Preliminary study of the neotectonic in Luyeh area
I-Chin, Yen 顏一勤

Abstract
Taiwan is setting on the boundary between Philippine Sea plate and Eurasian plate, the suture of two plates is remain at Longitudinal Valley of eastern Taiwan. The active seismicity and rapid rates of horizontal and vertical deformation are revealing active of orogeny. In the southern of Longitudinal Valley is exists many neotectonic geomorphologic evidences. In this study, we were investigated at field work on the geomorphology and geology to analysis the neotectonic feature of southern of Longitudinal Valley. We interpreted that Longitudinal Valley Fault still push the southern of Longitudinal Valley move to west and the block will be lock at Luyeh Fault. The different geometry of surface deformation cause by the fault tip variant of the Luyeh Fault, and the structures die out by plunging in the south and north.

Introduction

Taiwan island is located the boundary between Philippine Sea plate and Eurasian plate (Fig. 1). Rapid rates of horizontal and vertical deformation and active seismicity amply demonstrate the current vigor of the orogeny. In southern Taiwan, the South China Sea lithosphere is subducting eastward under the Luzon arc. In north-eastern Taiwan, the Philippine Sea plate is subducting northward under the Ryukyu arc. The active Taiwan orogen is in the transfer zone between the two opposite-facing arcs, and is a tandem suturing and tandem disarticulation of a volcanic arc and a continental sliver to and from the Eurasian continental margin.

Fig. 1

The Longitudinal Valley between the Central Range and the Coastal Range thus marks the boundary between rocks of Eurasian affinity and rocks that originated on the Philippine Sea plate. GPS geodesy shows that the rate of the North Luzon arc moving towards the Asian continental margin is about 8.2 cm/yr in an azimuth of 310°, and also shows that the rate of closure is about 40 mm/yr in the south part of Longitudinal Valley. The dominant neotectonic structure of the south part of Longitudinal Valley is the eastern dipping Longitudinal Valley fault, along the eastern edge of the Valley. The Luyeh fault is a reverse-slip fault which marks the westernmost boundary of the suture zone with eastern dipping, and it reactivated and caused the terrace to form a flexure scarp. No historic earthquake rupture appeared in southern Longitudinal Valley, but Quaternary terrace deforme revealed active faulting. The Longitudinal Valley fault and Luyeh fault both are active fault in southern Longitudina; Valley.

In this study, we separate three units-PingDin, GaoTai and Peinanshan, of the south part of Longitudinal Valley. Our field investigations on the geomorphology and geology was based on interpreted geomorphologic feature with air-photo and 40m digital elevation model (DEM) of Taiwan island in laboratory. Our goals in this paper is present the neotectonic proof and try to explain the fault mechanism in southern Longitudinal Valley area.

Geomorphologic Analysis

Based on air-photo and 40m DEM of Taiwan island interpretation, we classified three types of terraces-fluvial terrace, alluvial terrace, and landslide terrace; and four types of structure features-thrust fault, normal fault, syncline form and anticline form, in southern part of Longitudinal Valley(Fig. 2). After field investigation of geomorphology and geology, the final display is characterization as following:

Fig. 2
1. Terraces:
a. Fluvial terrace: The fluvial terraces in this area could be separated to Luyeh River and Peinan River terraces by different drainage. The Luyeh River terraces development along the both sides of Luyeh River with 5 steps. Lungtien terraces are the widest and along the north side of Luyeh River, and deformed at 60m high terrace. The south side of Luyeh River remains several tilting small terrace in the northern Peinanshan tableland. The Peinan River terraces development along eastern and western side of the Peinan River. In PingDin area, 4 steps were been formatted. In the eastern side of Peinan River, the terraces setting on the western of Costal Range in 5 steps. They also can find that terraces cover over basement rocks.
b. Alluvial terrace: The old alluvial terraces in this study are distribution on the top of the GaoTai and Peinanshan tableland and had laterited. The young alluvial terraces are in front of small valley with alluvial fan. This kind of terrace is difficult to recognize tilting or not.
c. Landslide terrace: Landslide deposits are cover over on western side of the Costal Range hills. In generally, the landslide deposits contain large angular gravels with poorly sorted, and included volcanic rocks that transport from Costal Range hills .

2. Structure features
a. Thrust fault: The thrust faults in PingDin area are along western scarp of three different steps terrace, and both with eastern dipping. According to the height of basement rocks decrease from eastern to western terrace, the thrust fault is younger from eastern to western. The youngest one was formed only 2m height scarp at southwestern terrace of PingDin platform, and also with eastern dipping. The oldest one was extend to south to connect with eastern dipping Longitudinal Valley fault, and across two parts of western Costal Range hills and northeastern corner of Peinanshan tableland (Fig.2 LVF). The other important eastern dipping fault in westernmost of southern part of Longitudinal Valley was formed the double thrust fault system in GaoTai area and extend to south from Lungtien terrace to Taitung city with single thrust fault. The specially in GaoTai is exist a western dipping thrust fault in front of eastern side scarp of tableland.
b. Normal fault: The normal faults in this study were presented in the northwestern part of Peinanshan tableland, and trend to northwestern to southeastern way.. It was revealed extension component in this region.
c. Syncline form: The syncline form extended from central part of Peinanshan tableland in southern to under southeastern part of Lungtien terraces. It’s the main structure controlled by regional syncline deformation.
d. Anticline form: The anticline form of this study almost behind of thrust faults. In the GaoTai area, the anticline along the ridge at west of tableland and back of westernmost thrust fault, and also exist in the top of tableland along northeastern to southwestern way. The anticline in Peinanshan area is along geomorphologic high extend in north to south way, and plunging to central part of tableland.

In the PingDin area, three eastern dipping thrust faults in front of second, third and fourth step of terrace. From the relief of each step of terrace, the fourth step terrace more high than other terraces, and also exposed basement rocks under the terrace. According to basement rocks high than west terraces, the Longitudinal Valley Fault can be extend to this thrust fault. And the others thrust fault in the west side could be a new fault propagate from eastern thrust fault (Fig. 3AA’).

In the GaoTai area, the western thrust fault was revealed double thrust fault with anticline. And the opposite dipping thrust fault was existed in the eastern side of the tableland, maybe formed by a back thrust fault of anticline back limb. The Luyeh Fault, westernmost thrust fault, is also active result in tilting terrace in the western of tableland (Fig. 3BB’). The Longitudinal Valley Fault in this region without exposure, and maybe exist under the Peinan River.

In the Peinanshan area, the Longitudinal Valley Fault in eastern side of tableland, and cut through northeastern tableland and a part of western Coastal Range hills. It also made syncline along western side, and terminal in central tableland. The anticline setting between syncline and westernmost thrust fault, and plunge to south at central tableland. It was formed by westernmost fault cause of fault propagation folds mechanism (Fig. 3CC’ and 3DD’).

Fig. 3

Discussion

Based on the field investigation of geomorphology and geology, this study can conclude that structure mechanism in study region is may be dominant by fault propagation fold with variant fault tip line. The fault plane is different height of tip line under westernmost thrust fault, and plunge to south and north with higher fault tip at central part of fault. The Longitudinal Valley Fault in this study also showed that was active fault with younger terrace tilting, and the fault trace almost along Peinan River bed.

According to geodetic and leveling survey shows that the horizontal velocity of eastern side of Longitudinal Valley Fault is move at ~310° and more faster than western side, the velocity between Longitudinal Valley Fault and Luyeh Fault is decrease from eastern to western (see Fig. 4) . It’s reveal the Longitudinal Valley Fault is more active than Luyeh Fault, and fault plane lock at Luyeh Fault. The leveling data of coseismic uplift rate about two earthquakes at 2003~2004 in Luyeh and Luyeh-Juayuan area, shows that Longitudinal Valley Fault has uplift in eastern and deform with anticline in west of Longitudinal Valley Fault (see Fig. 5a and 5b). The Luyeh-Juayuan area also presented two different uplift rate between 0308 to S193, it is provide the thrust fault in PingDin platform still active.

Both Longitudinal Valley Fault and Luyeh Fault are active and eastern dipping. From the horizontal velocity distribution near the fault, the Coastal Range still push to west and Longitudinal Valley block was moved to west by deformation and locked at westernmost thrust fault. The different fault tip depth made fold structure plunge to north and south, and also formed fan-shaped distribution of GPS vector direction.

Fig. 4

            

Fig. 5a                                                                  Fig. 5b