Detailed analysis of the spreading behaviour of the East Pacific Rise in the past 83 Ma reveals time-dependent variations that are difficult to explain in terms of changes in slab pull forces, and suggest that forces acting at the ridge axis - possibly related to a region of intense dynamic upwelling revealed by mantle convection modelling - are also an important control on the evolution of this ridge system.

1. 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

2. The East Pacific Rise since 83 Ma
• East Pacific Rise (EPR)
Chron
34ny
(83 Ma)
is the remnant of
much longer PacificFarallon Ridge.
• Has produced ~45%
of reconstructable
oceanic lithosphere
since 83 Ma (Rowley
2008).
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)
Monday, 6 January 14

3. EPR in the mantle reference frame
Indo-Atlantic hotspot frame, Lord Howe circuit.
Rowley et al., submitted.
Unlike other spreading ridges, EPR axis has remained fixed
over one region of the mantle.
Monday, 6 January 14

4. EPR in the mantle reference frame
Indo-Atlantic hotspot frame, Lord Howe circuit.
Rowley et al., submitted.
Unlike other spreading ridges, EPR axis has remained fixed
over one region of the mantle.
Monday, 6 January 14

5. EPR in the mantle reference frame
Indo-Atlantic hotspot frame, Lord Howe circuit.
Rowley et al., submitted.
Unlike other spreading ridges, EPR axis has remained fixed
over one region of the mantle.
Monday, 6 January 14

6. Spreading asymmetry & its significance
Chron 24.3no
(53.35 Ma)
Pacific isochron
Monday, 6 January 14

7. Spreading asymmetry & its significance
Chron 24.3no
(53.35 Ma)
Pacific isochron
50.78 Ma
C24.3no
Predicted
Nazca
isochron
Monday, 6 January 14

8. Spreading asymmetry & its significance
Chron 24.3no
(53.35 Ma)
Pacific isochron
50.78 Ma
C24.3no
Predicted
Nazca
isochron
Long term Pacific
spreading fraction ≈ 0.42
Monday, 6 January 14

9. Spreading asymmetry & its significance
Chron 24.3no
(53.35 Ma)
Pacific isochron
50.78 Ma
C24.3no
Predicted
Nazca
isochron
Long term Pacific
spreading fraction ≈ 0.42
Without asymmetric spreading,
EPR would not remain fixed.
Monday, 6 January 14
symmetric
since 50 Ma

10. Spreading asymmetry & its significance
Chron 24.3no
(53.35 Ma)
Pacific isochron
50.78 Ma
C24.3no
Predicted
Nazca
isochron
Long term Pacific
spreading fraction ≈ 0.42
Without asymmetric spreading,
EPR would not remain fixed.
Monday, 6 January 14
symmetric
since 50 Ma
& 83 Ma

11. Stable mantle upwelling beneath EPR
650 km depth
Rowley et al., submitted.
cm/yr
Predicted mantle flow based on
buoyancy distribution model TX2008
(Simmons et al. 2009) and ‘V2’ viscosity
profile (Mitrovica & Forte 2004).
Monday, 6 January 14

12. Stable mantle upwelling beneath EPR
250 km depth
Rowley et al., submitted.
cm/yr
Predicted mantle flow based on
buoyancy distribution model TX2008
(Simmons et al. 2009) and ‘V2’ viscosity
profile (Mitrovica & Forte 2004).
Monday, 6 January 14

13. Stable mantle upwelling beneath EPR
250 km depth
cm/yr
Predicted mantle flow based on
buoyancy distribution model TX2008
(Simmons et al. 2009) and ‘V2’ viscosity
profile (Mitrovica & Forte 2004).
Monday, 6 January 14
shaded area: radial flow
velocity>2cm/yr

14. Mantle flow & spreading behaviour
-10˚
-20˚
-30˚
180˚ -170˚ -160˚ -150˚ -140˚ -130˚ -120˚ -110˚ -100˚ -90˚ -80˚ -70˚ -60˚
0
depth (km)
400
800
1200
1600
2000
2400
2800
0
10
20
5 cm/yr
-0.5
30
40
50
60
70
90
100
distance (∆)
0.0
δρ/ρ (%)
It is also strongly
asymmetric.
Monday, 6 January 14
80
0.5
110
120
Divergent mantle
flow in uppermost
mantle leads rather
than lags overriding
plate motions.

15. Mantle flow & spreading behaviour
Pacific & Nazca plates have both
slowed down in past 5-10 Ma...
Pacific
Age
Monday, 6 January 14

16. Mantle flow & spreading behaviour
Pacific
Pacific & Nazca plates have both
slowed down in past 5-10 Ma...
...matching modelled effects of
changing mantle flow.
Age
Forte et al. 2008
Monday, 6 January 14

17. Spreading rate & asymmetry
Rowan & Rowley, in revision
more Pacific plate
more Nazca plate
Monday, 6 January 14
50 Myr record of
spreading
asymmetry:
clear variability

18. Spreading rate & asymmetry
Rowan & Rowley, in revision
more Pacific plate
more Nazca plate
50 Myr record of
spreading
asymmetry:
clear variability
Increasing
asymmetry
appear linked to
increases in
spreading rate.
Monday, 6 January 14

19. More than slab pull?
Distribution of
slab pull forces
are consistent
with absolute
motions of Pacific
and Nazca plates.
(Conrad & LithgowBertolli, 2002,2004)
Pacific
Monday, 6 January 14
Nazca

20. More than slab pull?
Distribution of
slab pull forces
are consistent
with absolute
motions of Pacific
and Nazca plates.
(Conrad & LithgowBertolli, 2002,2004)
Pacific
Nazca
But changes
induced by a time
varying ‘plume
push’* at ridge axis
could increase
spreading rate &
asymmetry.
*(cf. Cande & Stegman,
2011)
Monday, 6 January 14

21. Absolute motions of Pacific
& Nazca/Farallon plates
Pacific
Nazca
E
N
W
calculated near ridge at 15º S
Monday, 6 January 14

22. Absolute motions of Pacific
& Nazca/Farallon plates
Before 50 Ma: both
plates speed up & slow
down in concert. Faster
rates associated with
more northerly drift.
Pacific
Nazca
E
N
W
calculated near ridge at 15º S
Monday, 6 January 14

23. Absolute motions of Pacific
& Nazca/Farallon plates
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
?
Pacific
Nazca
E
N
W
calculated near ridge at 15º S
Monday, 6 January 14

24. Absolute motions of Pacific
& Nazca/Farallon plates
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
?
Pacific
Nazca
E
N
These intervals also
coincide with periods
of high asymmetry.
W
calculated near ridge at 15º S
Monday, 6 January 14

25. Explaining absolute motions
Rowley et al., submitted
Slowdown of the Pacific
plate may be explained
by upwelling being
slightly west of centre...
Monday, 6 January 14

26. Explaining absolute motions
Rowley et al., submitted
Slowdown of the Pacific
plate may be explained
by upwelling being
slightly west of centre...
Monday, 6 January 14
Pacific
Nazca

27. Ridge migration in mantle frame
E
ridge
perpendicular
Ridge perpendicular wobbles that average out to roughly zero...
Monday, 6 January 14

28. Ridge migration in mantle frame
N,E
E
ridge parallel
ridge
perpendicular
Ridge perpendicular wobbles that average out to roughly zero...
...superposed on (mostly N) ridge parallel drift.
Monday, 6 January 14

29. Ridge migration in mantle frame
N,E
E
ridge parallel
ridge
perpendicular
Ridge perpendicular wobbles that average out to roughly zero...
...superposed on (mostly N) ridge parallel drift.
Linked changes in mantle drift & spreading behaviour
Monday, 6 January 14

30. Time variation of coupling signals
Radial
mantle flux
Faster
Slower
Spreading
Rate
Faster
Slower
Asymmetry
Higher
Lower
Absolute
NAZ/PAC
motions
Migration
over mantle
Monday, 6 January 14
More ridge Less ridge
orthogonal orthogonal
Slower
Faster
A 15-25 Myr cycle?

31. Time variation of coupling signals
Radial
mantle flux
Faster
Slower
Spreading
Rate
Faster
Slower
Asymmetry
Higher
Lower
Absolute
NAZ/PAC
motions
Migration
over mantle
Monday, 6 January 14
More ridge Less ridge
orthogonal orthogonal
Slower
Faster
A 15-25 Myr cycle?

32. 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

33. 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

34. Monday, 6 January 14