Rheological Controls on Strain Partioning during Continental Extension (When does E=MC 2 ?)

1. Chris Wijns, Klaus Gessner, Roberto Weinberg, Louis Moresi Rheological Controls on Strain Partioning during Continental Extension(When does E=MC2 ?)

2. Dynamical modelers’ joke There are only 10 types of people in this world • those that understand binary • and those that don’t If you don’t think this is funny you’ll realize that modelers don’t necessarily think like other people.

3. A Meta-benchmark … • How do you know to trust dynamic models ? • If you trust a black box model, then what ? • Why would you want a dynamic model and not a kinematic one ? • When the kinematics is ambiguous • When you want to predict general behaviours • Example - what happens when geologists get hold of a modeling code !

4. Outline I. Generic crustal extension models • physical and numerical model • end-member modes: distributed faulting vs. mcc • continuum of behaviour and secondary factors II. Field Examples • western Turkey • conceptual models of mcc and rolling hinges • related numerical modelling results

5. Conclusion: the vertical rheological contrast between upper and lower crust is the key to fault spacing and the mode of extension (in the absence of heterogeneities) I. Generic Extension Models

6. T=0 oC T=400 oC T=1200 oC Physical and numerical model d /dt = 6.3x10-15 s-1 = 3.1 mm/yr = 100% extension in 5 Ma

7. Crustal strength profile • Byerlee coeff = 0.44 • maximum shear stress = 250 Mpa • crustal thickness = 60 km

8. End-member: distributed faulting • strong lower crust • many closely-spaced faults; limited slip; contiguous upper crust

9. End-member: metamorphic core complexes • weak lower crust • few, widely-spaced faults; large strain; block and fault rotation; exhumed lower crust

10. Two basic modes

11. Two basic modes

12. Continuum of behaviour • r = ratio of integrated maximum shear stress of upper to lower crust

13. Continuum of behaviour: r

14. Continuum of behaviour: rh

15. Continuum of behaviour: fault spacing • empirical relationship predicts mode of extension

16. Secondary factors: fault weakening • crustal necking instead of planar fault zones

17. Secondary factors • fault weakening • buoyancy

18. Validation test Central Menderes mcc

19. Conclusions part I • ratio of upper to lower crust “strength” controls fault spacing and mode of extension • strong lower crust = distributed faulting • weak lower crust = mcc • note: pre-existing weaknesses may change the mode • secondary controls: ratio of upper to lower crust thickness, fault weakening, lower crust buoyancy

20. II. Field Examples and Conceptual Models Numerical models explain some field observations or suggest new observations

21. Western Turkey: Central Menderes

22. Conceptual models: rolling hinge vs. from Gessner et al. (2001) [Wernicke, 1981; Spencer, 1984; Buck, 1988]

23. Initial low angle detachment from Davis, Lister, and Reynolds (1986)

24. Analogue modelling from Koyi and Skelton (2001)

25. upper crust: 12.5 km • lower crust: 25 km • upper mantle: 9.375 km • ß =1.7 • velocity: 1.25 cm / yr each side • d /dt = 6.3x10-15 • time: 3.52 Ma More modelling reults

26. Single fault: “rolling hinge” • in mcc mode

27. Temperature evolution uniform T contours, i.e., single T “top” as in Snake Range

28. Low-angle “detachment fault” • very low friction coefficient (yield strength) for lower crust  near lithostatic pore pressure

29. Reproducible field observations   

30. Not modelled

31. Conclusions part II • current-like lateral flow of lower crust relative to upper crust segments • thermal structure of metamorphic domes • ductile shear zone operates continuously from surface to mid-crustal levels • flow patterns of exhumed footwall match kinematics of exhumed mylonitic fronts in mcc • mylonites may be a secondary feature, not an exhumed part of a primary, lithospheric scale shear zone