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Abstract Accurately imaging seismogenic structures is crucial for seismic hazard assessment. This is especially important around the deformation front (DF) region off SW Taiwan, where seismic potential is high but the fault structure and seismic activity are still poorly understood. Here, we present a comprehensive delineation of seismogenic structures along the onshore and offshore DF of SW Taiwan, utilizing hypocenter relocations and first-motion focal mechanism solutions from January 1991 to March 2019. Five distinctive seismogenic structures are delineated. Particularly, the energetic seismic activity initialled after the 2017 Mw5.3 Tainan earthquake and migrated northeastward along the western side of the DF. The offshore trace of the DF could serve as a boundary delimiting the seismic activity driven by the flexural stress of the subducted Eurasia Plate to the west of the DF. This observation could constrain the magnitude estimation of future earthquakes offshore SW Taiwan. Meanwhile, the areas with higher seismic moment release are located near the DF rather than known faults, indicating that the DF may dominate the seismic moment release. However, further study is warranted into the intricate relationship among seismic strain variation, the structure of DF, and seismic activity to further understand the seismic potential. Our first-motion focal mechanism solutions show that several earthquakes in SW Taiwan were characterized by P-axis orientations parallel to the strike of the Taiwan orogeny. Those events are preferably explained by the lateral compression induced by the ongoing collision between the Eurasia and Philippine Sea plates.

Accurately imaging seismogenic structures is crucial for seismic hazard assessment. This is especially important around the deformation front (DF) region off SW Taiwan, where seismic potential is high but the fault structure and seismic activity are still poorly understood. Here, we present a comprehensive delineation of seismogenic structures along the onshore and offshore DF of SW Taiwan, utilizing hypocenter relocations and first-motion focal mechanism solutions from January 1991 to March 2019. Five distinctive seismogenic structures are delineated. Particularly, the energetic seismic activity initialled after the 2017 Mw5.3 Tainan earthquake and migrated northeastward along the western side of the DF. The offshore trace of the DF could serve as a boundary delimiting the seismic activity driven by the flexural stress of the subducted Eurasia Plate to the west of the DF. This observation could constrain the magnitude estimation of future earthquakes offshore SW Taiwan. Meanwhile, the areas with higher seismic moment release are located near the DF rather than known faults, indicating that the DF may dominate the seismic moment release. However, further study is warranted into the intricate relationship among seismic strain variation, the structure of DF, and seismic activity to further understand the seismic potential. Our first-motion focal mechanism solutions show that several earthquakes in SW Taiwan were characterized by P-axis orientations parallel to the strike of the Taiwan orogeny. Those events are preferably explained by the lateral compression induced by the ongoing collision between the Eurasia and Philippine Sea plates. Key Points: The deformation front has high seismic potential but seismogenic structure remains unclear. The deformation front confined seismic activity and released relatively large seismic moments. The role of the deformation front must be considered in seismic hazard assessment. [ABSTRACT FROM AUTHOR]

Abstract The Taiwan orogenic belt is formed by the strong convergence between the Philippine Sea Plate and the Eurasian Plate. The detailed mountain building process is still under debated largely due to the poor constraint of deep crustal structures, particularly the geometry at the Moho-depth. Here the Moho-refracted P waves are identified from the seismic data recorded by a dense seismic array (Formosa Array) in northern Taiwan. Although the refracted seismic energy is often weak at each individual station, the waveform similarity recorded at the nearby stations provides a reliable constraint for estimating the apparent velocity recorded by the dense seismic array. The forward modeling of the observed Moho-refracted P waves shows a larger crustal thickness (~ 52 km) beneath the Backbone Ranges than beneath the adjacent Hsuehshan Ranges (~ 36 km). Such a result is not only confirming the Moho variations along a few of the NW-SE profiles from the previous studies, but also showing the strong Moho variation is well extended along the NE-SW direction. The large change in the crustal thickness across the Taiwan orogeny strongly indicate that the orogenic deformation in Taiwan might extend beyond the shallow crust, possibly involving in the deep crust and upper mantle. The Taiwan orogeny may not be reaching to the isostatic equilibrium yet.

The Taiwan orogenic belt is formed by the strong convergence between the Philippine Sea Plate and the Eurasian Plate. The detailed mountain building process is still under debated largely due to the poor constraint of deep crustal structures, particularly the geometry at the Moho-depth. Here the Moho-refracted P waves are identified from the seismic data recorded by a dense seismic array (Formosa Array) in northern Taiwan. Although the refracted seismic energy is often weak at each individual station, the waveform similarity recorded at the nearby stations provides a reliable constraint for estimating the apparent velocity recorded by the dense seismic array. The forward modeling of the observed Moho-refracted P waves shows a larger crustal thickness (~ 52 km) beneath the Backbone Ranges than beneath the adjacent Hsuehshan Ranges (~ 36 km). Such a result is not only confirming the Moho variations along a few of the NW-SE profiles from the previous studies, but also showing the strong Moho variation is well extended along the NE-SW direction. The large change in the crustal thickness across the Taiwan orogeny strongly indicate that the orogenic deformation in Taiwan might extend beyond the shallow crust, possibly involving in the deep crust and upper mantle. The Taiwan orogeny may not be reaching to the isostatic equilibrium yet. [ABSTRACT FROM AUTHOR]

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]

In order to tackle the relationship between the exhumed Taiwan Orogeny and the associated sediment routing systems, new results from zircon fission track (ZFT) dating, calcareous nannofossil biostratigraphy, and the occurrence of the diagnostic magnetic mineral pyrrhotite in modern and ancient syn-orogenic sediments from pro-foreland basins in western Taiwan are reported. The ZFT age spectra of modern river sediments correspond well to the thermochronometric ages of exposed bedrock and indicate a rapidly exhumed source with a young ZFT peak age of 1.7 Myr in central Taiwan, which has not been widely reported. ZFT results from the Pleistocene successions show diachronous deposition of totally-reset zircons from south to north, accompanied by a trend of younging up-section. The progression of these ZFT signals in conjunction with changes in petrography and pyrrhotite occurrence suggest transient developments of sediment routing systems in central Taiwan and the importance of paleo sediment routing systems. In the Dadu River, totally-reset zircons did not appear until 1–0.45 Myr, when the flux of thermally mature material became prominent. The course of the Choshui river migrated during the past 1 Myr and the reorganization of the river reflects activity of frontal thrusts in Pleistocene times. Exhumation rates recorded by detrital sediments of the Choshui River catchment have been consistently higher than those of the Dadu River, suggesting inherited heterogeneity of exhumation in the orogen. • Modern and ancient pro-foreland deposits of Taiwan were subjected to ZFT, calcareous nannofossils, and pyrrhotite studies. • Detrital ZFT spectra revealed reorganization of sediment routing systems in central Taiwan. • ZFT spectra of modern sediments recognize fast exhumed source areas with the young 1.7 Myr peak age. • Differences of exhumation rates suggest heterogeneity of unroofing between catchments. [ABSTRACT FROM AUTHOR]

The Tulungwan-Chaochou Fault system in southern Taiwan represents the boundary between a slate belt of moderate metamorphic grade and a relatively unmetamorphosed fold-and-thrust belt. The offset between hanging wall and footwall of this fault ranges from 7 to 11 km and is considered one of the major tectonostratigraphic faults in Taiwan. This 75-km-long fault system is also one of the most conspicuous topographic features in Taiwan. The geometry, kinematic history and associated subsidiary structures have not been resolved. Field mapping of fabrics and brittle faults show that a 45-km-long west-northwest-vergent antiform defined by folded slaty cleavage exists in the hanging wall of the fault. This antiform has not been previously described and apparently formed in a brittle environment. The flat crest and tight forelimb of the antiform suggests a two-stage deformation model composed of a fault-bend fold followed by a trishear fold. We infer that regional scale fold is associated with a thrust that splays upward from the main detachment. [ABSTRACT FROM AUTHOR]

Abstract: The Taiwan orogen sitting at the pre-rifted Eurasian margin and bracketed by two sub-orthogonal subduction systems is subject to complicated geotectonics, motivating numerous seismic tomographic studies in local and regional scale. Most of them obtained P-wave velocities (Vp), or with Vp/Vs ratio jointly, but few were for S-wave velocities (Vs). With unprecedented amount of S-wave data, in this study a new set of well-tuned Vp, Vs, and Vp/Vs models were determined by an elaborate joint-inversion scheme, integrating the data of P- and S-wave travel times, S–P times, and the borehole logging data (for near-surface correction) into one system. This allowed us to revisit the subduction-collision process in detail by comparing Vp, Vs, and Vp/Vs ratio simultaneously. With enhanced slab imaging of Philippine Sea Plate (PSP) under northeast Taiwan, we are able to discriminate the plausible missing Luzon forearc basement along the plate boundary and propose a “skateboarding” edge model, in which the westernmost PSP rides on the east-subducted forearc basement and subducts northward underneath the Eurasian lithosphere. The underlain forearc basement along with the PSP subduction then results in the anomalous shallow double seismic zone observed only near Taiwan Island. Meanwhile, at around 121°E the north-subducting PSP seems dipping west to collide with the deep Eurasian lithosphere, shaping an irregular corner wedge structure. In addition, we evaluate our model with previous ones by waveform modeling approach and show the current performance of travel-time tomography in Taiwan region. [Copyright &y& Elsevier]

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]

Abstract: In order to constrain the tectonic consequences of the collision of the Luzon arc with the SE Chinese continental margin during the Neogene Taiwan orogeny, we assess the thermal history of the poly-metamorphic para-autochtthonous rocks of the continental margin in Taiwan. To accomplish this we report a new SIMS U–Pb age of prograde metamorphic zircons and interpret this age in the context of existing geochronology and thermochronology. Zircons extracted from the Fenniaolin amphibolite near Nanao, Taiwan, in the contact aureole of a granodiorite, are interpreted to have crystallized during the first lower amphibolite facies prograde metamorphism based on [1] their equant shape rather than the prismatic shape typical of magmatic zircon, [2] their microcrystalline or “micro-saccharoidal” aggregate structure and patchy and sector zoning rather than oscillatory zoning visible in cathodoluminescent imaging, [3] their low concentrations of U (most<40ppm), [4] their chlorite inclusions, and [5] their inclusion within prograde metamorphic amphiboles. The apparent absence of magmatic zircon is also consistent with the low Zr concentrations of the N-MORB metabasaltic protolith. SIMS analyses of 18 zircon grains yield a concordant 206Pb/238U age of 86±1Ma, analytically indistinguishable from the age of the adjacent pluton that caused the contact metamorphism. We thus conclude that the contact metamorphism of this intrusion caused the first lower amphibolite facies metamorphism of these rocks in the mid-Cretaceous. Existing 40Ar/39Ar and Rb–Sr cooling ages of both amphibole and muscovite at ~80Ma (e.g. Lan, 1990; Lo and Onstott, 1995) show that the granodiorite and its contact aureole cooled rapidly from ~700° to <400°C in~5m.y., indicating that the ambient temperature of the country rocks at the time of intrusion was less than the closure temperature of Ar diffusion in muscovite. Existing thermobarometry on rocks in the contact aureole yield pressures of ~5kb, equivalent to a depth of 15–20km. Cooling ages of biotite and K-feldspar in the literature reflect slow cooling at <2°C/m.y. for more than 50m.y. to at least 20Ma, suggesting atectonic cooling. The crustal history reconstructed from these data shows that the regional ‘gneissic’ terrane of the northern Tananao complex may have been caused by coalescing contact aureoles of multiple mid-Cretaceous intrusions (the ‘Nanao intrusive event’), and not by the Neogene Taiwan orogeny. On the contrary, the Tananao rocks probably achieved middle greenschist conditions and 15–20km depth in a forearc setting in the Early Cretaceous. This intrusive event caused ‘regional contact metamorphism’ at about 85Ma, after which the passive margin setting preserved these rocks at mid-crustal depths until the beginning of the Taiwan orogeny (~5Ma). These depths (10–12km) and temperatures (~200–250°C) were high enough to account for the late Miocene lower greenschist facies cleavages and the Pliocene–Pleistocene fission track cooling ages without additional loading and heating from the Taiwan orogeny. [Copyright &y& Elsevier]

Overflows through oceanic gateways govern the exchange of water masses in the world's ocean basins. These exchanges also involve energy, salinity, nutrients, and carbon. As such, the physical features that control overflows can exert a strong influence on regional and global climate. Here, we present the first description of sedimentary processes generated by the overflow of Pacific Deep Water (OPDW). This mass flows southward at approximately 2000–3450 m water depth within the Luzon Trough (gateway) from the Pacific Ocean into the South China Sea. OPDW can be divided into: a) a lower, denser layer (including an associated weak counter-current), which has generated a large contourite depositional system (CDS-1) that includes large erosional (channel and moat), depositional (mounded and plastered drift), and mixed (terrace) contourite features along the trough bottom and walls, and b) an upper mixing layer, which has not generated any significant depositional or erosional contourite features. Where OPDW does not reach the seafloor, it is underlain by bottom water that circulates more sluggishly but has generated a second contourite depositional system (CDS-2) made of a large sheet-like drift. The OPDW flow has generally enhanced since the middle to late Miocene, except in the shallower northernmost corridor. In the deeper main trough, reductions in width and depth of the gateway by Taiwan orogenic events have likely accelerated the overflow. The latest significant enhancening may promote widespread development of contourite depositional systems along the South China Sea's lower continental slope and adjacent deeper areas. This work highlights the importance of gateway-confined overflows in controlling the morphology and sedimentary evolution of adjacent deep marine sedimentary systems. A clear understanding of overflow processes and their products is essential for decoding tectonic control in oceanographic or paleoceanographic processes. • Multidisciplinary study of the overflow of Pacific Deep Water along the Luzon Trough. • The overflow overlies a more sluggish layer at depth greater than 3450 m. • The overflow lower layer has formed a contourite depositional system. • The upper (mixing) layer of the overflow has not generated significant contourite features. • Gateway and overflow have influenced South China Sea morphology and sedimentary evolution. [ABSTRACT FROM AUTHOR]