The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence se...The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three-four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic-Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126-139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K-Ar age of amphibole in garnet amphibolite in the ophiolite melange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics： （1） They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 （0.7-1.47%）, low MgO （mostly less than 8%）, and low SiO2 （mostly less than 53%）. （2） The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element （LREE）-depleted rare earth element （REE） patterns. （3） The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high-field-strength element （HFSE） patterns with left oblique features. （4） No protogenetic olivine and clinoenstatite was detected. （5） Some dikes show low展开更多
Tectonically active areas,such as forearc regions,commonly show contrasting relief,differential tectonic uplift,variations in erosion rates,in river incision,and in channel gradient produced by ongoing tectonic deform...Tectonically active areas,such as forearc regions,commonly show contrasting relief,differential tectonic uplift,variations in erosion rates,in river incision,and in channel gradient produced by ongoing tectonic deformation.Thus,information on the tectonic activity of a defined area could be derived via landscape analysis.This study uses topography and geomorphic indices to extract signals of ongoing tectonic deformation along the Mexican subduction forearc within the Guerrero sector.For this purpose,we use field data,topographical data,knickpoints,the ratio of volume to area（Rva）.the stream-length gradient index（St）,and the normalized channel steepness index（k_（sn））.The results of the applied landscape analysis reveal considerable variations in relief,topography and geomorphic indices values along the Guerrero sector of the Mexican subduction zone.We argue that the reported differences are indicative of tectonic deformation and of variations in relative tectonic uplift along the studied forearc.A significant drop from central and eastern parts of the study area towards the west in values of R_（VA）（from ~500 to^300）,St（from ~500 to ca.400）,maximum St（from ~1500-2500 to ~ 1000） and k_（sn）（from ~150 to ~100） denotes a decrease in relative tectonic uplift in the same direction.We suggest that applied geomorphic indices values and forearc topography are independent of climate and lithology.Actual mechanisms responsible for the observed variations and inferred changes in relative forearc tectonic uplift call for further studies that explain the physical processes that control the forearc along strike uplift variations and that determine the rates of uplift.The proposed methodology and results obtained through this study could prove useful to scientists who study the geomorphology of forearc regions and active subduction zones.展开更多
Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges,back arc basins,or forearcs during subduction initiation,and are key indicators of plate tectonics.Although it is ...Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges,back arc basins,or forearcs during subduction initiation,and are key indicators of plate tectonics.Although it is widely agreed that ophiolites are remnants of oceanic crust and associated depleted mantle preserved in orogens,the recognition of ophiolites and their tectonic significance is still a topic of discussion and disagreement.We propose that ophiolites can be recognized in the geologic record by some combination of genetically related pillow basalt,layered gabbro,sheeted dykes,podiform chromite,harzburgite or/and dunite.Mafic igneous rocks have either ocean-ridge basalt or immature oceanic arc basalt chemical compositions.Using a scoring system of 1–11 for ophiolite confidence level,scores of8 are considered confident,6–8 probable,and<6 questionable or unlikely ophiolites.Most ophiolites with scores6 are<900 Ma.The oldest confident ophiolite(score of 8)is the Zunhua ophiolite in eastern China at 2550 Ma,and the oldest well documented sheeted dykes occur in the Jormua and Purtuniq ophiolites at 2000–1950 Ma.Ophiolites do not become geographically widespread until after 900 Ma,and most ophiolites of all ages formed in a forearc(subduction initiation)tectonic setting.If ophiolite production requires plate tectonics,subduction must have begun at least locally by 2700 Ma but did not become widespread until after 2000 Ma.The abundance of ophiolites after 900 Ma may reflect better preservation of subduction-related ophiolites,or to an increasing global network of interconnected plates.Ophiolite frequency peaks in the geologic record partially reflect geographic regions where ophiolites have been extensively studied rather than monitoring the production rate of ophiolites,but the scarcity of>900 Ma ophiolites is probably real.展开更多
Methane gas hydrate related bottom-simulating reflectors(BSRs)are imaged based on the in-line and cross-line multi-channel seismic(MCS)data from the Andaman Forearc Basin.The depth of the BSR depends on pressure and t...Methane gas hydrate related bottom-simulating reflectors(BSRs)are imaged based on the in-line and cross-line multi-channel seismic(MCS)data from the Andaman Forearc Basin.The depth of the BSR depends on pressure and temperature and pore water salinity.With these assumptions,the BSR depth can be used to estimate the geothermal gradient(GTG)based on the availability of in-situ temperature measurements.This calculation is done assuming a 1D conductive model based on available in-situ temperature measurement at site NGHP-01-17 in the study area.However,in the presence of seafloor topography,the conductive temperature field in the subsurface is affected by lateral refraction of heat,which focuses heat in topographic lows and away from topographic highs.The 1D estimate of GTG in the Andaman Forearc Basin has been validated by drilling results from the NGHP-01 expedition.2D analytic modeling to estimate the effects of topography is performed earlier along selected seismic profiles in the study area.The study extended to estimate the effect of topography in 3D using a numerical model.The corrected GTG data allow us to determine GTG values free of topographic effect.The difference between the estimated GTG and values corrected for the 3D topographic effect varies up to~5℃/km.These conclude that the topographic correction is relatively small compared to other uncertainties in the 1D model and that apparent GTG determined with the 1D model captures the major features,although the correction is needed prior to interpreting subtle features of the derived GTG maps.展开更多
The Cretaceous-Eocene Xigaze forearc basin is a crucial data archive for understanding the tectonic history of the Asian continental margin prior to and following collision with India during the early Cenozoic Era. Th...The Cretaceous-Eocene Xigaze forearc basin is a crucial data archive for understanding the tectonic history of the Asian continental margin prior to and following collision with India during the early Cenozoic Era. This study reports apatite and zircon(U-Th)/He thermochronologic data from fourteen samples from Albian-Ypresian Xigaze forearc strata to determine the degree and timing of heating(burial) and subsequent cooling(exhumation) of two localities along the Yarlung suture zone(YSZ) near the towns of Saga and Lazi. Thirty-seven individual zircon He ages range from 31.5 ± 0.8 Ma to6.06 ± 0.18 Ma,with the majority of grains yielding ages between 30 Ma and 10 Ma. Twenty apatite He ages range from 12.7 ± 0.5 Ma to 3.9 ± 0.3 Ma,with the majority of grains yielding ages between 9 Ma and 4 Ma. These ages suggest that the Xigaze forearc basin was heated to 140-200 ℃ prior to cooling in Oligocene-Miocene time. Thermal modeling supports this interpretation and shows that the samples were buried to maximum temperatures of ～140-200 0 C by 35-21 Ma, immediately followed by the onset of exhumation. The zircon He and apatite He dataset and thermal modeling results indicate rapid exhumation from ～21 Ma to 15 Ma, and at ～4 Ma. The 21-15 Ma thermochronometric signal appears to be regionally extensive, affecting all the lithotectonic units of the YSZ, and coincides with movement along the north-vergent Great Counter Thrust system. Thrusting, coupled with enhanced erosion possibly related to the paleo-Yarlung River, likely drove Early Miocene cooling of the Xigaze forearc basin.In contrast, the younger phase of rapid exhumation at ～4 Ma was likely driven by enhanced rock uplift in the footwall of north-striking rifts that cross-cut the YSZ.展开更多
The Yarlung Zangbo Suture Zone (YZSZ), a major lineament in Tibet geological framework, which is accepted as the collision site between India and Asia (Allègre et al., 1984; Coulon et al., 1986; Dewey et al., 199...The Yarlung Zangbo Suture Zone (YZSZ), a major lineament in Tibet geological framework, which is accepted as the collision site between India and Asia (Allègre et al., 1984; Coulon et al., 1986; Dewey et al., 1990; Yin et al., 1994), is an extremely complicated tectonic zone. It includes seven different tectonic\|sedimentary units from north to south as follows: Gangdese arc complex keeping the Sangri Group inside, the Qiuwu Formation, the Giabulin Formation, the Xigaze Group, ophiolitic massifs, the Liuqu Group, and melange zones (Wang et al., 1999). Current models, which mainly focus on researches at the unit of ophiolitic massifs, propose that most of the Tethyan oceanic lithosphere was subducted into one single subduction zone active during the Middle Cretaceous or the Late Cretaceous, and closed during the Paleogene India\|Asia collision. In this report, we present latest research results on units in the Xigaze forearc basin and others in YZSZ after 6\|year\|period of comprehensive investigations. Chronostratigraphy framework, sedimentology, and evolution of the Xigaze forearc basin are discussed in details. Four thrust systems in YZSZ are named. Dynamic evolution of the YZSZ including two subductions of Tethys is presented.展开更多
Interpretation of new multichannel seismic reflection data from the Andaman Forearc Basin(AFB) in the northern Indian Ocean is presented here. The highquality multichannel seismic data from the Andaman Forearc region ...Interpretation of new multichannel seismic reflection data from the Andaman Forearc Basin(AFB) in the northern Indian Ocean is presented here. The highquality multichannel seismic data from the Andaman Forearc region enable us to examine the seismic characters and to demarcate seismic sequences bounded by distinct unconformities. Ages of marked seismic horizons have been calibrated with available litholog data from nearby industry boreholes. Seismic interpretation of new data shows that the AFB is filled with * 4.5-s-two way travel time(TWT) thick Neogene to Recent sediments. The entire basin assemblage exhibits two distinct major sequences pertaining to the Neogene and Quaternary times. A large part of the basin is filled with intermittent mass transport deposits(MTD). We infer that the episodic uplift of the Invisible Bank, protuberance of the outerarc and regular deformation through reactivation of preexisting normal faults since the Pleistocene could be attributed as causal mechanisms for the MTDs. Strong bottom simulating reflectors are identified in the Late Miocene and younger sediments of the outerarc and AFB at a depth of * 0.6 s TWT and correspond to the presence of gas hydrates in this region. Our interpretations have significant implications for geodynamic as well as resource exploration in the AFB.展开更多
The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granu...The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granulite to LT-LP greenschist facies.It incorporates mafic volcanic/plutonic protoliths mixed with pelagic,hemi-pelagic and coarser epiclastic sedimentary protoliths.New LA-ICP-MS U–Pb concordia ages,trace elements,and Ti-in-zircon geothermometry for -250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported.Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma,respectively,that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite,which formed at a supra-subduction zone(SSZ)forearc spreading center.A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected30 m below the peridotite contact of the Table Mountain massif.Zircon crystals analyzed have similar size and morphologies,subparallel rare earth element(REE)variation patterns,and steep heavy REE-enrichments((Lu/Gd)_(cn)>20),significant positive Ce anomalies(dominantly>5)and slight positive to dominantly negative Eu anomalies(1.2–0.4).Zircon shows Th/U mean values of 0.37–0.48 with little to no rim to core variation.Minimum Ti-in-zircon mean crystallization temperatures range from764–787℃.These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites.Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge,that is,a consequence of the newl展开更多
Strata of the Late Cretaceous Niobrara Formation and Pierre Shale Group include bentonites that provide a distal record of volcanic activity taking place to the west. Detailed stratigraphic analysis combined with mine...Strata of the Late Cretaceous Niobrara Formation and Pierre Shale Group include bentonites that provide a distal record of volcanic activity taking place to the west. Detailed stratigraphic analysis combined with mineralogy and geochemistry of the bentonites indicates the following timing of events: 1) Eustatic sea level fall as a result of the end of the Niobrara Cycle;2) Tectonic deformation of the Western Interior Seaway coincident with tectonism on the Absoroka Thrust in Wyoming and Late Canyon Range Thrust in Utah;3) Backarc volcanism in Montana associated with the Little Elkhorn Mountain volcanic complex;4) Forearc volcanism in the Cascades area indicates subduction of a hot oceanic crust where plagioclase in the oceanic crust is being incorporated into the melt;5) Cessation of tectonic activity results in a return of sedimentation patterns to north-south trending belts with the Boyer Bay and Burning Brule members of the Sharon Springs Formation deposited to the east and the Mitten Black Shale Formation deposited in the basin.展开更多
Gravimetric and geologic data show that the reactivation of the Neogene Interandean depression and/or the ~75 - 65 Ma ophiolite suture into the modern dynamic of the Andes controlled the Gulf of Guayaquil Tumbes basin...Gravimetric and geologic data show that the reactivation of the Neogene Interandean depression and/or the ~75 - 65 Ma ophiolite suture into the modern dynamic of the Andes controlled the Gulf of Guayaquil Tumbes basin (GGTB) location and evolution during the past 1.8 - 1.6 Myr at least. Depending on whether the remobilization occurred along the interandean depression or the ophiolite suture, the GGTB evolved trough pure or simple shear mechanisms, respectively. Because the GGTB exhibits an along strike tectonic asymmetry associated with a pervasive seismic gap, the simple shear solution is more likely. Tectonic inversion occurred along a mid-crust detachment (the Mid-Crust detachment hereafter) matching the ophiolite suture that accommodates the North Andean Block (NAB) northward drift. The so-called Decoupling Strip located at the shelf slope break accommodated the tensional stress rotation from N-S along the shelf area i.e. NAB-drift induced to E-W along the continental margin i.e. subduction-erosion-induced. The landward dipping Woollard detachment system located at the Upper-Lower slope boundary connects the subduction channel at depth, allowing the Upper slope to evolve independently from the Lower slope wedge. The long-term recurrence interval between earthquakes, the strong interplate coupling, and the aseismic creeping deformation acting along the main low-angle detachments i.e. the Woollard and the Mid-Crust detachments may account for the pervasive seismic gap at the GGTB area. Because the subduction channel exhibits no record of significant seismic activity, no evidence exists to establish a link between the GGTB sustained subsidence and a basin-centered asperity. Because the GGTB is a promising site of hydrocarbon resources, to understand processes at the origin of this escape-induced forearc basin has a major economic interest.展开更多
Serpentinites,which contain up to 13 wt%of water,are important reservoirs for chemical recycling in subduction zones.In the past two decades,forearc mantle serpentinites were identified in different locations around t...Serpentinites,which contain up to 13 wt%of water,are important reservoirs for chemical recycling in subduction zones.In the past two decades,forearc mantle serpentinites were identified in different locations around the world.Here,we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount,which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench.Nearly all the collected samples are extensively hydrated,and olivine grains in ultramafic rocks are replaced by serpentine minerals,with only one sample preserving remaining trace of orthopyroxene.Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich(~42 wt%),but have low contents in Al2O3,CaO,rare earth and high field strength elements,which is consistent with the overall depleted character of their mantle protoliths.Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25%partial melting of the presumed fertile mantle source,which is generally lower than those of peridotites from Torishima Forearc Seamount,Conical Seamount and South Chamorro Seamount(mostly>25%).All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements(Li,Sr,Pb and U).In details,Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb,and display remarkably high abundances of U.These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid,without or with little contribution from slab-derived fluids.The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.展开更多
Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived fr...Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab,and later interacted with seawater after emplacement at or near the seafloor.This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites.We measured trace element concentrations of peridotites from the southern Mariana forearc.The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly,and La shows good correlation with other REEs (except Ce).In addition,there is a great enrichment of U,Pb,Sr and Li elements,which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram.For the seawater-like REE pattern,we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor,by serpentinization or by marine weathering.Furthermore,the anomalous behavior of Ce,compared with other rare earth elements in these samples,may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater.Positive U,Pb,Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.展开更多
We present major and trace element data of lava recovered from the northern Yap Trench in the western Pacific and discuss their petrogenesis and tectonic implications within the framework of interactions between the C...We present major and trace element data of lava recovered from the northern Yap Trench in the western Pacific and discuss their petrogenesis and tectonic implications within the framework of interactions between the Caroline Ridge and Yap Trench.Rocks were collected from both landward and seaward trench slopes and exhibited geochemical characteristics similar to backarc basin basalt(BABB)and mid-ocean ridge basalt(MORB),including high Fe content,tholeiitic affinity,high TiO_(2) value at a given FeO_(T)/MgO ratio,Ti/V ratio between 20 and50,low Ba/Nb ratio and Th/Nb ratio,and trace element patterns commonly displayed by BABB and MORB,which are distinct from arc lava.These rocks seem to have been generated during mantle upwelling and decompression melting at a spreading center.However,compared with typical forearc lava produced by seafloor spreading in the Mariana forearc region,such as the early Eocene forearc basalts and late Neogene forearc lava in the southernmost Mariana Trench,the Yap Trench lava is derived from a more fertile mantle and feature a more minor subduction component;thus,they cannot be the products of forearc mantle decompression melting.We suggest that the landward slope lava represents backarc basin crust that was overthrust onto the forearc lithosphere during the collision of the Caroline Ridge with the Yap Trench(20–25 Ma),which played a key role in the evolution of the Yap subduction system.Moreover,the seaward slope lava represents the subduction plate crust that accreted onto the deep trench during the collision.This collision event resulted in the cessation of Yap Arc magmatism;thus,the Yap Trench volcanic rocks(<25 Ma)previously suggested to be arc magma products may actually represent the nascent island arc lava with a lower subduction component than in the mature Mariana Arc lava.展开更多
Carbon in sedimentary carbonates dominates the global carbon input flux in subduction zones,the fate of which makes an impact on the global carbon cycle.At forearc depths,~32%of subducting water is released through sl...Carbon in sedimentary carbonates dominates the global carbon input flux in subduction zones,the fate of which makes an impact on the global carbon cycle.At forearc depths,~32%of subducting water is released through slab dehydration and may greatly promote sedimentary carbon migration to the forearc mantle.However,it is controversial that considering the infiltration of external aqueous fluids,whether extremely limited or a significant portion of sedimentary carbon is liberated from subducting slabs in the forearc region.To explore to what extent hydrous fluids could facilitate carbon migration at forearc depths,hydrous carbonate-dominated sediment(1.14 wt.%H2O)-harzburgite reaction(layered)experiments have been performed at 1.5 GPa and 600–1000℃with various durations.For comparison,an anhydrous sediment-harzburgite reaction experiment was conducted to investigate the role of water on carbon migration.In hydrous experiments under subsolidus conditions(600–900℃),(1)a reaction zone comprised of clinopyroxene+dolomite forms at the sediment-harzburgite interface due to the metasomatic reaction;(2)the Ca#(100×Ca/[Ca+Mg+Fe],in molar)of calcite in the sediment layer drastically deceases when approaching the reaction zone;(3)newly formed dolomite and pargasite occur in the upper harzburgite layer.The above phenomena were not observed in the anhydrous experiment.Under a supersolidus condition(1000℃),a reaction zone composed of olivine+clinopyroxene+pargasite+CO_(2)formed as a result of hydrous carbonate melt-harzburgite interaction.The experiments demonstrate that aqueous fluids could significantly promote the chemical reaction and component exchange between sediments and mantle peridotite,and also induce subducting sedimentary carbon migration to the forearc mantle.It is estimated roughly that globally,~50%of subducting sedimentary carbon may be released at forearc depths.The carbon and water would be stabilized as carbonates(e.g.,dolomite)and hydrous minerals(e.g.,pargasite)in the forearc mantle,implying that 展开更多
Seamounts on the drifting oceanic crust are inevitably carried by plate motions and eventually accreted or subducted.However,the geochemical signatures of the subducted seamounts and the significance of seamount subdu...Seamounts on the drifting oceanic crust are inevitably carried by plate motions and eventually accreted or subducted.However,the geochemical signatures of the subducted seamounts and the significance of seamount subduction are not well constrained.Hundreds of seamounts have subducted beneath the Philippine Sea Plate following the westward subduction of the Pacific Plate since the Eocene(~52 Ma).The subducted oceanic crust and seamount materials can be exhumed from the mantle depth to the seafloor in the Mariana forearc region by serpentinite mud volcanoes,providing exceptional opportunities to directly study the subducted oceanic crust and seamounts.The International Ocean Discovery Program(IODP)expedition 366 has recovered a few metamorphosed mafic clasts exhumed from the Mariana forearc serpentinite mud volcanoes,e.g.,the Fantangis?a and Asùt Tesoru seamounts.These mafic clasts have tholeiitic to alkaline affinities with distinct trace elements and Nd-Hf isotopes characteristics,suggesting different provenances and mantle sources.The tholeiites from the Fantangisna Seamount have trace element characteristics typical of mid-ocean ridge basalt.The Pacific-type Hf-Nd isotopic compositions,combined with the greenschist metamorphism of these tholeiites further suggest that they came from the subducted Pacific oceanic crust.The alkali basalts-dolerites from the Fantangisna and Asùt Tesoru seamounts show ocean island basalt(OIB)-like geochemical characteristics.The OIB-like geochemical signatures and the low-grade metamorphism of these alkali basalts-dolerites suggest they came from subducted seamounts that originally formed in an intraplate setting on the Pacific Plate.The Pacific Plate origin of these metabasites suggests they were formed in the Early Cretaceous or earlier.Two types of OIBs have been recognized from alkali metabasites,one of which is geochemically similar to the HIMU-EMI-type OIBs from the West Pacific Seamount Province,and another is similar to the EMII-type OIBs from the Samoa Island in southern展开更多
The sub-arc mantle that experienced hydrous melting is commonly characterized by refractory geochemical compositions. Nevertheless, minor lherzolites with fertile compositions have also been reported for mantle perido...The sub-arc mantle that experienced hydrous melting is commonly characterized by refractory geochemical compositions. Nevertheless, minor lherzolites with fertile compositions have also been reported for mantle peridotites from subduction zone. The petrogenesis and mantle source of the lherzolites are still controversial. The New Caledonia ophiolite(Peridotite Nappe) has been regarded as an allochthonous body of forearc lithosphere. This is supported by refractory compositions of its dominant mantle rocks.A few isolated lherzolitic massifs have also been observed in the northern part of New Caledonia.Those lherzolites are compositionally similar to abyssal peridotites, with negligible subduction-related modification. Here, we present new comprehensive geochemical compositions, in particular highprecision Sr-Nd-Hf isotope data, for the lherzolites. The initial^(176) Hf/^(177) Hf ratios display moderate correlations with sensitive indicators for the extent of melting(i.e., olivine Fo, whole-rock Mg# and Yb contents in clinopyroxene) and whole-rock initial^(187) Os/^(188) Os ratios. Some samples have ancient radiogenic Hf isotopes and unradiogenic Os isotope compositions, implying the preservation of ancient depletion signals in the lherzolites. The Nd isotope compositions, together with trace elements and mineral micro-textures, suggest that the lherzolites have been overprinted by a recent melt-rock interaction event. The high equilibrium temperatures of the studied samples have been estimated by the twopyroxene REE thermometer, yielding temperatures of 1066–1315 ℃. The lherzolites have more depleted Nd-Hf isotope compositions and higher equilibrium temperatures than the New Caledonia harzburgites.This indicates that the lherzolites may represent the residues of asthenosphere mantle trapped within the forearc region. Our studies on the New Caledonia lherzolites with ancient depletion signals suggest that ancient mantle domains in the convective mantle can be emplaced in forearc region by the upwelling of asthe展开更多
Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of thes...Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of these seaways were diachronous events, resulting in E–W variations in the timing of oceanic crust production and ophiolite emplacement. The Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, isotopic fingerprints, and sedimentary cover types, pointing to major differences in their mantle melt sources and tectonic and paleogeographic settings of magmatic construction(Dilek and Furnes, 2019). The Jurassic Western Alpine and Ligurian ophiolites in Europe and their counterparts in southern and northern Iberia formed in a narrow basin(Western Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle, and the ophiolites display a Hess–type oceanic crustal architecture with MORB geochemical signatures(Dilek and Furnes, 2011). All these ophiolites were incorporated into continental margins from the downgoing oceanic lithosphere of the Western Tethys. Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways(Dilek et al., 1990), and their rift–drift, seafloor spreading and suprasubduction zone(SSZ) magmatic construction involved multiple episodes of melting, depletion and refertilization of previously or actively subduction metasomatized mantle sources. Deep mantle recycling processes through subduction zone tectonics and/or plume activities played a major role in their melt evolution, and in the incorporation of mantle transition zone(MTZ) materials into their peridotites(Fig. 1;Dilek and Yang, 2018;Xiong et al., 2019). Tectonic mélanges structurally beneath these ophiolites include Permo–Triassic, OIB–type extrusive rocks, indicating that the initial dismantling of the Pangea supercontinent that led to the opening of the Triassic and展开更多
Based on structural deformation analysis in the oblique Sumatra subduction system, we review uplift mechanisms of the forearc high and formation of the forearc basin. The development of the forearc high has been attri...Based on structural deformation analysis in the oblique Sumatra subduction system, we review uplift mechanisms of the forearc high and formation of the forearc basin. The development of the forearc high has been attributed to the flexural uplift, basin inversion, uplift of older accretion wedge, and backthrust in the landward margin of the accretion wedge. Observation of recently acquired seismic reflection data shows that the interplay between trenchward-vergent thrusts and arcward-vergent backthrusts has played a major role in the uplift of forearc high. The uplifted sediments on the forearc high were previously formed in a forearc basin environment. The present-day morphology of the forearc high and forearc basin is related to the uplift of the accretionary wedge and the overlying forearc basin sediments during Pliocene. Regardless of obliquity in the subduction system, the Sumatran forearc region is dominated by compression that plays an important role in forming Neogene basin depocenters that elongated parallel to the trench.展开更多
基金jointly supported by the Geological Survey Project of Chinese (Grant No.1212010911070)National Science Foundation of China (Grant No.41072167)Institute of Geology, Chinese Academy of Geological Sciences (Grant No. J1120)
文摘The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three-four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic-Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126-139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K-Ar age of amphibole in garnet amphibolite in the ophiolite melange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics： （1） They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 （0.7-1.47%）, low MgO （mostly less than 8%）, and low SiO2 （mostly less than 53%）. （2） The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element （LREE）-depleted rare earth element （REE） patterns. （3） The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high-field-strength element （HFSE） patterns with left oblique features. （4） No protogenetic olivine and clinoenstatite was detected. （5） Some dikes show low
基金funding provided by CONACYT-SEP Ciencia Basica(Grant No.129456):Active Tectonic Deformation along the Pacific Coast of Mexico and by the research grants PAPIIT IN110514 and DGAPA-PASPA 2015-2016a postdoctoral fellowship provided through the DGAPA-UNAM program
文摘Tectonically active areas,such as forearc regions,commonly show contrasting relief,differential tectonic uplift,variations in erosion rates,in river incision,and in channel gradient produced by ongoing tectonic deformation.Thus,information on the tectonic activity of a defined area could be derived via landscape analysis.This study uses topography and geomorphic indices to extract signals of ongoing tectonic deformation along the Mexican subduction forearc within the Guerrero sector.For this purpose,we use field data,topographical data,knickpoints,the ratio of volume to area（Rva）.the stream-length gradient index（St）,and the normalized channel steepness index（k_（sn））.The results of the applied landscape analysis reveal considerable variations in relief,topography and geomorphic indices values along the Guerrero sector of the Mexican subduction zone.We argue that the reported differences are indicative of tectonic deformation and of variations in relative tectonic uplift along the studied forearc.A significant drop from central and eastern parts of the study area towards the west in values of R_（VA）（from ~500 to^300）,St（from ~500 to ca.400）,maximum St（from ~1500-2500 to ~ 1000） and k_（sn）（from ~150 to ~100） denotes a decrease in relative tectonic uplift in the same direction.We suggest that applied geomorphic indices values and forearc topography are independent of climate and lithology.Actual mechanisms responsible for the observed variations and inferred changes in relative forearc tectonic uplift call for further studies that explain the physical processes that control the forearc along strike uplift variations and that determine the rates of uplift.The proposed methodology and results obtained through this study could prove useful to scientists who study the geomorphology of forearc regions and active subduction zones.
文摘Ophiolites are fragments of oceanic lithosphere that are produced at spreading centers at ocean ridges,back arc basins,or forearcs during subduction initiation,and are key indicators of plate tectonics.Although it is widely agreed that ophiolites are remnants of oceanic crust and associated depleted mantle preserved in orogens,the recognition of ophiolites and their tectonic significance is still a topic of discussion and disagreement.We propose that ophiolites can be recognized in the geologic record by some combination of genetically related pillow basalt,layered gabbro,sheeted dykes,podiform chromite,harzburgite or/and dunite.Mafic igneous rocks have either ocean-ridge basalt or immature oceanic arc basalt chemical compositions.Using a scoring system of 1–11 for ophiolite confidence level,scores of8 are considered confident,6–8 probable,and<6 questionable or unlikely ophiolites.Most ophiolites with scores6 are<900 Ma.The oldest confident ophiolite(score of 8)is the Zunhua ophiolite in eastern China at 2550 Ma,and the oldest well documented sheeted dykes occur in the Jormua and Purtuniq ophiolites at 2000–1950 Ma.Ophiolites do not become geographically widespread until after 900 Ma,and most ophiolites of all ages formed in a forearc(subduction initiation)tectonic setting.If ophiolite production requires plate tectonics,subduction must have begun at least locally by 2700 Ma but did not become widespread until after 2000 Ma.The abundance of ophiolites after 900 Ma may reflect better preservation of subduction-related ophiolites,or to an increasing global network of interconnected plates.Ophiolite frequency peaks in the geologic record partially reflect geographic regions where ophiolites have been extensively studied rather than monitoring the production rate of ophiolites,but the scarcity of>900 Ma ophiolites is probably real.
文摘Methane gas hydrate related bottom-simulating reflectors(BSRs)are imaged based on the in-line and cross-line multi-channel seismic(MCS)data from the Andaman Forearc Basin.The depth of the BSR depends on pressure and temperature and pore water salinity.With these assumptions,the BSR depth can be used to estimate the geothermal gradient(GTG)based on the availability of in-situ temperature measurements.This calculation is done assuming a 1D conductive model based on available in-situ temperature measurement at site NGHP-01-17 in the study area.However,in the presence of seafloor topography,the conductive temperature field in the subsurface is affected by lateral refraction of heat,which focuses heat in topographic lows and away from topographic highs.The 1D estimate of GTG in the Andaman Forearc Basin has been validated by drilling results from the NGHP-01 expedition.2D analytic modeling to estimate the effects of topography is performed earlier along selected seismic profiles in the study area.The study extended to estimate the effect of topography in 3D using a numerical model.The corrected GTG data allow us to determine GTG values free of topographic effect.The difference between the estimated GTG and values corrected for the 3D topographic effect varies up to~5℃/km.These conclude that the topographic correction is relatively small compared to other uncertainties in the 1D model and that apparent GTG determined with the 1D model captures the major features,although the correction is needed prior to interpreting subtle features of the derived GTG maps.
基金supported by the U.S. National Science Foundation Continental Dynamics Program (EAR-1008527Lead PI, P. Kapp)
文摘The Cretaceous-Eocene Xigaze forearc basin is a crucial data archive for understanding the tectonic history of the Asian continental margin prior to and following collision with India during the early Cenozoic Era. This study reports apatite and zircon(U-Th)/He thermochronologic data from fourteen samples from Albian-Ypresian Xigaze forearc strata to determine the degree and timing of heating(burial) and subsequent cooling(exhumation) of two localities along the Yarlung suture zone(YSZ) near the towns of Saga and Lazi. Thirty-seven individual zircon He ages range from 31.5 ± 0.8 Ma to6.06 ± 0.18 Ma,with the majority of grains yielding ages between 30 Ma and 10 Ma. Twenty apatite He ages range from 12.7 ± 0.5 Ma to 3.9 ± 0.3 Ma,with the majority of grains yielding ages between 9 Ma and 4 Ma. These ages suggest that the Xigaze forearc basin was heated to 140-200 ℃ prior to cooling in Oligocene-Miocene time. Thermal modeling supports this interpretation and shows that the samples were buried to maximum temperatures of ～140-200 0 C by 35-21 Ma, immediately followed by the onset of exhumation. The zircon He and apatite He dataset and thermal modeling results indicate rapid exhumation from ～21 Ma to 15 Ma, and at ～4 Ma. The 21-15 Ma thermochronometric signal appears to be regionally extensive, affecting all the lithotectonic units of the YSZ, and coincides with movement along the north-vergent Great Counter Thrust system. Thrusting, coupled with enhanced erosion possibly related to the paleo-Yarlung River, likely drove Early Miocene cooling of the Xigaze forearc basin.In contrast, the younger phase of rapid exhumation at ～4 Ma was likely driven by enhanced rock uplift in the footwall of north-striking rifts that cross-cut the YSZ.
文摘The Yarlung Zangbo Suture Zone (YZSZ), a major lineament in Tibet geological framework, which is accepted as the collision site between India and Asia (Allègre et al., 1984; Coulon et al., 1986; Dewey et al., 1990; Yin et al., 1994), is an extremely complicated tectonic zone. It includes seven different tectonic\|sedimentary units from north to south as follows: Gangdese arc complex keeping the Sangri Group inside, the Qiuwu Formation, the Giabulin Formation, the Xigaze Group, ophiolitic massifs, the Liuqu Group, and melange zones (Wang et al., 1999). Current models, which mainly focus on researches at the unit of ophiolitic massifs, propose that most of the Tethyan oceanic lithosphere was subducted into one single subduction zone active during the Middle Cretaceous or the Late Cretaceous, and closed during the Paleogene India\|Asia collision. In this report, we present latest research results on units in the Xigaze forearc basin and others in YZSZ after 6\|year\|period of comprehensive investigations. Chronostratigraphy framework, sedimentology, and evolution of the Xigaze forearc basin are discussed in details. Four thrust systems in YZSZ are named. Dynamic evolution of the YZSZ including two subductions of Tethys is presented.
文摘Interpretation of new multichannel seismic reflection data from the Andaman Forearc Basin(AFB) in the northern Indian Ocean is presented here. The highquality multichannel seismic data from the Andaman Forearc region enable us to examine the seismic characters and to demarcate seismic sequences bounded by distinct unconformities. Ages of marked seismic horizons have been calibrated with available litholog data from nearby industry boreholes. Seismic interpretation of new data shows that the AFB is filled with * 4.5-s-two way travel time(TWT) thick Neogene to Recent sediments. The entire basin assemblage exhibits two distinct major sequences pertaining to the Neogene and Quaternary times. A large part of the basin is filled with intermittent mass transport deposits(MTD). We infer that the episodic uplift of the Invisible Bank, protuberance of the outerarc and regular deformation through reactivation of preexisting normal faults since the Pleistocene could be attributed as causal mechanisms for the MTDs. Strong bottom simulating reflectors are identified in the Late Miocene and younger sediments of the outerarc and AFB at a depth of * 0.6 s TWT and correspond to the presence of gas hydrates in this region. Our interpretations have significant implications for geodynamic as well as resource exploration in the AFB.
基金Funding to J.F.Casey for mapping,sample collections,and geochemical work in the Bay of Islands region were derived from U.S.National Science Foundation grants EAR80-26445,EAR-83-09535,EAR-88-04756a University of Houston Departmental of Earth and Atmospheric Sciences grant in 2017.
文摘The welded metamorphic sole at the base of the Bay of Islands Ophiolite Complex(BOIC)in the Northern Appalachians of Newfoundland shows a typical inverted pressure-temperature(P-T)metamorphic gradient from HT-MP granulite to LT-LP greenschist facies.It incorporates mafic volcanic/plutonic protoliths mixed with pelagic,hemi-pelagic and coarser epiclastic sedimentary protoliths.New LA-ICP-MS U–Pb concordia ages,trace elements,and Ti-in-zircon geothermometry for -250 zircon analyses from three metabasites of the upper HT sole amphibolites with N-MORB-like protoliths are reported.Two samples collected within meters of the ophiolite peridotite-sole contact of the Blow Me Down Mountain and North Arm Mountain massifs yielded the oldest comparable concordia ages of 487.7±2.6 Ma and 489.1±3.1 Ma,respectively,that are both within error of the igneous age of 488.3±1.5 Ma of the directly overlying BOIC ophiolite,which formed at a supra-subduction zone(SSZ)forearc spreading center.A third slightly younger age of 484.2±2.4 Ma was obtained for an upper HT amphibolite sample with similar phase assemblages but collected30 m below the peridotite contact of the Table Mountain massif.Zircon crystals analyzed have similar size and morphologies,subparallel rare earth element(REE)variation patterns,and steep heavy REE-enrichments((Lu/Gd)_(cn)>20),significant positive Ce anomalies(dominantly>5)and slight positive to dominantly negative Eu anomalies(1.2–0.4).Zircon shows Th/U mean values of 0.37–0.48 with little to no rim to core variation.Minimum Ti-in-zircon mean crystallization temperatures range from764–787℃.These neocrystallized zircon crystals appear to be derived from thin leucosomes within the three amphibolites.Two other samples also from the upper HT sole show evidence of inherited detrital zircon with core dates spanning the Cambrian Notre Dame Arc through older Laurentian-like basement and rift age ranges.Subcretion of the sole took place below a hot forearc asthenospheric wedge,that is,a consequence of the newl
文摘Strata of the Late Cretaceous Niobrara Formation and Pierre Shale Group include bentonites that provide a distal record of volcanic activity taking place to the west. Detailed stratigraphic analysis combined with mineralogy and geochemistry of the bentonites indicates the following timing of events: 1) Eustatic sea level fall as a result of the end of the Niobrara Cycle;2) Tectonic deformation of the Western Interior Seaway coincident with tectonism on the Absoroka Thrust in Wyoming and Late Canyon Range Thrust in Utah;3) Backarc volcanism in Montana associated with the Little Elkhorn Mountain volcanic complex;4) Forearc volcanism in the Cascades area indicates subduction of a hot oceanic crust where plagioclase in the oceanic crust is being incorporated into the melt;5) Cessation of tectonic activity results in a return of sedimentation patterns to north-south trending belts with the Boyer Bay and Burning Brule members of the Sharon Springs Formation deposited to the east and the Mitten Black Shale Formation deposited in the basin.
文摘Gravimetric and geologic data show that the reactivation of the Neogene Interandean depression and/or the ~75 - 65 Ma ophiolite suture into the modern dynamic of the Andes controlled the Gulf of Guayaquil Tumbes basin (GGTB) location and evolution during the past 1.8 - 1.6 Myr at least. Depending on whether the remobilization occurred along the interandean depression or the ophiolite suture, the GGTB evolved trough pure or simple shear mechanisms, respectively. Because the GGTB exhibits an along strike tectonic asymmetry associated with a pervasive seismic gap, the simple shear solution is more likely. Tectonic inversion occurred along a mid-crust detachment (the Mid-Crust detachment hereafter) matching the ophiolite suture that accommodates the North Andean Block (NAB) northward drift. The so-called Decoupling Strip located at the shelf slope break accommodated the tensional stress rotation from N-S along the shelf area i.e. NAB-drift induced to E-W along the continental margin i.e. subduction-erosion-induced. The landward dipping Woollard detachment system located at the Upper-Lower slope boundary connects the subduction channel at depth, allowing the Upper slope to evolve independently from the Lower slope wedge. The long-term recurrence interval between earthquakes, the strong interplate coupling, and the aseismic creeping deformation acting along the main low-angle detachments i.e. the Woollard and the Mid-Crust detachments may account for the pervasive seismic gap at the GGTB area. Because the subduction channel exhibits no record of significant seismic activity, no evidence exists to establish a link between the GGTB sustained subsidence and a basin-centered asperity. Because the GGTB is a promising site of hydrocarbon resources, to understand processes at the origin of this escape-induced forearc basin has a major economic interest.
基金The National Natural Science Foundation of China under contract Nos 41506047,41876044 and 91858214the Chinese Academy of Sciences’ Strategic Priority Research Program Grant under contract Nos XDB06030103 and XDB06030204
文摘Serpentinites,which contain up to 13 wt%of water,are important reservoirs for chemical recycling in subduction zones.In the past two decades,forearc mantle serpentinites were identified in different locations around the world.Here,we present petrology and whole rock chemistry of ultramafic and mafic rocks dredged from the Hahajima Seamount,which is located 24–40 km west to the junction of the Izu-Bonin Trench and the Mariana Trench.Nearly all the collected samples are extensively hydrated,and olivine grains in ultramafic rocks are replaced by serpentine minerals,with only one sample preserving remaining trace of orthopyroxene.Our new results show that the Hahajima serpentinized peridotite samples are all MgO-rich(~42 wt%),but have low contents in Al2O3,CaO,rare earth and high field strength elements,which is consistent with the overall depleted character of their mantle protoliths.Model calculations indicate that these Hahajima peridotite samples were derived from 10%–25%partial melting of the presumed fertile mantle source,which is generally lower than those of peridotites from Torishima Forearc Seamount,Conical Seamount and South Chamorro Seamount(mostly>25%).All the serpentinites from these four forearc seamounts show strong enrichment in fluid-mobile and lithophile elements(Li,Sr,Pb and U).In details,Hahajima Seamount serpentinites do not have obvious enrichment in Cs and Rb,and display remarkably high abundances of U.These observations indicate that the serpentinization of Hahajima peridotites occurred by addition of seawater or low temperature seawater-derived hydrothermal fluid,without or with little contribution from slab-derived fluids.The geochemical signature of serpentinites from Hahajima Seamount could be interpreted as the result of the combination of extensive partial melting and subsequent percolation of seawater through the mantle wedge.
基金Supported by the Pilot Project of Knowledge Innovation Project,Chinese Academy of Sciences (Nos.KZCX2-YW-211, KZCX3-SW-223)the National Natural Science Foundation of China (No.40830849)the Special Foundation for the Eleventh Five-Year Plan of COMRA (No.DYXM-115-02-1-03)
文摘Peridotites from the southern Mariana forearc were sampled on the landward trench slope of the Izu-Bonin-Mariana (IBM) subduction zone by dredging.These mantle wedge peridotites underwent hydration by fluid derived from a dehydrated descending slab,and later interacted with seawater after emplacement at or near the seafloor.This study investigates how these two different rock-fluid interaction processes influenced trace element distribution in the southern Mariana forearc peridotites.We measured trace element concentrations of peridotites from the southern Mariana forearc.The southern Mariana forearc peridotites are characterized by a distinct seawater-like REE pattern with an obvious negative Ce anomaly,and La shows good correlation with other REEs (except Ce).In addition,there is a great enrichment of U,Pb,Sr and Li elements,which show a distinct positive anomaly relative to adjacent elements in the multi-element diagram.For the seawater-like REE pattern,we infer that REEs are mainly influenced by seawater during peridotite-seawater interactions after their emplacement at or near the seafloor,by serpentinization or by marine weathering.Furthermore,the anomalous behavior of Ce,compared with other rare earth elements in these samples,may indicate that they have undergone reactions involving Ce (IV) when the peridotites interacted with seawater.Positive U,Pb,Sr and Li anomalies are inferred to be related to seawater and/or fluids released during dehydration of the subducting slab.
基金The National Key R&D Program of China under contract No.2017YFC1405502the Scientific Research Fund of the Second Institute of Oceanography,Ministry of Natural Resources under contract Nos QNYC1901 and JG2002+1 种基金the National Natural Science Foundation of China under contract No.41976072the“13th Five-Year Plan”for Resources and Environment Projects of the China Ocean Mineral R&D Association(COMRA)under contract No.DY135-G2-1-01。
文摘We present major and trace element data of lava recovered from the northern Yap Trench in the western Pacific and discuss their petrogenesis and tectonic implications within the framework of interactions between the Caroline Ridge and Yap Trench.Rocks were collected from both landward and seaward trench slopes and exhibited geochemical characteristics similar to backarc basin basalt(BABB)and mid-ocean ridge basalt(MORB),including high Fe content,tholeiitic affinity,high TiO_(2) value at a given FeO_(T)/MgO ratio,Ti/V ratio between 20 and50,low Ba/Nb ratio and Th/Nb ratio,and trace element patterns commonly displayed by BABB and MORB,which are distinct from arc lava.These rocks seem to have been generated during mantle upwelling and decompression melting at a spreading center.However,compared with typical forearc lava produced by seafloor spreading in the Mariana forearc region,such as the early Eocene forearc basalts and late Neogene forearc lava in the southernmost Mariana Trench,the Yap Trench lava is derived from a more fertile mantle and feature a more minor subduction component;thus,they cannot be the products of forearc mantle decompression melting.We suggest that the landward slope lava represents backarc basin crust that was overthrust onto the forearc lithosphere during the collision of the Caroline Ridge with the Yap Trench(20–25 Ma),which played a key role in the evolution of the Yap subduction system.Moreover,the seaward slope lava represents the subduction plate crust that accreted onto the deep trench during the collision.This collision event resulted in the cessation of Yap Arc magmatism;thus,the Yap Trench volcanic rocks(<25 Ma)previously suggested to be arc magma products may actually represent the nascent island arc lava with a lower subduction component than in the mature Mariana Arc lava.
基金supported by the Key R&D Program of China(Grant No.2019YFA0708400)the MOST Special Funds of the State Key Laboratory of Geological Processes and Mineral Resources(Grant No.MSFGPMR01)。
文摘Carbon in sedimentary carbonates dominates the global carbon input flux in subduction zones,the fate of which makes an impact on the global carbon cycle.At forearc depths,~32%of subducting water is released through slab dehydration and may greatly promote sedimentary carbon migration to the forearc mantle.However,it is controversial that considering the infiltration of external aqueous fluids,whether extremely limited or a significant portion of sedimentary carbon is liberated from subducting slabs in the forearc region.To explore to what extent hydrous fluids could facilitate carbon migration at forearc depths,hydrous carbonate-dominated sediment(1.14 wt.%H2O)-harzburgite reaction(layered)experiments have been performed at 1.5 GPa and 600–1000℃with various durations.For comparison,an anhydrous sediment-harzburgite reaction experiment was conducted to investigate the role of water on carbon migration.In hydrous experiments under subsolidus conditions(600–900℃),(1)a reaction zone comprised of clinopyroxene+dolomite forms at the sediment-harzburgite interface due to the metasomatic reaction;(2)the Ca#(100×Ca/[Ca+Mg+Fe],in molar)of calcite in the sediment layer drastically deceases when approaching the reaction zone;(3)newly formed dolomite and pargasite occur in the upper harzburgite layer.The above phenomena were not observed in the anhydrous experiment.Under a supersolidus condition(1000℃),a reaction zone composed of olivine+clinopyroxene+pargasite+CO_(2)formed as a result of hydrous carbonate melt-harzburgite interaction.The experiments demonstrate that aqueous fluids could significantly promote the chemical reaction and component exchange between sediments and mantle peridotite,and also induce subducting sedimentary carbon migration to the forearc mantle.It is estimated roughly that globally,~50%of subducting sedimentary carbon may be released at forearc depths.The carbon and water would be stabilized as carbonates(e.g.,dolomite)and hydrous minerals(e.g.,pargasite)in the forearc mantle,implying that
基金financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(XDA22050103,XDB42020303,XDB18020102)the National Natural Science Foundation of China(Grant No.41803002)+2 种基金China Postdoctoral Science Foundation(No.2018 M642708)the Qingdao National Laboratory for Marine Science and Technology(2017ASKJ02)the Taishan Scholar Foundation of Shandong Province(ts201712075)。
文摘Seamounts on the drifting oceanic crust are inevitably carried by plate motions and eventually accreted or subducted.However,the geochemical signatures of the subducted seamounts and the significance of seamount subduction are not well constrained.Hundreds of seamounts have subducted beneath the Philippine Sea Plate following the westward subduction of the Pacific Plate since the Eocene(~52 Ma).The subducted oceanic crust and seamount materials can be exhumed from the mantle depth to the seafloor in the Mariana forearc region by serpentinite mud volcanoes,providing exceptional opportunities to directly study the subducted oceanic crust and seamounts.The International Ocean Discovery Program(IODP)expedition 366 has recovered a few metamorphosed mafic clasts exhumed from the Mariana forearc serpentinite mud volcanoes,e.g.,the Fantangis?a and Asùt Tesoru seamounts.These mafic clasts have tholeiitic to alkaline affinities with distinct trace elements and Nd-Hf isotopes characteristics,suggesting different provenances and mantle sources.The tholeiites from the Fantangisna Seamount have trace element characteristics typical of mid-ocean ridge basalt.The Pacific-type Hf-Nd isotopic compositions,combined with the greenschist metamorphism of these tholeiites further suggest that they came from the subducted Pacific oceanic crust.The alkali basalts-dolerites from the Fantangisna and Asùt Tesoru seamounts show ocean island basalt(OIB)-like geochemical characteristics.The OIB-like geochemical signatures and the low-grade metamorphism of these alkali basalts-dolerites suggest they came from subducted seamounts that originally formed in an intraplate setting on the Pacific Plate.The Pacific Plate origin of these metabasites suggests they were formed in the Early Cretaceous or earlier.Two types of OIBs have been recognized from alkali metabasites,one of which is geochemically similar to the HIMU-EMI-type OIBs from the West Pacific Seamount Province,and another is similar to the EMII-type OIBs from the Samoa Island in southern
基金financially supported by the National Natural Science Foundation of China (Grant 41902061)the Opening Foun-dation of the Laboratory for Marine Geology Qingdao National Laboratory for Marine Science and Technology (Grant MGQNLMKF201813)the China Postdoctoral Science Foundation (Grant2019M652292)。
文摘The sub-arc mantle that experienced hydrous melting is commonly characterized by refractory geochemical compositions. Nevertheless, minor lherzolites with fertile compositions have also been reported for mantle peridotites from subduction zone. The petrogenesis and mantle source of the lherzolites are still controversial. The New Caledonia ophiolite(Peridotite Nappe) has been regarded as an allochthonous body of forearc lithosphere. This is supported by refractory compositions of its dominant mantle rocks.A few isolated lherzolitic massifs have also been observed in the northern part of New Caledonia.Those lherzolites are compositionally similar to abyssal peridotites, with negligible subduction-related modification. Here, we present new comprehensive geochemical compositions, in particular highprecision Sr-Nd-Hf isotope data, for the lherzolites. The initial^(176) Hf/^(177) Hf ratios display moderate correlations with sensitive indicators for the extent of melting(i.e., olivine Fo, whole-rock Mg# and Yb contents in clinopyroxene) and whole-rock initial^(187) Os/^(188) Os ratios. Some samples have ancient radiogenic Hf isotopes and unradiogenic Os isotope compositions, implying the preservation of ancient depletion signals in the lherzolites. The Nd isotope compositions, together with trace elements and mineral micro-textures, suggest that the lherzolites have been overprinted by a recent melt-rock interaction event. The high equilibrium temperatures of the studied samples have been estimated by the twopyroxene REE thermometer, yielding temperatures of 1066–1315 ℃. The lherzolites have more depleted Nd-Hf isotope compositions and higher equilibrium temperatures than the New Caledonia harzburgites.This indicates that the lherzolites may represent the residues of asthenosphere mantle trapped within the forearc region. Our studies on the New Caledonia lherzolites with ancient depletion signals suggest that ancient mantle domains in the convective mantle can be emplaced in forearc region by the upwelling of asthe
文摘Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of these seaways were diachronous events, resulting in E–W variations in the timing of oceanic crust production and ophiolite emplacement. The Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, isotopic fingerprints, and sedimentary cover types, pointing to major differences in their mantle melt sources and tectonic and paleogeographic settings of magmatic construction(Dilek and Furnes, 2019). The Jurassic Western Alpine and Ligurian ophiolites in Europe and their counterparts in southern and northern Iberia formed in a narrow basin(Western Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle, and the ophiolites display a Hess–type oceanic crustal architecture with MORB geochemical signatures(Dilek and Furnes, 2011). All these ophiolites were incorporated into continental margins from the downgoing oceanic lithosphere of the Western Tethys. Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways(Dilek et al., 1990), and their rift–drift, seafloor spreading and suprasubduction zone(SSZ) magmatic construction involved multiple episodes of melting, depletion and refertilization of previously or actively subduction metasomatized mantle sources. Deep mantle recycling processes through subduction zone tectonics and/or plume activities played a major role in their melt evolution, and in the incorporation of mantle transition zone(MTZ) materials into their peridotites(Fig. 1;Dilek and Yang, 2018;Xiong et al., 2019). Tectonic mélanges structurally beneath these ophiolites include Permo–Triassic, OIB–type extrusive rocks, indicating that the initial dismantling of the Pangea supercontinent that led to the opening of the Triassic and
文摘Based on structural deformation analysis in the oblique Sumatra subduction system, we review uplift mechanisms of the forearc high and formation of the forearc basin. The development of the forearc high has been attributed to the flexural uplift, basin inversion, uplift of older accretion wedge, and backthrust in the landward margin of the accretion wedge. Observation of recently acquired seismic reflection data shows that the interplay between trenchward-vergent thrusts and arcward-vergent backthrusts has played a major role in the uplift of forearc high. The uplifted sediments on the forearc high were previously formed in a forearc basin environment. The present-day morphology of the forearc high and forearc basin is related to the uplift of the accretionary wedge and the overlying forearc basin sediments during Pliocene. Regardless of obliquity in the subduction system, the Sumatran forearc region is dominated by compression that plays an important role in forming Neogene basin depocenters that elongated parallel to the trench.
