Signals of dynamic coupling between mantle and lithosphere beneath the axis of the East Pacific Rise Christopher J. Rowan , David B. Rowley, Alessandro Forte, Nathan Simmons & Stephen Grand. with thanks to CIFAR, Chuck DeMets, and Pavel Doubrovine Monday, 6 January 14
Signals of dynamic coupling between mantle and lithosphere beneath the axis of the East Pacific Rise - AGU 2013
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Signals of dynamic coupling between mantle and lithosphere beneath
the axis of the East Pacific Rise
Christopher J. Rowan, David B. Rowley, Alessandro Forte, Nathan Simmons & Stephen Grand.
with thanks to CIFAR, Chuck DeMets, and Pavel Doubrovine
Monday, 6 January 14
The East Pacific Rise since 83 Ma
Isochrons generated from interpolating crossing data from Atwater & Severinghaus (1989), Cande & Haxby (1991), Munschy et al. (1996), Wilder (2003) & age grid of Müller et al. (2008)
Chron 34ny
(83 Ma)
• East Pacific Rise (EPR) is the remnant of much longer Pacific-Farallon Ridge.
• Has produced ~45% of reconstructable oceanic lithosphere since 83 Ma (Rowley 2008).
Monday, 6 January 14
EPR in the mantle reference frame
Rowley et al., submitted.
Unlike other spreading ridges, EPR axis has remained fixed over one region of the mantle.
Indo-Atlantic hotspot frame, Lord Howe circuit.
Monday, 6 January 14
EPR in the mantle reference frame
Rowley et al., submitted.
Unlike other spreading ridges, EPR axis has remained fixed over one region of the mantle.
Indo-Atlantic hotspot frame, Lord Howe circuit.
Monday, 6 January 14
EPR in the mantle reference frame
Rowley et al., submitted.
Unlike other spreading ridges, EPR axis has remained fixed over one region of the mantle.
Indo-Atlantic hotspot frame, Lord Howe circuit.
Monday, 6 January 14
Spreading asymmetry & its significance
Chron 24.3no (53.35 Ma)
Pacific isochron
Monday, 6 January 14
Spreading asymmetry & its significance
Chron 24.3no (53.35 Ma)
Pacific isochron
Chron 24.3no (53.35 Ma)
Pacific isochron
C24.3no Predicted
Nazca isochron
50.78 Ma
Monday, 6 January 14
Spreading asymmetry & its significance
Chron 24.3no (53.35 Ma)
Pacific isochron
Chron 24.3no (53.35 Ma)
Pacific isochron
C24.3no Predicted
Nazca isochron
50.78 Ma
Long term Pacific spreading fraction ≈ 0.42
Monday, 6 January 14
Spreading asymmetry & its significance
Chron 24.3no (53.35 Ma)
Pacific isochron
Chron 24.3no (53.35 Ma)
Pacific isochron
C24.3no Predicted
Nazca isochron
50.78 Ma
Long term Pacific spreading fraction ≈ 0.42
Without asymmetric spreading, EPR would not remain fixed.
symmetric since 50 Ma
Monday, 6 January 14
Spreading asymmetry & its significance
Chron 24.3no (53.35 Ma)
Pacific isochron
Chron 24.3no (53.35 Ma)
Pacific isochron
C24.3no Predicted
Nazca isochron
50.78 Ma
Long term Pacific spreading fraction ≈ 0.42
Without asymmetric spreading, EPR would not remain fixed.
symmetric since 50 Masymmetric since 50 Ma
& 83 Ma
Monday, 6 January 14
Stable mantle upwelling beneath EPR
cm/yr
Predicted mantle flow based on buoyancy distribution model TX2008
(Simmons et al. 2009) and ‘V2’ viscosity profile (Mitrovica & Forte 2004).
Rowley et al., submitted.650 km depth
Monday, 6 January 14
Stable mantle upwelling beneath EPR
cm/yr
Predicted mantle flow based on buoyancy distribution model TX2008
(Simmons et al. 2009) and ‘V2’ viscosity profile (Mitrovica & Forte 2004).
Rowley et al., submitted.250 km depth
Monday, 6 January 14
Stable mantle upwelling beneath EPR
cm/yr
Predicted mantle flow based on buoyancy distribution model TX2008
(Simmons et al. 2009) and ‘V2’ viscosity profile (Mitrovica & Forte 2004).
250 km depth
shaded area: radial flow velocity>2cm/yr
Monday, 6 January 14
Mantle flow & spreading behaviour
0
400
800
1200
1600
2000
2400
2800
de
pth
(km
)
0 10 20 30 40 50 60 70 80 90 100 110 120
distance (∆)
-0.5 0.0 0.5
δρ/ρ (%)
5 cm/yr
180˚ -170˚ -160˚ -150˚ -140˚ -130˚ -120˚ -110˚ -100˚ -90˚ -80˚ -70˚ -60˚-30˚
-20˚
-10˚
Divergent mantle flow in uppermost
mantle leads rather than lags overriding
plate motions.
It is also strongly asymmetric.
Monday, 6 January 14
Mantle flow & spreading behaviour
Pacific & Nazca plates have both slowed down in past 5-10 Ma...
Pacific
Age
Monday, 6 January 14
Mantle flow & spreading behaviour
Forte et al. 2008
Pacific & Nazca plates have both slowed down in past 5-10 Ma...
Pacific
Age
...matching modelled effects of changing mantle flow.
Monday, 6 January 14
Spreading rate & asymmetry
more Nazca plate
more Pacific plate
50 Myr record of spreading
asymmetry: clear variability
Rowan & Rowley, in revision
Monday, 6 January 14
Spreading rate & asymmetry
more Nazca plate
more Pacific plate
50 Myr record of spreading
asymmetry: clear variability
Increasing asymmetry
appear linked to increases in
spreading rate.
Rowan & Rowley, in revision
Monday, 6 January 14
More than slab pull?
Distribution of slab pull forces are consistent with absolute motions of Pacific and Nazca plates.(Conrad & Lithgow-Bertolli, 2002,2004)
NazcaPacific
Monday, 6 January 14
More than slab pull?
But changes induced by a time varying ‘plume push’* at ridge axis could increase spreading rate & asymmetry.*(cf. Cande & Stegman,
2011)
Distribution of slab pull forces are consistent with absolute motions of Pacific and Nazca plates.(Conrad & Lithgow-Bertolli, 2002,2004)
NazcaPacific
Monday, 6 January 14
Absolute motions of Pacific & Nazca/Farallon plates
N
E
W
Nazca
Pacific
calculated near ridge at 15º SMonday, 6 January 14
Absolute motions of Pacific & Nazca/Farallon plates
N
E
W
Nazca
Pacific
Before 50 Ma: both plates speed up & slow down in concert. Faster rates associated with more northerly drift.
calculated near ridge at 15º SMonday, 6 January 14
Absolute motions of Pacific & Nazca/Farallon plates
N
E
W
Nazca
Pacific
?Before 50 Ma: both plates speed up & slow down in concert. Faster rates associated with more northerly drift.
After 50 Ma: Pacific plate slows down and Nazca plate speeds up as they bear more W & E
calculated near ridge at 15º SMonday, 6 January 14
Absolute motions of Pacific & Nazca/Farallon plates
N
E
W
Nazca
Pacific
?Before 50 Ma: both plates speed up & slow down in concert. Faster rates associated with more northerly drift.
After 50 Ma: Pacific plate slows down and Nazca plate speeds up as they bear more W & E
These intervals also coincide with periods of high asymmetry.
calculated near ridge at 15º SMonday, 6 January 14
Explaining absolute motions
Slowdown of the Pacific plate may be explained
by upwelling being slightly west of centre...
Rowley et al., submitted
Monday, 6 January 14
Explaining absolute motions
NazcaPacific
Slowdown of the Pacific plate may be explained
by upwelling being slightly west of centre...
Rowley et al., submitted
Monday, 6 January 14
Ridge migration in mantle frameE
ridge perpen-dicular
Ridge perpendicular wobbles that average out to roughly zero...
Monday, 6 January 14
Ridge migration in mantle frameEN,E
ridge parallel
ridge perpen-dicular
Ridge perpendicular wobbles that average out to roughly zero......superposed on (mostly N) ridge parallel drift.
Monday, 6 January 14
Ridge migration in mantle frameEN,E
ridge parallel
ridge perpen-dicular
Ridge perpendicular wobbles that average out to roughly zero......superposed on (mostly N) ridge parallel drift.
Linked changes in mantle drift & spreading behaviourMonday, 6 January 14
Time variation of coupling signalsRadial
mantle flux Faster Slower
Spreading Rate Faster Slower
Asymmetry Higher Lower
Absolute NAZ/PAC motions
More ridge orthogonal
Less ridge orthogonal
Migration over mantle Slower Faster
A 15-25 Myr cycle?
Monday, 6 January 14
Time variation of coupling signalsRadial
mantle flux Faster Slower
Spreading Rate Faster Slower
Asymmetry Higher Lower
Absolute NAZ/PAC motions
More ridge orthogonal
Less ridge orthogonal
Migration over mantle Slower Faster
A 15-25 Myr cycle?
Monday, 6 January 14
More than slab pull!
The spreading behaviour of the EPR can only be fully explained
in terms of a significant dynamic contribution from mantle flow
under the ridge axis.
This contribution appears to have varied in
magnitude (~15-25 Myr periodicity) and may have changed fundamentally in
nature at ~50 Ma.
Monday, 6 January 14
More than slab pull!
The spreading behaviour of the EPR can only be fully explained
in terms of a significant dynamic contribution from mantle flow
under the ridge axis.
This contribution appears to have varied in
magnitude (~15-25 Myr periodicity) and may have changed fundamentally in
nature at ~50 Ma.
Monday, 6 January 14
Monday, 6 January 14
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