Controls of metamorphic grade in shear belts

Abstract

The relationship between metamorphic grade and deformation is examined for shear belts occurring in Precambrian shields. A particular case, where isotherms have fallen continuously relative to an originally horizontal datum surface, for some time prior to the initiation of the shear belt and throughout its life, is examined in detail. Such behaviour of isotherms appears to be common. A type of syn-tectonic metamorphic boundary occurring within shear belts and not coinciding with a strain gradient is discussed in detail. Such boundaries are called active facies boundaries, as mineral assemblages on each side tend to maintain perfect equilibrium with metamorphic conditions during deformation. The orientation of active facies boundaries depends on the vertical displacement rate. In ductile thrust zones horizontal gradients in metamorphic conditions can be set up, and folIowing erosion once active facies boundaries may be exposed. The metamorphic history of rocks in such zones will vary vertically, and, under certain circumstances, laterally. A model is set up to predict these variations, and is applied to the northem boundary of the Ikertoq shear belt, western Greenland.