Strength of the lithosphere Introduction to mantle rheology from laboratory approach

1. Strength of the lithosphereIntroduction to mantle rheology fromlaboratory approach Shun-ichiro KaratoYale UniversityNew Haven, USA EURISPET

2. Yale University EURISPET

3. 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 EURISPET

4. EURISPET

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

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

7. 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 EURISPET

8. Processes controlling the “strength” ductile deformation brittle deformation EURISPET

9. Strength in the brittle regimeByerlee’s law EURISPET

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

11. brittle versus plastic deformation Material dependence (opx versus olivine) P-dependence Water dependence EURISPET

12. strength strength depth depth ductile branch ductile branch EURISPET

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

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

15. How has a continental root survived? ~200 km thick continental lithosphere has survived for ~3Gyrs EURISPET

16. 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) 2 3 EURISPET

17. 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. EURISPET

18. 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 EURISPET

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

20. 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 EURISPET

21. 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. EURISPET

22. 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 EURISPET

23. opx (Ohuchi and Karato (2009)) ol opx ol EURISPET

24. opx (Ohuchi and Karato (2009) stress strain EURISPET

25. Role of a weak opx on the strength of an oli + opx mixture EURISPET

26. opx (IWL)-model ol (LBF)-model (Ohuchi and Karato (2009)) EURISPET

27. 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. EURISPET

28. Temperature difference? Continent versus ocean: temperature difference EURISPET

29. 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? EURISPET

30. Water weakening Strain rate low-P data Mei and Kohlstedt (2000) water content --> EURISPET

31. partial melting removes water EURISPET

32. strength depth Quantify the water weakening effect Quantify the P-effect on dry rheology EURISPET

33. Water weakeningneed to find a formula for extrapolation to high-pressures low-P data (<0.45 GPa) Mei and Kohlstedt (2000) EURISPET

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

35. 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) EURISPET

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

37. Need to know “dry” rheology to evaluate the effect of de-watering EURISPET

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

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

40. Synchrotron facility at Brookhaven National Lab EURISPET

41. Strain measurements by X-ray imaging EURISPET

42. Geometry of X-ray diffraction for the rotational Drickamer apparatus Diffracted X-ray Y 2q Incident X-ray Observed part EURISPET

43. wadsleyite EURISPET

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

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

46. EURISPET

47. 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. 2 3 EURISPET

48. 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? EURISPET

49. Conditions for the survival of continental roots EURISPET

50. Peridotite from the shear zone (Italy) modal fraction volume fraction of fine-grained region (%) opx ribbon EURISPET