Seismic Anisotropy of the Uppermost Mantle beneath Mid-ocean Ridges

Nature ◽  
1968 ◽  
Vol 218 (5141) ◽  
pp. 558-559 ◽  
Author(s):  
R. B. WHITMARSH
Keyword(s):  
Seismic Anisotropy ◽  
Uppermost Mantle ◽  
Ocean Ridges

scholarly journals Magmatic channelisation by reactive and shear-driven instabilities at mid-ocean ridges: a combined analysis

2021 ◽  
Author(s):  
D W Rees Jones ◽  
H Zhang ◽  
R F Katz
Keyword(s):  
Seismic Anisotropy ◽  
Linear Instability ◽  
Linear Growth Rate ◽  
Reactive Flow ◽  
High Porosity ◽  
Ridge Axis ◽  
Feedback Mechanisms ◽  
Solid Flow ◽  
Melt Extraction ◽  
Ocean Ridges

Summary It is generally accepted that melt extraction from the mantle at mid-ocean ridges is concentrated in narrow regions of elevated melt fraction called channels. Two feedback mechanisms have been proposed to explain why these channels grow by linear instability: shear flow of partially molten mantle and reactive flow of the ascending magma. These two mechanisms have been studied extensively, in isolation from each other, through theory and laboratory experiments as well as field and geophysical observations. Here, we develop a consistent theory that accounts for both proposed mechanisms and allows us to weigh their relative contributions. We show that interaction of the two feedback mechanisms is insignificant and that the total linear growth rate of channels is well-approximated by summing their independent growth rates. Furthermore, we explain how their competition is governed by the orientation of channels with respect to gravity and mantle shear. By itself, analysis of the reaction-infiltration instability predicts the formation of tube-shaped channels. We show that with the addition of even a small amount of extension in the horizontal, the combined instability favours tabular channels, consistent with the observed morphology of dunite bodies in ophiolites. We apply the new theory to mid-ocean ridges by calculating the accumulated growth and rotation of channels along streamlines of the solid flow. We show that reactive flow is the dominant instability mechanism deep beneath the ridge axis, where the most unstable orientation of high-porosity channels is sub-vertical. Channels are then rotated by the solid flow away from the vertical. The contribution of the shear-driven instability is confined to the margins of the melting region. Within the limitations of our study, the shear-driven feedback does not appear to be responsible for significant melt focusing or for the shallowly dipping seismic anisotropy that has been obtained by seismic inversions.

Seismic Anisotropy of the Uppermost Mantle under Oceans

Nature ◽  
1964 ◽  
Vol 203 (4945) ◽  
pp. 629-631 ◽  
Author(s):  
H. H. HESS
Keyword(s):  
Seismic Anisotropy ◽  
Uppermost Mantle

Seismic anisotropy of the uppermost mantle beneath the Rio Grande rift: Evidence from Kilbourne Hole peridotite xenoliths, New Mexico

2011 ◽  
Vol 311 (1-2) ◽  
pp. 172-181 ◽  
Author(s):  
Takako Satsukawa ◽  
Katsuyoshi Michibayashi ◽  
Elizabeth Y. Anthony ◽  
Robert J. Stern ◽  
Stephen S. Gao ◽  
...  
Keyword(s):  
New Mexico ◽  
Seismic Anisotropy ◽  
Rio Grande ◽  
Rio Grande Rift ◽  
Uppermost Mantle ◽  
Peridotite Xenoliths

Crust-Mantle Kinematics in and around the Hellenic Arc Elucidated by Local Shear Wave Splitting and Receiver Function Analyses

2021 ◽  
Author(s):  
Derya Keleş ◽  
Tuna Eken ◽  
Judith M. Confal ◽  
Tuncay Taymaz
Keyword(s):  
Seismic Anisotropy ◽  
Receiver Function ◽  
Receiver Functions ◽  
Data Sets ◽  
S Wave ◽  
Uppermost Mantle ◽  
Hellenic Arc ◽  
Wave Splitting ◽  
Harmonic Decomposition ◽  
Local Shear

<p>The fundamental knowledge on seismic anisotropy inferred from various data sets can enhance our understanding of its vertical resolution that is critical for a better interpretation of past and current dynamics and resultant crustal and mantle kinematics in the Hellenic Trench and its hinterland. To investigate the nature of deformation zones, we perform both local S-wave splitting (SWS) measurements and receiver functions (RFs) analysis. Our preliminary findings from the harmonic decomposition technique performed on radial and tangential RFs suggest relatively more substantial anisotropic signals in the lower crust and uppermost mantle with respect to upper and middle crustal structure in the region. Apparent anisotropic orientations obtained from RFs harmonic decomposition process show several consistencies with those discovered from local SWS measurements at selected stations. The actual anisotropic orientation for the structures, however, requires further modelling of the receiver functions obtained.</p>

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 (<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 (>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.

scholarly journals Seismic anisotropy in the uppermost mantle, back-arc region of the northeast Japan arc: Petrophysical analyses of Ichinomegata peridotite xenoliths

2006 ◽  
Vol 33 (10) ◽  
pp. n/a-n/a ◽  
Author(s):  
Katsuyoshi Michibayashi ◽  
Natsue Abe ◽  
Atsushi Okamoto ◽  
Takako Satsukawa ◽  
Kenta Michikura
Keyword(s):  
Seismic Anisotropy ◽  
Uppermost Mantle ◽  
Peridotite Xenoliths ◽  
Northeast Japan Arc ◽  
Back Arc

scholarly journals Three-dimensional seismic anisotropy in the crust and uppermost mantle beneath the Taiwan area revealed by passive source tomography

2015 ◽  
Vol 120 (11) ◽  
pp. 7814-7829 ◽  
Author(s):  
Ivan Koulakov ◽  
Andrey Jakovlev ◽  
Yih-Min Wu ◽  
Nikolay L. Dobretsov ◽  
Sami El Khrepy ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Three Dimensional ◽  
Uppermost Mantle ◽  
Taiwan Area

Estimating bend-faulting and mantle hydration at the Marianas trench from seismic anisotropy

2021 ◽  
Author(s):  
Hannah Mark ◽  
Douglas Wiens ◽  
Daniel Lizarralde
Keyword(s):  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Seismic Refraction ◽  
Deep Ocean ◽  
Oceanic Lithosphere ◽  
Spatial Variations ◽  
Uppermost Mantle ◽  
Hydrous Minerals ◽  
Seismic Refraction Data ◽  
Mantle Velocity

<p><span>Bend faults formed in oceanic lithosphere approaching deep ocean trenches promote water circulation and the formation of hydrous minerals. As the plate subducts, these minerals can dehydrate into the mantle wedge, generating the melts that feed arc volcanoes, or subduct fully into the deeper mantle. Balancing the global water budget requires an estimate of the amount of water recycled to the mantle by subduction, but current estimates for water fluxes at subduction zones span several orders of magnitude, mainly because of large uncertainties in the amount of water carried in the lithospheric mantle of the incoming plate. </span></p><p><span>We use active source seismic refraction data collected on the incoming plate at the Marianas trench to measure azimuthal seismic anisotropy in the uppermost mantle, and assess the degree of faulting and associated serpentinization of the uppermost mantle based on spatial variations in the observed anisotropy. We find that the fast direction of anisotropy varies with distance from the trench, rotating from APM-parallel at the eastern side of the study area to approximately fault-parallel near the trench. The fast direction orientations suggest that a coherent set of bend-faults are beginning to form at least 200 km out from the trench, although the extrinsic anisotropy signal from the faults does not substantially overprint the signal from preexisting mineral fabrics until the plate is ~100 km from the trench. The average (isotropic) mantle velocity decreases slightly as the plate nears the trench. Preliminary interpretation suggests that the observed spatial variations in anisotropy can be explained by serpentinization localized along pervasive, trench-parallel faults or joints.</span></p>

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