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Ultrahigh-pressure (UHP) eclogites often show strong plastic deformation and anisotropy of seismic properties. We report in
this paper the seismic velocity and anisotropy of eclogite calculated from the crystallographic preferred orientations (CPOs)
of constituent minerals (garnet, omphacite, quartz and rutile) and single crystal elastic properties...
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... are the best approximation to the chemical compo- sitions of garnet and omphacite in the experimentally deformed eclogites (GB244 & GB320) and the naturally deformed eclogites (D95-21 & MB98-08). The elastic constants are estimated to be the average of the elastic constants of their end members, assuming that the sec- ond-order elastic constants C ij are related linearly to the molar fraction of constituent end-members (Table 2) [34] . The seismic properties of garnet single crystal show that the directions of fast P-wave velocity (9.15 km/s) corre- spond to the <100> directions of garnet while the direc- tions of slow P-wave velocity (9.00 km/s) are the <111> directions (Figure 2a). ...
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Citations
... Amphibole exists in subducting slabs under lower-pressure and lower-temperature conditions than eclogite stability conditions, and in the middle to lower crust [33], whereas the LPO development of amphibole is known to produce a strong seismic anisotropy [18,[34][35][36]. Therefore, deformed retrograded eclogites may represent seismic velocity and anisotropy in relatively shallow subducting slabs [5,6,20,31,[37][38][39][40][41][42][43][44][45][46][47][48][49]. Measuring the LPOs of omphacite, amphibole, and garnet in retrograded eclogites can therefore be useful in understanding the seismic velocity and anisotropy of subducting slabs in subduction zones. ...
Various rock phases, including those in subducting slabs, impact seismic anisotropy in subduction zones. The seismic velocity and anisotropy of rocks are strongly affected by the lattice-preferred orientation (LPO) of minerals; this was measured in retrograded eclogites from Xitieshan, northwest China, to understand the seismic velocity, anisotropy, and seismic reflectance of the upper part of the subducting slab. For omphacite, an S-type LPO was observed in three samples. For amphibole, the <001> axes were aligned subparallel to the lineation, and the (010) poles were aligned subnormal to foliation. The LPOs of amphibole and omphacite were similar in most samples. The misorientation angle between amphibole and neighboring omphacite was small, and a lack of intracrystalline deformation features was observed in the amphibole. This indicates that the LPO of amphibole was formed by the topotactic growth of amphibole during retrogression of eclogites. The P-wave anisotropy of amphibole in retrograded eclogites was large (approximately 3.7–7.3%). The seismic properties of retrograded eclogites and amphibole were similar, indicating that the seismic properties of retrograded eclogites are strongly affected by the amphibole LPO. The contact boundary between serpentinized peridotites and retrograded eclogites showed a high reflection coefficient, indicating that a reflected seismic wave can be easily detected at this boundary.
... AVs = 0.60%), which is similar to other dry (i.e. mostly Grt and Omp) eclogite (e.g., Abalos et al., 2011;Bascou et al., 2001;Kim et al., 2018;Llana-Funez and Brown, 2012;Sun et al., 2012a;Wang et al., 2009;Wang et al., 2005b;Zhang et al., 2008). The second least anisotropic sample is P1502-5 (AVp = 2.45%, max. ...
Crystal preferred orientation (CPO) of high pressure metamorphic rocks is an important contributor to seismic anisotropy in the subduction interface and continental crust. However, the mechanisms of deformation and CPO development, as well as the effect of CPO patterns on the seismic anisotropies remain not fully solved. By analyzing the microstructures, CPOs and seismic properties of the high pressure rocks from the Central Qiangtang metamorphic belt in the northern Tibetan Plateau, we found that the mineral CPOs (e.g., omphacite and amphibole) are developed dominantly by non-dislocation based deformation mechanisms in the eclogites and amphibolites, while quartz and phengite CPOs are primarily formed by dislocation creep in the garnet-quartz-mica schists. The variations of CPO patterns are mainly controlled by the strain geometry and/or strain magnitude, which also regulate the first-order symmetry patterns of seismic anisotropies among samples. Besides, amphibole enrichment can remarkably increase the anisotropies and decrease the velocities of seismic waves in the eclogite‒amphibolite suite. A combination of seismic parameters such as Vp, Vs, Vp/Vs ratio, intensities and symmetries of P-wave anisotropy (AVp) and shear wave splitting (AVs), and P- or S-wave reflection coefficients can be used to discriminate eclogites, amphibolites and their garnet-quartz-mica schists country rocks at depth when high resolution seismic methods can be applied. Notably, amphibolite and garnet-quartz-mica schists showing moderate-to-large AVp and max. AVs can make considerable contributions to field-observed crustal anisotropy near the strike-slip continental shear zones or mantle wedge anisotropy in the subduction zones.
... Investigating the seismic properties (i.e., P-and S-wave velocities and their anisotropies) of eclogite is crucial for constraining the presence of eclogite in the deep crust and upper mantle, which has profound implications for interpreting the composition, density, and thermal and mechanical structures of the subducted crust, continental lithosphere, and upper mantle, as well as for reconstructing the geodynamic evolutions of the subduction and collision zones [9][10][11][12][13][14][15][16][17][18][19]. For this purpose, numerous previous studies have addressed the seismic properties of dry eclogite (i.e., bi-mineralic eclogite), e.g., [20][21][22][23][24][25][26][27]; retrograded eclogite (i.e., amphibolized eclogite), e.g., [16,19,23,[28][29][30][31]; epidote/glaucophane eclogite [12,14,28]; lawsonite eclogite [13,32]. However, because of the sample rarity, the seismic properties of the olivine and orthopyroxene rich eclogite have not been studied yet. ...
Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth’s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flemsøya in the Nordøyane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0–2.5%, max. AVs = 0.6–2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist. Furthermore, we found that amphibole CPOs were the main contributors to the higher seismic anisotropies in some amphibole-rich samples. The average seismic velocities of Flem eclogite were greatly affected by the relative volume proportions of omphacite and amphibole. The Vp (8.00–8.33 km/s) and Vs (4.55–4.72 km/s) were remarkably larger than the hydrous-phase-bearing eclogite, blueschist, and gabbro, but lower than dry eclogite and peridotite. The Vp/Vs ratio was almost constant (avg. ≈ 1.765) among Flem eclogite, slightly larger than olivine-free dry eclogite, but similar to peridotite, indicating that an abundance of olivine is the source of their high Vp/Vs ratios. The Vp/Vs ratios of Flem eclogite were also higher than other (non-)retrograded eclogite and significantly lower than those of gabbro. The seismic features derived from the Flem eclogite can thus be used to distinguish olivine-rich eclogite from other common rock types (especially gabbro) in the deep continental crust or subduction channel when high-resolution seismic wave data are available.
... The single-crystal seismic anisotropy of garnet (< 1%) is weaker than that of omphacite (up to ~20% for P-wave seismic anisotropy and ~15% for S-wave seismic anisotropy: Bascou et al., 2001). The seismic anisotropy of eclogite is generally reported to be less than ~5%, which can be ascribed predominantly to the properties http://www.springer.com/journal/12303 of omphacite ( Bascou et al., 2001Bascou et al., , 2002Zhang et al., 2008;Ábalos et al., 2011;Cao et al., 2013;Worthington et al., 2013). Here we report the microstructures of a pristine eclogite that contains alternating garnet-and omphacite-rich layers from northern Victoria Land (NVL) in Antarctica. ...
We examined the microfabrics of omphacite and garnet in foliated eclogite to determine the influence of the layered structure on seismic observations in subduction zone. The analyzed eclogite, from the Lanterman Range, northern Victoria Land, Antarctica, is characterized by layering in which the modal abundances of garnet and omphacite vary. For garnet, the low aspect ratios, similar angular distribution of long axes relative to the foliation in both layers, uniform grain size distribution, near-random crystallographic preferred orientations (CPOs), and misorientation angle distributions are indicative of passive behavior during deformation. In contrast, omphacite shows relatively high aspect ratios, a low angle between the long axes of crystals and the foliation, a wide grain-size distribution, and distinctive CPOs, suggesting dislocation creep as the main deformation mechanism. The results of fabric analyses are consistent with strain localization into omphacite or omphacite-rich layers rather than garnet or garnet- rich layers. The single-crystal seismic anisotropy of garnet is very weak (AVP = 0.2%, AVS = 0.5–0.6%), whereas that of omphacite is much stronger (AVP = 3.7–5.9% and AVS = 2.9–3.8%). Seismic anisotropy of the omphacite-rich layers shows an increase of 329% for AVP and 146% for AVS relative to the garnet-rich layers. Our results demonstrate the importance of the layered structure in strain localization and in the development of the seismic anisotropies of subducting oceanic crust.
... [3] Previous studies on seismic properties of blueschist and eclogite mainly focused on the seismic velocities and anisotropies of these rocks, and most of these investigations were concentrated on medium-and high-temperature eclogites, which are simply composed of a bi-mineralic assemblage of garnet and omphacite [e.g., Abalos et al., 2011;Bascou et al., 2001;Christensen and Mooney, 1995;Fountain et al., 1994;Kern et al., 1999Kern et al., , 2002Llana-Fúnez and Brown, 2012;Mauler et al., 2000;Rudnick and Fountain, 1995;Sun et al., 2012;Wang et al., 2005Wang et al., , 2009Warner et al., 1996;Zhang et al., 2008]; only a few recent studies focused on blueschist [Bezacier et al., 2010;Fujimoto et al., 2010;Kim et al., 2013;Mookherjee and Bezacier, 2012]. Based on these studies, some general conclusions have been reached: (1) eclogite commonly shows very high P-wave (8.0-8.6 km/s) and S-wave velocities (4.5-4.9 km/s) comparable to those of mantle peridotite [Rudnick and Fountain, 1995], which are significantly higher than those of blueschist (V p = 7.3-7.6 ...
... We used the elastic stiffnesses from Bezacier et al. [2010] for glaucophane, Ryzhova et al. [1966] for epidote, Vaughan and Guggenheim [1986] for phengite, and McSkimin et al. [1965] for quartz. To obtain the appropriate single-crystal elastic properties for omphacite (Di 0.5 ,Jd 0.4 ,Ae 0.1 ) and garnet (Grs 0.25 ,Pyr 0.20 ,(Alm,Spss) 0.55 in eclogite) (Grs 0.35 ,Pyr 0.05 , (Alm,Spss) 0.60 in blueschist), an average of the elastic stiffness of the end-members was used, assuming that the elastic stiffnesses C ij were linearly related to the molar fraction of constituent end-members [Collins and Brown, 1998;Zhang et al., 2008]. The elastic constants for end-members of omphacite and garnet were from Aleksandrov et al. [1964]; Bass [1989]; Kandelin and Weidner [1988]; Levien et al. [1979]; O'Neill et al. [1991]; Wang and Simmons [1974]. ...
... [32] The patterns of P-wave anisotropy in omphacite aggregate resemble those of glaucophane and epidote, which show the fastest and slowest P-wave velocities parallel to the lineation and normal to the foliation, respectively (Figures 11 and S6). These results are consistent with those of most previous calculations and laboratory observations of deformed eclogites [Abalos et al., 2011;Bascou et al., 2001;Mauler et al., 2000;Zhang et al., 2008]. The patterns of S-wave polarization anisotropy (AVs) are more complicated, and the polarization direction of the fast shear wave is mostly sub-parallel to lineation for a foliation-normal incident seismic ray, except for the sample 08BSY-38. ...
[1] The potential seismological contributions of metamorphosed and deformed oceanic crust in a subduction zone environment were studied in a detailed petro-fabric analysis of blueschist and eclogite in the North Qilian suture zone, NW China. The calculated whole-rock seismic properties based on the measured lattice preferred orientations of the constituting minerals show increasing P-wave and S-wave velocities and decreasing seismic anisotropies from blueschist to eclogite, mainly due to the decreasing volume proportion and deformation extent of glaucophane. The low velocity of the upper layer in the subducting oceanic crust can be explained by the existence of blueschist and foliated eclogite, which induces a 3–12% reduction in velocity compared to that induced by the surrounding mantle rocks. This low-velocity layer may gradually disappear when blueschist and foliated eclogite are replaced by massive eclogite at a depth in excess of 60–75 km for the paleo North Qilian subduction zone. Trench-parallel seismic anisotropy with a moderate delay time (0.1–0.3 s) can only effectively contribute to deformed blueschist and eclogite in a high-angle (>45–60°) subducting slab, regardless of the direction of slab movement. The calculated reflection coefficients (Rc = 0.04–0.20) at the lithologic interfaces between eclogite and blueschist imply that it may be possible to detect eclogite bodies in shallow subduction channels using high-resolution seismic reflection profiles. However, the imaging of eclogite bodies located in deep subduction zones could be challenging.
... They provide a unique opportunity for studying the seismic anisotropy of subduction zone under HP/UHP conditions. Great efforts and progress have been made on documenting the seismic properties of eclogites from the continental collision zones in the past (e.g., Zhang et al., 2008;Wang et al., 2005a, b;Ji et al., 2003;Kern et al., 2002;Bascou et al., 2001). However, there are rare studies integrating both results from laboratory measurements and theoretical calculations Zhang et al., 2008). ...
... Great efforts and progress have been made on documenting the seismic properties of eclogites from the continental collision zones in the past (e.g., Zhang et al., 2008;Wang et al., 2005a, b;Ji et al., 2003;Kern et al., 2002;Bascou et al., 2001). However, there are rare studies integrating both results from laboratory measurements and theoretical calculations Zhang et al., 2008). Combination of laboratory measurement and theoretical calculation of seismic properties will enhance our understanding of the earth's seismic anisotropy and provide reasonable interpretation of field observations. ...
We report here lattice preferred orientations (LPOs) and seismic properties of eclogites from the Sulu (苏鲁) UHP terrane. Our
results show strong fabrics in omphacite and amphibole, and approximately random fabrics in garnet with or without strong
shape preferred orientations (SPOs). Dislocation creep is likely to be responsible for the observed omphacite fabrics that
vary with geometry and orientation of finite strain ellipsoid. Weak garnet LPOs suggest that garnet did not accommodate plastic
strain or was not deformed by dislocation creep with a dominant slip system. The calculated seismic properties of eclogites
and their component minerals show a strong correlation with their LPOs. Seismic anisotropies are mostly induced by omphacite
component in fresh eclogites and by amphibole component in retrograded eclogites, respectively. Retrogression of omphacite
to amphibole and quartz will increase seismic anisotropies but decreases seismic velocities of eclogite. Garnet component
increases the seismic velocities but decreases seismic anisotropies of eclogite. Comparison of the calculated and the measured
seismic properties of eclogites suggests that both methods resolve comparable results with some discrepancies. Compositional
layering can play a very important role in determining the seismic properties of eclogites in addition to LPO.
Key Wordseclogite-LPO-retrogression-seismic property-deformation mechanism
Rheology of rocks controls the deformation of the Earth at various space-time scales, which is crucial to understand the tectonic evolution of continental lithosphere. Researches of rock rheology are mainly conducted via high-pressure and high-temperature rheological experiments and multi-scale observations and measurements of naturally deformed rocks. At present, a large amount of data from such kinds of studies have been accumulated. This paper first provides an up-to-date comprehensive review of the rheological mechanisms, fabric types and seismic properties of the main rock-forming minerals at different depths of continental lithosphere, including olivine, orthopyroxene, clinopyroxene, amphibole, plagioclase, quartz and mica. Then, progress in high-pressure and high-temperature experiments and natural deformation observations is introduced, mainly regarding the rheological strength and behavior, seismic velocity and anisotropy of lithospheric mantle peridotite, eclogite, mafic granulite, amphibolite and felsic rocks. Finally, by taking the Tibetan Plateau as an example, the application of rock rheology for quantitative interpretation of seismic anisotropy data is discussed. The combination of mineral deformation fabrics and seismic anisotropy is expected to make an important breakthrough in understanding the rheological properties and structure of continental lithosphere.
The crust within collisional orogens is very heterogeneous both in
composition and grade of deformation, leading to highly variable physical
properties at small scales. This causes difficulties for seismic
investigations of tectonic structures at depth since the diverse and
partially strong upper crustal anisotropy might overprint the signal of
deeper anisotropic structures in the mantle. In this study, we characterize
the range of elastic anisotropies of deformed crustal rocks in the Alps.
Furthermore, we model average elastic anisotropies of these rocks and their
changes with increasing depth due to the closure of microcracks. For that,
pre-Alpine upper crustal rocks of the Adula Nappe in the central Alps, which
were intensely deformed during the Alpine orogeny, were sampled. The two
major rock types found are orthogneisses and paragneisses; however, small
lenses of metabasites and marbles also occur. Crystallographic preferred
orientations (CPOs) and volume fractions of minerals in the samples were
measured using time-of-flight neutron diffraction. Combined with single
crystal elastic anisotropies these were used to model seismic properties of
the rocks. The sample set shows a wide range of different seismic velocity
patterns even within the same lithology, due to the microstructural
heterogeneity of the deformed crustal rocks. To approximate an average for
these crustal units, we picked common CPO types of rock forming minerals
within gneiss samples representing the most common lithology. These data
were used to determine an average elastic anisotropy of a typical crustal
rock within the Alps. Average mineral volume percentages within the gneiss
samples were used for the calculation. In addition, ultrasonic anisotropy
measurements of the samples at increasing confining pressures were
performed. These measurements as well as the microcrack patterns determined
in thin sections were used to model the closure of microcracks in the
average sample at increasing depth. Microcracks are closed at approximately
740 MPa yielding average elastic anisotropies of 4 % for the average
gneiss. This value is an approximation, which can be used for seismic models
at a lithospheric scale. At a crustal or smaller scale, however, local
variations in lithology and deformation as displayed by the range of elastic
anisotropies within the sample set need to be considered. In addition,
larger-scale structural anisotropies such as layering, intrusions and brittle faults have to be included in any crustal-scale seismic model.
Subduction and exhumation are key processes in the formation of orogenic systems across the world, for example, in the European Alps. For geophysical investigations of these orogens, it is essential to understand the petrophysical properties of the rocks involved. These are the result of a complex interaction of mineral composition and rock fabric including mineral textures (i.e., crystallographic preferred orientations). In this study we present texture-derived elastic anisotropy data for a representative set of different lithologies involved in the Alpine orogeny. Rock samples were collected in the Lago di Cignana area in Valtournenche, in the Italian northwestern Alps. At this locality a wide range of units of continental and oceanic origin with varying paleogeographic affiliations and tectono-metamorphic histories are accessible. Their mineral textures were determined by time-of-flight neutron diffraction. From these data the elastic properties of the samples were calculated. The data set includes representative lithologies from a subduction-exhumation setting. In subducted lithologies originating from the oceanic crust, the P-wave anisotropies (AVPs [%]) range from 1.4 % to 3.7 % with average P-wave velocities of 7.20–8.24 km/s and VP / VS ratios of 1.70–1.75. In the metasediments of the former accretionary prism the AVPs range from 3.7 % to 7.1 %, average P-wave velocities are 6.66–7.23 km/s and VP / VS ratios are 1.61–1.76. Continental crust which is incorporated in the collisional orogen shows AVP ranging from 1.4 % to 2.1 % with average P-wave velocities of 6.52–6.62 km/s and VP / VS ratios of 1.56–1.60. Our results suggest that mafic and felsic rocks in subduction zones at depth may be discriminated by a combination of seismic signatures: lower anisotropy and higher VP / VS ratio for mafic rocks, and higher anisotropy and lower VP / VS ratio for felsic rocks and metasediments.
The upper crust within collisional orogens is very heterogeneous both in composition and grade of deformation, leading to very variable physical properties at small scales. This yields difficulties for seismic investigations of tectonic structures at depth since local changes in elastic anisotropy cannot be detected. In this study, we show elastic anisotropies of the range of typical lithologies within deformed upper crustal rocks in the Alps. Furthermore, we aim to model average elastic anisotropies for these rocks and their changes with increasing depth due to the closure of microcracks. We therefore sampled rocks in the Adula Nappe of the central Alps, which is typical for upper crust in collisional orogens. The two major rock types found are orthogneisses and paragneisses, however, small lenses of metabasites and marbles also occur. Crystallographic preferred orientations (CPOs) and volume fractions of minerals in the samples were measured using time-of-flight neutron diffraction. Combined with single crystal elastic anisotropies these were used to model seismic properties of the rocks. The sample set shows a wide range of different seismic velocity patterns even within the same lithology, due to the heterogeneity of deformed upper crust. To approximate an average for these upper crustal units, we picked common CPO types of rock forming minerals within the gneiss samples, which represent the most common lithology. These data were used to determine an average elastic anisotropy of a typical upper crustal rock within the Alps. Average mineral volume percentages within the gneiss samples were used for the calculation. In addition, ultrasonic measurements of elastic anisotropies of the samples at increasing pressures were performed. These measurements, as well as the microcrack pattern determined in thin sections of the samples were used to model the closure of microcracks in the average sample at increasing depth. At ≈740 MPa microcracks are assumed to be closed yielding average elastic anisotropies of 4 % for the average gneiss. This value is an approximation, which can be helpful for seismic models at a lithospheric scale. At a crustal or smaller scale, however, it is an oversimplification and local lithological as well as deformational changes shown by the range of elastic anisotropies within the sample set have to be considered.
