博格达造山带东段早石炭世火山岩地球化学特征及构造属性

博格达造山带东段克孜库都克地区七角井组玄武岩和中酸性火山碎屑岩及少量流纹岩在时空上构成双峰式火山岩组合。玄武岩LA-ICP-MS锆石U-Pb年龄为331.0±3.0Ma,属于早石炭世晚期;Si O2含量为47.68%~48.82%,Ti O2含量(1.83%~2.17%)略高于N型大洋中脊玄武岩,高Al(Al2O3含量为15.56%~16.09%),富钠贫钾(Na2O/K2O=5.44~7.76),低Mg(Mg O含量为5.97%~7.17%,Mg#为43~47),表明其原始岩浆发生过明显的橄榄石和辉石的分离结晶作用。玄武岩具有近于平坦的稀土配分模式,轻微负Eu异常(δEu=0.89~0.93),相对富集Rb、Ba、P,亏损Th、Nb、Ta、Sr、Ti等不相容元素。火山岩岩石地球化学特征表明:研究区玄武岩可能是亏损尖晶石相地幔橄榄岩向石榴石相地幔橄榄岩过渡的产物,且在其上升过程中受到较弱程度的地壳物质混染,形成于陆内裂谷环境,其地球动力学体制可能与古亚洲洋壳向先存的准噶尔-吐哈陆块斜向俯冲,产生的侧向撕裂力拉张陆块有关。克孜库都克地区早石炭世玄武岩构造属性的确立进一步证实了博格达造山带在石炭纪时期处于大陆裂谷演化过程的观点,为进一步深入理解博格达地区石炭纪构造岩浆演化过程提供了新的地质与年代学依据。     

Interactions between freshwater input, light, and phytoplankton dynamics on the Louisiana continental shelf

We examined the effects of freshwater flow and light availability on phytoplankton biomass and production along the Louisiana continental shelf in the region characterized by persistent spring–summer stratification and widespread summer hypoxia. Data were collected on 7 cruises from 2005 to 2007, and spatially-averaged estimates of phytoplankton and light variables were calculated for the study area using Voronoi polygon normalization. Shelf-wide phytoplankton production ranged from 0.47 to 1.75 mg C m⁻² d⁻¹ across the 7 cruises. Shelf-wide average light attenuation (k_d) ranged from 0.19–1.01 m⁻¹ and strongly covaried with freshwater discharge from the Mississippi and Atchafalaya Rivers (R²=0.67). Interestingly, we observed that the euphotic zone (as defined by the 1% light depth) extended well below the pycnocline and to the bottom across much of the shelf. Shelf-wide average chlorophyll a (chl a) concentrations ranged from 1.4 to 5.9 mg m⁻³ and, similar to k_d, covaried with river discharge (R²=0.83). Also, chl a concentrations were significantly higher in plume versus non-plume regions of the shelf. When integrated through the water-column, shelf-wide average chl a ranged from 26.3 to 47.6 mg m⁻², but did not covary with river discharge, nor were plume versus non-plume averages statistically different. The high integrated chl a in the non-plume waters resulted from frequent sub-pycnocline chl a maxima. Phytoplankton production rates were highest in the vicinity of the Mississippi River bird's foot delta, but as with integrated chl a were not statistically different in plume versus non-plume waters across the rest of the shelf. Based on the vertical distribution of light and chl a, a substantial fraction of phytoplankton production occurred below the pycnocline, averaging from 25% to 50% among cruises. These results suggest that freshwater and nutrient inputs regulate shelf-wide k_d and, consequently, the vertical distribution of primary production. The substantial below-pycnocline primary production we observed has not been previously quantified for this region, but has important implications about the formation and persistence of hypoxia on the Louisiana continental shelf.     

琼东南盆地地壳伸展深度依赖性及其动力学意义

地壳或岩石圈尺度内伸展因子随深度变化特征对于理解岩石圈演化有重要的指示意义.我们利用南海北部大陆边缘琼东南盆地区深反射地震剖面的地壳分层模型,计算了沿剖面上地壳与全地壳的伸展因子.结果表明:琼东南盆地区具有明显的地壳尺度内伸展的深度相关性（上地壳尺度伸展因子变化范围为1.0~2.0,全地壳尺度的伸展因子变化范围为1.2~2.5）;琼东南盆地各构造单元内的上地壳与全地壳伸展具有明显的非均一性(长昌凹陷上地壳尺度伸展最大,乐东—陵水凹陷其次,松南—宝岛凹陷最小;长昌凹陷和松南—宝岛凹陷的地壳尺度伸展因子较乐东—陵水凹陷大) 与各向异性(南东—北西剖面较之北东—南西向剖面地壳伸展因子大).这些结果预示着琼东南盆地区地壳伸展优势方向为北西向,盆地区东西部的伸展过程或伸展机制可能差异较大拟或存在太平洋岩石圈俯冲角空间差异或地幔岩浆产出时空差异.结合研究区相关研究成果,推断地壳伸展因子的深度相关性可能是共轭大陆边缘低角度拆离控制的简单剪切系统内伴随地幔挤出的动力学现象.     

Mesozoic tectono-magmatic activities in South China:Retrospect and prospect

The South China Block was formed through the collisional orogeny between the Cathaysia Block and the Yangtze Block in the Early Neoproterozoic.The northern,western and southern sides of the South China Block were affected by disappearance of the Paleo-Tethyan Ocean during the Paleozoic.The southern and northern sides of the South China Block were respectively collided with the Indo-China Block and North China Block in the latest Paleozoic to form the basic framework of the Eastern China.The Eastern China has been affected by the westward subduction of the Pacific Plate since the Mesozoic.Therefore,the South China Block was influenced by the three major tectonic systems,leading to a superposed compound tectonics.The comparative study of the Mesozoic geology between the South China Block and its surrounding areas suggests that although the Mesozoic South China Block was adjacent to the subduction zone of the western Pacific,no juvenile arc-type crust has been found in the eastern margin.The main Mesozoic geology in South China is characterized by reworking of ancient continental margins to intracontinental tectonics,lacking oceanic arc basalts and continental arc andesites.Therefore,a key to understanding of the Mesozoic geology in South China is to determine the temporal-spatial distribution and tectonic evolution of Mesozoic magmatic rocks in this region.This paper presents a review on the tectonic evolution of the South China Block through summarizing the magmatic rock records from the compressional to extensional tectonic process with the transition at the three juncture zones and using the deformation and geophysic data from the deep part of the South China continental lithosphere.Our attempt is to promote the study of South China’s geology and to make it as a typical target for development of plate tectonic theory.     

Thermal history of the Rio Muni (West Africa)–NE Brazil margins during continental breakup

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110–95 Ma (both margins) and 85–70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45–35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15–10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local ‘intra-basinal’ reorganization, and the amplifying effect of negative feedbacks between these processes.     

Collision of the Ganges–Brahmaputra Delta with the Burma Arc: Implications for earthquake hazard

We take a fresh look at the topography, structure and seismicity of the Ganges–Brahmaputra Delta (GBD)–Burma Arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential. The GBD, the world's largest delta, has been built from sediments eroded from the Himalayan collision. These sediments prograded the continental margin of the Indian subcontinent by  400 km, forming a huge sediment pile that is now entering the Burma Arc subduction zone. Subduction of oceanic lithosphere with > 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land. The prism starts at an apex south of the GBD shelf edge at  18°N and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit. The front of the prism is blind, buried by the GBD sediments. Thus, the deformation front extends 100 km west of the surface fold belt beneath the Comilla Tract, which is uplifted by 3–4 m relative to the delta. This accretionary prism has the lowest surface slope of any active subduction zone. The gradient of the prism is only  0.1°, rising to  0.5° in the forearc region to the east. This low slope is consistent with the high level of overpressure found in the subsurface, and indicates a very weak detachment. Since its onset, the collision of the GBD and Burma Arc has expanded westward at  2 cm/yr, and propagated southwards at  5 cm/yr. Seismic hazard in the GBD is largely unknown. Intermediate-size earthquakes are associated with surface ruptures and fold growth in the external part of the prism. However, the possibility of large subduction ruptures has not been accounted for, and may be higher than generally believed. Although sediment-clogged systems are thought to not be able to sustain the stresses and strain-weakening behavior required for great earthquakes, some of the largest known earthquakes have occurred in heavily-sedimented subduction zones. A large earthquake in 1762 ruptured  250 km of the southern part of the GBD, suggesting large earthquakes are possible there. A large, but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism, and is the only known candidate for a rupture of the plate boundary along the subaerial part of the GBD–Burma Arc collision zone.     

东南大陆边缘早侏罗世火成岩特征及其构造意义

东南大陆边缘早侏罗世火成岩主要呈双峰式火山岩、基性超基性杂岩体及A型花岗岩等形态产出。本文运用岩石学探针技术,通过早侏罗世火成岩岩石学与地球化学研究,并与晚中生代火成岩作对比,提出早侏罗世火成岩的形成与南岭东段近EW向张性断裂活动有关,标志着印支挤压造山的结束;之后东南大陆进入晚中生代NE向活动大陆边缘俯冲造山阶段,经历了挤压造山—剪切拉张过程,并在晚白垩世末期进入又一轮后造山拉张裂解阶段,即中生代时东南大陆边缘经历了早中生代(三叠纪—早侏罗世)和晚中生代(中侏罗世—晚白垩世)两期造山事件,其中早侏罗世的区域拉张作用是特提斯构造域向滨太平洋构造域转换的前奏,构造域转换可能始于中侏罗世(165Ma)。     

大陆岩石圈导电性的研究方法

随着地球科学的进展 ,大陆岩石圈导电性结构的研究越来越引起人们的重视 ,而超宽频带大地电磁测深则是目前用于探测岩石圈导电性最有效的地球物理先进技术。它把现代性能优良的宽频带大地电磁系统 (MT 2 4NS或V5 2 0 0 0 )与长周期智能化大地电磁系统 (LIMS)配套使用 ,采集地面上频率范围为n× 10 -4～n× 10 2 Hz的天然电磁场信号 ,并通过一系列数据处理和反演计算 ,获得深达下地壳和上地幔的地下导电性结构模型。此模型不仅可以提供有关岩石圈地质构造轮廓的信息 ,更重要的是可以间接反映现今地下深部的热结构特征和物质状态分布特点。多年来 ,应用超宽频带大地电磁测深对青藏高原岩石圈导电性结构的研究表明 ,高原的中、下地壳确实是良导电性的 ;它可能说明西藏地壳中普遍存在岩石的局部熔融 ,或地热流体。沿着应县—商河剖面的大地电磁测深研究结果从电性的角度证明了太行山山前断裂为一组向东倾斜的深断裂 ,华北地区岩石圈以其为界划分为东、西两区 ;东区为低阻区 ,与构造活动区的岩石圈导电性特征相符 ;西区为高阻区 ,表现出稳定大陆区岩石圈导电性结构的特点。     

东南沿海地区古近纪大陆岩石圈地幔特征及成因

东南沿海地区新生代玄武岩中的橄榄岩包体来自岩石圈地幔 ,上地幔橄榄岩包体的岩石学及地球化学特征都记录了地幔演化的历史。普宁橄榄岩包体斜方辉石含量与太古宙克拉通地幔类似 ,但在矿物学、REE、痕量元素和Sr Nd同位素上又与太古宙岩石圈地幔不同。橄榄岩包体的岩相学、矿物学、REE、痕量元素特征都提供了含H2 O富Si流体交代橄榄岩的证据 ,这种流体可能主要是洋壳物质局部熔融而成。流体交代使橄榄岩富Si,同时富Sr、Pb和强不相容元素等大洋岩石圈物质。这表明普宁大陆岩石圈地幔既保留太古宙岩石圈地幔的特征 ,又具有大洋俯冲地幔的特征 ,它是古老岩石圈地幔向大洋岩石圈地幔转换的一部分 ,这种转换可能是大洋岩石圈与大陆岩石圈地幔相互作用的结果。