scholarly journals Serpentinization, Deformation, and Seismic Anisotropy in the Subduction Mantle Wedge

2020 ◽  
Vol 21 (4) ◽  
Author(s):  
Charis Horn ◽  
Pierre Bouilhol ◽  
Philip Skemer
Keyword(s):  
Mantle Wedge ◽  
Seismic Anisotropy

scholarly journals Overriding plate, mantle wedge, slab, and subslab contributions to seismic anisotropy beneath the northern Central Andean Plateau

2016 ◽  
Vol 17 (7) ◽  
pp. 2556-2575 ◽  
Author(s):  
Maureen D. Long ◽  
C. Berk Biryol ◽  
Caroline M. Eakin ◽  
Susan L. Beck ◽  
Lara S. Wagner ◽  
...  
Keyword(s):  
Mantle Wedge ◽  
Seismic Anisotropy

Experimental investigation of antigorite dehydration fabrics at high pressure and high temperature

2020 ◽  
Author(s):  
Junfeng Zhang ◽  
Wenlong Liu ◽  
Yongfeng Wang
Keyword(s):  
Mantle Wedge ◽  
Seismic Anisotropy ◽  
Coarse Grained ◽  
Anisotropic Fluid ◽  
Plate Boundaries ◽  
Intermediate Depth ◽  
Fine Grained ◽  
Serpentinized Peridotite ◽  
Type C ◽  
Convergent Plate Boundaries

<p>Antigorite dehydration is well known as a key process in convergent boundaries for the genesis of mantle wedge partial melting and intermediate-depth earthquakes. However, the crystallographic preferred orientations (CPOs) of prograde minerals from antigorite dehydration and its effects on seismic anisotropy of subducting slabs remain ambiguous and controversial. Here we report hydrostatic dehydration experiments on foliated serpentinized peridotite at pressures of 0.3-6 GPa and temperatures of 700-900 °C. Our results show that the orientations of prograde olivine inherit orientations from adjacent olivine grains in the olivine-rich layer by epitaxial growth. In contrast, olivine CPOs evolved with the grain size from fiber-[001] featuring clear [100] point maxima and [001] girdles for fine-grained olivine to orthorhombic patterns characterized by clear [100] and [001] point maxima for coarse-grained olivine, i.e., type-C CPO. We propose that the fine-grained fiber-[001] CPO is developed by topotactic growth at the onset of dehydration, while the orthorhombic type-C CPO for the coarse-grained olivine, especially the [001] point maximum along the lineation, is mainly developed by anisotropic growth resulting from anisotropic fluid flow during the dehydration. The developed olivine type-C CPO in the antigorite-rich layer after antigorite dehydration could explain the trench or strike parallel seismic anisotropy observed at convergent plate boundaries.</p>

scholarly journals Rock seismic anisotropy of the low-velocity zone beneath the volcanic front in the mantle wedge

2009 ◽  
Vol 36 (12) ◽  
Author(s):  
Katsuyoshi Michibayashi ◽  
Tatsuya Oohara ◽  
Takako Satsukawa ◽  
Satoko Ishimaru ◽  
Shoji Arai ◽  
...  
Keyword(s):  
Mantle Wedge ◽  
Seismic Anisotropy ◽  
Volcanic Front ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
Velocity Zone

scholarly journals Deep Anisotropic Structure under the Central Volcanic Region, New Zealand

2021 ◽  
Author(s):  
◽  
Sonja Melanie Greve
Keyword(s):  
Subduction Zone ◽  
Shear Wave ◽  
Large Scale ◽  
Mantle Wedge ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Small Scale ◽  
Mantle Flow ◽  
Delay Times ◽  
Wave Splitting

<p>Seismic anisotropy across the Hikurangi subduction zone measured from shear-wave splitting exhibits strong lateral changes over distances of about 250 km. Teleseismic S-phases show trench-parallel fast polarisations with increasing delay times across the forearc and arc region. In the arc region, delay times reach up to 4.5 s, one of the largest delay times measured in the world. Such large delay times suggest strong anisotropy or long travel paths through the anisotropic regions. Delay times decrease systematically in the backarc region. In contrast, local S-phases exhibit a distinct change from trench-parallel fast orientations in the forearc to rench-perpendicular in the backarc, with average delay times of 0.35 s. In the far backarc, no apparent anisotropy is observed for teleseismic S-phases. The three different anisotropic regions across the subduction zone are interpreted by distinct anisotropic domains at depth: 1) In the forearc region, the observed "average" anisotropy (about 4%) is attributed to trench-parallel mantle flow below the slab with possible contributions fromanisotropy in the slab. 2) In the arc region, high (up to 10%) frequency dependent anisotropy in the mantle wedge, ascribed to melt, together with the sub-slab anisotropy add up to cause the observed high delay times. 3) In the far backarc region, the mantle wedge dynamic ends. The apparent isotropy must be caused by different dynamics, e.g. vertical mantle flow or small-scale convection, possibly induced by convective removal of thickened lithosphere. The proposed hypothesis is tested using anisotropicwave propagation in two-dimensional finite difference models. Large-scale models of the subduction zone (hundreds of kilometres) incorporating the proposed anisotropic domains of the initial interpretation result in synthetic shear-wave splittingmeasurements that closely resemble all large-scale features of real data observations across the central North Island. The preferred model constrains the high (10%) anisotropy to the mantle wedge down to about 100 kmunder the CVR, bound to the west by an isotropic region under the western North Island; the slab is isotropic and the subslab region has average (3.5%) anisotropy, down to 300 km. This model succeeds in reproducing the constant splitting parameters in the forearc region, the strong lateral changes across the CVR and the apparent isotropy in the far backarc region, as well as the backazimuthal variations. The influence of melt on seismic anisotropy is examined with different small-scale (tens of kilometres) analytical modelling approaches calculating anisotropy due to melt occurring in inclusions, cracks or bands. Conclusions are kept conservative with the intention not to over-interpret the data due to model complexities. The models show that seismic anisotropy strongly depends on the scale of inclusions and wavelengths. Frequency dependent anisotropy for local and teleseismic shear-waves, e.g. for frequency ranges of 0.01-1Hz can be observed for aligned inclusions on the order of tens of meters. To test the proposed frequency dependence in the recorded data, two different approaches are introduced. Delay times exhibit a general trend of -3 s/Hz. A more detailed analysis is difficult due to the restricted frequency content of the data. Future studies with intermediate frequency waves (such as regional S-phases) are needed to further investigate the cause of the discrepancy between local and teleseismic shear-wave splitting. An additional preliminary study of travel time residuals identifies a characteristic pattern across central North Island. Interpretation highlights the method as a valuable extension of the shear-wave splitting study and suggests a more detailed examination to be conducted in future.</p>

scholarly journals Microstructural Evolution of Amphibole Peridotites in Åheim, Norway, and the Implications for Seismic Anisotropy in the Mantle Wedge

Minerals ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 345 ◽  
Author(s):  
Sejin Jung ◽  
Haemyeong Jung ◽  
Håkon Austrheim
Keyword(s):  
Mantle Wedge ◽  
Seismic Anisotropy ◽  
Microstructural Analysis ◽  
Dislocation Creep ◽  
Hydrous Minerals ◽  
Fine Grained ◽  
Lattice Preferred Orientation ◽  
Initial Stage ◽  
Dominant Deformation Mechanism ◽  
Rich Condition

The microstructure of amphibole peridotites from Åheim, Norway were analyzed to understand the evolution of the lattice-preferred orientation (LPO) of olivine throughout the Scandian Orogeny and its implication for the seismic anisotropy of the subduction zone. The Åheim peridotites had a porphyroclastic texture and some samples contained an abundant amount of hydrous minerals such as tremolite. Detailed microstructural analysis on the Åheim peridotites revealed multiple stages of deformation. The coarse grains showed an A-type LPO of olivine, which can be interpreted as the initial stage of deformation. The spinel-bearing samples showed a mixture of B-type and C-type LPOs of olivine, which is considered to represent the deformation under water-rich conditions. The recrystallized fine-grained olivine displays a B-type LPO, which can be interpreted as the final stage of deformation. Microstructures and water content of olivine indicate that the dominant deformation mechanism of olivine showing a B-type LPO is a dislocation creep under water-rich condition. The observation of the B-type LPO of olivine is important for an interpretation of trench-parallel seismic anisotropy in the mantle wedge. The calculated seismic anisotropy of the tremolite showed that tremolite can contribute to the trench-parallel seismic anisotropy in the mantle wedge.

scholarly journals Deep Anisotropic Structure under the Central Volcanic Region, New Zealand

2021 ◽  
Author(s):  
◽  
Sonja Melanie Greve
Keyword(s):  
Subduction Zone ◽  
Shear Wave ◽  
Large Scale ◽  
Mantle Wedge ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Small Scale ◽  
Mantle Flow ◽  
Delay Times ◽  
Wave Splitting

<p>Seismic anisotropy across the Hikurangi subduction zone measured from shear-wave splitting exhibits strong lateral changes over distances of about 250 km. Teleseismic S-phases show trench-parallel fast polarisations with increasing delay times across the forearc and arc region. In the arc region, delay times reach up to 4.5 s, one of the largest delay times measured in the world. Such large delay times suggest strong anisotropy or long travel paths through the anisotropic regions. Delay times decrease systematically in the backarc region. In contrast, local S-phases exhibit a distinct change from trench-parallel fast orientations in the forearc to rench-perpendicular in the backarc, with average delay times of 0.35 s. In the far backarc, no apparent anisotropy is observed for teleseismic S-phases. The three different anisotropic regions across the subduction zone are interpreted by distinct anisotropic domains at depth: 1) In the forearc region, the observed "average" anisotropy (about 4%) is attributed to trench-parallel mantle flow below the slab with possible contributions fromanisotropy in the slab. 2) In the arc region, high (up to 10%) frequency dependent anisotropy in the mantle wedge, ascribed to melt, together with the sub-slab anisotropy add up to cause the observed high delay times. 3) In the far backarc region, the mantle wedge dynamic ends. The apparent isotropy must be caused by different dynamics, e.g. vertical mantle flow or small-scale convection, possibly induced by convective removal of thickened lithosphere. The proposed hypothesis is tested using anisotropicwave propagation in two-dimensional finite difference models. Large-scale models of the subduction zone (hundreds of kilometres) incorporating the proposed anisotropic domains of the initial interpretation result in synthetic shear-wave splittingmeasurements that closely resemble all large-scale features of real data observations across the central North Island. The preferred model constrains the high (10%) anisotropy to the mantle wedge down to about 100 kmunder the CVR, bound to the west by an isotropic region under the western North Island; the slab is isotropic and the subslab region has average (3.5%) anisotropy, down to 300 km. This model succeeds in reproducing the constant splitting parameters in the forearc region, the strong lateral changes across the CVR and the apparent isotropy in the far backarc region, as well as the backazimuthal variations. The influence of melt on seismic anisotropy is examined with different small-scale (tens of kilometres) analytical modelling approaches calculating anisotropy due to melt occurring in inclusions, cracks or bands. Conclusions are kept conservative with the intention not to over-interpret the data due to model complexities. The models show that seismic anisotropy strongly depends on the scale of inclusions and wavelengths. Frequency dependent anisotropy for local and teleseismic shear-waves, e.g. for frequency ranges of 0.01-1Hz can be observed for aligned inclusions on the order of tens of meters. To test the proposed frequency dependence in the recorded data, two different approaches are introduced. Delay times exhibit a general trend of -3 s/Hz. A more detailed analysis is difficult due to the restricted frequency content of the data. Future studies with intermediate frequency waves (such as regional S-phases) are needed to further investigate the cause of the discrepancy between local and teleseismic shear-wave splitting. An additional preliminary study of travel time residuals identifies a characteristic pattern across central North Island. Interpretation highlights the method as a valuable extension of the shear-wave splitting study and suggests a more detailed examination to be conducted in future.</p>

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