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 - 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 ...

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 Signals of dynamic coupling between mantle and lithosphere beneath the axis of the East Pacific Rise - AGU 2013 Presentation 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 • 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
    • 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
    • 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
    • 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
    • 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 50.78 Ma C24.3no Predicted Nazca isochron Monday, 6 January 14
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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.
    • 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 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
    • 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
    • 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
    • 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
    • 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
    • Absolute motions of Pacific & Nazca/Farallon plates Pacific Nazca E N W calculated near ridge at 15º S Monday, 6 January 14
    • 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
    • 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
    • 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
    • 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
    • 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
    • Ridge migration in mantle frame E ridge perpendicular Ridge perpendicular wobbles that average out to roughly zero... Monday, 6 January 14
    • 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
    • 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
    • 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?
    • 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?
    • 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