1. 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)

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

3. 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

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

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

6. Numerical modeling approach
-Velocity boundary conditions extend the lithosphere

7. 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

8. 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

9. Numerical models of lithosphere extension
Symmetric rifts

10. Numerical models of lithosphere extension

11. Asthenosphere upwelling

12. Dynamic topography
t=16.8 m.y.

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

14. Synthetic seismic wave velocities
rift
(%)

15. Small-scale convection within rift

16. Western US geology

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

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

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

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

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

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

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

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

25. 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?

26. 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)