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I. Mechanics of Thrusts and Nappes |
Department of Geology, Sedgwick Museum, Downing Street, Cambridge CB2 3EQ, England
Although thrust belts at present-day subduction zones are sub-parallel to such zones, several ancient thrust belts dip in the opposite direction to the inferred dip of the contemporaneous subduction zone. Gravity spreading, gravity sliding or compressional stresses transmitted from the subduction zone are unlikely causes of such thrust belts. The driving force is attributed instead to the hydrostatic head of a fluid-like rock welt. If the welt is hot—as in a granitic/metamorphic belt—foreland thrusts may form.
The emplacement stresses are similar in magnitude to those of gravity spreading, but the thrust wedge need not be weak. The essential requirements of a thrust belt created by a fluid welt are a downhill surface slope in the direction of thrust transport and weak decollement horizon(s).
In the Mesozoic foreland thrust belts of western North America, the volume of rock pushed onto the foreland is comparable to the volume of the batholiths in the orogenic core and to the volume of new material added by subduction to the overriding American Plate.
After collision with another continent or an island arc the recognition of thrust belts emplaced by fluid welts is more difficult, particularly when ophiolites are present.
