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Yingjie Yang - One of the best experts on this subject based on the ideXlab platform.
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penetration of mid crustal Low Velocity Zone across the kunlun fault in the ne tibetan plateau revealed by ambient noise tomography
Earth and Planetary Science Letters, 2014Co-Authors: Chengxin Jiang, Yingjie Yang, Yong ZhengAbstract:Abstract The NE Tibetan Plateau, composed of the Mesozoic accretions of Lhasa, Qiangtang and Songpan-Ganze Terranes, are bounded by the east Kunlun–Qaidam Block in the north with the boundary delineated by the Kunlun Fault. The NE Tibetan Plateau is at a nascent stage of plateau growth resulting from the collision between the Indian and Eurasian plate starting ∼50 million years ago, and is one of the best areas to study the growth mechanism of the Tibetan Plateau. In this study, we process continuous ambient noise data collected from ∼280 stations during 2007 and 2010 and generate Rayleigh wave phase Velocity maps at 10–60 s periods with a lateral resolution of ∼ 30 – 50 km for most of the study region. By adopting a Bayesian Monte Carlo method, we then construct a 3-D Vsv model of the crust using the Rayleigh wave dispersion maps. Our 3-D model reveals that strong LVZs exist in the middle crust across the NE Tibetan Plateau; and the lateral distribution of LVZs exhibit significant west–east variations along the Kunlun Fault. In the west of 98°E, LVZs are confined to regions of the Kunlun Fault and the eastern Kunlun Ranges, but absent beneath the Qaidam Basin; while in the east of 98°E, LVZs extend and penetrate northward into the east Kunlun and Qinling Orogens over ∼ 100 km across the Kunlun Fault. The strong contrast of the LVZs distribution along the Kunlun Fault may be related to the distinct neighboring tectonic units in the north: a strong crust of the Qaidam Basin in the west blocking the penetration of LVZs, but a weak crust in the Qinling Orogens facilitating the extrusion of LVZs. The distribution of LVZs in the NE Tibetan Plateau is consistent with the crustal channel fLow model, which predicts a branch of north-eastward mid-crustal channel fLow. Our 3D model clearly delineates the north extent of the mid-crustal LVZs, probably reflecting the status of channel fLow in the NE Tibetan Plateau.
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a synoptic view of the distribution and connectivity of the mid crustal Low Velocity Zone beneath tibet
Journal of Geophysical Research, 2012Co-Authors: Yingjie Yang, Michael H Ritzwoller, Yong Zheng, Weisen Shen, A L LevshinAbstract:[1] Based on 1–2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around Tibet, Rayleigh wave phase Velocity maps are constructed from 10 s to 60 s period. A 3-D Vsv model of the crust and uppermost mantle is derived from these maps. The 3-D model exhibits significant apparently inter-connected Low shear Velocity features across most of the Tibetan middle crust at depths between 20 and 40 km. These Low Velocity Zones (LVZs) do not conform to surface faults and, significantly, are most prominent near the periphery of Tibet. The observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear Zones, rather than being localized horizontally near the edges of rigid blocks. The prominent LVZs are coincident with strong mid-crustal radial anisotropy in western and central Tibet and probably result at least partially from anisotropic minerals aligned by deformation, which mitigates the need to invoke partial melt to explain the observations. Irrespective of their cause in partial melt or mineral alignment, mid-crustal LVZs reflect deformation and their amplification near the periphery of Tibet provides new information about the mode of deformation across Tibet.
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seismic attenuation near the east pacific rise and the origin of the Low Velocity Zone
Earth and Planetary Science Letters, 2007Co-Authors: Yingjie Yang, Donald W. Forsyth, D S WeeraratneAbstract:Abstract Low shear wave velocities beneath mid-ocean ridges and in the Low-Velocity Zone beneath oceanic plates commonly have been attributed to the presence of melt or dissolved water, but several recent studies have challenged that interpretation. The alternative is that the anelastic effects of increasing temperature may cause the observed drop in Velocity along with a predicted increase in attenuation. We report the first measurements of surface wave attenuation within regional arrays of seismometers on the seafloor. Near the East Pacific Rise, there is much less attenuation than is predicted by models in which the Velocity is controlled solely by the direct elastic and anelastic effects of changing temperature, suggesting that melt and water concentration do play an important role. There also is somewhat less attenuation than is found in global studies; we speculate that scattering from unresolved Velocity heterogeneities contribute to the apparent attenuation in global studies.
Akira Hasegawa - One of the best experts on this subject based on the ideXlab platform.
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anomalous deepening of a seismic belt in the upper plane of the double seismic Zone in the pacific slab beneath the hokkaido corner possible evidence for thermal shielding caused by subducted forearc crust materials
Earth and Planetary Science Letters, 2010Co-Authors: Saeko Kita, Tomomi Okada, Akira Hasegawa, Junichi Nakajima, Toru MatsuzawaAbstract:Abstract Hypocenter relocations of earthquakes in the Pacific slab have shown an anomalous deepening of a seismic belt in the upper-plane of the double seismic Zone at depths of 80–120 km beneath the Hokkaido corner, while it is located at depths of 70–90 km in the surrounding Tohoku and eastern Hokkaido areas. Seismic tomographic inversions performed beneath the Hokkaido corner have shown that a Low-Velocity Zone having seismic velocities of crust materials exists in the mantle wedge just above the Pacific slab and makes direct contact with the upper surface of the Pacific slab. These observations suggest that: 1) the Low-Velocity Zone just above the Pacific slab is part of the subducted Kuril forearc sliver. 2) The contact with the subducted, and so relatively cold, sliver materials prevents the mantle wedge from heating the Pacific slab and causes a Lower temperature condition in the Pacific slab crust beneath the Hokkaido corner than in the surrounding areas. 3) As a result, a delay of eclogite-forming phase transformations occurs and enhances the local deepening of the seismic belt in the slab crust there.
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tomographic evidence for hydrated oceanic crust of the pacific slab beneath northeastern japan implications for water transportation in subduction Zones
Geophysical Research Letters, 2008Co-Authors: Yusuke Tsuji, Junichi Nakajima, Akira HasegawaAbstract:[1] We estimate detailed seismic-Velocity structure around the Pacific slab beneath northeastern Japan by double-difference tomography. A remarkable Low-Velocity Zone with a thickness of ∼10 km, which corresponds to much hydrated oceanic crust, is imaged coherently along the arc at the uppermost part of the slab. The Zone gradually disappears at depths of 70–90 km, suggesting the occurrence of intensive dehydration reactions there. The concentration of intraslab earthquakes at these depths supports dehydration-embrittlement hypothesis as a mechanism for generating intraslab earthquakes. A Low-Velocity Zone imaged immediately above the slab at depths >70 km probably reflects a hydrous layer that absorbs water expelled from the slab and carries it to deeper depths along the slab. Our observations suggest that an along-arc variation in arc volcanism might be related to that in the development of the hydrous layer above the slab.
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imaging the source area of the 1995 southern hyogo kobe earthquake m7 3 using double difference tomography
Earth and Planetary Science Letters, 2007Co-Authors: Tomomi Okada, Akira Hasegawa, J Suganomata, Dapeng Zhao, Haijiang Zhang, Clifford H ThurberAbstract:Abstract To understand the generation process of inland earthquake, we determined the seismic Velocity structure in and around the source area of the 1995 southern Hyogo (Kobe) earthquake (M7.3) in SW Japan. We adopted the double-difference (DD) tomography method [Zhang, H. and C. Thurber. Double-Difference Tomography: the method and its application to the Hayward Fault,California. Bull Seism Soc Am 93 (2003) 1875–1889.]. We inverted arrival times recorded by a dense temporary seismic network for aftershocks and seismic networks routinely operated by Japanese Universities. Obtained results are summarized as folLows: (1) Low-Velocity Zones of a few kilometers' width are distributed along the fault or along the aftershock alignment, suggesting that the fault of the 1995 earthquake is located primarily in a Low-Velocity Zone. (2) Amount of Velocity decrease within this Low-Velocity Zone varies along the strike of the fault. Most of large slip areas (asperities) seem to correspond to higher Velocity areas relative to the surroundings on the fault, rather than to Lower Velocity areas.
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anomalous Low Velocity Zone and linear alignment of seismicity along it in the subducted pacific slab beneath kanto japan reactivation of subducted fracture Zone
Geophysical Research Letters, 2006Co-Authors: Junichi Nakajima, Akira HasegawaAbstract:[1] A detailed investigation of the hypocenter distribution beneath Kanto, Japan, reveals a NW-SE-trending linear alignment of seismicity within the subducted Pacific slab. We estimate the 3D seismic Velocity structure in the Pacific slab to understand the factors controlling the genesis of such intraslab earthquakes. A narrow Low-Velocity Zone is imaged within the subducted slab over a length of ∼150 km, which partly penetrates into the mantle portion of the slab. The Low-Velocity Zone correlates in space with the NW-SE-trending earthquake cluster. A reactivation of hydrated fracture Zone formed prior to subduction is probably related to the Low-Velocity anomaly. Dehydration reactions of the hydrated oceanic mantle as well as the oceanic crust might Lower the seismic Velocity along the fossil fracture Zone, accompanied by intraslab earthquakes. These observations support the hypothesis of dehydration embrittlement as the most viable mechanism for generating intraslab earthquakes.
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detailed imaging of the fault planes of the 2004 niigata chuetsu central japan earthquake sequence by double difference tomography
Earth and Planetary Science Letters, 2006Co-Authors: Tomomi Okada, Akira Hasegawa, Toru Matsuzawa, Haijiang Zhang, Tadashi Yaginuma, Norihito Umino, Clifford H ThurberAbstract:Abstract Imaging of the hypocentral faults of the Niigata–Chuetsu earthquake sequence was performed by double-difference tomography using data obtained by a temporary aftershock observation network densely deployed in the hypocentral region. Two parallel “unfavorly oriented” fault planes steeply inclined to the WNW are revealed, that of the main shock and that of the largest aftershock, as well as a Zone of Velocity change between the high-Velocity footwall and the Low-Velocity hanging wall for both P- and S-waves. This suggests that the main shock and the largest aftershock are caused by the reactivation of normal faults in the Miocene under the current compressional stress regime. Parts of the fault planes are also found to be located within the Low-Velocity Zone. This Low-Velocity Zone continues to the Lower crust, which is imaged by an inversion from the regional data, and would correspond with highly fractured Zone around the fault plane with high fluid pressure, which reactivated the “unfavorly oriented” fault and triggered the present earthquake. Large coseismic slip area appears to extend to the northeastern portion with relatively high Velocity. This suggests that the asperity (large coseismic slip area) of the main shock possibly correspond with a higher Velocity region along the fault plane.
Fabrice Gaillard - One of the best experts on this subject based on the ideXlab platform.
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Unravelling partial melt distribution in the oceanic Low Velocity Zone
Earth and Planetary Science Letters, 2020Co-Authors: Emmanuel Gardès, Malcolm Massuyeau, Mickaël Laumonier, Fabrice GaillardAbstract:The widespread Low seismic Velocity Zone (LVZ) in the shalLow oceanic mantle has long been debated in terms of mantle melting. At LVZ depths, volatiles (CO2 and H2O) are present in minute amounts, which implies mantle incipient melting down to beLow 1000 °C with the production of minute amounts of volatile-rich melt, well beLow 1 vol.%. However, melt compositions and distributions in the incipient melting regime have only been inferred from experiments departing from actual mantle conditions. Here, we experimentally reproduce incipient melting by re-equilibrating a naturally CO2- and H2O-bearing mantle rock at mantle temperatures and pressure. By using cutting-edge microscopy characterizations, we evidence that minute amounts of volatile-rich melts fully interconnect in mantle rocks down to lithospheric temperatures, enabling thus the modification of geophysical signals from the mantle. These findings and the correspondence of the domain of local, sharp drops in shear wave Velocity (Vs) with the domain of (CO2+H2O)-melting in the LVZ strongly supports that these geophysical anomalies relate to mantle melting. Geophysical surveys image in situ the very Low and highly heterogeneous distribution of melt in the mantle generated by the very Low and highly heterogeneous distribution of volatiles probed by surficial geochemical surveys. The global-scale geophysical signature of the LVZ appears mainly unaffected because the average background melt fraction is very Low, estimated at ∼0.03-0.05 vol.% melt. However, enhanced geophysical signals arise from sporadic, localized areas where melt fraction is increased, such as the ∼0.2 vol.% melt estimated for detecting sharp Vs drops using SS precursors. In-depth deciphering of the dynamics of melt and volatiles in the LVZ calls for investigations on the seismic Velocity, permeability and rheology of partially molten mantle rocks covering the diversity of mantle melt compositions, fractions and temperatures.
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petrological mapping of a Low Velocity Zone lvz induced by co 2 h 2 o bearing incipient melts
2014 AGU Fall Meeting, 2014Co-Authors: Malcolm Massuyeau, Emmanuel Le Trong, Yann Morizet, Emmanuel Gardès, Fabrice GaillardAbstract:The link between volatiles and mantle melting has so far been illuminated by experiments, revealing that ppm concentration levels of carbon and other volatiles in the Earth’s mantle induce partial melting. Pressure-temperature conditions of incipient melting for CO2-H2O-peridotite [1] match fairly well with the upper part of the LVZ, as the redox melting [2] with the Lower part. Recent experimental studies about the Earth mantle conductivity have shown the importance of small amounts of hydrated CO2-rich melts in the geophysical signature of the LVZ [3]. Although such melts are stable under the P-T-fO2 conditions of the LVZ [1-2, 4-6], the variability of these parameters complicates the definition of their chemical composition. Using Margules’ formalisms, we established a multi-component model describing the Gibbs free energy of melt produced by mantle melting in presence of CO2-H2O, that are carbonatite-carbonated melt-nephilinite-basanite and basalt with increasing degree of partial melting. This parameterization is calibrated on crystal-liquid, redox, fluid-liquid and liquid-liquid equilibria obtained by experimental studies in the P-T range 1-10 GPa and 900-1800°C. We propose a calculation of the composition of melts produced in the oceanic LVZ as a function of ages (temperature) and chemical heterogeneities (water, alkalis). At about 80 km depth, we show that the composition of the melts is > 30 wt% SiO2 for ages < 30 Ma, and comes closer to the carbonatitic terms for older lithosphere. Besides lateral chemical variations, our model calculates the melt composition along an oceanic ridge adiabat, predicting an abrupt compositional transition between a H2O-rich carbonatitic melt and a carbonated silicate melt, between 130 km and 100 km. We propose a chemical mapping of the melt composition (and of the degree of partial melting) as a function of the distance to the ridge and of the depth. Our model represents an innovating attempt to connect the chemical variations between carbonated and silicated melts with the geophysical observations. [1] Wallace & Green, 1988, Nature 335, 343-346 [2] Stagno et al., 2013, Nature 493, 84-88 [3] Sifre et al., 2014, Nature 509, 81-85 [4] Presnall & Gudfinnsson, 2005, SPGSA 388, 207-216 [5] Hirschmann et al., 2009, PEPI 176, 54-68 [6] Dasgupta et al., 2013, Nature 493, 211-215
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towards the definition of a petrological Low Velocity Zone lvz
2014Co-Authors: Malcolm Massuyeau, Yann Morizet, Fabrice GaillardAbstract:Carbon and others volatiles that are present in the Earth's mantle at ppm concentration levels, induce partial melting. CO2-H2O-rich melts are stable under the P-T-fO2 conditions of the Low Velocity Zone (LVZ). Recent experimental studies about the Earth mantle conductivity have shown the primordial importance of small amounts of hydrated CO2-rich melts in the geophysical signature of the LVZ. Nevertheless, the chemical composition of these melts is difficult to capture as it depends on T-P and redox state. Using Margules formalisms, we established a multi-component model describing the Gibbs free energy of melt produced by mantle melting in presence of CO2-H2O that are carbonatite-carbonated melt-nephilinite-basanite and basalt with increasing degree of partial melting. This parameterization is calibrated on crystal-liquid, redox, fluid-liquid and liquid-liquid equilibria obtained by experimental studies in the P-T range 1-10 GPa and 900-1800°C. We propose a calculation of the composition of melts produced in the oceanic LVZ as a function of age. At about 80 km depth, we show that the composition of the melts is >30 wt% SiO2 for ages <20 Ma, and comes closer to the carbonatitic terms for older lithosphere. Besides lateral chemical variations, our model calculates the melt composition along an oceanic ridge adiabat, predicting an abrupt compositional transition between a H2O-rich carbonatitic melt and a carbonated silicate melt, between 140 km and 160 km. With the distance to the ridge, this transition is shifted to Lower depths between 70 and 90 km. We propose a chemical mapping of the melt composition (and of the degree of partial melting) as a function of the distance to the ridge and of the depth. The chemical variations between carbonated and silicated melts may be related to the geophysical observations.
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electrical conductivity during incipient melting in the oceanic Low Velocity Zone
Nature, 2014Co-Authors: David Sifre, Fabrice Gaillard, Malcolm Massuyeau, Emmanuel Gardès, Leila Hashim, Saswata HiermajumderAbstract:The Low-viscosity layer in the upper mantle, the asthenosphere, is a requirement for plate tectonics1. The seismic Low velocities and the high electrical conductivities of the asthenosphere are attributed either to subsolidus, water-related defects in olivine minerals2, 3, 4 or to a few volume per cent of partial melt5, 6, 7, 8, but these two interpretations have two shortcomings. First, the amount of water stored in olivine is not expected to be higher than 50 parts per million owing to partitioning with other mantle phases9 (including pargasite amphibole at moderate temperatures10) and partial melting at high temperatures9. Second, elevated melt volume fractions are impeded by the temperatures prevailing in the asthenosphere, which are too Low, and by the melt mobility, which is high and can lead to gravitational segregation11, 12. Here we determine the electrical conductivity of carbon-dioxide-rich and water-rich melts, typically produced at the onset of mantle melting. Electrical conductivity increases modestly with moderate amounts of water and carbon dioxide, but it increases drastically once the carbon dioxide content exceeds six weight per cent in the melt. Incipient melts, long-expected to prevail in the asthenosphere10, 13, 14, 15, can therefore produce high electrical conductivities there. Taking into account variable degrees of depletion of the mantle in water and carbon dioxide, and their effect on the petrology of incipient melting, we calculated conductivity profiles across the asthenosphere for various tectonic plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (more than five million years old), incipient melts probably trigger both the seismic Low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas in young plates4, where seamount volcanism occurs6, a higher degree of melting is expected.
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experimental determination of electrical conductivity during deformation of melt bearing olivine aggregates implications for electrical anisotropy in the oceanic Low Velocity Zone
Earth and Planetary Science Letters, 2011Co-Authors: Luca Caricchi, Fabrice Gaillard, Julian Mecklenburgh, Emmanuel Le TrongAbstract:Abstract A novel experimental setup was used to measure in-situ variations of electrical conductivity (EC) during deformation in torsion (simple shear) at 300 MPa confining pressure and temperatures between 873 and 1473 K. This setup is designed to test if deformation of partially molten systems can produce electrical anisotropy. The motivation for this study comes from the observation that the Lithosphere–Asthenosphere Boundary (LAB) at mid-ocean ridges and in particular at the East Pacific Rise is strongly electrically anisotropic. In an initial set of calibration experiments, the variation of EC with temperature (873–1473 K) was determined for Carrara marble, Aheim dunite and basalt-bearing olivine aggregates. EC was then monitored during deformation experiments at 1473 K and measured in the frequency range between 6 MHz and 1 Hz. The electrical response of the different materials tested as a function of frequency, changes significantly depending on the presence, absence, proportion and distribution of melt contained in the specimen. Melt-free samples show a single conduction mechanism whereas melt-bearing samples display two conduction mechanisms linked in series, reflecting the contribution of isolated and connected melt. Impedance was measured along the sample radius, in a direction parallel to the shear gradient inherent in torsion experiments. During the tests, increasing values of the impedance measured suggest that the long range melt connectivity decreases radially, and melt drains from Low to high shear stress regions. The conductivity, calculated from impedance measurements, is Low and comparable to values measured along mid-ocean ridges. We suggest that electrical anisotropy of the LAB reflects an alternation of melt-enriched and melt-depleted channels elongated in the spreading direction possibly induced by spreading Velocity gradients along the ridge. This implies that the observed electrical anisotropy reveals larger scale processes than strain-induced generation of crystallographic preferred orientations. Such large-scale processes could influence the distribution of seamounts and chemical variations of mid-ocean ridge basalts.
Elizabeth S Cochran - One of the best experts on this subject based on the ideXlab platform.
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Low Velocity damaged structure of the san andreas fault at parkfield from fault Zone trapped waves
Geophysical Research Letters, 2004Co-Authors: John E Vidale, Elizabeth S CochranAbstract:[1] We used dense linear seismic arrays across and along the San Andreas Fault (SAF) at Parkfield, California to record fault Zone trapped waves generated by explosions and microearthquakes in 2002. Prominent trapped waves appeared at stations close to the SAF main fault trace while some energy was trapped in the north strand at the array site. Observations and 3-D finite-difference simulations of trapped waves at 2–5 Hz show evidence of a damaged core Zone on the main SAF. The Zone from the surface to seismogenic depths is marked by a Low-Velocity waveguide ∼150 m wide, in which Q is 10–50 and shear velocities are reduced by 30–40% from wall-rock velocities, with the greatest Velocity reduction at shalLow depth. We interpret that this distinct Low-Velocity Zone on the main SAF is a remanent of damage due to past large earthquakes on the principal fault plane at Parkfield. A less-developed Low-Velocity Zone may be evident on the north strand that experienced minor breaks in the 1966 M6 event.
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Low Velocity damaged structure of the san andreas fault at parkfield from fault Zone trapped waves preparing for the san andreas fault observatory at depth earthquakes and crustal structure
Geophysical Research Letters, 2004Co-Authors: John E Vidale, Elizabeth S CochranAbstract:We used dense linear seismic arrays across and along the San Andreas Fault (SAF) at Parkfield, California to record fault Zone trapped waves generated by explosions and microearthquakes in 2002. Prominent trapped waves appeared at stations close to the SAF main fault trace while some energy was trapped in the north strand at the array site. Observations and 3-D finite-difference simulations of trapped waves at 2-5 Hz show evidence of a damaged core Zone on the main SAF. The Zone from the surface to seismogenic depths is marked by a Low-Velocity waveguide ∼150 m wide, in which Q is 10-50 and shear velocities are reduced by 30-40% from wall-rock velocities, with the greatest Velocity reduction at shalLow depth. We interpret that this distinct Low-Velocity Zone on the main SAF is a remanent of damage due to past large earthquakes on the principal fault plane at Parkfield. A less-developed Low-Velocity Zone may be evident on the north strand that experienced minor breaks in the 1966 M6 event.
Yong Zheng - One of the best experts on this subject based on the ideXlab platform.
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penetration of mid crustal Low Velocity Zone across the kunlun fault in the ne tibetan plateau revealed by ambient noise tomography
Earth and Planetary Science Letters, 2014Co-Authors: Chengxin Jiang, Yingjie Yang, Yong ZhengAbstract:Abstract The NE Tibetan Plateau, composed of the Mesozoic accretions of Lhasa, Qiangtang and Songpan-Ganze Terranes, are bounded by the east Kunlun–Qaidam Block in the north with the boundary delineated by the Kunlun Fault. The NE Tibetan Plateau is at a nascent stage of plateau growth resulting from the collision between the Indian and Eurasian plate starting ∼50 million years ago, and is one of the best areas to study the growth mechanism of the Tibetan Plateau. In this study, we process continuous ambient noise data collected from ∼280 stations during 2007 and 2010 and generate Rayleigh wave phase Velocity maps at 10–60 s periods with a lateral resolution of ∼ 30 – 50 km for most of the study region. By adopting a Bayesian Monte Carlo method, we then construct a 3-D Vsv model of the crust using the Rayleigh wave dispersion maps. Our 3-D model reveals that strong LVZs exist in the middle crust across the NE Tibetan Plateau; and the lateral distribution of LVZs exhibit significant west–east variations along the Kunlun Fault. In the west of 98°E, LVZs are confined to regions of the Kunlun Fault and the eastern Kunlun Ranges, but absent beneath the Qaidam Basin; while in the east of 98°E, LVZs extend and penetrate northward into the east Kunlun and Qinling Orogens over ∼ 100 km across the Kunlun Fault. The strong contrast of the LVZs distribution along the Kunlun Fault may be related to the distinct neighboring tectonic units in the north: a strong crust of the Qaidam Basin in the west blocking the penetration of LVZs, but a weak crust in the Qinling Orogens facilitating the extrusion of LVZs. The distribution of LVZs in the NE Tibetan Plateau is consistent with the crustal channel fLow model, which predicts a branch of north-eastward mid-crustal channel fLow. Our 3D model clearly delineates the north extent of the mid-crustal LVZs, probably reflecting the status of channel fLow in the NE Tibetan Plateau.
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a synoptic view of the distribution and connectivity of the mid crustal Low Velocity Zone beneath tibet
Journal of Geophysical Research, 2012Co-Authors: Yingjie Yang, Michael H Ritzwoller, Yong Zheng, Weisen Shen, A L LevshinAbstract:[1] Based on 1–2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around Tibet, Rayleigh wave phase Velocity maps are constructed from 10 s to 60 s period. A 3-D Vsv model of the crust and uppermost mantle is derived from these maps. The 3-D model exhibits significant apparently inter-connected Low shear Velocity features across most of the Tibetan middle crust at depths between 20 and 40 km. These Low Velocity Zones (LVZs) do not conform to surface faults and, significantly, are most prominent near the periphery of Tibet. The observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear Zones, rather than being localized horizontally near the edges of rigid blocks. The prominent LVZs are coincident with strong mid-crustal radial anisotropy in western and central Tibet and probably result at least partially from anisotropic minerals aligned by deformation, which mitigates the need to invoke partial melt to explain the observations. Irrespective of their cause in partial melt or mineral alignment, mid-crustal LVZs reflect deformation and their amplification near the periphery of Tibet provides new information about the mode of deformation across Tibet.
