An overview of coastal development in a Young Mountain Belt-Taiwan

Abstract

The coastal morphology of Taiwan exhibits characteristics of collisional tectonism superimposed upon global sea level changes. The island's eastern coast is situated on the accreted Luzon arc alongside the NE–SW trending plate boundary. To the west, the Central Range, Hsueshan Range and Western Foothills are the fold and thrust belts of the island. They are the raised continental shelf and slope sediments or meta-sediments of the Chinese continental margin. The northern and southern coasts of the island are in the two tips of these belts. The western coastal area is mostly in the foreland of the orogen. Situated on the accreted arc proximal to the collision zone, the middle and southern portions of the eastern coast have high uplift rates of up to 9–10 mm/yr. Holocene transgression deposits reach 50 m amsl or more. There are commonly 3–4 flights of Holocene terraces. Thrusts and warping are recognizable along the northern and southern coasts because they trend perpendicular to the main fold and thrust belts of the island. The Holocene uplift rates are typically about 0–5 mm/yr. Higher rates occur in the upthrown limbs of the thrusts or areas proximal to up-warping axes. Sitting on the foreland basin, the development of the western coast is related to the westwardly propagation of fold and thrust belts which in turn are influenced by the lithological rigidity of the underlying basement. Coastal areas underlain by two Mesozoic basement highs are relatively stable or have low subsiding rates (about 2–3 mm/yr). Between these basement highs lie an area occupied by the west front of the fold and thrust belts, which has an uplift rate similar to the northern and southern coasts. To the south of the basement highs, the coastal plains probably conceal newly developed folds or faults where SW trending, paleo-incised valleys and domes of Late Pleistocene and Holocene age exist beneath the coastal plains’ surfaces as indicated by several tens of drilling cores from the plains. This suggests an incipient stage of fold and thrust belt development. The southwestern-most coastal plains are believed to be a piggy-back basin of the fold and thrust belts, showing a higher subsidence rate (6–8 mm/yr) than other parts of the western coast.