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2011 Vol. 22, No. 2

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Some Issues on the Strength of the Lithosphere
Shun-ichiro Karato
2011, 22(2): 131-136. doi: 10.1007/s12583-011-0164-y
The strength of the oceanic and continental lithosphere has important controls on some of the major geological processes on earth including the operation of plate tectonics and the long-term stability of the continental roots. However, explaining these major geological features from the experimental and theoretical studies on the strength of rocks is challenging and some of the existing models for the strength of the lithosphere do not explain these main geological observations. A brief review is provided to summarize recent progress in these areas with the emphasis on experimental studies.
Melt Evolution above a Spontaneously Retreating Subducting Slab in a Three-Dimensional Model
Guizhi Zhu, Taras Gerya, David A Yuen
2011, 22(2): 137-142. doi: 10.1007/s12583-011-0165-x
Dehydration of the subducting slab favors the melting of the surrounding mantle. Water content and melt evolution atop a spontaneously retreating subducting slab are reported in a three-dimensional (3-D) model. We find that fluids, including water and melts in the rocks, vary substantially along the trench, which cannot be found in two-dimensional (2-D) models. Their maxima along the subducting slab are mainly located at about 50 to 70 and 120 to 140 km. Volumetric melt production rate changes spatially and episodically atop the slab, which may reflect the intensity and variations of volcanoes.
Influences of Lower-Mantle Properties on the Formation of Asthenosphere in Oceanic Upper Mantle
David A Yuen, Nicola Tosi, Ondrej Čadek
2011, 22(2): 143-154. doi: 10.1007/s12583-011-0166-9
Asthenosphere is a venerable concept based on geological intuition of Reginald Daly nearly 100 years ago. There have been various explanations for the existence of the asthenosphere. The concept of a plume-fed asthenosphere has been around for a few years due to the ideas put forth by Yamamoto et al.. Using a two-dimensional Cartesian code based on finite-volume method, we have investigated the influences of lower-mantle physical properties on the formation of a low-viscosity zone in the oceanic upper mantle in regions close to a large mantle upwelling. The rheological law is Newtonian and depends on both temperature and depth. An extended-Boussinesq model is assumed for the energetics and the olivine to spinel, the spinel to perovskite and perovskite to post-perovskite (ppv) phase transitions are considered. We have compared the differences in the behavior of hot upwellings passing through the transition zone in the mid-mantle for a variety of models, starting with constant physical properties in the lower-mantle and culminating with complex models which have the post-perovskite phase transition and depth-dependent coefficient of thermal expansion and thermal conductivity. We found that the formation of the asthenosphere in the upper mantle in the vicinity of large upwellings is facilitated in models where both depth-dependent thermal expansivity and conductivity are included. Models with constant thermal expansivity and thermal conductivity do not produce a hot low-viscosity zone, resembling the asthenosphere. We have also studied the influences of a cylindrical model and found similar results as the Cartesian model with the important difference that upper-mantle temperatures were much cooler than the Cartesian model by about 600 to 700 K. Our findings argue for the potentially important role played by lower-mantle material properties on the development of a plume-fed asthenosphere in the oceanic upper mantle.
Regional Stress Fields under Tibet from 3D Global Flow Simulation
Siqi Zhang, H L Xing, David A Yue, Huai Zhang, Yaolin Shi
2011, 22(2): 155-159. doi: 10.1007/s12583-011-0167-8
Tibetan area is the most active continental collision zone on earth. Several major earthquakes occurred around the boundaries of Tibetan plateau and caused massive damages and casualties. The dynamics of this area is not well understood due to the complex structure of Tibet and its surrounding area. In this study, a 3D global flow simulation with only viscous rheology is applied to studying the stress distribution in this area, and the interaction between Tibet and its surrounding areas is investigated. Finally, the possibility of combining regional modeling with global models is also discussed.
Small-Scale Mineralogical Heterogeneity from Variations in Phase Assemblages in the Transition Zone and D” Layer Predicted by Convection Modelling
Arie P van den Berg, David A Yuen, Michael H G Jacobs, Maarten V de Hoop
2011, 22(2): 160-168. doi: 10.1007/s12583-011-0168-7
Small-scale heterogeneity in the deep mantle is concentrated in the upper-mantle transition zone (TZ), in the depth range 410–660 km and also at the bottom 250 km D″ region. This encourages a more detailed investigation of the potential for seismic reflectivity imaging by modelling heterogeneous structures in mantle convection models including phase transitions of the TZ and D″ regions. We applied finite elements with variable spacing near the boundary layers in 2-D cylindrical geometry that allow for sufficient spatial resolution. We investigated several models including an extended Boussinesq (EBA) model, focused on the D″ region, and a compressible (ALA) model for the TZ region. The results for the D″ region show typical lens-shaped structures of post-perovskite (PPV) embedded in the perovskite (PV) background mantle, where the thickness of the lenses, at 200–400 km, strongly depends on the Clapeyron slope of the PV-PPV transition. A second phase transition (double crossing) occurs in case the core temperature is higher than the intercept temperature Ti. Our phase-dependent rheology results in contrasting effective viscosity between PV and PPV. Our model results reveal a distinctly clear mechanical weakening of the PPV lenses with about an order of magnitude lower viscosity. The shear wave-speed distributions computed from our convection results are strongly correlated with the heterogeneous distribution of the mineral phase. Gradients in the seismic wave-speed that are the target of seismological reflectivity imaging are clearly revealed. The wave-speed results show a clear resolution of the top and bottom interfaces of the PPV lenses. Our ALA model for the TZ is based on a thermodynamical model for the magnesium end-member of an olivine-pyroxene mantle. The model predicts a much more complex distribution of mineral phases, compared to our D″ results, in agreement with the greater number of mineral phases involved in the olivine-pyroxene phase diagram for theP, T conditions of the transition zone. Near cold downwelling flows representing subducting lithospheric slabs, where the local geotherm can differ by up to 1 000 K from the horizontal average, and small-scale lateral variations in the mineral phases can occur.
Ab Initio Molecular Dynamic Simulation on the Elasticity of Mg3Al2Si3O12 Pyrope
Li Li, Donald J. Weidner, John Brodholt, Dario Alfè, G David Price
2011, 22(2): 169-175. doi: 10.1007/s12583-011-0169-6
We calculated thermo-elastic properties of pyrope (Mg3Al2Si3O12) at mantle pressures and temperatures using Ab initio molecular dynamic simulation. A third-order Birch-Murnaghan equation at a reference temperature of 2 000 K fits the calculations with bulk modulus, K0=159.5 GPa, K0′=4.3, V0=785.89 Å3, Grüneisen parameter, γ0=1.15, q=0.80, Anderson Grüneisen parameter δT =3.76 and thermal expansion, α0=2.93×10−5 K−1. Referenced to room temperature, where V0=750.80 Å3, γ0 and α0 become 1.11 and 2.47×10−5 K−1. The elastic properties of pyrope are found to be nearly isotropic at transition zone conditions.
Parallel Numerical Analysis on the Rheology of the Martian Ice-Rock Mixture
Huimin Jing, Huai Zhang, Han Li, David A Yuen, Yaolin Shi
2011, 22(2): 176-176. doi: 10.1007/s12583-011-0170-0
The distribution and amount of ground ice on Mars is an important issue to be addressed for the future exploration of the planet. The occurrence of interstitial ice in Martian frozen ground is indicated by landforms, such as fluidized ejecta craters, softened terrain, and fretted channels. However, experimental data on the rheology of ice-rock mixture under Martian physical conditions are sparse, and the amount of ground ice that is required to produce the viscous deformation observed in Martian ice-related landforms is still unknown. In our study, we put forward a three-dimensional non-Newtonian viscous finite element model to investigate the behavior of ice-rock mixtures numerically. The randomly distributed tetrahedral elements are generated in regular domain to represent the natural distribution of ice-rock materials. Numerical simulation results show that when the volume of rock is less than 40%, the rheology of the mixture is dominated by ice, and there is occurrence of a brittle-ductile transition when ice fraction reaches a certain value. Our preliminary results contribute to the knowledge of the determination of the rheology and ice content in Martian ice-rock mixture. The presented model can also be utilized to evaluate the amount of ground ice on Mars.
On the Layering Artifacts in Seismic Imageries
Huawei Zhou
2011, 22(2): 182-194. doi: 10.1007/s12583-011-0171-z
A common feature in seismic imageries of the crust and mantle is a layering pattern. Layering structures do exist in multiple scales, such as layered strata and unconformities in local and regional scales, and undulating seismic discontinuities in the crust and mantle. However, layering artifact also exists due to limitations in seismic processing and data coverage. There is a tendency for seismic stacking methods to over-map reflectors and scatters into along-isochron layers. In contrast, seismic tomography methods tend to under-map sub-horizontal layers with along-raypath smears and artifacts of various de-mean processes. To better identify signals and artifacts in seismic imageries, it is necessary to understand the origins of various artifacts and make careful comparison between the solutions of different data and methods.
Three-Dimensional S-Wave Velocity Structure in Eastern Tibet from Ambient Noise Rayleigh and Love Wave Tomography
Xiaoming Xu, Hongyi Li, Meng Gong, Zhifeng Ding
2011, 22(2): 195-204. doi: 10.1007/s12583-011-0172-y
We apply ambient noise tomography to continuous three-component broadband seismic data between January 1, 2008 and December 31, 2008 from the regional networks of 76 stations deployed by China Earthquake Administration. Ambient noise cross-correlations were performed to produce the Green's functions of each station-pair. Within the period from 6 to 50 s, Rayleigh and Love wave dispersion curves were measured using the multiple filter analysis method. Then three-dimensional (3-D) S-wave velocity structures from the surface down to 70 km are inverted from both Rayleigh and Love wave dispersion results. The obtained S-wave velocity maps show strong lateral variations and correlate well with the distinct geological and tectonic features in the study area. The Sichuan basin displays low velocity in shallow depth due to thick sedimentary deposits but high velocity in the mid-lower crust; the eastern Tibetan plateau is clearly featured with a low-velocity zone in its mid-to-lower crust which is consistent with the crustal flow model proposed to explain the mechanism of uplift and pattern of deformation for the Tibetan plateau. Meanwhile, our results also exhibit that the crustal thickness decreased from the eastern Tibetan plateau to the Sichuan basin.
Crustal and Upper-Mantle Seismic Reflectors beneath the Three Gorges Reservoir Region
Zhihui Zou, Huawei Zhou, Wulin Lioa
2011, 22(2): 205-213. doi: 10.1007/s12583-011-0173-x
Seismic studies of the crustal structure beneath the Three Gorges Reservoir (TGR) region in Central China have been limited by the sparse and uneven distribution of seismic stations. To increase the station coverage, we made three deployments of a mobile seismologic array in the TGR region during the three summers from 2008 to 2010. Here we present interpretations along a west-east profile through the central TGR region based on new seismic waveform data and a velocity model constrained by regional earthquake data. Two strong mid-crustal reflection interfaces at depths around 10 and 20 km are seen under the TGR. The shallow reflector defines the bottom of the Zigui (秭归) basin. The new waveform data show that the amplitude of the Moho reflection is quite weak, and beneath the Moho, there is a strong reflector around 54-km depth. It is likely that in the TGR region, the Moho is a gradient rather than a sharp boundary. We speculate that the gradient Moho and the 54-km-deep reflector in the upper mantle in the TGR region may be by-products of the Qinling (秦岭)-Dabie (大别) orogen.
Active Source Tomography in Northwestern Xinjiang, China: Implication for Mineral Distribution
Bao Mei, Yixian Xu, Hui Qian
2011, 22(2): 214-225. doi: 10.1007/s12583-011-0174-9
The main aim of this work is to understand the distribution of minerals by obtaining a shallow velocity structure around the Karatungk (喀拉通克) region. Data were acquired in 2009 by a denser array in deploying a transportable seismometer with 4.5 Hz vertical geophone. All the P-wave arrival times are picked automatically with Akaike information criterion, and then checked manmachine interactively by short-receiver geometry. The database for local active-source tomographic inversion involves 4 241 P-wave arrival time readings from 96 shots and three quarry blasts. Checkerboard tests aimed at checking the reliability of the obtained velocity models are presented. The resulting Vp distribution slices show a complicated 3-D structure beneath this area and offer a better understanding of three well-defined mineral deposits. Near the surface we observe a series of zones with slightly high-velocity which probably reflect potential deposits. Based on features of metallic ores we attempt to delimit their distributions and stretched directions.
Effective Dispersion Curve and Pseudo Multimode Dispersion Curves for Rayleigh Wave
Shuangxi Zhang
2011, 22(2): 226-230. doi: 10.1007/s12583-011-0175-8
The hi-energy bands in the dispersion image are usually interpreted as the true dispersion phase velocities. However, the multiple dispersion modes of Rayleigh wave in layered media stack in space, producing the effective dispersion curve and the pseudo multimode dispersion curves in dispersion image. The effective dispersion curve has the maximum energy with lower phase velocities than pseudo dispersion phase velocities, and thus is often misunderstood as the fundamental mode. Within the tolerable misfit, the effective dispersion curve can approach the true fundamental mode. Different from the true multimode dispersion curves, the pseudo multimode dispersion curves are related to the effective dispersion curve. A numerical model is adapted to simulate the true dispersion curves, effective dispersion curve, and pseudo multimode dispersion curves. Their differences and mutual relations are demonstrated.
Influence of Uneven Trace Spacing on Rayleigh Wave Dispersion
Shuangxi Zhang, Mengkui Li
2011, 22(2): 231-240. doi: 10.1007/s12583-011-0176-7
Rayleigh wave dispersion signals are significant to underground investigation. Traditionally, uniformed trace spacing is employed in surface wave surveys. In some cases, however, uneven trace spacing is often encountered because of the limitations of the site condition. In order to study the influence of uneven trace spacing on the dispersion data construction of Rayleigh waves, data acquisition is performed based on a 2.5D field layout with a linear array of geophones fixed and a mobile source. The observation direction controls the trace spacing of the measurement. The final results demonstrate that the trace nonuniformity has significant influence on the Rayleigh wave dispersion feature constructed. When the observation angle is over 45°, the dispersion image will be too distorted to extract dispersion data correctly.
Layer-Stripping Full Waveform Inversion with Damped Seismic Reflection Data
Aifei Bian, Wenhui Yu
2011, 22(2): 241-249. doi: 10.1007/s12583-011-0177-6
Full waveform inversion (FWI) directly minimizes errors between synthetic and observed data. For the surface acquisition geometry, reflections generated from deep reflectors are sensitive to overburden structure, so it is reasonable to update the macro velocity model in a top-to-bottom manner. For models dominated by horizontally layered structures, combination of offset/time weighting and constant update depth control (CUDC) is sufficient for layer-stripping FWI. CUDC requires ray tracing to determine reflection traveltimes at a constant depth. As model complexity increases, the multi-path effects will have to be considered. We developed a new layer-stripping FWI method utilizing damped seismic reflection data, which does not need CUDC and ray tracing. Numerical examples show that effective update depth (EUD) can be controlled by damping constants even in complex regions and the inversion result is more accurate than conventional methods.
Comparison of Results of Recent Seismic Profiles with Tectonic Models of the North China Craton
Timothy M Kusky
2011, 22(2): 250-259. doi: 10.1007/s12583-011-0178-5
The geometry and timing of amalgamation of the North China craton (NCC) have been controversial, with three main models with significantly different interpretations of regional structure, geochronology, and geological relationships. The model of Zhao G C et al. suggests that the eastern and western blocks of the NCC formed separately in the Archean, and an active margin was developed on the eastern block between 2.5 and 1.85 Ga, when the two blocks collided above an east dipping subduction zone. The model of Kusky et al. presumes that the eastern block rifted from an unknown larger continent at circa 2.7 Ga, and experienced a collision with an arc (perhaps attached to the western block) above a west-dipping subduction zone at 2.5 Ga, and the 1.85 Ga metamorphism is related to a collision along the northern margin of the craton when the NCC joined the Columbia supercontinent. The model of Faure et al. suggests two collisions in the central orogenic belt, at 2.1 and 1.88 Ga. Recent seismic results support both the models of Kusky et al. and Faure et al., showing that subduction beneath the central orogenic belt (COB) was west-directed, and that there is a second, west-dipping paleosubduction zone located to the east of the COB dipping beneath the western block (Ordos craton). The boundaries identified through geophysics do not correlate with the boundaries of the Trans-North China orogen suggested in the Zhao et al. model, and the subduction polarity is opposite that predicted by that model. The seismic profiles are consistent with an Archean collision above a west-dipping subduction zone beneath the COB predicted by the models of Kusky et al., and the second west-dipping subduction zone is consistent with the two events suggested in the Faure et al. model.
Lithospheric Structure in the North China Craton Constrained from Gravity Field Model (EGM 2008)
Yuanyuan Li, Yushan Yang, Timothy M. Kusky
2011, 22(2): 260-272. doi: 10.1007/s12583-011-0179-4
A detailed knowledge of the thickness of the lithosphere in the North China craton (NCC) is important for understanding the significant tectonic reactivation of the craton in Mesozoic and Cenozoic. We achieve this goal by applying the newly proposed continuous wavelet transform theory to the Gravity Field Model (EGM 2008) data in the region. Distinct structural variations are identified in the scalogram image of profile Alxa-Datong (大同)-Qingdao (青岛)-Yellow Sea (profile ABC), transversing the main units of NCC, which we interpret as mainly representing the Moho and lithosphere-asthenosphere boundary (LAB) undulations. The imaged LAB is as shallow as 60–70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior. A rapid change of about 30 km in the LAB depth was detected at around the boundary between the Bohai (渤海) Bay basin (BBB) and the Taihang (太行) Mountains (TM), roughly coincident with the distinct gravity decrease of more than 100 mGal that marks the North-South Gravity Lineament (NSGL) in the region. At last we present the gravity modeling work based on the spectral analysis results, incorporating with the observations on high-resolution seismic images and surface topography. The observed structural differences between the eastern and western NCC are likely associated with different lithospheric tectonics across the NSGL. Combined with seismic tomography results and geochemical and petrological data, this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton.
Postrift Rapid Subsidence Characters in Qiongdongnan Basin, South China Sea
Xinyi Yin, Jianye Ren, Chao Lei, Shan Wang, Junxia Zhang
2011, 22(2): 273-279. doi: 10.1007/s12583-011-0180-y
In this article, the backstripping technique was used in studying the subsidence characters of the Qiongdongnan (琼东南) basin (QDNB) in order to understand its dynamic mechanism of formation and evolution. Meanwhile, the geothermal characteristics of this area were summarized, and the stretching factors (β) of the upper crust, the whole crust, and the whole lithosphere were calculated. The QDNB is characterized by high subsidence rate, high geothermal gradient, high geothermal heat flow, and the lithosphere stretching and thinning of this area are depth dependent. An asthenosphere zone must have been confined under the lithosphere of Southeast Asian continent because of the mutual subductions of the Eurasian plate, the Pacific plate, the Indian-Australian plate, and the Philippine Sea plate. These characters indicate that strong mantle convection occurred and the lower crust materials flowed away in the domain, which lead to the rapid flexural isostasy subsidence of the upper crust and the uplift of the asthenosphere.
Investigation of the Co-seismic Gravity Field Variations Caused by the 2004 Sumatra-Andaman Earthquake Using Monthly GRACE Data
Jin Li, Wenbin Shen
2011, 22(2): 280-291. doi: 10.1007/s12583-011-0181-x
Gravity Recovery and Climate Experiment (GRACE) observations have been used to detect the co-seismic and post-seismic gravity field variations due to the Mw=9.3 Sumatra-Andaman earthquake that occurred on December 26, 2004. This article focuses on investigating some gravitational effects caused by this huge earthquake. We computed the geoid height changes, the equivalent water height (EWH) changes, and the gravity changes using the GRACE Level-2 monthly spherical harmonic (SH) solutions released by University of Texas Center for Space Research (UTCSR). The GRACE results agree well with the prediction by a dislocation model and are consistent with the results obtained by some previous scholars. In particular, we calculated the three components of the gravity gradient variations and found that they can recover the seismic-related signature more sensitively due to a certain degree of amplification of the signals. A positive-negative-positive mode predominates in the spatial distribution of the horizontal components of the gravity gradient variations, which is possibly attributed to the anomalies in the crustal density distribution caused by the uplift-subduction effect of the dip-slip earthquake. Moreover, the latitude components of the gravity gradient changes show strong suppression of the north-south stripes, which is due to the along-orbit measurements of the two GRACE satellites. We conclude that the positive-negative-positive mode in latitude gravity gradient changes would be a more sensitive feature to detect the deformations of some major dip-slip earthquakes by GRACE data.
Resistivity Logs of the Chinese Continental Scientific Drilling Main Hole: Implication for the Crustal Electrical Structure of Dabie-Sulu Terrane, Central-Eastern China
Miao Luo, Heping Pan, Yonggang Zhao
2011, 22(2): 292-292. doi: 10.1007/s12583-011-0182-9
To understand the electrical structure of the Sulu (苏鲁) terrane, we analyzed the resistivity logs of the Chinese Continental Scientific Drilling main hole (CCSD-MH) and obtained the statistical characteristics of the resistivity log of ultrahigh-pressure (UHP) metamorphic rocks. To check the logs' quality, we compared the resistivity log data with the core test data. The two datasets show a good coherence. On the whole, the resistivity of the UHP metamorphic rocks from the CCSD-MH is very high (mostly higher than 1 000 Ω·m). From orthogneiss to paragneiss, amphibolite, retrograded eclogite, rutile eclogite, phengite eclogite, and ultramafic rock, the resistivity decreases gradually. The fracture zone, enrichment of metal minerals, and serpentinization of peridotite can probably cause the decrease of resistivity. Comparing the electrical structure of the crust of Dabie (大别) terrane and Sulu terrane, we can conclude that (1) the electrical structure of the crust of Dabie terrane and Sulu terrane shows a very big difference, (2) the high-resistivity zone of the north Sulu orogen extended to a very great depth (60 km), but the high-resistivity zone of Dabie orogen is over the depth of 25 km.