In the thin-kinned approximation, both the critical taper of an active fold-and-thrust belt and the rate of brittle frictional heat generation depend on only two uncertain parameters: the effective coefficient of friction on the basal decollement fault, μb*= (1-λb)μb and the effective Coulomb wedge strength,μ* = (1-λ)sin ϕ/(1-sin ϕ). An increase in μb* increases the critical taper, whereas an increase in μ* decreases the taper; the basal and internal heat generation and therefore the surface heat flow are, in contrast, increased by an increase in either μb* or μ*. We exploit this difference in sensitivity to constrain μb* and μ* in the well-studied active fold-and-thrust belt of Taiwan. The measured surface heat flow and known geometry of the Taiwan wedge are best fit by an effective coefficient of basal friction μb* = 0.16 ±0.06. If the basal pore fluid:lithostatic pressure ratio is everywhere similar to that measured in several oil wells near the deformation front, then the coefficient of sliding friction on the Taiwan decollement fault is μb = 0.50 ±0.20.