Forearc Research Articles

From the southern Gangdese Yeba arc to the Bangong-Nujiang Ocean: Provenance of the Upper Jurassic-Lower Cretaceous Lagongtang Formation (northern Lhasa, Tibet)

The stratigraphic records of sedimentary basins situated on the Lhasa block provide crucial evidence to reconstruct the early history of the Tibetan Plateau associated with the successive closure of oceanic branches and related magmatic activities. The Upper Jurassic to Lower Cretaceous Lagongtang Formation of the Nagqu Basin located in the northern part of the Lhasa block chiefly consists of fine-grained quartzose to litho-quartzose sandstones, siltstones and shales deposited in continental shelf to bathyal environments. Provenance data, including sandstone petrography, U-Pb ages and in situ Hf isotopes of detrital zircon, together with paleocurrent indicators, suggest that the Lagongtang Formation was mainly fed by recycling of quartz-rich siliciclastic strata of the central-southern Lhasa block. Fresh felsitic volcanic rock fragments, plagioclase prevailing among detrital feldspars, and zircon grains yielding the same age as that indicated by biostratigraphic analysis testify to penecontemporaneous magmatic activity. Volcanic detritus may have been derived from the Middle Triassic-Lower Cretaceous Yeba and/or Sangri volcanics exposed in the southern Lhasa block to the south as well as from Middle Triassic-Lower Cretaceous magmatic rocks of the central Lhasa block. Although the Nagqu Basin is adjacent to the Bangong–Nujiang suture, no debris from Bangong–Nujiang suture and Qiangtang was found in the Lagongtang Formation here. Our provenance data rule out the hypothesis that the Lagongtang Formation could have been deposited in a forearc basin associated with the southward subduction of the Bangong-Nujiang Ocean or in a basin related to the supposed Shiquanhe-Namco Ocean.

Lithogeochemical features and formation environment of the Late Jurassic–Early Creaceous tuff-terrigenous rocks of Ust-Belsky Mountains (North-west of Koryak highlands)

The paper presents the results of studying the lithochemical features of the Late Jurassic-Early Cretaceous tuff-terrigenous rocks of the Ust-Belsky and Algan terranes. Tuff-terrigenous rocks represented by medium-fine-pelitic-grained quartz-feldspar and feldspar greywackes. According to the results, they were formed due to the erosion of intermediate and basic composition volcanic formations, in the marine environment, near the coast. The main source was associated with the island arc in the northwest of the region. Tuff-terrigenous rocks of Ust-Belsky terrane formed in different parts of the forearc basin. Tuff-terrigenous rocks of Algan terrane formed in the trenchslope basin.

Paleoenvironmental Evolution of a Forearc in Response to Forcings by Drainage, Climate, Volcanism, and Tectonics: The Quillagua Depocenter, Chile

Abstract The Late Miocene and Pliocene Quillagua depocenter lake system existed in a forearc basin on the west side of the Andes Mountains in northern Chile, alternating between standing-water and salar conditions. Quaternary incision of the Loa River Canyon resulted in bypass of the prior depositional surface and drainage of groundwater from the abandoned depocenter. Systematic regional geological mapping, 32 new chronological constraints on the strata in the basin, outcrop-scale facies analyses, and geophysical data underpin a revised evaluation of the controls on the lake system. The progressive stages, ages, and causes of the Quaternary destruction of the lake system are reconstructed based on mapped distributions of superficial fluvial sediments, chronological studies of terrace deposits, and landform analysis. The lake system occurred at the junction of small catchments draining the slowly rising western Andean foothills and the large paleo-Loa River catchment draining the Andean volcanic arc, during a time span of intense caldera activity. Small magnitude climate variability affected both the hyperarid low elevation sectors and arid upper sectors of the catchments. By 10 Ma, the regional climate was extremely arid, limiting water and sediment to small amounts, and during the Late Miocene and Pliocene, there was no surface-water outlet to the Pacific. Hydrological variations from 9 to 2.6 Ma led to sediment accumulation in variable lake environments, alternating with long hiatuses. Minor deformation within the Quillagua depocenter shifted the topographic axis and groundwater outlets. Simultaneous headward erosion from the Pacific shore captured the Loa River, which triggered large-magnitude incision that persists today. The progression of surface water environmental change was accompanied by changing composition and amount of surface and groundwater, which determined deposition of primary evaporite minerals, extensive diagenesis, and eventually, complex patterns of dissolution expressed as karst.

Comment on “The Chongón/Colonche orocline transrotational bending and the coeval opening of the Progreso basin in southwestern Ecuador” by Alemán et al

• Lack of evidence to support a counterclockwise rotation of the Piñon block and the coeval opening of the Progreso Basin. • Forearc basin controlled by the complex interplay of oblique subduction, rise of an outer forearc high, and tectonic escape. • Post-accretion strain partitioning across an outer wedge with a varying trench-parallel oriented deformation.

Geochronology and geochemistry of the Huntington Formation, Olds Ferry terrane, Blue Mountains province, northern U.S. Cordillera: Implications for accreted terrane correlation and assembly

AbstractThe Olds Ferry terrane is the more inboard of two accreted volcanic arc terranes in the late Paleozoic–early Mesozoic Blue Mountains province of the northern U.S. Cordillera. We present geologic, geochronologic, and geochemical data from the volcano-sedimentary Huntington Formation of the Olds Ferry arc that place the terrane within a firm temporal and tectonomagmatic context, and establish its identity as a fringing arc terrane along the Triassic to Early Jurassic Cordilleran margin. The Huntington Formation is divided into two unconformity-bounded informal members: a Norian (ca. 220 Ma) lower member comprising a sequence of mafic-intermediate volcanics, massive volcaniclastic breccias, and minor carbonates deposited unconformably onto the 237.7 Ma Brownlee pluton and intruded by the 210.0 Ma Iron Mountain pluton; and a Rhaetian through Pleinsbachian (<210–187.0 Ma) upper member composed of massive conglomerates, abundant rhyodacite to rhyolite effusive and pyroclastic flows, and interlayered sandstone turbidites, deposited with angular unconformity onto the lower member. An erosional hiatus and regional tilting produced an angular unconformity separating the Huntington Formation from the overlying basal conglomerates of the late Early to Middle Jurassic Weatherby Formation of the Izee forearc basin transgressive onlap sequence. Huntington Formation volcanic rocks are isotopically enriched relative to depleted mantle and coeval igneous rocks in the outboard Wallowa terrane. A temporal evolution to more radiogenic 87Sr/86Sr ratios (0.7036–0.7057) and εNd values (+5.4 to +3.1) in the upper member volcanics suggests increasing involvement of continental-derived material in their petrogenesis. Precambrian xenocrystic zircons in both lower and upper member volcaniclastic rocks strongly support a proximal location of the Olds Ferry terrane to cratonal North America during much of its history. The chronology and tectonostratigraphic architecture of the Olds Ferry terrane allows its robust correlation to other fringing-arc terranes along the U.S. and Canadian Cordillera.

Cenozoic sequence stratigraphy and sedimentary evolution of forearc basin in Ryukyu Island Arc region

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。;The Ryukyu forearc basin is located at the convergence of the northern Philippine Sea plate and the Eurasian plate. It is developed between the accretionary wedge in the northern Ryukyu trench and the Ryukyu Island Arc. It is a component unit of a typical 'trench-arc-basin' system. Now, based on the multi-channel seismic data, the sequence stratigraphic framework established for the first time in the study area is used to analyze the Cenozoic sequence stratigraphy and sedimentary evolution characteristics, and discuss the main source of sediment. According to the characteristics of seismic reflection and the contact relationship of reflection layer group, it is revealed that the Cenozoic in the study area experienced the process of shallow marine facies, semi-deep marine facies and deep-sea facies. It is considered that island arc metamorphic basement sedimentation developed in Eocene, and the basement sedimentation period of residual extensional basin from Late Eocene to Early Miocene belongs to the basement sedimentation period of fore arc basin, which is a shallow sea environment and mainly develops pyroclastic lithofacies; The Middle Miocene to Quaternary is the main sedimentary period of the forearc basin. It transited from the bathyal to the deep-sea sedimentary environment. The local volcanic clastic facies developed many typical sedimentary systems, such as gravity flow, turbidite fan, underwater delta and turbidite channel. It is inferred that the South Ryukyu Islands in the northern part of the Middle Miocene was the main provenance in the forearc basin period. Since the Late Miocene, the land area in the northeast of Taiwan Island has become the largest provenance, and the South Ryukyu Islands is in the second place. The results are an extension of the study on the basin control of the Ryukyu Island Arc and its surrounding structures, and have important guiding significance for the study of the 'source-sink' system of the land and eastern sea area of Taiwan Island.

Arc tempos of the Gangdese batholith, southern Tibet

The character of arcs varies over time with significant temporal fluctuations in the quantity and spatiotemporal patterns of magmatism. However, the driving mechanisms for this episodic behavior of arcs need more constraints. This paper analyzed the published data along with our new zircon U-Pb dating and Hf isotopic and whole-rock geochemical data of plutonic rocks in the Gangdese belt in southern Tibet to explore the features, potential drivers, and tectonic implications of episodic arc activity in the Gangdese arc. A comprehensive compilation of U-Pb ages and Lu-Hf isotopic analyses of zircon grains from igneous rocks in the Gangdese belt, sedimentary rocks in trench fill sequences, forearc basins and foreland basins, and sands from modern river reveals that: 1) Gangdese arc activity was episodic during Late Cretaceous to Middle Eocene, displaying two magmatic flare-ups (ca. 100–80 and 65–45 Ma) and one magmatic lull (ca. 80–65 Ma), and 2) both flare-up magmas show relatively positive εHf(t) values (+5 ~ +15) indicative of juvenile sources suggesting these magmas are dominated by contributions from the depleted mantle. In contrast, the magmatic lull between these two magmatic flare-ups could be caused by flat subduction of the Neotethyan slab beneath the southern margin of the Lhasa terrane. These flare-ups likely contributed greatly to the crustal thickening of the Gangdese belt. Constraints from paleo-elevation and geochemical proxies for crustal thickness showed that the ~100–80 Ma flare-up was accompanied by the formation of a thick arc root while the ~65–45 Ma flare-up likely developed in a thinner crust without an arc root.

Metallogenic age and genesis of the Zhaishang gold deposit in Minxian, Gansu Province

寨上金矿位于西秦岭大规模卡林-类卡林型金矿床集中地之中，是近二十年来探获的又一超大型金矿。通过对寨上金矿的6件矿石样品中的载金矿物黄铁矿进行Rb、Sr、Re、Os同位素分析，获得Rb-Sr、Re-Os等时线年龄分别为273±10Ma、273Ma±22Ma。该数据大于前人所获的二期成矿阶段年龄，在西秦岭已探明的金矿床中尚无报道，反映了寨上金矿更早一期的成矿年龄为273Ma，这也为该区域寻找金矿床拓宽了时间轴。Sr、Os、S同位素及稀土和微量元素研究表明，寨上金矿成矿物质为深源与壳源物质的混合。二叠纪时位于岷礼前陆盆地的吴家山隆起经历了强烈的隆升延展作用，岩浆热液沿断裂通道上升到有较高Au背景值盆地中的褶皱带，使地层中的Au活化富集，在构造有利部位卸载成矿，形成早期寨上金矿床，后期钨矿化及金矿化叠加在早期金矿床之上。寨上金矿第一期金成矿的年龄在时空上与阿尼玛卿洋盆闭合、共和坳拉谷俯冲碰撞、江里沟及中川喜马拉雅型岩体的侵入时间基本吻合，符合“地壳加厚有利于形成金矿床及喜马拉雅型花岗岩与金矿关系密切”的结论，对在该区域寻找同时代金矿床具有一定的指示意义。;Zhaishang gold deposit is located in the concentrated area of large-scale carlin-like gold deposits in West Qinling Mountains, and it is the second super-large gold deposit discovered in the past 20 years. Rb, Sr, Re, Os isotopes of the pyrite, the gold-bearing mineral of Zhaishang gold deposit, in six ore samples were tested in this paper to determine their metallogenic ages and ore-forming materials. The measured Rb-Sr and Re-Os isochron ages of the pyrite are 273±10Ma and 273Ma±22Ma, which are older than the ages of two metallogenic stages of the Zhaishang deposit obtained by the predecessors, i.e., a earlier metallogenic stage (the first stage of 273Ma) might be exist in this area. This new chronological data may broaden the time axis for searching gold deposits in this area since no same metallogenic ages have ever been reported in the explored gold deposits in West Qinling. Furthermore, analyses results of Sr, Os, S isotopes, REE and trace elements compositions indicate that the ore-forming material of the Zhaishang gold deposit is the mixture of deep source and crust source materials. Tectonically, the Wujiashan Rise which is located in the Minli fore-arc basin experienced a strong geothermal uplift and extension in the Late Hercynian Period, which resulted in the hydrothermal fluids rising along the fault channels to the folded belts of the basin with a high Au background, and the activated Au enriched in the formation and unloaded in suitable structural spaces to form the first stage Zhaishang gold deposit. The later two stages of W and Au mineralization superimposed on the early gold deposit. Compared with regional geological evolutionary events, the age of gold mineralization in the first stage of Zhaishang gold deposit is consistent with the time of the closure of the A'nyemaqen ocean basin in the West Qinling, the subduction collision of Gonghe aulacogen, and the intrusion of Jiangligou and Zhongchuan Himalayan-type plutons, which obeys the rule crustal thickening is conducive to the formation of gold deposits and Himalayan granites are closely related to gold deposits. It is of indicative significance for searching contemporary gold deposits in this area.

Geochemistry of Upper Eocene-Oligocene sandstones from Tuzgölü Basin (Central Anatolia)

In this study, mineralogical and geochemical features of Upper Eocene-Oligocene sandstones exposed by the Tuzgölü Fault Zone (TFZ) at the eastern border of the Tuzgölü Basin were investigated. The absence of zircon enrichment in the Zr/Sc-Th/Sc diagram indicated no sedimentary recycling. This shows that the sandstones are first cycle sediments, that is, the material transported directly from the source. Critical element ratios for provenance such as La/Sc, La/Co, Th/Sc, Th/Co, Th/Cr, Zr/Sc, Zr/Co, Ba/Sc, and Ba/Co, Th/Sc-Eu/Eu* diagram and average Rare Earth Element (REE) pattern suggest a provenance in “intermediate magmatic” composition. The variation in the negative Ce anomaly effect observed between the lower and upper parts of the sequence indicates variation in the oxygen level of the water. In each of the La-Th-Sc, Th-Co-Zr/10, and Th-Sc-Zr/10 tectonic setting discrimination diagrams, the sandstone average fell onto the “Continental Island Arc” position. This tectonic setting defines the arc that developed along the continental margin of the subduction zone. The tectonic setting found for the basin, supported the evolution model that the Tuzgölü Basin developed as a fore-arc basin.

Zircon U-Pb and molybdenite Re-Os geochronology and geochemistry of the Tieling deposit in the Eastern Tianshan, NW China: Insights into the timing of mineralization and tectonic setting

The Aqishan-Yamansu belt in the Eastern Tianshan (NW China) contains many Carboniferous granitic intrusive rocks and volcanic rocks and hosts numerous large to medium sized Fe (-Cu) deposits. The Tieling deposit consisting of 89 iron ore bodies in the Aqishan-Yamansu belt has been mined since discovered in 1970 s. Mo mineralization was newly identified in concealed rocks of the Tieling deposit, and molybdenite is extensively distributed as a disseminated phase in the biotite moyite and occurs as aggregates in quartz veins. The molybdenite Re-Os model ages (300.6 ∼ 298.7 Ma) are consistent with the zircon U-Pb age (304.5 ± 2.4 Ma) of the ore-causative biotite moyite within error, which indicating that the Tieling Mo mineralization occurred during the Late Carboniferous. The Late Carboniferous biotite moyite exhibits calc-alkaline and peraluminous characteristics, with negligible Eu anomalies, akin to I-type granites. Dominantly positive εHf(t) values (+13.6 to + 15.2), variable Mg# values (31 to 49), and limited variation in (87Sr/86Sr)i ratios (ISr = 0.70358–0.70390), and positive εNd(t) values (+4.55 to 6.39), along with the juvenile TDM2 age (548 to 697 Ma), suggest that the biotite moyite was probably originated from partial melting of the juvenile lower crust with involvement of mantle-derived magmas. Integrating published data and our work, we conclude that the Aqishan-Yamansu belt evolved from the pre- Early Carboniferous forearc basin (ca. 450–350 Ma) to the Carboniferous island arc environment (ca. 350–310 Ma) triggered by the bidirectional subduction of the Kanggur Ocean, then the extensional geodynamic setting (ca. 310–270 Ma) is conducive to the porphyry Mo mineralization of the Tieling deposit and Ni-Cu-Au deposits in the Eastern Tianshan.

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

Abstract The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.

Origin of the Intra‐Oceanic Silverwood Block (New England Orogen, Australia): Evidence From Radiolarian Biostratigraphy and Detrital Zircon Petrochronology

AbstractCambrian to Triassic subduction processes in eastern Australia produced a complex and highly contorted assembly of supra‐subduction units. However, regardless of the long history of oceanic subduction, relatively little evidence exists on oceanic terranes whose accretion onto the continental margin may have accompanied subduction processes. We present new radiolarian and petrochronological data from Devonian and Carboniferous rocks exposed in the middle of a tight orocline. Based on radiolarian biostratigraphy and U‐Pb dating of detrital zircon grains, we show that volcaniclastic rocks from the Silverwood Group and Alice Creek beds were deposited during the Late Devonian, which is later than previously thought. Trace‐element compositions of the Devonian zircons are characteristic of crystallization of magmas in an oceanic environment, thus supporting previous suggestions that the Silverwood Group was derived from an intra‐oceanic arc system. Carboniferous zircons from a nearby forearc basin unit (Mount Barney beds) exhibit a continental affinity, indicating that accretion of the Silverwood Block likely occurred before the establishment of this continental arc. A serpentinite belt that occurs adjacent to the Silverwood Block might represent a remnant ophiolitic suture of the now‐consumed oceanic domain, which once separated the Silverwood Block from the eastern Gondwanan margin. Based on the assumption that such an intervening ocean existed, we discuss alternative scenarios for the origin and accretion of the Silverwood Block. The most likely scenario involves intra‐oceanic magmatism in a marginal oceanic basin whose development was driven by trench retreat, and a subsequent accretion in response to trench advance and/or slab flattening.

Provenance and tectonic setting of the Sumdo Formation in the Lhasa Terrane, Tibet: Implications for early subduction evolution of the Sumdo Paleo–Tethys Ocean

The tectonic setting of the Tangjia–Sumdo metamorphic belt, which separates the North Lhasa Block (NLB) and South Lhasa Block (SLB), remains controversial. Moreover, the sedimentary evolution of the Tangjia–Sumdo metamorphic belt has been little studied, particularly during the early subduction evolution of the Sumdo Paleo-Tethys Ocean (SPTO). Here we present petrological, geochemical, and zircon U-Pb geochronological data on the clastic rocks and metabasaltic interlayers collected from the Sumdo Formation in the Sumdo area. The Sumdo Formation consists of metamorphosed sandstone, graywacke, phyllite, and interlayered metabasalts. Detrital zircon U-Pb dating of the metasandstone and quartz schist identified a youngest zircon age group of 389–316 Ma, which have negative εHf(t) values (−20.5 to −1.5) and old Hf crustal model ages (TCDM = 1450–2654 Ma). The zircon trace element features and Hf isotopic compositions show that the sedimentary detritus was sourced from the NLB, which indicates that this terrane experienced Late Devonian–early Carboniferous magmatism. The ages of the youngest detrital zircons (YC2σ age of 324 ± 13 Ma) indicate that the Sumdo Formation formed mainly during the late Carboniferous–early Permian. Combined with geochemical data of the metabasalts and other regional geological evidence, we conclude that the Sumdo Formation formed in an initial forearc basin. The Sumdo Formation was likely deposited along an active continental margin according to the in–situ suture model, where northward subduction of the Sumdo Paleo–Tethys oceanic lithosphere occurred beneath the NLB. As such, the Sumdo Formation provides a sedimentary record of the early subduction evolution of the SPTO.

A Messinian Gypsum Deposit in the Ionian Forearc Basin (Benestare, Calabria, Southern Italy): Origin and Paleoenvironmental Indications

This study reports the first accurate record of the Messinian Resedimented Gypsum in the forearc and back-arc basins connected to the Calabrian-Peloritan orogen. A multidisciplinary approach has been used to investigate a gypsum deposit located in the Benestare’s area (Calabria, Southern Italy). Such deposit is made of bedded gypsrudites displaying clastic selenite with chaotical textures. On the top, the gypsrudites are interspersed with gypsum lenses belonging to the branching-like facies. Despite these two facies seem different macroscopically, they show petrographic features, fluid inclusions, organic matter and Strontium isotopic values very similar to each other. On the other hand, both facies show fractured and folded crystals. Crystals are only locally corroded and preserve primary structure relict as well as allochthonous (organic debris) and autochthonous putative microbial remains. All crystals are rich in fluid inclusions but these are visibly affected by stretching and leaking (re-equilibration processes) suggesting a moderate plastic deformation during re-sedimentation and subsequent burial. Minimal transport of the deposit is testified by subangular shapes of the gypsum crystals. The gypsrudite and branching-like facies reveal an 87Sr/86Sr average value of 0.709045 and 0.709082, respectively. These values suggest a strong connection with the global Ocean and reduced freshwater input. The Benestare’s deposit originated from the partial to complete dismantling of selenite crystals related to the first stage (5.97–5.60 Ma) of the Messinian Salinity Crisis through gravitational collapse due to local controlling factors.

Petrology and geochemistry of ultramafic and mafic rocks in the late Silurian-early Devonian Darbut ophiolitic mélange of west Junggar (NORTHWESTERN CHINA): implications for petrogenesis and tectonic evolution

ABSTRACT New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite.

Study on Medium-Thick Al-Alloy T-Joints by Dual P-GMAW Bilateral Synchronous Welding

The T-joints of medium-thick 6082 Al-alloy plates created by dual pulsed gas metal arc welding (P-GMAW) and bilateral synchronous welding were investigated to improve weld quality using the adaptive deposition method, which calculates the minimum amount of deposition according to the welding condition, groove size, and cross-sectional area, effectively reducing the heat input and deformation of the welds on the basis of weld filling. The optimized linear energy with a wire feed speed (WFS) of 9.5 m/min can ensure a well-formed weld with a complete root fusion, and high-quality T-joint welds were obtained both in root openings of 0 mm and 1 mm. The biggest penetration was 4 mm, which was four times more than that of the result from a single torch welding process. When the distance between the two welding torches exceeded 20 mm, the molten pool was completely separated, and process pores were observed in the unfused root zone. Influenced by the thermal cycles in asymmetric welding, the hardness distribution changed: the width of the softer zone at the base plate with the fore arc was smaller than that zone with the rear arc. Furthermore, dual P-GMAW bilateral synchronous welding with an asymmetric heat source can further reduce the deformation of the welded joint by about 20% compared to that of symmetric welding.

Paleomagnetism of the Carboniferous–Permian Myall blocks, Tamworth Belt, southern New England Orogen: Permian counterclockwise rotations and Triassic clockwise rotation

This paleomagnetic study on the Myall blocks of the southern New England Orogen complements studies on four other Tamworth Belt blocks aiming to unravel evolution of the Tamworth Belt and the Manning Orocline in particular. Focus is on ignimbritic successions that have partly interlinked stratigraphies across the five blocks and can retain primary magnetisations despite prevalent overprinting. This makes the Tamworth Belt prospective to oroclinal testing by comparing poles and pole paths for individual blocks across its curved structure. Paleomagnetic, rock magnetic and magnetic fabric results are detailed from an upper lower Carboniferous to lower Permian succession, sampled across forearc basin and arc sites (64) of the western Myall Block and forearc basin sites (16) of the eastern Myall Block. Predominantly thermal demagnetisations show a widely present low-temperature component, attributed to late Oligocene weathering, intermediate-temperature primary and overprint components confined to the Nerong Volcanics, and high-temperature primary and overprint components. Overprinting proved more prevalent in the Myall blocks than in other Tamworth Belt blocks, with five phases dated from mid-Carboniferous to mid-Triassic. Comparison of primary and overprint poles against Carboniferous, Permian and Triassic reference paths shows counterclockwise rotations ranging from 20–45° for the western Myall Block forearc basin, to 30° to 90° west to east across the southwestern Myall Block arc complex, to 85–110° for the eastern Myall Block forearc basin, occurring from the latest Carboniferous–early Permian Tablelands phase probably to the mid- to late Permian initial pulse of the Hunter-Bowen phase. The southwestern Myall Block also shows a mid-Triassic, or later, clockwise rotation ranging west to east from 50° to 110°. KEY POINTS Paleomagnetism of upper lower Carboniferous to lower Permian successions of the Myall blocks constrains evolution of the Manning Orocline. The Myall blocks are heavily affected by overprints, with five phases dated from mid-Carboniferous to mid-Triassic. Primary and overprint magnetisations show various counterclockwise and clockwise rotations of the western, eastern and southwestern parts of the Myall blocks. Counterclockwise rotations occurred from latest Carboniferous–earliest Permian probably to mid- to late Permian and clockwise rotation during or after mid-Triassic.

Comparison Study of Crustal Structure in Aceh Region based on Volcanic Arc System using Receiver Function Method

Aceh region has a very complex crustal structure from the forearc ridge to the backarc basin. This study aims to determine the velocity model of P and S waves and the depth of Moho discontinuity. This research was conducted using teleseismic earthquake data (30°-90° from the station) with M>6 from four seismic stations belonging to the BMKG in Aceh region. The stations are qualified based on the volcanic arc system zone. Furthermore, the velocity model determined by result of forward modelling, while the depth of the Moho layer estimated by migrated receiver function from time domain to the depth domain. At station SNSI that represented the forearc ridge zone, the depth of Moho is ±28 km, at station TPTI represent the forearc basin is ±16 km, while at zone with higher topography, namely volcanic arc zone represented by station KCSI, the Moho depth was identified at ±38 km, and the backarc basin represented by station LASI with ±40 km depth of Moho. This variation occurs because the composition of the earth’s layers below the station is diverse also different topography for each station.

Analysis of fault patters based on erarthquake data on the land of Sumatera island

Geodynamics of Sumatra is interesting to study because of the unique geological setting Geodynamics of Sumatra is interesting to study because of the unique geological setting and high seismicity. This high seismicity is caused by the many faults found on land and in the surrounding waters. This paper presents the results of research that aims to determine fault patterns both on land and in the waters around Sumatra based on earthquake data from 1960-2000. The area under study is at the coordinates of 6°N - 6°S and 95°E - 109°E at an epicenter depth of <60 km with a magnitude between 4-10. The area is divided into two zones, namely the front arc and the Barisan arc. Data were obtained from ISC and Global CMT. By using the Focal Mechanism Method, the results show that the fault pattern in the Sumatran forearc Basin zone is dominated by an Reverse Fault located in the accretion zone while on the mainland of Sumatra it is dominated by Strike Slip along the Sumatran Fault System. By knowing the position and pattern of the fault, especially on the mainland of the island of Sumatra, it can be used as a reference for spatial planning. In addition, further studies will also be able to learn about the dangers or disasters caused by the fault pattern.

Identification of Crustal Thickness in Central Part of Sumatra Using Teleseismic Receiver Function Method

The Central part of Sumatra is a region that has a high potential for earthquakes. This research intended to determine the crustal thickness of the earth, P and S wave velocity profiles, and vp/vs value in the Central part of Sumatra using stacking H-k and inversion techniques based on the analysis of receiver function. This study utilized teleseismic earthquake data with a distance of 30° to 90° from the station and magnitude more than 6 (M>6). The stations used in this study were 3 BMKG broadband stations located in 3 zones, the fore arc ridge zone (SISI), the volcanic zone (PLSI) and the back arc zone (TPRI). The crustal thickness varies in the fore arc ridge zone (SISI) estimated 17.8 km, volcanic zone (PLSI) reaches 29.7 km and the back arc zone (TPRI) reaches 34 km. The crustal thickness is quite thick under the PLSI and thicker beneath TPRI station. These possibly due to the effect of topography and isostatic compensation in the station. However, whether there is a correlation between crustal thickness and topography needs further research using more stations. The highest vp/vs value was found in the volcanic zone of 1.9, that might be associated with the presence partial melting beneath the station. Meanwhile, the vp/vs value in the back arc zone is 1.72, indicating a relatively more homogeneous structure.