Taiwan is the type example of an arc-continent collision. Numerous tectonic models have been proposed for this orogen, and include both thin-skinned and thick-skinned lithospheric deformation. These models predict very different structures at middle and lower crustal depths, but insufficient geophysical data exist to unequivocally distinguish between them. Long-period magnetotelluric (MT) data were collected in central Taiwan in 2006-2007 to constrain the crustal resistivity structure. A two-dimensional inversion of these MT data revealed a prominent electrical conductor that extends across the décollement predicted by the thin-skinned model. This feature is interpreted to be due to 1%-2% saline fluids, and is inconsistent with the thin-skinned model. In contrast, the thick-skinned model predicts this feature since fluids are generated in the crustal root through metamorphism. Quantitative correlation of the resistivity and seismic velocity models supports small-volume, high-salinity fluids in a thickened crust as the cause of this conductor.