Chronological constraints on the tectonic evolution of the Chinese Tianshan Orogen through detrital zircons from modern and palaeo-river sands

The Chinese Tianshan Orogen marks prolonged and complicated interactions between the southwestern Palaeo-Asian Ocean and surrounding blocks. New and previously published detrital zircon chronological data from modern and palaeo-river sands were compiled to reveal its tectonic evolution. It is characterized by predominant Palaeozoic as well as minor Mesozoic and Precambrian detrital zircon ages with a multimodal characteristic. The oldest Phanerozoic zircon population (peaking at 475 Ma) is a result of subduction and closure of the early Palaeozoic Terskey Ocean. However, the absence of this peak in the Chinese North and southern South Tianshan suggests that subductions of the North and South Tianshan oceans may not have initiated until the Late Ordovician with subsequent 460–390 and 360–320 Ma arc magmatism. Similar to the magmatic suite in classic collisional orogens, the youngest massive 320–270 Ma magmatism is suggested to be post-collisional. The North and South Tianshan oceans therefore probably had their closure to form the Chinese Tianshan Orogen during the late Carboniferous. The weak Mesozoic intra-plate magmatism further rejects a late Permian–Triassic Tianshan Orogen due to a lack of extensive syn- and post-collisional magmatism. Moreover, diverse Precambrian detrital zircon age patterns indicate that the surrounding blocks have distinct evolutionary processes with short-term amalgamation during the Meso- to Neoproterozoic.     

再论印度与亚洲大陆何时何地发生初始碰撞

印度与亚洲大陆碰撞形成了喜马拉雅造山带.该造山带是当今固体地球科学研究的重点和热点,是建立新的大陆动力学理论的最佳天然实验室.印度与亚洲大陆碰撞时限是正确认识和理解该造山带形成与演化、高原隆升的动力学过程等的起点.近南北向陆陆碰撞的最直接证据是碰撞带两侧块体在古纬度上的相互重叠.本文拟通过对相关古地磁资料的分析,结合近年来在拉萨地块南缘林子宗群火山岩和沉积岩夹层上获得的最新古地磁结果,探索当今古地磁数据所限定的印度和亚洲大陆发生初始碰撞的时间和古地理位置.结果表明,拉萨地块林子宗群形成时期(约64~44 Ma)古亚洲大陆最南缘的古地理位置(~10°N)限定了印度与亚洲大陆的初始碰撞最可能发生在65~50 Ma之间;如果以由印度洋海底地形所限定的东冈瓦纳大陆裂解前的印度板块形状为大印度模型,则印度与亚洲大陆的初始碰撞很可能发生在60~55 Ma之间.     

Recognition of Paleo-Tethys Suture Zone in eastern Myanmar

The site of final closure of Paleo-Tethys Ocean during the Triassic-Jurassic was marked by the colli-sional event between the Shan Massif and Indochina plate, in eastern Myanmar. Previous site of consumption of Paleo-Tethys is now occupied by a complex zone of suturing and deformation along Than Lwin River and in the east of it, formed by drastic tectonic evolution. It is geographically located approximately between longitudes 98E and 100E, and it extends northwards to Yunnan and southwards into West Thailand. The sigmoidal wrench struc-tures of suture zone is situated in the east of step-like structures of Shan Massif, and the western boundary of the suture zone must be immediately on the west side of Than Lwin River and the eastern boundary is near the east border of Myanmar. The area is largely covered by younger rocks of Paleozoic and Mesozoic age. The Upper Pa-leozoic units of Carboniferous to Permian comprise limestone and dolomite of the Plateau Limestone overlying unconformably the Lower Paleozoic rocks. Metasedimentary rocks of the Carboniferous and their metamorphic equivalents and the flysch-like Triassic-Jurassic units are found as folded and thrusted beds in the area. Large complex of granitic rocks of Late Triassic are also found. Metavolcanics, gabbro and a large number of chromite occur along with these units. A preliminary delineation of a suture zone as the site of closure of Paleo-Tethys Ocean in Myanmar territory is made for the first time and this zone is believed to be a tectonic linkage between Changning-Menglian belt of West China and Inthanon Zone of West Thailand.     

Paleomagnetic constraints on Miocene rotation in the central Japan Arc

We present new Middle Miocene paleomagnetic data for the central Japan Arc, and discuss their implications for Miocene rotation. To obtain a refined paleodirection, we made magnetic measurements on basaltic to andesitic lavas and intrusive rocks from 12 sites in the Tsugu volcanic rocks (ca 15 Ma) in the northern part of the Shitara area, Japan. Significant secondary magnetizations in samples with strong magnetic intensities are interpreted as lightning‐induced components. Mean directions carried by magnetite and/or titanomagnetite were determined for all sites. An overall mean direction with a northerly declination was obtained from dual‐polarity site means for nine sites. This direction is indistinguishable from the mean direction for coeval parallel dikes in the northern part of the Shitara area, and also indistinguishable from the Miocene reference direction derived from the paleopole for the North China Block in the Asian continent. These comparisons suggest little or no rotation or latitudinal motion in the study area with respect to the North China Block since 15 Ma. We obtained a refined early Middle Miocene paleodirection (D = 9.7°, I = 52.5°, α₉₅ = 4.8°; 30 sites) and paleopole (82.0°N, 230.8°E, A₉₅ = 5.6°) for Shitara by combining data from the Tsugu volcanic rocks and a coeval dike swarm. An anomalous direction found at three sites could be a record of an extraordinary field during a geomagnetic polarity transition or excursion. Paleomagnetic data from Shitara suggest that: (i) the western wing of the Kanto Syntaxis, a prominent cuspate geologic structure in central Honshu, underwent a counterclockwise rotation with respect to the main part of the southwestern Japan Arc between ca 17.5 Ma and 15 Ma; (ii) collision between the Japan and Izu–Bonin (Ogasawara) Arcs began prior to 15 Ma; and (iii) clockwise rotation of the entire southwestern part of the Japan Arc had ceased by 15 Ma.     

Chronology of subduction and collision along the İzmir-Ankara suture in Western Anatolia: records from the Central Sakarya Basin

ABSTRACT

Western Anatolia is a complex assemblage of terranes, including the Sakarya Terrane and the Tauride-Anatolide Platform that collided during the late Cretaceous and Palaeogene (80–25 Ma) after the closure of the Izmir-Ankara Ocean. Determining the precise timing at which this ocean closed is particularly important to test kinematic reconstructions and geodynamic models of the Mediterranean region, and the chronology of suturing and its mechanisms remain controversial. Here, we document the Cretaceous-Eocene sedimentary history of the Central Sakarya Basin, along the northern margin of the Neotethys Ocean, via various approaches including biostratigraphy, geochronology, and sedimentology. Two high-resolution sections from the Central Sakarya Basin show that pelagic carbonate sedimentation shifted to rapid siliciclastic deposition in the early Campanian (~ 79.6 Ma), interpreted to be a result of the build-up of the accretionary prism at the southern margin of the Sakarya Terrane. Rapid onset of deltaic progradation and an increase in accumulation rates in the late Danian (~ 61 Ma), as well as a local angular unconformity are attributed to the onset of collision between the Sakarya Terrane and the Tauride-Anatolide Platform. Thus, our results indicate that though deformation of the subduction margin in Western Anatolia started as early as the Campanian, the closure of the ?zmir-Ankara Ocean was only achieved by the early Palaeocene.     

青藏高原东南缘哀牢山构造带泥质高压麻粒岩的发现及其构造意义

哀牢山构造带泥质高压麻粒岩主要由石榴石、夕线石、钾长石和斜长石变斑晶及尖晶石、铁假蓝宝石、蓝晶石、石英、金红石和钛铁矿包裹体组成,为确定印支地块和华南地块的边界提供了关键性标志。石榴石-黑云母-斜长石-石英地质温压计(GBPQ)计算结果及标志性高温矿物组合(Spl+Qz)表明泥质高压麻粒岩的形成和演化经历了高压/高温进变质到中温/低压退变质的顺时针P-T演化过程。其中:1)高压/高温进变质阶段的矿物组合为Ky+Sil+Grt1+Kf1+Pl1+Spr+Ter(Kf+Pl)+Bt1+Spl+Qtz+Ilm1+Rut1,形成于850～919℃,≥10.4kbar;2)中温/低压退变质阶段的矿物组合为Grt2+Bt2+Pl2+Ms+Qtz+Ilm2+Rut2,早期和晚期的温压条件分别为664～754℃,4.9～6.5kbar和572～576℃,3.5～3.9kbar。反映陆壳物质在碰撞过程中俯冲到地下深处(≥30km)经高压高温变质后快速折返到中上地壳的动力学演变轨迹。     

Identification of Archaean plate tectonic processes from multidimensional discrimination diagrams and probability calculations

We applied our group's previously published multidimensional diagrams in 2006–2012 and corresponding probability estimates in 2011–2012 to geochemical data for Archaean rocks compiled from cratons in Australia, South Africa, Brazil, Canada, and India. Tectonic processes similar to present-day plate tectonics evidently were active at least since the Palaeoarchaean (?3570 Ma). This seems to be true in spite of a presumably hotter Earth at that time. For the eastern part of the Pilbara craton (Australia), a Palaeoarchaean (3570–3450 Ma) and Mesoarchaean (2900 Ma) continental arc setting apparently evolved to a collision (Col) setting during the Neoarchaean (2600 Ma). We infer an island arc (IA) environment for Kambalda (Australia) during the Neoarchaean (2700 Ma). For the Barberton belt (South Africa), a transition from a mid-ocean ridge regime during the older part of the Palaeoarchaean (3470 Ma) to an IA setting during the younger part (3300–3260 Ma) is likely. We inferred an arc environment for the São Francisco craton (Brazil) and the Rio Maria terrane (Brazil) during the Mesoarchaean (3085–2983 Ma and 2870 Ma, respectively), whereas a within-plate setting is clearly indicated for the Carajás metallogenic province (Brazil) during the Neoarchaean (2740–2700 Ma). We also recognize an IA regime for the Mesoarchaean (3000 Ma) North Caribou and Neoarchaean (2700 Ma) Abitibi greenstone belts (Canada), and for the Gadwal greenstone belt (India) during the Neoarchaean (2700–2500 Ma). A Col setting was inferred for the Archaean sanukitoid suite (Canada) and the Kolar suture zone (India) during the Neoarchaean (2700–2660 Ma and 2630–2520 Ma, respectively).     

Kinematics of the Guerrero terrane accretion in the Sierra de Guanajuato,central Mexico: new insights for the structural evolution of arc–continent collisional zones

The Guerrero terrane comprises Middle Jurassic–Early Cretaceous arc successions that were accreted to the North American craton in the late Early Cretaceous, producing closure of the Arperos oceanic basin and the formation of an approximately 100 km-wide fold–thrust belt. Such a suture is key to investigating the structural evolution related to Guerrero terrane accretion and, in general, to arc–continent collisional zones. The Sierra de Guanajuato is an exposure of the Guerrero terrane suture belt and consists of a complex tectonic pile that formed through at least three major shortening phases: D1SG, D2SG, and D3SG (SG, Sierra de Guanajuato). During the D1SG and D2SG phases, the Upper Jurassic–Lower Cretaceous successions of the Arperos Basin piled up, forming a doubly vergent imbricate fan of thrust sheets that accommodated substantial NE–SW shortening. Mylonite microtextures, as well as syntectonic minerals, indicate that the D1SG and D2SG deformation events took place under low greenschist-facies metamorphic conditions. We relate these deformation phases to the progressive NE migration of the Guerrero terrane, which triggered the collapse and closure of the Arperos Basin. During D3SG, the El Paxtle arc assemblage of the Guerrero terrane was tectonically emplaced onto the previously deformed successions of the Arperos Basin. However, D3SG structures indicate that during this deformational stage, the main shortening direction was oriented NW–SE and that contraction was accommodated mostly by SE-vergent ductile thrusts formed under low greenschist-facies metamorphic conditions. We suggest that the top-to-the-SE emplacement of the El Paxtle assemblage may be a result of the tectonic escape of the arc produced by the continuous NE impingement of the Guerrero terrane during its collisional addition to the Mexican mainland.     

雅鲁藏布江缝合带中段构造特征及成因模式新见解

雅鲁藏布江缝合带中的蛇绿岩在西藏南部不同地段,表现形式并不相同.通过对缝合带中段的构造特征及蛇绿岩组合特征的深入研究,认为印度板块的大陆地壳北缘具有特殊的波状弯曲的几何边界--东西两端为向北突出的犄角,构造结之间为向南突出的弧形边界.这种特殊的边界条件,在新特提斯洋俯冲碰撞过程中,新特提斯洋首先在突出结点处完成关闭,而...     

Petrology and Geochemistry of Post-Collisional Early Miocene Volcanism in the Karacada? Area (Central Anatolia, Turkey)

Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism.