Signals of dynamic coupling between mantle and lithosphere beneath the axis of the East Pacific Rise - AGU 2013

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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.

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Signals of dynamic coupling between mantle and lithosphere beneath the axis of the East Pacific Rise - AGU 2013

  1. 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. 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. 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. 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. 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. 6. Spreading asymmetry & its significance Chron 24.3no (53.35 Ma) Pacific isochron Monday, 6 January 14
  7. 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. 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. 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. 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. 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. 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. 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. 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. 15. Mantle flow & spreading behaviour Pacific & Nazca plates have both slowed down in past 5-10 Ma... Pacific Age Monday, 6 January 14
  16. 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. 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. 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. 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. 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. 21. Absolute motions of Pacific & Nazca/Farallon plates Pacific Nazca E N W calculated near ridge at 15º S Monday, 6 January 14
  22. 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. 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. 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. 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. 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. 27. Ridge migration in mantle frame E ridge perpendicular Ridge perpendicular wobbles that average out to roughly zero... Monday, 6 January 14
  28. 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. 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. 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. 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. 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. 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. 34. Monday, 6 January 14

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