ABSTRACT

Plio-Pleistocene sandstones from the Coastal Range of eastern Taiwan provide a record of shifting source areas and unroofing events in adjacent arc and accretionary-wedge terranes during the early stages of are-continent collision. In latest Miocene and early Pliocene time, arc-derived sediments were deposited in a precollisional forearc basin between the Luzon volcanic arc and the Manila Trench. Sandstones of this age are rich in zoned plagioclase, volcanic lithic fragments, and pelagic foraminifera and contain only minor quartz and sedimentary-metasedimentary lithic fragments. In contrast, lower Pliocene to lower Pleistocene sandstones are poor in feldspar and volcanic fragments and contain abundant sedimentary and metasedimentary lithic fragments. This reflects a provenance shift f om the volcanic arc to the uplifted (meta) sedimentary accretionary wedge of proto-Taiwan, which resulted from the early Pliocene onset of arc-continent collision.

A refined classification of sedimentary and metasedimentary lithic fragments is introduced here and used to interpret the early evolution of the collisional orogen. Lithic fragments are rich in pelitic minerals and are divided into: 1) sedimentary (Ls) = mudstone, shale, siltstone, chert and argillite; 2) low-grade metamorphic (Lm1) = slate, slatey siltstone, and quartzite; and 3) medium-grade metamorphic (Lm2) = phyllite-schist, phyllitic quartzite and quartz-mica-albite aggregate. A good correlation is observed between these lithic fragment types and bedrock lithologies exposed in the present-day Central Range of Taiwan. Based on this correlation, the change in relative abundance of sedimentary and metasedimentary lithic fragments through time has been used to interpret an unroofing sequence for the collisional fold-and-thrust belt.

Lower Pliocene Taiwan-derived sandstones are rich in sedimentary lithic fragments (Ls), which were shed from the accretionary wedge during the earliest stages of arc-continent collision. From early Pliocene to early Pleistocene time, sandstones became progressively depleted in Ls lithic fragments and enriched first in Lm1 and later in Lm2 fragments, as deeper levels in the metamorphic complex were uplifted and exposed to erosion. Regional metamorphism of biotite-grade rocks (= Lm2) occurred at about 4.5 Ma at depths of 12-15 km (Ernst 1983) and were uplifted to the surface by about 1.4 Ma (this study). These ages give an uplift rate of 4 to 5 km/m.y., which is in good agreement with independent estimates based on fission-track studies, raised coral reefs, Quaternary elevation changes, and present-day denudation rates. Thus, it appears that sandstone detrital modes of clastic orogenic sequences can be used to interpret specific aspects of unroofing and tectonic evolution in nearby active mountain belts.