What makes the lithosphere strong
and the asthenosphere weak?
Karen M. Fischer, Brown University
Thanks to: David Abt, Heather Ford, Scott French,
Kate Rychert, Stéphane Rondenay
Z
Zaranek
k (2006)
Crust
M tl
Mantle
Lithosphere
dry, depleted?
melt-free
Asthenosphere
LAB
hydrated?
partial melt?
Temperature
Viscosity
Shear-wave velocity
LAB
Temperature
p
onlyy
Temperature + water in asthenosphere
Temperature + water & melt in asthenosphere
Nettles and Dziewonski (2008)
What can we learn from scattered/converted waves?
What is the depth of the lithosphere-asthenosphere boundary
beneath continents?
How sharp is the lithosphere-asthenosphere velocity gradient?
Wh is
Why
i the
th asthenosphere
th
h weakk - temperature,
t
t
water,
t melt?
lt?
Zaranek (2006)
Sp
p or Ps “receiver functions”
421 Events
Cru
ust
Lith
hosphere
0.2
HRV Sp: 421 events
0.15
Moho
01
0.1
0.05
0
LAB
0
100
200
Depth (km)
1)
Decompose the wavefield into its incident P and S components using a freesurface transform (Kennett, 1991)
2)
Deconvolve S from P (Sp) or P from S (Ps) using a simultaneous frequencydomain approach (Bostock,
(Bostock 1998) or an iterative time
time-domain
domain approach (Ligorria
and Ammon, 1999) and migrate to depth in 1D with corrections for lateral
heterogeneity
300
Combined inversions of
Ps and Sp
6 0-9 6%
6.0-9.6%
< 5-11 km
5.3-7.4%
Rychert et al. (2005 & 2007)
Combined inversions of
Ps and Sp
Zaranek (2006)
6 0-9 6%
6.0-9.6%
< 5-11 km
5.3-7.4%
Rychert et al. (2005 & 2007)
North America: Sp
Abt et al.
al (2009)
• Strong LAB phase
beneath western U.S.
andd portions
i
off
Appalachians
• No LAB phase
beneath cratons
• Discontinuity
within cratonic
lithosphere
North America: Sp
Abt et al.
al (2009)
• Sharp LAB beneath
younger continent < 30 km
k water/melt in
p
asthenosphere
• Gradual/weak LAB
beneath cratons > 50 km temperature alone
Australia: Sp
(Ford et al., in prep)
A 0
(a)
(b)
500
Distance (km)
1000 1500 2000 2500 3000
TOO YNG ARMA
EIDS
CTAO
A’
COEN
• Sharp LAB
beneath
Phanerozoic
E. Australia
< 30 km water/melt
t / lt in
i
asthenosphere
0
50
100
150
200
250
B 0
(c)
500
Distance (km)
1000 1500 2000 2500 3000
NWAO KMBL FORT
BBOO STKA
YNG
B’
C 0
(d)
0
0
50
50
100
100
150
150
200
200
250
250
MBWA
500
FITZ
Distance (km)
1000 1500 2000 2500 3000
WRAB
CTAO
C’
• Gradual/weak
LAB beneath
craton > 50-90
50 90 km
k temperature
alone
Sp conversion point depths: LAB in Southern California
French et al.
Conclusions
Phanerozoic North America and Australia
• LAB at 50 km to 110 km
• LAB velocity gradient sharp - water/melt in asthenosphere
Craton
• No “LAB” phase at depths comparable to base of fast lid
• LAB velocity gradient small or gradual - consistent with
purely thermal boundary
• Velocity drop internal to lithosphere at 60 to 115 km
Southern California
• Lith
Lithosphere
h
iis:
• < 60 km beneath southern CA Borderlands
• locally thinned (10-15 km) beneath San Jacinto Fault