new geomorphic data on the active taiwan orogen: a multisource approach

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. B10, PAGES 20,243-20,266, OCTOBER 10, 1994

New geomorphic data on the active Taiwan orogen: A multisource approach

B. Deffontaines, J.-C. Lee, J. Angelier, J. Carvalho, and J.-P. Rudant Department of Geoteetonies, Universit6 Pierre et Marie Curie, and URA 1759 CNRS, Paris, France

Abstract. A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital elevation model (DEM), side-looking airborne radar (SLAR), and satellite (SPOT) imagery, aerial photographs, and control from independent field data. This analysis enables us not only to present an integrated geomorphic description of the Taiwan orogen but also to highlight some new geodynamic aspects. Well-known, major geological structures such as the Longitudinal Valley, Lishan, Pingtung, and the Foothills fault zones are of course clearly recognized, but numerous, previously unrecognized structures appear distributed within different regions of Taiwan. For instance, transfer fault zones within the Western Foot- hills and the Central Range are identified based on analyses of lineaments and general morphology. In many cases, the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently. In turn, the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic features in other areas. Examples are the conjugate pattern of strike-slip faults within the Central Range and the oblique fold-and-thrust pattern of the Coastal Range. Furthermore, neotectonic and morphologic analyses (drainage and erosional surfaces) hae been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in Taiwan, such as for the Longitudinal Valley Fault. Next, at a more general scale, numerical processing of digital elevation models, resulting in average topography, summit level or base level maps, allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance. Finally, a preliminary morphotectonic sketch map of Taiwan, combining information from all the sources listed above, is presented.

Introduction

The island of Taiwan, where collision and mountain building occur, provides a key example for analyzing the relation between morphology and tectonics (morphoteeton- ies) in an active compressional environment. In this paper, we present an overview of the major morphoteetonie characteristics of Taiwan. The regional-scale overview of morpl'/ology, taking into account the contributions of neoteetonic field studies already carried out in another mountain belt [Deffontaines et al., 1993], allows one to better understand its tectonic significance in the active orogen. In order to characterize the major tectonic and morphological erosional features in an active collisional setting, we combine different morphoteetonie approaches based on both manual and computer-based analyses of maps at the scale of the whole island. We present the results of general analyses of topography (including the drainage pattern), of remote sensing (including SLAR and

satellite imagery), and of tectonic mechanisms (as revealed by independent field studies). In the last sections of the paper, these differents aspects are summarized in maps. In addition to our interpretation, these maps provide a basis for neotectonic mapping and for further morphological analyses.

Plate Tectonic and Geological Setting of Taiwan

Before addressing morphotectonic problems, it is worthwhile to briefly present the major geological and geophysical characteristics of Taiwan. Neotectonic studies in the

field are of special interest in our analysis, because they provide geometrical and mechanical constraints in the morphological analysis of the active mountain belt. These results have been published elsewhere, so that there is no need to describe local structures again; however, their relation to morphology is discussed in some specific cases in later sections of this paper.

Copyright 1994 by the American Geophysical Union.

Paper number 94JB00733. 0148-0227/94/94JB-00733505.00

Plate Tectonic Setting

Taiwan is located along a segment of the convergent boundary between the Philippine Sea and Eurasian plates

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20,244 DEFFONTAINES ET AL.: NEW GEOMORPHIC DATA ON TAIWAN OROGEN

(Figures la and lb). Northeast of Taiwan, the Philippine Sea plate is subducting to the northwest beneath the Eur- asian continental margin, whereas south of Taiwan the South China Sea floor, belonging to the Eurasia plate, is subdueting to the southeast beneath the Philippine Sea plate [Angelier, 1986, 1990]. Between these major arc-and- trench systems (Ryukyu and Luzon-Manila, respectively), where active subduction occurs, collision and oblique convergence [Suppe, 1984] dominate across the Taiwan segment of the plate boundary. As a consequence, a typical orogen developed in Taiwan during the late Cenozoic, especially since 5 Ma [Ho, 1986a].

It is important to observe that the western tip of the Ryukyu trench is not located near the northeastern tip of the Taiwan orogenic belt at latitude 25øN, but southeast off Hualien, at latitude 23øN (Figure la). On land, the most active fault zone is situated in eastern Taiwan, in the Longitudinal Valley trending NNE-SSW between Hualien and Taitung (7 in Figure la). The Taiwan mountain belt is still growing, as shown by continuing uplift and shortening [Yu and Liu, 1989] and by widespread seismic activity [Tsai, 1986]. In addition to its present-day activity [Tsai et al., 1977], the Longitudinal Valley Fault Zone of eastern Taiwan represents a major geological boundary between the Luzon are (to the Eas0 and the Chinese continental margin (to the West) for the Late Cenozoic [Ho, 1982]; it was thus commonly considered as the main boundary between the Philippine Sea plate and Eurasia. Note, however, that recent and active westward thrusting occurs at the front of the mountain belt near Kaohsiung and Taichung at longitude 120.5øE, like a northern extension of the Manila Trench (Figure la). Note also that before Plio-Quaternary time, the location of the main suture zone was probably different (i.e., inside the present orogen; Lu and Hsa [1992]; see also Stephan et al. [1986]).

Both the geological structure of Taiwan [e.g., Ho, 1986a, b] and the distribution of earthquakes [e.g., Tsai, 1986], taking into account the obliquity of the collision [Suppe, 1984], indicate that the lithospheric structure under Taiwan is characterized by a complex pattern of slabs that must be viewed in three dimensions (Figure lb).

Geological Setting

Although the major aim of this paper deals with morphology in relation to tectonics in Taiwan, it is necessary to give a brief geologic and lithologic overview in order to understand topography in relation to erosion of the different units. The major tectonic units of the mountain belt have contrasting lithological properties, such as the Hsiiehshan Range where hard sandstones prevail, and the Backbone Range where much less resistant slates are common (3a and 3b, respectively, in Figure la). From the geological point of view (Figures 2a and 2b), Taiwan can be divided into two main provinces [Ho, 1982, 1986a, b] separated by the Longitudinal Valley (Figure la). The eastern province, which includes the Coastal Range and its southward extension in small islands (Figure 2a), resem- bles the Luzon Arc and thus belongs to the westernmost part of the Philippine Sea plate. In the Coastal Range of Taiwan, the Miocene calc-alkaline volcanics, with shallow water marine sediments, are overlain by thick flysch-like series, Plio-Pleistocene in age. In contrast, the mountains

of the central and western parts of Taiwan (Figure 2a) are composed mainly of Tertiary sediments of the Eurasian continental passive margin, which were folded, faulted and metamorphosed during the late Cenozoic [Suppe, 1981; Ho, 1982; Jahn et al., 1986; Faure et al., 1991].

In more detail, the central and western parts of Taiwan are divided into several lithostratigraphic and metamorphic units [Ho, 1986b, 1988; Teng et al., 1991; Chen et al., 1983]. From east to west (Figures la and 2a), the major units are (1) the Central Range divided into eastern Central Range (polyphase metamorphic basement unconformably overlain by marine argillaceous sediments) and western Central Range (slates and sandstones of Backbone Range and Hsiieshan Range as discussed before); (2) the Western Foothills composed of nonmetamorphic shallow marine to shelf elastic sediments, Miocene to early Pleistocene in age, affected by numerous northwest vergent folds and low-angle thrust faults; and (3) the Coastal Plain, which represents a foredeep filled with Pliocene and Quaternary elastic deposits. In addition, the Plio-Pleistocene andesitic Tatun volcano, at the northern tip of Taiwan, is interpreted as the westernmost extension of the Ryukyu volcanic arc [Teng et al., 1992]. The contrasting lithologies of these different units must be considered carefully in the geomorphic analyses of further sections.

From the tectonic point of view, the main geological units of Taiwan, (Figure la, compare with Figure 2a), are separated by large faults described hereafter from west to east (i.e., from the foreland to the inner belt). First, the frontal thrusts have been active during Plio-Quaternary time in western Taiwan, with the Western Foothills moving westwards over the Coastal Plain foredeep (1 and 2, respectively, in Figure la). Second, most thrusts and reverse faults of the Foothills and the Central Range are west vergent, with some exceptions related to backthrusting which plays an important role in the inner parts of the metamorphic belt [e.g., Stanley et al., 1981]. The NNE- SSW trending Lishan Fault (L in Figure la), in particular, is a complex, polyphase tectonic line, which developed early in the geological history of Taiwan as a Tertiary normal fault [Teng et al., 1991], moved later as a probable west vergent thrust [Lu and Hsa, 1992] during the late Miocene, maybe also as a left-lateral strike-slip fault [Biq, 1989], and underwent late reactivation as a probable backthrust [Clark et al., 1991] during the Plio-Quaternary collision. Third, the NNE-SSW trending Longitudinal Valley Fault Zone (Figure la) is a major west vergent reverse fault with a component of left-lateral strike-slip motion. This active fault [Tsai, 1986] also played a major role earlier, during the Plio-Pleistocene; the left-lateral component of slip decreased with time and the thrust component increased, both movements continuing to occur [Barrier and •lngelier, 1986; Angelier et al., 1990].

Topography: The Digital Elevation Model

We obtained two Taiwan digital elevation models (hereafter, DEM) by using two independent sources of data: the statistical map of altitudes of Taiwan (scale 1'400,000, [Hsieh et al., 1975]) for the first one, and the Operation Navigational Chart (ONC: J12 and H12, scale 1:1,000,000 [Defense Mapping Agency Aerospace Center, 1972]) for the second one.

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The first set of DEM maps (derived from the statistical map of altitudes of Taiwan and discussed in a later section) allows accurate identification of the altitudes of the relief

summits and valleys, because it is numerically digitized from numerous accurate topographic contours. This first DEM model is illustrated in Figures 3a and 3b. Especially, there is no or little smoothing effect affecting the eleva- tions of highest and lowest points in the topography, contrary to the previous construction. From a technical point of view, to build the second DEM (Figure 7a), we digitized Taiwan topography isocontours from ONC with a PC-Microstation (Intergraph). Then the bicubic method derived from the theory of regionalized variables [d'Auturne, 1976; Hottier, 1977] was used for interpolation. The advantage of using the first of these two sets of maps lies in the presence of reliable data even in the flat lowlands such as in and around the Coastal Plain (Figure 3a). The spatial resolution of the data used in this DEM is 500 m x 500 m in horizontal plane and about 75 m (250 feet) in elevation; this resolution is rather poor for such a rugged terrain as the Central Range. Note at this stage that this intrinsic bound in the resolution of our DEM plays an important role in the analysis, because (1) it allows an easy overview of the Taiwan topography by considering the island as a whole; and (2) only the major topographic features are thus analyzed and access to details of the morphology must be obtained by other means. These two sets are thus complementary. In both cases, the use of digital models allows extensive numerical analysis and various types of geometrical processing which would be difficult to carry out otherwise.

In order to recognize major morphological features, the Taiwan DEM is not simply examined but more information is obtained through the analysis of paramaters derived by numerical means, such as the Taiwan slope map and the hill-shading DEM (Figure 3a). The program "Ombrage" (J. Carvalho, unpublished program, 1990) provided hill- shading illustrations such as in Figures 3a and 3b, which were obtained with variable trends and plunges of illumination. All trends were used with an azimuth interval of 20 ø

and a constant light inclination of 30 ø. This process was adopted in order to define the major topographic lineaments because structures perpendicular to the light source are better illuminated [Wise, 1982; Wise et al., 1985]. We have selected herein two contrasting hill shading directions (N60øE and N120øE, shown in Figures 3a and 3b, respectively), in order to better show the major structural features perpendicular and parallel, respectively, to the dominating trend of the Taiwan belt. To analyze and interpret such documents, we used a lineament analysis based on structure, texture, shape, and color (intensity hue and saturation [O'Leary et al., 1976; Scanvic, 1983]), which corresponds to classical qualitative interpretation criteria in remote sensing. For instance, alignments (shape) of pixels characterized by similar values (color) may have a lithological or structural origin.

Even with the relatively poor resolutions discussed above, simple observation of the DEM shown in Figure 3a and preliminary analysis and interpretation (Figure 3c) allow identification of some major structural elements of the Taiwan orogen. These properties must be carefully considered in the rugged terrain of the Central Range, despite the smoothing effect resulting from the poor DEM

resolution. The major geological and topographic longitudinal features of Taiwan, such as the N20øE Longitudinal Valley, the N30 ø Lishan Fault (L in Figure la), the frontal thrusts of the Western Foothills (which vary in trend from NE-SW in northern Taiwan to N-S in the south), and the N-S Pingtung fault (P in Figure la), are easily recognized (Figure 3). Less important features which also trend approximately parallel to the mountain belt are also clearly identifiable (e.g., the Western Foothills, NNE of Kaohsiung). A majority of these longitudinal structures were already shown in the geological map of Taiwan [Ho, 1986b].

More interesting, in the DEM-derived maps, many linear features strike obliquely or perpendicularly to the major structural trends, with less extent and continuity than for the well-known major features. For instance, linear features trending N90ø-Nl10øE are clearly visible within the Central Range (Figure 3), despite their lack of continuity discussed above. Most of these oblique and transverse features have tectonic significance (as discussed below) and, in contrast to longitudinal structures, many of them were not shown in the geological map of Taiwan [Ho, 1986] and even in more detailed local maps. Geological studies in the field resulted in identification of fracture

sets, major and minor, at hundreds of sites in the Central Range and Western Foothills. Although part of these geological data still remains unpublished, a good account of such fracture orientation analyses was given by Chu [1990]. A comparison between these local orientation data and the location of major morphological linear features (not done in Chu's dissertational study) indicates that the latter correspond in many cases to segments of N90 ø- N l10øE valleys following strike-slip faults. Although the main strike-slip fault in a valley cannot be observed in most cases, its existence is commonly ascertained by the presence of numerous minor strike-slip faults observed in outcrops along a valley, where large sets of minor faults strike parallel to the valley axis. In some cases, however, the major faults themselves could be directly observed in the field. This observation is also valid for other types and orientations of major faults, such as in the Lishan Valley (J.C. Lee, unpublished data and dissertational work, 1994). Note again that our conclusion results from comparisons between statistical analyses of orientations of brittle features at many sites [e.g., Chu, 1990] and overall topographic trends (Figure 3). The strike-slip character of fault lines, and even the identification of linear features as major fractures, is generally impossible to recognize solely from the DEM analysis, but is easily ascertained by independent geological analysis. Note also in Figure 3 the capability of the basic morphological treatment to highlight large-scale structural or lithological features in the Central Range. In contrast, few topographic lineaments are revealed in the lowlands of the Westernmost Foothills and Coastal Plain

(Figure 3c). We conclude that general analyses of the DEM of Tai-

wan, carried out with both the models mentioned earlier and despite the relatively poor resolution, allow easy identification of the major linear morphological features, not only those corresponding to the main longitudinal fault zones and folds independently recognized and mapped by structural geologists [e.g., Ho, 1982, 1986b] but also the features which are less continuous and generally torre-

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Figure la. Geodynamic framework of Taiwan. 1, Coastal plain; 2, Western Foothills; 3, Western Central Range (3a, HsQehshan Range, and 3b, Backbone Range); 4, Eastern Central Range; 5, extension related to Okinawa Basin opening; 6, Tatun volcano; 7, Longitudinal Valley; 8, Coastal Range; 9, Ilan plain; 10, direction of convergence (Philippine Sea plate relative to Eurasia), with linear velocity; L: Lishan Fault; P: Pingtung Fault.

spond to various types of faults, tight folds, and steep monoclines as shown by field observation (although they are often not shown in geological maps). This DEM analysis was also very useful in order to analyze the general morphology of Taiwan, as discussed in the last section of this paper. Both these analyses (linear features and general shapes) obviously do not allow identification of small morphostructures due to their poor accuracy. As a result, comparisons between DEM analyses and geological information obtained in the field may be difficult where most geological structures observed in the field are small (such as for minor faults and fractures) and do not trend parallel

to large topographic features. This gap between observation at contrasting scales can be filled by using other sources of informations at various intermediate scales

(SLAR and satellite images and aerial photographs) as discussed below. From the structural point of view, the major interest of these general analyses of the topography, togeth- er with geological mapping and tectonic study in the field, lies in the demonstration that the development of linear topographic elements is strongly controlled by the presence of major structural elements (such as the Lishan Valley FaulO or medium-scale structural elements (such as in the transverse valleys of the Central Range).

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Figure lb. Lithospheric scale schematic Taiwan block diagram of the Eurasian plate-Philippine Sea plate convergence [after ,4ngelier, 1986]. 1, Oceanic crust; 2, continental crust; 3, crust of volcanic arcs (Luzon and Ryukyu). The Philippine Sea plate is subducting beneath the Eurasian plate northeast of Taiwan (Ryukyu volcanic arc), while the Eurasian Plate is subducting beneath the Philippine Sea plate south of Taiwan (Manila trench).

Drainage Pattern and Thalweg Distribution

Taiwan Drainage Pattern

The Taiwan hydrographic drainage pattern (Figure 4a) has been simply extracted manually from topographic maps, (D.Boutin and B.Delcaillau, unpublished data) of scale 1:100.000. In these maps, the drainage pattern is accurately described where all streams, including small creeks, are mapped. The regional drainage pattern of Taiwan identified in the map of Figure 4a belongs to different basic drainage pattern types, which have been defined based on studies in other countries that resulted in

a descriptive drainage classification [Howard, 1967; Deffontaines and Chorowicz, 1991]. This classification is based on considering parameters such as the confluence angle of tributaries, drainage symmetry, drainage frequency (number of streams per unit area), and drainage density (length of stream per unit area).

Using this classification, one thus distinguishes the following features in Taiwan (Figure 4a), from west to east, the parallel drainage of the western Coastal Plain (mainly along W-E trends, see Figure l a for location) and the spectacular development of a dendritic drainage pattern characterized by a high stream frequency in the Western foothills. From north to south, a contorted dendritic and a trellis pattern compose the Central Range drainage. The Longitudinal Valley and the Coastal Range are characterized by a parallel and a trellis drainage, respectively. In more detail, some scattered, radial, outward (centrifugal) draina• •t, x xespond either to ' ' • (Tatun ' ' .... v oJcanoe• v oJ• m. Ju,

northern Taiwan)or to highly differentiated relief (North- eastern Central Range).

In any preliminary interpretation based on the identification of these types of drainage patterns, differences in lithology should be carefully considered. For instance, the southeastern portion of the Coastal Plain and Western

Foothills is characterized by strong changes in drainage frequency and density partly related to the systematic. occurrence of mudstones. As another example, the drainage patterns of mountainous areas such as the Hsiiehshan Range (with abundant hard sandstones) and the adjacent Backbone Range (with the slates of the Lushan Formation) differ markedly, due to strong lithological contrast revealed by erosion. Geological structure also plays a major role, as in northern Taiwan between Taipei and Chilung, where the relations between structural trends in the folded-faulted

series of the Foothills and the drainage pattern are obvious. One should also consider the role of the major faults, such as for the Lishan, Longitudinal Valley and Pingtung fault zones (Figure 4), which correspond not only to strong lithological contrasts between major units (Figures 1 and 2) but also to weaker sheared rocks, making erosion easier. The relationship between the type of drainage pattern and neotectonic deformation is complex, due to interactions between (1) lithology and structure, (2) climatic and erosional factors, and (3) uplift and shortening. As a consequence, a complete evaluation of such relationships in the active Taiwan orogen would require many detailed regional analyses, in addition to the examples discussed herein.

Taiwan Drainage Anomalies

Drainage network anomalies [Hobbs, 1904; Howard, 1967; Deffontaines, 1991; Deffontaines and Chorowicz, 1991; Deffontaines et al., 1992b] are revealed using channel patterns and their local modifications. Figure 4b represents Taiwan drainage anomalies which have been extracted manually from the following criteria: (1) local modifications of the drainage pattern: radial, centripetal (negative) or centrifugal (positive), annular or parallel when it should be dendritic if not influenced, for instance within the Western Coastal Plain [Howard, 1967; Deffontaines and Chorowicz, 1991]; (2) local modifications


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Figure 2a. Lithological map of Taiwan: (modified from Ho [1986]). Cenozoic stratigraphic section: Q, Quaternary Alluvium; Qa, Terrace Deposits; Qb' and Qb, Upper and Lower Toukoshan Formation; Qc, Hengchun reefal limestone; Pa, Plio-Quaternary Takangkou Formationof the Coastal Range; Li, Lichi M61ange of eastern Taiwan; K, Kenting M61ange of southern Taiwan; P1, Pleistocene Formation of western Taiwan; F, Foothills; Ms, Late Miocene to Pliocene Formation of Coastal Range; Mj, Miocene Formation of Kenting area; T, Miocene Calcalkalic volcanics and shallow marine deposits of Tulouanshan Formation; Lu, Miocene Backbone Range; H, Hsuehshan Range; $e, Eocene Slate, Back- bone Range; $c, Schist, Central Range; Ma, Marble; G, Gneiss, Central Range; a, volcanism of North Taiwan; aa, volcanism of $E Taiwan Luzon volcanic arc.

of the channel pattern such as rectilinear, curvilinear, or compressed meanders and oriented distribution of marshes, lakes, ponds, and aquatic vegetation, e.g. the Longitudinal Valley north of Taitung, where active faulting occurs); (3) modification of the confluence angle (e.g., streams usually converge at a 60 ø cofluence angle, locally it can be as large as 120 ø when associated with meanders, indicating depressed zone in alluvial plain, s• Western Coastal Plain); (4) divergence of streams which indicates raised zones, or volcanic aprons (Tatun volcano, Figure 4a); (5)

alignment of high angle curves, confluences, springs, and tributaries (Central Range, Figure 4b); (6) symmetry of tributaries (Western Coastal Plain); (7) flow directions reverse or oblique to the shoreline (Longitudinal valley rivers); and (8) low or high frequencies of streams (numbers per unit area) provide evidence for drainage anomalies (e.g. southern part of the Western Coastal Plain).

Drainage anomalies are widely distributed all around the island of Taiwan (Figure 4b). Their analysis is particularly useful in the areas where geological field work is difficult.

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Figure 2b. Structural sketch map of Taiwan (modified from Ho [1986]). 1, Known fault mapped on the geological map of Taiwan; LV, Longitudinal Valley Fault Zone.

This is the case for the flat lowlands of the Coastal plain due to extreme anthropic activity (rice crop) and, more interesting from our point of view, for the Central Range where access is limited due to the lack of roads and to the

presence of high-density jungle (except near the summits). Considering the drainage anomalies in itself cannot allow characterization of active features. They must also be compared with the distribution of orographic alignments, and detailed analysis of Landsat and SPOT images are especially necessary to get data in interfluve areas (especially Central Range, Coastal Plain, and Hsiiehshan Range; see later section). Taking into special account the occur- rences of dry thalwegs (i.e., without waterflow) also provides significant information.

Taiwan DEM Drainage Pattern

The DEM drainage pattern (Figure 4c) was automatically extracted from the digital elevation model illustrated in

Figures 3a and 3b. The "hydrological flow modeling" algorithm described by Chorowicz et al. [1992] has been used for this extraction (according to a procedure provided by C. Ichoku (personal communication, 1993)). In the process of extracting flow channels from the DEMs, the algorithm simulates surface runoff. First, the DEM is scanned in order to identify probable "channel heads"; flow routing is then performed by pixel to pixel transition. At each point along a channel, all contiguous neighbors of the current pixel are examined, and flow is routed into that which presents the steepest downward slope. In this way, each channel is carved until it meets an already carved one and "flows" into it, or until it reaches the image boundary. Otherwise, if it "flows" into a depression (in which case all contiguous neighbours of the current pixel are higher than it), an "outflow" route is sought. Among all saddle points adjoining the sources of channels flowing into this depression, the lowest one should be the natural "outflow" point (assuming the depression were filled with water). The


Figure 3a. N60øE Hill shading of Taiwan digital elevation model (DEM) showing features oblique and perpendicular to the mountain belt: Azimuth N60øE and light inclination of 30 ø . Note that relief perpendicular to the beam is emphasized, parallel relief being less visible. This is the reason for illustrating two contrasting trends of illumination with the same model (N60 and N120øE).

channel originating from this saddle (now, "outflow") point is reversed and redirected into the basin on its opposite side, where the flow routing continues.

The DEM drainage pattern (Figure 4c) was derived automatically from the morphometric model (DEM), independently of the drainage pattern extracted manually from topographic maps. As a consequence, numerous thalwegs in flat lowlands are not shown in Fig 4c because of the poor resolution of the DEM (extraction from the DEM puts emphasis on large valleys), whereas some well- marked dry valleys are not shown in Figure 4a.

Owing to the rough topographic nature of the terrain in the mountain belt and the poor resolution, the network extracted is composed of only narrow (skeletal) flow channels. Wide valleys and flat bottoms are absent, except in the Coastal Plain area, the Ilan plain, the Longitudinal Valley, few narrow flats, and some valleys in the mountains. The extracted channel network is comparable to the manually extracted network and can best portray linear structural features on which the analysis in this work will be based.

Figure 3b. N120øE Hill shading of Taiwan digital elevation model (DEM) showing longitudinal features principally: Azimuth N120øE and light inclination of 30 ø. Compare with Figure 3a.

Hydrographic and DEM Drainage Patterns: A Comparison

A comparison between the drainage pattern obtained manually from hydrographic information (Figure 4a) and that inferred from the DEM (called "topographic drainage" herein, Figure 4c) can be carried out despite the differences in resolution. Not surprisingly, there is an excellent consistency between the two patterns taking into account the difference in scales (e.g., all large streams of the topographic maps and the automatically extracted thalwegs from the DEM). However, some interesting differences exist due to the difference in origin of these maps' some pixels in the DEM drainage pattern, which appear only on the automatically extracted drainage patterns, correspond either to important dry thalwegs (without water flow) or to stream capture (Figure 4c). For example, the Western Foothills are characterized by numerous capture phenomena probably due to active thrusting deformation (development of ramps and piggy back basins), which result in the presence of numerous dry thalwegs (in the Pakuashan recent anticline of western Taiwan, for instance, B. Delcaillau (manuscript in preparation, 1994)). In contrast,

20,252 DEFFONTAINES ET AL.' NEW GEOMORPHIC DATA ON TAIWAN OROGEN

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Figure 4b. Drainage network anomalies of Taiwan, extracted from hydrographic information in topographic maps. Only drainage anomalies are shown as their lines (1). They include alignments of confluences, tributaries, springs, and compressed meanders (see text for further explanations).

because of the high value of the incidence angle. In order to obtain more radiometric information, it would be interesting to work directly on numerical data (ERS1, the data of the European radar satellite are not yet available). The scale of available SLAR radar photographs that we used (1:250,000) is compatible with that of the analyses and interpretation presented herein.

Two different maps were derived from the SLAR images of Figure 5a as follows. First, the interpretation map of Figure 5b shows the location of faults, Quaternary deposits, antiforms and synforms, and traces of lithological bedding that are clearly identifiable in both the SLAR imagery and geological maps. This analysis did not aim at constructing a general lineament map but rather at high- lighting close comparisons with geologic data independently obtained (however, several faults defined in the western fold-and-thrust belt were not recognized in the literature). This type of lineament analysis results in a structural map with scattered but well-controlled information. Second, the interpretative map of Figure 5c presents a summary of all linear features identified in SLAR images which have a probable structural significance. As a result, the lineament

pattern is denser than in Figure 5b. For instance, the Coastal Range and the Central Range have widely distributed SLAR lineaments, most of which correspond to faults and folds. In Figure 5c, note (1) the large number of features parallel to the structural grain of the Western Foothills and Central Range, (2) the presence of oblique NW-SE trends southwest of Taichung (corresponding to transfer zones across the Foothills and the outer Central

Range, as discussed in other sections of this paper), and (3) the en 6chelon pattern of the Coastal Range, with numerous NE-SW linear features discussed in detail in a

later section.

The SLAR image appears to be a major source of data despite its intrinsic distortion; it is of special interest as far as the geological structure is concerned. Furthermore,

Figure 4c. Drainage network of Taiwan, extracted from the DEM. Note the good correspondence between the drainage pattern of the topographic maps Figure 4a and the drainage network extracted from the DEM Figure 4c. Differences are however present: some thalwegs appear on the drainage network Figure 4c whereas they do not on the drainage pattern Figure 4a, which may correspond to dry thalwegs present in morphology Figure 4b but not taken into account in the hydrographic drainage pattern. In contrast, some drainage patterns not visible on the drainage network may be due to vertical movements characterized by stream captures. Note also the contrast between the hydrographic and relief information in the flat lowlands of Western Taiwan, where there are dense drainage patterns in the absence of marked talwegs.


Figure 5a. SLAR image south looking of Taiwan [MRSO and Mars aerial remote sensing Incorporation image, 1981] Within the Coastal Plain, numerous villages and towns appear in white. In the northern part, east to Taipei, different terraces and old alluvial fans are clearly distinguished.

because the accuracy is better than that of the DEM, a better account of linear features is obtained (compare Figures 3c and 5c, obtained by comparable techniques from the images of Figures 3a and 5a, respectively).

SPOT Images

Four SPOT panchromatic scenes (10 m x 10.m ground resolution) of eastern Taiwan (KJ: 300 - 301-4, 1987, 1988) have been acquired, processed, and analyzed. Fo- cusing on the Longitudinal Valley and the Coastal Range, the SPOT images allow a morphostructural overview (with much less distortion effects than SLAR images and better accuracy). This analysis allows modification and improvement of the structural information already existing in geological maps [Hsu, 1956; Ho, 1986b]. The analysis of SPOT-type images not only supplements radar analysis but also constitutes an indispensable link between the scale of the DEM and that of the geological observation discussed in the next subsection.

Let us consider a single SPOT image (KJ: 300-303, January 16, 1987), shown as an example. In this image (compare Figures 6a and 6b), the area of the Coastal Range reveals numerous NE-SW trending major topographic crests in the Tuluanshan formation (the volcanic arc). Locally, in the southern part of Figure 6a, folds and thrusts, which were not detectable in SLAR imagery, are evidenced. Different alluvial fans within the Longitudinal Valley may also be distinguished. Furthermore, this spatial analysis allows better mapping of the Central Range in the areas where access is difficult (not shown in Figure 6b). Owing to increased accuracy relative to the radar image, the geological features recognized are more numerous despite the presence of high-density jungle in many areas. It is possible, using SPOT images, to analyze large areas (60 km x 60 km for a single image); this is not the case at more derailed scales.

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Figure 5b. Morphological and tectonic analysis of Tai- wan SLAR images: Both the north and south looking images have been analyzed. The major lineaments in terms of morphology and tectonics are traced in the map. Location of faults, Quaternary deposits, antiforms and synforms, and traces of lithological bedding are compared with geologic data independently obtained (however, several faults defined in the western fold-and-thrust belt were not recognized in the literature). 1, inferred faults; 2, faults; 3, strike-slip faults; 4, reverse faults; 5, syncline fold axes; 6, anticline fold axes; 7, joint trends.

20,254 DEFFONTA1NES ET AL.: NEW GEOMORPHIC DATA ON TAIWAN OROGEN

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Figure 5c. Interpretation of SLAR photographs of Tai- wan: A summary of linear features identified in SLAR images, which have a probable structural significance, are presented here. Note (1) the large number of features parallel to the structural grain of the Western Foothills and Central Range; (2) the presence of oblique NW-SE trends southwest of Taichung (corresponding to transfer zones across the Foothills and the outer Central Range, as discussed); and (3) the en 6chelon pattern of the Coastal Range, with numerous NE-SW linear features.

Aerial Photographs

Local analyses of aerial photographs were undertaken at different places and in different ways, depending on the neotectonic problems addressed (e.g., Pinanshan, Fuli, and Chihshang area within the Longitudinal Valley; Pakua area in the Western Foothills near Taitung; Lishan fault zone in northern Taiwan). These analyses allowed accurate identification of numerous structures. They cannot be extensively discussed herein because they are too detailed for the scale of our present study. However, their contribution to a comprehensive morphotectonic analysis must be acknowl- edged, and such photographs should be taken into account in order to build a complete and accurate morphotectonic sketch map of Taiwan. From the tectonic analysis point of view, the detail displayed in aerial photographs is the only

information accurate enough to be directly and systemati- cally correlated with the results of tectonic observation in the field. In contrast, analyses of other images with poorer resolution, reveal features which in some cases cannot be directly correlated with structures observed in outcrops (e.g., the SPOT and SLAR images discussed before).

As an example, a local map of a characteristic fault- fracture pattern (two sets of conjugate strike-slip faults, some reverse faults, and one set of tension fractures, all geometrically and mechanically consistent according to field tectonic analysis) was published by Barrier et al. [1982], based on the analysis of aerial photographs in the Pinanshan conglomerates of the southern Longitudinal Valley (the active collision zone). In this case, most strike- slip faults and tension fractures, which range in size from tens to hundreds meters, were easily detected in aerial photographs because they correspond to large numbers of small creeks that dissect the Pinanshan Conglomerate massif. These small tectonic lines are not detectable in our

satellite images (and, of course, in the DEM), which reveal major structural trends only. Their accurate geometry and nature (i.e., tension or shear and slip orientation for faults) could not be identified with certainty in aerial photographs but were unambiguously recognized in several sites in the field. This example (not illustrated herein; see Barrier et al. [1982] and Barrier and Angelier [1983] for details), like several other examples in different areas of Taiwan, confirms that the aerial photographs constitute an indispensable link between the analyses at extremely different scales (e.g., satellite imagery of Taiwan and field observations in outcrops).

To conclude, the spatial and aerial approach presented herein is characterized by increasingly detailed data resolution (SLAR, SPOT, and aerial photographs, successively). In many cases led us to ascertain the validity of different morphotectonic features previously described. For instance, the interpretation of SLAR images not only allows general mapping of linear features at the scale of Taiwan (Figure 5) but also reveals oblique and transverse structures as discussed before for the Central Range and the Coastal Range. The spatial approach (SPOT imagery, see Figure 6) raises less distortion problems than the SLAR images; from a morphometric point of view, it provides data within the interfiuves (e.g., in summits, saddles, and flanks of the topography) which were not taken into account in the previous drainage analyses. Finally, due to its high resolution, the study of aerial photographs allows accurate local analysis (e.g., the Pinanshan case mentioned above) and direct comparison between features observed in outcrops and general images.

Contribution of Tectonic Analysis Although some major features such as the Lishan or the

Longitudinal Valley Faults are clearly visible and inter- pretable in most maps shown before, it is necessary to combine the results of interpretations in maps with those of independent tectonic analyses in the field. Especially, structural and tectonic analyses have provided valuable information on the geometry and mechanism of major structures of the orogen (especially faults), information which cannot be obtained unambiguously from general maps-•d images, although it is indispensable in any morphotectonic interpretation.

DEFFONTAINES ET AL.- NEW GEOMORPHIC DATA ON TAIWAN OROGEN 20,255

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Structures

The Longitudinal Valley of eastern Taiwan is clearly identified in all the documents shown before, including the DEM (Figures 3a and 3b), the drainage maps (Figure 4a), and the SLAR and satellite images (Figures 5 and 6). This major feature, which separates the Central Range and the Coastal Range (Figure 1), is very active [Tsai, 1986]. Its influence on the development of the drainage pattern is of particular interest (Figure 4). Whereas the analysis of the SLAR image mostly reveals the importance of the pattern of faults trending N20øE along the Longitudinal Valley (Figure 5), the analysis of the SPOT image shows the interaction between the active fault zone and the alluvial

fans (Figure 6). From the seismological point of view, the

main active zone of the Longitudinal Valley Fault strikes N20øE and dips approximately 50 ø to the east [Tsai et al., 1977]. Focal mechanisms of earthquakes are of reverse and strike-slip types, corresponding to combined thrusting and left-lateral slip [Barrier and Angelier, 1986]. This geometrical reconstruction of motion could not be done in the

absence of field studies; it allows better interpretation of morphotectonic features related to the Coastal Range uplift, to the shortening due to thrusting, and to the presence of a significant strike-slip component. Owing to active tectonics along the Longitudinal Valley Fault Zone and despite the high rates of clastic sedimentation (fans, alluvia), lateral offsets of drainage networks are observables. The interpretation of the SLAR image shown in Figure 5c highlights the presence of numerous N40øE features in the Coastal

DEFFONTAINES ET AL.: NEW GEOMORPHIC DATA ON TAIWAN OROGEN 20,257

Range east of the Longitudinal Valley, corresponding to NW vergent thrusts, to tight fold axes and steeply dipping fold flanks. Note that both the presence of these compressional features trending NE-SW and the oblique motion, reverse and left-lateral, of the Longitudinal Valley Fault, are in agreement with the NE-SW direction of the major Pleistocene compression identified based on detailed fault slip analyses in eastern Taiwan [Barrier, 1985]. This particular example shows that any interpretation of the morphostructural pattern needs to be supported by evidence of mechanical consistency provided by independent field tectonic analyses.

From the geological point of view, the uppermost section of the Takangkou formation, early Pleistocene in age, is affected by large tight folds and major thrusts throughout the Coastal Range; these features are clearly visible in the SLAR and satellite images (Figures 5 and 6). The Quaternary terraces of the Longitudinal Valley are also affected by thrusting, tilting, and gentle warping; these structures are principally observed in aerial photographs. Folds and thrusts affecting the Plio-Quaternary strike either N20øE, parallel to the Longitudinal Valley (especially along its eastern edge), or approximately N40øE, oblique to the trend of the plate boundary (especially within the Coastal Range); this distribution of the two major structural trends of compressional features is clearly visible in the SPOT image of Figures 6a and 6b.

West of the Longitudinal Valley, numerous linear features are observable in the Central Range, as Figures 3-5 show. They correspond in many cases to transverse fractures trending W-E to NW-SE, with deep valleys crosscut- ting the major NNW-SSE structural grain. Such transverse fractures are visible even in the DEM of Figures 3a and 3b, despite the poor resolution. Geological analysis in the field revealed in many cases that these linear features trend parallel (1) to tension fractures, and (2) to strike-slip faults as discussed earlier (the latter resulting in many cases from the reactivation of the former). The strike-slip fault movements were generally dextral along E-W trends and sinistral along NW-SE ones. The longitudinal features correspond to a wider variety of structures independently recognized in the field (fold axes and flanks, regional cleavages, thrusts and reverse faults), the most important ones being clearly observable in the DEM (Figure 3 and earlier discussion). The map of the drainage pattern (Figure 4) also reveals the presence of segments of valleys which trend parallel to the main structural grain; such features are especially highlighted in the map of drainage anomalies (Figure 4b). Field tectonic analyses showed that the thrusts and reverse faults which trend parallel to the belt axis correspond in most cases to pure reverse slip or to oblique reverse sinistral slip, with some exceptions near the northeastern tip of the Taiwan orogen. In the $LAR map (Fig- ure 5), the presence of all these types of transverse and longitudinal features is clearly illustrated (compare with Figures 3 and 4).

Many large morphological and structural features shown in the maps discussed above are clearly related to sets of minor tectonic structures observed in the field, despite differences in scales and limitations due to the distribution

of outcrops (e.g., transverse strike-slip faults in the Central Range as described before). However, the structural pattern is complicated so that its tectonic interpretation is

hazardous unless mechanisms of tectonic deformation are

taken into careful account (as mentioned earlier for the Coastal Range).

Mechanisms

The palcostress reconstructions previously carried out throughout the island provide an indispensable key to interpret the structural pattern which is obviously complex (Figures. 1-5). For instance, palcostress reconstructions were made at numerous sites in the Coastal Range [Barri- er, 1985; Barrier and Angelier, 1986], indicating a consistent NW-SE trend of the maximum compressional stress. In the Longitudinal Valley itself, some palcostress reconstructions were made in Quaternary terraces, indicating trends of compression that range from NW-SE to W-E [Barrier and Angelier, 1983]. The analysis of the focal mechanisms of earthquakes and microearthquakes also allowed reconstruction of NW-SE compression in this area [Barrier and Angelier, 1986; Yeh et al., 1991]. Analyses of present-day deformation by means of geodetic analyses [Yu and Liu, 1989; Yu et al., 1990; Lee and Angelier, 1993] reveal a consistent NW-SE direction of shortening across NNE-SSW trending faults. The Quaternary deformation in this region is dominated by oblique, left-lateral thrusting, due to NW-SE compression across a NNE-SSW boundary. Palcomagnetic studies [Lee et al., 1991] indicate that the different segments of the Coastal Range underwent clockwise rotations of about 20 ø which vary in age from about 3 Ma at the northern tip of the Coastal Range to less than 0.5 Ma at its southern tip. In agreement with the structural pattern of oblique folds and thrusts [Hsu, 1956], these palcomagnetic results suggest that the collision of the Luzon arc (the Coastal Range) against the Chinese continental margin (the Central Range) migrated progressively from north to south during the Plio-Quaternary, resulting in an en 6chelon pattern of NE-SW second-order fold-and- thrust ranges created by NW-SE compression. This structural pattern has a clear morphological expression in the present relief of the Longitudinal Valley and the Coastal Range, and the independent identification of a dominating NW-SE compression resulted in this case in unambiguous geodynamic interpretation (association of thrusting and left- lateral slip). The en 6chelon distribution of folds and thrusts along the Coastal Range is well expressed in the interpretative map obtained from SLAR image analysis (Figure 5c; see also SPOT image analysis, Figure 6).

In a similar way, palcostress analyses in the Western Foothills and the Central Range allow reconstruction of the compressional trends related to the major tectonic events since the late Miocene. Despite some complexites, this evolution is generally dominated by a change from NW-SE trends of compression to E-W ones during the late Pliocene and the Quaternary [Angelier et al., 1986, 1990]. Such variations in the trend of compression explain the common reactivation of tension fractures as left-lateral strike-slip faults and the common occurrence of successive dextral

and then sinistral slips on the WNW-ESE trending subvertical fractures within the mountain belt as observed

in many outcrops [Chu, 1990]. The importance of transverse and oblique linear features independently observed in interpretative maps obtained from SLAR images (Figure 5c), and even from the DEM (Figure 3c), is thus explained. Limited clockwise rotation complicates the pattern


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at the northeastern tip of the Taiwan mountain belt, as was suspected from paleostress analyses first [Angelier et al., 1986] and demonstrated later from paleomagnetic data [Angelier et al., 1990; Lee et al., 1991]

Finally, the structural development of the Plio-Quaterna- ry orogen in Taiwan can be entirely explained in terms of (1) inherited structures such as the extensional fault patterns of the earlier (Miocene) Chinese margin and the eastern boundary of the Central Range which probably corresponds to a major pre-Pliocene left-lateral strike-slip fault zone reactivated later as the Longitudinal Valley Fault, (2) average SE-NW compression resulting in the

development of the major structures, and (3) change from SE-NW to E-W in the average direction of compression during the latest Pliocene-Pleistocene.

We conclude that although tectonic and morphological analyses were generally carried out separately because of the different techniques involved, a generalized mechanical interpretation of the orogen development certainly provides a key for better understanding the distribution of the features identified in the maps presented before (Figure 3- 5). In turn, the distribution of morphologic features impose constraints and allows improvement in the mechanical interpretation. This complementarity has been illustrated

20,260 DEFFONTAINES ET AL.- NEW GEOMORPHIC DATA ON TAIWAN OROGEN

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before such as for the structural pattern of the Coastal Range [Hsu, 1956; Barrier and Angelier, 1986], the transverse-oblique features of the Central Range [Chu, 1990], and in more detail the brittle tectonic pattern of the Pinanshan massif near Taitung [Barrier et al., 1982; Barrier and Angelier, 1983]: in these cases the structural information was available but not correlated with, or enriched by, morphological observation.

Mountain Building and Erosion

In the previous section of this paper, the general consistency between the structural patterns identifiable from morphology and remote sensing analyses in Taiwan and those independently mapped by geologists in the field was highlighted. In the present section, we aim at illustrating the general relationships between the erosional phenomena


and the structural development at the scale of the whole island, especially in terms of vertical movements. The DEM (Figures 3a, 3b, 7a, and 8a) is here of great help to automatically obtain numerous treatments of the Taiwan topographic surface, such as average topography map (hereafter ATM), summit level map (SLM) and base level map (BLM), which would be difficult to get otherwise (due to time consuming operations, subjectivity in the choice of selected points, and so on). All these maps are shown in Figure 7.

Prior to any processing, the isocontour map (Figure 7a) shows many topographic features: for instance, the Coastal Range of eastern Taiwan is characterized by a pattern of oblique NE-SW and N-S topographic crests and bounded by the low and narrow rectilinear Longitudinal Valley. Farther west, many major topographic discontinuities strike NE-SW, and E-W to NW-SE, within the Central Range. Both the Longitudinal Valley and the Lishan Valley are still visible after ATM processing (compare Figures 7a, 7b, and 7f). In western Taiwan, the Foothills present NNE-SSW and N-S ridges south of Taichung, in contrast with northern Taiwan where NE-SW morphological alignments dominate (Figure 7a). We first examine the ATM, before defining highest and lowest surfaces as well as their significance from a geodynamic point of view.

Taiwan Average Topography Map

On basis of the second DEM discussed earlier, we used simple numerical techniques with 5x5 edge filtering, in order to obtain a smoothed map with pixels of 500 x 500 m in the horizontal plane. For each pixel, we considered the 24 surrounding pixels and the average of the altitudinal 5 x 5 matrix, which is affected to the pixel considered (ATM, Figures 7b and 7f). Imbricated series of averaging calculations result in the map presented herein. The topography of mountain areas is thus smoothed, whereas the flat lowlands are not affected. The major mountain units are still identifiable (Hsiiehshan, Central, and Coastal Ranges, Tatun volcano in Figures 7b and 7f; see Figure la for location). The Yushan Range massif (Y in Figure 7b) deserves particular attention; it appears to be a major and high topographic feature, roughly circular, of this smoothed surface. The smoothing effect also allows easier visual identification of the major drainage paths (compare Figures 4a and 7f).

A north-south cross section of the Central Range based on this smoothed topography (Figure 7e) reveals, from south to north, a regular northward slope (1') interrrupted by a discontinuity at latitude 23'20', then a kind of plateau (Yushan area), and finally a southward slope (1'30'). This distribution is consistent with models of mountain building such as that presented by Suppe [1981] for Taiwan, within the framework of oblique arc-continent collision migrating from north to south. The central plateau appearing in this profile thus represents tlae area of largest total tectonic accumulation since the beginning of the major collision (assuming that the uplift-erosion ratio is approximately constant). The northern slope corresponds to decreasing volumes, because recent collision is less active to the north. The southern slope corresponds to the most actively growing portion of the orogen, as collision migrates southward due to oblique convergence across the plate

boundary. At the scale considered, similar conclusions are obtained while examining the SLM (see Figure 7 and next subsection). A transverse topographic profile of Taiwan is also shown for reference. Note that they are apparent discrepancies between original (DEM) and smoothed profiles in both cases (longitudinal and transverse cross sections). These discrepancies are present because calculations were made in three dimensions so that smoothing effects are influenced not only by the topography in the section but also by the presence of topographic features on both sides of a profile (e.g., the northern high in the SLM, influenced by the Hsiiehshan Range summits near the profile).

From a geodynamic point of view, note that the existence of the Yushan Range as a major massif is also related to the influence of the "Peikang high" in the Coast- al Plain. This subsurface horst buried beneath Plio-Quater- nary sediments and revealed by seismic reflection profiles and drill holes data [e.g., Chow et al., 1989] has probably induced a discontinuity in the collisional mechanism. Ac- cording to this interpretation, the strongly uplifted Yushan Range massif would be bounded by a major transfer zone that trends N140*E: the "Pakua Transfer Fault Zone"

southeast of Taichung (PTFZ, see Plate lb for location). Parallel to this tectonic zone, there are several secondary transfer fault zones which are, from north to south: the Sanyi Transfer Fault Zone (STFZ, Plate lb), the Chukou Transfer Fault Zone (ChuTFZ, Plate lb) bounding the southwestern part of Yushan Range, and the Chishan Transfer Fault Zone (ChiTFZ, Plate lb) located north of Pingtung Plain. We conclude that major and minor transfer zones, visible in different interpretative maps shown herein (Figure 5 and Plate 1), do exist and accommodated different northwestward movements of the Western Foothills and

Central Range thrust units, relative to the stable foreland, during the Late Cenozoic collision. Most of these faults are left-lateral (consistent with the recent WNW-ESE compression, see earlier section), but right-lateral movements may have occurred during collision while fold-and-thrust units underwent differential movements toward the NW. Further

tectonic studies along these oblique fault zones are needed.

Taiwan Summit Level Map

The next step in this morphological analysis consists of characterizing the three-dimensional summit envelope of Taiwan mountains (SLM, Figures 7c and 7g). Despite uncertainties, this summit level map (hereafter SLM) more or less reflects the general dynamic level of erosion in the mountain belt. This conclusion was proposed by $uppe [1981], based on series of E-W topographic cross sections. A map of present-day summit levels can be elaborated from a DEM by numerical selection of spot heights [Pannekoek, 1967; Howard, 1973; Prud'hornrne, 1972; Dernoulin, 1986; Deffontaines et al., 1992a, b; Chorowicz and Deffontaines, 1993]. The SLM may be compared to a "virtual sheet" resting on the high points of the relief of a region, defined in order to eliminate irregularities of the topographic surface and to reconstruct a surface depending closely on the summits of the regional relief which subtend it. This map based on the morphometric approach to summit levels was established at different scales by the "filling-in of erosional hollows". We applied the same


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Plate 1. (a) Superimposition of different morphotectonic data of Taiwan. 1, drainage anomalies; 2, hill shading analyses; 3, known faults; 4, SLAR analyses (see text for further explanations). (b) Preliminary morphotectonic sketch map of Taiwan. Extraction of the major morphotectonic features of Plate la. Lines with triangles correspond to reverse or thrust faults; PTFZ, Pakua Transfer Fault Zone; STFZ, Sanyi Transfer Fault Zone; ChuTFZ, Chukou Transfer Fault Zone; ChiTFZ, Chishan Transfer Fault Zone; 1, major geomorphic structures; 2, meverse faults; 3, major strike-slip fault; 4, major normal fault.


methodology as for building the DEM. We used, as a database, the DEM derived from ONC (see above, Figure 7a, scale 1:1,000,000). These data were processed numerically, based on the morphometric approach to summit levels at different scales. This interpolation method does not require a high density of extracted summit level points. It results in a summit level surface passing through all definition points (continuous surface) and also guarantees that the interpolated surface is smoothed across the whole extent of the summit level surface (tangent plane continuity requirement). The calculation procedure is based on series of successive approximations, and the DEM is progressively densifted and adapted to the external constraints. The summit level map is progressively densifted by doubling and is thus adjusted step by step on known points. The process is repeated until the summit level map (Figure 7g) has the same spatial resolution as the initial digital elevation model. Given that the summit level surface starts from

a random distribution of points, this technique can be used for other purposes, in particular to construct DEMs, valley or subsurface level surfaces. The resulting surface corresponds to a 3-D smoothed envelope of the topographic summits, the degree of smoothing depending on user's choices (Figures 7c and 7g; see also corresponding profiles in Figures 7e and 8b).

Of course, this surface should not be interpreted as a direct key for reconstructing the total recent deformation. It does not represent an initial geomorphological erosional or structural surface, because tectonic uplift and associated erosion is only partly taken into account. It rather represents a "dynamic" erosional surface corresponding to the steady state of a fast continuing uplift (due to collision), with high rainfall and important linear river erosion. Three major regions are distinguished (Figures 7c and 7g). The northern one is the Hsiiehshan Range, characterized by an NE-SW crest and asymmetric wedgelike shape, with a gentle northwestern dipping slope contrasting with steep southeastern flanks. The Central Range is characterized by a rather flat plateau (in reality, dissected by strong erosional drainage such as for the Lishan Valley), bounded in its eastern part by a flank with a very high slope gradient, in contrast to the western flank. These general shapes reflect the combined effects of uplift (related to shortening) and erosion, taking into account the contrasts in lithology (e.g., the slate belt and the inner metamophic belt of the eastern Central Range, see Figure 2).

The differences between the summit level map (Figures 7c, 7g, and 8b) and the digital elevation model (Figures 7a and 8a), give a minimum value for the total volume eroded due to the total physical denudation (perspective view in Figure 8c). This erosion balance corresponds to the fill-in of hollows previously mentioned in the SLM subsection. A previous study of "instantaneous" erosion [Li, 1976] was based on considering (1) physical denudation, with sus- pended particle loads in rivers from throughout the island, and (2) chemical denudation estimated by using bimonthly chemical data collection for soluble materials and the water

flow data for these rivers. The type of operation suggested in this study provides a lower bound to the total cumulative estimate of the physical denudation and prepares the deter- mination of a more complete 3-D erosion balance. Note that the actual total amount of erosion is of course larger than that suggested in Figure 8c, because the summit level

surface is itself a dynamic erosional surface [Suppe, 1981; Knuepfer, 1988; Liew et al., 1993].

Taiwan Base Level Map

After having considered the highest surface of the topography (the SLM), we consider the lowest surface linking the valleys, called hereafter the base level map (BLM, Fig- ures 7d and 7h). From a technical point of view, the BLM is obtained by automatic extraction of valley level points (DEM drainage) and altimetric data interpolation, following the same methodology described above (DEM and SLM). Again, the degree of smoothing depends on user's choices. This map illustrates the presence of the principal slopes in the DEM drainage and major knickpoints of river longitudinal profiles, which can be due either to the influence of structure and lithology or to confluence of tributaries. Further studies aim at linking the BLM and uplift data, based on longitudinal profile analysis [Rhea, 1993]. The differences between the DEM and the BLM give a rough preliminary estimate of minimum volume which may be eroded in the near future; note, however, that such estimates are difficult to obtain, because as for the summit level surface, the base level surface is in fact a dynamic surface which corresponds to local erosional base levels.

Conclusion' Perspective for a Neotectonic Map of Taiwan

Despite its preliminary character, the study of Taiwan presented herein corresponds to an integrated approach combining different multisource and multiscale data and analyses. As a final step, we have combined in Plate la, with the help of a multisource database and a geological information system (GIS), the morphostructural interpretations obtained in the whole island. Interpretations were based on analyses of several general sources of information (topography, drainage patterns and anomalies, SLAR and satellite imagery: Figures.3-6), taking field data into additional account.

An automatic geomorphic approach using the DEM and derived surfaces (such as average topographic, summit level and base level maps) has good potential for future analyses of the recent geodynamic evolution of the whole island of Taiwan, including quantification of erosion- uplift/shortening balance (work in progress).

These analyses involved different techniques discussed before, such as various types of filtering, hill-shading analyses, and so on (e.g., Figures 3a, 3b, 4b, 4c, 5b, 5c, 7, and 8). It is important to note that most techniques required preliminary acquisition or conversion of data in numerical formats. In turn, we obtained a multisource database, which is available for more sophisticated forth- coming analyses at the same general scale and may be improved. In addition, the relation to geology (Figures 1-2) was carefully considered, resulting in an understanding of the morphotectonic significance that would have otherwise been difficult to obtain (e.g., Figure 6b). This compilation (Plate 1) results in a preliminary multisource morphotectonic map of Taiwan.

The juxtaposition of the most frequently recognized discontinuities results in denser patterns in the map of Plate


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Figure 8. (a) Perspective view of the digital elevation model of Taiwan (SLM). Azimuth N140øE, elevation 45 ø, vertical exaggeration x 2. (b) Perspective view of the summit level map of Taiwan (DEM). Azimuth N140øE, elevation 45 ø, vertical exaggeration x 2. (c) Perspective view of the erosion balance of Taiwan (SLM-DEM). Azimuth N140øE, elevation 45 ø, vertical exaggeration x 2.


la, where the origin of linear features is indicated by color codes. This densification is somewhat artificial because in

most cases, for a given feature, it is simply due to very slight differences in location (distortion). These minor misfits have been maintained in Plate l a in order to highlight the importance of the major features identified in maps from different sources, as a kind of relative weighting. The major structures thus recognized from various analyses are shown in Plate lb, obtained from Plate la through manual extraction.

In addition to well known structures, several important structures are recognized in the final maps of Plates la and lb. These include (1) the presence of N140øE transfer fault zones between major folds and thrusts within the Western Foothills and outer Central Range; (2) the occurrence of more densely spaced features in the northern part than in the central and southern part of Taiwan for the thrusts and folds of the Western Foothills zone; and (3) the presence of numerous oblique and transverse discontinuities (relative to the trend of the belt) in the Central Range, related to strike-slip faulting. Other examples of identifiable structures have been mentioned or discussed throughout the text. Thus the composite map of Plate 1 does not correspond to a simple multisource cartographic database but also allows identification of structures that were poorly described or interpreted before. Further studies should focus on the characterization and quantification of major topographic discontinuities wich have neotectonic significance (tectonic evolution, ages, and mechanisms), such as the Longitudinal Valley and the Coastal Range, the Lishan Fault, and the Pakua thrust anticline in the Western Foot- hills. In the preliminary morphotectonic map of Plate 1, however, the interpretation of the narrow Coastal Range appears to be difficult, probably due to inadequate scale (compare with Figure 6, better adapted for structural analysis). It was also pointed out that in many cases (e.g., the Pinanshan conglomerates), most images fail to show features which are very significant despite their small size (as analyses at the scale of aerial photographs shows). These examples bring strong confirmation that, in numerous cases, more detailed studies should be carried out on specific geological targets, involving accurate analyses of local topography, aerial photographs, and field work.

As the mass of morphological and geological information is large, it not only provides a database but also leads us to better understand local morphology and geology in comparison with other geological methods (tectonics analysis, stratigraphy, etc.). The multisource morphotectonic approach presented in this paper provides a basis for the establishment of a Taiwan neotectonic map in the near future. It is worthwhile, however, to point out again that extensive field work is indispensable in order to reach this stage.

Acknowledgments. This research has been done with various supports from the Taiwan-France cooperation program supported by the French Institute in Taipei (IFT) and the National Science Council, the Central Geological Survey of Taiwan, the University Pierre et Marie Curie, the National Taiwan University, the French Programme National de T616d6tection $patiale (PNTS), the CROU$, and the Ministry of Education of Taiwan. The authors thank C.Ichoku (Department of Structural Geology, Geomorphol- ogy and Remote Sensing) and D. Boutin and B. Delcaillau (Uni- versity Toulouse Le-Mirail) for the automatic and manual extrac-

tion of the DEM drainage pattern. Most figures are due to the computing skills of J.F. Brouillet; SPOT images have been printed by M. Danrde and M. Moroni. The authors are also grateful to P. Knuepfer, D. Merritts, and an anonymous reviewer for their numerous and constructive comments which allowed

major improvement of the manuscript. Support for reproduction of color plates has been provided by NASA Grant 3338 to Michael A. Ellis.

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(Received April 2, 1993; revised February 21, 1994; accepted March 17, 1994.)

new geomorphic data on the active taiwan orogen: a multisource approach

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