Seismic anisotropy and mantle deformation in NW Iran through splitting measurements of SKS and direct S phases

2020 ◽  
Author(s):  
Shiva Arvin ◽  
Farhad Sobouti ◽  
Keith Priestley ◽  
Abdolreza Ghods ◽  
Seyed Khalil Motaghi ◽  
...  
Keyword(s):  
Flow Field ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Central Iran ◽  
Plate Motion ◽  
Propagation Path ◽  
Mantle Flow ◽  
South Caspian Basin ◽  
Nw Iran ◽  
S Waves

<p><span>The present tectonics of Iran has resulted from the continental convergence of the Arabian and Eurasian plates. Our study area, in NW Iran comprises a part of this collision zone and consists of an assemblage of distinct lithospheric blocks including the central Iranian Plateau, the South Caspian Basin, and the Talesh western Alborz Mountains. A proper knowledge of mantle flow field is required to bettwer constrain mantle kinematics in relation to the dynamics of continental deformation in NW Iran. To achieve this aim, we examined splitting of teleseismic shear waves (e.g. SKS and S) arriving with steep arrival angles beneath the receiver, which provide excellent lateral resolution in the upper mantle. We used data from 68 temporary broadband stations with varying operation periods (4 to 31 months) along 3 linear profiles. We perfomed splitting analyses on SK(K)S and direct S waves. </span>Resultant splitting parameters obtained from both shear phases exhibit broad similarities. Relatively large time delays observed for direct S-waves, however, are anticipated since these waves travel longer than SKS along a non-vertical propagation path in an anisotropic layer. Overall, the fast polarization directions (FPDs) in the Alborz, Talesh, Tarom Mountain and in NW Iran indicate a strong consistency with NE-SW anisotropic orientations. Besides, we observe a good accordance between S and SKS results. A comparison of splitting parameters with the absolute plate motion (APM) vector and structural trends in Iran and eastern Turkey suggests asthenospheric flow field as the dominant source for observed seismic anisotropy. The lithospheric layer beneath these regions is relatively thin (compared to the adjacent Zagros region), explaining why it appears to only make a partial contribution to the observed anisotropy. The stations located in central Iran just southwest of the Alborz yield angular deviations from the general NE-SW trend as this may be explained by changing style of deformation across the different tectonic blocks. These stations indicate significant misfit between SK(K)S and direct S-waves that could be caused by local heterogeneities developed due to a diffuse boundary from the flow organization in the upper mantle of central Iran. Another possibility for large differences between two types of waves might be reflect the anisotropic structure of a remnant slab segment or a foundered lithospheric root beneath central Iran with a volume small enough to be detected by SKS phases, but not by the direct S waves.</p>

scholarly journals Upper Mantle Anisotropy beneath the Western Segment, NW Indian Himalaya, Using Shear Wave Splitting

Lithosphere ◽  
2020 ◽  
Vol 2020 (1) ◽  
pp. 1-9 ◽  
Author(s):  
Shubhasmita Biswal ◽  
Sushil Kumar ◽  
Sunil K. Roy ◽  
M. Ravi Kumar ◽  
W. K. Mohanty ◽  
...  
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Western Himalaya ◽  
Shear Wave Splitting ◽  
Plate Motion ◽  
Indian Plate ◽  
Deformation Pattern ◽  
Mantle Flow ◽  
Wave Splitting

Abstract This study investigates the upper mantle deformation pattern beneath the Indo-Eurasia collision zone utilizing the core-refracted (S(K)KS) phases from 167 earthquakes recorded by 20 broadband seismic stations deployed in the Western Himalaya. The 76 new shear wave splitting measurements reveal that the fast polarization azimuths (FPAs) are mainly oriented in the ENE-WSW direction, with the delay times varying between 0.2 and 1.7 s. The FPAs at most of the stations tend to be orthogonal to the major geological boundaries in the Western Himalaya. The average trend of the FPAs at each station indicates that the seismic anisotropy is primarily caused due to strain-induced deformation in the top ~200 km of the upper mantle as a result of the ongoing Indo-Eurasian collision. A contribution from the mantle flow in the direction of the Indian plate motion is possible. The mantle strain revealed in the present study may be due to a combination of basal shear resulting from plate motion and ductile flow along the collision front due to compression.

scholarly journals Seismic anisotropy: an original tool to understand the geodynamic evolution of the Italian peninsula

1997 ◽  
Vol 40 (3) ◽  
Author(s):  
L. Margheriti ◽  
C. Nostro ◽  
A. Amato ◽  
M. Cocco
Keyword(s):  
Upper Mantle ◽  
Common Property ◽  
Seismic Waves ◽  
Seismic Anisotropy ◽  
Structural Features ◽  
Relative Strength ◽  
Plate Motion ◽  
Mantle Minerals ◽  
Anisotropic Bodies ◽  
Shallow Crust

Anisotropy is a common property of the Earth's crust and the upper mantle; it is related to the strain field of the medium and therefore to geodynamics. In this paper we describe the different possible origins of anisotropic behavior of the seismic waves and the seismological techniques used to define anisotropic bodies. In general it is found that the fast polarization direction is parallel to the absolute plate motion in cratonic areas, to the spreading direction near rifts or extensional zones, and to the main structural features in transpressive regimes. The delay times between fast and slow waves reflect the relative strength and penetration at depth of the deformation field. The correspondence between surface structural trends and anisotropy in the upper mantle, found in many regions of the world, strongly suggest that orogenic processes involve not only the shallow crust but the entire lithosphere. Recently in Italy both shear wave splitting analysis and Pn inversion were applied to define the trend of seismic anisotropy. Along the Northern Appeninic arc fast directions follow the strike of the arc (i.e., parallel to the strike of the Miocene-Pleistocene compressional features), whereas in the Tyrrhenian zone fast directions are about E-W SW-NE; parallel to the post-Miocene extension that is thought to have reoriented the mantle minerals fabric in the astenosphere.

scholarly journals Supplemental Material: Seismic anisotropy in southern Costa Rica confirms upper mantle flow from the Pacific to the Caribbean

2020 ◽  
Author(s):  
Vadim Levin ◽  
et al.
Keyword(s):  
Data Analysis ◽  
Costa Rica ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Data Sources ◽  
Mantle Flow ◽  
Analysis Methodology ◽  
The Pacific ◽  
Wave Splitting ◽  
The Caribbean

Data sources, details of data analysis methodology, and additional diagrams and maps of shear wave splitting measurements.<br>

scholarly journals Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

2020 ◽  
Vol 6 (28) ◽  
pp. eabb0476
Author(s):  
Jorge C. Castellanos ◽  
Jonathan Perry-Houts ◽  
Robert W. Clayton ◽  
YoungHee Kim ◽  
A. Christian Stanciu ◽  
...  
Keyword(s):  
Pacific Northwest ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Azimuthal Anisotropy ◽  
Mantle Flow ◽  
Near Surface ◽  
Vertical Loading ◽  
The Pacific ◽  
Anisotropy Model ◽  
Lithosphere Dynamics

Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to control the evolution of the lithosphere. While tectonic plate motions provide evidence for this relation, the mechanism by which mantle processes influence near-surface tectonics remains elusive. Here, we present an azimuthal anisotropy model for the Pacific Northwest crust that strongly correlates with high-velocity structures in the underlying mantle but shows no association with the regional mantle flow field. We suggest that the crustal anisotropy is decoupled from horizontal basal tractions and, instead, created by upper mantle vertical loading, which generates pressure gradients that drive channelized flow in the mid-lower crust. We then demonstrate the interplay between mantle heterogeneities and lithosphere dynamics by predicting the viscous crustal flow that is driven by local buoyancy sources within the upper mantle. Our findings reveal how mantle vertical load distribution can actively control crustal deformation on a scale of several hundred kilometers.

Seismic anisotropy in southern Costa Rica confirms upper mantle flow from the Pacific to the Caribbean

Geology ◽  
2020 ◽  
Vol 49 (1) ◽  
pp. 8-12 ◽  
Author(s):  
Vadim Levin ◽  
Stephen Elkington ◽  
James Bourke ◽  
Ivonne Arroyo ◽  
Lepolt Linkimer
Keyword(s):  
Costa Rica ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Volcanic Eruptions ◽  
Central American ◽  
Mantle Flow ◽  
Dynamic Topography ◽  
The Pacific ◽  
The Caribbean ◽  
Surrounding Areas

Abstract Surrounded by subducting slabs and continental keels, the upper mantle of the Pacific is largely prevented from mixing with surrounding areas. One possible outlet is beneath the southern part of the Central American isthmus, where regional observations of seismic anisotropy, temporal changes in isotopic composition of volcanic eruptions, and considerations of dynamic topography all suggest upper mantle flow from the Pacific to the Caribbean. We derive new constraints on the nature of seismic anisotropy in the upper mantle of southern Costa Rica from observations of birefringence in teleseismic shear waves. Fast and slow components separate by ∼1 s, with faster waves polarized along the 40°–50° (northeast) direction, near-orthogonally to the Central American convergent margin. Our results are consistent with upper mantle flow from the Pacific to the Caribbean and require an opening in the lithosphere subducting under the region.

Mantle flow under the Central Alps: Constraints from non-vertical-ray SKS shear-wave splittting

2020 ◽  
Author(s):  
Eric Löberich ◽  
Götz Bokelmann
Keyword(s):  
Shear Wave ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Single Layer ◽  
Depth Resolution ◽  
Shear Wave Splitting ◽  
Mantle Flow ◽  
Central Alps ◽  
Upper Mantle Anisotropy ◽  
Wave Splitting

&lt;p&gt;The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation &lt;em&gt;&amp;#966;&lt;/em&gt; and delay time &lt;em&gt;&amp;#916;t&lt;/em&gt;, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations &lt;em&gt;&amp;#966;&lt;/em&gt; show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters &lt;em&gt;&amp;#966;&lt;/em&gt; and &lt;em&gt;&amp;#916;t&lt;/em&gt;, and in the backazimuthal variation of &lt;em&gt;&amp;#948;&amp;#966;&lt;/em&gt; and &lt;em&gt;&amp;#948;&amp;#916;t&lt;/em&gt;, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.&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.

Detailed Investigation of Seismic Anisotropy in the Upper Mantle of the Northern Aegean Region

2020 ◽  
Author(s):  
Ceyhun Erman ◽  
Seda Yolsal-Çevikbilen ◽  
Tuna Eken ◽  
Tuncay Taymaz
Keyword(s):  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Broad Band ◽  
Mantle Flow ◽  
Data Set ◽  
Study Region ◽  
Waveform Data ◽  
Seismic Stations ◽  
Aegean Region ◽  
Fast Polarization

&lt;p&gt;Seismic anisotropy studies can provide important constraints on geodynamic processes and deformation styles in the upper mantle of tectonically active regions. Seismic anisotropy parameters (e.g. delay time and fast polarization direction) can give hints at the past and recent deformations and can be most conventionally obtained through core-mantle refracted SKS phase splitting measurements. In order to explore the complexity of anisotropic structures in the upper mantle of a large part of the Aegean region, in this study, we estimate splitting parameters beneath 25 broad-band seismic stations located at NW Anatolia, North Aegean Sea and Greece mainland. To achieve this we employ both transverse energy minimization and eigenvalue methods. Waveform data of selected earthquakes (with M&lt;sub&gt;w&lt;/sub&gt; &amp;#8805; 5.5; 2008-2018 and with epicentral distances between 85&amp;#176;&amp;#8211;120&amp;#176;) were retrieved from Earthquake Data Center System of Turkey (AFAD; http://tdvm.afad.gov.tr/) and European Integrated Data Archive (EIDA; http://orfeus-eu.org/webdc3/). A quite large data set, the majority of which have not been studied before, were evaluated in order to estimate reliable non-null and null results. In general, station-averaged splitting parameters mainly exhibit the NE-SW directed fast polarization directions throughout the study area. These directions can be explained by the lattice-preferred orientation of olivine minerals in the upper mantle induced by the mantle flow related to the roll-back process of the Hellenic slab. We further observe that station-averaged splitting time delays are prone to decrease from north to south of the Aegean region probably changing geometry of mantle wedge with a strong effect on&amp;#160; the nature of mantle flow along this direction. The uniform distribution of splitting parameters as a function of back-azimuths of earthquakes refers to a single-layer horizontal anisotropy for the most part of the study area. However, back azimuthal variations of splitting parameters beneath most of northerly located seismic stations (e.g., GELI, SMTH etc.) imply the presence of a double-layer anisotropy. To evaluate this, we performed various synthetic tests especially beneath the northern part of study region. Yet, it still remains controversial issue due to the large azimuthal gap and thus requires further modelling which may involve the use of joint data sets.&lt;/p&gt;

Northwest U.S. crustal seismic anisotropy suggests crustal flow driven by vertical loads in the underlying mantle

2020 ◽  
Author(s):  
Eugene Humphreys ◽  
Jorge Castellanos ◽  
Robert Clayton ◽  
Jonathan Perry-Houts ◽  
YoungHee Kim ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Anisotropy Field ◽  
Azimuthal Anisotropy ◽  
Mantle Flow ◽  
Crustal Anisotropy ◽  
Vertical Loading ◽  
Close Proximity ◽  
Mantle Anisotropy ◽  
Tectonic Forces

&lt;p&gt;Azimuthal anisotropy in the NW U.S. crust is derived using 3-17 s Rayleigh waves derived using ambient noise from about 300 broadband stations. Velocity is resolved between all station pairs in close proximity, and velocity as a function of azimuth is determined for each station. Azimuthal anisotropy orientations point strongly toward tomographically-imaged high-velocity structures in the underlying mantle, but show no relation to the underlying mantle anisotropy field. We suggest that the crustal anisotropy is decoupled from lateral tectonic forces and is created by upper mantle vertical loading, which in turn generates lateral pressure gradients that drive channelized flow in the ductile mid and lower crust. This idea is tested with geodynamic modeling. Using reasonable values for crustal viscosity and mantle buoyancy structure, we find that the local buoyancy sources within the upper mantle will drive the viscous crustal flow in a manner that reproduces well the imaged crustal anisotropy. We conclude that mantle vertical loading, acting independently from mantle flow, can actively control crustal deformation on a scale of several hundred kilometers.&lt;/p&gt;

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