龙门山推覆构造带中段山前断裂晚第四纪活动的地质与地球物理证据

龙门山逆冲推覆构造带中段山前断裂的存在和最新活动时代一直是个争论的问题.石油地震探测资料和浅层地震剖面揭示该断裂的存在,并断错了第四系;野外调查表明,龙门山中段山前存在明显的线性地貌特征,山前断裂断错晚了更新世晚期的洪积台地;探槽剖面揭示距今约1500 a之前在山前断裂上曾发生过一次地表破裂型事件,而该断裂未来具备发生强震的潜势.断错地貌的差分GPS测量和年代学分析显示山前断裂晚第四纪垂直滑动速率大于0.36 mm/a,其与龙门山中段主干断裂活动强度相当,说明龙门山山前断裂在龙门山逆冲推覆构造带的变形中也承担着重要的作用.该研究不仅能为成都平原的地震危险性评价提供基础资料,也有助于全面理解青藏高原东缘的隆升机制.     

喜马拉雅及南藏的地壳俯冲带——地震学证据

地质学的证据表明 ,在喜马拉雅的冲断层带MCT和MBT处有大规模的地壳缩短 ;在雅鲁藏布缝合带附近也观测到冲断层 .但是 ,迄今还不知道这些冲断层向下俯冲多深 .我们根据地震学的证据 ,认为喜马拉雅及南藏的冲断层向下延伸至 80— 1 0 0km ,然后停止 .在MCT、MBT以及雅鲁藏布缝合带下面的冲断层与喜马拉雅以及南藏的多次地壳俯冲有密切关系 .这个现象为印度—欧亚的碰撞过程设定一个十分重要的框架 .该地区的地壳俯冲有一定深度 ,由于入侵的地壳太轻 ,使俯冲不能更深 ;此时由于印度板块的继续向北推进 ,在原俯冲带后方 ,出现另一个新的地壳俯冲带 .喜马拉雅与南藏的多重地壳俯冲与该地区地质活动的多幂性相吻合 .首先 ,在雅鲁藏布缝合带产生地壳俯冲 ,在到达 80— 1 0 0km处停止 .然后 ,在雅鲁藏布以南的MCT和MBT相继产生新的地壳俯冲 .它们也在 80— 1 0 0km的深处停止 .除了喜马拉雅和雅鲁藏布向北倾斜的地震带外 ,另外还观测到一个自地表从唐古拉山向南缓慢倾斜并到达雅鲁藏布地壳底部的地震带 .它可以解释为在唐古拉山附近的地壳向北仰冲 .喜马拉雅及南藏的多重地壳...     

大别造山带地壳结构反演及其动力学意义

大别造山带是全球最大的碰撞造山带之一,三叠纪时期,扬子板块深俯冲至地幔的200km处,经历了超高压变质作用。白垩纪早期,该造山带发生了强烈的伸展和垮塌,以及大规模的后造山地幔源岩浆侵入和火山活动。本研究收集了大别造山带及其邻区(29°~34°N、114°~119°E)的震相资料,采用双差层析成像技术,对大别造山带地壳结构进行反演,研究地壳结构与后造山地幔源岩浆侵入和火山活动之间的关系。结果显示,大别造山带中上地壳存在低速结构,该低速结构可能是熔融的幔源侵入物质,由于俯冲板片断裂,或下地壳/岩石圈发生拆沉,导致软流圈物质上涌至地壳底部、侵入地壳中,形成大别造山带地壳中的低速结构;同时,合肥盆地显示为低速区,可能是受浅部沉积层影响。研究中横切大别山的4条剖面显示,该地区下方存在北向倾斜高速结构,该高速结构可能是襄樊-广济断层,或者是扬子板块向华北板块下方俯冲的遗迹。     

青藏高原东缘地壳密度结构及其地球动力学意义

青藏高原东缘是高原物质向E及SE扩展的重要通道,掌握青藏高原东缘的地壳密度结构对研究青藏高原的隆升、变形机制具有重要意义。文中在前人研究成果的基础上,选取了地面实测的9条交叉的重力测线数据,以深地震反射剖面为约束,采用人机交互模式反演得到了青藏高原东缘地下的二维密度结构,并通过克里金插值法获取了三维密度结果。反演结果表明,青藏高原东缘地区具有巨厚的地壳,莫霍面埋深最深约为61km,而四川盆地的莫霍面埋深约为42km,以龙门山-安宁河-小金河断裂为界,两侧形成了莫霍面深度变化梯度带;从反演得到的沉积层厚度来看,沉积层在青藏高原东缘几个块体内呈现中心普遍厚度较大、边缘厚度较薄的特点。结合该地区的地震空间分布特征分析,青藏高原东缘的莫霍面和沉积层厚度分布与该地区的地震分布均具有很强的相关性,这对未来地震预测也具有重要的参考价值。     

秦岭造山带及其两侧区域地壳剪切波分裂

本文分析了陕西区域地震台网2006年1月到2015年7月的近震波形资料,采用剪切波分裂系统分析方法（SAM）,获得了秦岭造山带及两侧区域17个台站的快波偏振方向和慢剪切波时间延迟.结果显示,剪切波分裂参数具有明显的分区特征.鄂尔多斯内部和渭河盆地的台站表现出NE-SW和NEE-SWW向的快波偏振方向,与华北地块区域主压应力方向一致;鄂尔多斯西南部,处于多个地块过渡区,积累了大量的应变,该区快波偏振方向较为复杂,反映了该区复杂的区域构造;南秦岭以勉略缝合带为界,北部为秦岭微板块,南部为扬子地块北缘.秦岭微板块地区台站快剪切波优势偏振方向为NWW-SEE向,与华南地块应力方向一致;扬子地块北缘地区台站表现出NE-SW向的快波优势偏振方向,可能受到青藏高原向东北扩张的影响.本文得到的快剪切波优势偏振方向与秦岭造山带被商丹缝合带和勉略缝合带分割的构造划分方案一致.研究区内,扬子地块北缘地区慢剪切波时间延迟最大,可能反映该区各向异性强于其他区域.本文的研究结果为进一步了解区域应力场特征和动力学过程提供了重要参考.

     

喜马拉雅及南藏的地壳俯冲带——地震学证据

地质学的证据表明 ,在喜马拉雅的冲断层带MCT和MBT处有大规模的地壳缩短 ;在雅鲁藏布缝合带附近也观测到冲断层 .但是 ,迄今还不知道这些冲断层向下俯冲多深 .我们根据地震学的证据 ,认为喜马拉雅及南藏的冲断层向下延伸至 80- 1 0 0km ,然后停止 .在MCT、MBT以及雅鲁藏布缝合带下面的冲断层与喜马拉雅以及南藏的多次地壳俯冲有密切关系 .这个现象为印度-欧亚的碰撞过程设定一个十分重要的框架 .该地区的地壳俯冲有一定深度 ,由于入侵的地壳太轻 ,使俯冲不能更深 ;此时由于印度板块的继续向北推进 ,在原俯冲带后方 ,出现另一个新的地壳俯冲带 .喜马拉雅与南藏的多重地壳俯冲与该地区地质活动的多幂性相吻合 .首先 ,在雅鲁藏布缝合带产生地壳俯冲 ,在到达 80- 1 0 0km处停止 .然后 ,在雅鲁藏布以南的MCT和MBT相继产生新的地壳俯冲 .它们也在 80- 1 0 0km的深处停止 .除了喜马拉雅和雅鲁藏布向北倾斜的地震带外 ,另外还观测到一个自地表从唐古拉山向南缓慢倾斜并到达雅鲁藏布地壳底部的地震带 .它可以解释为在唐古拉山附近的地壳向北仰冲 .喜马拉雅及南藏的多重地壳...     

地球物理综合观测揭示秦岭—桐柏—大别复合造山带地壳及上地幔结构

秦岭—桐柏—大别复合造山带(以下称为秦岭大别造山带)属于中国中央造山带的一部分,由华北克拉通与扬子克拉通汇聚形成.对于秦岭大别造山带及其周缘地区的研究,可以为这一大陆碰撞造山带的形成与演化过程提供重要信息.本文整合研究区域的接收函数与背景噪声数据,采用H-κ叠加分析、接收函数与背景噪声联合反演、克希霍夫偏移成像等方法,...     

大别山超高压变质带地壳结构及其构造意义

根据大别山深地震测深剖面的资料,取得了大别山地区的地壳结构模型．二维地壳结构显示了碰撞造山带的特征和超高压变质带的深部地球物理证据．地壳上部的三维速度结构表明：在2 km深度上速度分布与地表的地质构造明显相关,在5～10 km深度上超高压变质带显示相对高速．布格重力异常的观测资料显示大别山地区有较大范围的负异常．在由上部地壳引起的布格重力异常中,超高压变质带则为正异常区．由于在地震剖面的二维地壳模型中,造山带的山根仅有4～5 km厚,且上地幔顶部的速度横向变化不明显,因此可以认为,观测与计算的布格重力异常不一致的原因主要来自地壳,至少在中上地壳内应产生负布格重力异常．超高压变质带中地壳内的物质密度应比周围地区低．这一较低密度的物质与扬子地壳向北俯冲到100 km以下的深度,然后返回地壳的碰撞过程有关．     

华北克拉通中西部地壳S波速度结构及其地质意义

华北克拉通是世界上最古老的克拉通之一.我们利用布设于华北中部的ChinArray计划461个宽频带地震台阵的连续波形资料, 基于背景噪声成像技术, 获得了克拉通中西部5~45 s的Rayleigh波群速度频散曲线, 并利用线性反演方法获得了研究区地壳上地幔顶部的S波速度结构.密集流动地震台阵使我们能够揭示研究区精细的地壳上地幔顶部速度变化, 以深入探讨华北克拉通中西部深部结构及其对岩浆和地震的控制作用.8 km深度的S波速度切片显示低速与高速异常分别与地表的盆地和山脉对应良好.不同经度和纬度方向的S波速度剖面均表明, 西部克拉通地壳大致可以分为上、中、下地壳三层.克拉通西部鄂尔多斯块体的下地壳S波速度介于3.7~3.8 km·s^-1, 暗示其下地壳以长英质岩石为主.大同火山区下方的S波低速异常从中地壳延伸至上地幔顶部, 推测源自软流圈的地幔热流提供了近垂直的主干上涌通道, 并控制了该区新生代岩浆活动.强震集中分布在上地壳高速体内部或高低速相间区, 其下地壳乃至上地幔顶部都呈现明显的低速异常, 推测源自上地幔/下地壳的深部热流沿地壳尺度的陡深断裂上侵, 诱发上覆高应力刚性块体发生蠕动破裂与应力释放, 进而诱发大震.

     

兴蒙造山带及邻区地壳速度结构特征

兴蒙造山带位于中亚造山带东段, 作为古亚洲构造域的重要组成部分, 由西伯利亚板块与华北板块碰撞拼合而成, 其经历了大陆裂解、洋盆扩张、洋壳俯冲消减和碰撞拼合造山等复杂的构造演化过程.为了利用壳幔结构约束造山带演化的深部过程, 跨越华北地块北缘、松辽—锡林浩特地块、兴安地块以及索伦—西拉木伦缝合带和二连—贺根山缝合带, 实施了一条520 km长的深地震测深剖面, 获得了高质量的人工源大当量的宽角反射和折射地震资料, 并采用地震动力学射线方法获得地壳速度结构.结果显示: (1)研究区地壳平均速度为6.15~6.3 km·s^-1, Pn波速度为7.8~8.2 km·s^-1; (2)地壳厚度约为36.1~42.2 km, 最厚位置(~42.2 km)对应地表大兴安岭主峰, 说明大兴安岭在此位置存在山根; (3)地壳速度在1.5~6.8 km·s^-1范围内, 认为在该区地壳内不存在洋壳物质; (4)主要断裂带或缝合带位于速度等值线变化剧烈的梯度带上; (5)速度结构显示研究区具有明显的横向分区和纵向分层的特点.地壳内速度剧烈变化特征表明兴蒙造山带的地壳物质组成不均匀, 尤其中下地壳, 速度等值线起伏剧烈.这种复杂的地壳速度结构应该与中生代以来多板块汇聚引发的多期区域性伸展和挤压作用有关.

     

Geophysical evidence for thrusting of crustal materials from orogenic belts over both sides of the Yangtze Block and its geological significance

Based on deep geophysical detections, we have reconstructed the crustal structure from the eastern margin of the Tibetan Plateau to the Jiangnan-Xuefeng orogenic belt. The results suggest that the Yangtze Block was overthrusted by crustal materials in its NW direction from the eastern Tibetan Plateau but in its SE direction from the Jiangnan orogen. These overthrusting effects control the crustal structure from the western Sichuan to the western area of the Jiangnan orogen-Xuefeng orogenic belt. The eastward extruded materials from the eastern Tibetan Plateau were blocked by the rigid basement in the Sichuan Basin, where upper-middle crust was overthrusted whereas the lower crust was underthrusted beneath the Sichuan Basin. The underthrusted unit was absorbed by crustal folding, shortening and thickening in the Yangtze Block, forming the Xiongpo and Longquan Mountains tectonic belts and resulting in the NW-directed thrusting of the Pujiang-Chengdu-Deyang fault, and the western hillsiden fault in the Longquan Mountain. These results provide resolution to the controversy where the eastward extrusion material from the Qinghai-Tibet Plateau had gone. Overall, that Yangtze Block was subjected to thrusting of the crustal materials from the orogenic belts over its both sides. This finding has implications for the study of the intracontinental orogenic mechanism in South China, the reconstruction of tectonic evolutionary history and the kinematics processes during the lateral extrusion of the Tibet Plateau.     

Discovery of Paleogene palynological assemblages from the Wanbaogou Group-complex in western part of the Eastern Kunlun orogenic belt and its geological significance

The studied region is located in the Northern Qing- hai-Tibet Plateau, the southern fringe of the Qaidam Basin, about 80 km south away from Golmud City of Qinghai Province (Fig. 1). The Eastern Kunlun oro- genic belt tectonically belongs to a joint zone between the Paleo-Asian Tectonic Domain and the Tethyan- Himalayan Tectonic Domain. Owing to the repeated tectonic movements, its geological structures have become much complicated and the degrees of defor- mation, metamorphism and disl…     

从重力场识别与提取地壳变形带信息的方法研究

表面形貌识别是指用具体的参数表征表面各区段几何形态及属性并最终对不同类型的表面形貌进行识别,表面刻痕识别是其中的一种.以随机过程理论为基础的表面刻痕识别技术可以确切地识别表面形貌的各项特征,其各阶谱矩及统计不变量可以对表面的刻痕以及各向异性进行详细地刻画.区域重力场上的各向异性刻痕主要反映地壳线状变形带,它们常常是区域大地构造单元的边界.本文用重力场研究地壳变形带信息识别方法,将二阶谱矩、统计不变量等参数赋予一定的地质构造含义,并定义了脊形化系数、边界脊形化系数等,从而更详细地表征地壳变形带及大陆构造单元边界信息.理论模型与实际数据试验结果均表明,地壳变形带信息识别方法不但对重力场上地壳变形产生的线形刻痕进行了有效的刻画,而且提取出的刻痕信息,可为大陆构造单元划分提供客观依据.     

Discovery of deerite from the Aksu Precambrian blueschist terrane and its geological significance

Deerite [Fe ₁₂ ²⁺ Fe ₆ ³⁺ Si₁₂O₄₀ (OH)₁₀] was first discovered in the metamorphic magnetite-bearing quartzites which are interlayered with blueschists in Aksu Precambrian blueschist terrane. The deerite-bearing mineral association includes (1) deerite + riebeckite + stilpnomelane + ilvaite + magnetite + quartz and (2) deerite + stilpnomelane + magnetite +quartz based on the investigation under microscope. The study of mineral chemistry shows that the deerites from Aksu Precambrian blueschist are Mn-poor deerite similar to that from Alps. But the deerite from Aksu is the nearest to the end member of deerite. According to the equilibrium P-T region of deerite determined by Lattard and Breton (1994), the deerites in metamorphic magnetite-bearing quartzites from Aksu Precambrian blueschists were formed under the conditions of pressure lower than 1.0 Gpa, temperature ranging from 300 to 400°C and about the l0°C/km geothermal gradient. The deerite in metamorphic magnetite-bearing quartzites from Aksu blueschists is the only one Precambrian deerite reported now. This suggests that the earth began to cool, and the modern cooling subduction regime between plates started at Late Proterozoic. The Late Proterozoic may be the important period during the evolution of the earth.     

Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation

The Taupo Volcanic Zone (TVZ) of New Zealand is characterised by extensive volcanism and by high rates of magma production. Associated with this volcanism are numerous high-temperature (> 250 °C) geothermal systems through which the natural heat output of 4200 ± 500 MW is channelled. Outside the geothermal fields the heat flow is negligible. The average heat flux from the central 6000 km² of the TVZ, which contains most of the geothermal fields, is 700 mW/m³. This heat flux appears to be more concentrated along the eastern margin of the TVZ.Schlumberger resistivity measurements (AB/2 of 500 m and 1000 m) have identified 17 distinct geothermal fields with natural heat outputs greater than 20 MW. An additional six, low-heat-output geothermal fields also occur, and may represent formerly more active systems now in decline. Two extinct fields have also been identified. The average spacing between fields is 10–15 km. The distribution of geothermal fields does not appear to be directly associated with individual volcanic features except for the geothermal system that occurs within Lake Taupo and which occupies the vent of the 1800 yr.B.P. Taupo eruption. The positions of the geothermal fields do not appear to have varied for at least the last 200,000 years. These data are consistent with a model of large-scale convection occurring throughout the TVZ, in which the geothermal fields represent the upper portion of the rising, high-temperature, convective plumes. The majority of the recharge to the convection system is provided by the downward movement of cold meteoric water between the fields which suppresses the heat flow in these regions.Gravity measurements indicate that to a depth of about 2.5 km the upper layers of the TVZ consist of low-density pyroclastic infill. A seismic refraction interface with velocity change from 3.2 km/s to 5.5 km/s occurs at a similar depth. The cross-sectional area of the convection plumes (identified electrically) appears to increase at depths of 1–2 km, consistent with a decrease in permeability at the depth at which the velocity and density increase.The seismicity is dominated by swarm activity which accounts for about half of all earthquakes and is highly variable in both space and time. The small number of seismic events (and swarms) that have well determined depths show a cut off of seismicity at depths of 7–9 km. The depth of the transition from brittle to ductile behaviour of the rocks is identified with the transition from a regime where heat is transported by (hydrothermal) convection and pore pressures are near-hydrostatic to a regime where heat transport is dominantly conductive and pore pressures are lithostatic. Within the convective region, temperatures are moderated by the circulation of water so that the depth of the transition from convective to conductive heat transfer can be linked to the bottom of the seismogenic zone. Rocks must become ductile within about 1 km of the bottom of the overlying convective zone.Seismic refraction studies suggest that the crust beneath the TVZ is highly thinned with a seismic velocity of about 7.5 km/ s, typical of the upper mantle, occurring at depth of 15 km. Seismological studies indicate the upper mantle is highly attenuating beneath the TVZ. Conductive heat transfer between the bottom of the convective system, at about 8 km, and the base of the material with crustal velocities, at 15 km, is not able to provide all the heat that is discharged at the surface. Repeated intrusion from the mantle may provide the additional heat transport required.     

川滇地块西部差异性旋转的构造意义:青藏高原东南缘白垩纪红层古地磁学新证据

印度-欧亚板块碰撞以来青藏高原内部及其周缘地区经历了复杂的构造演化,复杂构造变形区的复合构造使得古地磁的数据解释究竟代表区域的构造旋转还是只能反映局部的构造变形一直是备受关注的问题.本文通过采集川滇地块西缘渔泡江断裂东侧三岔河地区白垩纪红层古地磁样品,揭示采样区差异性旋转并探讨川滇地块西部自中新世以来的构造演化规律.前人的地质调查表明川滇地块渔泡江断裂东侧上白垩统赵家店组地层发育倾伏褶皱.三岔河剖面以三岔河镇为界分为南北两段,三岔河南段剖面高温剩磁分量平均方向在倾斜校正后D_s=29.3°,I_s=45.7°,k_s=54.3,α₉₅=6.6°,倾伏地层产状校正后D_s=30.6°,I_s=46.6°,k_s=69.3,α₉₅=5.8°;而三岔河北侧剖面高温剩磁分量平均方向在倾斜校正后D_s=350.4°,I_s=42.1°,k_s=69.4,α₉₅=9.2°,倾伏地层产状校正后D_s=347.4°,I_s=41.9°,k_s=96.6,α₉₅=7.8°;两组高温剩磁分量均通过了褶皱检验,表明其获得于褶皱形成之前.相对于东亚稳定区80 Ma古地磁极,三岔河南侧剖面发生了20.5°±4.8°的顺时针构造旋转量,与楚雄盆地核部之间不存在差异性旋转;但三岔河镇以北剖面却发生了22.7°±6.6°的逆时针旋转.综合分析川滇地块内部的古地磁数据表明自中新世以来川滇地块南部楚雄盆地经历了约20°的顺时针构造旋转,而三岔河镇北侧经历了约20°逆时针旋转.进一步分析表明三岔河北侧剖面相对于南侧剖面经历了约40°的逆时针旋转,可能由于研究区的滑脱构造导致岩石薄弱层拆离滑脱所引起.

     

Revised apparent polar wander path of the Yangtze Block and its tectonic implications

Paleornagnetic data, during the Phanerozoic obtained in the last 20 years in the Yangtze Block (YZB), are critically reviewed. A new apparent polar wander path (APWP) for the YZB is constructed by selected poles with objective reliable criteria, with a goal of placing constraints on the models of the formation and subsequent deformation and reconstructing the tectonic evolution history of this region.     

下扬子天目山盆地火山岩锆石LA-ICP-MS定年及地质意义

天目山盆地是下扬子江南隆起带保存较完整的中生代火山盆地,中生代火山岩系岩性自下而上主要为流纹岩-英安岩-安山岩。对盆地内黄尖组下段流纹岩和英安岩分别进行了锆石 LA-ICP MS定年,分别获得了133．6±1．5 Ma（MSWD＝0．73）和135．0±2．1 Ma（MSWD＝0．78）的锆石U-Pb年龄,指示天目山盆地黄尖组火山岩时代为早白垩世。天目山盆地火山活动起始时间和长江中下游地区晚中生代火山活动基本一致,说明江南隆起带和长江中下游地区在早白垩世均处于强烈拉张环境。