scholarly journals Lithospheric dynamics in the vicinity of the Tengchong volcanic field (southeastern margin of Tibet): an investigation using P receiver functions

2020 ◽  
Vol 224 (2) ◽  
pp. 1326-1343
Author(s):  
Hengchu Peng ◽  
José Badal ◽  
Jiafu Hu ◽  
Haiyan Yang ◽  
Benyu Liu
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Volcanic Field ◽  
Receiver Functions ◽  
Indian Plate ◽  
Fast Wave ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Velocity Anomaly ◽  
Lateral Extrusion

SUMMARY Tengchong volcanic field (TVF) in the northern Indochina block lies in a critical area for understanding complex regional dynamics associated with continent–continent convergence between the Indian and Eurasian plates, including northeastward compression generated by subduction of the Indian Plate beneath the Burma Arc, and southeastward lateral extrusion of the crust from below central Tibet. We gathered 3408 pairs of P receiver functions with different frequencies and calculated the splitting parameters of the Moho-converted Pms phase. An anisotropic H-κ stacking algorithm was used to determine crustal thickness and Vp/Vs ratios. We also inverted for the detailed S-velocity structure of the crust and upper mantle using a two-step inversion technique. Finally, we mapped the topography of the lithosphere–asthenosphere boundary. Results show fast-wave polarization directions with a dominant NE–SW orientation and delay times varying between 0.19 and 1.22 s, with a mean of 0.48 ± 0.07 s. The crustal Vp/Vs ratio varies from 1.68 to 1.90 and shows a maximum value below the central part of the TVF, where there is relatively thin crust (∼35–39 km) and a pronounced low-velocity anomaly in the middle–lower crust. The depth of the lithosphere–asthenosphere boundary ranges from 53 to 85 km: it is relatively deep (∼70–85 km) in the vicinity of the TVF and relatively shallow in the south of the study area. In the absence of low shear wave velocity in the upper mantle below the TVF, we propose that the low-velocity anomaly in the lower crust beneath the TVF derives from the upper mantle below the neighbouring Baoshan block.

Crust-mantle velocity structure in Shanxi rift, Central North China Craton

2020 ◽  
Author(s):  
Yan Cai ◽  
Jianping Wu
Keyword(s):  
Upper Mantle ◽  
High Velocity ◽  
North China Craton ◽  
Lower Crust ◽  
North China ◽  
Velocity Structure ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
The North ◽  
Shanxi Rift

<p>North China Craton is the oldest craton in the world. It contains the eastern, central and western part. Shanxi rift and Taihang mountain contribute the central part. With strong tectonic deformation and intense seismic activity, its crust-mantle deformation and deep structure have always been highly concerned. In recent years, China Earthquake Administration has deployed a dense temporary seismic array in North China. With the permanent and temporary stations, we obtained the crust-mantle S-wave velocity structure in the central North China Craton by using the joint inversion of receiver function and surface wave dispersion. The results show that the crustal thickness is thick in the north of the Shanxi rift (42km) and thin in the south (35km). Datong basin, located in the north of the rift, exhibits large-scale low-velocity anomalies in the middle-lower crust and upper mantle; the Taiyuan basin and Linfen basin, located in the central part, have high velocities in the lower crust and upper mantle; the Yuncheng basin, in the southern part, has low velocities in the lower crust and upper mantle velocities, but has a high-velocity layer below 80 km. We speculate that an upwelling channel beneath the west of the Datong basin caused the low velocity anomalies there. In the central part of the Shanxi rift, magmatic bottom intrusion occurred before the tension rifting, so that the heated lithosphere has enough time to cool down to form high velocity. Its current lithosphere with high temperature may indicate the future deformation and damage. There may be a hot lithospheric uplift in the south of the Shanxi rift, heating the crust and the lithospheric mantle. The high-velocity layer in its upper mantle suggests that the bottom of the lithosphere after the intrusion of the magma began to cool down.</p>

Integrated geophysical-petrological modelling of the Eifel region

2020 ◽  
Author(s):  
Agnes Wansing ◽  
Jörg Ebbing ◽  
Eva Bredow
Keyword(s):  
Mantle Plume ◽  
Upper Mantle ◽  
West Germany ◽  
Small Scale ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Quaternary Volcanism ◽  
Velocity Anomaly ◽  
Type Composition ◽  
Eifel Region

<p>We present an integrated geophysical-petrological model of the Eifel region. The Eifel is a volcanic active region in West Germany that exhibits Tertiary as well as Quaternary volcanism. One suggestion for the source of this volcanism is a small-scale upper mantle plume.</p><p>The 3D model includes the crust and upper mantle and was generated by combined modelling of topography and the gravity field with constraints from seismology and geochemistry. In the best-fit model, the subcontinental lithospheric mantle is associated with a Phanerozoic-type composition, resulting in a depth of 80 km for the lithosphere-asthenosphere boundary (LAB) beneath the Eifel and in comparison 110 - 130 km beneath the Paris basin. A Proterozoic-type composition in contrast results in a LAB depth of 120 km in the Eifel. While the model fits the geophysical observables and features a thin lithosphere, it does not lead to a plume-like structure and does not feature a seismic low-velocity anomaly.</p><p>The measured low-velocity anomaly can be reproduced by introducing (1) an even thinner lithosphere or (2) a plume-like body above the thermal LAB with a composition based on data from Eifel xenoliths, which have a mainly basanitic composition. This additional structure results in a thermal anomaly and has an effect on the isostatic elevation of c. 360 m, but it does not result in a significant signal in the gravity anomalies. Further modelling showed how crustal intrusions could additionally mask the gravitational effect from such a small-scale upper mantle plume.</p><p>The model does not conclusively explain the source of the Eifel volcanism, but the models and the calculation of synthetic dispersion curves help to assess the possibility to resolve a small-scale upper mantle plume with joint inversion in future analysis.</p>

Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

1992 ◽  
Vol 29 (7) ◽  
pp. 1530-1548 ◽  
Author(s):  
B. C. Zelt ◽  
R. M. Ellis ◽  
R. M. Clowes ◽  
E. R. Kanasewich ◽  
I. Asudeh ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Velocity Structure ◽  
P Wave ◽  
Seismic Reflection Profile ◽  
Wide Angle ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Middle Crust ◽  
Refraction Data

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

scholarly journals Constraints on the rheology of lower crust in a strike-slip plate boundary: Evidence from the San Quintin xenoliths, Baja California, Mexico

2017 ◽  
Author(s):  
Thomas van der Werf ◽  
Vasileios Chatzaras ◽  
Leo M. Kriegsman ◽  
Andreas Kronenberg ◽  
Basil Tikoff ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Baja California ◽  
Volcanic Field ◽  
Strike Slip ◽  
Mafic Granulite ◽  
Dislocation Creep ◽  
Diffusion Creep ◽  
Plate Boundaries ◽  
Crust And Upper Mantle

Abstract. The rheology of lower crust and its time-dependent behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of mafic granulite and lherzolite xenoliths from the Holocene San Quintin volcanic field, of northern Baja California, Mexico. The San Quintin volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. Combining microstructural observations, geothermometry and phase equilibria modeling we constrain that crystal-plastic deformation took place at temperatures of 750–900 °C and pressures of 400–580 MPa, corresponding to 15–22 km depth. A hot crustal geothermal gradient of 40 °C/km is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is dry. Microstructural evidence suggests that the mafic granulite and peridotite xenoliths were dominantly deforming by processes transitional between dislocation creep and diffusion creep. Recrystallized grain size paleopiezometry yields similar differential stresses in both the uppermost lower crust and upper mantle. Using dry-plagioclase and dry-olivine flow laws we demonstrate that the viscosity of the lower crust and upper mantle is low (2.2 × 1018 – 1.4 × 1020 Pa s). Comparing the viscosity structure of the lithosphere constrained from the San Quintin xenoliths with results from post-seismic relaxation studies from western US, we suggest that lower crust is stronger during transient deformation (e.g., post-seismic relaxation period) while the upper mantle is stronger during long-term deformation (e.g., interseismic period).

Imaging the East Asia using waveform tomography with massive datasets 

2021 ◽  
Author(s):  
Hui Dou ◽  
Sergei Lebedev ◽  
Bruna Chagas de Melo ◽  
Baoshan Wang ◽  
Weitao Wang
Keyword(s):  
East Asia ◽  
Mantle Plume ◽  
Upper Mantle ◽  
High Velocity ◽  
Deep Structure ◽  
Indian Plate ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
The Pacific ◽  
Velocity Anomalies

<p>We present a new shear-wave velocity model of the upper mantle beneath the East Asia, ASIA2021, derived using the Automatic Multimode Inversion technique. We use waveform fits of over 1.3 million seismograms, comprising waveforms of surface waves, S and multiple S waves.  In total, data from 9351 stations and 23344 events constrain ASIA2021, which maps in detail the structure of the lithosphere and underlying mantle beneath the region. Our model reveals deep structure beneath the tectonic units that make up East Asia. It shows agreement with previous models at larger scales and, also, sharper and stronger velocity anomalies at smaller regional scales. High-velocity continent roots are mapped in detail beneath the Sichuan Basin, Tarim Basin, Ordos Block, and Siberian Craton, extending to over 200 km depths. The lack of a high-velocity continental root beneath the Eastern North China Craton (ENCC), underlain, instead, by a low-velocity anomaly, is consistent with the destruction of this Archean nucleus. Strong low-velocity anomalies are mapped within the top 100 km beneath Tibet, Pamir, Altay-Sayan area, and back-arc basins. At greater depths, ASIA2021 shows high-velocity anomalies related to the subducted and underthrusted lithosphere of India beneath Tibet and the subduction of the Pacific and other plates in the upper mantle. In the mantle transition zone (MTZ), we find high-velocity anomalies probably related to deflected subducted slabs or detached portions of ancient continent cratons. In particularly, ASIA2021 reveals separate bodies, probably originating from the Indian Plate lithosphere beneath central Tibet, with one at 100-200 km beneath Songpan-Ganzi Block (SGFB) and the other in the MTZ. A strong low-velocity anomaly extending from the surface to the lower mantle beneath Hainan volcano and South China Sea is consistent with the hypothesis of the Hainan mantle plume. The high-velocity anomaly beneath ENCC in MTZ can be interpreted as a detached Archean continent root. The Pacific Plate subducts beneath the eastern margin of Asia into the MTZ and appears to deflect and extend horizontally as far west as the Songliao Basin. The absence of major gaps in the stagnant slab is consistent with the origin of Changbaishan volcano above being related to the Big Mantle Wedge, proposed previously. The low-velocity anomalies down to ~ 700 km depth beneath the Lake Baikal area suggest a hot upwelling (mantle plume) feeding the widely distributed Cenozoic volcanoes in central and western Mongolia.</p>

Reflections from discontinuities beneath antarctica

1971 ◽  
Vol 61 (5) ◽  
pp. 1441-1451
Author(s):  
R. D. Adams
Keyword(s):  
North American ◽  
Upper Mantle ◽  
Deep Structure ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Nuclear Explosions ◽  
Novaya Zemlya ◽  
The Earth ◽  
Velocity Channel ◽  
Upper Boundary

abstract Early reflections of the phase P′P′ recorded at North American seismograph stations from nuclear explosions in Novaya Zemlya are used to examine the crust and upper mantle beneath a region of eastern Antarctica. Many reflections are observed from depths less than 120 km, indicating considerable inhomogeneity at these depths in the Earth. No regular horizons were found throughout the area, but some correlation was observed among reflections at closely-spaced stations, and, at many stations, reflections were observed from depths of between 60 and 80 km, corresponding to a likely upper boundary of the low-velocity channel. Deeper reflections were found at depths of near 420 and 650 km. The latter boundary was particularly well-observed and appears to be sharply defined at a depth that is constant to within a few kilometers. The boundary at 420 km is not so well defined by reflections of P′P′, but reflects well longer-period PP waves, arriving at wider angles of incidence. This boundary appears to be at least as pronounced, but not so sharp as that near 650 km. The deep structure beneath Antarctica presents no obvious difference from that beneath other continental areas.

scholarly journals Constraints on the rheology of the lower crust in a strike-slip plate boundary: evidence from the San Quintín xenoliths, Baja California, Mexico

Solid Earth ◽  
2017 ◽  
Vol 8 (6) ◽  
pp. 1211-1239 ◽  
Author(s):  
Thomas van der Werf ◽  
Vasileios Chatzaras ◽  
Leo Marcel Kriegsman ◽  
Andreas Kronenberg ◽  
Basil Tikoff ◽  
...  
Keyword(s):  
Grain Size ◽  
Upper Mantle ◽  
Lower Crust ◽  
Baja California ◽  
Plate Boundary ◽  
Volcanic Field ◽  
Strike Slip ◽  
Low Viscosity ◽  
The Pacific ◽  
Flow Laws

Abstract. The rheology of lower crust and its transient behavior in active strike-slip plate boundaries remain poorly understood. To address this issue, we analyzed a suite of granulite and lherzolite xenoliths from the upper Pleistocene–Holocene San Quintín volcanic field of northern Baja California, Mexico. The San Quintín volcanic field is located 20 km east of the Baja California shear zone, which accommodates the relative movement between the Pacific plate and Baja California microplate. The development of a strong foliation in both the mafic granulites and lherzolites, suggests that a lithospheric-scale shear zone exists beneath the San Quintín volcanic field. Combining microstructural observations, geothermometry, and phase equilibria modeling, we estimated that crystal-plastic deformation took place at temperatures of 750–890 °C and pressures of 400–560 MPa, corresponding to 15–22 km depth. A hot crustal geotherm of 40 ° C km−1 is required to explain the estimated deformation conditions. Infrared spectroscopy shows that plagioclase in the mafic granulites is relatively dry. Microstructures are interpreted to show that deformation in both the uppermost lower crust and upper mantle was accommodated by a combination of dislocation creep and grain-size-sensitive creep. Recrystallized grain size paleopiezometry yields low differential stresses of 12–33 and 17 MPa for plagioclase and olivine, respectively. The lower range of stresses (12–17 MPa) in the mafic granulite and lherzolite xenoliths is interpreted to be associated with transient deformation under decreasing stress conditions, following an event of stress increase. Using flow laws for dry plagioclase, we estimated a low viscosity of 1.1–1.3×1020 Pa ⋅ s for the high temperature conditions (890 °C) in the lower crust. Significantly lower viscosities in the range of 1016–1019 Pa ⋅ s, were estimated using flow laws for wet plagioclase. The shallow upper mantle has a low viscosity of 5.7×1019 Pa ⋅ s, which indicates the lack of an upper-mantle lid beneath northern Baja California. Our data show that during post-seismic transients, the upper mantle and the lower crust in the Pacific–Baja California plate boundary are characterized by similar and low differential stress. Transient viscosity of the lower crust is similar to the viscosity of the upper mantle.

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