Small scale convection at the edge of the Colorado Plateau?

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Miles Conley
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1 Small scale convection at the edge of the Colorado Plateau? Jolante van Wijk & David Coblentz, Rick Aster, Jeroen van Hunen, Saskia Goes, Steve Grand, Scott Baldridge, Jim Ni 1

2 La RISTRA seismic experiment 2

3 Sine et al. GRL 2008 Surface expression of Colorado Plateau = Colorado Plateau mantle lithosphere Short wavelength velocity variations in asthenosphere Two drip like anomalies below Plateau 3

4 150 km depth 200 km depth Obrebski et al., GJI

5 Two fast velocity features below CP in asthenosphere Large lateral variations in seismic wave velocities Low seismic wave velocities below CP edge Cold lithospheric downwellings? Convective removal of CP lithosphere? 5

6 Colorado Plateau tectonically stable, not so much affected by extension that formed Basin & Range Province and Rio Grande Rift Coblentz et al

7 As a consequence, Colorado Plateau lithosphere thicker than surrounding extending provinces Coblentz et al

8 BRP and RGR lithosphere thinned Step in LAB between CP and surrounding extended provinces May induce edge driven convection and form downwellings King and Andersen,

9 CitCom finite element code ( Conservation of mass, momentum, thermal energy, incompressible fluid, Boussinesq approximations (with adiabatic heating/cooling) Visco plastic rheology High viscosity lithosphere 9

10 Van Wijk et al

11 Van Wijk et al

12 Seismic wave velocities are predicted from model temperatures including elastic and anelastic effects and variations of mineral phase composition with pressure and temperature Van Wijk et al

imaged Composition changes absolute velocities but not anomalies

13 If these are downwellings: Vs and Vp contrast between CP and BRP similar in magnitude to imaged Amplitude of downwellings slightly smaller than imaged Composition changes absolute velocities but not anomalies Additional contribution due to melt in BRP likely required Van Wijk et al

14 Model predictions: Uplift of Colorado Plateau edge Coblentz et al

15 Model predictions: Uplift of Colorado Plateau edge Magmatism at Colorado Plateau edge Van Wijk et al

16 Model predictions: Uplift of Colorado Plateau edge Magmatism at Colorado Plateau edge Late Neogene recent pulse of magmatism (that was preceded by a period of relative quiescence following the middle Cenozoic pulse) Van Wijk et al

17 Model predictions: Uplift of Colorado Plateau edge Magmatism at Colorado Plateau edge Thinning of Colorado Plateau edge, BRP and RGR lithosphere Levander et al. (2011) 17

18 Rio Grande rift: Small scale convection below rift Low velocity anomaly results from upwelling asthenosphere Van Wijk et al

19 Levander et al. (2011): Crustal thinning above downwelling Young feature Delamination above drip? 19

20 La RISTRA Moho (from receiver functions) and surface elevation: Thick crust beneath Colorado Plateau Old feature Wilson et al. GRL (2010) 20

How does the lithosphere deform above a downwelling? Gale (www.geodynamics.

21 How does the lithosphere deform above a downwelling? Gale ( Layered lithosphere Visco plastic rheology Van Wijk et al (in prep) 21

22 Material (Lower) crustal thickening above downwelling Drip fed mainly by BRP mantle lithosphere Van Wijk et al (in prep) 22

23 Temperature Up to 400 C temperature difference between downwelling and asthenosphere Van Wijk et al (in prep) 23

24 Is the downwelling detectable by GPS? Probably not, v_x amplitudes are order of magnitude smaller than lithosphere/mantle flow and upper crust rests on lower crust. v_x sign change offset from drip location Van Wijk et al (in prep) 24

25 Stresses: upper crust under compression above drip, under extension in Basin & Range Province Strain rates: high values Van Wijk et al (in prep) 25

26 Uplift/subsidence of surface: Subsidence somewhat offset from downwelling 26

27 Yielding in lower crust: detachment? Crustal thickening: eclogite 27

28 Downwelling detachment? High strain rate & yielding indicate that detachment in lower crust may follow downwelling 28

29 Geodynamic models: these could be downwellings 200 km depth CP affected by Farallon slab: rehydration and metasomatism may have lowered CP viscosity (more conducive to destabilization and convection) Obrebski et al., GJI

High resolution receiver function imaging reveals Colorado Plateau lithospheric architecture and mantle supported topography

GEOPHYSICAL RESEARCH LETTERS, VOL. 37,, doi:10.1029/2010gl044799, 2010 High resolution receiver function imaging reveals Colorado Plateau lithospheric architecture and mantle supported topography David