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In Taiwan's Central Range mountains, fundamental constraints on depositional ages and the timing of deformation and metamorphism remain a problematic issue preventing a consensual chronology of orogenic events to be established. In this contribution, we report detrital zircon U-Pb detrital ages for the Chulai Formation, the easternmost strip of metamorphic sediments depositionally overlying the metamorphic, high-pressure Yuli belt. We demonstrate that the maximum depositional age of this unit is 11.2 ± 0.2 Ma (Upper Miocene, Tortonian), making it the youngest pervasively deformed and metamorphosed unit of the Central Range. Detrital zircon ages suggest an almost exclusively continental origin of the sediments similar to the Yuli belt's matrix detrital age zircon spectra. Sedimentary relationships and structural considerations indicate that the Chulai Formation underwent essentially the same deformation history as the underlying Yuli belt, and thus the maximum depositional age of 11.2 ± 0.2 Ma is interpreted as the upper limit for the start of pervasive deformation of the eastern Central Range geological units. When considering the existing geochronological constraints on the metamorphism, we argue that the timing for the Cenozoic metamorphism of the Taiwan orogen is likely to be ~6 - 8 Ma. [ABSTRACT FROM AUTHOR]

Taiwan is one of the most typical arc-continental collision systems in the world. The Taiwan orogeny is often explained by the thin-skin model, where a shallow decollement, with depths of less than 10 km, is considered as the crustal deformation boundary for generating significant mountains on the island of Taiwan. In this study, the seismic data generated by crustal earthquakes, recorded by seismic stations in the Chia-Yi area of western Taiwan was examined to find the crustal structures beneath the Taiwan orogeny. A significant velocity boundary below the decollement was found from unambiguous multiple secondary waves and phases of primary waves converted to secondary waves. Careful comparison between the observations and calculated results of the travel-time and ray-path indicates that this boundary is dipping to the west at ~10 degrees, which, surprisingly, is dipping to the opposite direction of the proposed decollement. It is also interesting to note that this boundary is not only a velocity boundary but also consistent with a seismicity boundary. This westdipping boundary is the major seismogenic boundary in the upper crust, while the east-dipping decollement proposed earlier is a geological boundary. As a result, the westward dipping boundary observed in this study is more suitable, than the eastward dipping decollement, to be the seismogenic boundary in the Chia-Yi area. [ABSTRACT FROM AUTHOR]

Abstract: Large gneiss bodies have been reported in the metamorphic complex in northern and eastern Taiwan for decades. Some of them are cut or intruded by granitic pegmatite dikes. However, increasing evidence suggests that the gneiss bodies are more likely to be granites or meta-granites. To validate the existence of the granites/meta-granites and propose their potential distribution in the metamorphic complex of northeastern Taiwan, a geological reconnaissance along with a crooked long-distance VLF-EM survey line of 19km and a 4.4km controlled experimental line were conducted in the Hoping geological area of the northeastern Taiwan mountain belt. The VLF-EM data were initially processed by using the Fraser linear filter and a nonlinear filtering method based on the ensemble empirical mode decomposition (EEMD) technique to enhance the signal and to evaluate the data quality. A skin-depth added Karous–Hjelt filter was performed to generate the equivalent current density model. With the aid of the 3-D topographic representation, the equivalent current density model clearly indicates that a vast area of granites/meta-granites in the survey area is highly possible. In spite of a large uncertainty of the pseudo-quantitative model, the geological implication of our finding agrees with the tectonic framework that Taiwan and the adjacent southern Ryukyu arc system could be part of the rifted China continental margin before the collision of the Luzon and Ryukyu arcs started in late Cenozoic. [Copyright &y& Elsevier]