Abstract

Overthrust sheets of granitic rock have numerous features in common with overthrust sheets of ultramafic rock. Regardless of rock type, these overthrust masses are 5 to 10 km thick or more, tens of kilometers wide, and up to hundreds of kilometers long. Their upper parts may consist of undeformed and unmetamorphosed igneous or sedimentary rocks, but near their base the crystalline rocks become tectonites, and retrograde metamorphism is widespread.

A necessary condition for the formation of these sheets is a region of unusually steep geothermal gradient such as may be found behind volcanic arcs or in any region of upwelling mantle (for example, young ocean floor). At the time of detachment, the relatively brittle and rigid cover moves away from its underlying thermally softened base. At first, displacement between cover and base is penetrative, but as strain softening (and shear heating?) proceeds, displacement becomes localized along a fault zone that will be parallel to isotherms in the rock, and the overthrust crystalline-based sheet is thus freed from its base. During movement, the underlying rocks of the detached sheet may be metamorphosed, and fragments of the overridden rocks may be picked up. When movement ends, the cooled sheet is emplaced on top of cooler rocks. Fluids driven from overridden rocks during movement, and following emplacement, will act to reduce frictional resistance and will result in retrograde metamorphism of the overlying crystalline mass. K-Ar dates in the lower part of the overriding sheet may be set during movement.

Such a model explains the observed structural relations, metamorphic history, and thinness of the overthrust sheets. Rigidity of the detached cover and isostasy prevent the cover (suprastructure) from folding; these conditions require that the cover be displaced laterally for great distances. Lateral constraints and lithologic anisotropy will dictate the direction of overthrusting.

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