seismic anisotropies of the songshugou peridotites...

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Tectonophysics

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Seismic anisotropies of the Songshugou peridotites (Qinling orogen, centralChina) and their seismic implications

Yi Caoa,b,⁎, Haemyeong Junga,⁎, Shuguang Songb

a Tectonophysics Laboratory, School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of KoreabMOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China

A R T I C L E I N F O

Keywords:Seismic anisotropyOlivineCrystal preferred orientationPeridotiteForearc mantleQinling orogen

A B S T R A C T

Though extensively studied, the roles of olivine crystal preferred orientations (CPOs or fabrics) in affecting theseismic anisotropies in the Earth's upper mantle are rather complicated and still not fully known. In this study,we attempted to address this issue by analyzing the seismic anisotropies [e.g., P-wave anisotropy (AVp), S-wavepolarization anisotropy (AVs), radial anisotropy (ξ), and Rayleigh wave anisotropy (G)] of the Songshugouperidotites (dunite dominated) in the Qinling orogen in central China, based on our previously reported olivineCPOs. The seismic anisotropy patterns of olivine aggregates in our studied samples are well consistent with theprediction for their olivine CPO types; and the magnitude of seismic anisotropies shows a striking positivecorrelation with equilibrium pressure and temperature (P–T) conditions. Significant reductions of seismic ani-sotropies (AVp, max. AVs, and G) are observed in porphyroclastic dunite compared to coarse- and fine-graineddunites, as the results of olivine CPO transition (from A-/D-type in coarse-grained dunite, through AG-type-likein porphyroclastic dunite, to B-type-like in fine-grained dunite) and strength variation (weakening: A-/D-type →AG-type-like; strengthening: AG-type-like → B-type-like) during dynamic recrystallization. The transition ofolivine CPOs from A-/D-type to B-/AG-type-like in the forearc mantle may weaken the seismic anisotropies anddeviate the fast velocity direction and the fast S-wave polarization direction from trench-perpendicular totrench-oblique direction with the cooling and aging of forearc mantle. Depending on the size and distribution ofthe peridotite body such as the Songshugou peridotites, B- and AG-type-like olivine CPOs can be an additional(despite minor) local contributor to the orogen-parallel fast velocity direction and fast shear-wave polarizationdirection in the orogenic crust such as in the Songshugou area in Qinling orogen.

1. Introduction

It has long been recognized that the mantle flow and seismic ani-sotropy in the Earth's interior are intrinsically linked by crystal pre-ferred orientations (CPOs or fabrics) of mantle-forming minerals, suchas olivine and pyroxenes (e.g., Hess, 1964; Jung, 2017; Karato et al.,2008; Mainprice, 2007; Montagner and Guillot, 2002; Nicolas andChristensen, 1987; Savage, 1999). Olivine, as the most abundant mi-neral in the Earth's upper mantle, plays a dominant role. However, thediversity of olivine CPO types and their strain- or time-dependentevolution characteristics (i.e. CPO patterns change with strain or time)dramatically increase the complexity on interpreting the flow pattern ofthe mantle (e.g., flow direction and geometry) based on the observedseismic anisotropy (e.g., azimuthal and polarization anisotropies)(Karato et al., 2008; Skemer and Hansen, 2016).

Over the past two decades, six major types of olivine CPOs werefound by high P–T deformation experiments: A-type or [100](010)

pattern (Jung and Karato, 2001; Zhang and Karato, 1995), B-type or[001](010) pattern and C-type or [001](100) pattern (Jung and Karato,2001; Jung et al., 2006; Katayama and Karato, 2006), D-type or axial[100] pattern (Bystricky et al., 2000; Hansen et al., 2014; Zhang et al.,2000), E-type or [100](001) pattern (Katayama et al., 2004), and AG-type or axial [010] pattern CPOs (Holtzman et al., 2003; Mainprice,2007). These olivine CPOs are characterized by distinctive orientationsand symmetries of three principal crystallographic axes (i.e. [100],[010] and [001] axes) in the kinematic framework (experimental fra-mework: shear plane and shear direction; natural framework: foliationand lineation). Because the [100] and [010] axes are the fastest(Vp = 9.77 km/s at ambient condition) and slowest (Vp = 7.65 km/s atambient condition) directions in a single olivine crystal, respectively,their orientations and symmetries in an aggregate dictate the patternsof seismic anisotropy (Michibayashi et al., 2016; Nicolas andChristensen, 1987). For example, in the case of shear-wave splitting inan olivine aggregate—when a polarized S-wave enters an anisotropic

https://doi.org/10.1016/j.tecto.2017.11.017Received 22 August 2017; Received in revised form 2 November 2017; Accepted 14 November 2017

⁎ Corresponding author.E-mail addresses: [email protected] (Y. Cao), [email protected] (H. Jung).

Tectonophysics 722 (2018) 432–446

Available online 20 November 20170040-1951/ © 2017 Elsevier B.V. All rights reserved.

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medium such as olivine and its aggregates, it splits into two perpen-dicularly polarized S-waves with different velocities (fast one namedVs1 and slow one named Vs2)—the Vs1 tends to polarize along fast [100]axes and deviating from slow [010] axes. This characteristic results in astrong splitting of S-wave and large delay time (a path-dependentparameter that describes the arrival time difference between Vs1 andVs2 received at a seismic station) when the seismic wave travels sub-parallel to the [001] axes in an anisotropic olivine dominant medium.Applying an olivine CPO, the 3D pattern of seismic anisotropy can bepredicted and placed into a kinematic framework.

Given a mantle flow pattern at a tectonic setting, the obtained oli-vine CPOs—acquired from either conceptual or numerical CPO evolu-tion modeling or a natural sample—predict a seismic anisotropy patternthat can be tested by comparing it with actual seismological observa-tions [i.e. forward approach, see Skemer and Hansen, 2016]. Con-versely, combining the seismological observations with inferred (e.g.,outcomes from experiment or microphysical modeling) or observedolivine CPOs at a given tectonic setting, one can deduce the historical orcurrent patterns of mantle flow in the studied region [i.e. inverse ap-proach, see Skemer and Hansen, 2016]. The olivine CPO is pivotal in

bridging mantle flow patterns and seismological observations in bothforward and inverse methods. Among all input sources of olivine CPOs,the naturally observed olivine CPO bears irreplaceable merit, because itoffers the most direct insights in the actual CPO composition and de-formation pattern in the studied mantle region. However, one must becautious that the sporadic outcrops and biased sampling of mantleperidotites on the Earth's surface may sometimes restrict the inter-pretation of mantle flow and seismological observation at a large spatialscale. To minimize this uncertainty, extensive sampling and analyzingof olivine CPOs in peridotite outcrops would be helpful.

In this paper, the forward approach is employed to explore the rolesof natural olivine CPOs on affecting the observed seismic anisotropy.The mineral CPOs (olivine, pyroxene and amphibole) used in this paperwere mostly taken from our previous study (Cao et al., 2017), in whichthe characteristics and geneses of various olivine CPOs were describedand discussed. Since Songshugou peridotite body was originally afragment of forearc mantle and is currently situated in the Qinlingorogenic belt in central China (Cao et al., 2016), the calculated seismicanisotropies from mineral CPOs provide us an attempt to constrain theirroles in the forearc mantle and Qinling orogenic belt.

115°

30°30°

40°

40°

95°

95° 115°

Yangtze Craton

North China Craton

Sulu

Qinling-Dabie

East Kunlun

Tarim Craton

Xi'an

Beijing

500 km

Qaidam

Block

North

Qilia

n

Xining

Songshugou

Qilia

n B

lock

(a)

(b)

Fushui

2 km

N

Fushui Complex

So

ng

sh

ug

ou u

ltram

afic m

assif

Shangnan

Qinling

Complex

Garnet-

pyroxenite

Gabbro-diorite

complex

Amphibolite

/gneiss

Ultramafic

rocks

Shangdan

Fault

(c)

E109°55'

N33°33'

N33°38'

E109°55'

E110°00'

E110°00'

N33°33'

(b)

Songsh

ugou p

erid

otite b

ody

A

B(c)

200 mN

Harzburgite

Olivine clinopyroxenite

Banded/podiform

chromitite

Coarse-grained dunite

Fine-grained &

porphyroclastic dunite

(c)

avg.~70

~200-240

Fig. 1. (a) Geological map showing the major tectonic units in China. The location of the Songshugou region is indicated by the red box [modified after Song et al., 2013]. (b) Geologicalmap showing the major structural and lithological units in the Songshugou region. A nearly NS profile “A–B” is marked in a red line segment and shown in Fig. 13. (c) Geological mapshowing the spatial distribution of various ultramafic rocks in the study area, which is indicated by the red box in (b) [modified after Su et al., 2005]. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of this article.)

Y. Cao et al. Tectonophysics 722 (2018) 432–446

433

Fig. 2. Optical photomicrographs showing the microstructures of six types of dunite and harzburgite (XZ-plane thin sections, cross-polarized light). Yellow dashed line denotes thefoliation defined by the SPO or layering of olivine grains. White dashed outline delineates the shapes and orientations of spinel crystals. (a) A coarse-grained dunite shows sub-perpendicular SPOs of olivine and spinel crystals (i.e. anomalous foliation; Group 1, 08S-32). (b) A coarse-grained dunite shows consistent SPOs of olivine and spinel crystals (i.e. normalfoliation; Group 2, S03-2A). (c) A coarse-grained harzburgite presents sub-parallel SPOs of olivine, orthopyroxene, and spinel grains (i.e. normal foliation; Group 3, S03-2B). (d) A coarse-grained dunite showing and two thin spinel-rich layers aligning parallel to the orientated olivine crystals (i.e. normal foliation; Group 4, SSG-2). Inset is an enlarged area (white rectangle)displaying the textures of largely elongated olivine crystals. (e) A porphyroclastic dunite with normal foliation displays a large and elongated olivine porphyroclast with subgrainboundaries aligning oblique to foliation (upper left) and a fresh fine-grained polygonal olivine matrix showing mosaic textures (dominantly 120° triple junctions) (Group 5, SSG-1). (f) Aconspicuous normal foliation is defined by the layering of fine-grained olivine and oriented needle-shaped amphibole in fine-grained dunite (Group 6, S91-03). All figures are modifiedafter Cao et al. (2017). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


434

2. Tectonic and deformation histories of Songshugou peridotites

The Qinling orogen is a giant NWW-SEE-trending orogenic belt lo-cated in central China. It is formed by multiple and prolonged

subduction-accretion-collision events that were ultimately finalized bythe continental collision between the North China Craton to the northand the Yangtze Craton (or South China Craton) to the south (Fig. 1a)[more detailed tectonic maps and introductions can be referred to therecent review by Dong and Santosh, 2016]. The Songshugou peridotitebody belongs to the North Qinling belts in the eastern part of theQinling orogenic belt. It is a large fault-bounded, lentoid, NW-SE-trending block embedded in a suite of meta-mafic rocks (e.g., amphi-bolite, garnet amphibolite, garnet pyroxenite, and high-pressure gran-ulite) in the core region of the Qinling Complex, which is considered asa Meso-Neoproterozoic crustal fragment (Bader et al., 2013b) (Fig. 1b).This peridotite body is emplaced during the early-Paleozoic(~480–515 Ma), based on recent metamorphic U–Pb ages of zircon andmonazite in garnet amphibolite and granulite country rocks (Baderet al., 2013a; Liu et al., 2009).

Based on mineral and whole-rock textures and chemistries, as wellas multiphase inclusions in olivine and spinel, Cao et al. (2016) pro-posed that the Songshugou peridotite body represents a fragment of thelithospheric mantle that was originally situated in a warm forearc re-gion (i.e. juvenile subduction zone), where it experienced high-degreepartial melting (F > 25%), generation of boninite, and high-T(> 1100 °C) boninitic melt–rock reactions. Subsequently, owing tocontinuous cooling, the Songshugou peridotite body underwent threestages of low-T (~600–1100 °C) fluid–rock reactions that produced Ti-rich spinel, Cr-chlorite, and amphiboles, and reflects a tectonic

Table 1Abbreviations of rock types.

Cao et al. (2017) G-cubed This study

Acronym Full name Short name

SP-CGD-AF Spinel-poor coarse-grained dunite withanomalous foliation

→ Group 1

SP-CGD-NF Spinel-poor coarse-grained dunite with normalfoliation

→ Group 2

SP-CGH-NF Spinel-poor coarse-grained harzburgite withnormal foliation

→ Group 3

SR-CGD-NF Spinel-rich coarse-grained dunite with normalfoliation

→ Group 4

SP-PCD-NF Spinel-poor porphyroclastic dunite with normalfoliation

→ Group 5

SP-FGD-NF Spinel-poor fine-grained dunite with normalfoliation

→ Group 6

– Spinel-poor olivine-clinopyroxenite with normalfoliationa

→ Group 7

Letters in bold correspond to the acronym.a Olivine clinopyroxenite was not acronymed in Cao et al. (2017).

Fig. 3. CPOs of olivine in dunite and harzburgite. Pole figures are presented on equal-area lower hemisphere plots and are contoured with a half width of 15° and cluster size of 5°. Thewhite solid lines in the pole figure of [100] axes denote the authentic foliation that is defined by the consistent SPOs or layering of olivine and spinel. The white solid and dashed lines in(a, b) Group 1 in the pole figure of [100] axes indicate the authentic and false foliations that are defined by SPOs or layering of spinel and olivine, respectively. X, Y and Z denote thelineation, direction perpendicular to lineation and parallel to foliation, and foliation-normal direction, respectively. N: number of grains, M: M-index, and J: J-index. J-index is calculatedusing a ‘de la Vallée Poussin’ kernel with a half-width of 10°. This figure is modified after Cao et al. (2017), in which detailed descriptions and interpretations about these olivine CPOs areavailable.


435

evolution through a cooling forearc mantle (i.e. mature subductionzone) to a continental crust accompanying its ultimate exhumation intoa collisional orogenic belt. In a follow-up study, Cao et al. (2017) re-cognized and classified two major types of olivine CPOs, being high-T(at low stress) and low-T (at high stress) CPOs, in the dunites andharzburgites, which are produced by ductile deformation associatedwith the above high-T and low-T geological/tectonic events, respec-tively.

3. Rock types and olivine CPOs

The Songshugou peridotite body (and the studied samples, see Fig. 2and Table 1) can be lithologically and texturally grouped into spinel-poor coarse-grained dunite (CGD) with anomalous foliation (Fig. 2a,Group 1, i.e. inconsistent foliation defined by the shape preferred or-ientations (SPOs) between olivine and spinel) or normal foliation(Fig. 2b, Group 2, i.e. consistent foliation defined by the SPOs of olivineand spinel), spinel-poor coarse-grained harzburgite (CGH) with normalfoliation (Fig. 2c, Group 3), spinel-rich coarse-grained dunite (CGD)with normal foliation (Fig. 2d, Group 4), spinel-poor porphyroclasticdunite (PCD) (Fig. 2e, Group 5) and fine-grained dunite (FGD) (Fig. 2f,Group 6) with normal foliation, olivine clinopyroxenite with normalfoliation (Group 7), and banded/podiform chromitite (Cao et al., 2017;Song et al., 1998; Su et al., 2005). The porphyroclastic and fine-graineddunite (Groups 5 and 6) are the dominant rock types that occupy ~85%of the massif area, whereas coarse-grained dunite and harzburgite(Groups 1–4) take ~10% of the massif area (Fig. 1c). Olivine clin-opyroxenite (Group 7) takes a minor portion (~5%) and chromititeonly sporadically occurs (Fig. 1c). In the field, the porphyroclastic andfine-grained dunite (Groups 5 and 6) display distinct and continuous

steep foliation (trending subparallel to the strike of massif with anaverage dip angle of ~70°) and lineation (plunging sub-perpendicularto the strike of massif in the range of ~200° − 240°) (Fig. 1c, the de-tailed distribution of foliation and lineation in the whole peridotitebody is not mapped at present) as the result of dominant top-to-SWshear sense during exhumation (Song et al., 1998). The foliation andlineation are weak in coarse-grained dunite and harzburgite (Groups1–4) and olivine clinopyroxenite, and their spatial orientations are thusnot known in the field.

Since the olivine CPOs in dunite Groups 1–6 were reported in ourprevious work, we here briefly summarize the key points of these re-sults. For details about the approach to determine structural framework(i.e. foliation and lineation) in the hand specimen and the electronbackscatter diffraction (EBSD) method that was used for analyzingmineral CPOs, as well as naming and characteristics of olivine CPOtypes and their formation mechanisms (e.g., M- and J-index, see alsoFig. 3), the readers are referred to Cao et al. (2015) and Cao et al.(2017). As shown in Fig. 3 and the Fig. 5 in Cao et al. (2017), we ob-served a stronger A-/D-type CPO in Group 1 (Fig. 3a and b), a weak C-type-like CPO in Group 2 (Fig. 3c), a random CPO in Group 3 (Fig. 3d),a stronger B-type-like CPO in Group 4 (Fig. 3e and f), a weak AG-type-like CPO in Group 5 (Fig. 3g and h) and a weak B-type-like CPO inGroup 6 (Fig. 3i and j). The olivine CPOs in Groups 1–4 belong to low-stress (at high T) CPOs (> 1100 °C,< 8–11 MPa), whereas the olivineCPOs in Groups 5 and 6 are high-stress (at low T) CPOs (~800–1100 °C,~24–30 MPa) (Cao et al., 2017). Additional CPOs of amphibole andorthopyroxene for Group 3 (S03-2B) and of olivine (weak AG-type-like),clinopyroxene and amphibole for Group 7 (S91-32) are given in Figs. S1and S2.

4. Seismic anisotropies

4.1. Method

Because dunite is the dominant rock type (~90 vol%, Fig. 1c) inSongshugou peridotite body and olivine is the dominant phase(> 90 vol%) in the dunite, the seismic anisotropies of Songshugouperidotite body can be approximated by dunite that is represented bymonomineralic olivine aggregate. To calculate the seismic anisotropy ofolivine aggregates, CPO, elastic stiffness tensor< Cij > and density ofthe olivine aggregate are needed. The elastic stiffness tensor< Cij > ofthe aggregate can be obtained by integrating the olivine elastic stiffnesstensor (Cij) over the individual orientations of all olivine grains (i.e.olivine CPO), assuming an averaging scheme (e.g., Vogit, Reuss, VRHand Geometric). The density of the olivine aggregate is the same as thatof a single olivine crystal (ρol). The< Cij > and ρol are inserted intothe Christoffel equation to solve for propagation-direction-dependentseismic velocity triplets (i.e. Vp, Vs1, and Vs2) in an olivine aggregate ina full 3D space. In this study, the calculation was accomplished usingthe petro-physical software originally developed by Mainprice (1990)[http://www.gm.univ-montp2.fr/PERSO/mainprice/W_data/CareWare_Unicef_Programs/]. The variations in the calculated seismicanisotropies depend largely on the single-crystal elastic stiffness tensor(Cij) and the averaging scheme.

To understand the influence of different P–T conditions, we calcu-lated the seismic anisotropies of the olivine aggregate at three differentP–T conditions [ambient (room P and T), 1.5 GPa and 850 °C, and1.5 GPa and 1200 °C] using a constant VRH (or Hill) averaging scheme.The ambient condition roughly represents the shallow crustal depthwhere the Songshugou peridotite body is currently dwelling, whereasthe latter two conditions approximately correspond to the stages atwhich low-T and high-T CPOs are formed, respectively (Cao et al.,2017; Cao et al., 2016). To achieve this calculation, we first calculatedthe elastic stiffness tensors of a single-crystal olivine at high P–T con-ditions using the equation provided by Mainprice (2007):

Table 2Elastic stiffness tensor (Cij) (unit in GPa) of olivine single crystal at various conditions.

i\j 1 2 3 4 5 6

Ambient conditiona

1 320.5 68.2 71.6 0 0 02 196.5 76.8 0 0 03 233.5 0 0 04 64.0 0 05 77.0 06 78.7density 3.355 g/cmb

1.5 GPa & 850 °Cc

1 293.8 65.4 69.8 0 0 02 179.9 77.2 0 0 03 213.8 0 0 04 56.0 0 05 90.9 06 68.6Densityb 3.211 g/cmb

1.5 GPa & 1200 °Cc

1 284.7 61.7 66.7 0 0 02 169.4 75.2 0 0 03 201.9 0 0 04 51.6 0 05 95.6 06 63.2Densityb 3.140 g/cmb

a The elastic stiffness of olivine at ambient condition is taken from Abramson et al.(1997).

b The density of olivine at high PT conditions are calculated using the Excel spread-sheet provided by Hacker and Abers (2004). An olivine composition of Fo# of 90 isemployed.

c The elastic stiffness of olivine at high PT conditions are calculated using the equationprovided by Mainprice (2007), in which the reference elastic stiffness (ambient condi-tion), first- and second-order pressure derivatives are taken from Abramson et al. (1997)and first-order temperature derivative is from Isaak (1992).


436

http://www.gm.univ-montp2.fr/PERSO/mainprice/W_data/CareWare_Unicef_Programs

http://www.gm.univ-montp2.fr/PERSO/mainprice/W_data/CareWare_Unicef_Programs

= + ⋅ + +

⋅ + ⋅ ⋅

C P T C P T dC dP ΔP d C dP dC dT

ΔT d C dPdT ΔP ΔT

( , ) ( , ) ( / ) 12

( / ) ( / )

( / )

ij ij ij ij ij

ij

0 02 2

2 (1)

where Cij(P0,T0) is the reference tensor of elastic stiffness (e.g., ambientconditions), dCij/dP,dCij/dT,d2Cij/dP2and d2Cij/dPdTare first- and/orsecond-order pressure and temperature derivatives. Values of theseparameters for olivine are taken from Abramson et al. (1997) and Isaak(1992). The densities of olivine (Fo# = 90) at high P–T conditions arecalculated using the Excel spreadsheet provided by Hacker and Abers(2004). The calculated stiffness tensors (Cij) and densities of a singleolivine crystal at various conditions are shown in Table 2.

The P-wave anisotropy (AVp) is expressed as:

= × − +V V V V VA 200 ( )/( )p p p p pmax min max min (2)

where Vpmax and Vpmin are the highest and lowest P-wave velocities,respectively.

The P-wave anisotropy in foliation plane [Ap(F)] is simply definedas:

= × − +F V X V Y V X V YA ( ) 200 | ( ) ( )|/( ( ) ( ))p p p p p (3)

in which Vp(X) and Vp(Y) are the P-wave velocities along X and Y di-rections in the foliation plane (F).

The magnitude of shear-wave splitting (AVs) is expressed as:

= × − +V V V V VA 200 ( )/( )s s s s s1 2 1 2 (4)

where Vs1 and Vs2 are the velocities of fast and slow S-waves propa-gating along a direction, respectively. To calculate the radial anisotropy

(ξ) and Rayleigh wave azimuthal anisotropy (G), foliation is alignedhorizontal by convention. The radial anisotropy (ξ) is expressed as:

= =ξ V V N L( / ) /SH SV2 (5)

where VSH and VSV are the velocities of S-wave that are polarizedhorizontally and vertically, respectively. The values of ξ > 1 andξ < 1 indicate positive and negative radial anisotropies, respectively.The Rayleigh wave azimuthal anisotropy (G) is defined as:

= × − +G V V V V200 ( )/( )SVmax SVmin SVmax SVmin (6)

wherein VSV (max. and min.) are dependent on the azimuth (θ) alongwhich S-wave propagates:

= + +ρV F G θ G θcos 2 sin 2C SSV2 (7)

The parameters F, N and L (constant term) and GC and GS (2θ term)are the linear combinations of elastic coefficients Cij given by Crampin(1984) and Montagner and Nataf (1986).

4.2. Results

The P-wave anisotropy (column 1), shear wave splitting (column 2),and the polarization direction of Vs1 (Vs1 Pol. Dir., column 3) are pre-sented in Figs. 4–6. Though at different P–T conditions, the patterns ofseismic anisotropies for each sample are similar (Figs. 4–6). In allsamples, the Vpmax and the Vs1 Pol. Dir. always tend to align subparallelto the maximum of [100] axes in the pole figures (Figs. 4–6). For theA-/D-type CPO in Group 1, the Vpmax and Vs1 Pol. Dir. are alignedsubparallel to the lineation (Figs. 4a,b, 5a,b and 6a,b), whereas the

Vp Contours (km/s) AVs Contours (%) Vs1 polarization Planes

(a) 08S-32

Group 1

A-/D-type

(b) S03-2C

Group 1

A-/D-type

(c) S03-2A

Group 2

C-type-like

(d) S03-2B

Group 3

random

(e) S95-31

Group 4

B-type-like

(f) SSG-2

Group 4

B-type-like

Ambient

condition

Max. = 8.77 Min. = 8.11

Anisotropy = 7.7%

Max. = 5.34 Min. = 0.22

Max. = 8.69 Min. = 8.11

Anisotropy = 6.9%

Max. = 4.86 Min. = 0.10

Max. = 8.56 Min. = 8.21

Anisotropy = 4.3%

Max. = 3.27 Min. = 0.08

Max. = 8.41 Min. = 8.25

Anisotropy = 1.9%

Max. = 1.72 Min. = 0.04

Max.= 8.81 Min. = 8.02

Anisotropy = 9.5%

Max. = 6.48 Min. = 0.19

Max. = 8.62 Min. = 8.18

Anisotropy = 5.2%

Max. = 4.20 Min. = 0.04

Max. = 8.52 Min. = 8.18

Anisotropy = 4.1%

Max. = 2.52 Min. = 0.04

(g) S91-26

Group 5

AG-type-like

Max. = 8.49 Min. = 8.15

Anisotropy = 4.2%

Max. = 2.52 Min. = 0.00

(h) SSG-1

Group 5

AG-type-like

Max. = 8.64 Min. = 8.11

Anisotropy = 6.3%

Max. = 4.71 Min. = 0.12

(i) S91-03

Group 6

B-type-like

Max. = 8.55 Min. = 8.18

Anisotropy = 4.4%

Max. = 3.14 Min. = 0.02

(j) S91-05

Group 6

B-type-like


8.32

8.48

8.64

1.0

2.0

3.0

4.0

5.0

5.34

0.22

8.24

8.40

8.56

1.5

2.5

3.5

4.5

4.86

0.10

8.35

8.45

8.55

1.0

1.5

2.0

2.5

3.0

3.27

0.08

8.28

8.32

8.36

8.40

.40

.80

1.20

1.60

1.72

0.04

8.24

8.40

8.56

8.72

2.0

3.0

4.0

5.0

6.0

6.48

0.19

8.30

8.40

8.50

8.60

1.0

2.0

3.0

4.0

4.20

0.04

8.30

8.40

8.50

.50

1.00

1.50

2.00

2.50

2.52

0.04

8.25

8.35

8.45

.50

1.00

1.50

2.00

2.50

2.52

0.00

8.24

8.32

8.40

8.48

1.5

2.5

3.5

4.5

4.71

0.12

8.30

8.40

8.50

1.0

1.5

2.0

2.5

3.0

3.14

0.02

X

Z

Y

X

Z

Y

Fig. 4. Seismic anisotropies of olivine aggregates in dunite and harzburgite at ambient condition. The solid bold lines in the pole figures denote the authentic foliation, whereas thedashed bold lines in pole figures (a, b) indicate the false foliation. X, Y and Z denote the lineation, direction perpendicular to lineation and parallel to foliation, and foliation-normaldirection, respectively. Data were presented in the lower-hemisphere using an equal-area projection.


437

Vpmax and Vs1 Pol. Dir. (when S-wave travels subparallel to XZ plane)tend to align sub-perpendicular to lineation and at low-to-moderateangle to foliation for B-type-like olivine CPOs in Group 4 (Figs. 4e,f, 5e,fand 6e,f) and Group 6 (Figs. 4i,j and 5i,j). For PCD that shows an AG-type-like CPO, the Vpmax and Vs1 Pol. Dir. align at a moderate angle tolineation and subparallel to foliation (Figs. 4g,h and 5g,h). In contrast,the C-type-like olivine CPO in Group 2 presents Vpmax and Vs1 Pol. Dir.at high angles to foliation (Figs. 4c, 5c and 6c). The random CPO inGroup 3 is characterized by a weak Vpmax at a moderate angle to bothlineation and foliation (Figs. 4d, 5d and 6d).

The relationships between the patterns of seismic anisotropies andCPOs of olivine aggregates can also be depicted using Vp–Flinn diagram(Ji et al., 2015; Michibayashi et al., 2016; Sun et al., 2016). Except fortwo Group 4 samples (yellow circles), all samples are well located in theregions for their distinctive olivine CPOs (Fig. 7 and Table 4). The shiftof Group 4 to lower Vp(X)/Vp(Y) ratios than the typical value of B-typeCPO (θ= 28°) is likely attributed to a higher concentration of [010]axes (slowest direction) and a lower concentration of [100] and [001]axes approaching the lineation (cf. Figs. 3e,f and 2i,j), which jointlyreduce Vp(X) and the Vp(X)/Vp(Y) ratio. Although weak in P-waveanisotropy, the plot of Group 3 indicates a weak C-type affinity despiteits nearly random CPO (grey circle). The Groups 1, 2 and 4 exhibitsoverall higher P-wave and S-wave polarization anisotropies thanGroups 5 and 6 (Figs. 7 and 8; Table 3). The Group 3 presents the lowestAVp (~2%) and max. AVs (~2%), whereas the highest AVp (~10%)and max. AVs (~6–10%) are found in one Group 4 sample (S95-31)

(Fig. 8 and Table 3). The Ap(F) is also overall higher in Groups 1, 2 and4 compared to Groups 3, 5 and 6 (Fig. 10 and Table 4). The weak Ap(F)in Group 5 is considered to result from their weak CPO strengths andAG-type-like CPOs which show largely transverse isotropy of elasticity.

The radial anisotropies are negative [ξ < 1, VSV > VSH or Vp(Z) > Vp(X) and Vp(Y)] in Groups 2 and 3 (Fig. 9 and Table 3),agreeing with their C-type-like signatures in these samples (Fig. 7). Incontrast, positive radial anisotropies [ξ > 1, VSV < VSH or Vp(Z) < Vp(X) or Vp(Y)] are observed in Groups 1, 4, 5 and 6 (Fig. 9 andTable 3), which are consistent with their A/D-type, B- and AG-type-likeCPOs (Fig. 7). For samples showing positive radial anisotropy, themagnitudes of anisotropy (value of ξ) do not vary significantly betweendifferent rock types (Fig. 9). In a similar manner to P-wave and S-wavepolarization anisotropies, except for one Group 4 sample, Rayleighwave azimuthal anisotropy (G) is overall greater in Groups 1 and 4(~4–7%) compared to Group 5 (~0.5–2%) and 6 (~2–3.5%) (Fig. 9and Table 3). Because both Ap(F) and G represent the azimuthal ani-sotropies of seismic waves travelling on the horizontal foliation plane asassumed for calculation, a strong positive correlation between Ap(F)and G is observed (R2 = 0.88, Fig. 10).

The influences of different P–T conditions are mainly manifested inthe magnitudes of seismic anisotropies (AVp, max. AVs, ξ and G). Asshown in Fig. 8 and Table 3, both AVp and max. AVs increase sig-nificantly with increasing P–T conditions. The tie lines connectingvarious P–T conditions for each sample display slopes larger than 1(Fig. 8), implying that the variation of the magnitude of shear wave


(a) 08S-32

Group 1

A-/D-type

(b) S03-2C

Group 1

A-/D-type

(c) S03-2A

Group 2

C-type-like

(d) S03-2B

Group 3

random

(e) S95-31

Group 4

B-type-like

(f) SSG-2

Group 4

B-type-like

1.5 GPa

850 °C

Max. = 8.69 Min. = 8.00

Anisotropy = 8.2%

8.24

8.40

8.56

Max. = 7.02 Min. = 0.06

3.0

5.0

7.0

7.02

0.06

Max. = 8.61 Min. = 7.99

Anisotropy = 7.4%

8.16

8.32

8.48

Max. = 6.06 Min. = 0.21

2.0

3.0

4.0

5.0

6.0

6.06

0.21

Max. = 8.50 Min. = 8.11

Anisotropy = 4.6%

8.25

8.35

8.45

Max. = 4.18 Min. = 0.08

1.0

2.0

3.0

4.0

4.18

0.08

Max. = 8.32 Min. = 8.16

Anisotropy = 1.9%

8.22

8.26

8.30

Max. = 1.82 Min. = 0.04

.60

1.00

1.40

1.80

1.82

0.04

Max.= 8.72 Min. = 7.89

Anisotropy = 9.9%

8.08

8.24

8.40

8.56

Max. = 8.91 Min. = 0.29

2.0

4.0

6.0

8.0

8.91

0.29

Max. = 8.51 Min. = 8.07

Anisotropy = 5.4%

8.20

8.30

8.40

8.50

Max. = 5.36 Min. = 0.15

1.0

2.0

3.0

4.0

5.0

5.36

0.15

Max. = 8.45 Min. = 8.07

Anisotropy = 4.6%

8.20

8.30

8.40

Max. = 4.01 Min. = 0.21

1.0

2.0

3.0

4.0

4.01

0.21

(g) S91-26

Group 5

AG-type-like

Max. = 8.43 Min. = 8.03

Anisotropy = 4.8%

8.10

8.20

8.30

8.40

Max. = 4.25 Min. = 0.06

1.0

2.0

3.0

4.0

4.25

0.06

(h) SSG-1

Group 5

AG-type-like

Max. = 8.56 Min. = 7.99

Anisotropy = 6.9%

8.16

8.32

8.48

Max. = 6.39 Min. = 0.17

2.0

3.0

4.0

5.0

6.0

6.39

0.17

(i) S91-03

Group 6

B-type-like

Max. = 8.46 Min. = 8.08

Anisotropy = 4.6%

8.15

8.25

8.35

8.45

Max. = 4.33 Min. = 0.06

1.0

2.0

3.0

4.0

4.33

0.06

(j) S91-05

Group 6

B-type-like


X

Z

Y

X

Z

Y

Fig. 5. Seismic anisotropies of olivine aggregates in dunite and harzburgite at 1.5 GPa and 850 °C. The solid bold lines in the pole figures denote the authentic foliation, whereas thedashed bold lines in pole figures (a, b) indicate the false foliation. X, Y and Z denote the lineation, direction perpendicular to lineation and parallel to foliation, and foliation-normaldirection, respectively. Data were presented in the lower-hemisphere using an equal-area projection.


438

splitting is more P–T sensitive compared to P-wave anisotropy. It isestimated that a change in conditions from 1.5 GPa and 1200 °C toambient conditions causes AVs to drop by ~30–40% and AVp by~15–20%. Except for one Group 4 sample (SSG-2) in which G decreaseswith P–T conditions and Group 3 that shows almost constant ξ and Gvalues, the magnitudes of radial anisotropy (|ξ − 1|) and Rayleighwave azimuthal anisotropy also increase with P–T conditions (Fig. 9and Table 3). Unlike AVp and max. AVs (Fig. 8), the slopes of tie linesconnecting various P–T conditions in ξ–G space differ between differentrock types (Fig. 9).

5. Discussion and implication

5.1. Olivine CPO transition, seismic anisotropy variation and seismicimplications in the forearc mantle

As previously pointed out by Cao et al. (2017), the Group 1, whichhas coarser grain size and comparatively stronger A-/D-type olivineCPO, is probably the precursor of finer grain-sized Groups 5 and 6which form through a dynamic recrystallization process and show weakolivine AG- and B-type-like CPOs. In contrast to the observation that the

Vp Contours (km/s)

Max. = 8.62 Min. = 7.89

Anisotropy = 8.9%

8.00

8.16

8.32

8.48

AVs Contours (%)

Max. = 8.17 Min. = 0.15

2.0

4.0

6.0

8.0

8.17

0.15

Vs1 polarization Planes

(a) 08S-32

Group 1

A-/D-type

Max. = 8.54 Min. = 7.88

Anisotropy = 8.0%

8.00

8.16

8.32

8.48

Max. = 6.84 Min. = 0.32

2.0

4.0

6.0

6.84

0.32

(b) S03-2C

Group 1

A-/D-type

Max. = 8.42 Min. = 8.00

Anisotropy = 5.1%

8.10

8.20

8.30

8.40

Max. = 4.75 Min. = 0.04

1.5

2.5

3.5

4.5

4.75

0.04

(c) S03-2A

Group 2

C-type-like

Max. = 8.23 Min. = 8.06

Anisotropy = 2.1%

8.10

8.14

8.18

8.22

Max. = 2.12 Min. = 0.04

.40

.80

1.20

1.60

2.00

2.12

0.04

(d) S03-2B

Group 3

random

Max. = 8.65 Min. = 7.77

Anisotropy = 10.6%

8.00

8.20

8.40

8.60

Max. = 10.32 Min. = 0.08

2.0

4.0

6.0

8.0

10.0

10.32

0.08

(e) S95-31

Group 4

B-type-like

Max. = 8.43 Min. = 7.96

Anisotropy = 5.8%

8.10

8.20

8.30

8.40

Max. = 6.26 Min. = 0.06

2.0

3.0

4.0

5.0

6.0

6.26

0.06

(f) SSG-2

Group 4

B-type-like

1.5 GPa

1200 °C

X

Z

Y

X

Z

Y

Fig. 6. Seismic anisotropies of olivine aggregates in duniteand harzburgite at 1.5 GPa and 1200 °C. The seismic ani-sotropies in Groups 5 and 6 are not shown, because theirolivine CPOs are formed at low T conditions(~800–1100 °C). The solid bold lines in the pole figuresdenote the authentic foliation, whereas the dashed boldlines in pole figures (a, b) indicate the false foliation. X, Yand Z denote the lineation, direction perpendicular tolineation and parallel to foliation, and foliation-normaldirection, respectively. Data were presented in the lower-hemisphere using an equal-area projection.


439

intermediate AG-type CPO presents the highest strength (Ronda peri-dotite, southern Spain) (Précigout and Hirth, 2014), the intermediateGroup 5 shows lower AG-type-like CPO strength than both Groups 1

and 6 (Fig. 3; Cao et al., 2017). Because the intensity of seismic ani-sotropy is largely controlled by CPO strength (Cao and Jung, 2016),Group 5 also presents lower seismic anisotropies (AVp, Ap(F), max. AVs,and G) than both Groups 1 and 6 (Figs. 8–10). These results suggest that(1) the grain boundary sliding process associated with dynamic re-crystallization (Cao et al., 2017) significantly weakened a pre-existingolivine CPO (A-/D-type in Group 1) and reduced the seismic aniso-tropies, and (2) further slip of dislocation formed and strengthened anew olivine CPO (B-type-like in Group 6) that results in an increase inseismic anisotropies.

To model the variation of seismic anisotropies in response to theolivine CPO transition from A-/D-type (CGD, Group 1), through AG-type-like (PCD, Group 5), to B-type-like (FGD, Group 6), we calculatedthe seismic anisotropies of the mixtures of Groups 1 and 6 with varyingGroup 6 fractions. With the increase of Group 6 fraction, the Vpmax andVs1 Pol. Dir. (when S-wave travels sub-perpendicularly to foliation)gradually displace from the lineation-subparallel to lineation-sub-perpendicular direction within the foliation plane (Fig. 11a–k). BothAVp and max. AVs decrease steadily with an increasing fraction ofGroup 6 in a similar way and reach minimum values at ~60–70%Group 6, and increase again with further addition of Group 6 (Fig. 12).By comparing the patterns of seismic anisotropies, i.e. the spatial dis-tributions of max., min., and contours in Vp and AVs pole figures, wefound that the mixtures of Groups 1 and 6 that have 70–90% Group 6(Fig. 11h–j and modeled Group 5 in Fig. 12) display the best approx-imations of seismic anisotropy patterns with the observed Group 5(Fig. 11l and observed Group 5 in Fig. 12). Further comparison on themagnitudes of seismic anisotropies also shows that fractions of~60–90% Group 6 in the mixtures of Groups 1 and 6 are needed toaccount for the intensities of seismic anisotropies that are observed inGroup 5 (SSG-1 and S91-26) (stars in Fig. 12). This fraction of Group 6is coincident with the area percentage of PCD (Group 5) and FGD(Group 6) (~85%) in the field (Fig. 1c), confirming that the CGD(Group 1) may have been dynamically recrystallized to a degree of~70–90%, to produce the PCD (Group 5) and FGD (Group 6) as ob-served.

Our modeling method of seismic anisotropy variations is relied onthe counteracting effect of seismic anisotropies between two differentolivine CPOs. The two different olivine CPOs are linked by a transi-tional AG-type-like CPO due to dynamic recrystallization process,which first weakens the original A-/D-type CPO and later forms andstrengthens a new B-type-like CPO. Therefore, this modeling methodcannot be implemented to the case such as Ronda peridotites, in whichAG-type CPO shows higher strength than A- and B-type CPOs (Précigoutand Hirth, 2014). The reason for their opposite fabric strength patternwith strain is not clear. It may reside in the relatively greater role ofdislocation glide (strengthening CPO) compared to grain boundarysliding (weakening CPO) during dynamic recrystallization (Cao et al.,2017; Précigout and Hirth, 2014) and/or the recrystallization (eradi-cation) of large crystals that are orientated unfavorably for in-tracrystalline slip (Ji and Mainprice, 1990). The modeling of seismicanisotropies of AG-type-like CPO (PCD, Group 5) requires A-/D-type(CGD, Group 1) and B-type-like (FGD, Group 6) endmember CPOs to berepresentative of their own steady states (i.e. relatively constant CPOstrengths in their own categories). This requirement might be met byaveraging A-/D-type and B-type-like endmember CPOs from severalGroups 1 and 6 samples, respectively (Figs. 11 and 12).

The transition of olivine CPOs from low-stress (at high T) A-/D-typeto high-stress (at low T) B-type-like accompanied with cooling mayhave a potential effect on the seismic anisotropies in the forearc mantlewhere Songshugou peridotite body once situated. In the context of 2Dcorner flow, the fast velocity direction (FVD) and Vs1 Pol. Dir. are ex-pected to deviate progressively from trench-perpendicular to trench-oblique direction and the intensities of seismic anisotropies would alsobe reduced when A-/D-type fabric is gradually replaced by B- and AG-type-like fabrics during the cooling and aging of forearc mantle

Group 1 (A-/D-type)

Group 2 (C-type-like)

Group 3 (random)

Group 4 (B-type-like)

Group 5 (AG-type-like)


0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1.06

1.07

0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03 1.04 1.05 1.06

Vp

(X

)/V

p(Y

)

Vp(Y)/Vp(Z)

= −28°(B-type)

= 0°(AG-type)

= 4

5°

= 6

3°(A

-ty

pe)

=9

(°

0D

-typ

e)

= 1

06°(E

-ty

pe

)

= 158°(C-type)

2%

4%

8%

10%

AVp =

12%

Vp(Z)

Vp(Y)

Vp(X)

Fig. 7. Vp–Flinn diagram for different types of olivine CPO (solid lines) and plots of ourstudied samples (colored circles). θ is the Fabric-Index Angle between the horizontal axisand solid lines. The dashed lines are contours of AVp in 2% interval for each olivine CPOtypes. Vp(X), Vp(Y) and Vp(Z) are the P-wave velocities propagating parallel to X, Y and Zdirections (see inset) at ambient conditions, respectively. The background of this figure ismodified after Michibayashi et al. (2016). (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

1

2

3

4

5

6

7

8

9

10

11

1 2 3 4 5 6 7 8 9 10 11

ma

x. A

Vs (

%)

AVp (%)

1.5 GPa & 850 °C

Ambient condition

1.5 GPa & 1200 °C

Group 1 (A-/D-type)


Group 3 (random)




k=1

Fig. 8. P-wave anisotropy (AVp) and max. S-wave polarization anisotropy (max. AVs) ofolivine aggregates in the studied samples at three representative P–T conditions: ambientcondition (circles), 1.5 GPa & 850 °C (squares), and 1.5 GPa & 1200 °C (triangles).


440

(Fig. 12). Besides that, the decreasing temperature (from 1200 °C at1.5 GPa to 850 °C at 1.5 GPa) would further reduce the seismic aniso-tropies (by ~10% AVp and ~15% max. AVs, Fig. 8).

Hitherto, many studies have addressed the olivine CPOs in theforearc region (e.g., Cao et al., 2015; Cordellier et al., 1981; Hariganeet al., 2013; Ji et al., 1994; Jung, 2009; Jung et al., 2014; Kaczmarek

et al., 2015; Kim and Jung, 2015; Mehl et al., 2003; Michibayashi et al.,2007; Mizukami et al., 2004; Palasse et al., 2012; Park and Jung, 2015;Soustelle et al., 2013; Soustelle et al., 2010; Tasaka et al., 2008;Tommasi et al., 2006; Wang et al., 2016). Consistent with the naturalobservations, Cao et al. (2015) recently conceived that the forearc li-thospheric mantle is dominated by A-/D-type CPOs, whereas B-, C- andE-type CPOs mainly occur in the deeper asthenospheric mantle. Theexample of Songshugou peridotites adds more details to the distributionpattern of olivine CPOs by suggesting that the lower forearc litho-spheric mantle may also contain minor B-, C-type and random CPOs dueto the effects of high-T melt during forearc infancy (Cao et al., 2017).Nevertheless, the dominance of A-/D-type and B-/AG-type-like CPOs inSongshugou dunites still agrees with the overall composition of olivineCPOs in forearc region.

The above-discussed seismic implications are based on the seismicproperties of olivine aggregate, because dunite is predominant in theSongshugou peridotite body. However, harzburgite and lesser lherzoliteare also found in the forearc mantle, suggesting that the seismic con-tributions of pyroxenes (especially orthopyroxene) should not be ig-nored. The roles of pyroxenes in affecting the seismic anisotropies ofperidotite mainly depend on the modal proportion and CPO types of

Table 3Seismic anisotropies of olivine aggregates in studied samples.

Sample Rock type Olivine J-indexa Ambient condition 1.5 GPa & 850 °C 1.5 GPa & 1200 °Cb

AVp (%) max. AVs (%) ξ G (%) AVp (%) max. AVs (%) ξ G (%) AVp (%) max. AVs (%) ξ G (%)

08S-32 Group 1 2.30 7.7 5.34 1.043 5.15 8.2 7.02 1.066 6.16 8.9 8.17 1.078 6.85S03-2C Group 1 2.00 6.9 4.86 1.012 3.95 7.4 6.06 1.023 4.77 8.0 6.84 1.027 5.36S03-2A Group 2 1.56 4.3 3.27 0.970 0.65 4.6 4.18 0.960 1.27 5.1 4.75 0.955 1.66S03-2B Group 3 1.41 1.9 1.72 0.988 1.45 1.9 1.82 0.988 1.38 2.1 2.12 0.987 1.44S95–31 Group 4 2.74 9.5 6.48 1.024 5.35 9.9 8.91 1.044 5.90 10.6 10.32 1.053 6.45SSG-2 Group 4 2.25 5.2 4.20 0.999 2.96 5.4 5.36 1.010 2.17 5.8 6.26 1.014 2.06S91–26 Group 5 1.77 4.1 2.52 1.012 0.75 4.6 4.01 1.035 0.93 – –SSG-1 Group 5 1.63 4.2 2.52 1.015 2.00 4.8 4.25 1.040 2.21 – –S91–03 Group 6 2.07 6.3 4.71 1.046 3.06 6.9 6.39 1.077 3.40 – –S91–05 Group 6 1.51 4.4 3.14 1.008 2.01 4.6 4.33 1.018 2.16 – –

a J–index was calculated using a ‘de la Vallée Poussin’ kernel with a half width of 10°. Data from Cao et al. (2016a)b The seismic anisotropies of PCD and FCD at 1.5 GPa and 1200 °C are not shown because of their low deformation temperatures (~800–1100 °C).

Table 4P-wave velocities along principal structural axes with their ratios and anisotropies atambient condition.

Sample Rock type Vp (X)(km/s)a

Vp (Y)(km/s)b

Vp (Z)(km/s)c

Vp(X)/Vp(Y)

Vp(Y)/Vp(Z)

Ap(F)(%)d

08S-32 Group 1 8.70 8.20 8.20 1.06 1.00 5.92S03-2C Group 1 8.56 8.24 8.28 1.04 1.00 3.81S03-2A Group 2 8.30 8.25 8.48 1.01 0.97 0.60S03-2B Group 3 8.33 8.32 8.39 1.00 0.99 0.12S95-31 Group 4 8.20 8.64 8.24 0.95 1.05 5.23SSG-2 Group 4 8.22 8.42 8.30 0.98 1.01 2.40S91-26 Group 5 8.40 8.35 8.27 1.01 1.01 0.60SSG-1 Group 5 8.42 8.37 8.25 1.01 1.01 0.60S91-03 Group 6 8.40 8.50 8.12 0.99 1.05 1.18S91-05 Group 6 8.32 8.40 8.27 0.99 1.02 0.96

a P–wave velocity along lineation direction (X).b P–wave velocity along lineation-perpendicular and foliation-parallel direction (Y).c P–wave velocity along foliation-perpendicular direction (Z).d P–wave anisotropy in foliation plane (X-Y plane).

0

1

2

3

4

5

6

7

8

0.90 0.92 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10

Ra

yle

igh w

ave a

zim

uth

al

an

iso

tro

py, G

(%

)

Radial anisotropy

Group 1 (A-/D-type)


Group 3 (random)




1.5 GPa & 850 °C

Ambient condition

1.5 GPa & 1200 °C

Po

sitive

Ne

ga

tive

Fig. 9. Radial anisotropy (ξ) and Rayleigh wave azimuthal anisotropy (G) of olivine ag-gregates in the studied samples at three representative P–T conditions: ambient condition(circles), 1.5 GPa & 850 °C (squares), and 1.5 GPa & 1200 °C (triangles). Horizontal fo-liation is assumed.

0

1

2

3

4

6

7

0 1 2 3 4 5 6 7

G (

%)

Ap(F) (%)

5

Group 1 (A-/D-type)


Group 3 (random)




G = 0.74*Ap(F) + 1.14

2R = 0.88

Fig. 10. P-wave anisotropy in foliation plane [Ap(F)] versus Rayleigh wave azimuthalanisotropy (G) assuming horizontal foliation plane. The seismic anisotropies are calcu-lated for ambient conditions. The X and Y directions on the foliation are used in thecalculation of Ap(F) and the foliation is aligned horizontal to calculate G.


441

pyroxenes, as well as their potential influences on the rheologicalproperties and CPO development of olivine (Hansen and Warren, 2015;Jung et al., 2010; Tasaka et al., 2014). Commonly, an addition ofpyroxenes tends to reduce the intensities of whole-rock seismic aniso-tropies but does not significantly change their patterns in comparison toa purely olivine aggregate sample (Precigout and Almqvist, 2014).Therefore, the calculated seismic anisotropies based on solely olivineCPO can provide the first-order approximation and implication to theseismic anisotropies in the forearc mantle.

5.2. Implications for seismic anisotropies in Qinling orogen

The Songshugou peridotite body is currently located in Qinlingorogenic belt, which is formed by the continental collision between theNorth China Craton and the Yangtze Craton (Fig. 1a; Dong and Santosh,2016). This peridotite body shows an elongated shape with trend (NW-SE) aligning subparallel to the strike of the Qinling orogen (NWW-SEE)(Fig. 1a and b). Previous field surveys suggest that the Songshugouperidotite body is characterized by a continuous steeply SW-dipping

foliation (avg. dip angle ~ 70°) and a lineation that is oriented~200–240° (Song et al., 1998), indicating that this peridotite body wasexhumed mainly by top-to-SW normal-slip, rather than strike-slip ex-trusion (Fig. 13; Song et al., 1998). This structural framework of theSongshugou peridotite body can allow us to evaluate the role of olivineCPOs on the seismic anisotropies in the Songshugou area of Qinlingorogenic belt.

As illustrated in Fig. 13, Groups 1, 3 and 7 all present the fastest Pwave subparallel to the lineation that is plunging steeply south-westward, whereas Groups 5 and 6 have a Vpmax sub-perpendicular tothe lineation that plunges nearly horizontally in the NW–SE direction.For a shear wave (e.g., SKS, SKKS, P-to-S converted waves) that pro-pagates mainly upwards within an angle of 30–45° to vertical direction(i.e. seismic wave sampling window), this spatial configuration be-tween the structural framework and seismic anisotropy pattern resultsin weak splitting of shear waves for Groups 1, 3 and 7 and compara-tively stronger shear wave splitting for Groups 5 and 6 with the Vs1 Pol.Dir. aligning subparallel to the strike of peridotite body or Qinlingorogenic belt. For surface wave dispersion and P-wave anisotropy


(a)

100%

Group 1

1.5 GPa

850 °C

Max. = 8.64 Min. = 8.05

Anisotropy = 7.0%

8.16

8.24

8.32

8.40

8.48

Max. = 6.02 Min. = 0.10

2.0

3.0

4.0

5.0

6.0

6.02

0.10

Max. = 8.60 Min. = 8.06

Anisotropy = 6.5%

8.16

8.24

8.32

8.40

8.48

Max. = 5.61 Min. = 0.19

2.0

3.0

4.0

5.0

5.61

0.19

(b) 90%

Group 1

+

10%

Group 6

Max. = 8.56 Min. = 8.06

Anisotropy = 6.0%

8.15

8.25

8.35

8.45

8.55

Max. = 5.20 Min. = 0.08

1.0

2.0

3.0

4.0

5.0

5.20

0.08

(c)

80%

Group 1

+

20%

Group 6

Max. = 8.52 Min. = 8.06

Anisotropy = 5.5%

8.20

8.30

8.40

8.50

Max. = 4.83 Min. = 0.08

1.5

2.5

3.5

4.5

4.83

0.08

(d)

70%

Group 1

+

30%

Group 6

Max. = 8.48 Min. = 8.07

Anisotropy = 5.0%

8.15

8.25

8.35

8.45

Max. = 4.48 Min. = 0.21

1.0

2.0

3.0

4.0

4.48

0.21

(e)

60%

Group 1

+

40%

Group 6

Max. = 8.46 Min. = 8.07

Anisotropy = 4.7%

8.15

8.25

8.35

8.45

Max. = 4.20 Min. = 0.17

1.0

2.0

3.0

4.0

4.20

0.17

(f)

50%

Group 1

+

50%

Group 6

Max. = 8.44 Min. = 8.07

Anisotropy = 4.5%

8.20

8.30

8.40

Max. = 4.05 Min. = 0.08

1.0

2.0

3.0

4.0

4.05

0.08

(g)

40%

Group 1

+

60%

Group 6

Max. = 8.43 Min. = 8.07

Anisotropy = 4.4%

8.20

8.30

8.40

Max. = 4.07 Min. = 0.11

1.0

2.0

3.0

4.0

4.07

0.11

(h)

30%

Group 1

+

70%

Group 6

Max. = 8.45 Min. = 8.06

Anisotropy = 4.6%

8.20

8.30

8.40

Max. = 4.30 Min. = 0.13

1.0

2.0

3.0

4.0

4.30

0.13

(i)

20%

Group 1

+

80%

Group 6

Max. = 8.47 Min. = 8.06

Anisotropy = 5.0%

8.15

8.25

8.35

8.45

Max. = 4.66 Min. = 0.08

1.5

2.5

3.5

4.5

4.66

0.08

(j)

10%

Group 1

+

90%

Group 6

Max. = 8.50 Min. = 8.05

Anisotropy = 5.5%

8.10

8.20

8.30

8.40

8.50

Max. = 5.15 Min. = 0.04

1.0

2.0

3.0

4.0

5.0

5.15

0.04

(k)

100%

Group 6

Max. = 8.43 Min. = 8.09

Anisotropy = 4.1%

8.20

8.30

8.40

Max. = 3.59 Min. = 0.08

1.5

2.5

3.5

3.59

0.08

(l)

Group 5

180° CW

rotated


X

Z

Y

Fig. 11. Seismic anisotropies of olivine aggregates in (a–k) mixtures of CGD (Group 1) and FGD (Group 6) at 1.5 GPa and 850 °C. The mixture increases from 100% Group 1 to 100%Group 6 with 10% intervals of Group 6. The seismic anisotropies of olivine aggregates of (a) 100% Group 1 and (k) 100% Group 6 are averaged from two Group 1 samples (08S-32 andS03-2C; Fig. 5a and b) and two Group 6 samples (S91-03 and S91-05; Fig. 5i and j), respectively. For comparison, the seismic anisotropies of olivine aggregates in PCD Group 5 is shown in(l), which is averaged from two Group 5 samples (S91-26 and SSG-1; Fig. 5g and h). The pole figures of Group 5 are rotated 180° clockwise to match the seismic anisotropy patterns inmixtures of Groups 1 and 6. The solid bold lines in the pole figures denote the authentic foliation. X, Y and Z denote the lineation, direction perpendicular to lineation and parallel tofoliation, and foliation-normal direction, respectively. Data were presented in the equal-area lower-hemisphere.


442

Vp (km/s) AVs (%)

mod.

Group 5

obs.

Group 5

100% CGD (Group 1) 100% FGD (Group 6)

20% Group 1

80% Group 6

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

An

iso

tro

py (

%)

Fraction of Group 6

Averaged (Group 5)

S91-26 (Group 5)

SSG-1 (Group 5)

100% Group 1 100% Group 6

A- D-type B-type-likeAG-type-like

AVp

max. AVs

Fig. 12. Variations of patterns and intensities of seismic anisotropies(AVp and max. AVs) with a varying fraction of FGD (Group 6) in themixture of CGD (Group 1) and FGD (Group 6) at 1.5 GPa and 850 °C.The seismic anisotropies of two PCD (Group 5) samples (S91-26 andSSG-1) and their averaged sample are shown for comparison. Theaveraged sample shows remarkably weaker seismic anisotropiesthan its make-up individuals, possibly owing to the counteractioncaused by the slight differences in seismic anisotropy patterns be-tween S91-26 and SSG-1. Solid and dashed colored horizontal linesdenote the values of AVp and max. AVs, respectively. The coloredstars indicate their right-hand intercepts with a mixture of Groups 1and 6. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

Amphibolite/gneiss

Fine-grained & Porphyroclastic

dunite

Coarse-grainedharzburgite

Coarse-graineddunite

Amphibole vein

Olivineclinopyroxenite

A B

200 m

Gro

up 5

and

6

Gro

up 1

Gro

up 3

Gro

up 7

seismic wavesampling window

30-45

FVD or Vs1 Pol. Dir.

SW NE

XZ

Y

Fig. 13. A cross-section (profile “A–B” in Fig. 1b) showing the distribution of rock types in the Songshugou peridotite body [modified after Song et al., 1998] and their induced seismicanisotropy patterns at ambient conditions. For Groups 5 and 6, foliation and lineation are plunging SW at high dip angles (avg. ~70°) due to dominant top-to-SW shear sense. For Groups1, 3 and 7, the orientations of foliation and lineation are assumed to be the concordant with those of Groups 5 and 6. X and Z denote lineation and foliation-normal direction, respectively.The seismic anisotropies of Groups 3 and 7 were calculated using CPOs and volume proportions of major minerals in polymineralic aggregates [Group 3 (S03-2B): Ol ~ 50%, Opx ~ 10%,Amp ~ 40%, see Fig. S1 and Cao et al., 2017; Group 7 (S91-32): Ol ~ 25%, Cpx ~ 50%, Amp ~ 25%, see Fig. S2]. The proportion of olivine clinopyroxenite (Group 7) in this profile islarger than that shown in the area map (Fig. 1c), possibly owing to spatial heterogeneity.


443

tomography (seismic waves travelling subparallel to the horizontalplane are required), one predicts null to negative radial anisotropy(ξ ~ 0.92–0.99) and weak Rayleigh wave azimuthal anisotropy(G ~ 0.4–0.8%) for Groups 1, 3 and 7, whereas almost null radial ani-sotropy (ξ ~ 0.98–1.00) and comparatively larger Rayleigh wave azi-muthal anisotropy (G ~ 1.3–2.6%) for Groups 5 and 6 are predicted(foliation dip angle of 70° is used in the calculation, Table 5). SinceGroups 5 and 6 are the dominant rock types in the Songshugou peri-dotite body (Fig. 1c), and Groups 1, 3 and 7 have weak FVD and Vs1 Pol.Dir., the seismic anisotropy patterns of the Songshugou peridotite bodycan be approximately represented by those of Groups 5 and 6. In thiscontext, strike-subparallel FVD and Vs1 Pol. Dir. with significant S-wavepolarization anisotropy (AVs up to 4%), moderate Rayleigh wave azi-muthal anisotropy (G up to 3%), and nearly null radial anisotropy canbe induced by the presence of B-/AG-type-like olivine CPOs in theSongshugou peridotite body (Fig. 13).

Seismological studies have reported orogen-parallel NWW-SEE Vs1Pol. Dir. in the Qinling-Dabie Mountain regions using the SKS shearwave splitting data (Fig. 14a) (Chang et al., 2008; Chang et al., 2011;Hu et al., 2011; Li et al., 2011; Tian and Santosh, 2015; Yu and Chen,2016). These studies proposed that the orogen-parallel Vs1 Pol. Dir. inQinling orogen may result from olivine A-/D-type CPOs caused by li-thospheric shortening due to the convergence between North China

Craton and Yangtze Craton (Chang et al., 2011; Tian and Santosh,2015) or eastward extrusion of the Tibetan asthenospheric mantle (Yuand Chen, 2016). In the Songshugou area, Vs1 Pol. Dir. aligns NWW-SEEconsistent with those of other areas in the Qinling orogen, but presentsa moderate delay time (~0.7 s) compared to the eastern and westernneighboring regions (> 1.0 s) (Fig. 14b).

Besides the contributions of A-/D-type CPOs in the present-dayupper mantle, B-/AG-type-like olivine CPOs in the emplaced peridotitebody that yield strike-parallel Vs1 Pol. Dir. can be an additional con-tributor to the orogen-parallel Vs1 Pol. Dir. in the Songshugou area. Theseismic contribution of olivine B-/AG-type-like CPOs depends on thesize and distribution of peridotite body. Considering a max. AVs of 4%,~85 km thick mantle slice that consists of purely B-/AG-type olivineCPOs in the orogenic crust of Songshugou area is needed to account forthe delay times of ~0.7 s (Fig. 14b). Although the lateral and depthextents of Songshugou peridotite body in the crust is unknown, itsnarrow width (~2 km, Fig. 1b) implies that its spatial distribution islikely limited and seismic anisotropy contribution is likely minor. Inaddition, the effects of fractures and cracks on seismic anisotropies arealso of importance in the crustal depth (Almqvist and Mainprice, 2017),which may counteract the intrinsic seismic anisotropies caused by CPOs(Ji et al., 2003) and is, however, not considered here. Actually, thecontributors to orogen-parallel seismic anisotropies are likely multifold.For example, depending on the distribution and abundance, the CPOs ofserpentines (e.g., antigorite and lizardite) in the lithospheric shear zoneare also considered to induce the orogen-parallel Vs1 Pol. Dir. (Ji et al.,2013; Shao et al., 2014). Therefore, although an orogen-parallel Vs1Pol. Dir. can indeed be caused by B-/AG-type-like olivine CPOs, theircontribution is constrained to be minor and localized.

6. Conclusion

The seismic anisotropies of the Songshugou peridotites (dunitedominated) were calculated based on mineral CPOs, and these resultsmay increase our understandings on the roles of mineral CPOs affectingthe seismic anisotropies of forearc mantle and orogenic belt.

(1) The olivine aggregates are characterized by distinct patterns ofseismic anisotropies (e.g., P-wave anisotropy and S-wave splitting),which correspond well to their olivine CPO types (e.g., A-/D-type,

Table 5Seismic anisotropies of different rock types at ambient condition.

Sample Ambient conditiona

AVp (%) max. AVs (%) ξ G (%)

Coarse-grained duniteb 6.6 4.54 0.924 0.36Porphyroclastic dunitec 3.4 2.16 0.984 1.26Fine-grained dunited 5.0 3.74 1.003 2.55Coarse-grained harzburgitee 3.2 1.64 0.988 0.18Olivine clinopyroxenitef 4.2 1.93 0.985 0.77

a A dip angle of 70° for foliation is used for calculation.b Averaged from olivine aggregates in two Group 1 samples (08S-32 and S03e2C).c Averaged from olivine aggregates in two Group 5 samples (S91–26 and SSG-1).d Averaged from olivine aggregates in two Group 6 samples (S91–03 and S91–05).e Represented by one Group 3 sample (S03-2B: Ol ~ 50%, Opx ~ 10%, Amp ~ 40%).f Represented by one Group 7 sample (S91–32: Ol ~ 25%, Cpx ~ 50%, Amp ~ 25%).

Songshugou

-

Songshugou

(a) (b)

Fig. 14. (a) Vs1. Pol. Dir. of SKS shear wave splitting analysis and (b) spatial distribution of delay times in the Qinling-Dabie orogen and its adjacent regions. Songshugou area is markedby red edged yellow stars. Arrow with APM and purple arrows in (b) are absolute plate movement direction and GPS velocity vectors, respectively. Figures are modified after Yu and Chen(2016). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


444

B-, C- and AG-type-like).(2) The seismic anisotropies (AVp, max. AVs, ξ and G) of olivine ag-

gregates show a positive correlation with equilibrium P–T condi-tions. From 1.5 GPa at 1200 °C to 1.5 GPa at 850 °C, AVp and max.AVs drop by ~10% and ~15%, respectively. From 1.5 GPa at1200 °C to ambient condition, AVs drops by ~30–40% and AVp by~15–20%.

(3) Accompanied by olivine CPO transition from A-/D-type in CGD(Group 1), through AG-type-like in PCD (Group 5), to B-type-like inFGD (Group 6), seismic anisotropies [AVp, Ap(F), max. AVs, and G]reduce significantly from Group 1 to 5, and then increase fromGroup 5 to 6. The intensities and seismic anisotropy patterns ofobserved PCD are well modeled by dynamic recrystallization de-grees of ~70–90% of CGD (Group 1).

(4) The olivine CPO transitions from A-/D-type to AG- and B-type-likemay lead FVD and Vs1 Pol. Dir. to progressively weaken and deviatefrom trench-perpendicular to trench-oblique direction with theaging and cooling of forearc mantle.

(5) Depending on the size and distribution of peridotite body such asthe Songshugou peridotites, B- and AG-type-like olivine CPOs in theorogenic crust can be an additional (despite minor) local con-tributor to the orogen-parallel FVD and Vs1 Pol. Dir. such as in theSongshugou area in Qinling orogen.

Supplementary data to this article can be found online at https://doi.org/10.1016/j.tecto.2017.11.017.

Acknowledgements

We are grateful to Jacques Précigout and two anonymous reviewersfor their thoughtful comments, as well as Editor Zheng-Xiang Li forpaper handling. This research was supported by the KoreaMeterological Administration Research and Development Programunder Grant, KMIPA2017-9020 to H.J. and a BK21+ postdoctoralfellowship to Y.C. in Korea, as well as the National Natural ScienceFoundation of China (Grant Nos. 41572040, 41372060) and the MajorState Basic Research Development Program (2015CB856105) to S.G.Sin China.

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