1. June 29, 2009EURISPET1 Strength of the lithosphere Introduction to mantle rheology from laboratory approach Shun-ichiro Karato Yale University New Haven, USA

2. June 29, 2009EURISPET2 Yale University

3. June 29, 2009EURISPET3 Outline rheology and geological problems plate tectonics, survival of continents fundamentals of non-elastic deformation oceanic lithosphere importance of opx continental lithosphere water, pressure effects

4. June 29, 2009EURISPET4

5. June 29, 2009EURISPET5 Plate Tectonics (bending) Survival of Continents (strength of the oceanic lithosphere) (oceanic lithosphere) (strength of the continental lithosphere) Geodynamic issues in subduction zones related to rheological properties

6. June 29, 2009EURISPET6 A conventional model of lithosphere strength (Kohlstedt et al., 1995) This model does not explain major geological features: too strong oceanic lithosphere for plate tectonics too weak continents to preserve deep continental roots oceanic lithospherecontinental lithosphere

7. June 29, 2009EURISPET7 ABC of rock deformation How to construct a strength profile? Brittle deformation generation and propagation of a fault Plastic deformation permanent strain due to microscopic atomic motion

8. June 29, 2009EURISPET8 Processes controlling the “strength” brittle deformation ductile deformation

9. June 29, 2009EURISPET9 Strength in the brittle regime Byerlee’s law

10. June 29, 2009EURISPET10 Ductile deformation by thermally activated processes  ~  1/n exp (G*/nRT) G*: material dependent, P dependent (G*=E*+PV*-TS*) The rate depends on defect concentration. G*.

11. June 29, 2009EURISPET11 brittle versus plastic deformation Material dependence (opx versus olivine) P-dependence Water dependence

12. June 29, 2009EURISPET12 depth strength ductile branch

13. June 29, 2009EURISPET13 homogeneous deformation Rheology (of oceanic lithosphere) and mantle convection (Tackley, 2000) plate tectonics stagnant lid (Solomatov-Moresi, 1996, 1997)

14. June 29, 2009EURISPET14 Plate tectonics would not occur on Earth for this model. (Kohlstedt et al., 1995) plate tectonics

15. June 29, 2009EURISPET15 How has a continental root survived? ~200 km thick continental lithosphere has survived for ~3Gyrs

16. June 29, 2009EURISPET16 In order to preserve the deep continental root, it must have a high viscosity (>10 -10 higher than the surrounding mantle). Lenardic and Moresi (1999) 23

17. June 29, 2009EURISPET17 A conventional model (Kohlstedt et al., 1995) Continental roots would be weaker than deep oceanic mantle --> continental roots would not have survived for this model.

18. June 29, 2009EURISPET18 A conventional model of lithosphere strength (Kohlstedt et al.,1995) fails to explain the most important features of geological processes: plate tectonics and long-term stability of continents. What are wrong with that model ?  Limited experimental conditions (low pressure)  Uncertainties in water content in the continental mantle  olivine-based model  continental lithosphere was assumed to be “wet”  water, P-effects are poorly constrained

19. June 29, 2009EURISPET19 needs for deformation experiments at higher P 1.Deformation of minerals that are stable only at high P (opx, wadsleyite, ringwoodite etc.) 2.Characterization of water and pressure effects

20. June 29, 2009EURISPET20 Deformation apparatus Paterson apparatus P<0.5 GPa, T<1550 K Rotational Drickamer apparatus P<17 GPa, T<2300 K Griggs apparatus P<3 GPa, T<1600 K Oceanic lithosphere: P to 3 GPa, T to 1500 K Continental lithosphere: P to 10 GPa, T to 1700 K

21. June 29, 2009EURISPET21 Oceanic lithosphere (why plate tectonics on Earth?) Oceanic lithosphere is (nearly) dry and cold. brittle fracture + dry olivine (power-law creep) --> too strong How can one make the lithosphere weak at low T (and dry)? Plastic deformation is material sensitive. Lithosphere is made of olivine + opx. How about opx (orthopyroxene)? Little previous studies on opx deformation. Opx is stable only above ~1 GPa (at high T) A conventional gas-apparatus can be used only below 0.5 GPa.

22. June 29, 2009EURISPET22 Plastic deformation of opx (Ohuchi and Karato, 2009) Griggs apparatus (1.3 GPa, 973-1273 K) CsCl pressure medium Simple shear With a small amount of water

23. June 29, 2009EURISPET23 (Ohuchi and Karato (2009)) opx ol opx ol

24. June 29, 2009EURISPET24 (Ohuchi and Karato (2009) strain stress opx

25. June 29, 2009EURISPET25 Role of a weak opx on the strength of an oli + opx mixture

26. June 29, 2009EURISPET26 (Ohuchi and Karato (2009)) opx (IWL)-model ol (LBF)-model

27. June 29, 2009EURISPET27 How has a continental root survived? (Kohlstedt model) Continental roots would be weaker than deep oceanic mantle --> continental roots would not have survived. Rheology of the deep continental roots.

28. June 29, 2009EURISPET28 Temperature difference? Continent versus ocean: temperature difference

29. June 29, 2009EURISPET29 Causes for a strong continent Temperature difference is too small. Water content difference? Water enhances deformation. Continental upper mantle is “depleted”(large degree of partial melting). --> hardening of continental roots by partial melting?

30. June 29, 2009EURISPET30 Water weakening low-P data Mei and Kohlstedt (2000) water content --> Strain rate

31. June 29, 2009EURISPET31 partial melting removes water

32. June 29, 2009EURISPET32 depth strength Quantify the water weakening effect Quantify the P-effect on dry rheology

33. June 29, 2009EURISPET33 Water weakening need to find a formula for extrapolation to high-pressures low-P data (<0.45 GPa) Mei and Kohlstedt (2000)

34. June 29, 2009EURISPET34 Data from a broad pressure range are needed to characterize the water effect. Mei-Kohlstedt Karato-Jung (Karato, 1989)

35. June 29, 2009EURISPET35 Pressure effects on creep strength of olivine (“wet”) Variation in the strength of olivine under “wet” conditions is different from that under “dry” conditions. The strength changes with P in a non-monotonic way. High-P data show much higher strength than low-P data would predict. Karato and Jung (2003)

36. June 29, 2009EURISPET36 A two-parameter (r, V*) equation fits nicely to the data. Karato and Jung (2003)

37. June 29, 2009EURISPET37 Need to know “dry” rheology to evaluate the effect of de-watering

38. June 29, 2009EURISPET38 High-P deformation gas-medium apparatus P<0.5 GPa, T<1550 K Rotational Drickamer apparatus P to 17 GPa, T<2300 K Griggs apparatus P<3 GPa, T<1600 K

39. June 29, 2009EURISPET39 RDA (rotational Drickamer apparatus) 1.High P-T (good support, nearly homogeneous T (P)) 2.Large strain (torsion tests) 3. Relatively large sample size (broad range of grain-size)

40. June 29, 2009EURISPET40 Synchrotron facility at Brookhaven National Lab

41. June 29, 2009EURISPET41 Strain measurements by X-ray imaging

42. June 29, 2009EURISPET42 Incident X-ray  Geometry of X-ray diffraction for the rotational Drickamer apparatus Diffracted X-ray 22 Observed part

43. June 29, 2009EURISPET43 wadsleyite

44. June 29, 2009EURISPET44 (dry) olivine, deformation Important to conduct high-P experiments (low-P experiments are not useful even though they are high-resolution). Kawazoe et al. (2009)

45. June 29, 2009EURISPET45 Hardening due to de-watering (  T=0) oceanic, wedge mantle continental lithosphere

46. June 29, 2009EURISPET46

47. June 29, 2009EURISPET47 Summary-I  In order to obtain critical data on the rheological properties from experimental studies, one needs to conduct deformation experiments beyond ~1 GPa.  With a pure olivine lithosphere, plate tectonics is difficult to operate: opx may weaken the lithosphere to allow plate tectonics to operate.  The de-watering in the deep upper mantle can increase the viscosity ~10 - 10 times that would stabilize the continental roots. 23

48. June 29, 2009EURISPET48 Summary-II (issues to be studied further)  Role of opx in an opx+ol mixture: experimental study on deformation of an opx-ol mixture, study of naturally deformed rocks  Is the continental lithosphere really “dry”?  Does subduction help growth of continents or destroy them?

49. June 29, 2009EURISPET49 Conditions for the survival of continental roots

50. June 29, 2009EURISPET50 volume fraction of fine-grained region (%) modal fraction Peridotite from the shear zone (Italy) opx ribbon

51. June 29, 2009EURISPET51 water

52. June 29, 2009EURISPET52 Big Mantle Wedge (BMW) model (Zhao et al., 2004) Big Mantle Wedge (1) Shallow & deep slab dehydration (Ohtani et al., 2004); (2) Corner flow (convection) in the Big Mantle Wedge; (3) Thinning & fracture of the continental lithosphere; (4) Upwelling of the asthenospheric materials to form the intraplate volcanoes.

53. June 29, 2009EURISPET53 Topography Vp tomography at 600 km depth Huang & Zhao (2006) JGR Bouguer gravity North-South gravity lineament Ma (1989); Xu (2007) The western edge of the stagnant slab roughly coincides with the surface topographic boundary & NSGL. The stagnant slab has affected the surface structure and tectonics?

54. June 29, 2009EURISPET54 Zhao (2004) PEPI 146, 3-34. Global mantle tomography

55. June 29, 2009EURISPET55 Deformation mechanism map

56. June 29, 2009EURISPET56 Ductile rheology Plastic deformation in minerals occurs due to the thermally activated motion of crystalline defects.

57. June 29, 2009EURISPET57 Rate of deformation (strain-rate) (density of defect)*(velocity of defect motion) (velocity of defect motion) (driving force [stress])*(mobility) (mobility) exp[-H  C  /RT]: depends on mechanisms (density of defect): function of T,  C: depends on mechanisms Many mechanisms exist for plastic deformation. -> “strength” depends on mechanisms.

58. June 29, 2009EURISPET58 (Ohuchi and Karato (2009)

59. June 29, 2009EURISPET59 (Ohuchi and Karato (2009)