Taiwan is a modern mobile belt in the classical sense, and an orogeny formed by the collision of an island arc with the Asian continental margin. The main island of Taiwan comprises in part a metamorphic basement covered by Cenozoic deposits to a thickness of more than 10,000 meters. The north-south axis of deposition has shifted progressively westward with successive orogenic episodes. A considerable part of the Tertiary sediments of Taiwan has been subjected to different grades of induration or metamorphism. Large igneous intrusions are rare, but there are several important volcanic fields in northern Taiwan, eastern Taiwan, and the Taiwan Strait. Taiwan is located on a convergent plate boundary between the Asiatic continental plate and the Philippine Sea oceanic plate; the present convergence rate is 7 cm/yr. in a northwest direction.
The major rock formations of Taiwan form long narrow belts roughly parallel to the long axis of the island. These rock belts young progressively westward from the central backbone range to the west coast. The prevailing structural pattern is that of an elongate arc convex to the west. The northern, shorter bend of the arc strikes east-northeast whereas the southern, major arm of the arc extends mainly north-south. All the major structures of Taiwan, including the important faults and fold axes, follow fairly closely this arcuate structure throughout the island.
The metamorphic basement of Taiwan is the oldest geologic-tectonic element, and was formed in Late Paleozoic to Mesozoic times when a thick sequence of sandstone, shale, siltstone, limestone and volcanic rocks was deposited along with accompanying acid to intermediate magmatic activity. This old geologic-tectonic element experienced several phases of orogenic deformation, magmatism and metamorphism, culminating in a Late Mesozoic orogeny, locally known as the Nanao Orogeny. These pre-Cenozoic rocks were then folded into mountain ranges and intensely metamorphosed to form the main metamorphic complex of Taiwan. The detailed geologic history of this metamorphic basement remains difficult to unravel.
This metamorphic complex is exposed along the eastern flank of the Central Range extending north from Wuyenchio on the northeastern coast southward to the Tamali stream in Taitung-hsien. The total length is nearly 250 kilometers and the width ranges from 10 to 30 kilometers. This metamorphic belt is made up of several kinds of schists and metamorphosed limestone, with subordinate amounts of gneiss and amphibolite which are exposed mostly in the northern part. The schists can be differentiated into three main categories: greenschist, black schist, and siliceous schist. In the eastern part, scattered exposures of mafic to ultramatic rocks, mostly altered to serpentinite, represent rocks of oceanic affinities. The age of these metamorphic rocks is poorly constrained by scarce fossils and limited radiometric age determinations. A few fusulinids found in the mamorized limestone of this metamorphic series have been assigned a Permian age. The limited radiometric dating of the metamorphic rocks show an age spread from 90 to 3 millions years. The younger ages could be the result of resetting of the radiometric clock by successive deformation events. The metamorphic complex is thus considered as pre-Tertiary in general, possibly ranging in age from Late Paleozoic to Mesozoic. This metamorphic complex could be composed of a complicated mass of thus-far undifferentiated rock formations of varying ages, and may record several periods of tectonism and igneous activity.
All the basement rocks in the metamorphic complex are grouped under the general stratigraphic term "Tananao Schist." Three major episodes of diastrophism and metamorphism are tentatively identified in this complex: (1) formation of Late Mesozoic paired metamorphic belts in the Tananao Schist: the western Tailuko low P/T belt and the eastern Yuli high P/T belt; (2) late-Miocene (?) metamorphism and recrystallization of these two metamorphic belts; and (3) Plio-Pleistocene collision-related metamorphism of both metamorphic basement and its cover strata.
The metamorphic complex forms the basement over which accumulated most of the Tertiary sedimentary deposits of Taiwan. The earliest deposits laid down in the Tertiary basin are dominantly dark argillaceous sediments that were later indurated and metamorphosed, forming a thick, dark gray argillite-slate-phyllite series. Sandstone members are locally common, the sandstone being very well indurated or of a quartzitic texture. White sandstone containing carbonaceous interlayers is found mostly toward the northwestern part of the argillaceous belt near the continental shelf of the basin. Basaltic pyroclastic deposits and minor lava flows are common in some parts of the argillite-slate belt. Small disseminated limy to marly lenses and irregular conglomerate layers or lentils are also intercalated in the slate, mostly in the high ridges and the southern part of the Central Range. All these rocks are now exposed along the crest of the Central Range and on its western and southeastern flanks.
Fossils are sparsely distributed in this undifferentiated metamorphosed shaly sequence. The prominent organisms are foraminifers, calcareous nannofossils, and mollusks. Their ages extend from Eocene to early or early middle Miocene. However, it is very difficult to subdivide this mighty slate sequence into adequate mappable units with exact age assignment due to the monotonous lithologic succession, paucity of fossils, complicated structural features, and lack of recognizable datum planes for regional correlation. Correlation of stratigraphic units among different parts of the argillite-slate series is also difficult.
Two different lithotectonic zones can be recognized in the argillite-slate series based on differences in lithology, age, and metamorphism grade. They are the western Hsuehshan Range belt and the eastern Backbone Range belt. The tectonic relationship between these two lithotectonic belts is not yet clear. The western Hsuehshan Range belt is characterized by the widespread occurrence of thick carbonaceous sandstone and the near absence of limestone lenses. Conglomerate and pyroclastic effusives are better developed toward the middle part of this western belt. Due to the relatively common sandstone horizons, it is comparatively easy to subdivide the rocks of the western belt into mappable rock units. The age of these rocks has been ascribed to Eocene, Oligocene, and early to middle Miocene. The deformation of the rocks in this belt produced repeated anticlinal and synclinal folds, which are usually separated by strike faults. The folds are either broad and open or closed and isoclinal. The sediments are indurated or metamorphosed to varying degrees.
The highest part and the southern part of the Central Range together form the Backbone Ridge belt which is the least known geologic terrain in Taiwan. Although this belt covers quite a large extent in the Central Range, only two broad stratigraphic units are distinguished on the map: the Miocene Lushan Formation and the Eocene Pilushan Formation. The chief lithology of both formations is principally metamorphosed argillaceous sediment, mostly slate and phyllite, although sandstone interbeds are more abundant in the Pilushan Formation. The Pilushan Formation also contains more limy and conglomerate lenses and more volcanic intercalations than does the Lushan Formation. The apparent stratigraphic gap between the Eocene and Miocene slates suggests an unconformity between the two, but thus far no good evidence of an angular discordance has been discovered, beyond a faunal break recorded in most localities.
Non-metamorphosed Late Cenozoic rocks are exposed on the west of the Central Range, forming the western foothills and extending beneath the western coastal plain and tablelands. Recent study of nannofossils indicates that the lower part of this young clastic sequence is Oligocene in age. Only fault contacts are known between these non-metamorphosed deposits in the western foothills and the metamorphosed Eocene to Miocene argillaceous sediments in the Central Range. The stratigraphic and structural relations between these two Cenozoic belts are still not clear. As the non-metamorphosed Late Cenozoic rocks in the western foothills and the metamorphosed Eocene to Miocene argillaceous sediments in the Central Range differ markedly in degree of deformation as well as in lithofacies, they have clearly had contrasting structural and burial histories despite their partially overlapping age ranges.
The clastic deposits in the Late Cenozoic basin of the western foothills range in age from late Oligocene to early Pleistocene. They are mostly in continuous sequences with only minor breaks or diastems. The lithology is marked by alternating beds of sandstone, siltstone, and shale with little intercalated limestone and tuff lentils. The accumulated thickness of the Cenozoic deposits in the western basin is 8,000 meters or more.
The Oligocene to Miocene sediments may be divided into two facies types on the basis of inferred depositional environments: the shelf type and the basin type. The strata of the shelf type are characterized by near-shore deposits of mixed marine and continental origin. They are mainly white to light gray orthoquartzitic or arkosic sandstones, thin coal beds, dark shales, and thin interlaminations of silt, clay, and sand. The strata of the basin type were deposited under conditions of greater subsidence and rapid accumulation and are exclusively marine. They are represented by poorly sorted clastic sediments, either as thick sandstone or shale members or in a monotonous sequence of alternating fine-grained sandstones, subgraywackes, and dark shales or claystones. These two types of sediments are usually in interfingering or intertonguing relationships and occur in repetitive succession in the northern part of the western basin. They are characterized by three Miocene coal-bearing formations in rhythmic alternation with basin-type marine formations, forming three sedimentary cycles in the Miocene of northern Taiwan. In each cycle, early coal-bearing formation is overlain by an intervening sequence of marine strata.
Disseminated basaltic tuff lenses or irregular bodies including a minor amount of associated lava flows and tuffaceous sediments are scattered in all the Miocene units in the northern part of this western basin. These are mainly the result of submarine eruptions that formed small pyroclastic cones of varying sizes. They are best developed in the lower Miocene sedimentary cycle of northern Taiwan. Volcanism was rare in central Taiwan in the Miocene, but there was minor volcanism during the late Miocene in southern Taiwan.
The exposed Pliocene sedimentary deposits in northern and central Taiwan are mainly marine and of basin facies. They are composed of shales, siltstones, and fine sandstones, reaching a maximum thickness of more than 2,000 meters. Marine fossils are abundant in these rocks.
The character of Neogene sedimentation changes markedly toward the south, where the Neogene deposits are generally thicker, more shaly, and finer grained. The Miocene sedimentary rocks in southern Taiwan are marked by the progressive decrease in abundance of shelf facies (coal-bearing formations) to the exclusive development of basinward marine facies. This indicates that water in the basin deepened toward the south. Neogene rocks exposed in southern Taiwan are mainly alternating successions of dark gray shale or claystone, mudstone, and a little sandstone. They are mainly of middle or late Miocene to Pliocene in age. A mudstone series more than 4,000 meters thick was piling up in southern Taiwan during the Pliocene and Pleistocene times. This mudstone represents a mass of submarine mudflow in the southern part of the Neogene basin. Some biohermal to biostromal reef limestone bodies are scattered in the Pliocene and Pleistocene rocks of southern Taiwan, more abundantly developed in the Pleistocene.
The Neogene sedimentation in western Taiwan ended with the deposition of thick early Pleistocene conglomerate which indicates the incipient phase of a major orogenic paroxysm. Strong and widespread orogeny accompanied and followed the accumulation of this conglomerate in this western basin in Pleistocene time. This is the Penglai Orogeny, the most important diastrophism of Taiwan. All the Cenozoic units of the western basin were folded and thrust up into mountain ranges. The Eocene to Miocene argillaceous sediments to the east were also deformed and altered into an argillite-slate belt exposed in the Central Range. The pre-Tertiary metamorphic basement was reactivated and brought up on the eastern flank of the Central Range. Distinct post-orogenic deposits are represented by lateritic and non- lateritic tableland gravels that cover extensive areas of western Taiwan, especially toward the north. Southward the newly emerged Taiwan island provided shallow-water environments for the buildup of Pleistocene reefal limestone, which uncon-formably overlies all older formations in the coastal areas of southern Taiwan.
Following early Pleistocene orogeny, Neogene units of the western basin were deformed by a combination of folds and thrusts. Tight and asymetric folds and low-angle thrusts prevail in the eastern part. The rocks were deformed into elongate, nearly parallel anticlines and synclines, crowd closely against one another. As a general rule, the anticlines generally have steep or overturned northwestern flanks and the synclines have steep or overturned southeastern flanks. The folds verge toward the northwest and have been separated from each other by thrust faults, forming a series of southeast-dipping imbricate thrust blocks. Many of these faults are low-angle thrusts traceable for long distances. The deformation was characterized by lateral compression from the east, perhaps assisted by gravitational forces.
This imbricate thrust zone is delimited on the west by several continuous to en echelon sole faults, which are the major tectonic structures in western Taiwan. These sole thrusts form a structural front along which the strongly deformed thick rock sequences in the fold and thrust belt of the western foothills give way to gently warped thinner sediments in the foreland toward the west. West of this structural front are typical foreland structures, comprising broad and gentle folds and minor faults.
Although the rocks in the western foothills are strongly folded and steeply dipping, basement rocks are not involved in the folding, and have been little disturbed by the tectonism. The younger Cenozoic strata in the western basin were sheared off from stratigraphically lower formations along a decollement surface that has been recognized.
Two sets of strike-slip faults are recognized in different places of western Taiwan. One set is dextral or right-slip, striking chiefly WNW. The other set is sinistral or left-slip, trending mainly NNE. These two sets of strike-slip faults represent two complementary shear fractures in a fold belt dominated by a principal compressive stress acting in a NW-SE direction. They are symmetrically disposed about the main fold axes in the fold belt.
During and after the early Pleistocene orogeny, andesitic eruptions took place in northern Taiwan and in the northern and northeastern offshore islands. The volcanic activities formed two important volcano groups in northern Taiwan: the Tatun Volcano Group in the west, characterized by andesitic flows; and the Chilung Volcano Group in the east, marked by dacite flows and associated gold and copper deposits. At the same time, Pleistocene plateau basalt was extruded from fissures in the Late Cenozoic strata on the Penghu Island Group in the Taiwan Strait. Extensive basaltic flows cover the entire island group except one southwest islet, forming typical mesa-type landforms.
The present Coastal Range in eastern Taiwan is the site of a Neogene volcano- sedimentary basin, representing a volcano/magmatic arc situated on the western leading edge of the Philippine Sea plate. To the west, across the linear Longitudinal Valley, the remainder of Taiwan is located on the eastern margin of the Eurasian plate. Therefore the eastern Neogene basin (Coastal Range) is different from the western Neogene basin (western foothills) in sedimentary facies and in geologic development. Deposition in the Coastal Range is typically of turbiditic flysch-type facies, characterized by more abundant volcanic components.
The eruption of andesitic lava flows in the early Miocene marked the beginning of the known geologic history of the east basin. These andesitic flows form an igneous complex of composite activities, exposed mainly in the middle part of the Coastal Range in the drainage area of the Hsiukuluanchi. The two southeastern offshore islands, Lutao and Lanhsu, are also formed of this andesite and related pyroclastic deposits, lying in the same Mio-Pliocene volcanic belt.
The eruption of the andesite was followed by the accumulation of a thick sequence of andesitic agglomerate and other tuffaceous sediments, reaching nearly 1,500 meters thick. Thin limestone lenses yielding mainly fossils of early Pliocene age are found in places on top of the thick agglomerate formation, indicating that the agglomerate series is chiefly of late Miocene age. The andesite flows and the overlying thick pyroclastic series represent the magmatic arc on the leading edge of the Philippine Sea plate in eastern Taiwan. Based on radiometric dating, the age of andesite in the igneous complex of this arc is younger toward the south. The age is mainly Miocene in the Coastal Range, but volcanism continued into the Pliocene and Plio-Pleistocene toward the southeastern offshore islands and further south.
Overlying the volcanic series is a Pliocene volcaniclastic unit. An obscure disconformity may separate this unit from the underlying igneous rocks. The volcaniclastic unit may reach 3,000 meters in maximum thickness and its upper part may be Pleistocene in age. It is composed of turbiditic sandstones, siltstones, mudstones, and conglomerate. These rocks are characterized by the predominance of well-developed turbidite sedimentary structures and flysch-type depositional features. This unit contains various amounts of volcanic constituents in different sections.
A chaotic and non-stratified, muddy to clayey formation containing many exotic blocks of different sizes, ages, and lithologies was emplaced in late Pliocene to early Pleistocene time in the Coastal Range. This formation may have resulted from submarine gliding, in which massive mudflows with various Miocene and Pliocene rock fragments and exotic blocks of ophiolitic lithologies slid down along a steep submarine slope or fault scarp in the south China Sea basin on the west. This massive chaotic unit is a typical melange including abundant blocks of oceanic crust and upper mantle material, and probably formed at the time of major orogeny in eastern Taiwan. Also in early Pleistocene time, a massive piedmont deposit was laid down along the eastern front of the Central Range, generally intertonguing with the chaotic melange formation toward the east. Fault contact between these two formations has been reported in some places. The piedmont deposit is represented by a conglomerate that crops out mainly on the Pinanshan hill northwest of the city of Taitung. It is composed of pebbles derived from the metamorphic complex of the Central Range west of the Taitung valley. Total thickness of the conglomerate is estimated to be nearly 1,400 meters. After the early Pleistocene orogeny, strata of the eastern Neogene basin were folded, faulted, and uplifted into mountains. The Neogene rocks in the Coastal Range were then folded into subparallel anticlines and synclines cut by several longitudinal faults.
The early Pleistocene orogeny produced widespread emergence of Taiwan. In Pleistocene time, local marine transgressions took place over many areas on the borders of the rising land area. Reef limestones continued to grow around the rim of the early Taiwan island, mainly on the Hengchun Peninsula. Elsewhere mainly continental detritus was laid down. These subaerial Quaternary deposits are composed of thin and widely scattered surficial sediments of valley fills consisting of sandstone, siltstone, claystone, and sand and gravel beds. Lateritic to non-lateritic terrace gravel covered a considerable part of this island, unconformably overlying all older rocks. Probably as a result of extensional relaxation of compressional forces after a major orogeny, mild and intermittent regional uplifting, block faulting, broad warping, and tilting are characteristic of the Pleistocene history of much of Taiwan accompanied by extensive volcanism in northernmost Taiwan as mentioned earlier. Taiwan is a presently active mobile belt, characterized by continued crustal mobility expressed as frequent earthquakes, some of which accompany strike-slip faulting and recurrent displacement along existing ruptures.
The Plio-Pleistocene plate tectonic setting of Taiwan is the convergence and collision between the western Eurasian plate and the easterly impinging Philippine Sea plate. The Longitudinal Valley between Hualien and Taitung in eastern Taiwan has been recognized as the suture zone between these two plates. In northeastern Taiwan, convergence is marked by northward subduction of the Philippine Sea plate beneath the Ryukyu Arc System on the Eurasian plate along the Ryukyu Trench. The subduction polarity changes at 24cX N near Hualien in eastern Taiwan. There plate convergence is marked by eastward subduction of the Eurasian plate underneath the Luzon Arc System on the Philippine Sea plate along the Eastern Longitudinal Valley.
|Figure2. Geologic provinces of Taiwan
Most of the Cenozoic rock formations of Taiwan vary widely in lithofacies, thickness, and structure from place to place. A given section of any formation may change from area to area and is thus likely not to be representative at all places. In order to understand the geologic history of this island more fully, it is essential to study the rocks deposited in several geologically distinct regions during each geologic interval. Rocks of the same age are often represented by different facies and lithology due to different sedimentary or tectonic environments in these regions. It is thus desirable to discuss first the major geologic provinces and their bearing on the geologic development of Taiwan. Each province is characterized by geologic and geographic features that can be readily distinguished on the geologic map. Taiwan can be broadly divided into three major geologic provinces (Fig. 2):
- Central Range, subdivisible into two subprovinces:
Western flank and crest zone, including Hsuehshan and Yushan
Eastern flank of the Central Range
- Western foothills
- Coastal Range
The first geologic province of Taiwan is the Central Range, which forms the backbone ridge of the island. On the map in Fig. 2, the Central Range is defined purely from the geologic point of view. This is the region including all the Tertiary submetamorphic rocks and the pre-Tertiary metamorphic complex east of a major boundary fault, the Chuchih fault, which is called the Laonungchi or Chaochow fault to the south. The Central Range is bounded on the east by the Eastern Longitudinal Valley. Much of the stratigraphy and structure in this province are still poorly known due to metamorphism and insufficient field mapping. This geologic province can be subdivided geologically into two subprovinces, eastern and western. The western Central Range belt (IV) is a broad Tertiary submetamorphic belt that is exposed along the western flank, the crest zone, and the southern part of the Central Range. It extends also as a narrow rim on the southeastern flank of the Central Range. The western subprovince can be further subdivided into two lithotectonic belts: the western Hsuehshan Range belt (IVa) and the eastern Backbone Ridge belt (IVb). The eastern Central Range belt (V) is underlain by the pre-Tertiary metamorphic complex exposed largely on the eastern flank of the Central Range. This subprovince is subdivided into the western Tailuko belt (Va) and the eastern Yuli belt (Vb).
The western foothills province (III) is composed of Oligocene to Pleistocene clastic sediments. The rocks are mainly alternations of sandstones and shales with locally interspersed limestone and tuff lenses. The total thickness is 8,000 meters or more. This is the best-known geologic province of Taiwan as a result of extensive mineral exploration and other activities in this region. More detailed geologic mapping has been completed here and more structural details are known about this province than about the other two provinces. To the west, the Coastal Plain (II) and the Penghu islands (I), two physiographic units, can also be included in the western foothills province.
The third geologic province is the Coastal Range (VII) in eastern Taiwan. This province is also underlain by Neogene sediments but of a different geologic environment as compared with the Neogene rocks in the western foothills. The rocks are characterized by the abundance of volcanic derivatives, poorly sorted volcaniclastic sediments, turbiditic clastic rocks, and chaotic melange. The Coastal Range is separated from the Central Range by the Taitung Longitudinal Valley (VI). This is an important tectonic feature and may be considered an independent subprovince.