夹层厚度对瑞利面波频散曲线的影响

文章以弹性波横波速度为参数,利用瑞利面波频散曲线的快速标量传递算法进行正演模拟计算,系统研究软弱层厚度对瑞利面波基阶模频散曲线的影响,分析和总结了频率与相速度（f-c）、频率与波数（f-k）、深度与相速度（h-c）等瑞利面波基阶模曲线随软弱层厚度变化的规律。     

基于瑞雷面波频散曲线反演的岩体力学参数求取方法

岩体力学参数是岩土工程中表征岩体力学性质的重要参数,应用二维傅里叶变换获得高信噪比的频散曲线,采用阻尼最小二乘法自动反演横波速度,进而计算岩体力学参数。结合内蒙古二连盆地瑞雷面波勘探实例,得到了动态岩体力学参数剖面,且在采集和处理参数选取合理的情况下,最大勘探深度可达120m。相比于其他岩体力学参数测试技术,瑞雷面波法具有快速、高效、经济、无榀的特点．     

基于OCCAM算法的瑞雷波频散曲线快速稳定反演

OCCAM算法在大地电磁测深资料反演中已获得了成功应用。将其引入到瑞雷波频散曲线反演中,应用权重矩阵,提高了解的纵向分辨率;应用拉格朗日乘子光滑参数改善了迭代效果并提高了解的稳定性,使其可以在解的精度和模型分辨率中取最佳折中解。实测资料试算结果表明:用OCCAM算法对低速软弱夹层瞬态瑞雷波频散曲线进行反演,不但具有稳定性好、精度高、分辨能力强的特点,而且能自动分层和反演地层参数。     

Lithospheric structure of the Tien Shan region constrained by joint inversion of receiver function and Rayleigh wave dispersion

We obtain a lithospheric shear‐wave velocity model across the Tien Shan orogenic belt by jointly inverting Rayleigh wave group velocities and teleseismic P‐wave receiver functions at 61 broadband seismic stations deployed in this region. Our new model reveals prominent lateral variations of shear‐wave velocity in both the crust and uppermost mantle. This model reveals different structures in the upper and middle crust across the Talas Fergana Fault, which may suggest the presence of a tectonic boundary between the western and central Tien Shan beneath the fault. According to the velocity images, the depth extent of the fault is ~40 km and this is confined to the crust. Pronounced low‐velocity anomalies are imaged in the middle crust and uppermost mantle beneath the southern and middle Tien Shan, implying that the upwelling of the materials from the upper mantle could have played an important role in the mountain building.     

On the role of heat flow, lithosphere thickness and lithosphere density on gravitational potential stresses

Gravitational potential stresses (GPSt) are known to play a first-order role in the state of stress of the Earth's lithosphere. Previous studies focussed mainly on crust elevation and structure and little attention has been paid to modelling GPSt using realistic lithospheric structures. The aim of the present contribution is to quantify gravitational potential energies and stresses associated with stable lithospheric domains. In order to model realistic lithosphere structures, a wide variety of data are considered: surface heat flow, chemical depletion of mantle lithosphere, crustal thickness and elevation. A numerical method is presented which involves classical steady-state heat equations to derive lithosphere thickness, geotherm and density distribution, but additionally requires the studied lithosphere to be isostatically compensated at its base. The impact of varying surface and crustal heat flow, topography, Moho depth and crust density on the signs and magnitudes of predicted GPSt is systematically explored. In clear contrast with what is assumed in most previous studies, modelling results show that the density structure of the mantle lithosphere has a significant impact on the value of the predicted GPSt, in particular in the case of thick lithospheres. Using independent information from the literature, the method was applied to get insights in the state of stress of continental domains with contrasting tectono-thermal ages. The modelling results suggest that in the absence of tectonic stresses Phanerozoic and Proterozoic lithospheres are spontaneously submitted to compression whereas Archean lithospheres are in a neutral to slightly tensile stress state. These findings are in general in good agreement with global stress measurements and observed geoid undulations.     

S-wave velocity structure of the North China from inversion of Rayleigh wave phase velocity

We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ.     

Objective regionalization of Rayleigh wave dispersion data by clustering algorithms: an application to the Mediterranean basin

The main target of the present study is an objective and automated regionalization of Rayleigh wave dispersion data for the Mediterranean basin, without a priori seismotectonic constraints, and to determine the corresponding regional shear-velocity structures. The database used is formed by almost 200 Rayleigh wavetrains corresponding to 42 regional events, with surface-wave magnitude greater than 4.5, recorded at the MedNet very-broad-band stations in the Mediterranean area. Path-averaged group velocities for the Rayleigh wave fundamental mode are derived for each available epicentre-station trajectory crossing the Mediterranean basin. After this, a principal component analysis and a clustering process are applied to local group velocities, obtained for 13 different periods from 10 to 70 s, in order to classify the Mediterranean basin into several homogeneous regions. The stochastic inversion of the averaged group velocity dispersion curve obtained for each region provides the respective shear-velocity structures, down to a depth of 150–160 km. The characteristics of these areas and their possible correlation with the main seismotectonic features of the Mediterranean region are discussed. The regional models reveal significant lateral changes in the elastic structure, with the main differences concerning particularly the upper 35–40 km. Within this depth range, low shear velocities, varying from 2.8 to 3.9 km s⁻¹, characterize the Eastern Mediterranean, whereas higher velocities, ranging from 3.0 to 4.2 km s⁻¹, are deduced for the Western Mediterranean. These results suggest a thicker crust in the eastern part, but with a greater thickness of sedimentary layers. However, for depths of between 80 and 110 km, lower shear velocities are obtained in the Western part, while higher shear velocities are derived for the Eastern Mediterranean Sea, in the Aegean Sea, Greece, the south of Italy, Sicily and Tunisia. This velocity pattern suggests an averaged thicker lithosphere under the latter areas, as the top of the asthenosphere is detected at a mean depth of 75 km for the remaining regions. This thicker lithosphere can be related to processes associated with the convergence of the Eurasian and African plates and subduction under the Calabrian and Hellenic Arcs.     

中国大陆地壳和岩石圈化学成分模型可信吗? ——来自大地热流值的检验

大地热流值是大陆地壳和岩石圈U，Th,K丰度的直接约束；根据地球化学元素丰度值推算出的大地构造单元的区域地壳热流值，必定不能大于区域的平均热流值，根据700余个实测大地热流数据，对目前发表的中国大陆地壳和岩石圈的化学成分模型进行了检验，结果表明，多数模型不能满足大地热充约束，如黎彤等的关于中国大陆及其内部构造单元的地壳和岩石圈成分模型，倪守斌等提出的新疆北部地壳生热率模型，以及高山等提出的扬子地台北缘地壳成分模型，这些模型的U，Th,K丰度值不太可靠，其他强不相容元素的丰度值的可信程度亦值得怀疑，而迟清华，鄢明才提出的华北地台地壳成分模型和高山等建立的中国东部及华北地台和秦岭造山带的地壳成分模型通过了区域大地热流的检验。     

Local modification of the lithosphere beneath the central and western North China Craton: 3-D constraints from Rayleigh wave tomography

We have imaged the lithospheric structure beneath the central and western North China Craton (NCC) with Rayleigh wave tomography. The Rayleigh waveforms of 100 teleseismic events recorded by 208 broadband stations are used to yield high-resolution phase velocity maps at 13 periods from 20 s to 143 s. A 3-D S-wave velocity model is constructed based on the phase velocity maps. Our S-wave velocity model is broadly consistent with the results of previous tomography studies, but shows more detailed variations within the lithosphere. The Trans-North China Orogen (TNCO) is generally characterized by low-velocity anomalies but exhibits great heterogeneities. Two major low-velocity zones (LVZs) are observed in the north and south, respectively. The northern LVZ laterally coincides with sites of Cenozoic magmatism and extends to depths greater than 200 km. We propose that a small-scale mantle upwelling is present, confined to the north of the TNCO. A high-velocity patch in the uppermost mantle is also observed between the two LVZs adjacent to the narrow transtensional zone of the Cenozoic Shanxi–Shaanxi Rift (SSR). We interpret this as the remnant of a cratonic mantle root. The Ordos Block in the western NCC is associated with high-velocity anomalies, similarly reflecting the existence of cratonic mantle root, but a discernible low-velocity layer is observed at depths of 100–150 km in this location. We interpret that this mid-lithospheric structure was probably formed by metasomatic processes during the early formation of the NCC. Based on the observations from our S-wave velocity model, we conclude that the current highly heterogeneous lithospheric structure beneath the TNCO is the result of multiphase reworking of pre-existing mechanically weak zones since the amalgamation of the craton. The latest Cenozoic lithospheric reworking is dominated by the far-field effects of both Pacific plate subduction and the India–Eurasia collision.     

2D inversion of the magnetotelluric data from Mahallat geothermal field in Iran using finite element approach

The natural-field magnetotelluric (MT) method has proven very useful for mapping the geothermal fields as resistivity sections. The depth of investigation of the MT method is sufficiently large to penetrate deep into the upper crust. MT soundings along two transects across Mahallat geothermal field in Iran were carried out to determine the crustal structure in the region. The selected MT profiles in the region cross over the hydrothermally altered zones and different geological structures. Data were acquired along two profiles crossing the Mahallat hot springs with a total of 28 MT stations in a frequency range of 8,000 to 0.008 Hz. Spacing between stations was kept 500 m for a good resolution. We have used the code MT2DInvMATLAB for inversion using the method of finite elements for forward modeling. Apparent resistivity and phase data of transverse electric (TE), transverse magnetic (TM), and TE + TM modes along each profile were modeled. The geothermal fluid reservoir is resolved at 1,000 to 3,000 m depth and the geothermal resource is estimated to be located at 7,000 m or deeper.     

Rayleigh面波各阶模式频散曲线对横波速度和层厚的敏感性探讨

Rayleigh面波勘探的目的在于有效利用频散曲线反演地层厚度及横波速度,而不同模式的频散曲线对横波速度和层厚的敏感性不同。通过求取介质参数变化10%后与参数不变化时的二组频散曲线的差值,得到各阶模式的频率～相速度差曲线,分析了Rayleigh面波各模式频散曲线对横波速度、层厚的敏感性。试验结果表明,基阶模式对于浅层的横波速度和层厚比较敏感,敏感区域主要集中在较窄的频带范围内。而高阶模式对于相对较深层的横波速度和层厚比较敏感,且频率范围分布较大,敏感性强的频段分布比较分散。研究结果可以为Rayleigh面波多模式联合反演提供理论依据。     

Age, thermal evolution and history of the Black Sea Basin based on heat flow and multichannel reflection data

Multichannel reflection data (Tugolessov et al., 1985) have revealed two deeps in the basement topography of the Black Sea which are filled with sediments from 12 to 15 km thick. The deeps lack the “granitic layer” and are underlain by oceanic-type crust which we assume to be generated by seafloor spreading processes. The age of the deeps was interpreted previously, in a highly controversial manner, as being from the Paleozoic — Early Mesozoic to the Recent. In the paper, age estimations were undertaken using surficial heat flow data, assuming that they are related to deep-seated age-dependent heat flow generated by the cooling oceanic lithosphere, but that they are strongly distorted by the heating of continuously accumulating sediments as well as by additional heat input from radiogenic production within sediments. Using reliable thermophysical parameters of compacting sediments, the distorted heat flow in the sediments was evaluated numerically. This allowed us to estimate the age of the Black Sea deeps floor. The results show that the West Black Sea deep is 130 to 95 m.y. old, and the East Black Sea deep is nearly 110 m.y. old. These figures support an interpretation of the Black Sea deeps as remnants of a Late Mesozoic back-arc basin that evolved behind the Lesser Caucasian — Pontide island arc. The inferred Middle Cretaceous age of the deeps is the first estimate obtained quantitatively, and corresponds well with available heat flow and multichannel reflection data.     

Multi-stage granitic magmatism during exhumation of subducted continental lithosphere: Evidence from the Wulong pluton,South Qinling

Detailed petrology and zircon U–Pb dating data indicate that the Wulong pluton is a zoned granitic intrusive, formed from successive increments of magmas. An age range of at least 30 Ma is recorded from the 225–235 Ma quartz diorite on the pluton margin, the ca. 218 Ma granodiorite in the intermediate zone, and the ca. 207 Ma monzogranite at the pluton center. All the granitoids display evolved Sr–Nd–Pb isotopic compositions, with ⁸⁷Sr/⁸⁶Sr(i) of 0.7044–0.7062, unradiogenic Nd (ε_Nd(t) values of − 6.1 to − 3.0, Nd model ages of 1.1–1.3 Ga, and moderately radiogenic Pb compositions (²⁰⁶Pb/²⁰⁴Pb(i) = 17.500–17.872, ²⁰⁷Pb/²⁰⁴Pb(i) = 15.513–15.549, ²⁰⁸Pb/²⁰⁴Pb(i) = 37.743–38.001), in combination with variations in zircon Hf isotopic compositions (with ε_Hf(t) values in each stage span 12 units) and the Hf isotopic model ages of 800–1600 Ma. These features suggest that the granitoids might have been derived from the reworking of an old lower crust, mixed with Paleozoic and Proterozoic materials. The rocks also display an adakitic affinity with Sr (479–973 ppm), high Sr/Y ratios (mostly > 60) and negligible Eu anomalies (Eu/Eu* = 0.78–0.97) but low Rb/Sr ratios, low Y (4.6–17 ppm), HREE (Yb = 0.95–1.7 ppm), Yb/Lu (6–7) and Dy/Yb (1.9–2.4) ratios, suggesting the absence of plagioclase and presence of garnet + amphibole in their residue. Considering a large gap among their crystallization ages, we propose that the geochemical evolution from pluton margin to center was controlled mainly by melting conditions and source compositions rather than fractional crystallization. Mafic enclaves that were hosted in the quartz diorite and granodiorite are mainly syenogabbroic to syenodioritic in composition, and are metaluminous and enriched in LREE and LILEs, but are depleted in HFSE, and display an evolved Sr–Nd–Pb isotopic composition, suggesting that they may have been derived from the partial melting of an enriched mantle lithosphere, which was metasomatized by adakitic melts and fluids from a subducted continental crust.In combination with the results of the Triassic ultra-high pressure metamorphic rocks in the Dabie orogenic belt, we apply a model involving the exhumation of subducted continental crust to explain the formation of the Wulong pluton. At the first stage, a dense and refractory mafic lower crust that was trapped at mantle depth by continental subduction witnessed melting under high temperature conditions to produce the quartz diorite magma, characterized by low SiO₂ (60.65–63.98 wt.%) and high TiO₂ (0.39–0.86 wt.%). The magma subsequently interacted with mantle peridotite, leading to high Mg# (57–67) and the metasomatism of the overriding mantle wedge. At the second stage, an asthenosphere upwelling that was probably caused by slab break-off at ca. 220 Ma melted the enriched sub-continental lithospheric mantle (SCLM) to produce mafic magmas, represented by the mafic enclaves that are hosted in the quartz and granodiorite, resulting in the partial melting of the shallower subducted crust, and generating the granodiorite that is distinguished by high SiO₂ (69.16–70.82 wt.%), high Al₂O₃ (15.33–16.22 wt.%) and A/CNK values (mostly > 1.05). At the third stage, the final collapse of the Triassic Qinling–Dabie Orogenic Belt at ca. 215–205 Ma caused extensive partial melting of the thickened orogenic lower crust to produce the monzogranite, which is characterized by high SiO₂ (67.68–70.29 wt.%), low TiO₂ (mostly < 0.35 wt.%) and high Sr/Y ratios of 86–151.     

The resistivity structure of the Penola Trough,Otway Basin from magnetotelluric data

We present inversion results for a 100 site, broadband magnetotelluric (MT) survey in the Penola Trough, Otway Basin, South Australia. The Penola Trough is host to several petroleum reservoirs and has more recently been a target for unconventional geothermal exploration. We present two interpretations of the MT data. A 1D anisotropic interpretation, where anisotropy is determined within the Otway Basin sequence and basement in the northeastern Penola Trough, fits the impedance tensor well. However, the anisotropy strike is inconsistent with the known orientation of electrically conductive fractures in the Penola Trough. On the other hand, a 3D interpretation, which incorporates lateral variations in resistivity, requires no anisotropy yet it matches the data equally well. Both the 1D and 3D inversions resolve several layers within the Otway Basin sequence, which correspond to stratigraphic units defined in wells and in the coincident Haselgrove–Balnaves 3D seismic survey. These include the Eumeralla and Dilwyn formations, which are poorly resolved in the seismic data. The basin architecture, defined in the 3D inversion, in particular the depth to basement, is consistent with previous interpretations based on seismic reflection data that show that the Otway Basin thins in the northeastern Penola Trough. This does not occur in the anisotropic model. We therefore conclude that the subsurface resistivity appears to be isotropic in the Penola Trough. This contrasts with the anisotropic resistivity structure determined in a previous study in the Koroit region, eastern Otway Basin. The difference in the MT responses between the two regions is supported by resistivity and permeability information from well logs and may reflect differences in the orientation of subsurface fractures, or differences in the present-day stress field, between the two regions.     

Crustal structure of the Ruby Mountains and southern Carlin Trend region, Nevada, from magnetotelluric data

We have collected about 150 magnetotelluric (MT) soundings in northeastern Nevada in the region of the Ruby Mountains metamorphic core complex uplift and southern Carlin mineral trend, in an effort to illuminate controls on core complex evolution and deposition of world-class gold deposits. The region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation, which have contributed to the total expression of electrical resistivity. Most of the soundings reside in three east–west profiles across increasing degrees of core uplift to the north (Bald Mountain, Harrison Pass, and Secret Pass latitudes). One short cross-line was also taken to assess an east–west structure to the north of the northern profile. Model resistivity cross-sections were derived from the MT data using a 2-D inversion algorithm, which damps departures of model parameters from an a priori structure. Geological interpretation of the resistivity combines previous seismic, potential field and isotope models, structural and petrological models for regional compression and extension, and detailed structural/stratigraphic interpretations incorporating drilling for petroleum and mineral exploration. To first order, the resistivity structure is one of a moderately conductive, Phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks. Late Devonian and early Mississippian shales of the Pilot and Chainman Formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance (conductivity–thickness product) from east to west. These increases are attributed to graphitization caused by Elko–Sevier era compressional shear deformation and possibly by intrusive heating. The resistive crystalline central massifs adjoin the host stratigraphy across crustal-scale, steeply dipping fault zones. The zones provide pathways to the lower crust for heterogeneous, upper crustal induced, electric current flow. Resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west. The numerous crustal breaks imaged with MT may contribute to the low effective elastic thickness (T_e) estimated regionally for the Great Basin and exemplify the mid-crustal, steeply dipping slip zones in which major earthquakes nucleate. An east–west oriented conductor in the crystalline upper crust spans the East Humboldt Range and northern Ruby Mountains. The conductor may be related to nearby graphitic metasediments, with possible alteration by middle Tertiary magmatism. Lower crustal resistivity everywhere under the profiles is low and appears quasi one-dimensional. It is consistent with a low rock porosity (<1 vol.%) containing hypersaline brines and possible water-undersaturated crustal melts, residual to the mostly Miocene regional extension. The resistivity expression of the southern Carlin Trend (CT) in the Pinon Range is not a simple lineament but rather a family of structures attributed to Eocene intrusion, stratal deformation, and alteration/graphitization. Substantial reactivation or overprinting by core complex uplift or Basin–Range extensional events seems likely. We concur with others that the Carlin Trend may result in part from overlap of the large Eocene Northeast Nevada Volcanic Field with Precambrian–Paleozoic deep-water clastic source rocks thickening abruptly to the west of the Pinon Range, and projecting to the north–northwest.     

New heat flow data from the immediate vicinity of the Kola super-deep borehole: Vertical variation in heat flow confirmed and attributed to advection

We present new heat flow values and other geothermal data in the upper crystalline crust in the immediate vicinity of the 12.4-km deep Kola super-deep borehole, NW Russia. Our results show a systematic vertical increase in geothermal gradient and heat flow density as deep as we could measure (1.6 km). Our results confirm earlier results on vertical heat flow trends of in the uppermost part of the Kola super-deep hole, and imply that the thermal regime is not in steady-state conductive conditions. In an area of 3-km × 5-km measurements were performed in 1–2-km deep boreholes surrounding the Kola super-deep hole and on core samples from these holes. Temperature logs are available from 36 holes. Core data exists from 23 boreholes with a total length of 11.5 km at a vertical resolution of 10 m. We carried out a very detailed study on thermal conductivity with regard to anisotropy, inhomogeneity and temperature dependence. Tensor components of thermal conductivity were determined on 1375 core samples from 21 boreholes in 3400 measurements. Additionally, we measured specific heat capacity, heat generation rate, density, porosity, and permeability on selected subsets of core samples. Heat flow from 19 boreholes varies between 31 and 45 mW m⁻² with an average value of 38 mW m⁻². In most boreholes the vertical heat flow profiles show a considerable variation with depth. This is consistent with observations in the upper part of the Kola super-deep borehole. We conclude that this variation is not caused by technical operations but reflects a natural process. It is considered to be due to a combination of advective, structural and paleoclimatic effects. Preliminary 3-D numerical modeling of heat and flow in the study area provides an indication of relative contributions of each of these factors: advective heat transfer turns out to have a major influence on the vertical variation of heat flow, although transient changes in surface temperature may also cause a significant variation. Heterogeneity of the rocks in the study area is less important.     

Estimates of heat flow from Cenozoic seafloor using global depth and age data

The total heat output of the Earth constrains models of mantle and core dynamics. Previously published estimates (42–44 TW) have recently been questioned because the measured conductive heat flow on young oceanic lithosphere is about a factor of 2 less than the expected heat flow based on half-space cooling models. Taking the conductive ocean heat flow values at face value reduces the global heat flow from 44 to 31 TW, which has major implications for geodynamics and Earth history. To help resolve this issue, we develop a new method of estimating total oceanic heat flow from depth and age data. The overall elevation of the global ridge system, relative to the deep ocean basins, provides an independent estimate of the total heat content of the lithosphere. Heat flow is proportional to the measured subsidence rate times the heat capacity divided by the thermal expansion coefficient. The largest uncertainty in this method is due to uncertainties in the thermal expansion coefficient and heat capacity. Scalar subsidence rate is computed from gradients of depth and age grids. The method cannot be applied over very young seafloor (< 3 Ma) where age gradient is discontinuous and the assumption of isostasy is invalid. Between 3 and 66 Ma, the new estimates are in agreement with half-space cooling model. Our model-independent estimate of the total heat output of Cenozoic seafloor is 18.6 to 20.5 TW, which leads to a global output of 42 to 44 TW in agreement with previous studies.