Brittle frictional mountain building: 3. Low-grade metamorphism

Abstract

An active fold-and-thrust belt in unchanging tectonic and climatic conditions exhibits a dynamic steady state, with the flux of rocks accreted at the toe balanced by the flux of rocks eroded off the top. Rocks entering the toe are buried and heated before they are uplifted and eroded; this results in a characteristic map pattern of low-grade metamorphism on the surface. Metamorphic isograds are generally parallel to the regional strike of a fold-and-thrust belt, with the grade increasing progressively from unmetamorphosed and zeolite facies near the deformation front up to greenschist facies in the highest mountains; such a pattern is observed in the active fold-and-thrust belt of Taiwan. This paper examines the origin of this low-grade syntectonic metamorphism, using a previously developed mechanical and thermal model of a steady state fold-and-thrust belt as a basis. To model the metamorphism, a petrogenetic grid is developed for rocks of basaltic composition, considering phases within the chemical system CaO-MgO-Al2O3-SiO2-H2O. Equilibria diagnostic of stable low-grade mineral assemblages are mapped onto a cross section of the Taiwan fold-and-thrust belt, using the calculated fluid pressure and temperature distributions within the wedge. -from Authors