东海超高压变质带花岗质片麻岩的岩石学及其成因特征

东海地区的花岗质岩石主要由二长花岗质片麻岩、角碱长花岗质片麻岩和霓石二长花岗质片麻岩组成。根据该类岩石区域产状、化学成分、稀土和微量元素特征,结合前人理论和实验资料,确定研究区花岗质片麻岩是由云母片麻岩类岩石,在超高压变质作用峰期(M2)及近等温减压初期阶段(M3),于相对富水流体体系中经深熔作用而成。该项研究对于深入探讨苏鲁超高压变质带变质作用-岩浆作用相互间的成因关系及其动力学过程有着重要的科学意义     

Rock Damage Structure of the South Longmen-Shan Fault in the 2008 M8 Wenchuan Earthquake Viewed with Fault-Zone Trapped Waves and Scientific Drilling

This article is to review results from scientific drilling and fault-zone trapped waves(FZTWs) at the south Longman-Shan fault(LSF) zone that ruptured in the 2008 May 12 M8 Wenchuan earthquake in Sichuan, China. Immediately after the mainshock, two Wenchuan Fault Scientific Drilling(WFSD) boreholes were drilled at WFSD-1 and WFSD-2 sites approximately 400 m and 1 km west of the surface rupture along the Yinxiu-Beichuan fault(YBF), the middle fault strand of the south LSF zone. Two boreholes met the principal slip of Wenchuan earthquake along the YBF at depths of 589-m and 1230-m, respectively. The slip is accompanied with a 100-200-m-wide zone consisting of fault gouge, breccia, cataclasite and fractures. Close to WFSD-1 site, the nearly-vertical slip of ~4.3-m with a 190-m wide zone of highly fractured rocks restricted to the hanging wall of the YBF was found at the ground surface after the Wenchuan earthquake. A dense linear seismic array was deployed across the surface rupture at this venue to record FZTWs generated by aftershocks. Observations and 3-D finite-difference simulations of FZTWs recorded at this cross-fault array and network stations close to the YBF show a distinct low-velocity zone composed by severely damaged rocks along the south LSF at seismogenic depths. The zone is several hundred meters wide along the principal slip, within which seismic velocities are reduced by ~30–55% from wall-rock velocities and with the maximum velocity reduction in the ~200-m-wide rupture core zone at shallow depth. The FZTW-inferred geometry and physical properties of the south LSF rupture zone at shallow depth are in general consistent with the results from petrological and structural analyses of cores and well log at WFSD boreholes. We interpret this remarkable low-velocity zone as being a break-down zone during dynamic rupture in the 2008 M8 earthquake. We examined the FZTWS generated by similar earthquakes before and after the 2008 mainshock and observed that seismic velocities within fault core zone was reduced by ~10% due to severe damage of fault rocks during the M8 mainshock. Scientific drilling and locations of aftershocks generating prominent FZTWs also indicate rupture bifurcation along the YBF and the Anxian-Guangxian fault(AGF), two strands of the south LSF at shallow depth. A combination of seismic, petrologic and geologic study at the south LSF leads to further understand the relationship between the fault-zone structure and rupture dynamics, and the amplification of ground shaking strength along the low-velocity fault zone due to its waveguide effect.     

Pressure dependence and anisotropy of P-wave velocities in ultrahigh-pressure metamorphic rocks from the Dabie–Sulu orogenic belt (China): Implications for seismic properties of subducted slabs and origin of mantle reflections

The compressional wave velocities (Vp), pressure derivatives (Vp′) and anisotropy (A) of three types of eclogites and country rocks from the Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt, China, have been measured under confining pressures up to 800 MPa. Type-1 eclogites, which are coarse-grained and subjected to almost no retrograde metamorphism, experienced recovery-accommodated dislocation creep at peak metamorphic conditions (in the diamond stability field). Type-2 eclogites are fine-grained reworked Type-1 materials that experienced recrystallization-accommodated dislocation creep under quartz/coesite boundary conditions during the early stage of exhumation. Type-3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust. Type-1 eclogites are richer in Al₂O₃ and MgO but poorer in SiO₂ and Na₂O than Type-2 and Type-3 eclogites. Anisotropy of Type-1 and Type-2 eclogites is generally low (<4%) because volumetrically important garnet is elastically quasi-isotropic, while Type-3 eclogites can exhibit high anisotropy (>10%) due to the presence of strongly anisotropic retrograde minerals such as amphibole, plagioclase and mica. The transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure (P_c), which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks. The Vp–pressure relationship can be expressed by Vp=a(lnP)²+blnP+c (P≤P_c) and Vp=V₀+DP (P≥P_c), where P is the confining pressure, a and b are constants describing the closure of microcracks below P_c, c is the velocity when P is equal to one (MPa), V₀ is the projected velocity of a crack-free sample at room pressure, and D is the intrinsic pressure derivative above P_c. When data are curve-fit, pressure derivatives and anisotropy as functions of pressure are determined. The average Vp of the eclogites in the linear regime is 8.42+1.41×10⁻⁴P for Type-1, 7.80+1.58×10⁻⁴P for Type-2, and 7.33+2.04×10⁻⁴P for Type-3, where Vp is in km/s and P in MPa. The decrease in V₀ and increase in D from Type-1 to Type-3 eclogites are attributed to a decrease in garnet content and an increase in retrograde minerals. The NE–SW trending, NW-dipping, slab-like high Vp anomaly (8.72 km/s at a depth of 71 km) which extends from the Moho to at least 110 km beneath the Dabie–Sulu region, can be interpreted as the remnant of a subducted slab which is dominated by Type-1 eclogites and has frozen in the upper mantle since about 200–220 Ma. Such relic crustal materials, subducted and preserved as eclogite layers intercalated with felsic gneiss, garnet–jadeite quartzite, marble and serpentinized peridotite, could be responsible for regionally observed seismic reflectors in the upper mantle.     

藏南冈底斯带中段早侏罗世岩浆作用的厘定及其大地构造意义

以藏南冈底斯带中段的东嘎花岗岩体和奴玛花岗岩体为研究对象,对其进行了锆石U-Pb年代学、Lu-Hf同位素和全岩地球化学综合分析,据此探讨其岩石成因和大地构造意义。东嘎花岗岩体主要由石英、长石以及少量黑云母、角闪石组成;奴玛花岗岩体主要由石英、长石以及少量黑云母组成。锆石LA-ICP-MS U-Pb定年结果显示,东嘎花岗岩体的加权平均年龄为185.8±2.3 Ma,奴玛花岗岩体的加权平均年龄为185.9±2.2Ma和185.1±1.7 Ma,两个岩体具有一致的成岩年龄,为同一期岩浆事件的产物。在地球化学组成上,两个岩体均富集轻稀土(LREE)和大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具有明显的俯冲带岩浆岩特征;里特曼指数和铝饱和指数显示,两个岩体属于弱过铝质钙碱性I型花岗岩。锆石Lu-Hf结果显示,两个岩体均具有显著亏损的锆石Hf同位素组成和极高的εHf(t)值(分别为+11.14~+15.55,平均值+13.76;+9.49~+15.52,平均值+13.56)以及相对年轻的二阶段模式年龄(T_(DM2)),反映了它们的源区以新生地壳的部分熔融为主。综合研究表明:早侏罗世(普林斯巴期)时,藏南冈底斯地区应为典型的活动大陆边缘环境,花岗质岩石的形成和新特提斯洋向拉萨地体的北向俯冲有关,其成因与幔源岩浆的底侵引起新生下地壳的部分熔融有关。     

Deformation Characteristics and Tectonic Evolution of the Eastern Qinling Orogenic Belt

The Qinling Mountains separating the northern from the southern China plate is a key region for the study of structural evolution of eastern Asia. It is composed of the Palaeozoic fold belt in its northern part and the Variscan and Indosinian fold belts in its southern part. The evolution of the former is marked by the closure of a northward subducting oceanic basin in the early stage, followed by southward obduction of ophiolites and intracontinental thrusting during the Variscan; whereas that of the latter is represented by intracontinental, shallow crustal deformation on the basis of a large-scale detachment structure(with a horizontal slip of at least of 100 km). Since the late Palaeozoic, however, both of the belts have been cut by a series of east-west sinistral strike-slip faults.     

大陆俯冲作用及青藏高原周缘造山带的崛起

青藏高原周缘造山带于新生代时崛起。周缘造山带中古老变质地体的折返与3种挤 出作用方式有关：喜马拉雅"逆冲－伸展"型挤出、祁连山"反向逆冲"型挤出和阿尔金"逆冲－转换"型挤出。据地质与地球物理综合研究推测,造山带的折返与周缘大陆岩石圈向内的俯冲作用有关：印度板块岩石圈向北俯冲至雅鲁藏布江缝合带下约 200 km处,西伯利亚板块往南低角度插入祁连山 40 km以下,塔里木地块沿阿尔金北缘边冲断层呈铲式往南俯冲于阿尔金山下100km处,扬子地块呈入体插入青藏高原东部中地壳下面。是否存在扬子地块往西运动及大陆俯冲作用尚待探究。     

海底粉质土波致液化的判别程序与计算方法

海底土体在波浪作用下能否产生液化是海岸工程所关心的问题。借鉴地震液化判别使用的砂土液化判别方法,将海底粉质土波致液化的判别分为初判和复判2个阶段。初判以所致海床土体发生破坏的临界循环应力比界限指标来判别,以土质基本特征和波浪条件为参数,对某海域海底液化形成判断;复判以波致海床土体中剪应力与实际土体的动剪切强度比较来判别。结合已有研究成果给出了波致土体液化判别的具体方法。     

伽马蜡烷--水体分层的地球化学标志

密执安盆地等海相蒸发环境伽马蜡烷的分布表明,伽马蜡烷作为咸水环境的生物标志化合物与高盐度环境并没有绝对的对应关系;松辽盆地淡水－微咸水环境较高含量的伽马蜡烷表明其形成与湖海沟通引起的水体分层事件有关。伽马蜡烷重碳同位素值及微生物学证据均表明了伽马蜡烷的形成与水体分层有关,由于高盐环境往往伴随着密度分层,从而解释了伽马蜡烷经常与高盐环境伴生,但并不仅仅局限于高盐环境的现象。     

Micro-area Chemical Composition and Preserved P-T Evolution Trace of Phengite in Albite Gneiss from the Donghai Ultrahigh-Pressure Metamorphic Area, East China

Study of micro-area chemical compositions indicates that phengite in albite gneiss from hole ZK2304 of the Donghai region has evident compositional zoning. SiO2 and tetrahedrally coordinated Si contents decrease, and Al2O3, AlIV and AlVI contents increase gradually from core to rim. However, K2O, MgO and FeO contents basically remain unchanged from core to rim. According to P-T estimates obtained from geothermometers and barometers, combined with previous experimental data, the core belt (micro-area I) of phengite was formed at T=637-672℃ and P=1.55-1.73 GPa, and the transitional belt (micro-area II) of the phengite were formed at r=594-654℃ and P=1.35-1.45 GPa. Towards the rim belt (micro-area III), the temperature decreased slightly, but the pressure decreased rapidly with r=542-630℃ and P=1.12-1.19 GPa. The P-T evolution path recorded by the compositional zoning of phengite is characterized by significant near-isothermal decompression, revealing that the gneiss has. undergone high-pressure-ultrahi