3D and 2D topographic correction to estimated geothermal gradient from the base of gas hydrate stability zone in the Andaman Forearc Basin

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.

CHRONOSTRATIGRAPHY,SEDIMENTATION AND EVOLUTION OF THE XIGAZE FOREARC BASIN: IMPLICATIONS FOR DYNAMIC EVOLUTION OF THE YARLUNG ZANGBO SUTURE ZONE

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.

Burial and exhumation history of the Xigaze forearc basin, Yarlung suture zone. Tibet

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.

Late Cenozoic seismic stratigraphy of the Andaman Forearc Basin,Indian Ocean

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.

Geomorphic indices and relative tectonic uplift in the Guerrero sector of the Mexican forearc

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.

Trace element composition of peridotites from the southern Mariana forearc:Insights into the geochemical effects of serpentinization and/or seafloor weathering

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.

A Forearc(Guleman,Elazi?)Ophiolite:Evidence from Peridotite Mineral Geochemistry

The Guleman ophiolite,one of the most important ophiolitic massifs of the Southeast Anatolian Ophiolitic Belt,consists of a core of serpentinized mantle rocks overlain by an ultramafic sequence,layered and isotropic gabbro,and sheeted dykes.The ophiolite structurally overlies the Lower Miocene Lice Formation and is overlain by young sandstones and shales of the Upper Maashtrichtian-Lower Eocene Hazar Complex and Middle Eocene Maden Complex.The Guleman ophiolite tectonically overlain by Precambrian to Upper Triassic Bitlis metamorphic massif.The mantle peridotites compose mainly of fresh and in place serpentinized harzburgite tectonite with local bands and lenses of dunites with large-sized chromitite pods.The Guleman peridotites commonly show porphyroclastic texture,high-temperature fabrics such as kink-bands in olivines.According to microprobe analyses,the harzburgite and dunite have low Ca O and Al2O3 abundance similar to Mariana forearc,and their average Cr-(=Cr/(Cr+Al)atomic)ratio of Cr-spinelsis surprisingly high(>0.63)besides Fo content of olivine is between 90.9 to 92.3 in peridotites.According to Mg#(Mg/(Mg+Fe2+))versus Cr#in spinel diagram,the degree of partial melting is higher than 35%and spinel values plot in the forearc peridotites field.The Gulemanharzburgites have low Ca O,Al2O3 and Ti O2 contents in orthopyroxene and clinopyroxene lammelles,resembling those of depleted harzburgites from modern forearcs and different from moderately depleted abyssal peridotites.Consequently,we propose that the Guleman peridotites form in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between Arabian and Anatolian plates.

Geochemistry of subducted metabasites exhumed from the Mariana forearc:Implications for Pacific seamount subduction

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 Pacific,with negative Nb-Ta-Ti anomalies and enriched Nd-Hf isotopes.Generally,these alkali metabasites are sourced from the heterogeneous mantle sources that are similar to the present South Pacific Isotopic and Thermal Anomaly.This study provides direct evidence for seamount subduction in the Mariana convergent margins.We suggest seamount subduction is significant to element cycling,mantle heterogeneity,and mantle oxidation in subduction zones.

Petrology and geochemistry of serpentinized peridotites from Hahajima Seamount in Izu-Bonin forearc region

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.

Ancient depletion signals in lherzolites from forearc region:Constraints from Lu-Hf isotope compositions

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 asthenosphere during the early stage of subduction initiation.

A Review on Tectonic Record of Strain Buildup and Stress Release across the Andean Forearc along the Gulf of Guayaquil-Tumbes Basin (GGTB) near Ecuador-Peru Border

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.

Growth of forearc highs and basins in the oblique Sumatra subduction system

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.

Synchronous formation of the‘forearc’Bay of Islands ophiolite and its basal high-temperature metamorphic sole constrained by U-Pb zircon ages

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 newly-formed BOIC forearc spreading center extending from the back arc to a triple junction along the westward-(or paleo-northward)verging trench of the Notre Dame arc.The early HT sole formation age at ca.489–488 Ma is long prior to initiation of obduction at ca.470 Ma and long after initiation of subduction beneath the paleo-northward verging Notre Dame peri-Laurentian arc at ca.514 Ma.This indicates Newfoundland sole ages of the BOIC and St.Anthony Complex are correlated with the age of SSZ spreading,but not necessarily subduction initiation because previously existing and self-sustaining subduction was ongoing.Sole ages are then not correlated with the younger age of obduction-related orogenic events(e.g.,proposed Taconic I and II)in the Newfoundland Appalachians.

东昆仑二叠系格曲组火山岩年代学、地球化学特征及其构造意义

东昆仑古特提斯造山带经历了从洋壳俯冲到陆-陆碰撞及后碰撞伸展的造山过程,但对古特提斯洋闭合时间一直存在争议,争论的焦点集中在晚二叠世格曲组沉积盆地原型及中上二叠统之间不整合关系代表的构造事件。本文对东昆仑红石山地区格曲组火山岩开展了锆石U-Pb年代学与岩石地球化学研究,结果表明,岩石富集Rb、Th、Ba等大离子亲石元素(LILE)而亏损Nb、Ta、Ti、P等高场强元素(HFSE),具有大陆岩浆弧的地球化学特征。火山岩具有高Ba/Th比值(53.6)、Th/Ce比值(0.23)、Nb/Ta比值(18.35)及低Dy/Yb比值(1.42),表明晚二叠世火山岩由枕状玄武岩和洋底沉积物等洋壳物质俯冲至60 km左右深度时熔融所形成。锆石U-Pb定年表明,格曲组下部碎屑岩段流纹质凝灰岩和英安质沉凝灰岩206Pb/238U加权平均年龄分别为(257.5±2.5)Ma和(256.2±4.8)Ma,上部灰岩段粗安质晶屑凝灰岩206Pb/238U加权平均年龄为(251.8±2.3) Ma。结合区域上已报道的格曲组及花岗岩研究成果,认为古特提斯洋在晚二叠世开始向北俯冲消减,沉积于弧前盆地的格曲组为古特提斯洋向北初始俯冲的沉积响应,而格曲组与下伏地层之间的角度不整合关系记录了古特提斯洋晚二叠世开始向北俯冲这一构造事件。

北祁连西段昌马地区晚奥陶世弧前沉积-构造演化

北祁连早古生代多岛洋盆的古地理格局和构造演化一直存在争议。位于北祁连南、北蛇绿岩之间的昌马地区广泛出露了一套早古生代深水火山-碎屑建造。早期填图工作普遍认为其形成于寒武纪至早奥陶世。本次研究对昌马西部的鹰嘴山和车路沟山南侧剖面开展系统的野外地质调查。并对采集到的浊积岩和火山岩样品开展了锆石LA-ICP-MSU-Pb定年工作。研究结果表明,原填为寒武系a岩组的鹰嘴山剖面所采集的砂岩样品2307NQL-13最年轻的锆石给出了(456±4)Ma的加权平均年龄;车路沟山南侧,原划归为下奥陶统阴沟群的安山岩样品2307NQL-06给出了(450±4)Ma的加权平均年龄,证明研究区大面积存在晚奥陶世沉积地层,现有年代地层方案需要重新审视。砂岩样品中的碎屑锆石以寒武纪—奥陶纪年龄为主,主要源于岩浆弧的剥蚀;32颗新元古代和古元古代碎屑锆石揭示古老基底物质的大量加入,表明晚奥陶世昌马地区沉积了大量来自中祁连的碎屑物质。来自中祁连基底的碎屑物质暗示北祁连洋的南分支已经闭合;碎屑物质形成于俯冲相关构造环境暗示北祁连洋的北分支俯冲仍在继续。本次研究给出的新数据将为昌马地区进一步的地层和沉积演化提供可靠的同位素年代约束,也为探讨北祁连洋早古生代复杂的俯冲、闭合过程提供重要依据。

H_(2)O-induced sedimentary carbon migration from subducting slabs to the forearc mantle

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 the forearc mantle may be an important carbon reservoir.Our study explains the fate of a portion of carbon that is not returned to the atmosphere through arc volcanism.

俯冲起始的机制、地质记录及其研究意义

板块俯冲起始是固体地球科学的重大前沿领域之一,是板块构造理论发展的关键和新方向,主要包括俯冲起始时间和机制两个关键科学问题。目前,关于俯冲起始机制存在多种模型,但根据其驱动力的来源不同,可以分为自发俯冲起始和诱发俯冲起始两类,前者包括转换断层塌陷、被动陆缘塌陷和地幔柱头周缘垮塌,后者包括俯冲传递和极性反转。不论是自发还是诱发俯冲起始过程,均会产生相应的地质记录,主要包括俯冲带(SSZ)型蛇绿岩、前弧玄武岩、玻安岩和变质底板。深入研究俯冲起始的地质记录是解开俯冲起始机制的基础,更是检验俯冲起始机制最直接、最有效的手段。另外,通过俯冲起始的地质记录研究,可对地幔属性提供制约,揭示不同源区对大陆地壳增生的贡献,并有效恢复古大洋及相关构造单元的演化历史,进而对其动力过程提供制约。虽然地质记录是解决板块俯冲起始问题的关键,但这些记录具有碎片化、非系统性的特征。在今后的研究中,应将这些地质记录进行横向、大面积对比,从而形成系统性约束;另外,需要注意现有的地质记录和俯冲起始可能并不是一一对应的关系,即没有地质记录的地方不一定就没有俯冲起始。但无论如何,通过俯冲起始研究,可以深入理解特定板块的几何学、运动学及动力学过程,并预测板块行为。同时,对比类地行星(如火星等)的演化过程,将有助于揭开地球独特性和宜居性等未解之谜。

马里亚纳弧前蛇纹岩泥火山角砾蛇纹岩流体流动性元素富集特征及影响因素

马里亚纳弧前蛇纹岩泥火山被认为是认识俯冲作用的直接窗口,其发育的角砾状蛇纹岩富含的流体流动性元素(FMEs)记录了引起蛇纹石化作用的流体和俯冲带的流体-岩石相互作用及元素循环等信息。本文整理了马里亚纳弧前物源深度逐渐增加的5座蛇纹岩泥火山(Yinazao、Fantangis?a、Asùt Tesoru、South Chamorro和Conical)发育角砾蛇纹岩的FMEs数据,通过不同泥火山、同一泥火山不同深度样品及同一块状样微区子样品间的FMEs特征对比,分析了蛇纹岩泥火山引起的蛇纹石化作用的流体性质、来源和形成机制。5座蛇纹岩泥火山角砾蛇纹岩的FMEs均较亏损地幔(DM)显示富集特征:B、Cs、As强烈富集(常>100×DM),Li、Rb、Sb中等富集(>10×DM),Ba、Sr、Pb弱富集(<10×DM)。浅源泥火山角砾蛇纹岩中B、Sr、Ba、Pb含量最高,且由浅源至深源泥火山其含量呈逐渐减少的趋势。而Li、Rb、Cs、As、Sb含量显示相反特征,浅源泥火山的角砾蛇纹岩中含量最低,且从浅源到深源泥火山其含量逐渐增加。不同泥火山角砾蛇纹岩FMEs具有系统性变化特征,反映了俯冲板片衍生流体是其发生蛇纹石化作用的主要流体。离海沟较近的泥火山下部俯冲板片成岩作用以蛋白石脱水为主,形成的板片衍生流体极富B,略富Li、Ba、Sr、Pb;离海沟较远的泥火山下部俯冲板片成岩或进变质作用主要为碳酸盐矿物分解和少量蚀变洋壳脱水及黏土矿物转化,板片衍生流体富Ba、Sr、Li、Rb、Cs;居于上述2类泥火山之间的泥火山,下部俯冲板片主要成岩作用为黏土矿物脱水及转化,板片衍生流体以富集B、Li、Rb、Cs、Ba为特征。海底以下深度<50 m的样品FMEs含量最高,指示海水风化对FMEs含量有一定影响,尤其是B、Sr。同一块角砾蛇纹岩不同微区结构的FMEs含量不同,纯蛇纹石区域其含量最高,指示蛇纹石化产物对FMEs有一定影响;绢石结构中Li、Rb、Cs含量高于橄榄石蚀变的蛇纹石,说明原始矿物类型也对部分FMEs富集有一定影响。

扬子北缘神农架地区郑家垭组碎屑锆石年代学及其构造意义

神农架地区位于扬子陆块北缘,东与黄陵地区前寒武纪变质基底相邻,对该区东部郑家垭组碎屑锆石开展U-Pb年龄谱研究可为中新元古代神农架地区与黄陵地区的相互关系和构造演化研究提供新的依据。本文运用LA-ICP-MS锆石U-Pb方法,对郑家垭组2件砂岩碎屑锆石进行U-Pb年龄测定,共获得了103组U-Pb有效年龄。碎屑锆石的谐和年龄主要集中分布在0.98-1.2Ga、1.3-1.6Ga、1.8-2.3Ga、2.45-2.9Ga 4个区间,其中最年轻的一组锆石谐和年龄为980-995Ma,表明郑家垭组的沉积作用持续到了新元古代早期。与周缘地质体中碎屑锆石年龄谱系进行比较,认为所研究样品的碎屑锆石物源一部分来自于神农架微陆块,一部分来自于庙湾蛇绿岩,具有双向物源特征。结合现有的资料,认为郑家垭组可能为神农架弧与庙湾SSZ型蛇绿岩之间的弧前盆地沉积,指示扬子克拉通内存在格林威尔运动早期的俯冲记录。

南海北部陆缘东部中生代沉积的地震反射特征

本文介绍了中美合作在南海北部陆缘进行的双船地震（合成排列剖面）工作，讨论了新生代沉积之下的中生代沉积之地震反射特征。在今日陆架新生代沉积之下的中生代沉积之地震反射表现杂乱、低振幅和不连续特性，而在今日上陆坡新生代沉积之下的中生代沉积之地震反射呈现连续、较强振幅和可长距离对比的特征。根据地球物理场特征及区域地质资料，我们指出燕山运动时间广东大陆边缘的构造格架：今日东沙群岛—彭湖列岛—带为火山弧，今日陆架区及广东大陆为弧后区，而今日陆坡区则为弧前区。故今日陆架下的中生代沉积为弧背盆地之沉积，表现为陆相沉积；今日陆坡下的中生代沉积为前弧盆地之沉积，表现为海相沉积。