Metamorphic core complexEarth 238-26

*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

Metamorphic core complexEarth 238-26

*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).

Metamorphic core complexEarth 238-26

*Slab break-offhttp://www.mantleplumes.org/Anatolia.html

Metamorphic core complexEarth 238-26

*Lithospheric delamination

http://www.gfz-potsdam.de/pb2/pb24/projects/indepth/twolines/twolines.html

Metamorphic core complexEarth 238-26

*Characteristics and geometry

Metamorphic core complexEarth 238-26

«Basin and Range » province

Metamorphic core complexEarth 238-26

Metamorphic core complexEarth 238-26

Metamorphic core complexEarth 238-26

*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.

Metamorphic core complexEarth 238-26

*Isostatic Uplift: Werneke and Axen (1988)

*Initial faulting at high angle

*Isostatic uplift which causesthe rotation of the fault. Mohr-Coulomb criteria is respected.

Metamorphic core complexEarth 238-26

*Combination of Isostatic Uplift, simple shearing and pure shearing: Lister and Davis (1989)

Metamorphic core complexEarth 238-26

*Flexural Uplift: Spencer, 1984

Metamorphic core complexEarth 238-26

*To sum up:

Metamorphic core complexEarth 238-26

Metamorphic core complexEarth 238-26

*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.

Metamorphic core complexEarth 238-26

*Magmatic Underplating or Intrusion

Metamorphic core complexEarth 238-26

*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.

*Crustal Flow Model: (divergent crustal flow)

Metamorphic core complexEarth 238-26

Metamorphic core complexEarth 238-26

*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 modellingEx: 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 brittle-ductile 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)