Comprehensive in situ constraints on LPO fabric of fast-spreading - - PowerPoint PPT Presentation

Comprehensive in situ constraints on LPO fabric of fast-spreading oceanic lithosphere from seismic anisotropy

Joshua B. Russell1, Hannah F. Mark2,3, James B. Gaherty1, Daniel Lizarralde2, Pei-Ying (Patty) Lin4, John A. Collins2, Greg Hirth5, Rob L. Evans2

1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA 2Woods Hole Oceanographic Institution, Woods Hole, MA, USA 3MIT/WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering, MA, USA 4Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan 5Geological Sciences Department, Brown University, Providence, Rhode Island, USA

DI11A-02

DI11A-02

Geodynamic models simulate LPO fabric formation and evolution at mid-ocean ridge Observations:

▸ Hand-sample peridotite fabrics ▸ 10

• 3–10

2 m length scale

▸ Seismic anisotropy observations ▸ 10

3–10 7 m length scale

Motivation

2

Karato et al., 2008 Annu. Rev. Blackman et al., 2017 GJI

DI11A-02

Geodynamic models simulate LPO fabric formation and evolution at mid-ocean ridge Observations:

▸ Hand-sample peridotite fabrics ▸ 10

• 3–10

2 m length scale

▸ Seismic anisotropy observations ▸ 10

3–10 7 m length scale

3

Karato et al., 2008 Annu. Rev. Blackman et al., 2017 GJI

Motivation

x1’ x3’ x2’ [100]

Michibayashi et al., 2016 EPSL

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4

Karato et al., 2008 Annu. Rev. Blackman et al., 2017 GJI Eddy et al., 2018 GJI

Motivation

x1’ x3’ x2’ [100]

Michibayashi et al., 2016 EPSL

Geodynamic models simulate LPO fabric formation and evolution at mid-ocean ridge Observations:

▸ Hand-sample peridotite fabrics ▸ 10

• 3–10

2 m length scale

▸ Seismic anisotropy observations ▸ 10

3–10 7 m length scale

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5

NoMelt (~70 Ma)

Karato et al., 2008 Annu. Rev. Blackman et al., 2017 GJI

Motivation

0° 10°N 20°N 160°W 150°W 140°W 50 60 6 100 110 90 80 70 −7000 −5000 −3000 −1000 1000 Depth (m)

600 km 400 km

x1’ x3’ x2’ [100]

Michibayashi et al., 2016 EPSL

Geodynamic models simulate LPO fabric formation and evolution at mid-ocean ridge Observations:

▸ Hand-sample peridotite fabrics ▸ 10

• 3–10

2 m length scale

▸ Seismic anisotropy observations ▸ 10

3–10 7 m length scale

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Olivine LPO fabric types

6

(010)[100] (0kl)[100] (001)[100]

Slip systems

After Skemer et al., 2012 G3 Karato et al., 2008 Annu. Rev. ; Jung et al., 2006

[100] [001] [010]

E-type

[100] [010] [001]

D-type

a b c

[100] [010] [001]

A-type

Fast Slow Intermediate

LPO fabric development depends on stress, H2O content, and temperature

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NoMelt anisotropy observations

7

Surface Waves:

Love 2θ & 4θ (5–7.5 s) Rayleigh 2θ (5–150 s)

Russell et al. in review fossil spreading 7.5 s

2θ + 4θ 2θ 2θ 4θ

• 2
• 1

1 2

c/c (%)

2θ + 4θ 2θ 2θ 4θ b)

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

• 100

100

2θ + 4θ 2θ 2θ 4θ

• 100

100

2θ + 4θ 2θ 2θ 4θ 2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

Love 7.5 s

= +

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

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NoMelt anisotropy observations

8

Surface Waves:

Love 2θ & 4θ (5–7.5 s) Rayleigh 2θ (5–150 s)

Russell et al. in review fossil spreading 7.5 s

2θ + 4θ 2θ 2θ 4θ

• 2
• 1

1 2

c/c (%)

2θ + 4θ 2θ 2θ 4θ b)

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

• 100

100

2θ + 4θ 2θ 2θ 4θ

• 100

100

2θ + 4θ 2θ 2θ 4θ 2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

Love 7.5 s

= +

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

• 100

100

7.5 s

2θ + 4θ 2θ 2θ 4θ

• 4
• 2

2 4

c/c (%)

2θ + 4θ 2θ 2θ 4θ a)

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

Rayleigh 7.5 s

Shear Parameters G: 2θ variation of VSV E: 4θ variation of VSH

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NoMelt anisotropy observations

9

• 100

100

7.5 s

2θ + 4θ 2θ 2θ 4θ

• 4
• 2

2 4

c/c (%)

2θ + 4θ 2θ 2θ 4θ a)

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

Rayleigh 7.5 s

Pn anisotropy Surface Waves:

Mark et al. in review

Love 2θ & 4θ (5–7.5 s) Rayleigh 2θ (5–150 s)

Russell et al. in review fossil spreading 7.5 s

2θ + 4θ 2θ 2θ 4θ

• 2
• 1

1 2

c/c (%)

2θ + 4θ 2θ 2θ 4θ b)

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

• 100

100

2θ + 4θ 2θ 2θ 4θ

• 100

100

2θ + 4θ 2θ 2θ 4θ 2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

• 100

100

2θ + 4θ 2θ 2θ 4θ

Love 7.5 s

= +

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Azimuth (º)

2θ + 4θ 2θ 2θ 4θ

Shear Parameters G: 2θ variation of VSV E: 4θ variation of VSH Compressional Parameters B: 2θ variation of VP

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Constraining the elastic tensor (Cij)

10

Cij =                   A + Bc + Ec A − 2N − Ec F + Hc

1 2Bs + Es

· A − Bc + Ec F − Hc

1 2Bs − Es

· · C Hs · · · L − Gc Gs · · · · L + Gc · · · · · N − Ec                  

13 elastic parameters required to constrain 13 elements of Cij

ρ VqP(θ)2 = A + Bc cos(2θ) + Bs sin(2θ) + Ec cos(4θ) + Es sin(4θ) ρ VqSV(θ)2 = L + Gc cos(2θ) + Gs sin(2θ) ρ VqSH(θ)2 = N − Ec cos(4θ) − Es sin(4θ)

Azimuthal Anisotropy:

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Cij =                   A + Bc + Ec A − 2N − Ec F + Hc

1 2Bs + Es

· A − Bc + Ec F − Hc

1 2Bs − Es

· · C Hs · · · L − Gc Gs · · · · L + Gc · · · · · N − Ec                  

Rayleigh waves (2θ)

▸ L, G, B, H

Love waves (2θ, 4θ)

▸ N, E, G

Pn (2θ, 4θ)

▸ A, B, E

Scaling relations

▸ C, H, F ▸ A, B below 7 km

Constraining the elastic tensor (Cij)

11

Cij =                   A + Bc + Ec A − 2N − Ec F + Hc

1 2Bs + Es

· A − Bc + Ec F − Hc

1 2Bs − Es

· · C Hs · · · L − Gc Gs · · · · L + Gc · · · · · N − Ec                  

(VSV) (VSH) (VPH) (VPV)

13 elastic parameters required to constrain 13 elements of Cij 9 terms 4 terms

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2 4 6 8

Strength (%)

50 100 150 200 250 300

G/L B/A H/F E/N

5 10 15 20 25 30 35 APM 60 90 120 150

Azimuth (°)

50 100 150 200 250 300 FSD FSD + 45° 5 10 15 20 25 30 35 3 3.5 4 4.5 5

VS (km/s)

50 100 150 200 250 300

Depth (km)

NF89 (52-110 My) Pa5 NoMelt 5 10 15 20 25 30 35 0.95 1 1.05 1.1 1.15 50 100 150 200 250 300 5 10 15 20 25 30 35

crust

G B E H Isotropic Radial Anisotropy Azimuthal anisotropy

ξ = (VSH / VSV)2

VSH > VSV VSH > VSV

Elastic model

12

Vs, ξ, G, B, H, E, ΨG, ΨB, ΨH, ΨE

Pn-constraints

• n E and B

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2 4 6 8

Strength (%)

50 100 150 200 250 300

G/L B/A H/F E/N

5 10 15 20 25 30 35 APM 60 90 120 150

Azimuth (°)

50 100 150 200 250 300 FSD FSD + 45° 5 10 15 20 25 30 35 3 3.5 4 4.5 5

VS (km/s)

50 100 150 200 250 300

Depth (km)

NF89 (52-110 My) Pa5 NoMelt 5 10 15 20 25 30 35 0.95 1 1.05 1.1 1.15 50 100 150 200 250 300 5 10 15 20 25 30 35

crust

G B E H Isotropic Radial Anisotropy Azimuthal anisotropy

ξ = (VSH / VSV)2

VSH>VSV VSH > VSV

Elastic model

13

Isotropic Radial Anisotropy Azimuthal anisotropy

Moho to 35km depth

G B E H Vs, ξ, G, B, H, E, ΨG, ΨB, ΨH, ΨE

Pn-constraints

• n E and B

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• 5

5

VqP (%)

NoMelt BIM98 (fast-spreading) PN78 (Mesozoic Average) PN78 (Harzburgite D-type)

• 4
• 2

2 4

VqSV (%)

• 2

2

VqSH (%)

50 100 150 200 250 300 350

Azimuth in x'1-x'2 plane (°)

0.9 1 1.1 1.2

( VqSH / V qSV )2

Azimuth in horizontal plane (º) Radial Anisotropy (VSH / VSV)2 VSH Anisotropy VSV Anisotropy VP Anisotropy

Comparison to petrofabrics

14

VP (km/s)

Anisotropy = 7.6% 8.5 8.6 8.7 8.8 8.9 9

Fast-shear polarisation

1 2 3 4 5

x1’ x3’ x2’ [100]

ρ VqP(θ)2 = A + Bc cos(2θ) + Bs sin(2θ) + Ec cos(4θ) + Es sin(4θ) ρ VqSV(θ)2 = L + Gc cos(2θ) + Gs sin(2θ) ρ VqSH(θ)2 = N − Ec cos(4θ) − Es sin(4θ)

NoMelt BIM98 (fast-spreading)

NoMelt (Moho to 35 km)

Azimuthal Anisotropy:

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• 5

5

VqP (%)

NoMelt BIM98 (fast-spreading) PN78 (Mesozoic Average) PN78 (Harzburgite D-type)

• 4
• 2

2 4

VqSV (%)

• 2

2

VqSH (%)

50 100 150 200 250 300 350

Azimuth in x'1-x'2 plane (°)

0.9 1 1.1 1.2

( VqSH / V qSV )2

Azimuth in horizontal plane (º) VSH Anisotropy VSV Anisotropy VP Anisotropy

▸ Excellent agreement of azimuthal

anisotropy

▸ NoMelt radial anisotropy is

relatively weak

Comparison to petrofabrics

15

VP (km/s)

Anisotropy = 7.6% 8.5 8.6 8.7 8.8 8.9 9

Fast-shear polarisation

1 2 3 4 5

VP (km/s)

Anisotropy = 10.0% 7.8 7.9 8 8.1 8.2 8.3 8.4

Fast-shear polarisation

1 2 3 4 5 6

x1’ x3’ x2’ [100]

Radial Anisotropy (VSH / VSV)2

Fast-spreading average

NoMelt BIM98 (fast-spreading) PN78 (Mesozoic Average)

NoMelt (Moho to 35 km)

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NoMelt BIM98 (fast-spreading) PN78 (Mesozoic Average) PN78 (Harzburgite D-type)

• 5

5

VqP (%)

• 4
• 2

2 4

VqSV (%)

• 2

2

VqSH (%)

50 100 150 200 250 300 350

Azimuth in x'1-x'2 plane (°)

0.9 1 1.1 1.2

( VqSH / V qSV )2

Azimuth in horizontal plane (º) VSH Anisotropy VSV Anisotropy VP Anisotropy

Comparison to petrofabrics

16

VP (km/s)

Anisotropy = 7.6% 8.5 8.6 8.7 8.8 8.9 9

Fast-shear polarisation

1 2 3 4 5

VP (km/s)

Anisotropy = 10.0% 7.8 7.9 8 8.1 8.2 8.3 8.4

Fast-shear polarisation

1 2 3 4 5 6

VP (km/s)

Anisotropy = 9.8% 8.1 8.2 8.3 8.4 8.5 8.6 8.7

Fast-shear polarisation

1 2 3 4 5 6

x1’ x3’ x2’ [100]

Radial Anisotropy (VSH / VSV)2

Fast-spreading average Antalya

• phiolite

harzburgite

PN78 (ophiolite average) NoMelt (Moho to 35 km) (Harzburgite)

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VP (km/s)

Anisotropy = 10.0% 7.7 7.8 7.9 8 8.1 8.2 8.3

Fast-shear polarisation

1 2 3 4 5 6

Comparison to petrofabrics: Rotated A-type fabric?

17

VP (km/s)

Anisotropy = 7.6% 8.5 8.6 8.7 8.8 8.9 9

Fast-shear polarisation

1 2 3 4 5

VP (km/s)

Anisotropy = 10.0% 7.8 7.9 8 8.1 8.2 8.3 8.4

Fast-shear polarisation

1 2 3 4 5 6

20º rotation

a b c

Rotated A-type

NoMelt BIM98 (fast-spreading) BIM98 (fast-spreading) 2 20° BIM98 (rotated 20º) NoMelt (Moho to 35 km)

x1’ x3’ x2’ [100]

• 5

5

VqP (%)

NoMelt BIM98 (fast-spreading) BIM98 (fast-spreading) 2 20

• 4
• 2

2 4

VqSV (%)

• 2

2

VqSH (%)

50 100 150 200 250 300 350

Azimuth in x'1-x'2 plane (°)

0.9 1 1.1 1.2

( VqSH / V qSV )2

Azimuth in horizontal plane (º) VSH Anisotropy VSV Anisotropy VP Anisotropy Radial Anisotropy (VSH / VSV)2

20º 20º

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a-axis rotation away from inferred shear plane ranging from 0º–60º for natural and laboratory samples

Rotated fabrics: observations

18

Skemer et al., 2012 G3

LPO evolution dependent on:

▸ finite strain ▸ pre-existing fabric ▸ deformation temperature ▸ orthopyroxene content ▸ grain size

20º–25º is consistent with shear strain <200–300%

A-type E-type D-type

NoMelt

Laboratory Natural Numerical

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Rotated fabrics: geodynamic modeling

19

0 Ma 20 Ma

Blackman et al., 2017 GJI

CPO development of fully- coupled, power-law (n=2), polycrystal material

▸ lithospheric a-axes

horizontally aligned

▸ shear strains in the

lithosphere too large

▸ cooling rate?

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Rotated fabrics: geodynamic modeling

20

CPO development of fully- coupled, power-law (n=2), polycrystal material

▸ lithospheric a-axes

horizontally aligned

▸ shear strains in the

lithosphere too large

▸ cooling rate?

0 Ma 20 Ma

Blackman et al., 2017 GJI

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Comparison to petrofabrics: E-type?

21

VP (km/s)

Anisotropy = 10.0% 7.7 7.8 7.9 8 8.1 8.2 8.3

Fast-shear polarisation

1 2 3 4 5 6

VP (km/s)

Anisotropy = 7.6% 8.5 8.6 8.7 8.8 8.9 9

Fast-shear polarisation

1 2 3 4 5

VP (km/s)

Anisotropy = 10.0% 7.8 7.9 8 8.1 8.2 8.3 8.4

Fast-shear polarisation

1 2 3 4 5 6

90º rotation

a c b a b c

“E-type”

NoMelt BIM98 (fast-spreading) BIM98 (fast-spreading) 2 20° BIM98 (E-type) NoMelt (Moho to 35 km)

x1’ x3’ x2’ [100]

• 5

5

VqP (%)

NoMelt BIM98 (fast-spreading) BIM98 (fast-spreading) 1 90

• 4
• 2

2 4

VqSV (%)

• 2

2

VqSH (%)

50 100 150 200 250 300 350

Azimuth in x'1-x'2 plane (°)

0.9 1 1.1 1.2

( VqSH / V qSV )2

Azimuth in horizontal plane (º) VSH Anisotropy VSV Anisotropy VP Anisotropy Radial Anisotropy (VSH / VSV)2

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VP (km/s)

Anisotropy = 9.2% 7.8 7.9 8 8.1 8.2 8.3

Fast-shear polarisation

1 2 3 4 5

Comparison to petrofabrics: D-type?

22

VP (km/s)

Anisotropy = 7.6% 8.5 8.6 8.7 8.8 8.9 9

Fast-shear polarisation

1 2 3 4 5

VP (km/s)

Anisotropy = 10.0% 7.8 7.9 8 8.1 8.2 8.3 8.4

Fast-shear polarisation

1 2 3 4 5 6

Average about fast axis

a b-c a b c

“D-type”

NoMelt BIM98 (fast-spreading) BIM98 (fast-spreading) 2 20° BIM98 (D-type) NoMelt (Moho to 35 km)

x1’ x3’ x2’ [100]

• 5

5

VqP (%)

• 4
• 2

2 4

VqSV (%)

• 2

2

VqSH (%)

50 100 150 200 250 300 350

Azimuth in x'1-x'2 plane (°)

0.9 1 1.1 1.2

( VqSH / V qSV )2

Azimuth in horizontal plane (º) VSH Anisotropy VSV Anisotropy VP Anisotropy Radial Anisotropy (VSH / VSV)2

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1 1.05 1.1 1.15 1.2

N/L

5 10 15

G/L (%)

Fabric types

23 [100] [001] [010]

E-type

[100] [010] [001]

A-type

[100] [010] [001]

D-type

Radial anisotropy Azimuthal anisotropy

E-type D-type

NoMelt

Laboratory Samples Natural Samples

DATA: Peselnick and Nicolas, 1978 Ben Ismaïl and Mainprice, 1998 Ben Ismaïl et al., 2001 Jung and Karato., 2001 Katayama et al., 2004 Jung et al., 2006 Karato, 2008

A-type

Modified from Karato et al., 2008

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▸ Anisotropy strength and direction consistent with oceanic

petrofabrics, bridging the gap between outcrop and seismic length scales

▸ Remarkably coherent LPO alignment across NoMelt

(~400x600km)

▸ Strong azimuthal anisotropy and relatively weak radial

anisotropy consistent with:

▸ (preferred) A-type fabric rotated 20º–25º, suggesting

lithospheric shear strains < 200%-300%

▸ or E-type fabric: moderate H2O concentration during fabric

formation near the ridge

▸ or D-type fabric: high stress, low H2O environment near

ridge

Conclusions

24

Rotated A-type Fabric (Preferred)

[100] [010] [001]

20º–25º

OR

E-type

[100] [001] [010] [100] [010] [001]

D-type

OR

We model the full anisotropic variability of surface- and Pn-waves, providing a first in situ elastic tensor for 70 Ma oceanic lithosphere.