Episodic Lithospheric Deformation in Eastern Tibet Inferred From Seismic Anisotropy

2020 ◽  
Vol 47 (3) ◽  
Author(s):  
Xuewei Bao ◽  
Xiaodong Song ◽  
David W. Eaton ◽  
Yixian Xu ◽  
Hanlin Chen
Keyword(s):  
Seismic Anisotropy ◽  
Lithospheric Deformation ◽  
Eastern Tibet

scholarly journals Lithospheric Deformation at the South Island Oblique Collision, New Zealand

2021 ◽  
Author(s):  
◽  
Sandra Bourguignon
Keyword(s):  
Wave Speed ◽  
Seismic Anisotropy ◽  
Plate Boundary ◽  
Southern Alps ◽  
Density Contrast ◽  
Mass Excess ◽  
Oblique Collision ◽  
Lithospheric Deformation ◽  
Shear Wave Speed ◽  
Central South

<p>Lithospheric deformation is investigated within the Southern Alps oblique collision zone of the Australian and Pacific plate boundary. Seismological methods and gravity modelling are used to estimate seismic anisotropy, wave-speed anomalies and mass anomalies in the uppermost mantle. While seismic anisotropy is generally interpreted to result from Cenozoic mantle shear, wave-speed and mass anomalies can be explained solely by thermal contraction of mantle rocks that results from the downward deflection of isotherms during mantle shortening. Along the eastern Southern Alps foothills and approximately 15' clockwise from their axis, earthquake Pn waves propagate at 8.54 +/- 0.20 km/s. This high wave speed is attributed to a high average Pn speed (8.3 +/- 0.3 km/s) and Pn anisotropy (7 - 13 %) in the mantle lid beneath central South Island. Two-dimensional ray-tracing suggests that the crustal thickness is 48 +/- 4 km beneath the Southern Alps' southern extent near Wanaka (western Otago). Such a thickness represents an 18 +/- 4 km thick crustal root that is thicker than necessary to isostatically sustain the approximately 1000 m topographic load of this region. A mass excess is proposed in the mantle below the region of over-thickened crust to compensate for the crustal root mass deficit. Assuming that the crustal root represents a -300 kg/m3 density contrast with the mantle lid, this mantle mass excess requires a minimum density contrast of 35 +/- 5 kg/m3, 110 +/-20 km width and 70 +/- 20 km thickness that will impart a downward pull on the overlying crust.</p>

scholarly journals Lithospheric Deformation at the South Island Oblique Collision, New Zealand

2021 ◽  
Author(s):  
◽  
Sandra Bourguignon
Keyword(s):  
Wave Speed ◽  
Seismic Anisotropy ◽  
Plate Boundary ◽  
Southern Alps ◽  
Density Contrast ◽  
Mass Excess ◽  
Oblique Collision ◽  
Lithospheric Deformation ◽  
Shear Wave Speed ◽  
Central South

<p>Lithospheric deformation is investigated within the Southern Alps oblique collision zone of the Australian and Pacific plate boundary. Seismological methods and gravity modelling are used to estimate seismic anisotropy, wave-speed anomalies and mass anomalies in the uppermost mantle. While seismic anisotropy is generally interpreted to result from Cenozoic mantle shear, wave-speed and mass anomalies can be explained solely by thermal contraction of mantle rocks that results from the downward deflection of isotherms during mantle shortening. Along the eastern Southern Alps foothills and approximately 15' clockwise from their axis, earthquake Pn waves propagate at 8.54 +/- 0.20 km/s. This high wave speed is attributed to a high average Pn speed (8.3 +/- 0.3 km/s) and Pn anisotropy (7 - 13 %) in the mantle lid beneath central South Island. Two-dimensional ray-tracing suggests that the crustal thickness is 48 +/- 4 km beneath the Southern Alps' southern extent near Wanaka (western Otago). Such a thickness represents an 18 +/- 4 km thick crustal root that is thicker than necessary to isostatically sustain the approximately 1000 m topographic load of this region. A mass excess is proposed in the mantle below the region of over-thickened crust to compensate for the crustal root mass deficit. Assuming that the crustal root represents a -300 kg/m3 density contrast with the mantle lid, this mantle mass excess requires a minimum density contrast of 35 +/- 5 kg/m3, 110 +/-20 km width and 70 +/- 20 km thickness that will impart a downward pull on the overlying crust.</p>

Spatial variation in seismic anisotropy beneath the eastern and northeastern Iranian plateau and its geodynamic implications

2021 ◽  
Author(s):  
Yifan Gao ◽  
Ling Chen ◽  
Morteza Talebian ◽  
Zimu Wu ◽  
Xu Wang ◽  
...  
Keyword(s):  
Large Scale ◽  
Seismic Anisotropy ◽  
Strike Slip ◽  
Surface Strain ◽  
Deformation Pattern ◽  
Strain Rates ◽  
Lithospheric Deformation ◽  
Iranian Plateau ◽  
Lut Block ◽  
Imaging Results

&lt;p&gt;The Iranian plateau is a natural laboratory for studying the early stage of continental collision and plateau development. The collisional front and northern plateau are the major areas accommodating the Arabia-Eurasia convergence. GPS observations suggest that the blocks of central Iran with minor shortening may be relatively rigid. However, recent seismic imaging results suggest that the lithosphere in this region might not be rigid for it is thin and not seismically fast. Widespread mantle-derived magmatism since Middle Miocene also lends support to a relatively hot and weak lithosphere. It may raise a question of why these blocks could behave rigidly when transmitting stresses to the north.&lt;/p&gt;&lt;p&gt;Deformation patterns of the lithosphere and asthenosphere in the northeastern and eastern Iranian plateau, which can be constrained by seismic anisotropy, may help to understand the nature of the lithosphere within the continental interior and its responses to the Arabia-Eurasia collision. We studied the seismic anisotropy of the region via teleseismic shear-wave splitting analysis on dense array data and compared the new results with multidisciplinary observations, particularly the surface strain rates and the structure of the lithosphere-asthenosphere system. In northeastern Iran around the Paleo-Tehtys suture, the dominant fast polarization direction (FPD) is NW-SE, subparallel to the strikes of thrust faults and orogenic belts. This combined with the relatively higher strain rates and thicker crust and lithosphere suggests that northeastern Iran with pre-existing weakness may have experienced considerable lithospheric shortening. The Lut block, which is a major block of eastern Iran bounded&lt;strong&gt; &lt;/strong&gt;by large-scale strike-slip faults and previously assumed rigid, shows a complex anisotropic structure. In its northern part where the strain rates are low, the average NE-SW FPD has no obvious link to active faults but is roughly parallel to the collision-induced asthenospheric flow. The area to the south around the Dasht-e-Bayaz fault shows high strain rates and a complex structure of Moho. The generally NW-SE FPDs are subparallel to the direction of the surface right-lateral shear, possibly reflecting a fault-controlled lithospheric deformation pattern. Further south is the central Lut area with moderate strain rates. It is characterized by a two-layer structure of anisotropy, with the FPDs in the upper and lower layers being similar to those of the area around the Dasht-e-Bayaz fault and the northern Lut block, respectively. This feature indicates that the anisotropy and deformation of the central Lut area could be affected by both large-scale strike-slip faults and collision-induced mantle flow.&lt;/p&gt;&lt;p&gt;Collectively, our observations suggest that both the collisional processes at the plate boundary and the nature and structural heterogeneities of the continental lithosphere may control the intracontinental deformation of the Iranian plateau. The observed minor deformation of the Lut block and also other blocks within this young plateau does not necessarily mean that these blocks are rigid, but is probably because of significant deformation preferentially taking place at not only the collision front but also mechanically weak zones in the hinterland, which may have accommodated most of the Arabia-Eurasia convergence.&lt;/p&gt;

scholarly journals Seismic anisotropy inferred from direct S-waves derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

2016 ◽  
Author(s):  
Ashwani Kant Tiwari ◽  
Arun Singh ◽  
Tuna Eken ◽  
Nitin Grewal ◽  
Chandrani Singh
Keyword(s):  
Seismic Anisotropy ◽  
Plate Motion ◽  
Reference Station ◽  
Sharp Change ◽  
Lithospheric Deformation ◽  
Mantle Dynamics ◽  
Study Region ◽  
S Waves ◽  
Anisotropy Parameters ◽  
Fast Polarization

Abstract. The present study deals with detecting seismic anisotropy parameters beneath southeastern Tibet near Namche Barwa Mountain using splitting of the direct S-waves. We employed the reference station technique to remove the effects of source side anisotropy. Seismic anisotropy parameters, splitting time delay and fast polarization directions were estimated through analyses on a total of 501 splitting measurements obtained from direct-S waves from 25 earthquakes (> 5.5 magnitude) that were recorded at 42 stations of Namchebarwa seismic network. We observed a large variation in time delays ranging from 0.64 to 1.68 s but in most cases it is more than 1 s, which suggests for a highly anisotropic lithospheric mantle in the region. A comparison between direct S- and SKS-derived splitting parameters generally shows a close similarity although some discrepancies exist where null or negligible anisotropy is reported earlier using SKS. The seismic stations with hitherto null or negligible anisotropy are now supplemented with new measurements with clear anisotropic signatures. Our analyses indicate a sharp change in lateral variations of fast polarization directions (FPDs) from consistent ENE-SSW or E-W to NW-SE direction at the southeastern edge of Tibet. Comparison of the FPDs with global positioning system (GPS) measurements, absolute plate motion (APM) directions and surface geological features signify that the observed anisotropy and hence inferred deformation patterns are not only due to asthenospheric dynamics but it is a combination of lithospheric deformation and sub-lithospheric (asthenospheric) mantle dynamics. Splitting measurement using direct-S waves proves their utility to supplement the anisotropic measurements in the study region and fills the missing links that remain rather illusive due to lack of SKS measurements.

scholarly journals Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

2020 ◽  
Vol 224 (1) ◽  
pp. 290-305
Author(s):  
Fenitra Andriampenomanana ◽  
Andrew A Nyblade ◽  
Michael E Wysession ◽  
Raymond J Durrheim ◽  
Frederik Tilmann ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Seismic Velocity ◽  
Plate Boundary ◽  
Active Regions ◽  
Shear Zones ◽  
Small Scale ◽  
Uppermost Mantle ◽  
Lithospheric Deformation ◽  
Pan African ◽  
Pn Velocity

SUMMARY The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low-Pn-velocity zones (&lt;8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (&gt;8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.

Des images du systeme lithosphere-asthenosphere sous la France et leurs implications geodynamiques; l'apport de la tomographie telesismique et de l'anisotropie sismique

2000 ◽  
Vol 171 (2) ◽  
pp. 149-167 ◽  
Author(s):  
Michel Granet ◽  
Sebastien Judenherc ◽  
Annie Souriau
Keyword(s):  
Seismic Anisotropy ◽  
Thermal Anomaly ◽  
Rift System ◽  
Low Velocity ◽  
Geological Structures ◽  
Tectonic Event ◽  
Lithospheric Deformation ◽  
Massif Central ◽  
The North ◽  
Armorican Massif

Abstract From seismic tomography and seismic anisotropy, images of the lithosphere-asthenosphere system beneath France for some remarkable tectonic areas have been computed : a continental rift system (the Upper Rhinegraben), an Hercynian structure reactivated by Neogene volcanism (Massif central), a region of a recent continental collision (Pyrenees) and finally a region of an ancient orogeny (Armorican Massif). These images have a horizontal spatial resolution of the order of 10 km and show not only the geometry of the deep geological structures but will also illustrate the link between surface observations and structures detected at depth. The images demonstrate the passive character of the Rhinegraben mainly because no low-velocity was found below the Moho, show the presence of a thermal anomaly beneath the Massif central interpreted as caused by a mantle plume in the decaying phase of its evolution and prove the lithospheric scale of the North Pyrenean fault and of the South-Armorican shear zone. The anisotropic measurements suggest a lithospheric deformation related to the most recent tectonic event. In the Pyrenees, the Armorican Massif or the Rhinegraben areas, the directions of the fast-polarisation azimuth (the polarisation direction of the fast shear wave) are parallel to the tectonic texture of the last events, but suggest also a reactivation of inherited Hercynian discontinuities. In the Massif central, the splitting parameters distinguish between two lithospheric units regions marked by a distinct fast-polarisation azimuth on each side of the Sillon Houiller fault zone.

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