Crustal accretion in the Manila trench accretionary wedge at the transition from subduction to mountain-building in Taiwan

• Seismic reflection image and wide-angle velocity model offshore southern Taiwan.
• Crustal-scale structure of early-stage arc–continent collision.
• Distal continental margin crust subducted at Manila trench and structurally underplated to prism.
• Crustal underplating may cause transition from lower to upper slope of prism.
• Exposed crust in Central Range onshore Taiwan may be derived from distal continental margin.

New marine seismic reflection and coincident wide-angle ocean-bottom seismometer data acquired offshore Taiwan provide high-resolution constraints on the crustal structure of an incipient mountain belt during the earliest stage of arc–continent collision. The new seismic reflection image and travel-time tomography velocity model show evidence for crust of the distal southern Chinese continental margin being subducted eastward beneath the Manila trench and underplated to the accretionary wedge before collision with the southern Chinese continental shelf. The distal margin crust consists of highly extended continental crust interspersed with volcanic bodies and a high-velocity lower crustal layer of likely magmatic intrusions. The distal margin crust is 10–14 km thick outboard of the trench, but thins to 6 km thick beneath the lower slope of the Manila trench accretionary wedge. Along the lower slope of the accretionary prism, we image westward-verging imbricate thrusts and folded strata up to 10 km thick. A sharp decrease in bathymetry marks the transition from lower to upper slope, where we observe a fast (>6.0 km/s) seismic velocity anomaly at the base of the wedge that we interpret as structurally underplated crust from the distal continental margin. Our results support a model of arc–continent collision in Taiwan where the accretionary wedge is first thickened by structural underplating of distal margin crust prior to collision with the continental shelf. The crustal rocks exposed throughout the Central Range in Taiwan may be similarly derived from subducted and structurally underplated crust from the highly extended distal continental margin.