Study on the Gravity Field and Deep-Seated Crustal Structure at the North Margin of the Qinghai-Tibet Plateau

1.I~ductionThenorthernmarginoftheQinghai-TibetplateauincludestheAltllnMis.,theQilianMis.,KunlunMis.,theQaidambasinandthesouthernTarimbasin.ThisareaistCctonicallycharacterizedbyintensiveCenozoicdeformationwithcomplicateddeformationalmechedsm(Molnaretal.,1987;Zheng,1991;Culetal.,1994;Ding,1995andXuetal.,1996).Thedeformationalmechanismsincludethrust-napping,strike-slipping,extensionandblockrotation,aswellassimultaneousupliftingandtypicalbasin-rangetectonics(CulandXu,1996).IntermsofCenozoi…     

P-SH conversions in a flat-layered medium with anisotropy of arbitrary orientation

P-SH conversion is commonly observed in teleseismic P waves, and is often attributed to dipping interfaces beneath the receiver. Our modelling suggests an alternative explanation in terms of flat-layered anisotropy. We use reflectivity techniques to compute three-component synthetic seismograms in a 1-D anisotropic layered medium. For each layer of the medium, we prescribe values of seismic velocities and hexagonally symmetric anisotropy about a common symmetry axis of arbitrary orientation. A compressional wave in an anisotropic velocity structure suffers conversion to both SV -and SH -polarized shear waves, unless the axis of symmetry is everywhere vertical or the wave travels parallel to all symmetry axes. The P-SV conversion forms the basis of the widely used 'receiver function' technique. The P-SH conversion occurs at interfaces where one or both layers are anisotropic. A tilted axis of symmetry and a dipping interface in isotropic media produce similar amplitudes of both direct ( P ) and converted ( Ps ) phases, leaving the backazimuth variation of the P-Ps delay as the main discriminant. Seismic anisotropy with a tilted symmetry axis leads to complex synthetic seismograms in velocity models composed of just a few flat homogeneous layers. It is possible therefore to model observations of P coda with prominent transverse components with relatively simple 1-D velocity structures. Successful retrieval of salient model characteristics appears possible using multiple realizations of a genetic-algorithm (GA) inversion of P coda from several backazimuths. Using GA inversion, we determine that six P coda recorded at station ARU in central Russia are consistent with models that possess strong (> 10 per cent) anisotropy in the top 5 km and between 30 and 43 km depth. The symmetry axes are tilted, and appear aligned with the seismic anisotropy orientation in the mantle under ARU suggested by SKS splitting.     

塔里木西南早第三纪古地磁和地壳缩短

属于帕米尔弧山前带的乌帕尔剖面和属于南天山山前带的巴对布拉克剖面，现今的纬度差为０．５６°，但是下第三系古地磁测定结果表明其古纬度差很大．乌帕尔剖面齐姆根组上部的古纬度为８°Ｎ，而巴什布拉克剖面巴什布拉克组第５段的古纬度为３６．１°Ｎ。若考虑到当时板块的相对运移速度，估计始新世早期巴什布拉克地区的古纬度大致为３１°Ｎ，与当时乌帕尔所处位置的纬差达２３°。又根据Ｋｌｏｏｔｗｉｊｋ测得的帕米尔西北缘利什坦层的古纬度值，在始新世末期．包括乌帕尔在内的帕米尔前缘与以巴什布拉区为代表的南天山山前带之间还有至少１０°的纬度差。古地磁资料表明，始新世早期，塔里木海宽达２０００ｋｍ，此时印度板块西北端已与欧亚板块局部碰撞，至始新世末，印度板块向北推进２０°，帕米尔弧前缘与南天山山前的距离缩短到约１０００ｋｍ，原塔里木海的两侧上升为山前平原。     

Attenuation differences in layer 2A in intermediate- and slow-spreading oceanic crust

In situ seismic attenuationQ⁻¹logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate- and slow-spreading oceanic crust.Q⁻¹logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in Ocean Drilling Program Holes 504B and 896A and in Hole 395A, located in 5.9–7.3 Ma crust on the Pacific and Atlantic plates, respectively. Our results strongly tie crustal properties to seismic measurables and observed geological structures: we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity. We interpret a distinct decrease in the Q⁻¹log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2A/2B boundary. The boundary is located at 465 m depth below the sea floor in both Hole 395A and 504B, although this is likely to be a coincidence of the sediment thickness at these sites. Layer 2A is estimated to be approximately 150 m thick in Hole 504B and > 300 m thick in Hole 395A. Cyclic sequences of high-porosity pillows and low-porosity massive units in the uppermost 100 m of volcanics in Hole 395A result in large velocity heterogeneities which cause > 5 times more attenuation in this layer than in Hole 504B. In Hole 896A, by contrast, fewer pillows, more massive flows, and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from Hole 504B. We conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration. The observed differences inQ⁻¹profiles and layer 2A thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid-ocean spreading centers for these Pacific and Atlantic ridge segments.     

陆源碎屑沉积作用对化学元素配分的制约——兼论五台地区前寒武纪碎屑沉积岩示踪源区陆壳成分的意义

为探讨粒度分选沉积作用对沉积物化学成分配分的影响,对比研究了五台群中石英岩、长石石英岩、粉砂岩和粘土岩等不同粒度沉积岩的化学成分特征,结果表明:不同粒度沉积岩的主元素及微量元素均具有较大的变化范围,SiO_2与Al_2O_3及TFe_2O_3之间呈线性负相关关系,这种负相关关系还表现在SiO_2与REE,Rb,Sr之间,这种特征可以用石英的稀释作用进行解释;铁族元素Cr,Ni,Co和REE,Rb,Sr等大离子亲石元素与Al_2O_3呈正相关关系,表明这些元素易在富铝的粘土岩中聚积。不同粒度成分沉积岩的稀土     

秦岭造山带地壳构造与楔入成山

横穿秦岭的河南叶县至湖北南漳的反射地震剖面揭示，秦岭造山带地壳由一系列由南向北楔入到中地壳的鳄鱼式构造所组成。南北溱岭上地壳分别形成南秦岭和北秦岭推覆系。北秦岭推覆系是组成基底的地壳沿脆转换面拆离冲叠形成的一个推覆系，可以分出栾川推覆体，瓦穴子推覆体，二郎坪推覆体及朱夏推覆体。     

洋岛,海山碳酸盐岩的沉积特征及其古地理意义

高出水面的洋岛和潜伏水下的海山普遍存在于现代地球表面的各大洋中，地质历史中存在的古洋岛和古海山也逐渐为人们所认识。笔者从地形特征，沉积学特征，成岩作用和生物学特征几个方面对洋岛，海山进行了分析，洋岛，海山通常具有洋岛型火山岩基底和碳酸盐盖层的双层式地层结构，具有低分异度，探讨了古洋岛，古海山的鉴别对于再造古海洋，古地理格局的现实意义。