metamorphic core complex

   Not all geologists agree on the interpretation of their formation, so these large geologic structures are still an interesting subject.
   They seem to have been formed by thermal upwelling within the crust of the Earth, accompanied by stretching forces that run in this area in a northeast-southwest direction.
   They make up many of the mountain ranges in southern and western Arizona, including South Mountain and the White Tank Mountains.  The broad, arched profile of South Mountain, for example, displays the domed and elongated structure, as contrasted with the other sharp, jagged peaks that dominate the skyline around Phoenix.
   Rocks within the complex have been metamorphosed (changed by heat and pressure); the stretched fabric is called mylonite.  The degree of metamorphism increases with depth, towards the core of the complex.
   Paralleling the surface of the domed structure is a detachment fault, where crustal displacement has occurred over many miles.
   These structures occur throughout the American West, along a trend running from southern British Columbia into Mexico.  

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