A seismological view of the lithosphereasthenosphere boundary!
!
Karen M. Fischer, Heather A. Ford, Vedran Lekic!
Department of Geological Sciences, Brown University!
David L. Abt!
ExxonMobil Exploration Company!
Zaranek (2006)!
Questions!
•  Is the
seismological LAB a
rheological LAB?!
Crust
Mantle
dry, depleted?
melt-free
Asthenosphere
LAB
•  Why is the
lithosphere strong
vs. the
asthenosphere temperature, water,
melt?!
•  Do the properties
of the lithosphereasthenosphere
boundary vary
between oceans,
young continents
and cratons?!
Lithosphere
hydrated?
partial melt?
Temperature
Viscosity
Shear-wave velocity
LAB
Fischer et al. (Ann. Rev., 2010)!
Temperature only
Temperature + water in asthenosphere *!
Temperature + water & melt in asthenosphere*!
*Assumes dry, depleted lithosphere!
Is the seismological LAB a rheological LAB?!
50
VTV
Craton
Depth (km)
100
150
200
250
4200
4300
4400
4500
4600
4700
4800
Shear Velocity (m/s)
Yuan et al. (GJI, 2011)!
4900
5000
50
50
VTV
VTV
Craton
100
Depth (km)
Depth (km)
100
150
250
250
4400
4500
4600
4700
4800
4900
5000
cr
at
on
35
Craton
Mg# 93
200
4300
500
Shear Velocity (m/s)
1325 C
150
200
4200
Craton
1000
1500 C
xe
no
40
lit
hs
1500
Temperature (K)
50
Depth (km)
100
VTV
150
Craton
Mg# 93
Craton
."
e=10-15 /s"
200
50 H/10**6 Si
250
Ford et al.!
18
10
20
10
22
10
Viscosity (Pa*s)
24
10
26
10
2000
50
Depth (km)
100
VTV
150
Craton
Mg# 93
Craton
x 40!
200
.
e=10-15 /s"
50 H/10**6 Si
250
18
10
20
10
22
10
Viscosity (Pa*s)
Ford et al.!
24
10
26
10
50
.
e=10-13 /s"
Depth (km)
100
VTV
150
Craton
Mg# 93
Craton
x 40!
200
.
e=10-15 /s"
50 H/10**6 Si
250
18
10
20
10
22
10
Viscosity (Pa*s)
Ford et al.!
24
10
26
10
50
.
e=10-13 /s"
Depth (km)
100
Craton
Mg# 93
VTV
150
Craton
x 40!
200
.
e=10-15 /s"
50 H/10**6 Si
250
1000 H/10**6 Si
18
10
20
10
22
10
Viscosity (Pa*s)
Ford et al.!
24
10
26
10
Yuan & Romanowicz (Nature, 2010)!
•  Why is the lithosphere strong vs. the asthenosphere temperature, water, melt?!
•  Do the properties of the lithosphere-asthenosphere boundary
vary between oceans, young continents and cratons?!
Surface wave tomography!
Ps receiver functions!
Rychert & Shearer (Science, 2009)!
Lekic & Romanowicz (EPSL, 2011)!
HRV Sp: 421 events!
!"#!"#$%&'()*+,
-"./0)(12#13#4(.,0)5!6
5789 7 789
Moho!
Depth (km)
Crust!
Lithosphere!
Sp or Ps !receiver functions"!
50
100
LAB!
150
1)  Decompose the wavefield into its incident P and S components using a
free-surface transform (Kennett, 1991)!
2)  Deconvolve S from P (Sp) or P from S (Ps) using a simultaneous
frequency-domain approach (Bostock, 1998) or an iterative time-domain
approach (Ligorria and Ammon, 1999) and migrate to depth in 1D with
corrections for lateral heterogeneity!
Measuring velocity gradients with Ps and Sp!
Ps and Sp synthetic seismograms!
Rychert et al. (JGR, 2007)!
Oceans!
Oceanic LAB <~100 km!
!
PA5: Gaherty et al. (1999)!
Tan & Helmberger (2007)!
!
ScS reverberations:!
Bagley & Revenaugh (2008)!
!
Ps and Sp receiver functions:!
Collins et al. (2002)!
Li et al. (2004)!
Kumar et al., (2007)!
Rychert & Shearer (2009)!
Kawakatsu et al. (2009)!
Kumar & Kawakatsu (2011)!
!
SS precursors:!
Schmerr (2011)!
Rychert & Shearer (2011) (<130 km)!
Nettles & Dziewonski (JGR, 2008)!
Phanerozoic continents!
Lithosphere-Asthenosphere (LAB)!
RES
Mid-Lithosphere (MLD)!
Ambiguous!
North America
Sp Discontinuity Depth
Abt et al. (JGR, 2010)
Ford et al. (in prep)!
YKW3
FRB
FFC
SCHQ
ULM
EYMN
!"
#"
$"
%"
&""
DepthDepth
(km) !
LABLAB
or MLD
&&"
&'"
Abt et al. (JGR, 2010)!
Basin and Range / Colorado Plateau!
Levander et al. (Nature, 2011)!
Sp receiver functions: Eastern Australia
Ford et al. (EPSL, 2010)!
Australia and North America!
•  Dominant Sp period ~ 10 s!
•  Sharp LAB beneath younger continent:
!H < 30-40 km, best fits <= 20 km !
!
How sharp is the LAB
velocity gradient?!
!
Combined
inversions
of Ps and
Sp:!
6.0-9.6%!
< 5-11 km!
5.3-7.4%!
Rychert et al. (Nature, 2005 & JGR, 2007)!
NE U.S. Ps and Sp:
!
!> 5-6 % velocity drop in 5-11 km!
!
!
!
!
!> 20˚C/km temperature gradient*!
Phanerozoic U.S. and Australia: !> 6% velocity drop in < 30-40 km!
!
!
!
!
!> 5˚C/km temperature gradient*!
! *T to Vs scaling from Faul and Jackson (2005)!
• ! Inconsistent with temperature gradients at base of lithosphere in
mantle flow models (e.g. King and Ritsema, 2000; Cooper et al., 2004;
Zaranek, 2006)!
•  Velocity gradient is too sharp or localized in depth to be defined by
temperature alone!
Zaranek (2006)!
Cooper et al. (2004)!
Sharper velocity gradients with composition or melt!
!
•  Hydration of the asthenosphere with respect to a
drier, more depleted lithosphere? !
!Chemical depletion < 2.5% (Lee, 2003)!
!
!
!
!
< ~1% (Schutt & Lesher, 2006)!
!Volatile enrichment in asthenosphere < 3.8%
(following Karato and Jung, 1998; Karato, 2003)!
!Depletion+hydration may be sufficient if T permits!
!Depletion+hydration+anisotropy?!
!
•  Partial melt in the asthenosphere?!
!Velocity drop large enough with 0.5 - 1.5%
(Hammond and Humphreys, 2000; Kawakatsu et al.,
2009) or ~2% melt (Takei and Holtzman, 2009a)!
!
Salton Trough!
Sp CCP Stack!
Lekic et al. (Science, 2011)!
Shear wave velocity from
Rayleigh wave tomography!
Rau and Forsyth (Geology,
2011)!
•  ~30 km of lithospheric thinning beneath Salton Trough and Inner
Borderlands!
•  LAB topography very well-correlated with surface geology/
deformation; varies over small length-scales!
•  High viscosity mantle lithosphere and localized strain!
!
!
Lekic et al. (2011)!
F
G
0
Depth (km)
20
40
60
80
100
120
140
0
E
E’
50
100
150
200
Distance Along Profile (km)
250
Cratons!
Lithosphere-Asthenosphere (LAB)!
RES
Mid-Lithosphere (MLD)!
Ambiguous!
North America
Sp Discontinuity Depth
Abt et al. (JGR, 2010)
Ford et al. (in prep)!
YKW3
FRB
FFC
SCHQ
ULM
EYMN
!"
#"
$"
%"
&""
DepthDepth
(km) !
LABLAB
or MLD
&&"
&'"
PLAL
4000
UTMT
3500
SLM
SIUC
3000
JFWS
EYMN
ULM
FFC
FCC
1000
YKW3
500
JERN
0
Distance (km)
1500
2000
2500
Depth (km)
0
50
100
150
200
JERN
YKW3
FCC
250
North America
Sp Receiver Functions
Ford et al. (in prep)!
FFC
No phase in LAB depth
range!
Do see discontinuity
internal to lithosphere!
ULM
EYMN
JFWS
SLM
SIUC
UTMT
PLAL
North American Craton
Miller & Eaton (GRL, 2010)!
!
Weak phase in LAB depth range at some stations
Also see discontinuity internal to lithosphere!
Ps receiver functions!
Cratons:!
No phase in LAB depth range!
Do see discontinuity internal to lithosphere!
Rychert & Shearer (Science, 2009)!
Sp receiver functions: Northern Australia
Ford et al. (EPSL, 2010)!
Craton:!
No phase in LAB depth range!
Do see discontinuity internal to lithosphere!
Australia and North America
(Ford et al., Abt et al.)!
•  Dominant Sp period ~ 10 s!
•  Sharp LAB beneath younger continent: !H
< 30-40 km, best fits <= 20 km !
•  No cratonic LAB phase:
!H > 50-60 km!
!
"#"'
"#"(
&'()*+,-.
Synthetic Sp
phases from
LAB velocity
gradients!
!
#/0'
%#/0'
'")*+
$#/0'
&")*+
%")*+
#
!"#"(
!"#"'
!"#"&
10% velocity drop!
!"#"%
!"#$
!"#
!$#
!6#
!5#
1.(+2/30'4
%!#
Cratonic lithosphere-asthenosphere boundary!
•  If no LAB phase beneath
cratons:
!H > 50-60 km!
•  If LAB phase beneath
cratons:
!H < 30-40 km!
•  Consistent with purely
thermal gradient!
•  Not consistent with
purely thermal gradient!
•  Gradual gradients in
composition or melt cannot
be ruled out!
•  Sharper gradients in
composition or melt
indicated!
Zaranek (2006)!
Oceans!
LAB!
200 km!
Western U.S.!
LAB!
Craton!
Appalachians!
MLD!
!
Origin of mid-lithospheric discontinuity at 60-120 km?!
!
•  Observed globally !
–
–
–
–
North America and Australia Sp: Abt et al., Ford et al. !
Global Ps, Sp: Rychert & Shearer, Rychert et al., others!
Shear velocity from tomography: Yuan & Romanowicz, Lekic et al.!
Active source profiles: Thybo et al.!
•  North America: MLD lies within transition between cratonic
lithospheric layers defined by azimuthal anisotropy (Yuan &
Romanowicz, 2010)!
Oceans!
LAB!
200 km!
Western U.S.!
LAB!
Craton!
Appalachians!
MLD!
!
Origin of mid-lithospheric discontinuity at 60-120 km?!
•  Hypothesis 1: Relict of cratonic mantle formation base of original dry, depleted lithosphere? !
•  MLD depth correlates with boundary between anisotropic layers!
•  Do the MLD and anisotropic boundary reflect the same
structure?!
•  Hypothesis 2: Reflects lithospheric alteration by upward
melt percolation - top of melt cumulate layer?!
•  No + phase from lower layer boundary; layer either thin (< 10
km) or lower boundary gradual (> 50 km)!
!
!
Conclusions!
The seismological LAB is a rheological boundary!
!
Oceans & Phanerozoic Continents !
•  LAB velocity gradient sharp (< 30-40 km in depth):
volatiles and/or melt in asthenosphere!
•  Salton Trough/Inner Borderlands: strong lithosphere
and localized strain in mantle!
Cratons!
•  Debate over LAB phase at depths comparable to
base of fast lid!
•  If no phase, LAB velocity gradual (> 50-60 km in
depth): consistent with purely thermal boundary!
•  Velocity drop internal to lithosphere at 60 to 120
km (MLD)!