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, we develop a petrogenetic grid 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. The variation of metamorphic grade along the surface is predicted by assuming that there is no retrograde metamorphism. The most important mechanical factor controlling the degree of metamorphism in an active fold‐and‐thrust belt is the amount of frictional heating, both on the basal decollement fault and within the deforming brittle wedge. Frictional heating raises temperatures in the deepest portions of the Taiwan fold‐and‐thrust belt by 200° C to 250° C; the resulting high temperatures, in excess of 400° C, are responsible for the extensive greenschist facies metamorphism. The amount of heating is well constrained by the observed heat flow anomaly, geochronological data, and the critical wedge taper; the best‐fitting coefficient of basal friction in Taiwan is μb = 0.5 ± 0.2. Underplating of footwall rocks by the assimilation of duplexes along the decollement fault is not considered to be a significant process in Taiwan, because of the observed balance between the accretionary influx at the toe and the erosive efflux off the top. In other fold‐and‐thrust belts, however, underplating can significantly increase the outcrop width of higher metamorphic grades on the surface by increasing the flux of rocks through the higher fluid pressure and temperature regions within the wedge.