Tsung-Kwei Liu

Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating

Abstract Based on analyses of about 970 zircon grain fission-track (FT) ages from 44 clastic rock samples collected from six transects and two pooled ages of apatite concentrates separated from a diabase body, the thermo-kinematic evolution of the Taiwan mountain belt since the last orogenic phase (the Penglai Orogeny) has been delineated for the first time. As a consequence of the active Penglai Orogeny since late Tertiary times, pre-orogenic FTs in detrital zircons and apatites have been subjected to varying degrees of annealing by geothermal heating. The spatial boundary between the partial and complete resetting of zircons coincides well with that corresponding to the 260°C isotherm between the greenschist facies and prehnite–pumpellite facies defined previously by crystallinity of potassic micas. The age distribution along each cross-section exhibits younger ages from the mountain front toward the rear plate boundary between the Eurasian and the Philippine Sea plates, suggesting asymmetric cooling and exhumation. Along the strike direction (roughly north to south) of the regional structure, the age distribution reflects southward propagation of the arc–continent collision and subsequent uplift-and-denudation. Spatially, the width of the zircon complete-reset zone gradually narrows down along the strike of the regional structure, reflecting the southward propagation of the arc–continent collision and subsequent uplift-and-denudation. Temporally, zircon FT ages for the western margin of the completely reset zone are progressively younger from 5–6 Ma in the northern part to ca. 2 Ma in the south-central part, then resume to ca. 6 Ma for the southern end, where collision is only in its initial stage. The previously tectonized pre-Tertiary Peikang Basement High on the Asian continental margin plays an important role in defining the uplift-and-cooling history and shaping the major salient-and-recess structure and neotectonics of the mountain belt.

Rates of cooling and denudation of the Early Penglai Orogeny, Taiwan, as assessed by fission-track constraints

Abstract As an attempt to define the timing of the peak temperature of the Penglai Orogeny and estimate the early cooling and denudation rates of the Taiwan mountain belt, fission-track dating of zircon and apatite was conducted on several key metamorphic rock samples. The zircon fission-track ages for the metasandstone clasts, collected from the central and northern parts of the Coastal Range, were determined as 4.0±0.5 and 3.6±0.3xa0Ma, respectively. Both of the clasts were identified as from the ancient Central Range which was metamorphosed at temperatures high enough to cause a complete reset of the zircon fission-track system. During 1–2xa0Ma, they were exhumed and deposited in the Coastal Range basin due to the later Penglai Orogeny of Taiwan. Obviously, they have not further been annealed since their deposition in the Coastal Range. The difference between the above fission-track ages and the stratigraphical age of the host sedimentary formation represent the ancient cooling ages when they were exposed on the early Central Range. These ancient cooling ages are comparable with the zircon fission-track age of a present-day outcrop of the Tananao Schist, 1.8±0.2xa0Ma. This accordance implies that at ca. 5xa0Ma the northern and central parts of the Central Range achieved the peak temperature of the Penglai Orogeny and then they began to emerge above sea level. Accepting this scenario, we calculate the rates of denudation and cooling of the Central Range to be ca. 2.5–4.6xa0mmxa0yr−1 and ∼120°Cxa0m.y.−1, respectively, for the last 4xa0Ma.

Holocene uplift and subsidence along an active tectonic margin southwestern Taiwan

Abstract Taiwan is located along a convergent plate boundary, where the Luzon Arc collides with the Eurasia continental margin. The Tainan Plain of southwestern Taiwan is incorporated into and deformed by a regional fold-and-thrust belt associated with this convergent plate margin. In this study we establish a tentative regional Holocene relative sea-level curve that allows us to analyze crustal uplift and subsidence rates of the Tainan Plain. The maximum Holocene uplift rates on the Tainan Plain occur on the Tainan Tableland and the Chungchou Terrace. These two areas have experienced long-term (Holocene) uplift rates of 5 and 7xa0mm/yr, respectively. The Tawan Lowland, located between these two areas, is subsiding at a long-term rate of about 1xa0mm/yr. Based on this pattern of the crustal movement, the Tainan Tableland is interpreted as a mud diapiric dome, and the Chungchou Terrace as the product of a blind thrust fault. The Holocene reference sea-level curve proposed in this study can be used to determine the pattern of crustal movements elsewhere in Taiwan. It also can suggest that the Holocene terrace development does not occur in area where the uplift rate exceeds 8xa0mm/yr, while late Holocene regression, the general trend around the western Pacific, is not recorded where the rate of subsidence is lower than 1xa0mm/yr.

Geomorphic evidence for prior earthquakes: Lessons from the 1999 Chichi earthquake in central Taiwan

The September 21, 1999, Mw 7.6 Chichi earthquake destroyed several thousand buildings and caused more than 2000 fatalities in central Taiwan. The earthquake occurred along the Chelungpu fault, a thrust fault on the western flank of the Taiwan fold-and- thrust belt. The surface rupture was more than 90 km long, and vertical displacements ranged from 3 to 8 m. Although pre-existing scarps were identified along the Chelungpu fault, the fault had previously been categorized as a less important active fault, owing to the lack of geochronologic evidence and the failure to recognize fault-related geomorphic features. Identifying geomorphic features at active faults in Taiwan will permit the delineation of future sur- face ruptures and the determination of past magnitudes of past earthquakes, thus contributing to hazard assessment.

Remnant magmatic activity in the Coastal Range of East Taiwan after arc–continent collision: fission-track data and 3He/4He ratio evidence

Abstract The magma activity of the North Luzon Arc is considered to have ceased, due to collision with the Asian continental margin, since late Miocene. New fission track dates of zircons taken from dike swarms in the central Coastal Range of East Taiwan show very young ages of ∼0.5 Ma . Furthermore, the high 3 He / 4 He ratios ( ∼2.5 R A ; RA is the air ratio) of hot spring gases near the same area indicate that more than 30% of a mantle-derived source component is necessary to account for the helium composition. These results suggest the existence of later remnant magmatic activity (more than 4xa0Ma) in the central Coastal Range. Therefore, arc magmatism may survive longer than the previously expected after collision.

A date for volcanic eruption inferred from a siltstone xenolith

We present a TL age determination of a heated siltstone xenolith found at Kueishantao, a volcanic island 10 km o! the northeastern shoreline of Taiwan. Before this study, no reliable numeric age determination had been obtained for the volcanic rocks. Its age has long been desired due to its active tectonic environment. Previous studies of the other radiometric dating methods indicate a very young age for the eruption of this volcano, possibly later than 10 ka. However, reliable age is not allowed because it is younger than the lower dating limit. Our result shows a TL age of 7

Thrust-related river terrace development in relation to the 1999 Chi-Chi earthquake rupture, Western Foothills, central Taiwan

0.7 ka for the lower part of the volcanic sequence, indicating that other volcanic eruptions occurred later than 7 ka. This nding suggests that the volcanic island, Kueishantao, has not ceased its igneous activity and is still active geologically. ( 2000 Elsevier Science Ltd. All rights reserved.

Luminescence dating of neotectonic activity on the southwestern coastal plain, Taiwan

The 21 September 1999 earthquake (MW 7.6) produced a surface rupture that extended for 100-km in a general north– south direction in front of the Western Foothills, central Taiwan. The study area, located in a segment of the fold –thrust belt of central Taiwan, provides an important natural laboratory to study the relationship between active thrust deformation and fluvial terrace development. We recognize, from west to east, three subparallel thrusts, the Sanyi, Houli, and Shihkang faults in the northern part of the earthquake disaster area. The 1999 earthquake rupture occurred only along the Shihkang fault, whereas the other two faults were inactive. Along the Tachia River in the hanging wall of the Shihkang fault, eight well-developed terrace levels are observed, and can be subdivided into lateritic (Lt5– Lt1) and non-lateritic terraces (Ft3 –Ft1). Five levels of lateritic terrace (Lt5 –Lt1) are located in the hanging wall of the Houli fault, and a lateritic terrace (Lt5) is present in the hanging wall of the Sanyi fault. Stratigraphic and elevation correlations between these well-developed fluvial terraces in relationship to nearby geologic structures show that the Sanyi fault is the oldest structure. Lateritic terraces of at least 10,000 yr BP, for example, overlie the Sanyi fault, but correlative terraces associated with the Houli fault are offset. These types of terrace correlations allow us to define the progressive development of the thrust system within the Western Foothills. q 2002 Published by Elsevier Science Ltd.

Variations of soil radon and thoron concentrations in a fault zone and prospective earthquakes in SW Taiwan

Abstract The southwestern coastal plain of Taiwan is heavily urbanised, with a population of more than 6 million. The area is known to be subsiding, and the resulting basin is filled with thick fluvial deposits, of at least 200xa0m thickness. In the past century, three large earthquakes have occurred in the area; two caused surface ruptures, and resulted in large property loss and more than 2000 casualties. There is an urgent need for an understanding of the dynamics and recurrence intervals of this neotectonic activity, but little is known of the chronology of the late Pleistocene deposits. Recently more than 20 cores longer than 250xa0m were taken from the coastal plain as part of a large hydrogeological investigation, and basic data on lithology, hydrogeology and palaeobiology were collected. The base of these cores is beyond the 14 C age limit, and so the application of luminescence dating to these sediments has been investigated. Optically stimulated luminescence methods have been applied to quartz sand-sized grains extracted from 29 samples. Dosimetry based on gamma spectrometry is also compared with ICP-MS and XRF analyses. In the age range up to ∼40xa0ka, radiocarbon ages are compared with the luminescence results, to give confidence that the initial bleaching of these sediments was sufficient. The luminescence ages are then discussed and differential rates of basin subsidence are deduced. It is clear from these data that the study area is tectonically active, and it may be that regions of similar subsidence rate correlate with identifiable geological structures.