Rheological Controls on Rheological Controls on Strain Partioning during Strain Partioning during Continental ExtensionContinental Extension(When does E=MC(When does E=MC22 ?) ?)

Chris Wijns, Klaus Gessner,

Roberto Weinberg, Louis Moresi

Dynamical modelers’ jokeDynamical 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 …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 !

OutlineOutline

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 ModelsI. 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 modelPhysical and numerical model

T=0 oC

T=1200 oC

T=400 oC

d/dt = 6.3x10-15 s-1 = 3.1 mm/yr = 100% extension in 5 Ma

Crustal strength profileCrustal strength profile

Byerlee coeff = 0.44

maximum shear stress = 250 Mpa

crustal thickness = 60 km

End-member: distributed faultingEnd-member: distributed faulting

• strong lower crust• many closely-spaced faults; limited slip;

contiguous upper crust

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End-member: metamorphic core End-member: metamorphic core complexescomplexes

● weak lower crust● few, widely-spaced faults; large strain; block

and fault rotation; exhumed lower crust

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Two basic modesTwo basic modes

Two basic modesTwo basic modes

Continuum of behaviourContinuum of behaviour

• r = ratio of integrated maximum shear stress of upper to lower crust

Continuum of behaviour: rContinuum of behaviour: r

Continuum of behaviour: rContinuum of behaviour: rhh

Continuum of behaviour: fault spacingContinuum of behaviour: fault spacing

• empirical relationship predicts mode of extension

Secondary factors: fault weakeningSecondary factors: fault weakening

• crustal necking instead of planar fault zones

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Secondary factorsSecondary factors

• fault weakening

• buoyancy

Validation testValidation test

Central Menderes mcc

Conclusions part IConclusions 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 II. Field Examples and Conceptual ModelsConceptual Models

Numerical models explain some field observations or suggest new observations

Western Turkey: Central MenderesWestern Turkey: Central Menderes

from Gessner et al. (2001) [Wernicke, 1981; Spencer, 1984; Buck, 1988]

Conceptual models: rolling hingeConceptual models: rolling hinge

vs.

Initial low angle detachmentInitial low angle detachment

from Davis, Lister, and Reynolds (1986)

from Koyi and Skelton (2001)

Analogue modellingAnalogue modelling

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 reultsMore modelling reults

Single fault: “rolling hinge”Single fault: “rolling hinge”

• in mcc mode

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Temperature evolutionTemperature evolution

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

Low-angle “detachment fault”Low-angle “detachment fault”

• very low friction coefficient (yield strength) for lower crust near lithostatic pore pressure

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Reproducible field observationsReproducible field observations

Not modelledNot modelled

Conclusions part IIConclusions 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