Download - Rheological Controls on Strain Partioning during Continental Extension (When does E=MC 2 ?)
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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
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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.
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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 !
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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
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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)
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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
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Crustal strength profileCrustal strength profile
Byerlee coeff = 0.44
maximum shear stress = 250 Mpa
crustal thickness = 60 km
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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
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Two basic modesTwo basic modes
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Continuum of behaviourContinuum of behaviour
• r = ratio of integrated maximum shear stress of upper to lower crust
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Continuum of behaviour: rContinuum of behaviour: r
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Continuum of behaviour: rContinuum of behaviour: rhh
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Continuum of behaviour: fault spacingContinuum of behaviour: fault spacing
• empirical relationship predicts mode of extension
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Secondary factors: fault weakeningSecondary factors: fault weakening
• crustal necking instead of planar fault zones
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Secondary factorsSecondary factors
• fault weakening
• buoyancy
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Validation testValidation test
Central Menderes mcc
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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
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II. Field Examples and II. Field Examples and Conceptual ModelsConceptual Models
Numerical models explain some field observations or suggest new observations
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Western Turkey: Central MenderesWestern Turkey: Central Menderes
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from Gessner et al. (2001) [Wernicke, 1981; Spencer, 1984; Buck, 1988]
Conceptual models: rolling hingeConceptual models: rolling hinge
vs.
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Initial low angle detachmentInitial low angle detachment
from Davis, Lister, and Reynolds (1986)
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from Koyi and Skelton (2001)
Analogue modellingAnalogue modelling
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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
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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
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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
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Not modelledNot modelled
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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