signals of dynamic coupling between mantle and lithosphere beneath the axis of the east pacific rise...
TRANSCRIPT
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