GROWTH OF AN ACCRETIONARY PRISM ASSOCIATED WITH ARC-CONTINENT COLLISION IN THE HENGCHUN PENINSULA, SOUTHERN TAIWAN

Sedimentologic, petrographic, and geochronologic analysis of uplifted accretionary prism strata in the Hengchun Peninsula of southern Taiwan indicate juxtaposition of deltaic foreland basin deposits, trench turbidites containing blocks of oceanic crust, and forearc basin deposits. These strata provide insight into accretionary prism growth associated with arc-continent collision. Oblique collision, beginning in the late Miocene, of the Luzon volcanic arc with southeastern China resulted in the progressive southward uplift of accretionary prism deposits and foreland basin development.

The Hengchun Peninsula consists of Holocene foreland basin deposits to the west and Miocene accretionary prism strata to the east, separated by the Hengchun reverse fault. We recognize five major stages of accretionary prism growth in the Hengchun Peninsula. Stage 1: Middle Miocene deposition of proximal trench submarine fan conglomerates derived from obducted oceanic crust previously incorporated into the accretionary prism. New ⁴⁰Ar/³⁹Ar ages from three clasts within these conglomerates have ages of 11.3 Ma, 14.7 Ma, and 13.2 Ma. Stage 2: Deposition of forearc basin submarine fan deposits that contain significant amounts of potassium feldspar. These deposits were later thrust westward over the accretionary prism strata. Stage 3: Incorporation into the accretionary prism and erosion of Stage 1 conglomerates, producing slide blocks that were deposited in the distal trench. Stage 4: Latest Miocene progradation of along-strike deltaic foreland basin systems into the trench. These deltaic strata contain abundant quartzite clasts and evidence of shallow marine conditions (detrital oyster shells, coal lenses, and trough cross-stratified sandstones) and are gradational with distal trench fan deposits. Stage 5: Regional uplift of the accretionary prism resulting in deposition of the Pleistocene Kenting mélange (olistostrome), possible thrusting of Stage 2 forearc basin strata over the accretionary prism strata, and initiation of Plio-Pleistocene foreland basin deposition. Thus, in the early stages of accretionary prism growth in southern Taiwan, sediments were derived mainly from seamount and arc sources, but during the later stages sediment was transported axially from the developing orogenic belt.