Lithospheric flexure under a critically tapered mountain belt: a new technique to study the evolution of the Tertiary Taiwan orogeny

A joint analysis of both the lithospheric flexure of an underlying plate and mountain-building mechanics is developed based on the critical wedge theory and the flexure response of an elastic plate over an inviscid asthenosphere. According to the analysis, the amount of deflection beneath a critically tapered mountain belt strongly depends upon the internal and basal friction coefficients of that belt and the flexural rigidity of the underlying plate rather than the horizontal stress induced by plate convergence. An increase in the internal friction of a mountain belt or a decrease in the friction along its basal decollement tends to enlarge the deflection of the underlying plate. The tendency toward an increased deflection of the underlying plate due to a decrease in flexural rigidity is suppressed by the development of a more narrowly tapered mountain belt. The newly developed technique presented here is used to analyze the evolution of the Tertiary Taiwan orogeny and shows that the optimal flexural rigidity of the Eurasian Plate in Taiwan is about 3×1022 Nm. It also demonstrates that the collision between the Eurasian Plate and the Luzon Arc might have initiated 7 Myr ago, a finding that would account for the burial history of the foreland basin in south-central Taiwan.