We use 110 000 small earthquakes to locate and map active faults in three dimensions within the Taiwan arc-continent collision. The structure is dominated by a nearly horizontal band of small earthquakes at ∼10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt, with other illuminated fault zones abutting the detachment zone. The zone steepens below eastern Taiwan to 30°–90° and reaches depths of 30–60 km. The three-dimensional shape of the detachment zone in relation to topography allows a new test of critical-taper wedge mechanics and suggests that the reversal of topographic slope across Taiwan is controlled by the shape of the detachment.

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