also to J.D. Horton who determined areal extents of individual igneous rock types. Constructive reviews by K.E. Watts and A.K. Gilmer are much appreciated and helped clarify data presentation and interpretation.

The Guaviare Complex is a new unit defined in the Colombian Amazonian Craton, which is part of the Precambrian basement located in southeastern Colombia. It is divided into three units according to their textural and compositional characteristics, termed Termales Gneiss, Unilla Amphibolite, and La Rompida Quartzite. Termales Gneiss rocks are petrographically classified as gneisses and quartz-feldspar granofels, with the local formation of blastomylonite-like dynamic rocks. The Unilla Amphibolite consists of only amphibolites, and La Rompida Quartzite consists of muscovite quartzites, quartz-feldspar granofels, and quartz-muscovite schists. The protoliths of Termales Gneiss and Unilla Amphibolite were formed in the Mesoproterozoic at 1.3 Ga due to bimodal magmatism (felsic and mafic) derived from mantle material, with some crust contamination that was probably related to the formation of extensional arcs associated with trans-arc basins in the NW section of the Amazon Craton. La Romp...