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Earth 238-26
Metamorphic core complex
*Geological context: syn to post-orogenic extension
-interpreted as MCC for the first time in 1980 in the
« Basin and Range » province (North American Cordillera)
-usually formed in a region of thick crust which is
gravitationally unstable, and occur in areas of
synextensional magmatism
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Metamorphic core complex
*Gravitational collapse of a thicken crust
The crust was gravitationally unstable and spread outward under its own
weight. The origin of the spreading coming from a reduction of viscosity
by a mantle derived heating event (Coney, 1987), thermal relaxation of
the overthickened crust (Sonder, et el., 1987), or collapse and steepening
of a previously shallow-dipping Laramide Benioff zone, which may have
reduced the regional stress and possibly started extension (Coney and
Harms, 1984).
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Metamorphic core complex
*Slab break-off
http://www.mantleplumes.org/Anatolia.html
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Metamorphic core complex
*Lithospheric delamination
http://www.gfz-potsdam.de/pb2/pb24/projects/indepth/twolines/twolines.html
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*Characteristics and geometry
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«Basin and Range » province
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Metamorphic core complex
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Metamorphic core complex
*Rolling Hinge: Werneke (1985): simple shear model
3 Ma
Low angle normal faulting reaches deep into
the crust. Shearing takes place and mylonites
form.
8 Ma
Mylonites are pulled to the surface by normal faulting
displacement, unroofing causes isostatic doming
14 Ma
Subaerial denudation, and core complex emplacement.
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*Isostatic Uplift: Werneke and Axen (1988)
*Initial faulting at high angle
*Isostatic uplift which causes
the rotation of the fault.
Mohr-Coulomb criteria is respected.
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*Combination of Isostatic Uplift, simple shearing and pure
shearing: Lister and Davis (1989)
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*Flexural Uplift: Spencer, 1984
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*To sum up:
Metamorphic core complex
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Metamorphic core complex
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Metamorphic core complex
*All these models postulate a low angle detachment is
present at the beginning of the extensional process.
*These models show an uplift of the Moho. However, a
lot of seismic profile surveys have shown a flat Moho
under a lot of Metamorhic Core Complex.
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*Magmatic Underplating or Intrusion
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*Crustal Flow Model: (convergent crustal flow)
When the hanging wall of a normal detachment fault is thinned, the
vertical load that acts on the layers below it is reduced. This creates a
horizontal pressure gradient at depth and will drive a lateral flow upward
to equalize the gradient.
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Metamorphic core complex
*Crustal Flow Model: (divergent crustal flow)
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*Remain a question: is a low angle detachment
necessary at the beginning of the crustal thinning ?
*Need to have a new approach: Numerical and
analogue modelling
Ex: for analogue modelling : the brittle crust is modelled by sand
and the ductile crust by silicone (1/3 of sand, 2/3 of silicone)
We can introduce also a viscosity anomaly at the interface brittleductile to model weakness zone
*Numerical and analogue modelling have shown that it
is not necessary to have a low angle detachment at
the initial stage. This flat shear zone could be the
consequence of the extension process.The shear zone
develops where weakness anomalies exist.
(C.Tirel,2004; Tirel et al., 2004)