Late Tertiary tectonic evolution of northern Iran: A case for simple crustal folding

Here we present a crustal folding or buckling mechanism to explain the rootless 3–5 km high Alborz Mountains in northern Iran as well as  10 km of Late Miocene to recent subsidence in the south Caspian basin and  3–6 km of subsidence in the central Iranian basin in the context of the middle Miocene to recent Arabia–Eurasia collision. A key element of the mechanism is the presence of lateral and vertical lithospheric strength contrasts between the north Iranian continental and south Caspian oceanic crusts: when compression from the collision is applied across the region, the strong south Caspian oceanic crust, buried under > 10 km of premiddle Miocene sediment, interacts with the bottom of the mechanically strong continental upper crust of northern Iran, resulting in upward buckling of the continental crust and downward buckling of the oceanic crust. We test this mechanism using a finite-element numerical model with a Maxwell rheology and obtain results that are consistent with the geological and geophysical observations. The observations compiled here and the model results demonstrate the potential for using this region as a natural laboratory for studying the early stages of continent–oceanic collision, including processes like basin inversion, fault localization and, potentially, subduction initiation.     

藏南冈底斯岩基日多地区花岗岩体形成时代和地球化学特征

冈底斯岩基广泛发育三叠纪-中新世的岩浆岩,是研究与新特提斯洋北向俯冲和印度-欧亚大陆碰撞相关的构造岩浆作用特征的天然实验室。日多地区花岗岩体位于藏南墨竹工卡县东侧日多乡附近,其主体为花岗岩,被花岗闪长玢岩脉侵入。锆石U-Pb地质年代学表明:主体花岗岩形成于62.7±0.5Ma,侵入其中的花岗闪长玢岩脉形成于59.5±1.5Ma,并捕获了大量的侏罗纪岩浆岩锆石(155.4±1.8Ma)。日多地区花岗岩体的全岩地球化学特征为:(1)高Si O_2、Na_2O、Al_2O_3,低Fe O~T、MgO、Ti O_2;(2)富集轻稀土(LREE),亏损重稀土(HREE)及高场强元素Nb、Ta、Ti、P元素;(3)具有Eu负异常,总体显示高钾钙碱性、过铝质花岗岩和岛弧型岩浆岩特征。锆石Hf同位素特征暗示其岩浆源区为基性下地壳物质。花岗闪长玢岩脉裹挟大量侏罗纪岩浆型锆石,表明冈底斯岩基拉萨以东地区可能经历了较广泛的晚侏罗世岩浆作用。     

克拉通岩石圈地幔的形成与破坏:大洋板块俯冲的贡献

克拉通是地球上最古老的大陆地块.克拉通之所以能够长期稳定存在,主要是因为它具有巨厚、刚性的岩石圈地幔.古老克拉通岩石圈地幔具有高度难熔(富Mg、贫Fe)的特点,其密度较下伏软流圈小,能够"漂浮"在软流圈之上而长期存在.古老克拉通岩石圈地幔是在地球早期壳幔分异之后形成的,其形成可能与大洋板块的俯冲作用密切相关.典型的克拉...     

Age and early metamorphic history of the Sanbagawa belt: Lu–Hf and P–T constraints from the Western Iratsu eclogite

Two distinct age estimates for eclogite-facies metamorphism in the Sanbagawa belt have been proposed: (i) c.  120–110 Ma based on a zircon SHRIMP age for the Western Iratsu unit and (ii) c.  88–89 Ma based on a garnet–omphacite Lu–Hf isochron age from the Seba and Kotsu eclogite units. Despite the contrasting estimates of formation ages, petrological studies suggest the formation conditions of the Western Iratsu unit are indistinguishable from those of the other two units—all ∼20 kbar and 600–650 °C. Studies of the associated geological structures suggest the Seba and Western Iratsu units are parts of a larger semi-continuous eclogite unit. A combination of geochronological and petrological studies for the Western Iratsu eclogite offers a resolution to this discrepancy in age estimates. New Lu–Hf dating for the Western Iratsu eclogite yields an age of 115.9 ± 0.5 Ma that is compatible with the zircon SHRIMP age. However, petrological studies show that there was significant garnet growth in the Western Iratsu eclogite before eclogite facies metamorphism, and the early core growth is associated with a strong concentration of Lu. Pre-eclogite facies garnet (Grt1) includes epidote–amphibolite facies parageneses equilibrated at 550–650 °C and ∼10 kbar, and this is overgrown by prograde eclogite facies garnet (Grt2). The Lu–Hf age of c.  116 Ma is strongly skewed to the isotopic composition of Grt1 and is interpreted to reflect the age of the pre-eclogite phase. The considerable time gap ( c.  27 Myr) between the two Lu–Hf ages suggests they may be related to separate tectonic events or distinct phases in the evolution of the Sanbagawa subduction zone.     

尼泊尔Ms8.1地震构造背景及对周边地区地震活动趋势的影响

分析印度板块与欧亚板块俯冲带、尼泊尔Ms8.1地震震中区地震构造及地震发震构造,讨论主震对余震触发及余震的时空强特征和地震对周边地区地震活动趋势的影响。初步分析认为,主边界断裂为本次地震的发震构造,属于低角度逆冲断层地震;余震分布范围与震源破裂面积和方式基本一致,具有向东迁移的时空特征;西藏定日Ms5.9、聂拉木Ms5.3地震不属于尼泊尔地震余震,是应力扰动的结果;沿俯冲带向东至喜马拉雅东构造结是大震发生的危险地段。     

试论板缘仰冲作用及其对板内构造的影响

板块学说发展史上存在着造山作用的仰冲和俯冲两种成因观点。俯冲说难以解释中脊俯冲、板缘构造、板内变形及海沟构造。仰冲说认为板块会聚边缘处在两个相向运动着的对流系交会带。根据板缘构造平、剖面特点,结合威尔逊旋回、板块运动史、地壳结构等分析,大陆仰冲板块是在克服属弱对流系一方的洋板块的抵抗,迫使贝尼奥夫带不断向洋退却过程中向前推进.而造成各种板缘和板内构造的变形机制的。据此,提出一个断续仰冲模式以代替传统的连续俯冲模式。用仰冲观点分析了形成板内变形的内、外条件和常见的变形型式。还试用板内仰冲作用来解释华南加里东期构造特点与钦州地槽之谜。     

P-waveform analysis for local subduction geometry south of Puget Sound,Washington

The local subduction geometry at a site south of Puget Sound in western Washington is investigated using teleseismicP-waveforms recorded on a three-component event triggered seismograph. The data are processed using source equalization deconvolution in order to isolate locally convertedP-to-S arrivals and stacked to improve the signal-to-noise ratio. Stable arrivals in the radial component indicate an oceanic Moho within the subducted slab at a depth of about 53 km beneath the station. Observed amplitude variations with azimuth in the radial data, as well as qualitative aspects of the tangential data, are used to establish a slab dip of 16° to the southeast. Our results are compatible with previous results from a site 60 km to the west, and further confirm a substantial warp in the regional geometry of the subducted Juan de Fuca plate.     

Characteristics of paleoproterozoic Subduction System in Western Margin of Yangtze Plate

INTRODUCTIONDebate centered on Proterozoic tectonic style and crustalevolution has existed for a long time.The customary viewbelieved that the Proterozoic undeveloped solid plate could notput into effect on the subduction because of the high heat flowin the earth,and the continental crustal growth was dominatedby mafic magm a vertical underplating (Wyborn,1988;Etheridge etal.,1987) .However,many observations recentlyobtained from Proterozoic mobile zones in the world suggeststhat Proter…     

Differential subduction and exhumation of crustal slices in the Sulu HP-UHP metamorphic terrane: insights from mineral inclusions, trace elements, U-Pb and Lu-Hf isotope analyses of zircon in orthogneiss

Based on new evidence the Sulu orogen is divided from south‐east to north‐west into high‐pressure (HP) crustal slice I and ultrahigh‐pressure (UHP) crustal slices II and III. A combined set of mineral inclusions, cathodoluminescence images, U‐Pb SHRIMP dating and in situ trace element and Lu‐Hf isotope analyses was obtained on zircon from orthogneisses of the different slices. Zircon grains typically have three distinct domains that formed during crystallization of the magmatic protolith, HP or UHP metamorphism and late‐amphibolite facies retrogression, respectively: (i) oscillatory zoned cores, with low‐pressure (LP) mineral inclusions and Th/U > 0.38; (ii) high‐luminescent mantles (Th/U < 0.10), with HP mineral inclusions of Qtz + Grt + Arg + Phe + Ap for slice I zircon and Coe + Grt + Phe + Kfs + Ap for both slices II and III zircon; (iii) low‐luminescent rims, with LP mineral inclusions and Th/U < 0.08. Zircon U‐Pb SHRIMP analyses of inherited cores point to protolith ages of 785–770 Ma in all seven orthogneisses. The ages recorded for UHP metamorphism and subsequent retrogression in slice II zircon (c. 228 and c. 215 Ma, respectively) are significantly older than those of slice III zircon (c. 218 and c. 202 Ma, respectively), while slice I zircon recorded even older ages for HP metamorphism and subsequent retrogression (c. 245 and c. 231 Ma, respectively). Moreover, Ar‐Ar biotite ages from six paragneisses, interpreted as dating amphibolite facies retrogression, gradually decrease from HP slice I (c. 232 Ma) to UHP slice II (c. 215 Ma) and UHP slice III (c. 203 Ma). The combined data set suggests decreasing ages for HP or UHP metamorphism and late retrogression in the Sulu orogen from south‐east to north‐west. Thus, the HP‐UHP units are interpreted to represent three crustal slices, which underwent different subduction and exhumation histories. Slice I was detached from the continental lithosphere at ～55–65 km depth and subsequently exhumed while subduction of the underlying slice II continued to ～100–120 km depth (UHP) before detachment and exhumation. Slice III experienced a similar geodynamic evolution as slice II, however, both UHP metamorphism and subsequent exhumation took place c. 10 Myr later. Magmatic zircon cores from two types of orthogneiss in UHP slices II and III show similar mid‐Neoproterozoic crystallization ages, but have contrasting Hf isotope compositions (εHf_(～785) = ?2.7 to +2.2 and ?17.3 to ?11.1, respectively), suggesting their formation from distinct crustal units (Mesoproterozoic and Paleoproterozoic to Archean, respectively) during the breakup of Rodinia. The UHP and the retrograde zircon domains are characterized by lower Th/U and ¹⁷⁶Lu/¹⁷⁷Hf but higher ¹⁷⁶Hf/¹⁷⁷Hf(t) than the Neoproterozoic igneous cores. The similarity between UHP and retrograde domains indicates that late retrogression did not significantly modify chemical and isotopic composition of the UHP metamorphic system.