OROGEN-SCALE DECOLLEMENTS

The continental lithosphere responds to stress by deforming as a gener ally layered medium. Deep seismic reflection data, coupled with a vari ety of ancillary geological and geophysical data, are interpreted to p rovide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers, and flat detachments in mech anically weak layers. This geometry is similar to ramp and flat struct ures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen-scale dec ollements in a manner that is analogous to so-called ''thin skin'' def ormation in sedimentary rocks. Decollements may occur within the crust (for example, near the base of a sedimentary section or in the middle crust), near the Moho, in the subcrustal lithosphere, or in the asthe nosphere. Even intracratonic basement-cored (''thick skin'') uplifts t hat occasionally occur in foreland regions such as the Wyoming provinc e are probably large (crustal) scale versions of ramp/flat features ob served in supracrustal rocks and are thus likely caused by the same fu ndamental tectonic processes.