The age of the Tunas formation in the Sauce Grande basin-Ventana foldbelt (Argentina): Implications for the Permian evolution of the southwestern margin of Gondwana

New SHRIMP radiogenic isotope dating on zircons in tuffs (280.8 ± 1.9 Ma) confirms the Early Permian (Artinskian) age of the uppermost section of the Tunas Formation. Tuff-rich levels in the Tunas Formation are exposed in the Ventana foldbelt of central Argentina; they are part of a deltaic to fluvial section corresponding to the late overfilled stage of the Late Paleozoic Sauce Grande foreland basin. Recent SHRIMP dating of zircons from the basal Choiyoi volcanics exposed in western Argentina yielded an age of 281.4 ± 2.5 Ma (Rocha-Campos et al., 2011). The new data for the Tunas tuffs suggest that the volcanism present in the Sauce Grande basin can be considered as the distal equivalent of the earliest episodes of the Choiyoi volcanism of western Argentina. From the palaeoclimatic viewpoint the new Tunas SHRIMP age confirms that by early Artinskian glacial conditions ceased in the Sauce Grande basin and, probably, in adajacent basins in western Gondwana.     

Crust and Upper Mantle Structure in the Caribbean Region by Group Velocity Tomography and Regionalization

An overview of the crust and upper mantle structure of Central America and the Caribbean region is presented as a result of the processing of more than 200 seismograms recorded by digital broadband stations from SSSN and GSN seismic networks. Group velocity dispersion curves are obtained in the period range from 10s to 40s by FTAN analysis of the fundamental mode of the Rayleigh waves; the error of these measurements varies from 0.06 and 0.09 km/s. From the dispersion curve, seven tomographic maps at different periods and with average spatial resolution of 500 km are obtained. Using the logical combinatorial classification techniques, eight main groups of dispersion curves are determined from the tomographic maps and eleven main regions, each one characterized by one kind of dispersion curves, are identified. The average dispersion curves obtained for each region are extended to 150s by adding data from a larger-scale tomographic study (Vdovin et al., 1999) and inverted using a nonlinear procedure. A set of models of the S-wave velocity vs. depth in the crust and upper mantle is found as a result of the inversion process. In six regions we identify a typically oceanic crust and upper mantle structure, while in the other two the models are consistent with the presence of a continental structure. Two regions, located over the major geological zones of the accretionary crust of the Caribbean region, are characterized by a peculiar crust and upper mantle structure, indicating the presence of lithospheric roots reaching, at least, about 200 km of depth.     

On the variability of the flow along the Meso-American Barrier Reef system: a numerical model study of the influence of the Caribbean current and eddies

A high resolution (3–8 km grid), 3D numerical ocean model of the West Caribbean Sea (WCS) is used to investigate the variability and the forcing of flows near the Meso-American Barrier Reef System (MBRS) which runs along the coasts of Mexico, Belize, Guatemala and Honduras. Mesoscale variations in velocity and temperature along the reef were found in seasonal model simulations and in observations; these variations are associated with meandering of the Caribbean current (CC) and the propagation of Caribbean eddies. Diagnostic calculations and a simple assimilation technique are combined to infer the dynamically adjusted flow associated with particular eddies. The results demonstrate that when a cyclonic eddy (negative sea surface height anomaly (SSHA)) is found near the MBRS the CC shifts offshore, the cyclonic circulation in the Gulf of Honduras (GOH) intensifies, and a strong southward flow results along the reef. However, when an anticyclonic eddy (positive SSHA) is found near the reef, the CC moves onshore and the flow is predominantly westward across the reef. The model results help to explain how drifters are able to propagate in a direction opposite to the mean circulation when eddies cause a reversal of the coastal circulation. The effect of including the Meso-American Lagoon west of the Belize Reef in the model topography was also investigated, to show the importance of having accurate coastal topography in determining the variations of transports across the MBRS. The variations found in transports across the MBRS (on seasonal and mesoscale time scales) may have important consequences for biological activities along the reef such as spawning aggregations; better understanding the nature of these variations will help ongoing efforts in coral reef conservation and maintaining the health of the ecosystem in the region.     

双地震带的影响因素探讨

讨论了全球39个俯冲带内的双地震带层间距、应力类型与俯冲参数的相互关系,这些俯冲参数包括动力学参数(板块年龄、热参数、板片拉力)、运动学参数(俯冲板块速度、上覆板块运动速度、海沟迁移速度、弧后形变特征)、几何形态参数(浅俯冲角、深俯冲角、俯冲深度、长度)及上覆板块性质等. 结果表明:(1)I型双地震带易形成于年龄较古老(>60 Ma)的俯冲板块,其层间距主要与动力学参数有关;(2)II型双地震带出现在较年轻(<60 Ma)的板块,其层间距及应力状态更容易受局部应力场的影响;(3)当热参数大于5000 km时仅能形成I型双地震带,而当热参数小于5000 km时,两类双地震带都能形成;(4)上覆板块的性质似乎决定着双地震带层间距的上下限,其中大陆型上覆板块所对应的双地震带层间距小于20 km;而海洋型对应DSZ层间距则大于15 km. 限于现有的观测条件,本文基于有限样本的统计结果仍需要进一步验证.     

华北板块南缘安徽青白口纪-早奥陶世层序地层与格架

华北地台南缘,主要出露青白口纪—早古生代地层,属被动大陆稳定克拉通盆地发育过程,以台地碳酸盐沉积为主。通过对十余条实测剖面及主干、辅助路线的观察研究,青白口纪—早古生代奥陶纪早期,测区可区分出二个巨层序、十二个三级层序。本文通过三级层序的划分,建立了华北地台南缘青白口纪—早古生代奥陶纪早期层序地层格架、探讨了各岩石地层单位的变化规律。     

松科二井早白垩世早期玄武安山岩的发现及地质意义

松科二井超深钻探获取了松辽盆地连续沉积-火山岩心,为系统研究中生代松辽盆地形成机制、揭示中国东北地区构造演化历史提供了极好的研究材料。本文对取自松科二井井深-6035~-6084m的玄武安山岩开展了系统的锆石U-Pb定年、全岩主微量元素和Sr-Nd同位素分析测试。研究结果表明,松科二井玄武安山岩隶属于火石岭组底部,其形成时代为早白垩世早期（141.6±1.4Ma）。玄武安山岩样品富集大离子亲石元素,亏损高场强元素,具有弧岩浆的地球化学特征。这些样品显示了一定程度的轻重稀土元素分异（（La/Yb）_N=8.40~11.88）,具有轻微亏损的Sr-Nd同位素特征（（⁸⁷Sr/⁸⁶Sr）_i=0.70496~0.70478,ε_Nd（t）=1.05~1.61）,表明它们很可能源自于岩石圈地幔。早白垩世时期中国东北地区主要受控于蒙古-鄂霍茨克洋的南向俯冲和东侧古太平洋俯冲过程,目前研究揭示了前者影响了同期的大兴安岭地区的火山活动,但其影响范围是否延伸至松辽盆地内部尚不清楚。古太平洋岩石圈开始俯冲后很可能诱发了松辽盆地西侧晚中生代火山岩的形成。早白垩世松辽盆地演化主要受蒙古-鄂霍茨克洋还是古太平洋板块的活动影响仍存在较多争议。松科二井玄武安山岩具有与板内玄武岩相近的Th/Hf比值（1.10~2.87）及Zr/Y比值（10.1~18.7）,表明它们具有板内岩浆的地球化学特征,且松科二井样品比同时代盆地西部的大兴安岭地区玄武岩具有更高的Nb/Ta比值,更高的Zr、Hf与Ti含量,以及更亏损的Nd同位素特征,可能指示与大兴安岭地区玄武岩形成于不同的构造背景。松辽盆地内早白垩世火山岩很可能为古太平洋板片回转诱发形成,指示早白垩世古太平洋板片回转诱发的岩石圈伸展促进了松辽盆地的形成。

     

塔里木板块东北部柳园粗面玄武岩带：软流圈地幔减压熔融的产物

在塔里木东北部,甘肃省西部敦煌市柳园镇南侧,有一条近东西向展布的粗面玄武岩带,长度约150km,宽度达10km,厚度为1900m。这条粗面石玄武岩带属下二叠统哲斯群,岩浆喷发方式以裂隙式溢流为主,枕状构造特别发育。在玄武岩层间夹有很薄的酸性凝灰岩和沉凝灰岩层,属典型的双峰式火山岩组合。按照岩石化学组成的 TAS 分类命名,主要岩石类型为玄武岩、夏威夷岩和橄榄粗安岩。主要造岩矿物为钠长石(Ab 91～100)和普通辉石。普通辉石斑晶、钠长石斑晶和部分基质中的钠长石具有中空结构,并被层状硅酸盐微晶充填。这些岩石基本属碱性玄武岩系列和过渡型系列,大部分属演化的岩浆。演化程度较低的样品具有轻稀土元素亏损型配分曲线,微量元素地球化学具有 MORB 亲和性,ε_(Nd)(t)= 10.14～10.89。玄武质岩浆与陆壳物质之间有较轻的同化混染作用。普通辉石、铁钛氧化物的分离结晶作用是岩浆演化的主要机制。玄武质岩浆源区属于软流圈地幔,部分熔融发生于尖晶石稳定域,是软流圈地幔减压(上涌)熔融的产物。     

Rapid evolution of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu, southwestern Japan

Abstract   Detailed geologic examination of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu revealed that the oceanic plate was composed of Paleocene to Lower Eocene mudstone and siliceous mudstone, lower Middle Eocene red mudstone, and mid-Middle Eocene trench-fill turbidite with siltstone breccia, successively overlying the pre-Eocene oceanic plate. This oceanic plate sequence was overlain by Upper Eocene siltstone. Deposition of the lower Middle Eocene red mudstone was accompanied by basalt flows and it is interbedded with continental felsic tuff, which indicates that the basalt and red mudstone were deposited near the trench just before accretion. The Hyuga Group has very similar geological structure to that of the chert–clastic complexes found in the Jurassic accretionary complexes in Japan: that is, a decollement fault formed in the middle of an oceanic plate sequence, and an imbricate structure formed only in the upper part of the sequence. Thus, it appears that the Hyuga Group was formed by the same accretionary process as the Jurassic accretionary complexes. No accretion occurred before the Middle Eocene, and the rapid accretion of the Hyuga Group was commenced by the supply of coarse terrigenous sediments in the mid-Middle Eocene, when the direction of movement of the Pacific Plate changed. The pre-Eocene oceanic basement and lower Middle Eocene volcanic activity suggest that the oceanic plate partly preserved in the Hyuga Group was very similar to the northern part of the present West Philippine Sea Plate.     

Shear partitioning in the Philippines: Constraints from Philippine Fault and global positioning system data

Abstract The Philippine Fault is a major left-lateral structure formed in an island arc setting. It accommodates a component of the oblique convergence between the Philippine Sea Plate and the Philippine archipelago. This observation is quantified through a series of global positioning satellite experiments between 1991 and 1996. The formation of the Fault marks the onset of a new geodynamic regime in the Philippine region. In the central Philippines, this event corresponds to the creation of a new tectonic boundary separating the Philippine Mobile Belt and the Philippine Sea Plate, following the latter's kinematic reorganization that occurred around 4 Ma ago. During this event, the Philippine Sea Plate changed its relative movement with respect to Eurasia from a northward to a north-westward motion, favoring the formation of a Philippine Fault–Philippine Trench system under a shear partitioning mechanism.     

辽宁北部早古生代弧－陆碰撞作用的发现及其构造意义

自然资源部中国地质调查局沈阳地质调查中心基础地质室构造研究团队在辽宁北部法库地区地质调查过程中,初步识别出了一次弧－陆碰撞作用.根据地质年代学研究,其主要发生在早古生代.该发现为华北板块北缘东段松辽盆地覆盖区的早古生代的构造演化史研究提供重要依据.