Rheological Controls on Strain Partioning during Continental Extension (When does E=MC 2 ?) Chris Wijns, Klaus Gessner, Roberto Weinberg, Louis Moresi

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.

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 !

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

I. Generic Extension Models 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)

Physical and numerical model T=0 o C T=1200 o C T=400 o C d  /dt = 6.3x s -1 = 3.1 mm/yr = 100% extension in 5 Ma

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

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

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

Two basic modes

End-member modes free-slip lower boundary zero-slip lower boundary basal velocity profile

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

Continuum of behaviour: r 

Continuum of behaviour: r h

Continuum of behaviour: fault spacing empirical relationship predicts mode of extension

Secondary factors: fault weakening crustal necking instead of planar fault zones

Secondary factors fault weakening buoyancy

Validation test Central Menderes mcc

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

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

Western Turkey: Central Menderes

Central Menderes from Gessner et al. (2001)

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

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

from Chéry (2001) Incipient vs mature detachment

from Koyi and Skelton (2001) Analogue modelling

Top vs. bottom driven extension from Tikoff et al. (2002) [Rey et al., 2001]

from Martinez et al. (2001) Buoyancy driven mcc

? upper crust: 12.5 km ? lower crust: 25 km ? upper mantle: km  ß =1.7 ! velocity: 1.25 cm / yr each side  d  /dt = 6.3x ! time: 3.52 Ma More modelling reults

Single fault: “rolling hinge” in mcc mode

Two faults: bivergent model for Central Menderes

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

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

   Reproducible field observations

Not modelled

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