The east coast of Coastal Range, Taiwan, is well known for the presence of multiple Holocene marine terraces and their very rapid uplift (e.g
. Liew et al
., 1993). We extended the study area from Hualien to Taitung over a ca
. 150-km long coastline. We present some new data on the ages and the heights of Holocene marine terraces and discuss their significance for Holocene coastal tectonics.
The Holocene marine terrace is subdivided into more than 10 steps at maximum. The highest and oldest terrace, mostly underlain by transgressive deposits, represents the sea level position at the culmination of Holocene sea-level rise and reaches to ca
. 80 m above sea level. Transgressive deposits are radiocarbon-dated at several sites, back to ca. 13000 yr BP. The lower terraces are mostly erosional one, overlain by thin coral beds and indicate successive lowering of relative sea-level during the late Holocene. Uplift rate of the coast is very high, ranging from 5 m/ka to 15 m/ka.
Three tectonic subregions, A, B and C, are identified, based on the pattern of marine terrace features and uplift rate. Subregion A, the northernmost area, is the Milun Upland, which is separated from the main part of Coastal Range by the Hualien River. The Milun Upland is a tilted block, bounded by the active Milun Fault at its western margin, and is fringed by a series of narrow lower terraces. The Milun Fault is a northern extension of the Taitung Longitudinal Valley Fault System and shows progressive deformation. Deformation of the Milun Fault is concordant to that of the 1951 earthquake. Uplift rate here ranges 5-8 m/ka. Subregion B, middle part of the coastal area, has an uplift rate of 5 m/ka. Subregion C is the southern most region, presents multiple terraces characterised by the highest uplift rate, 7-15 m/ka. Extensive alluvial fans overlie most of the highest marine terrace. Multiple lower terraces are also well preserved. The youngest terrace is dated at ca
. 400 yr BP. No onshore active fault is observed on terraces in the subregions B and C.
The presence of many steps during the Holocene time implies that intermittent uplifts, associated with large earthquakes at an interval of one thousand to several hundreds of years, have repeatedly occurred in this coastal area, but with different times and amounts at each subregion. The uplift of subregion A certainly resulted from an onshore active fault (Milun Fault). Causative faults for the uplift of subregions B and C are not identified. Offshore active faults, striking obliquely to the coastline (Lallemand et al
., 1999), appear to be causative faults. Another unsolved problem is to understand when such a high uplift started in this coastal area. This is difficult to determine at present, because no late or middle Pleistocene marine terraces are found in the area.
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