Thermal-rheological structure of the lithosphere beneath Jiyang Depression: Its implications for geodynamics

Thermal regime of the lithosphere is the scenario of the lithospheric thermal evolution, and the thermo-mechanical state of lithosphere definitively controls its deformation style and mechanism. Better understanding of the lithospheric deep thermal-rheo- logical structure of sedimentary basin will shed light on the formation and evolution dynamic process of the basin. Surface tectonics is the response of the deep structure, and is controlled by the lithospheric ther-mal-rheological properties.…     

南北地震带岩石圈S波速度结构面波层析成像

本文利用天然地震面波记录和层析成像方法,研究了南北地震带及邻近区域的岩石圈S波速度结构和各向异性特征.结果表明南北地震带的东边界不但是地壳厚度剧变带,也是地壳速度的显著分界.其西侧中下地壳的S波速度显著低于东侧,强震大多发生在低速区内部和边界.青藏高原东缘中下地壳速度显著低于正常大陆地壳,在松潘甘孜地块和川滇地块西部大约25～45 km深度存在壳内低速层;这些低速特征与高原主体的低速区相连,有利于下地壳物质的侧向流动.地壳的各向异性图像与下地壳流动模式相符,即下地壳物质绕喜马拉雅东构造结运动,东向的运动遇到扬子坚硬地壳阻挡而变为向南和向北东的运动.面波层析成像结果支持青藏高原地壳运动的下地壳流动模型.南北地震带的岩石圈厚度与其东侧的扬子和鄂尔多斯地块相似但速度较低.川滇西部地块上地幔顶部(莫霍面至88 km左右)异常低速;松潘甘孜地块上地幔盖层中有低速夹层(约90～130 km深度).岩石圈上地幔的速度分布图像与地壳显著不同,在高原主体与川滇之间存在北北东向高速带,可能会阻挡地幔物质的东向运动.上地幔各向异性较弱且与地壳的分布图像显然不同.因此青藏高原岩石圈地幔的构造运动具有与地壳不同的模式,软弱的下地壳提供了壳幔运动解耦的条件.     

Anisotropy and large-scale lateral inhomogeneity of the upper mantle

An investigation of travel-time residuals of P waves as compared to an average global travel-time curve with a base-line determined shows that about 400–500 km of the upper mantle at the boundary with the crust and a similar thickness of the lower mantle at the boundary with the core are laterally inhomogeneous. The upper mantle both under continents and under oceans consists of crust-mantle blocks distinguished by longitudinal wave velocity, surface tectonics and intensity of the heat flow. It has been revealed that within individual crust-mantle blocks there is an azimuthal relationship of P-wave travel-time indicative of possible anisotropy of upper mantle elastic properties down to depths of the order of several hundred kilometers.     

中国中部的地壳及上地幔构造

本文利用中国数字地震台网的资料,对昆仑山—柴达木盆地—秦岭—大别山纬向构造带的地壳及上地幔构造进行了研究。沿这一构造带沉积层平均厚度为8km,剪切波速度β=2.62km/s,壳内平均速度较低,为3.5km/s左右,在下地壳还存在低速层。地壳平均厚度为48km,自东而西相差不很大。上地慢低速层在70—90km深处开始出现。这种构造特征有别于地台和高原,具有块体间的过渡性质,表明这是中国地壳块体中的一个独立构造单元。     

A continental crustal model and its geothermal implications

The following crustal model based on realistic estimates of metamorphic rock volumes and H₂O content is proposed as a basis for geothermal calculations: (1) a surface zone of intermediate metamorphic rocks containing granitic intrusions and grading downward into (2) a more felsic migmatite zone, (3) a lower crustal zone of approximately andesitic composition crystallized in granulite or possibly amphibolite facies. Heat production values and thickness for the three zones are 3 HGU, 5 HGU, 0.5–1.5 HGU and 8, 8, and 18 km respectively. If the surface heat flow is 1.2 HFU, the model predicts a temperature of only 407°C at the Moho and an upper mantle heat flow of 0.3–0.5 HFU. The low temperatures resulting from this model rule out a seismic low-velocity zone in the crust produced by thermal effects.     

西藏高原当雄-亚东地带地壳与上地幔结构和速度分布的爆炸地震研究

本文概述了在西藏高原长达450公里左右的南北向测线上取得九次湖中水下爆炸地震记录的处理结果。通过数字处理、拟合和反演等计算,得出了该地区地壳与上地幔的成层结构和速度分布。 结果表明,该区整个沉积岩层厚约3-5公里,雅鲁藏布江以北到当雄地带,地壳巨厚达70-73公里;江南地区为68-45公里,并逐渐向南翘起。在成层地壳介质中发现下地壳中存在低速层,厚约10公里,速度为5.64公里/秒。分析认为,高原地形与巨厚地壳的形成是印度洋板块与欧亚板块碰撞以及长期挤压和内部物质运移的结果。     

Crustal structure and origin of the Cape Verde Rise

The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m^− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.     

长白山火山区壳幔S波速度结构研究

利用面波层析成像和远震接收函数方法对长白山地区的地壳上地幔速度结构进行了研究。结果表明:长白山火山区附近存在岩石圈减薄、上地幔软流圈增厚以及上地幔S波速度降低等与上地幔高温物质有关的现象,它表明长白山的岩浆系统一直延伸到上地幔软流圈范围。天池火山区地壳内部存在明显的S波低速层,在离天池火山口较近的WQD台附近,低速层顶部埋深约8km,厚度近20km,S波最小速度约2.2km/s。在距离天池火山北部50km的EDO台地壳中没有明显的低速层。火山区S波速度结构总体表现出距离天池越近,地壳的V_P/V_S越大,低速层的厚度和幅度增加的特征,表明天池火山口附近地壳内部存在高温物质或岩浆囊。CBS台站不同方位的接收函数及反演结果表明,地表低速层厚度以及莫霍面深度存在随方位的变化。地表低速层在南部方向明显较厚,莫霍面深度在南部天池火山口方向存在小幅度抬升。CBS台站附近特殊的近地表速度结构可能是该台站记录的火山地震波形主频较低的主要因素。天池火山口附近莫霍面的小幅度抬升意味着存在与火山作用有关的壳幔物质交换通道     

Temperature production in the continental crust due to radioactive heat sources

Summary The purpose of this paper is to present a method for computing the temperature distribution in the continental crust as a function of depth and time. Computed results indicate that the rise of the temperature due to the decay of long half-life radioactive elements K⁴⁰, U²³⁵, U²³⁸, and Th²³² will reach its maximum 90 m.y. after heat sources and temperature are redistributed in the upper part of the earth. This suggests that the lower part of the crust and upper mantle were periodically remelted before the present stage of the crust was reached. At the present time the decay of the radioactive elements U²³⁸ and Th²³² still significantly influences the crustal temperature. The present near-surface thermal gradient due to the radioactive heat sources alone may reach 20°C/km in a high radioactive heat source province.     

Shear-wave splitting beneath Yunnan area of Southwest China

Systematic analyses of seismic data recorded by the Yunnan regional seismograph network reveal significant crustal and upper mantle anisotropy. Splitting of the S phase of local earthquakes and teleseismic SKS, PKS, and SKKS phases indicates time-delays from 1.60 ms/km to 2.30 ms/km in the crust, and from 0.55 s to 1.65 s in the upper mantle which corresponds to an anisotropic layer with a thickness about between 55–165 km. The polarization orientations of fast shear waves in the crust are complicated with a predominantly north-south direction, and the mantle anisotropy has a nearly west-east direction. Our results show different deformation styles and mechanisms exist between the crust and upper mantle.     

华南陆缘高热流区的壳幔温度结构与动力学背景

华南陆缘是我国重要的矿产、地热资源区.晚中生代以来,在太平洋板块西向俯冲,地幔热对流活动共同作用下,该区出现多期岩浆-热事件和大规模爆发式成矿作用.在前人研究基础上,本文利用地表热流观测资料、地震剪切波资料、重力位球谐系数,计算了壳-幔温度结构,分析了动力学背景.计算结果表明：华南陆缘东南沿海地带,地壳10 km以浅温度达200℃以上,居里点温度475℃,莫霍面平均温度550℃.地壳浅层较热,花岗岩中放射性元素衰变放热是地壳浅层地下水热活动的重要热源,但地壳总体温度不高,为"冷壳热幔"型热结构.地幔中,90 km深度,温度950~1250℃;120 km深度,温度1050~1400℃;150 km深度,温度1200~1450℃;220 km深度,温度1500~1700℃."热"岩石圈底界深度在110~150 km之间,西深东浅.岩石圈内,地幔应力场为挤压-伸展相间格局;岩石圈之下,地幔应力场为一个以南昌为中心、长轴NE-SW向的椭圆.分析认为,晚中生代以来,太平洋板块的西向俯冲,导致华南陆缘在区域性SE向地幔对流背景上叠加局域性不稳定热扰动,在175~85Ma期间,上地幔物质向上流动,形成不同的岩浆活动高峰期.同时,岩石圈地幔受俯冲洋壳流体的影响,含水量高,黏度小,在地幔流切向应力场作用下,岩石圈底界由西向东"波浪"状减薄.现今岩石圈之下仍具备地幔小尺度热对流温度条件,但除地表浅层外,地壳整体温度不高,岩石圈构造稳定.     

鲁西地区的地壳结构及壳内近直立高速异常体的发现

长320 km横跨鲁西地区的聊城－连云港宽角反射地震剖面揭示了鲁西地区的地壳结构，上地壳为二层结构，总厚18～20 km，速度5.4～6.2 km/s；下地壳也分为二层结构，总厚度13～15 km，速度6.4～6.7 km/s.Moho深度33～35 km.Pn 速度为7.9 km/s.地壳速度分布在横向上有较大变化，且平均速度为6.2～6.5 km/s，较正常值偏高.研究结果发现地壳内有两个近直立的高速体.从下地壳延伸到上地壳并直达沉积盆地的底部.可能是幔源岩浆大量侵入地壳，使得地壳的局部平均速度增高.近直立的高速体可能是幔源岩桨上涌的通道.鲁西地壳结构的研究对于探讨古地台的裂解与沉陷机制具有一定的意义.     

云南西部地壳深部结构特征

在云南西部,穿过红河、小江断裂带完成了一条长360 km、呈北东向的深地震宽角反射/折射剖面.通过对该测线的观测资料进行一维、二维模拟解释,得到了沿剖面的二维地壳速度模型.研究结果显示,沿测线Moho界面埋深横线变化大,其西南侧Moho埋深约35 km,东北侧Moho最大埋深可达43 km.沿剖面从西南到北东方向,地壳平均P波速度从5.9 km/s逐渐增加到6.13 km/s,但显著低于全球大陆平均值.结合以往的接收函数和面波联合反演结果,我们推算沿测线从西南到东北,其下方地壳泊松比介于0.23~0.25之间.剖面西南侧上地壳具有异常低的P波速度和泊松比,暗示其下方上地壳以α-相长英质组分为主;而剖面东北上地壳相对较高的P波速度和泊松比则暗示其物质组成以花岗岩-花岗闪长岩为主.研究区下地壳的P波速度和泊松比分别介于6.25~6.75 km/s和0.24~0.26 km/s之间,暗示其上部组成以花岗岩相的片麻岩为主,而下部组成则以角闪石类岩石为主.红河断裂两侧地壳速度显著不同,从浅到深其速度差异逐渐变弱,但红河断裂两侧地壳厚度变化较大.而小江断裂下方两侧地壳速度和地壳厚度变化并没有红河断裂那么明显.     

青藏高原东西向差异形变与隆升机制

高精度布格重力异常约束下的三维空间域挠曲形变模拟显示,大约以90°E为界,青藏高原东、西两部分的岩石圈强度存在明显的差异.在90°E以东,岩石圈有效弹性厚度为35~45 km,该岩层厚度可使刚性的上地壳与上地幔岩石通过中下地壳柔塑性地层的黏滞流动产生构造解耦;地壳处于区域均衡状态,下地壳热物质的流动膨胀是地壳隆升的主控要素.而在90°E以西,断裂带严重削弱了该区域的岩石圈机械强度,岩石圈有效弹性厚度小于15 km,向西逐渐减小,至喀喇昆仑断裂带变为零,断裂切穿莫霍面进入地幔,发生纯剪切构造形变;这里的地壳接近局部均衡,厚皮逆冲是地形隆升的主要因素.震源深度大于80 km的地幔地震大多发生在青藏高原西部,其岩石圈深部具有的脆裂特征很好地支持了岩石圈机械强度模拟的结果.     

Some thermal and mechanical consequences of rapid uplift: an example from the Southern Alps, New Zealand

The thermal evolution of continental crust during active collision is modeled through numerical solutions of the two-dimensional heat conduction equation for a rapidly moving medium. The boundary conditions used in the modeling are derived from geological and geophysical observations from the active collision zone in the South Island of New Zealand. The problem domain over which the solutions are obtained consists of a 40 km horizontal by 25 km vertical spatial plane with a vertical discontinuity at 10 km from the western boundary. To the east of this discontinuity, vertical uplift rates of up to 10 mm/a occur over a timespan of up to 4 Ma. Temperature distributions are calculated at 10 ka intervals over the 4 Ma duration. A two-dimensional high-temperature region is established upon initiation of uplift of the eastern block due to the advective component carrying heat upwards more rapidly than it can be dissipated laterally from the problem domain. Temperatures within the upper 5 km are greater than 400°C after 2.25 Ma with geothermal gradients of up to 200°C/km attained within the upper 3 km. At times greater than 2.5 Ma, the vertical temperature distribution changes little while the anomalously high temperatures spread laterally into the stationary crust.Using rheological equations to describe the brittle behaviour of a water-saturated upper crust and the ductile behaviour of a quartz-dominated lower crust, together with the thermal distribution of the conduction models, the mechanical evolution of a collision zone is investigated. In addition to high crustal temperatures and associated high heat flow, rapid uplift produces a weakening of the crust by raising of the depth of transition from brittle to ductile behaviour. Within the zone of most rapid uplift, the brittle-ductile transition rises from 13 km to less than 5 km after 1.5 Ma of uplift. Further uplift reduces the brittle layer to 3 km thickness and causes lateral spreading of the low-strength zone. The reductions in crustal strength caused by the thermal weakening produce a high-strain zone within the region of maximum uplift which is incapable of sustaining large differential stresses. This causes horizontal and vertical stress transfer and results in shallow seismicity increases in the adjacent crust as well as in intermediate depth seismicity within the high-strength upper mantle.Because the thermal and mechanical anomalies discussed are a function of rapid uplift, all regions of active continental collision may be expected to exhibit similar behaviour. Some mechanical and thermal characteristics of the Himalayan collision zone are briefly examined in light of the numerical modeling.     

体波波形反演对青藏高原上地幔速度结构的研究

采用波形反演方法对青藏高原地区震中距8°-38°范围内的宽频带炸波波形进行拟合,研究该地区上地幔平均速度结构以及上地幔纵、横波速度的横向不均匀性结果表明青藏高原地区的平均地壳厚度约为68km,上地幔盖层平均厚度约为30－40km,速度约为8.10km／s雅鲁藏布江附近地壳厚度最大,约80km,相应的上地幔Pn速度为8．15km／s左右,青藏高原中部地区的地壳平均厚度约68-70km．位于拉萨地块北部的羌塘地块S波速度相对较低,其地壳和上地慢的平均S波速度分别比拉萨地块低1％和2％以上34°N以北,90°E附近的区域存在明显的上地幔P波低速异常区,P波的平均速度小于7．8km／s据此结果及前人工作,推断印度板块的俯冲可能以雅鲁藏布江缝合带附近为界,青藏高原巨大的地壳厚度是由于欧亚板块碰撞造成地壳缩短与增厚引起.     

青藏高原及邻近地区地壳与上地幔剪切波三维速度结构

本文利用30个基准台所记录的238条长周期面波资料,经过适配滤波和分格频散反演,得到中国大陆及邻区147个分格10-105s的纯路径频散,进而反演出青藏高原及邻近地区深至170km的剪切波三维速度结构.研究表明,青藏高原中西部地区和东部地区的地壳平均厚度分别为70±7km和65±7km,地壳平均剪切波速度分别为3.55和3.62km/s,上地幔顶盖平均速度分别为4.63和4.61km/s; 岩石层厚度均为120±10km;东部地区下地壳内30-40km深度处普遍存在低速层;青藏高原及其东侧的上地幔低速层内有横贯东西且明显向上隆起的低速腔.滇西缅北地区的地壳厚45±5km,上地壳及下地壳内都有低速层;上地幔顶盖的速度为4.42km/s,比青藏高原本体及恒河平原都低.恒河平原地壳厚34±2km,速度平均为3.45km/s;上地幔顶盖厚86±10km,速度平均为4.63km/s,顶盖内55-83km深处有一个低速夹层.     

新马德里地震带的岩石圈强度与板内形变

提出一个简单的假说来解释为什么在相对稳定的板块内部地区会存在高地震活动区与高构造形变区．首先,对于大多数板内地区而言,特别是大陆地盾地区与老的海洋盆地,下地壳与上地幔的温度相当低,那里的岩石相对坚硬在这些地区不可能发生明显的岩石圈变形,因为岩石图累积强度大大超过板块驱动力．相反,如果下地壳与上地幔温度相对较高,板块驱动力则主要由上地壳承受,因为下地壳与上地幔相对软弱在这种地区,由于岩石圈累积强度与板块驱动力大小相当,构造形变相对较快．本文将这种假说应用在位于美国中部的新马德里地震带与周围地区．地震带内部热流密度值约为60mw／m2,略高于本区背景热流密度值45mW／m2．计算得到的地温梯度与实验室结果所揭示的延性流动定律表明,在地震带内下地壳与上地幔相当软弱,板内应力主要由上地壳传递．那里的形变速率相对较高．与此相反,在周围地区热流值相对较低,岩石四累积强度大大超过板块驱动力,构造应力由地壳与上地幔共同承受热流值的大小和下地壳上地幔的受力状态是决定地震活动性在地震带内与周围地区强烈对比的主要因素．     

2008汶川Ms8.0地震发生的深层过程和动力学响应

汶川M_s8.0强烈地震发生在一条现今并不活动的龙门山构造带上,造成了以汶川、映秀为中心及其周边地域的严重破坏和人员的重大伤亡.然而强烈震发生前却未见有可能的确切征兆或浅表层异常活动,即浅层过程与地震发生的深层过程并不匹配.为此对这次强烈地震“孕育”、发生和发展的深层过程进行了分析和探讨,初步研究表明：①在印度洋板块与欧亚板块陆—陆碰撞、挤压作用下,喜马拉雅造山带东构造结向NNE方向顶挤、楔入青藏高原东北缘,迫使高原深部物质向东流展,在受到以龙门山为西北边界的四川盆地阻隔下,一部分物质则转而向东南侧向运移;②龙门山地带在地形上差达3500±500 m左右,而地壳厚度在龙门山西北部为60±5 km左右,四川盆地为40±2 km左右,而龙门山地带与其东、西两侧相比则为地壳厚度变化幅度达15~20 km的突变地域,即为应力作用的耦合地带;③中、下地壳和地幔盖层物质以地壳低速层、低阻层（深20~25 km）为第一滑移面,以上地幔软流层顶面为第二滑移面,且在四川盆地深部“刚性”物质阻 隔下,深部壳、幔物质以高角度在龙门山构造带和四川盆地的耦合地带向上运移（或称逆冲）,且在龙门山地表三条断裂构成的断裂系向下延伸到20 km左右深处汇聚,二者强烈碰撞、挤压、震源介质破裂;在物质与能量的强烈交换下,应力得到释放,故形成了这次M_s.0强烈地震.为此从深部初步揭示了这次强烈地震“孕育”、发生和发展的深层动力过程.     

大陆下地壳层流作用及其大陆动力学意义

大量的地质和地球物理资料表明 ,年轻的大陆构造活动区的下地壳可能因热软化而出现透入性非地震式顺层韧性流动 ,这种下地壳层流作用驱动大陆上地壳发生地震式脆性断块运动 ,形成盆山格局 ,发生圈层耦合。大陆下地壳低粘度物质顺层流动可能是在地幔岩浆底侵作用为下地壳提供热能和添加幔源物质的基础上 ,并在地幔上升派生的重力和剪切力作用下 ,造成大陆下地壳热软化物质从盆地下部的幔隆区顺层流向相邻造山带之下的幔拗区。在下地壳层流过程中 ,地温场和速度场发生变