Subduction Of Oceanic Ridge Research Articles

Ancient slabs beneath Arctic and surroundings: Izanagi, Farallon, and in-betweens

A detailed 3-D tomographic model of the whole mantle beneath the northern hemisphere (north of ~ 30°N latitude) is obtained by inverting a large amount of P-wave arrival time data (P, pP, and PP) to investigate transition of subducted slabs beneath Eurasia–Arctic–North America. We apply an updated global tomographic method that can investigate the whole mantle 3-D structure beneath a target area with high resolution comparable to that of regional tomography. The final tomographic model is obtained by performing independent calculations for 12 different target areas and stitching together the results. Our model clearly shows the subducted Izanagi and Farallon slabs penetrating into the lower mantle beneath Eurasia and North America, respectively. In the region from Canada to Greenland, a stagnant slab lying below the 660-km discontinuity is revealed. Because this slab has a texture that seems to be due to subducted oceanic ridges, the slab might be composed of the Farallon and Kula slabs that had subducted during ~60–50 Ma. During that period, a complex rift system represented by division between Canada and Greenland was developed. The oceanic ridge subduction and hot upwelling in the big mantle wedge above the stagnant slab caused a tensional stress field, which might have induced these complex tectonic events.

Tearing on the southward subducting Kelameili oceanic lithosphere in the early Devonian: Evidence from the magmatism in the Harlik arc, southern Altaids

The coexistence of adakites, high-Mg andesites (HMAs) and Nb-enriched basalts (NEBs) records special subduction geodynamics, for instance, subduction initiation, subduction of oceanic ridges or slab tearing, which are widespread in the circum-Pacific subduction zone. The Altaids in Central Asia shared similar tectonic evolution to the western Pacific. However, slab tearing has not been widely reported, which hinders a better comparison between the Altaids and the western Pacific. In this study, we report the contemporaneous early Devonian granodiorites and mafic microgranular enclaves (MMEs) in the Sidingheishan area of the Harlik arc, southern Altaids. The granodiorites have high Sr (534–683 ppm) but low Y (7.79–10.2 ppm) and Yb (0.71–1.68 ppm) contents, with corresponding high Sr/Y values (29.78–76.93). In combination with their juvenile isotopes (initial 87Sr/86Sr = 0.70333–0.70356, εNd(t) = +7.4 − +8.01) and high Mg# numbers (50.9–53), they indicate that their primitive magmas were derived from slab melting. The MMEs show similarly high Sr contents (507–696 ppm), and identically juvenile isotopic signatures (initial 87Sr/86Sr = 0.70346–0.70356, εNd(t) = +7.14 − +7.5) to their host granodiorites. However, their relatively low silica contents (55–60 wt%), evolved Mg# numbers (44.8–55.2) and high Nb contents (12.7–24.7 ppm) demonstrate that they are the fractionation products of NEBs, which were produced by the partial melting of the mantle that was previously metasomatized by these adakitic magmas. Considering all the available evidence, we propose a slab tearing model in which the southward subducting Kelameili oceanic lithosphere underneath the Harlik arc underwent rollback in the early Devonian and its differential rollback rates caused slab tearing. Subsequent asthenosphere upwelling stimulated the Kelameili oceanic lithosphere to melt, producing the adakitic magmas, some of which intruded the crust after limited interaction with the mantle peridotites and underwent crystallization of dominant amphibole in the crust. The rest reacted completely with the mantle peridotites, which underwent further melting to produce the Nb-enriched basaltic magmas. They intruded into the adakitic magma chamber after fractional crystallization of olivine, clinopyroxene and garnet in the deep, forming the MMEs and promoting the remobilization of those hornblendites. This study reveals the development of slab tearing that are closely associated with slab rollback in the Altaids, providing further evidence that argues for the similarity between the Altaids and the western Pacific.

Petrogenesis and geodynamic mechanisms of the Late Cretaceous magmatic ‘flare-up’ in the southern Lhasa Terrane, Tibet

Exploring magmatic ‘flare-up’ events can be used to unravel dynamic Earth processes including plate subduction and crustal growth. However, the cause and mechanism of magmatic ‘flare-up’ remain unresolved. The abundant magmatic records that are preserved in the Lhasa Terrane on the southern Tibetan Plateau make it an important region to study the episodic nature of the secular magmatism. This paper presents geochronological, mineralogical, and geochemical data that describes the Dazhuka diorites in the central-western and Sangri granites in the eastern parts of the southern Lhasa Terrane. Zircon LA–ICP–MS U Pb data indicate that these rocks were generated around ~95–90 Ma. Both are enriched in large ion lithophile elements and depleted in high field strength elements, indicating geochemical affinity with subduction-related magmatic rocks. The Dazhuka diorites have low SiO 2 contents (54.49–56.72 wt%), with moderate Mg# (41.7–42.7) and rare earth element fractionation ((La/Yb) N = 6.9–8.6), and homogeneous whole-rock 87 Sr/ 86 Sr i (0.704204–0.704480), ε Nd (t) (+3.5 − +4.1), and zircon ε Hf (t) (+10.2 − +13.3). These features suggest that the Dazhuka diorites originated in the spinel-garnet transition zone within the lithospheric mantle of a mantle wedge environment. The Sangri granites have high SiO 2 (64.36–69.54 wt%), Al 2 O 3 (14.27–15.61 wt%), and (La/Yb) N (4.6–49.6) and Sr/Y (34.7–119.3) ratios, indicating geochemical affinity with adakites. Low MgO (1.3–2.22 wt%) and depleted Sr-Nd-Hf isotopes (whole-rock 87 Sr/ 86 Sr i = 0.704099–0.705453, ε Nd (t) = +2.9 − +3.5, zircon ε Hf (t) = +7.1 − +9.3) suggest that the granites were derived from oceanic crust. The types of magmatic rocks that are distributed over the central-western and the eastern parts of the southern Lhasa Terrane differ, with higher magmatic temperatures in the east as compared to the central-west. These results indicate that different areas in the southern Lhasa Terrane may have experienced different geodynamic processes, with oceanic ridge subduction in the eastern part of the southern Lhasa Terrane and normal oceanic plate subduction in the central-western part. These different types of subduction processes are likely to have caused the magmatic ‘flare-up’ and variation in the magmatism of southern Tibet. • Both Dazhuka diorites and Sangri granites formed in early Late Cretaceous. • Dazhuka diorites were derived from the lithospheric mantle. • Sangri granites with adakitic affinity were sourced from the oceanic crust. • Multiply Neo-Tethys Ocean plate subduction processes result in magmatic ‘flare-up’.

Three‐Dimensional Variation of the Slab Geometry Within the South American Subduction System

AbstractSubduction of the Nazca plate results in the uneven distributions of earthquakes and arc volcanoes along the South America's western margin. Here, we construct a high‐resolution shear‐wave velocity model from immediately offshore to the backarc in South America, using advanced full‐wave ambient noise tomography. Our new model confirms and provides further constraints on three major features, including (a) extensive low‐velocity anomalies within the continental crust, (b) two high‐velocity flat slab segments located beneath southern Peru and central Chile, and (c) complex slab geometry at flat‐to‐normal transitional subduction. The flat slab segments roughly correlate with the volcanic gaps but not with the seismicity gaps. We suggest that variations of slab geometry along strike and down dip have significantly modified the flow patterns within the mantle wedge. Subduction of oceanic ridges has altered the slab dehydration processes, which can influence the distribution of arc volcanism and the occurrence of intermediate‐depth earthquakes.

Early Cretaceous magmatism of the Southern Qiangtang terrane and its tectonic significance

The tectonic evolution of the Bangong-Nujiang Tethys is known as the most important event in the late Mesozoic in the central Qianghai-Tibet Plateau. Magmatism is a probe and window for detecting deep crust, which also records the process of tectonic evolution. The magmatic arc formed at Early Cretaceous in the Southern Qiangtang subterrane, was products of the Mesozoic magmatism during the long-term subduction of the Bangong-Nujiang Tethys oceanic lithosphere and the subsequent Qiangtang-Lhasa terrane collision. This article summarizes zircon U-Pb geochronology, major and trace elements, and Sr-Nd-Pb-Hf isotopic data for the Early Cretaceous magmatic rocks located in the southern Qiangtang subterrane of central Tibet. However, there are three hypotheses about the tectonic background of the Early Cretaceous magmatism of the South Qiangtang subterrane: (1) oceanic ridge subduction; (2) slab roll-back; (3) slab break-off.

Late Carboniferous to Early Permian ridge subduction identified in the southeastern Central Asian Orogenic Belt: Implications for the architecture and growth of continental crust in accretionary orogens

The Central Asian Orogenic Belt (CAOB) was established in response to multiple subduction-accretion-collision processes. However, the origin and evolution of the accreted arc terranes remain unclear, as exemplified by the question how ancient and juvenile materials were accreted and constructed in the accretionary belts. This question is addressed here by presenting in-situ zircon UPb and Hf isotopic, whole-rock elemental and SrNd isotopic analyses of the four Late Paleozoic igneous rock units from Xilinhot in the southeastern CAOB. The new geochronological and geochemical data support the existence of two Late Carboniferous to Permian magmatic arc systems — Ulistai active continental margin and Baolidao arc belt. These arc systems are characterized by magmatic activity extending from 330 to 250 Ma, with two magmatic “flare-ups” (325–295 Ma vs. 295–270 Ma). The first magmatic “flare-up” (325–295 Ma) includes high-K calc-alkaline rhyolites, dacites and alkali-feldspar granites, which can be interpreted to have originated from re-melting of the juvenile lower crust beneath a continental arc induced by the Paleo-Asian oceanic ridge subduction. The second magmatic “flare-up” (295–270 Ma) reveals a seaward migration of magmatism due to slab rollback subsequent to ridge subduction. The Middle to Late Permian shoshonitic andesites witnessed the closure of the Hegenshan ocean during this period. In combination with previously reported Sr-Nd-Hf isotopic data, we suggest that the Baolidao arc belt and some other arc terranes (e.g., Bainaimiao arc) in the southeastern CAOB were built upon Precambrian microcontinents that have been largely fragmented and significantly reworked. In summary, ridge subduction accompanied by slab rollback played an important role in the rejuvenation of microcontinents and accretion of arc terranes in the southeastern CAOB.

Geodynamics of the divergent double subduction along the Bangong-Nujiang tethyan suture zone: Insights from late mesozoic intermediate-mafic rocks in central Tibet

Bangong-Nujiang Suture Zone (BNSZ) in central Tibet plays an important role in evaluating the formation and uplift mechanism of Tibetan Plateau. However, its Mesozoic tectonic evolution is ambiguous and intensely debated. In this study, Early Cretacesous adakites and sodium-rich arc rocks are identified in Western Qiangtang (WQ) and Northern Lhasa (NL) sub-terranes. Forty-four adakite samples from both WQ and NL have akin geochemical features, and are derived from partial melting of subducted oceanic crust with amphibole residual. Nineteen sodium-rich samples originated from a mixed source region between crustal sediment and enriched lithospheric mantle. These two parallel arc belts separated by the Bangong-Nujiang Suture Zone (BNSZ) represent the divergent double subduction of the Bangong-Nujiang Tethyan Ocean (BNTO). Combined with the previous studies, our new data suggest three significant magmatic flare-ups at ∼240–140 Ma, 135–105 Ma and 92–60 Ma in the WQ and BNSZ, and two at 135–105 Ma and 92–60 Ma in the NL. These asymmetrical magmatic activities indicate that the southern subduction may have commenced at about 135 Ma and experienced slab breakoff at the latest Early Cretaceous, and the northern subduction could trace back to L-Triassic (228 Ma) and experienced episodic low-angle subduction, slab rollback (190-140 Ma) and oceanic ridge subduction (135-100 Ma). The 100–92 Ma magmatic gap, 92–60 Ma magmatic flare-up and L-Cretaceous angular unconformities indicate that the double-sided subduction of the BNTO resulted in soft collision with oceanic lithosphere detachment.

Late-stage southwards subduction of the Mongol-Okhotsk oceanic slab and implications for porphyry Cu[sbnd]Mo mineralization: Constraints from igneous rocks associated with the Fukeshan deposit, NE China

Multistage igneous rocks identified in the newly discovered Fukeshan porphyry CuMo deposit, NE China give new insights into the late-stage southwards subduction of the Mongol-Okhotsk oceanic slab (MOOS) and CuMo mineralization during the late Mesozoic. The UPb ages of zircons from medium-grained monzogranite, coarse-grained monzogranite, ore-related quartz diorite porphyry, quartz monzonite, ganodiorite porphyry, diorite porphyry and andesitic porphyry associated with the Fukeshan deposit are 192.7 ± 1.9, 192.2 ± 2.7, 148.7 ± 0.8, 148.8 ± 0.9, 144.1 ± 1.1, 144.9 ± 0.9, and 144.8 ± 1.3 Ma, respectively. The Early Jurassic coarse-grained monzogranite, Late Jurassic quartz monzonite and quartz diorite porphyry, and Early Cretaceous granodiorite porphyry are characterized by low Yb and Y contents, and high Sr/Y ratios, indicating adakite affinities. The whole-rock geochemistry, and SrNd and zircon Hf isotopic features of the coarse-grained monzogranite and granodiorite porphyry indicate they were derived from the partial melting of a thickened basaltic lower crust. On the other hand, the quartz monzonite and quartz diorite porphyry probably originated from the melting of oceanic crust together with assimilation of enriched mantle and continental crust. The medium-grained monzogranites were possibly derived from a basaltic lower crust at shallower depths, and the diorite porphyry and andesitic porphyry probably originated from a metasomatized mantle source. Considering the Early Jurassic–Early Cretaceous magmatism and regional tectonic evolution of the Erguna Block, we propose that the MOOS was subducted towards the south during the Early Jurassic (195–170 Ma), slab retreat occurred during the early Late Jurassic (165–155 Ma), oceanic ridge subduction took place during the Late Jurassic (150–147 Ma). The final closure of the Mongol-Okhotsk Ocean occurred during the Early Cretaceous (145–143 Ma). The southwards subduction of a ridge on the MOOS provided a favorable tectonic setting for CuMo mineralization, and this is accordance with the discovery of many Late Jurassic–Early Cretaceous porphyry CuMo deposits in the eastern part of the Erguna Block.

Late Cretaceous volcanic rocks in the Sangri area, southern Lhasa Terrane, Tibet: Evidence for oceanic ridge subduction

Late Cretaceous magmatism in the southern Lhasa subterrane, Tibet, provides critical constraints on the subduction processes of the Neo-Tethyan oceanic plate. Here, we provide data on the whole-rock geochemistry, zircon UPb ages, trace element contents, and Hf isotopes of early Late Cretaceous volcanic rocks from Sangri County to Sangye Temple (SS) along the southern Lhasa subterrane. The SS volcanic rocks include basalts, andesites, and dacites. SIMS and LA–ICP–MS zircon UPb data indicate that the SS dacites were erupted at ca. 95 Ma, coeval with the early Late Cretaceous “flare-up” event in the southern Lhasa subterrane. The SS volcanic rocks belong to the calc-alkaline to high-K calc-alkaline series and have the geochemical features of subduction-related volcanic rocks with negative Nb, Ta, and Ti anomalies. The basalts have similar rare earth element (REE) patterns to E-MORB and are characterized by high Zr abundances (65–190 ppm) and Zr/Y ratios (3.3–6.5). They were possibly derived by high-degree partial melting of mantle peridotite that had been metasomatized by subduction-related fluids with an input of asthenospheric components. Positive whole-rock εNd(t) (+2.9 to +3.2) values for the andesites, and their variable Mg# values (35–61) and MgO (2.29–7.32 wt%), Cr (12–232 ppm), and Ni (10–127 ppm) contents can be attributed to medium-degree partial melting of mantle peridotite followed by fractional crystallization of clinopyroxene and olivine. The calc-alkaline SS dacites are similar to adakites with high Sr (663–1188 ppm) and low heavy rare earth element (HREE) and Y (10.7–11.5 ppm) contents. These adakitic dacites have positive whole-rock εNd(t) (+2.9 to +3.4) and zircon εHf(t) (+8.4 to +14.9) values with relatively high values of Mg# (36–57) and high contents of compatible elements, indicating that they were derived from the partial melting of subducted oceanic slab, and with the ascending magma interacting with mantle wedge peridotite. Our new data indicate a distinct rock association of coeval asthenosphere-component-involved basalts and slab-derived adakitic dacites. Such an association, together with the coeval high-temperature charnockites and high-temperature granulite-facies metamorphism of the charnockitic country rocks in the southern Lhasa subterrane, indicates an anomalous input of materials and heat during the early Late Cretaceous “flare-up” event. Such an event can probably be attributed to the northwards subduction of a Neo-Tethyan oceanic ridge that allowed for contributions from upwelling asthenosphere that differed from the typical results of normal-angle subduction or low-angle/flat subduction and subsequent slab rollback.

Geochemistry and tectonic implications of Early Permian granitic rocks in the Xingxingxia area of Chinese Central Tianshan Arc Terrane

The Chinese Central Tianshan Arc Terrane (CTA) in the southern Central Asian Orogenic Belt is characterized by voluminous Palaeozoic to Mesozoic (500–240 Ma) granites, with a magmatic “flare‐up” at Early Permian time (ca. 280 Ma). However, the tectonic setting of the Early Permian granites in the CTA is highly debated. In this study, Early Permian (ca. 280 Ma) granitic intrusions, including quartz diorite and dioritic enclave, amphibole granite, biotite granite, dioritic porphyry dyke, and a weakly mylonitic granitic dyke, were revealed from the Xingxingxia area of the CTA by LA‐ICP‐MS zircon UPb dating. These granitic rocks show magnesian and calc‐alkaline affinities and are characterized by Nb, Ta, P, Ti, and MREE depletion [(Dy/Yb)N = 1.02–1.51] with no significant Eu anomalies, suggesting geochemical characteristics of typical active continental margin magmatic rocks. Zircon trace element geochemistry of these Early Permian granitic rocks also indicates a continental arc setting with high U/Yb and Gd/Yb ratios. Among these granitic intrusions, the dioritic porphyry dyke has negative zircon εHf(t) values (−8.0 to −5.7), suggesting that the magma was produced by partial melting of the ancient crustal basements of the CTA. The rest intrusions show positive but largely varied zircon εHf(t) values of +4.1 to +10.9 (5 samples), suggesting that the magmas were probably derived from the mantle wedge with variable contributions from ancient crustal component during the magma evolution. These granitic intrusions were coeval with high‐temperature metamorphic rocks, mafic‐ultramafic rocks, and Cu‐Ni sulphide deposits in the CTA. Therefore, we propose a geodynamic model of oceanic ridge subduction of the South Tianshan Ocean during Early Permian involving upwelling of asthenospheric mantle through slab window and partial melting of the mantle wedge. The multiple‐derived magmas, including those from mantle wedge and/or ancient crustal basement, and their blendings formed the studied Early Permian granitic associations in the CTA.

Slab-derived adakites and subslab asthenosphere-derived OIB-type rocks at 156 ± 2 Ma from the north of Gerze, central Tibet: Records of the Bangong–Nujiang oceanic ridge subduction during the Late Jurassic

This paper reports zircon U–Pb age and Hf isotope, whole-rock major and trace element, and whole-rock Sr–Nd–Hf isotope data of the dacites from Rena Tso and mafic rocks (diabases and basalts) from Duobuza, north of Gerze, central Tibet. These data reveal the presence of a distinct rock association of slab-derived adakites (154±1Ma) and subslab asthenosphere-derived OIB-type (oceanic island basalt) mafic rocks (157.6±1.4Ma). The medium-K calc-alkaline dacites (SiO2=66–69wt.%) from Rena Tso are enriched in Sr (520–1083ppm) and depleted in heavy rare earth elements (HREEs) and Y (9.8–10.8ppm), resembling adakites. These adakitic dacites have low whole-rock initial 87Sr/86Sr ratios of 0.7043–0.7046, positive εNd(t) (+1.0 to +3.4), εHf(t) (+6.4 to +7.0), and zircon εHf(t) (+1.9 to +7.6) values, indicating an oceanic slab origin (crust and sediment). Considering the low Mg# (32–53) and (La/Yb)N (19–23), the adakitic dacites are most likely derived from the partial melting of the subducting slab at shallow depths and the subsequent interaction with peridotite in a thin mantle wedge during magma ascent. The diabases and basalts (SiO2=49–53wt.%) from Duobuza show an alkali signature with enrichment of high field strength elements (HFSEs) (e.g., Zr=213–285ppm) and exhibit positive Nb–Ta–Ti anomalies that are geochemically comparable to those of OIB. These samples show positive whole-rock εNd(t) values of +3.3 to +3.7, εHf(t) values of +4.7 to +5.7, and negative to positive zircon εHf(t) values of −1.5 to +5.2. These OIB-type mafic samples are interpreted as the products of low-degree decompression melting of the upwelling subslab asthenosphere with a minor contribution from the sub-continental lithospheric mantle (SCLM). Our new data indicate the presence of a distinct rock association of coeval slab-derived adakites and subslab asthenosphere-derived OIB-type rocks. Such an association along with the normal arc rocks further to the north of the Bangong-Nujiang suture zones (BNSZ) records the northward ridge subduction of the Tethyan Bangong-Nujiang Ocean at ca. 156±2Ma.

Subduction consequences along the Andean margin: thermal and topographic signature of an ancient ridge subduction in the Marañón Basin of Perú

All along the eastern border of the Andes lie foreland basins that are among the most prolific hydrocarbon provinces of the world. Their Cenozoic evolution was controlled by the Andean uplift and its consequences on deformation and sedimentation. In turn, the Andean uplift results from the interplay between the subducting Nazca oceanic plate and the South American continental plate. Although the process exists all along the margin, the subducting plate is not regular including bathymetric anomalies and segments that result in different response in the deformation and active volcanism of the overriding plate. In the Maranon Basin of Peru, evidences allow documenting the consequences of a topographic anomaly subduction on the thermal regime and deformation of the Andean foreland during the Neogene. In this basin, a maturity anomaly is difficult to explain by considering only the present day thermal regime. However, it spatially coincides with the trace of a lost subducting ridge, the Inca Plateau. Other features like differential uplift and erosion can be related to the same event in the area. We review the consequences of oceanic ridge subduction along the Andean margin. Their effects on the deformation and volcanism of the forearc and arc regions have been extensively described. Their influence on the present day foreland topography is testified by the existence of giant alluvial fans and displaced terraces. Their effect on magmatism and ore deposits formation has also been demonstrated. The example illustrated here shows that their influence on thermal regime, deformation, erosion and ultimately on petroleum systems must also be taken into account in the search for hydrocarbons in subduction related basins. All along the eastern border of the Andes lie foreland basins that are among the most prolific hydrocarbon provinces of the world. Their Cenozoic evolution was controlled by the Andean uplift and its consequences on deformation and sedimentation. In turn, the Andean uplift results from the interplay between the subducting Nazca oceanic plate and the South American continental plate. Although the process exists all along the margin, the subducting plate is not regular including bathymetric anomalies and segments that result in different response in the deformation and active volcanism of the overriding plate. In the Maranon Basin of Peru, evidences allow documenting the consequences of a topographic anomaly subduction on the thermal regime and deformation of the Andean foreland during the Neogene. In this basin, a maturity anomaly is difficult to explain by considering only the present day thermal regime. However, it spatially coincides with the trace of a lost subducting ridge, the Inca Plateau. Other features like differential uplift and erosion can be related to the same event in the area. We review the consequences of oceanic ridge subduction along the Andean margin. Their effects on the deformation and volcanism of the forearc and arc regions have been extensively described. Their influence on the present day foreland topography is testified by the existence of giant alluvial fans and displaced terraces. Their effect on magmatism and ore deposits formation has also been demonstrated. This example illustrated here shows that their influence on thermal regime, deformation, erosion and ultimately on petroleum systems, must also be taken into account in the search for hydrocarbons in subduction related basins.

Chronology and geochemistry of Neoarchean BIF-type iron deposits in the Yinshan Block, North China Craton: Implications for oceanic ridge subduction

The Dongwufenzi iron deposit is a typical Algoma-type Banded Iron Formation (BIF) in the Yinshan block of the North China Craton. The iron orebodies occur as layers interlayered with plagioclase amphibolite. LA–ICP-MS U–Pb dating of zircons from the plagioclase amphibolite yielded protolith crystallization age of 2538±9Ma (MSWD=1.01), followed by metamorphism at 2452±7Ma (MSWD=0.68). The geochemical characteristics of the ores including low Al2O3, TiO2 and HFSE contents, pronounced positive Eu anomalies, and the Y/Ho ratios (26–37) suggest that the BIFs precipitated from hydrothermal fluids discharged on the seafloor. The geochemical features of the plagioclase amphibolite suggest the protolith magma to be high-Fe basalt. Sm–Nd analyses of the plagioclase amphibolite yielded εNdt (t=2.54Ga) and single-stage depleted mantle Nd model ages (TDM) in the range of ca. +1.8 to +3.0 and 2.68 to 2.81Ga, respectively, suggesting an enriched mantle source. We link the source of Fe in the BIFs to the komatiitic complex in the same greenstone belt, and the source of Si with the high-Fe basalt. Integrated geochronological and geochemical data from the other Archean rocks in this area suggest that the Dongwufenzi BIF formed at 2.5Ga, coeval with the other BIFs in the North China Craton (NCC) as well as similar occurrences in other parts of the world. We propose a geodynamic model involving ridge subduction and slab window for the formation of the BIFs and the associated rock suites in the greenstone belt.

Deep electrical structure and geological significance of Tongling ore district

Abstract The Tongling ore district is one of the most important Cu-polymetal producers in the Lower Yangtze River metallogenic belt of China. To understand deep crust structure beneath the Tongling ore district more comprehensively and objectively and to explore important scientific issues, such as characteristics of lithospheric evolution, the relationship between the crustal structure and metallogenesis, it is necessary to study the variation of crust electrical structure in the study area. For this purpose, six magnetotelluric (MT) sounding profiles have been completed by SinoProbe-03 Project. Based on the imaging results of the six profiles, electrical conductivity structure of the crust has been analyzed. The result shows that the geology beneath the profiles is characterized by layering in vertical direction and block division in horizontal direction. The high-resistivity features are related to the transparent zones detected at the upper crust in reflection seismic surveys. The absence of reflectivity from the depths indicates a homogeneous zone, which could often be related to granitic intrusions. Combining the regional tectonic evolution, MT and other geophysical data, we suggest that the high-resistivity bodies crossing the Tongling uplift reveals the center of magma activity and the layering in vertical direction in the lower crust reveals a normal lower crust model and propose a small-scale underplating model which is different with both the “partial melting of a thickened or delaminated lower continental crust” model and the “oceanic ridge subduction”. The underplating might be related to small-scale lithospheric or lower crust delamination controlled by deep fault during the Early Cretaceous. The result of this MT survey is important for constraining lithospheric evolution and metallogenesis in the Lower Yangtze Metallogenic Belt, such as collision of North China Craton and Yangtze Craton, underplating models, deep structure, mechanisms of NE–SW trending rift as well as graben basin, and probable metallogenesis.

New model for the Early Cretaceous development of SW Japan based on basic rocks of the Chichibu Composite Terrane

The Kurosegawa Terrane in SW Japan is a fault zone that divides the Chichibu Composite Terrane into the Northern Chichibu and Southern Chichibu Terranes. It contains exotic tectonic blocks including granitoids, variously metamorphosed rocks and Siluro-Devonian deposits together with tectonic slices of the Jurassic-Early Cretaceous accretionary complexes. It is also intimately associated with Cretaceous forearc basin deposits. Understanding of the Korosegawa Terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. Detailed studies of the geochemistry of basic rocks of the Chichibu Composite Terrane in eastern Kii Peninsula revealed that HIMU basalts are included in basic rocks. The presence of HIMU basalts provides further evidence for the existence of the Kurosegawa Terrane in this area and supports our model on the tectonic evolution of the Kurosegawa Terrane as a transform fault zone formed due to oceanic ridge subduction in the late Early Cretaceous. This model can comprehensively describe not only the features of the Kurosegawa Terrane but also the coeval key situations of the western circum-Pacific orogenic belts.

Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma

Research Article| January 01, 2006 Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma F. Michaud; F. Michaud 1Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar J.Y. Royer; J.Y. Royer 2CNRS-UBO Domaines Océaniques, Place Copernic, 29280 Plouzané, France Search for other works by this author on: GSW Google Scholar J. Bourgois; J. Bourgois 3Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar J. Dyment; J. Dyment 4Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris, France Search for other works by this author on: GSW Google Scholar T. Calmus; T. Calmus 5Instituto de Geología, Universidad Nacional Autónoma de México, AP 1039, 83000, Hermosillo, Sonora, Mexico Search for other works by this author on: GSW Google Scholar W. Bandy; W. Bandy 6Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico D.F Search for other works by this author on: GSW Google Scholar M. Sosson; M. Sosson 7Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar C. Mortera-Gutiérrez; C. Mortera-Gutiérrez 8Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico D.F Search for other works by this author on: GSW Google Scholar B. Sichler; B. Sichler 9Ifremer, BP 70, 29280 Plouzané, France Search for other works by this author on: GSW Google Scholar M. Rebolledo-Viera; M. Rebolledo-Viera 10Instituto de Geofisica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Mexico D.F Search for other works by this author on: GSW Google Scholar B. Pontoise B. Pontoise 11Géosciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (1): 13–16. https://doi.org/10.1130/g22050.1 Article history received: 12 Jul 2005 rev-recd: 26 Aug 2005 accepted: 30 Aug 2005 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation F. Michaud, J.Y. Royer, J. Bourgois, J. Dyment, T. Calmus, W. Bandy, M. Sosson, C. Mortera-Gutiérrez, B. Sichler, M. Rebolledo-Viera, B. Pontoise; Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma . Geology 2006;; 34 (1): 13–16. doi: https://doi.org/10.1130/g22050.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The interaction of the Pacific-Farallon spreading centers with the North American convergent margin off Baja California, Mexico, supposedly ceased at 12 Ma, when plate convergence and seafloor spreading stopped. We propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23°N to 27°N (Famex cruise of the R/V L'Atalante, April 2002). The data unveil a major clockwise rotation of the Pacific-Farallon spreading direction, starting ca. 14 Ma, that formed a series of short spreading centers that became extinct ca. 8–7 Ma. We suggest that the transcurrent motion between the Pacific and North America along Baja California was accommodated by seafloor spreading and oblique convergence along the trench. This change in spreading direction was followed by a concomitant progressive demise of both Pacific-Farallon seafloor spreading and Farallon–North America subduction that are attributed to the break-off of the Farallon slab. This also resulted in the opening of a trench-parallel slab window beneath Baja California. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.

Subduction-stage pressure-temperature path of eclogite from the Sambagawa belt: Prophetic record for oceanic-ridge subduction

Petrologic studies of the Kotsu eclogite from the Sambagawa high-pressure ( P ), low-temperature ( T ) metamorphic belt show that the eclogite has recorded a very steep subduction-stage P-T path reaching 20 kbar and 600 °C; the dP / dT is >7.1 kbar/100 °C. Compilation of previously derived subduction-stage P-T paths from the Sambagawa belt and the result of this study show that these P-T paths are not on a straight line from the origin, but define a curve with dP / dT increasing with pressure. A new model incorporating the progressive approach of an oceanic ridge to a subduction zone shows that the Sambagawa P-T paths are best explained by a setting in which a ridge is close to being subducted at a slow rate relative to the plate-subduction rate. Exhumation of high- P , low- T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges.

Tertiary volcanism during extension in the Andean foothills of central Chile (33°15′–33°45′S)

This lithologic and geochemical study treats two Tertiary volcanic formations in the Andean foothills of central Chile deposited during and after an inferred culmination of crustal attenuation. The Abanico and Farellones Formations, which are described in their type localities just east of Santiago, formed from volcanic arcs in continental basins during the Oligocene and Early Miocene, respectively. Aphyric basic lavas of tholeiitic affinity, acid pyroclastic rocks, and lacustrine deposits constitute the >3100-m-thick Abanico Formation. The overlying >2100-m-thick Farellones Formation consists of calc-alkaline lavas (basalts absent) with thick pyroclastic deposits at the base. Both formations have Nd-Sr isotope signatures within the mantle array; the Abanico rocks (eNd ≈ +5.7) plot closer to N-MORB (normal mid-oceanic-ridge basalt) than the Farellones rocks (eNd = +3.9 to +5.1). The REE (rare earth element) patterns indicate greater depth to the mantle source and a smaller degree of partial melting with time. The Abanico lavas segregated within the stability field of spinel, whereas the lavas of the upper Farellones member show residual garnet in their source. Geochemical changes with time are systematic: the greatest contrast is between the middle and upper Farellones members in 1–2 m.y., e.g., for basaltic andesites, La/Yb increases from 4.3 (Abanico) to 5.6, 6.0, and 11.6 (lower, middle, and upper Farellones members). The bimodal composition of the Abanico Formation and the lower Farellones member indicates that volcanism took place during episodes of extensional conditions. Extension with subsidence is independently shown by the burial metamorphic pattern. Voluminous pyroclastic flows, structural relationships, and other evidence suggest recurrent caldera collapse. The first extensional episode ended with contraction and folding of the Abanico rocks, and the second episode resulted in uplift of the lower and middle Farellones members, followed by a more passive tectonic regime. Sequences showing many similarities with the Abanico and Farellones Formations occur along the Andean foothills of Chile. They decrease in age from Late Cretaceous– Paleocene at 23°S to Early Miocene–late Miocene at 35°S and might be explained by oblique subduction of oceanic ridge.

Organization of pre-Variscan basement areas at the north-Gondwanan margin

Pre-Variscan basement elements of Central Europe appear in polymetamorphic domains juxtaposed through Variscan and/or Alpine tectonic events. Consequently, nomenclatures and zonations applied to Variscan and Alpine structures, respectively, cannot be valid for pre-Variscan structures. Comparing pre-Variscan relics hidden in the Variscan basement areas of Central Europe, the Alps included, large parallels between the evolution of basement areas of future Avalonia and its former peri-Gondwanan eastern prolongations (e.g. Cadomia, Intra-Alpine Terrane) become evident. Their plate-tectonic evolution from the Late Proterozoic to the Late Ordovician is interpreted as a continuous Gondwana-directed evolution. Cadomian basement, late Cadomian granitoids, late Proterozoic detrital sediments and active margin settings characterize the pre-Cambrian evolution of most of the Gondwana-derived microcontinental pieces. Also the Rheic ocean, separating Avalonia from Gondwana, should have had, at its early stages, a lateral continuation in the former eastern prolongation of peri-Gondwanan microcontinents (e.g. Cadomia, Intra-Alpine Terrane). Subduction of oceanic ridge (Proto-Tethys) triggered the break-off of Avalonia, whereas in the eastern prolongation, the presence of the ridge may have triggered the amalgamation of volcanic arcs and continental ribbons with Gondwana (Ordovician orogenic event). Renewed Gondwana-directed subduction led to the opening of Palaeo-Tethys.