Metamorphic core complexEarth *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 *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 *Slab break-off

Metamorphic core complexEarth * Lithospheric delamination

Metamorphic core complexEarth *Characteristics and geometry

Metamorphic core complexEarth «Basin and Range » province

Metamorphic core complexEarth

Metamorphic core complexEarth

Metamorphic core complexEarth *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 *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.

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

Metamorphic core complexEarth *Flexural Uplift: Spencer, 1984

Metamorphic core complexEarth *To sum up:

Metamorphic core complexEarth

Metamorphic core complexEarth *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 *Magmatic Underplating or Intrusion

Metamorphic core complexEarth *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

Metamorphic core complexEarth *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 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)