The Ilan Basin of northern Taiwan forms the western limit of the Okinawa Trough, where the trough meets the compressional ranges of central Taiwan. Apatite fission-track ages of 1.2 ± 0.5 Ma and 3.5 ± 0.5 Ma, measured north and south of the basin, respectively, indicate faster exhumation rates in the Hsüehshan Range to the north (>1.6 mm/yr) than in the Backbone Range to the south (0.7 mm/yr). Reconstructed subsidence rates along the northern basin margin are also faster than in the south (6–7 compared with 3–5 mm/yr). Global positioning system (GPS) and active seismological data indicate motion of the southern basin margin to the east and southeast. We propose that the Ilan Basin is being formed as a result of extension of northern Taiwan, largely controlled by a major southeast-dipping fault, modeled at ∼30° dip, and mapped as a continuation of the Lishan Fault, a major thrust structure in the Central Ranges. Flexural rigidity of the lithosphere under the basin is low, with elastic thickness ∼3 km. A southwest-migrating collision between the Luzon Arc and southern China, accompanied by subduction polarity reversal in the Ryukyu Trench, has allowed crustal blocks that were previously held in compression between the Eurasian and Philippine Sea plates to move trenchward as they reach the northern end of the collision zone. Subduction polarity reversal permits rapid extension and formation of the Ilan Basin and presumably, at least, the western Okinawa Trough, as a direct consequence of arc-continent collision, not because of independent trench rollback forces. This conceptual model suggests that migrating arc-continent collision causes the rapid formation of deep marginal basins that are then filled by detritus from the adjacent orogen, and that these should be common features in the geologic record.

Abstract Taiwan is a spindle-shaped island located at the junction of the Ryukyu and Taiwan-Luzon arcs. Nearly 100 thermal springs are scattered throughout the island. Genetically, they can be classified into volcanic and nonvolcanic types. The hottest and most impressive thermal springs and fumaroles of volcanic origin are distributed in the Tatun volcanic region north of T'aipei, which was given priority in geothermal exploration. The nonvolcanic thermal springs are widely distributed over the island. They occur mostly in the submetamorphic Paleogene to lower Miocene rocks and less abundantly in the Neogene terrane. The heat sources of these nonvolcanic thermal springs can be attributed mainly to the geothermal gradient; however, a hidden intrusive body or magma chamber at depth may account for the heat supply of some thermal springs. It is unknown if, and to what extent, magmatic heat has played a role in the nonvolcanic thermal springs of Taiwan. Geothermal exploration in the Tatun volcanic region revealed that the central part is most prospective. Subsidence of the Miocene sedimentary formations may have occurred as a consequence of the volcanic activities, and a graben probably exists in the prospective area. The Miocene sandstones below the thick volcanic cover are considered to be the main reservoirs for geothermal fluids. The highest subsurface temperature recorded was 293°C. The power potential of the Tatun region has been estimated to be 5,000–14,000 Mw/year. Unfortunately, the high acidity of the hot water presents a serious corrosion problem, and development of this region is suspended at the present time. A province-wide geothermal exploration program was started in July 1972. After a preliminary investigation, 10 prospective thermal areas are thought to justify further exploration. Among them, the Tuchang thermal area, about 40 km southwest of Ilan in the northeastern part of Taiwan, seems most favorable and is presently being studied.