The island of Taiwan was formed approximately 4 to 5 million years ago at a convergent boundary between the Philippine Sea Plate and the Eurasian Plate. In a boundary running the length of the island and continuing southwards, oceanic crust of the Eurasian Plate is sliding under the Philippine Sea Plate whereas in the northeast of the island, the Philippine Sea Plate slides under continental crust of the Eurasian Plate (see 3D block diagram below). Most of the island comprises a huge fault block tilted to the west.
The island is active geologically, formed on a complex convergent boundary between the Yangtze Subplate of the Eurasian Plate to the west and north, the Okinawa Plate on the north-east, the Philippine Sea Plate on the east and south, and the Sunda Plate to the southwest. Subduction changes direction at Taiwan. The upper part of the crust on the island is primarily made up of a series of terranes, mostly old island arcs which have been forced together by the collision of the forerunners of the Eurasian Plate and the Philippine Sea Plate, which is moving to the northwest. These have been further uplifted as a result of the detachment of a portion of the Eurasian Plate as it was subducted beneath remnants of the Philippine Sea Plate, a process which left the crust under Taiwan more buoyant.
South of Taiwan, the Sunda Plate is subducting under the Philippine Sea Plate, forming the Luzon Volcanic Arc (including Green Island and Orchid Island). The east and south of the island are a complex system of belts formed by, and part of the zone of, active collision between the North Luzon Trough portion of the Luzon Volcanic Arc and the Eurasian Plate, where accreted portions of the Luzon Arc and Luzon forearc form the eastern Coastal Range and parallel inland Taitung Longitudinal Valley of Taiwan respectively.Clift, Schouten and Draut (2003) in Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes, p84–86
From the northeast of Taiwan and continuing eastwards in the Ryukyu Volcanic Arc, the Philippine Sea Plate is subducting under the Okinawa Plate, forming the Ryukyu Volcanic Arc.
Terranes
The Hai'an Range or Coastal Range terrane belongs to the Luzon Volcanic Arc (呂宋島火山弧), and is on the Philippine Sea Plate (菲律賓海板塊). West of this is the Longitudinal Valley or Huatung Valley, an active depositional zone in a rift valley. West of this is the Eastern Central Range terrane, part of the continental base that has been upturned. The western side of the range is the Western Central Range terrane, which is metamorphosed earlier Tertiary clastic sediments. The Lishan Fault separates the Hsuehshan Range terrane Eocene to Oligocene sandstone and shale. The Chukou Fault (觸口斷層) provides the boundary for the Western Foothills terrane, which is shallow marine detritus from Miocene to Pleistocene. A deformation front forms the boundary with the undeformed coastal plains alluvium, which is still depositing.
The Eastern Central Range terrane can be divided into the Tailuko belt on the west with greenschist metamorphism and the Yuli belt on the east with blueschist metamorphism. The two metamorphic belts are possibly separated by a fault called Shoufeng fault, but this is unproven.
Central Mountains
The oldest exposed rocks are heavily metamorphosed and found in the Central Mountains. Poor quality fossil evidence suggests an origin in the Permian Period. They form a band of rock from Nan'ao in the north to inland of Jinfeng on the eastern flank of the Central Mountain Range. The band is 30 km wide in the north and narrows to the south. These earlier rocks are heavily metamorphosed. They also occur as a basement beneath the western part of the island. They make up the Danana'ao Group or Tananao Schist. This group consists of the Kainanagang Gneiss, Tailuge Marble, and Changchun Schist.
The Yuli Schist is Cretaceous in age and consists of black schist and some greenschist, glaucophane schist, and spotted slate. It is the easternmost part and extends from Chian southwards to the west of the Chihpen hot springs in a belt about 150 km long. Although the dark colour of the black schist is due to carbon, carbon is less than 2% of the content. The minerals found are quartz, mica, chlorite, albite, sphene, and graphite.
The Changchun Schist being mostly greenschist is found on the western side and forms thick beds. It is found along with smaller amounts of chert, and black schist. The rock is foliated dark green rock containing chlorite, epidote, quartz, calcite, biotite, albite and actinolite. They are derived from mafic volcanic rocks. These rocks can host copper sulfide ores.
Siliceous schists are coloured grey, they are metamorphosed sandstone, rich in quartz. Quartzite and chert bands can be found, and they are commonly associated with the black schist.
The Chiuchu Formation or Tailuge Marble forms a band from Tailuko in the north to a point between Wulu and Kuanshan in the south. The limestone is mined in quarries south of Suao for cement manufacture. Although the colour is usually a shade of grey, there are also black or white limestone, which is chopped into blocks for building purposes. dolomite is also found along with the limestone particularly at Chingchangshan, Hopingchi, and Mukuashan.
The Kainangan Gneiss or Kanagan Formation occurs in several elongated bodies. The gneiss derived from sedimentary rocks contains coarse grained quartz, biotite, and albitic plagioclase. There is also orthogneiss derived from granite. This is light grey and contains sodic plagioclase, quartz, biotite, and muscovite. Within the gneiss there are pegmatite dikes containing quartz, feldspar and mica.
Other igneous rocks found in the old metamorphic belt are amphibolite and dolerite dikes. Serpentinite is found 5 km west of Fengtien (奉天). It contains exploited deposits of nephrite, talc and asbestos. Wanjung Station (萬榮) is close to more outcrops of serpentenite. Tamayenshan block northwest of Juisui (瑞穗) and the Tsenghuanshan block 10 km west of Juisui are significant ultramafic outcrops.
Green serpentinite from near Yuli makes a good quality building stone. A band of ultramafic rocks extends for 25 km north of Litao, Taitung (離島).
The area was subjected to the Nanao Orogeny around 85 Ma which involved the granite intrusion and regional metamorphism. Rifting that opened the South China Sea around 40 Ma may be connected with some dolerite intrusions. The Penglai Orogeny started about 10 Ma and continues at the present time.
Eastern stratigraphic region The most recently added part of the island is the Coastal Range on the east coast from Taitung City in the south to Hualien City in the north. The range is the continuation of the Luzon arc. The age is Neogene and the rock consist of melange and volcaniclastics. The rock units are firstly Chimei Volcanics Miocene andesite. Next is the Tuluanshan Formation, consisting of tuff, then Takangou Formation, consisting of sediments such as shale and sandstone that contain volcanic particles. The Lichi Formation consists of melange. It is made of mud containing blocks of other stone such as sandstone and ophiolite. It is found on the southern half of the west side of the Coastal range. When this erodes, badlands are formed. Because it is a mixture of stone, the Lichi Formation is hard to date, but is likely to be from Pliocene.
The Pinanshan conglomerate is found on Pinanshan Hill and along the Pinantachi stream north of Taitung (台東). The constituents are 5–15 cm cobbles erodes by freshwater from the Central Range. It formed somewhere from middle to late Pleistocene, and indicates the plate collision had taken place.
The Milun Conglomerate is to the north of Hualien. This has also gone under the names of Beiron Conglomerate Formation and Milunpi Conglomerate. It is tilted at 30°. It is undated but is likely from Pleistocene.
Central Mountains strata Shibachongxi Formation is from the Eocene consisting of slate and then beds of metamorphosed sandstone. The total thickness is up to 1000 m.
Dajian Formation consists of sandstone from the upper Eocene, up to 2700 meters thick.
Xichun Formation slate and phyllite is from the lower Oligocene. The thickness is from 0.6 to 3 km.
Siling Formation coarse sandstone in thick beds is from the Oligocene.
Shuichungliu Formation form the Oligocene contains argillaceous slate and graywacke.
Bilushan Formation contains slate and phyllite from the Eocene.
Lushan Formation from the Miocene contains shale slate and sandstone.
Aodi Formation contemporary
Sule Formation slate and sandstone from the later Miocene.
Western stratigraphic region The west part of the island exposes deformed and metamorphosed Cenozoic sediments, overlaid by Quaternary piedmont region in the flat plains on the west coast.
Cretaceous, Paleocene and Eocene deposits are not exposed on the surface, but are buried. They have been discovered by drilling.
The Yunlin Formation is from the Cretaceous and only known from boreholes. The rocks are siltstone, basalt, shale and limestone.
The Paleocene Wangong Formation consisting of volcaniclastics, sandstone, shale and limestone is over 1046 meters thick.
In the Eocene the Shuangji Formation formed mostly from volcanic particles in the form of tuff and tuffaceous sandstone. These beds are from 100 m to 3 km thick.
In the Oligocene the Wuzhishan Formation or Wuchihshan Formation (五指山組) formed thick beds of sandstone. The total thickness is 0.9 to 1.2 km.
The Yeliu Group from the Miocene is fine grained sandstone, with some beds of shale, basaltic tuff and three seams of coal. Possibly including Wushan Formation, Piling Shale, Peiliao Formation, Talu shale, Shihti Formation, Kuanyiongshan sandstone.
The Ruifang Group contains beds of sandstone, siltstone, shale and six thin coal beds. These beds are 0.8 to 1.6 km thick.
The Sangxia Group starts with medium grained sandstone, but in the upper layers increases shale. Eight thin coal seams are included. The total thickness of the deepest beds exceeds 5 km. This includes the Kueichulin Formation with the Yutengping Sandstone, Shihliufen Shale, Kuantaoshan Sandstone; the Nanchuang Formation including Shangfuchi Sandstone and Tungkeng Formation.
The Jinshui Formation or Chinshui Formation from the Pliocene is mostly shale interbedded with some mudstone and sandstone is between 80 and 400 m thick.
The Zhuolan Formation or Cholan Formation (卓蘭層) starts in the Pliocene and extends into the first stage of the Pleistocene. It consists of fine grained sandstone. This is 1.5 to 2.5 km thick. At the same time on the southern tip of the island, the Kending Formation or Kenting Formation (墾丁組) was formed consisting of mudstone with ophiolite melange. The ophiolite melange contains pebbles and blocks of Miocene age consisting of basic and ultrabasic rock from the ocean floor. The interpretation is that a wedge of seafloor was pushed above sea level, eroded and dropped fragments into the mud.
The Toukoshan Formation (Toukeshan) (頭嵙山層) commenced with fine sandstone with thin beds of conglomerate, and continued into mainly conglomerate with sandstone beds. This was formed in stage 1 and 2 of the Pleistocene. It is from 0.4 to 3 km thick.
The Szekou Formation is a light bluish gray siltstone, with shale and fine grained sandstone. The Maanshan Formation is very similar. It is overlain by Hengchun Limestone and may interfinger.
Western piedmont region The Hengchun Limestone was formed after the Penglai movement in the third Pleistocene stage. Also at this time laterite (紅土) and gravel on river terraces was formed. Holocene mud sand and coral form a wide belt on the west side of the island.
Volcanic rocks