Asthenosphere flow and mantle lithosphere instabilities below continental rifts and rifted margins Jolante van Wijk (University of Houston) Jeroen van.

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Presentation transcript:

Asthenosphere flow and mantle lithosphere instabilities below continental rifts and rifted margins Jolante van Wijk (University of Houston) Jeroen van Hunen (Durham University, UK) Saskia Goes (Imperial College, London)

Rio Grande rift S-wave tomography Detailed information on upper mantle seismic wave velocities Asthenosphere-lithosphere interactions? La RISTRA passive seismic experiment

Rio Grande rift S-wave tomography Shallow low seismic wave velocity anomaly below rift Low velocity anomaly has ‘crescent’ shape Fast velocity structures below margins of rift

La RISTRA Passive Seismic Experiment Results: Sine et al., GRL 2008 West et al., JGR 2004 Gao et al., JGR 2004

Numerical modeling approach -Citcom, finite element code Solve equations for conservation of mass, momentum and thermal energy Visco-plastic rheology

Numerical modeling approach -Velocity boundary conditions extend the lithosphere

Numerical modeling approach -Model domain does not change during tests, but surface elevation can be calculated by equating the surface normal stress to the topography

Synthetic seismic wave velocities 1) calculation of the phase diagram which provides the mineral proportions and mineral composition as a function of pressure and temperature (Goes et al., 2000; 2004; Cammarano et al. 2003; Connolly and Petrini, 2002; Stixrude and Lithogow-Bertelloni, 2005; van Wijk et al., 2008) 2) calculation of the mineral elastic parameters and density at the desired pressure-temperature conditions 3) addition of the dispersive effects on velocity of an-elasticity

Numerical models of lithosphere extension Symmetric rifts

Numerical models of lithosphere extension

Asthenosphere upwelling

Dynamic topography t=16.8 m.y.

Dynamic topography Meyer et al., GSA Spec. Paper 430, 2007

Synthetic seismic wave velocities rift (%)

Small-scale convection within rift

Western US geology

Rio Grande rift Rifting started in ~Early Miocene Low strain rates- extension rate 1-2 mm/yr Segmented character Northward propagation?

Rio Grande rift Gao et al., 2004 What explains the low-velocity perturbation below the Rio Grande rift?

Rio Grande rift What can explain the significant (~6%) low velocity anomaly below the rift? Compositional variations? Melt? Passive upwelling of mantle material? (temperature)

Rio Grande rift What can explain the significant (~8%) low velocity anomaly below the rift? Compositional variations? Melt? Passive upwelling of mantle material? (temperature)

Application to the Rio Grande rift Initial step in lithosphere thickness at edge of Great Plains No weak zone/seed in model

Application to the Rio Grande rift Great Plains Rio Grande rift Van Wijk et al., Geology, 2008

Rio Grande rift seismic velocity anomaly -predicted temperature structure -predicted S-wave anomaly (pyrolite composition) -La RISTRA tomography (West et al., 2004)

Rio Grande rift seismic velocity anomaly West et al., JGR 2004 pyrolite composition Archaean lithosphere (Griffin et al. 1999)

Rio Grande rift seismic velocity anomaly Amplitude and size are similar This anomaly can be explained thermally, even though the total amount of extension is small and extension rates have been low What explains the ‘moon’ shape?

Summary Asthenosphere flow below rifts: Upwelling asthenosphere below rifts limited to asthenospheric depths Dynamic topography of margins around breakup time Tomography of Rio Grande rift (low velocity anomaly, fast velocity structures)