Vertical mantle flow associated with a lithospheric drip beneath the Great Basin

1. Literature Review: Vertical mantle flow associated with a lithospheric drip beneath the Great Basin John D. West, Matthew J. Fouch, Jeffrey B. Roth, and Linda T. Elkins-Tanton

2. Term to Remember…Whether you want to or not! • Models are Consistent with a Lithospheric Drip. • Period.

3. Topical Overview • Shear Wave Splitting • Due to flow induced strain. • Lattice-Preferred Orientation (LPO) Olivine • Fast splitting LPO indicate direction of horizontal flow in upper mantle. • Seismic Tomography • Numerical Modelling of Lithospheric Drip • Mantle Flow Direction

4. Drips vs. Delamination • Lithospheric Drips • Analogous to “negative” plumes. • Begin as seed density anomaly. • “Fall” by means of Rayleigh-Taylor instability. • Central symmetrical core of downward flow. • Large vertical velocities relative to plate motion. • Lithospheric material feeds from larger lateral “disc”.

5. Drips vs. Delamination • Lithospheric Delamination • Peeling away from dense layered structure • Often occurs from base of thickened crust. • Delamination is a slower, broader process. • Tend to generate substantial modification of regional fabric. • Do not tend to exhibit strong variations in shear-wave splitting.

6. Previous Work • Lithospheric drips rarely detected in mantle. • Focus Areas • Surface uplift due to removal of eclogitic root. • Heating of lithosphere by plume head interaction. • Margins of active rift zones.

7. Important Considerations • Drips in young, thin lithosphere not usually considered. • Important to note that these areas are likely gravitationally unstable. • Drips may not exhibit surficial effects such as elevation change or regional volcanism.

8. Great Basin Province • Well known region of widespread extension and magmatism. • Several Episodes of Magmatism • ~80 - ~20 Mya • Ignimbrite flare up between 31 – 20 Mya • Crustal Extension • Initiated ~45 Mya • East/West direction at 10 – 15 mm/yr

9. Great Basin Province • High average elevation (1500 – 1700 m) • High average heat flow (~100 mW/m2) • Moderate crustal thickness (~35 km) • Thin Lithosphere (60 – 75 km)

10. Great Basin Province • Low Strain Rate (GPS) Kreemer et al. (2004)

11. Seismic Observations • Apparent absence of mantle fabric. • Consistent with lateral asthenospheric flow. • Absence of mantle fabric has led to range of mantle flow models for the region. From Sheehan et al. (1997)

12. Mantle Flow Models • Vertical Upgoing Flow from Mantle Plume. • Upward flow redirected by plate motion. From Savage and Sheehan (2000)

13. Mantle Flow Models • Inconsistencies of Mantle Plume model: • Magmatism at edges of Great Basin generated from shallow asthenospheric source rather than upwelling mantle. • Zone of regionally reduced heat flow. • Cold Nevada cylinder.

14. Mantle Flow Models • Toroidal (donut shaped) Flow Generated by Retreating Juan de Fuca Slab. • Creates anisotropy field that results in circular flow field. From Zandt and Humphreys (2008)

15. Mantle Flow Models • Inconsistencies in toroidal flow model: • Low splitting rates due to low strain rates at center of toroid. • Low strain rates would not erase mantle fabric.

16. Shear Wave Splitting • Seismic data recorded from EarthScope array. • Variations in seismic azimuthal anisotropy. From West et al. (2009)

17. Shear Wave Splitting • Splitting times drop to near zero values across central Great Basin (CGB). • Only region in western U.S. • Outside CGB splitting times range 1.25 – ≥2.25 seconds (Largest in N.A.) • South of Region • Northeast – Southwest fast directions • North of Region • East – West fast directions.

18. P-Wave Tomographic Model • Mantle thermal and compositional heterogeneity. • Near-vertical cylindrical zone of increased P-wave velocities. From West et al. (2009)

19. P-Wave Tomographic Model From West et al. (2009)

20. “Nevada Cylinder” • Approximate 100 km diameter. • Near-vertical extent from 75 km – 500 km. • Near 500 km depth, cylinder merges with separate zone of high velocity material. • Increased seismic velocities similar to those of subducting Juan de Fuca slab. • Suggests lithospheric origin.

21. Sublithospheric Mantle Fabric? • Evidenced By: • Shear wave splitting. • Constraint of thin lithosphere. • Similarity in fast directions over broad length scales. • Large splitting times due to lateral flow. • Mantle fabric due to horizontal strains. • But…what about the centrally located small splitting times?

22. “Small” Splitting Times • Isotropy resulting from small strains? • Complex anisotropy producing minimal shear-wave splitting. • Collocated with conduit of higher seismic velocity • Implies presence of lithospheric drip. • If you didn’t catch it earlier.

23. Gravitational Driven Convective Instabilities • 1% Density Anomaly / 10% Temp. Increase From West et al. (2009)

24. Higher Density Anomaly • Localized compositional variation. • Structural variations. • Due to accretion during early geologic development. • Or – Localized dense mafic accumulations following widespread regional magmatism.

25. Higher Temperature Anomaly • Asthenospheric warming due to slab window in Farallon plate. • Mechanical lithospheric thinning due to regional extension.

26. Relative Plate Motion From Fowler (1990)

27. Relative Plate Motion From Kreemer (2009)

28. Relative Plate Motion From Kreemer et al., Nevada Geodetic Laboratory (2010)

29. Orientation of Lithospheric Drip • Mantle flow is northeast relative to N.A. plate motion? From West et al. (2009)

30. 3D Mantle Flow Field • Drips contribute to more complex 3D flow than what most models represent. • Drips are a typical component of many mantle convection models. • Often disregarded due to relative size. • Great Basin drip may be closely related to rapid changes in plate boundaries. • OR – Simply natural component of mantle convection.

31. In Closing…. • Models are Consistent with a Lithospheric Drip. • It is a DRIP….can’t really identify the generating process. • ……but it is a DRIP!