Usos o alternativas para la gestión de decomisos

3.1 Introduction

The Huatung coast is located on the east margin of the Coastal Range where Taiwan faces the Pacific Ocean, This chapter aims to summarise the environmental background of the Huatung coast (Plate 3.1) and the characteristics o f the coastal landform and shore features. The discussion considers the Coastal Range as a whole in order to provide a better framework for the appreciation o f the relevant coastal characteristics.

The Coastal Range extends northeasterly (N20°E) over a length o f about 140 km and a width o f 10 km (Figure 3.1). To the west, it is separated by the Longitudinal Valley with the Central Range, the backbone o f the island o f Taiwan. All main ridges (sub-divides) o f the Coastal Range are located where major anticlines occur. Most o f them have an orientation around N30-35°E, with one exception o f N9°E (Shih 1977) (see also Figure 3.5 below). The main body o f the range lies between an altitude o f 100- 1000 m, and the topography o f the Range rises towards the south, with the highest point being 1,682 m at Hsingkang Peak in the south (Figure 3.1).

The topography o f the Coastal Range and associated coastal landforms are the result o f the interaction between the various endogenic and exogenic geomorphic factors over time. In contrast with the substantial literature on the regional tectonics and the characteristic landform - marine terraces, coastal geomorphology has largely been ignored until recently. The attributes o f the environmental settings that are important in shaping the coastal and shoreline characteristics o f the Huatung coast are introduced as below. Particular efforts have been made to examine the structure of the shoreline configuration and characteristic features along the shore.

3.2 Neotectonic History and Geological Setting

3.2.1 Neotectonic history

Located at the boundary of the Eurasian Plate and the Philippine Sea Plate, Taiwan Island itself is the product o f arc-continent oblique collision since the late Tertiary (Figure 3.2). This underlies the formation o f the Central Range and the Coastal Range. The Coastal Range, in particular, has evolved as the Luzon Arc o f the Philippine Sea

Plate 3.1: Physical environm ental characteristics o f the Huatung coast:

A . m o u n ta in o u s la n d s c a p e , h e a d la n d s and p r o tr u sio n s, and n arrow m a rin e terra ces;

B . w e ll- d e v e lo p e d H o lo c e n e m a rin e ter r a c e s, in c is e d c o a sta l rivers (n o t e lo c a tio n o f v illa g e s and c o a sta l h ig h w a y ); C . retrea tin g s e a c li f f s c o m p o s e d o f silts to n e w ith p r o te c tiv e tetrap od s;

S h u ilien H sinshe Fengpin Shihtiping Hsi uk ulu an R i v e r Mout h C h an g pin C h en g ku ng M a w u k u R i v e r M out h A H s in k a n g P e a k S e t t le m e n t s Taitung Hsingkang Peak (1682m ) 1 4 0 k m Shuilicn G 2 0 4 0 6 0 8 0 1 0 0 T f T T T T t

Taitung Tuian A/au’w/rw Fc/Z/ey Chengkung Changpin Hsiukuluan Valley Fenpin D is ta n c e from T aitu n g C ity (k m )

Huaiien

Figure 3.1: Topography o f the Coastal Range and neighbouring area. A. The expression by DTM (source data provided by Agricultural and Forestry Aerial Survey Institute); B. Longitudinal profiles o f the major divide o f Coastal Range, profile in gray colour is eastern divides o f the Muwuku Basin. Elevation is projected to a baseline oriented N20E (read off from 1:100,000 maps).

Plate has moved northwest in collision with the Eurasian Plate (Figure 3.3; for a detailed summary refer to Lin 1991a and Wu et al. 1994). Its existence demonstrates that a mountain chain can be built up rapidly; in geological terms, this kind o f orogeny can be considered almost instantaneous (Barrier and Angelier 1986) and is best explained with ‘thin-skin tectonics’ model (Suppe 1981), in which the Coastal Range works as a bulldozer, or ‘lithospheric collision m odel’ (Wu et al. 1997). The present convergent motion in the vicinity o f Taiwan is about 70 km Ma ' in the 305° azimuth (Seno 1977, Figure 3.2).

The Coastal Range, regarded as an accretionary wedge, is cut by a series of major,

en echelon, southeast-verging thrusts (Ho 1982). Recently, more models have been proposed to explain the evolution o f the Coastal Range. The timing o f the initial plate collision o ff Hualien, the northern tip o f the Coastal Range, has been variously estimated at around 4 Ma BP (Hsu, 1990, Wu et al. 1997), 2 Ma BP (Huang et al. 1992) or 1 Ma BP (Lin 1991c). In spite o f the discrepancies, most models agree that the formation o f the backbone o f the Coastal Range has involved a multiple-stage collision - at least three stages (Lin 1991a, 1991b, 1991c; (Figure 3.4A) or two stages (Huang et al. 1992) o f mountain growth. These results indicate a differential development stage and also the differential uplift rate along the Huatung coast. The following discussion will focus on the Holocene onwards.

Information on Holocene uplift along the Huatung coast have been derived from C '“^ and U-234/Th-230 dates on corals, fossil shells and driftwood (Lin 1969; Sung 1969; Peng et al. 1977; Lai 1987; Liew et al. 1988; Hsu 1988; Yamagugi 1990; Hsieh 1990; Lin 1991a; Liew et al. 1993). A number o f interpretative difficulties arise from the use o f different dating methods, the uneven spatial distribution o f the samples and stability o f the palaeo sea level (Lin 1991a).

Rapid and differential crustal uplift in Holocene is the common impression obtained from the various published studies. Peng et al. (1977), for example, indicated that the average Holocene uplift rate for the Coastal Range is 5.0±0.4 mm y f '. Chen et al. (1991) noted a significant difference between northern (5-9 mm y f ') and southern (14 mm yr ') parts o f the range, and Liew et al. (1993) reported rates o f uplift varying from 2.5-3.0 to 8.0 mm y f ' within a 65 km long coast north o f the Chengkung. The occurrence o f differential uplift is also evident from geomorphic expression (refer to section 3.2.3). Vita-Finzi and Lin (1998) pursued the question further by considering tracts o f coast between major thrusts. A fter the sea-level correction for elevation, these

A N T A R C riC PLATK. M a in la n d C h in a T a iw a n EURASIAN PLATE PHILIPPINE SEA PLATE

bathymeffic contour in metre

COASTAL RANGI E U R A S I A P L A T E E U R A S I A P L A T E P H I L I P P I N E S E A P L A T E O kinaw a Trench R yukyu Arc

N orth Luzon Trench

(X 1,000 m)

F ig u r e s.2: L ocation o f T aiw an (a b o v e ) (after O ilier, C. D. 1981) and m ain tecto n ic e le m e n ts o f the W estern P hilip p in e Sea Plate m argin (b e lo w ) (m o d ifie d from S en o , T. 1977 and H o, C .-S . 1982).

Figure 3.3; (A ) 3 -D sch em a tic plate tectonic settin g o f T aiw an (after H o 1986); (B ) Subm arine topography around Taiw an island (Source: h ttp ://d u ck 2.oc.n tu .ed u .tw /d ep th / taiw an3d.htm l).

-j On

- 2 0 ■