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.     

温压条件下岩石破坏前后的力学性质与波速

在温压条件下，对大理岩和砂岩进行了全过程应力应变试验，研究了温压条件对岩石的极限强度，残余强度，剪切破裂角的影响，以及岩石破坏前后的弹性模量与波速的变化。得到了在温压条件下，岩石变形全过程中有关弹性参数和力学性质的变化规律，并进行了简要分析。     

Compressional wave velocity of granite and amphibolite up to melting temperatures at 1 GPa

Compressional wave velocities (V_P) at above-solidus temperatures and at 1 GPa were obtained for a granite and amphibolite, which are considered to be major constituents of the continental crust. The temperature variation of velocities showed that the V_P values of granite decreased with rising temperature, but substantially increased beyond the melting temperature (850–900 °C). Such an increase may be caused by the α–β transition of quartz. The velocities of amphibolite decreased linearly with increasing temperature and dropped sharply at temperatures above the solidus (700 °C), indicating that partial melting of amphibolite acts to significantly lower the seismic velocities.     

Kinematic variations across Eastern Cordillera at 24°S (Central Andes): Tectonic and magmatic implications

The Eastern Cordillera (Central Andes,  24°S) consists of a basement-involved thrust system, resulting from Miocene–Quaternary eastward migrating compression, separating the Puna plateau from the Santa Barbara System foreland. The inferred Tertiary strains arising from shortening in the Eastern Cordillera and Santa Barbara System are similar, higher than in the Puna. Slip data collected on the major  N–S trending faults of Eastern Cordillera show a westward progression from dip-slip (contraction) to dextral and sinistral motions. This, consistently with established tectonic models, may result from partitioning due to the oblique Mio-Quaternary underthrusting of the Brazilian Shield north of 24°S. This strain partitioning has three main implications. (1) As the dextral and sinistral shear in the Eastern Cordillera are  62% and 29% of the compressive strain respectively, the Eastern Cordillera results more strained than Santa Barbara System foreland, contrary to previous estimates. (2) The partitioning in the Eastern Cordillera may find its counterpart in that to the west of the Central Andes, giving a possible structural symmetry to the Central Andes. (3) The easternmost N–S strike-slip structures in the Eastern Cordillera coincide with the easternmost Mio-Pliocene magmatic centres in the Central Andes, at  24°S. Provided that, further to the east, the crust is partially molten, the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the Eastern Cordillera.     

Key elements of regional seismic velocity models for long period ground motion simulations

Regional 3-D seismic velocity models used for broadband strong motion simulations must include compressional-wave velocity (Vp), shear-wave velocity (Vs), intrinsic attenuation (Qp, Qs), and density. Vs and Qs are the most important of these parameters because the strongest ground motions are generated chiefly by shear- and surface-wave arrivals. Because Vp data are more common than Vs data, many researchers first develop a Vp model and convert it to a Vs model. I describe recent empirical relations between Vs, Vp, Qs, Qp, and density that allow velocity models to be rapidly and accurately calculated. The U. S. government reserves the right to retain a non-exclusive, royalty free license in and to any copyright.     

有效应力变化对致密砂岩孔隙结构及弹性波响应的影响规律

致密砂岩气藏具有裂缝发育和有效应力高的特征,研究不同有效压力下孔、裂隙介质地震波传播特征,有利于地震解释与地下储层的识别.但是前人的研究较少考虑岩石内部微观孔隙结构特征与孔隙、裂隙间流体流动的关系.本文首先通过选取四川盆地典型致密砂岩岩样,在不同有效压力下对岩石样本进行超声波实验测量.然后基于实验测得的纵、横波速度进行裂隙参数反演,得到不同有效压力下致密砂岩样本的裂隙孔隙度.再将裂隙孔隙度和样本岩石物理参数代入双重孔隙介质模型,模拟得到不同有效压力下饱水致密砂岩样本纵横波速度频散和衰减的变化规律.结果表明模型预测的速度频散曲线与纵波速度实验测量结果能够较好的吻合.最后统计分析了致密砂岩裂隙参数,得到了致密砂岩储层裂隙参数随有效压力及孔隙度变化特征.依据实际岩石物理参数建立模型,其裂隙参数三维拟合结果能够较好描述致密砂岩裂隙结构与孔隙度、应力的关联,可为实际地震勘探中预测储层裂缝性质提供基础依据.     

Appraisal of CO2 storage potential in compressional hydrocarbon-bearing basins: Global assessment and case study in the Sichuan Basin (China)

Carbon capture and storage (CCS) has been proposed as a potential technology to mitigate climate change. However, there is currently a huge gap between the current global deployment of this technology and that which will be ultimately required. Whilst CO₂ can be captured at any geographic location, storage of CO₂ will be constrained by the geological storage potential in the area the CO₂ is captured. The geological storage potential can be evaluated at a very high level according to the tectonic setting of the target area. To date, CCS deployment has been restricted to more favourable tectonic settings, such as extensional passive margin and post-rift basins and compressional foreland basins. However, to reach the adequate level of deployment, the potential for CCS of regions in different tectonic settings needs to be explored and assessed worldwide. Surprisingly, the potential of compressional basins for carbon storage has not been universally evaluated according to the global and regional carbon emission distribution. Here, we present an integrated source-to-sink analysis tool that combines comprehensive, open-access information on basin distribution, hydrocarbon resources and CO₂ emissions based on geographical information systems (GIS). Compressional settings host some of the most significant hydrocarbon-bearing basins and 36% of inland CO₂ emissions but, to date, large-scale CCS facilities in compressional basins are concentrated in North America and the Middle East only. Our source-to-sink tool allows identifying five high-priority regions for prospective CCS development in compressional basins: North America, north-western South America, south-eastern Europe, the western Middle East and western China. We present a study of the characteristics of these areas in terms of CO₂ emissions and CO₂ storage potential. Additionally, we conduct a detailed case-study analysis of the Sichuan Basin (China), one of the compressional basins with the greatest CO₂ storage potential. Our results indicate that compressional basins will have to play a critical role in the future of CCS if this technology is to be implemented worldwide.     

1.0GPa和常温至1100℃条件下角闪石斜长片麻岩的Vp变化: 实验测量与理论计算

在1.0GPa、常温至1100℃条件下测量了角闪石斜长片麻岩的纵波速度（V_p）,并统计了不同温度实验产物中各种矿物的体积百分含量。结果显示,1.0GPa下,角闪石斜长片麻岩的V_p首先随温度升高（室温至700℃）缓慢降低约6％,然后（700~850℃）快速降低约6％,再（850~950℃）转而急剧升高15％~25％,最后（T>950℃）又快速下降。实验产物分析表明在高温高压下α石英-β石英相变和岩石部分熔融是岩石V_p异常变化的主要因素。由取样产物的矿物含量和弹性参数,计算了各温度条件下岩石的V_p,得出与实验测量相同的波速-温度变化趋势,即V_p随温度升高先缓慢降低,接着快速降低后又急剧升高,最后又快速减小。实验测量和理论计算对比研究表明,通过高温高压下岩石中的物相变化观测结果进行岩石波速的计算,是检验岩石弹性波速测量结果和研究地球内部地震波结构的一种有效方法。     

海原走滑断裂带及其尾端挤压构造

研究了青藏高原东北部地区海原活动断裂带的几何学、运动学和动力学特征、左旋位移总量及全新世滑动速率,对海原走滑断裂带东南端发育的尾端挤压区的构造特征及地壳缩短量进行了研究,分析了走滑断裂带的走滑量与端部挤压区地壳缩短量之间的转换平衡关系     

基于GPS、InSAR和强震数据联合反演2017年九寨沟MS7.0地震同震滑动分布

以多源观测为约束,利用近场GPS、InSAR同震位移和强震观测数据联合反演2017年九寨沟M_S7.0地震发震断层的几何形状和破裂模型,并初步分析其地震成因。结果表明,九寨沟地震的发震断层走向154°,倾角80°,以左旋走滑为主,兼有少量正断分量;地震破裂主要集中在2~12 km深度范围,最大滑移量为1.6 m,位于地下6 km深处,断层在近地表没有滑移;地震释放的地震矩为5.59×10¹⁸ Nm,矩震级为M_W6.44。结合余震分布认为,该地震的发震断层为隐伏的虎牙断裂北段。