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1.
The Okinawa Trough is a young, intracontinental backarc basin that has formed behind the Ryukyu arc–trench system since late Miocene time. In the Southernmost Part of the Okinawa Trough (SPOT), a cluster of active submarine volcanoes delineates a volcanic belt, which is located only ∼100 km above the Wadati–Benioff zone. We report herein new major and trace element data for the SPOT volcanic rocks. These rocks show a compositional range from medium-K andesite to rhyolite. Their geochemical characteristics are similar to those of pre-backarc rifting volcanic rocks from the central Ryukyu arc, and different from those of backarc basin lavas from the Middle Okinawa Trough and the post-backarc rifting Ryukyu arc volcanics. Therefore, despite being topographically contiguous with the rest of the Trough, the SPOT that developed in the Quaternary is not a simple backarc basin but instead an embryonic rift zone in which early arc volcanism occurs as a result of the Ryukyu subduction.  相似文献   

2.
琉球弧前盆地位于菲律宾海板块北部与欧亚板块汇聚部位,发育于琉球海沟北部增生楔与琉球岛弧之间,是典型“沟-弧-盆”体系的组成单元。现利用多道地震资料,首次建立琉球弧前盆地的层序地层格架,分析其新生代层序地层特征,阐明弧前盆地沉积充填演化过程,并探讨各盆地主要物源。通过地震剖面解释分析,表明:①始新世为岛弧变质基底沉积期,晚渐新世晚期-早中新世阶段发育残余伸展盆地基底沉积,属于浅海环境,主要受岩浆活动影响,发育火山碎屑岩相;②中中新世-第四纪时期是弧前盆地的主体沉积期,盆地从半深海沉积环境向深海环境过渡,发育典型深海沉积相,局部为火山碎屑岩相;中中新世时北部的南琉球群岛是弧前盆地主要物源区;晚中新世至第四纪时期,台湾岛东北部陆区成为对该弧前盆地贡献最大的物源区,而南琉球群岛的物源供给量降为次要地位。该研究结果是对琉球岛弧及周缘构造控盆作用研究的拓展,并对台湾岛陆地与东部海域“源-汇”系统研究有重要的指导意义。  相似文献   

3.
Recent interpretation of seismic sections and free-air gravity anomalies in offshore northern Taiwan reveals that the southern Taiwan–Sinzi Folded Zone began to form in late Middle Miocene, though it was mainly constructed in the Late Pliocene with strong reverse faulting and folding. Two westward progradational sequences were deposited in the shelf basin with sediments supplied from the southern Taiwan–Sinzi Folded Zone and the southern Ryukyu Arc. These two structures are displaced by several northwest-striking dextral strike–slip faults that were active in the early Quaternary when the clockwise-rotated southern Ryukyu Arc and the folded southern Taiwan–Sinzi Folded Zone were broken. It is believed that recent extension in the southern Okinawa Trough started in the early Quaternary because uplift on the southern Taiwan–Sinzi Folded Zone continued to latest Pliocene–early Quaternary. Paleogene–Miocene sediments of the East China Sea Shelf in the western part of the southern Okinawa Trough Basin are interpreted to indicate that the East China Sea Shelf Basin extended to the east of the southern Taiwan–Sinzi Folded Zone.  相似文献   

4.
The major and trace element and Pb–Sr–Nd isotopic compositions of Quaternary mafic lavas from the northern Ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution. Beneath the volcanic front in the northern part of the arc, the subducted slab of the Philippine Sea Plate bends sharply and steepens at a depth of ∼80 km. Lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements, consistent with the result of fluid enrichment processes related to dehydration of the subducting slab. New Pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south Kyushu and northern Ryukyu arc, which, in a parallel with data from the Lau Basin, appear to reflect mantle with affinities to Indian and Pacific-type mid-ocean ridge basalt (MORB). Indian Ocean MORB-type mantle, contaminated with subducted Ryukyu sediments can account for the variation of lavas erupted on south Kyushu, and probably in the middle Okinawa Trough. In contrast, magmas of the northern Ryukyu volcanic front appear to be derived from sources of Pacific MORB-type mantle contaminated with a sedimentary component. Along-arc variation in the northern Ryukyus reflects increasing involvement of a sedimentary component to the south. Compositions of alkalic basalts from the south Kyushu back-arc resemble intraplate-type basalts erupted in NW Kyushu since ∼12 Ma. We propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle, yielding Indian Ocean-type characteristics from a mantle upwelling zone beneath NW Kyushu and the East China Sea. This model also accounts for (1) extensional counter-clockwise crustal rotation (∼4–2 Ma), (2) voluminous andesite volcanism (∼2 Ma), and (3) the recent distinctive felsic magmatism in the south Kyushu region. Received: 30 November 1999 / Accepted: 20 July 2000  相似文献   

5.
Cathy Busby   《Tectonophysics》2004,392(1-4):241
Mesozoic rocks of the Baja California Peninsula form one of the most areally extensive, best-exposed, longest-lived (160 my), least-tectonized and least-metamorphosed convergent-margin basin complexes in the world. This convergent margin shows an evolutionary trend that may be typical of arc systems facing large ocean basins: a progression from highly extensional (phase 1) through mildly extensional (phase 2) to compressional (phase 3) strain regimes. This trend is largely due to the progressively decreasing age of lithosphere that is subducted, which causes a gradual decrease in slab dip angle (and concomitant increase in coupling between lower and upper plates), as well as progressive inboard migration of the arc axis.This paper emphasizes the usefulness of sedimentary and volcanic basin analysis for reconstructing the tectonic evolution of a convergent continental margin. Phase 1 consists of Late Triassic to Late Jurassic oceanic intra-arc to backarc basins that were isolated from continental sediment sources. New, progressively widening basins were created by arc rifting and sea floor spreading, and these were largely filled with progradational backarc arc-apron deposits that record the growth of adjacent volcanoes up to and above sea level. Inboard migration of the backarc spreading center ultimately results in renewed arc rifting, producing an influx of silicic pyroclastics to the backarc basin. Rifting succeeds in conversion of the active backarc basin into a remnant backarc basin, which is blanketed by epiclastic sands.Phase 1 oceanic arc–backarc terranes were amalgamated by Late Jurassic sinistral strike slip faults. They form the forearc substrate for phase 2, indicating inboard migration of the arc axis due to decrease in slab dip. Phase 2 consists of Early Cretaceous extensional fringing arc basins adjacent to a continent. Phase 2 forearc basins consist of grabens that stepped downward toward the trench, filled with coarse-grained slope apron deposits. Phase 2 intra-arc basins show a cycle of (1) arc extension, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions, followed by (2) arc rifting, characterized by widespread dike swarms and extensive mafic lavas and hyaloclastites. This extensional-rifting cycle was followed by mid-Cretaceous backarc basin closure and thrusting of the fringing arc beneath the edge of the continent, caused by a decrease in slab dip as well as a possible increase in convergence rate.Phase 2 fringing arc terranes form the substrate for phase 3, which consists of a Late Cretaceous high-standing, compressional continental arc that migrated inboard with time. Strongly coupled subduction resulted in accretion of blueschist metamorphic rocks, with development of a broad residual forearc basin behind the growing accretionary wedge, and development of extensional forearc (trench–slope) basins atop the gravitationally collapsing accretionary wedge. Inboard of this, ongoing phase 3 strongly coupled subduction, together with oblique convergence, resulted in development of forearc strike-slip basins upon arc basement.The modern Earth is strongly biased toward long-lived arc–trench systems, which are compressional; therefore, evolutionary models for convergent margins must be constructed from well-preserved ancient examples like Baja California. This convergent margin is typical of many others, where the early to middle stages of convergence (phases 1 and 2) create nonsubductable arc–ophiolite terranes (and their basin fills) in the upper plate. These become accreted to the continental margin in the late stage of convergence (phase 3), resulting in significant continental growth.  相似文献   

6.
How was Taiwan created?   总被引：4，自引：0，他引：4  
Since the beginning of formation of proto-Taiwan during late Miocene (9 Ma), the subducting Philippine (PH) Sea plate moved continuously through time in the N307° direction at a 5.6 cm/year velocity with respect to Eurasia (EU), tearing the Eurasian plate. Strain states within the EU crust are different on each side of the western PH Sea plate boundary (extensional in the Okinawa Trough and northeastern Taiwan versus contractional for the rest of Taiwan Island). The B feature corresponds to the boundary between the continental and oceanic parts of the subducting Eurasian plate and lies in the prolongation of the ocean–continent boundary of the northern South China Sea. Strain rates in the Philippines to northern Taiwan accretionary prism are similar on each side of B (contractional), though with different strain directions, perhaps in relation with the change of nature of the EU slab across B. Consequently, in the process of Taiwan mountain building, the deformation style was probably not changing continuously from the Manila to the Ryukyu subduction zones. The Luzon intra-oceanic arc only formed south of B, above the subducting Eurasian oceanic lithosphere. North of B, the Luzon arc collided with EU simultaneously with the eastward subduction of a portion of EU continental lithosphere beneath the Luzon arc. In its northern portion, the lower part of the Luzon arc was subducting beneath Eurasia while the upper part accreted against the Ryukyu forearc. Among the consequences of such a simple geodynamic model: (i) The notion of continuum from subduction to collision might be questioned. (ii) Traces of the Miocene volcanic arc were never found in the southwestern Ryukyu arc. We suggest that the portion of EU continental lithosphere, which has subducted beneath the Coastal Range, might include the Miocene Ryukyu arc volcanoes formed west of 126°E longitude and which are missing today. (iii) The 150-km-wide oceanic domain located south of B between the Luzon arc and the Manila trench, above the subducting oceanic EU plate (South China Sea) was progressively incorporated into the EU plate north of B.  相似文献   

7.
We determined high-resolution three-dimensional P- and S-wave velocity (Vp, Vs) structures beneath Kyushu in Southwest Japan using 177,500 P and 174,025 S wave arrival times from 8515 local earthquakes. A Poisson's ratio structure was derived from the obtained Vp and Vs values. Our results show that significant low-Vp, low-Vs and high Poisson's ratio zones are extensively distributed along the volcanic front in the uppermost mantle, which extend and dip toward the back-arc side in the mantle wedge. In the crust, low-Vp, low-Vs and high Poisson's ratio anomalies exist beneath the active volcanoes. The subducting Philippine Sea slab is clearly imaged as a high-Vp, high-Vs and low Poisson's ratio zone from the Nankai Trough to the back-arc. A thin low-velocity zone is detected above the subducting Philippine Sea slab in the mantle wedge, and earthquakes in the upper mantle are distributed along the transition zone between this thin low-velocity zone and the high-velocity Philippine Sea slab, which may imply that oceanic crust exists on the top of the slab and the forearc mantle wedge is serpentinized due to the slab dehydration. The seismic velocity of the subducting oceanic crust with basaltic or gabbroic composition is lower than that of the mantle according to the previous studies. The serpentinization process could also dramatically reduce the seismic velocity in the forearc mantle wedge.  相似文献   

8.
The Manus Basin to the northeast of Papua New Guinea is an actively spreading/rifting back-arc basin in the Bismarck Sea located between the inactive Manus-Kilinailau trench on the Pacific-plate side and the active New Britain trench on the Solomon-plate side. Spreading/rifting in the Manus Basin takes place in the last 0.78 Myr or so. We present major and trace elements, and Sr–Nd–Pb isotope compositions of rock samples taken from the South East Rift (SER) at the eastern end of the Manus Basin. The strong enrichment of Pb and LILE (large ion lithophile elements) relative to HFSE (high field strength elements) and REE (rare earth elements) in the SER lava is also quite similar to other island arc lavas, suggesting that substantial amount of subduction components were present in its source mantle. To investigate the origin of the subduction components in SER lavas, we compare the geochemical data of SER lavas to published data from New Britain Arc (NBA) and Tabar–Lihir–Tanga–Feni (TLTF) island chain. The volcanism in NBA is related to presently active subduction of the Solomon slab, whereas the TLTF volcanism is located in the forearc area of New Ireland arc which was formed during a former subduction of the Pacific slab. In other words, the NBA and TLTF lavas were influenced by subduction components from the present and former subduction, respectively. We argue that the subduction components in SER lava were incorporated in the mantle lithosphere during the active arc volcanism on New Ireland because the amount of the subduction component in SER decreases with increasing in distance from New Ireland. On the other hand, no relationships are found with respect to New Britain. The Sr–Nd–Pb isotopes indicate that SER lavas contain little sediment component and less amount of fluid component derived from altered oceanic crust compared to the TLTF lavas. This is probably due to the fact that SER is located in backarc settings in contrast to TLTF which is located in forearc setting with respect to the Pacific slab. Thus it is likely that the sediment was removed from the slab in the forearc and/or arc areas, and therefore little or none was introduced in the backarc mantle, which is the source region for SER magmas at present. Fluid derived from altered oceanic crust also may have made its way into the sub-forearc region more effectively than backare region by shallow dehydration process.  相似文献   

9.
The Antarctic Ross Orogen was built up during the early Paleozoic in the framework of the convergence between the Paleo-Pacific oceanic plate and the Gondwana continental margin. Models for the Ross Orogen in northern Victoria Land are based on terranes having a variable provenance with respect to the margin. However, recent studies provide evidence for the occurrence of different pieces of the lithospheric puzzle: (i) the Wilson continental magmatic arc, representing the main part of the active Gondwana margin, (ii) the Bowers arc–backarc system, (iii) the Admiralty crustal ribbon including continental material of the Wilson forearc, and (iv) the newly discovered, Cambrian oceanic magmatic Tiger arc, along the Ross Sea coast. An updated model is presented in which, after the Early Cambrian magmatic activity of the Wilson arc, a retreat of the subduction zone in the Early–Middle Cambrian gave way to boudinage of the Wilson forearc, trenchward arc migration, opening of the Bowers backarc basin and inception of the outboard Tiger subduction zone. Renewed convergence resulted in the development of the Middle Cambrian Bowers arc, closure of the backarc and deep underthrusting of portions of it at the Middle–Late Cambrian. Finally, in the latest Cambrian to earliest Ordovician, fast exhumation was coupled in the north with erosion and sediment shed to the northeast, and with extension and potassic magmatism in central and southern Victoria Land.  相似文献   

10.
《International Geology Review》2012,54(12):1369-1388
In the Quaternary, the Ryukyu Islands evolved from a continental margin arc to an island arc by backarc spreading of the Okinawa Trough, accompanied by subsidence and isolation of the islands, a process that has continued to the present. Trough-parallel half grabens were filled with marine siltstone. Similar sediments filling orthogonal fault-controlled and west-draining non-tectonic valleys record island separation. New Quaternary nannofossil biostratigraphic data date the deposition of the marine siltstone at 1.552 ± 0.154 Ma. At that time, the entire 1000 km-long island chain comprising the Ryukyu Islands separated from the Asian continent by rifting extending from the Okinawa Trough to the Tsushima Strait. The Tokara, Kerama, and Yonaguni gaps, branched or transverse rifts of the Okinawa Trough, separate the island chain into subgroups of the Osumi, Amami, Okinawa, and Yaeyama islands, and Taiwan. The shallow Taiwan Strait separated Taiwan from the Chinese mainland. The Kuroshio warm current that previously ran offshore of the continental margin arc began to enter the opening backarc basin through the Yonaguni gap and to exit through the Tokara gap, flowing along the axis of the Okinawa Trough. Under influence of the warm current and because of entrapment of continentally sourced detrital sediments by the Okinawa Trough, coral reefs formed around each island. These reefs make up a unit called the Ryukyu limestone. Subsidence continued through the deposition of this limestone, resulting in further isolation of each island. Some islands did not separate from the mainland but emerged above sea level later as a result of volcanic edifice construction or forearc uplift. Following initial isolation, the Japanese islands and Taiwan may have been connected to the mainland by land bridges during some sea level low stands related to glacial periods, whereas the other islands remained isolated. Based on ages of isolation of each island, a Quaternary palaeogeographic map and ‘phylogenetic tree’ of the islands can be drawn showing the separation time of each island from the mainland and from each other. This information should be useful for phylogenetic molecular biologists studying evolution of Ryukyu endemic species and vicariant speciation and could facilitate analysis of the DNA substitution rate.  相似文献   

11.
Kyushu Island, Japan, is located at the junction of the Southwest Japan arc and the Ryukyu arc. There are two major late Cenozoic epithermal gold-silver provinces in Kyushu, which are termed the Northern and Southern provinces. The provinces are characterized by: 1) Pliocene volcanism dominated by calc-alkaline andesite, followed by Quaternary volcanism including extrusion of both calc-alkaline and tholeiitic magmas; 2) formation of extensional grabens; 3) Pliocene to Pleistocene mineralization, which was dominated by abundant low sulfidation (LS) epithermal deposits with a few high sulfidation (HS) examples. The two epithermal gold-silver provinces have evolved differently since about 5 Ma; the Northern province has exhibited diminished hydrothermal activity from the Pliocene to Pleistocene, whereas the Southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions. Changes of tectonic setting from the Pliocene to Pleistocene account for the variable trends in epithermal gold deposit formation. Westward oblique subduction of the Philippine Sea plate beneath the Southwest Japan arc caused development of the Hohi graben and arc-related volcanism at about 6 Ma. This was associated with widespread LS mineralization in and surrounding the Hohi graben, as is represented by the Bajo and Taio deposits. The subduction of the relatively buoyant Kyushu-Palau ridge during the early Pliocene strengthened the coupling between the slab and overriding Ryukyu arc, leading to polygenetic andesite volcanism with associated HS (Kasuga, Iwato, and Akeshi) and LS (Kushikino) mineral deposits forming in the Southern province. A change of the subduction direction of the Philippine Sea plate, from west to north-northwest in the early Pliocene, increased the orthogonal convergence rate between the Southwest Japan arc and the Philippine Sea plate, resulting in a decrease of volcanic and hydrothermal activity in the Hohi graben of the Northern province. The more northerly subduction of the Philippine Sea plate shifted the locus of the Kyushu-Palau ridge subduction northward, resulting in underplating of the older (85–60 Ma), negatively buoyant Amami basin oceanic slab in the Southern province, rather than continued subduction of the young (27–15 Ma), buoyant Shikoku basin slab. This replacement caused steepening of the slab angle and slab-rollback in the Southern province, which was associated with regional extension, an eastward shift of the Ryukyu volcanic front, and development of the Kagoshima and Shimabara grabens, as well as the Okinawa trough. Rhyolite and basalt volcanism, in addition to andesite volcanism, have occurred since 2 Ma in the area of the Ryukyu back arc; coincident LS mineralization at Hishikari and Ohkuchi was affiliated with the rhyolite volcanism. Another change of the subduction direction of the Philippine Sea plate to the northwest occurred at 2–1 Ma. The forearc sliver of the Southwest Japan arc shifted westward, in association with right-lateral strike-slip faulting along the Median tectonic line, due to the increase of the westward convergence rate. This shift resulted in shortening and cessation of graben development in the Hohi area, restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben.  相似文献   

12.
利用日本气象厅(JMA)以及日本国立大学联合地震观测台网(JUNEC)记录到的3218个地震事件的231918条P波到时资料,反演求得西南日本160km深度范围内的三维P波速度结构。研究表明,在九州地区,俯冲的菲律宾海板块以高速为主要特征,该海洋板块在30～60km深度处的脱水使得弧前地幔楔顶端的橄榄石蛇纹岩化,在120km深度处的脱水使得地幔楔中的岩石局部熔融,融体上升引起该区的火山活动。在本州西部地区大山火山之下,低速异常显著,并伴随低频地震活动,说明该火山可能是个潜在的活火山,将来有喷发的可能性。  相似文献   

13.
扬子板块的地壳演化与地层对比   总被引：3，自引：2，他引：3  
扬子板块是在古元古－新太古宙的古陆核基础上发展演化而成。吕梁运动以前，以川中微型陆块为主体的若干个小型古陆块组成向北西突出的牛轭形陆块带。中元古宙在牛轭形陆块带的东西两侧形成海沟岛弧系，沉积一套弧前及弧后的优地槽和旰地槽建造。１０－１１亿年时的东川运动使沟弧系褶皱回返，固结为陆新元古宙早期产生大陆裂谷，形成五一套裂谷建造，晋宁运动使扬子板块整体固结新元古宙晚发育山前磨拉建造、冰碛层及碳酸盐盖层。全  相似文献   

14.
陆弧和弧前盆地是俯冲体系中具有密切联系的构造单元。中生代以来,华南受多期板块俯冲的控制,发育大规模岩浆岩带及海域广泛分布的弧前盆地。但陆域弧岩浆岩较少,海域又缺乏足够钻井,各时期陆弧的位置存在较大争议,同时,南海北部至东海一带弧前盆地也缺乏系统认识,因此,亟须新的研究思路深化对华南晚中生代俯冲体系和俯冲过程的认识。本文以前人研究为基础,对海域钻遇中生界的典型钻井进行了详细分析,系统开展了海域盆地区域构造和沉积对比,将弧前盆地发育与岛弧变迁相结合综合分析。结果表明早侏罗世—早白垩世陆弧位于南海北部—东海靠近陆域一侧,经历了早侏罗世局限陆弧、中晚侏罗世沿海陆弧带、早白垩世向海沟方向的迁移。在此过程中,华南海域弧前盆地群于中侏罗世正式形成,早白垩世发育盆缘角度不整合,粗碎屑相带向海沟方向迁移,晚白垩南海北部与东海各自进入新的构造体制,结束弧前盆地的发育。华南沿海海域中生代盆地的发育可为陆弧的展布提供重要约束,弧岩浆岩带的迁移控制了弧前盆地的演化。  相似文献   

15.
M.G. Audley-Charles   《Tectonophysics》2004,389(1-2):65-79
The bathymetry and abrupt changes in earthquake seismicity around the eastern end of the Java Trench suggest it is now blocked south–east of Sumba by the Australian, Jurassic-rifted, continental margin forming the largely submarine Roti–Savu Ridge. Plate reconstructions have demonstrated that from at least 45 Ma the Java Trench continued far to the east of Sumba. From about 12 Ma the eastern part of the Java Trench (called Banda Trench) continued as the active plate boundary, located between what was to become Timor Island, then part of the Australian proximal continental slope, and the Banda Volcanic Arc. This Banda Trench began to be obliterated by continental margin-arc collision between about 3.5 and 2 Ma.The present position of the defunct Banda Trench can be located by use of plate reconstructions, earthquake seismology, deep reflection seismology, DSDP 262 results and geological mapping as being buried under the para-autochthon below the foothills of southern Timor. Locating the former trench guides the location of the apparently missing large southern part of the Banda forearc that was carried over the Australian continental margin during the final stage of the period of subduction of that continental margin that lasted from about 12 Ma to about 3.5 Ma.Tectonic collision is defined and distinguished from subduction and rollback. Collision in the southern part of the Banda Arc was initiated when the overriding forearc basement of the upper plate reached the proximal part of the Australian continental slope of the lower plate, and subduction stopped. Collision is characterised by fold and thrust deformation associated with the development of structurally high decollements. This collision deformed the basement and cover of the forearc accretionary prism of the upper plate with part of the unsubducted Australian cover rock sequences from the lower plate. Together with parts of the forearc basement they now form the exposed Banda orogen. The conversion of the northern flank of the Timor Trough from being the distal part of the Banda forearc accretionary prism, carried over the Australian continental margin, into a foreland basin was initiated by the cessation of subduction and simultaneous onset of collisional tectonics.This reinterpretation of the locked eastern end of the Java Trench proposes that, from its termination south of Sumba to at least as far east as Timor, and probably far beyond, the Java-Banda Trench and forearc overrode the subducting Australian proximal continental slope, locally to within 60 km of the shelf break. Part of the proximal forearc's accretionary prism together with part of the proximal continental slope cover sequence were detached and thrust northwards over the Java-Banda Trench and forearc by up to 80 km along the southwards dipping Savu Thrust and Wetar Suture. These reinterpretations explain the present absence of any discernible subduction ocean trench in the southern Banda Arc and the narrowness of the forearc, reduced to 30 km at Atauro, north of East Timor.  相似文献   

16.
In Victoria, tholeiitic basalts and subvolcanic dolerites andgabbros in the Heatheote and Mount Wellington Greenstone Beltsoccur stratigraphically above boninites, and in the latter beltcontain interbedded arc-derived andesitic detritus. Intrusiverocks form concordant, sill-like bodies up to 1 km thick whichare often strongly differentiated, chemically and mineralogically.The volcanic section in both greenstone belts, between 1000and 1500m thick, is dominated by aphyric or sparsely augitephyrictholeiites which trend toward ferrotholeiite compositions andare comagmatic with underlying intrusives. The tholeiites are a low-K2O suite with chondritic REE patterns,initial Nd= +4?5 to +5?0, and trace element characteristicsmost like those shown by modern basalts erupted during the earlieststages of arc rifting and backarc basin opening. From virtuallyidentical parental magmas, the basalts in the Heathcote GreenstoneBelt show a very rapid increase in TiO2, Zr, Y, and La withincreasing differentiation relative to those in the Mount WellingtonGreenstone Belt. This is explained using O'Hara's (1977) modelfor open system fractionation, in which magma supply and eruptionrate is significantly higher for the Heathcote belt magma chambers. The proposed petrogenetic model for the Victorian Cambrian tholeiitesinvolves subduction of an oceanic spreading centre beneath anearly Cambrian forearc region. Continued magma generation atthe subducted spreading centre resulted in diapirs of 1400?CMORB-source mantle piercing into the over-riding plate and providingheat to locally generate boninites from shallow, refractory,hydrous upper mantle beneath the forearc. Subsequent partialmelting of the MORB-source diapirs yielded tholeiitic magmas,which ascended to erupt upon the very slightly older boninites,and eventually split the arc and open a backarc basin as magmasupply became more voluminous. In this forearc setting, thesource-to-eruption crustal thickness through which the earliesterupted tholeiites had to traverse was notably greater thanat typical spreading centres, and basaltic magmas were ableto differentiate through to ferrobasaltic compositions. Theboninite-tholeiite sections of the Heathcote and Mount WellingtonGreenstone Belts are therefore considered to represent a recordof the earliest stages of magrnatism accompanying ridge subductionand splitting of an arc to form a backarc basin.  相似文献   

17.
Recent tomographic investigations performed down to ~300?km depth in the Calabrian Arc region gave insight in favor of the hypothesis that the Ionian subducting slab is continuous in depth beneath the central part of the Arc, while detachment of the deep portion of the subducting structure may have already taken place beneath the edges of the Arc itself. In the present study, we perform new geophysical analyses to further explore the structure of the subduction system and the structure and kinematics of the crustal units in the study area for a more comprehensive view of the local geodynamic scenario. Local earthquake tomography that we address to the exploration of the upper 40?km in the whole region of southern Italy furnishes P-wave velocity domains, suggesting southeast-ward long-term drifting of the southern Tyrrhenian unit with an advancement front matching well with the segment of Calabrian Arc where the subducting slab was found continuous and trench retreat can be presumed to have been active in the most recent times. This scenario of retreating subduction trench inducing drifting of the lithospheric unit overriding the subducting slab is further supported by the analysis of gravity anomalies, allowing us to better constrain the transitional zones between different subduction modes (continuous vs. detached slab) along the Arc. Also, the relocation of recent crustal seismicity, associated with geostructural data taken from the literature, provides evidence for NW-trending seismogenic structures in northeastern Sicily and northern Calabria that we interpret as Subduction-Transform Edge Propagator (STEP) faults guiding the southeast-ward drifting process of the southern Tyrrhenian unit. Crustal earthquake relocations show also seismolineaments in southern Calabria corresponding to the NE-trending longitudinal structures of the Arc where the great shallow earthquakes of 28 December 1908, and 5 and 7 February 1783 occurred. Seismicity and the extensional stress regime detected in these structures find also reasonable location in the proposed scenario, being interpretable in terms of shallow response of the central segment of the Arc to slab rollback and trench retreat.  相似文献   

18.
We determine detailed 3-D Vp and Vs structures of the crust and uppermost mantle beneath the Kyushu Island, southwest Japan, using a large number of arrival times from local earthquakes. From the obtained Vp and Vs models, we further calculate Poisson’s ratio images beneath the study area. By using this large data set, we successfully image the 3-D seismic velocity and Poisson’s ratio structures beneath Kyushu down to a depth of 150 km with a more reliable spatial resolution than previous studies. Our results show very clear low Vp and low Vs anomalies in the crust and uppermost mantle beneath the northern volcanoes, such as Abu, Kujyu and Unzen. Low-velocity anomalies are seen in the mantle beneath most other volcanoes. In contrast, there are no significant low-velocity anomalies in the crust or in the upper mantle between Aso and Kirishima. The subducting Philippine Sea slab is imaged generally as a high-velocity anomaly down to a depth of 150 km with some patches of normal to low seismic wave velocities. The Poisson’s ratio is almost normal beneath most volcanoes. The crustal seismicity is distributed in both the high- and low-velocity zones, but most distinctly in the low Poisson’s ratio zone. A high Poisson’s ratio region is found in the forearc crustal wedge above the slab in the junction area with Shikoku and Honshu; this high Poisson’s ratio could be caused by fluid-filled cracks induced by dehydration from the Philippine Sea slab. The Poisson’s ratio is normal to low in the forearc mantle in middle-south Kyushu. This is consistent with the absence of low-frequency tremors, and may indicate that dehydration from the subducting crust is not vigorous in this region.  相似文献   

19.
We studied metasediments and mylonitic arc granitoids from the forearc–arc transition of southern Baja California, Mexico. Thin section analyses and field evidence show that metamorphism of the forearc–arc transition is of the high T/P active margin type. The heat was provided by Cretaceous arc intrusions. Field observations and thin section analyses, including the time/temperature deformation path, demonstrate that the study area was first affected by dextral, ductile shearing followed by ductile, sinistral, possibly transpressive strike-slip parallel to the magmatic arc during the Cretaceous. Both intervals are related to changing oblique plate convergence and, thus, identified as trench-linked strike-slip effects. The geometric relationship between arc-dipping foliation, stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing. The sinistral interval lasted up until regional cooling (Early Cenozoic?). Because the La Paz fault is closely associated with the forearc–arc transition, it must have the same Cretaceous to Early Cenozoic kinematic history. The northern segment of the La Paz fault is a modern, brittle, strike-slip fault interpreted as a dextral synthetic fault of the San Andreas system which opened the Gulf of California (Mar de Cortés/Golfo de California). We found no evidence for Miocene Basin and Range extension.  相似文献   

20.
Integration of petrographic and geochemical data on each island in the Banks Group, northern Vanuatu (New Hebrides) has revealed a decrease in K2O (and related incompatible elements) across the islands, away from the New Hebrides trench. The correlation with depth to the Benioff zone is therefore the reverse of that typically found in island arcs. REE evidence and source modelling indicates that this variation represents a progressive depletion in LREE/HREE in the upper mantle, laterally away from the trench and a progressive increase in partial melting of the source, in the same direction. These variations in chemistry are attributed to an earlier west-dipping Miocene subduction system, and the variation in degree of partial melting to the location of the islands with respect to the active back-arc basin. An enrichment of the upper mantle in the Miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements, from the west-dipping Benioff zone, and subsequent reaction with the overlying upper mantle. The model thus envisages approximately contemporaneous development of the Central Chain volcanoes and the backarc basin in the late Pliocene, with partial melting of hydrous, laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back-arc basin. The role of the present subduction regime in magma generation is considered to be very limited.  相似文献   

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