MISSING SOURCE TERRANE IN AN INCIPIENT ACCRETIONARY WEDGE: INDICATIONS FROM CONGLOMERATE AND SANDSTONE UNITS OF HENGCHUN PENINSULA, SOUTHERN TAIWAN

Pebbles embedded in Late Miocene to Pliocene sandstone members in southernmost Taiwan provide pivotal information on the pre-collisional tectonic configuration of the Taiwan orogen. The composition and size of pebbles and their host sandstones vary geographically throughout Hengchun Peninsula, southern Taiwan. Shemen, Shezetou, and Lilongshan sandstone members tend to increase in grain size westward. The Shemen member contains altered mafic (plutonic) and sparse low-grade metamorphic pebbles; Shezetou contains limestone, vein quartz, and sandstone pebbles; and the Lilongshan member has andesitic, anorthositic, gabbroic, amphibolitic, and meta-sandstone pebbles. In contrast, the Loshui member, exposed on the east side of the Peninsula, tends to increase in grain size eastward and contains cobbles of mostly sedimentary lithologies.

Pebble lithologies and grain-size variations in sandstone members in western Hengchun (Shemen, Shezetou, Lilongshan) suggest a nearby source terrane to the west that supplied relatively mature sand and a significant influx of pebbles to the basal portions of the exposed sandstone members. Pebble lithologies and grain-size variations in the Loshui (eastern) sandstones indicate a nearby source terrane to the east before the orogeny in Taiwan began at 5 Ma. This second source terrane supplied both turbiditic sandstone and material for siliceous mudstone clasts found in the very eastern part of the Loshui member.

Backtracking the Luzon Arc to 5 Ma places it 410 km southeast of its current location. Analysis of the Miocene units of Hengchun requires at least one terrane with continental affinity in the Luzon forearc and consumption of the vast area of the South China Sea between the Luzon Arc and Chinese continental margin to bring the different sandstone members together in their current configuration. The source terrane is likely related to metamorphosed sediments of the growing accretionary wedge in the early Taiwan collision, judging from the similarity in composition of Loshui sandstone detritus and the low-grade metamorphic belt of the island of Taiwan.