Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface

At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm x yr(-1) (ref. 1), resulting in the potential for destructive great earthquakes. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700) occurred before instrumental recording. Thermal and deformation studies indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of approximately 60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9) have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.     

Complex gas hydrate from the Cascadia margin

Natural gas hydrates are a potential source of energy and may play a role in climate change and geological hazards. Most natural gas hydrate appears to be in the form of 'structure I', with methane as the trapped guest molecule, although 'structure II' hydrate has also been identified, with guest molecules such as isobutane and propane, as well as lighter hydrocarbons. A third hydrate structure, 'structure H', which is capable of trapping larger guest molecules, has been produced in the laboratory, but it has not been confirmed that it occurs in the natural environment. Here we characterize the structure, gas content and composition, and distribution of guest molecules in a complex natural hydrate sample recovered from Barkley canyon, on the northern Cascadia margin. We show that the sample contains structure H hydrate, and thus provides direct evidence for the natural occurrence of this hydrate structure. The structure H hydrate is intimately associated with structure II hydrate, and the two structures contain more than 13 different hydrocarbon guest molecules. We also demonstrate that the stability field of the complex gas hydrate lies between those of structure II and structure H hydrates, indicating that this form of hydrate is more stable than structure I and may thus potentially be found in a wider pressure-temperature regime than can methane hydrate deposits.     

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.     

Subduction erosion along the Middle America convergent margin

'Subduction erosion' has been invoked to explain material missing from some continents along convergent margins. It has been suggested that this form of tectonic erosion removes continental material at the front of the margin or along the underside of the upper (continental) plate. Frontal erosion is interpreted from disrupted topography at the base of a slope and is most evident in the wake of subducting seamounts. In contrast, structures resulting from erosion at the base of a continental plate are seldom recognized in seismic reflection images because such images typically have poor resolution at distances greater than approximately 5 km from the trench axis. Basal erosion from seamounts and ridges has been inferred, but few large subducted bodies--let alone the eroded base of the upper plate--are imaged convincingly. From seismic images we identify here two mechanisms of basal erosion: erosion by seamount tunnelling and removal of large rock lenses of a distending upper plate. Seismic cross-sections from Costa Rica to Nicaragua indicate that erosion may extend along much of the Middle America convergent margin.     

Deep structure at northern margin of Tarim Basin

In this paper, a 2D velocity structure of the crust and the upper mantle of the northern margin of the Tarim Basin （TB） has been obtained by ray tracing and theoretical seismogram calculation under the condition of 2D lateral inhomogeneous medium using the data of seismic wide angle reflection/refraction profile from Baicheng to Da Qaidam crossing the Kuqa Depression （KD） and Tabei Uplift （TU）. And along the Baicheng to Da Qaidam profile, 4 of the 10 shot points are located in the northern margin of the TB. The results show that the character of the crust is uniform on the whole between the KD and TU, but the depth of the layers, thickness of the crust and the velocity obviously vary along the profile. Thereinto, the variation of the crust thickness mainly occurs in the middle and lower crust. The Moho has an uplifting trend near the Baicheng shot point in KD and Luntai shot point in TU, and the thickness of the crust reduces to 42 km and 47 km in these two areas, respectively. The transition zone between the KD and TU has a thickest crust, up to 52 km. In this transition zone, there are high velocity anoma- lies in the upper crust, and low velocity anomalies in the lower crust, these velocity anomalies zone is near vertical, and the sediment above them is thicker than the other areas. According to the velocity distributions, the profile can be divided into three sections： KD, TU and transition zone between them. Each section has a special velocity structural feature, the form of the crystalline basement and the relationship between the deep structure and the shallow one. The differences of velocity and tectonic between eastern and western profile in the northern margin of the Tarim Basin （NMTB） may suggest different speed and intensity of the subduction from the Tarim basin to the Tianshan orogenic belt （TOB）.     

Main Jurassic geological events along the eastern Paleo-Asian continental margin

The Jurassic is a period of intensive accretion along the eastern and southeastern margins of the Asian continent. The emphasis of the present study is the review of the Jurassic prism structure of the Russian Far Eastern Pacific Asian margin and comparison with its equivalent in the Japanese archipelago. The Jurassic accretion processes and geodynamic evolution of the Paleo-Asian continent eastern margins are outlined in the paper. During this period, the different ages and different facies fragments of the Paleo-Pacific, such as abyssal plain, intraoceanic volcanic seamountains and oceanic plateaux, were accreted onto the eastern margin of the Paleo-Asian continent as a result of step-wise subduction of oceanic lithosphere. Together with ocean-margin terrigenous deposits (trench-fill turbidites), these paleooceanic fragments formed a thick tectono-sedimentary complex of the Jurassic accretionary prism, causing the size of the continent to be expanded by an increase along its eastern margin.     

Tectonic affinity of Paleozoic stratigraphic slices along the northern margin in Dabie Orogen, central China: evidence from Pb isotope of rocks

Metavolcanic and metasedimentary rocks from the Paleozoic tectono-stratigraphic units along the northern margin in the Dabie orogen show that there are two types of Pb isotopic compositions, which have some affined regularity with tectonic subdivision. Compared with the main tectonic units of the Qinling orogen, the Dingyuan Formation should be the part of the South Qinling orogenic belt or the northern margin of the Yangtze craton, characterized by less radiogenic Pb isotopic composition, and the Guishan Formation, Nanwan Formation-Foziling Group and the Carboniferous should be the part of the North Qinling orogenic belt which is extended to the east, characterized by more radiogenic Pb isotopic composition. These facts combined with the distribution of the geological units suggest that the Paleozoic subduction zone between the Yangtze and North China cratons should be located in the north boundary of the Nanwan Formation-Foziling Group or somewhere that is covered by the Nanwan Formation-Foziling Group now. From the Pb isotopic evidence and suggestion presented here, one aspect of the constraint is provided for determining the position of the Triassic collision suture between the two cratons.     

Spatial and temporal distribution of MODIS and MISR aerosol optical depth over northern China and comparison with AERONET

Aerosol optical depths (AODs) from MODIS and MISR onboard the Terra satellite are assessed by comparison with measurements from four AERONET sites located in northern China for the period 2006-2009. The results show that MISR performs better than MODIS at the SACOL and Beijing sites. For the Xianghe and Xinglong sites, MODIS AOD retrievals are better than those of MISR. Overall, the relative error of the Angstrom exponent from MISR compared with AERONET is about 14%, but the MODIS error can reach 30%. Thus, it may be better to use the MISR Angstrom exponent to derive wavelength-dependent AOD values when calculating the aerosol radiative forcing in a radiative transfer model. Seasonal analysis of AOD over most of China shows two main areas with high aerosol loading: the Taklimakan Desert region and the southern part of North China and northern part of East China. The locations of these two areas of high aerosol loading do not change with season, but the AOD values have significant seasonal variation. The largest AOD value in the Taklimakan appears in spring when the Angstrom exponents are the lowest, which means the particle radii are relatively large. Over North and East China, the highest aerosol loading appears in summer. The aerosol particles are smallest in summer over both high-AOD areas.     

平滑算子在地震叠前深度域成像中的应用

对波动方程叠前深度偏移的成像条件和平滑算子进行了概述,推导了用平滑算子对成像条件进行平滑的公式,给出了同一平滑函数在不同平滑参数控制下以及不同平滑函数在相同平滑参数控制下的成像结果.模型试算结果表明:在合适的平滑参数控制下,平滑后的成像结果得到了明显改善;在相同的平滑参数控制下,Gaussian窗平滑算子平滑后得到的成像结果要好.     

沿河路基冲刷机理与冲刷深度

沿河路基路基冲刷是公路水毁中的一个主要类型，而最大冲刷深度将直接影响沿河路基的安全，在对沿河路基水毁的水力机理和泥沙运动规律进行理论分析的基础上，对凹岸最大冲刷深度的影响因素，进行了合理划分和适当简化，采用人工弯道概化模型试验方法进行了不同条件下的冲刷试验，结合野外调查资料，对试验数据进行回归分析，建立了最大冲刷深度与主要影响因素（水深，泥沙粒径，河宽，河弯半径）的关系式，经实测资料验证，该关系式能够比较准确的反映工程实际情况。     

地下室埋深对高层建筑地震反应的影响分析

为研究地下室埋深对高层建筑地震反应的影响,以实际工程中的某高层剪力墙结构为研究对象,采用理论分析和三维数值模拟相结合的方法,研究了地下室埋深和地基支撑刚度对高层钢筋混凝土结构抗震性能的影响。结果表明:随着地下室埋深的增加,结构的周期有变大的趋势;同时结构承受的地震作用将减少,使得结构抗震性能有较明显的提高;地基对结构支撑作用的变化对结构抗震性能的影响不大。     

Discovery of eclogite at northern margin of Qaidam Basin, NW China

Eclogite was first discovered at the northern margin of the Qaidam Basin in this study. It occurs as pods in the gneiss sequence of Middle to Upper Proterozoic age and is mainly composed of garnet, omphacite, phengite and rutile. The garnets contain 44%-62% of almandine, 15%-33% of grossular and 12%-30% of pyrope molecules, and the omphacites contain 40%-46% of jadeite. Applying garnet_clinopyroxene thermometry and jadeite geobarometry, the peak conditions of eclogite facies metamorphism occurred at about (722±123)℃ and at the pressure of up to c. 22 ×10 8 Pa.     

井间地震资料深度域合成记录的制作方法

在对深度域的井间地震资料进行地质解释时,用于层位标定的合成记录必须是深度域的。对深度域的地震资料合成记录制作方法进行了探讨。总结归纳深度域合成记录制作过程应包括:(1)利用声波、密度等测井曲线进行层位划分并建立深时关系,根据多层介质反射系数的波动方程推导公式,求取频率域的反射系数;(2)对时间域的井旁道记录利用同态反褶积方法提取地震子波,得到频率域的子波谱;(3)利用频率域褶积模型计算合成记录,作反傅里叶变换得到时间域合成地震记录,再根据时深关系求出每个时域采样点对应的深度点,按照所要求的深度采样间隔进行重采样,得到深度域合成地震记录。应用实例表明,利用该方法制作的合成记录与井旁道的吻合程度较高,可以用于井间资料的层位标定。     

井间地震资料深度域合成记录的制作方法

在对深度域的井间地震资料进行地质解释时，用于层位标定的合成记录必须是深度域的。对深度域的地震资料合成记录制作方法进行了探讨。总结归纳深度域合成记录制作过程应包括：(1)利用声波、密度等测井曲线进行层位划分并建立深-时关系，根据多层介质反射系数的波动方程推导公式，求取频率域的反射系数；(2)对时间域的井旁道记录利用同态反褶积方法提取地震子波，得到频率域的子波谱；(3)利用频率域褶积模型计算合成记录，作反傅里叶变换得到时间域合成地震记录，再根据时-深关系求出每个时域采样点对应的深度点，按照所要求的深度采样间隔进行重采样，得到深度域合成地震记录。应用实例表明，利用该方法制作的合成记录与井旁道的吻合程度较高，可以用于井间资料的层位标定。     

Discovery of Paleoproterozoic rapakivi granite on the northern margin of the Yangtze block and its geological significance

The Huashanguan rapakivi pluton in Zhongxiang,Hubei Province,China,is the first discovered Proterozoic rapakivi pluton in the Yangtze block.Based on field and petrographical observations,a typical rapakivi texture was found in the northern portion of the Huashanguan granitic pluton.Almost all the K-feldspar phenocrysts were round to oval in shape and most had plagioclase coatings known as rapakivi phenocrysts.Alkali feldspars and quartz had two or more generations.Petrochemically,the Huashanguan rapakivi granites were characterized as having high values of Si,K,Fe,Th,U,La,Ga,Ce,Sm and LREE,low values of Ca,Mg,Sr,Nb,Y and HREE,and a negative Eu anomaly.These geochemical characteristics of the Huashanguan granites were concordant with typical rapakivi granites,and had an affinity to A-type granites.LA-ICP-MS U-Pb zircon dating also was conducted.The dating yielded a 207Pb/206Pb weighted mean age of 1851±18 Ma (MSWD =1.2),which represents the age of the pluton emplacement.The age of 803±170 Ma at the lower intercept in the concordia diagram corresponds to the age of a later deformation event which affected the pluton,and suggests that the Huashanguan pluton was influenced by Neoproterozoic thermo-tectonic events after its formation.The discovery of Paleoproterozoic Huashanguan rapakivi granites indicates continental rifting or a post-orogenic extensional event that took place in the Paleoproterozoic in the Yangtze block.These events may be related to the breakup of the Paleoproterozoic Columbia supercontinent.     

柴达木盆地北缘地区逆冲推覆构造及其油气勘探意义

研究了柴北缘地区逆冲推覆构造的发育背景、发育模式 ,平面、剖面展布特征及其分带性 ,并对主干逆冲断裂带及其间的推覆体带、逆冲席带和低应变区的特征进行了详细描述 ,同时通过野外观察和室内构造反演来研究推覆构造的发展演变模式及其形成机理 ,指出其逆冲叠瓦扇带和隐伏逆冲前缘带是最有利的油气勘探目标 图 6,参 8     

柴达木盆地北缘下、中侏罗统古地理及盆地类型研究

柴达木盆地北缘侏罗系后期改造强烈,地层赋存复杂,从地面露头、钻井、地震等资料入手,采用单因素分析多因素综合作图法,对下、中侏罗统岩相古地理进行了定量研究.认为早中、侏罗世时期该区主要发育湖泊相,以滨浅湖-半深湖相为主;现今下、中侏罗统的分布范围并不代表其原始沉积盆地的边界,原始沉积盆地的北边界在南祁连山边界大断裂附近;北缘早、中侏罗世时期为伸展断陷盆地.