Luzon Arc Research Articles

Geochemical and zircon 18O–Hf–U/Pb isotopic study of volcanic rocks from Lutao Island in the northern Luzon magmatic arc

AbstractLutao is an islet off the eastern coast of the main island of Taiwan and is composed of pyroclastic agglomerates, volcanic breccias, lava flows, and dykes. There have been no reports of SIMS zircon oxygen isotopic data from Lutao Island so far, which has significantly hampered further understanding of its magmatic processes, such as crystallization, evolution, and generation of magma. In this study, we use zircon U–Pb dating, Lu–Hf and O isotopic analysis to constrain the age, origin, and magmatic processes of the volcanic rocks from Lutao Island. Integration of our new U–Pb and Hf–O isotope data with available published data allows us to better assess the timing and magmatic processes of Lutao Island in the context of the northern Luzon arc system. This work is the first report of SIMS zircon oxygen isotopic data of volcanic rocks from Lutao island. This study shows that the volcanic rocks from Lutao Island were generated by mantle-derived magma associated with high-T° hydrothermal alteration by seawater-derived fluids.

Melt generation and magma storage conditions of primitive arc lavas in the Macolod Corridor, southwestern Luzon arc, Philippines

Decoding the origin of primitive arc magmas from petrological, geochemical, and thermobarometric constraints is crucial for understanding their melt generation process and crustal storage conditions. In the Philippine arc setting, primitive basalts have been recognized in the Macolod Corridor, southwestern Luzon arc. The Macolod Corridor is a 30 by 60-km northeast-southwest striking, young rift system that hosts several Quaternary stratovolcanoes including the active Taal Volcano and Mt. Banahaw, lava domes, and ∼ 200 monogenetic centers classified as scoria cones, tuff cones, tuff rings, and maars. This study reports textural, petrological, and geochemical analyses of the Macolod primitive basalts to decipher their petrogenesis and elucidate their pre-eruptive magma storage conditions. We identified at least five distinct primitive lava compositions based on their modal mineralogy: clinopyroxene-olivine basalts, plagioclase-olivine-clinopyroxene basalts, olivine-plagioclase-clinopyroxene basalts, clinopyroxene-plagioclase-olivine basalts, and clinopyroxene-olivine-plagioclase basalts. Clinopyroxene-olivine basalts and clinopyroxene-plagioclase-olivine basalts occur as lapilli and bomb deposits. In contrast, plagioclase-olivine-clinopyroxene-basalts, olivine-plagioclase-clinopyroxene basalts, and clinopyroxene-olivine-plagioclase basalts occur as lapilli and volcanic bombs in monogenetic volcanoes and as basaltic blocky lava flows in small polygenetic volcanoes. Phenocrysts, glomerocrysts, and microphenocrysts assemblages include olivine, clinopyroxene, plagioclase ± spinel in a glassy matrix. The basalts are identified as subalkaline, medium-K, and medium-Fe tholeiitic basalts, based on their bulk-rock geochemistry. Adding 3–4 % equilibrium olivine to the Macolod primitive basalts generates magmas in equilibrium with mantle olivines with Fo90.68–90.82 and 0.392–0.395 wt% NiO compositions. Disequilibrium textures exhibited by olivines, clinopyroxenes, and plagioclases suggest that these are products of magma decompression and dissolution processes. Calculated melt based on olivines reveals that these primitive magmas last equilibrated at depths ranging from ∼36–42 km (1.03–1.23 GPa) at 1286°–1318 °C. Application of clinopyroxene-only thermobarometer results indicate clinopyroxene crystallization depths of around 7–16 and 10–19 km for hydrous and anhydrous estimates, respectively. The segregation depths estimated in this study translate to the uppermost mantle until near the Moho boundary whereas the storage depths correspond to prolonged magma storage regions in the upper crust as modeled by existing geophysical data (i.e., seismic travel-time tomography). Combining textural and geochemical results from this study with existing geophysical data provides new insights into the magma plumbing systems in the region and how these might operate through time.

A revolution in understanding SE Asia geodynamics since 20.5–18 Ma

We present an updated evaluation of SE Asian geodynamics that includes the interactions of the South China Sea (SCS) marginal basin with surrounding plates since the end of SCS spreading 20.5–18 Ma. Newly available Ar39/Ar40 ages of SCS oceanic crust drilled at IODP U1431 near the SCS East basin extinct spreading center are older than 18 Ma. Conversely, the oldest ages of the Luzon arc and forearc at Taiwan's Lanyu island, Coastal range and Lichi mélange are 17–18 Ma, suggesting that onset of the Manila subduction zone may have begun a few m.y. earlier. Before ∼20.5 Ma, the northern part of the Manila transcurrent fault (MTF), considered as the western boundary of the Ryukyu subduction zone, was a left-lateral lithospheric-scale shear zone. From ∼20.5–18 Ma to ∼7 Ma, this portion of MTF was connected to the Manila trench. Since ∼7 Ma, the MTF extended into the Taiwan Longitudinal valley and continued southwards to north Luzon island as near-vertical, left-lateral shear zone. Today, south of ∼24°N, the MTF protrudes down to 30 km depths and terminates above the deeper Manila slab. Since ∼7 Ma, the whole MTF shifted 400 km westward with respect to Eurasia and rotated ∼23° clockwise to become oriented ∼NS north of 16°N latitude. We identify a tear fault in the Eurasian (EU) plate north of the Ryukyu trench that is located south of the Myako and Yonaguni islands. Since ∼10 Ma, the tear continuously progressed westward within EU crust, with the Philippine Sea plate progressively subducting northwestward between the two lips of the tear fault. A RFF (ridge-fault-fault) triple junction was active in the EU crust before 20.5 Ma, from 10 to 7 Ma, and since 2 Ma. This triple junction was always located on the MTF with one branch of the MTF on each side of the triple junction, and the third branch being the spreading center.

Late Oligocene–Miocene evolution of deep-water circulation in the abyssal South China Sea: Insights from Nd isotopes of fossil fish teeth

Abstract A long-term neodymium isotope (εNd) record of fossil fish teeth was investigated to constrain the evolution of deep-water circulation in the abyssal South China Sea (SCS) during the late Oligocene–Miocene (27–10 Ma). Fish teeth samples were collected from the oceanic red beds at International Ocean Discovery Program Expedition 367 Site U1499 (water depth 3758 m). Seawater εNd values (from –7.1 to –4.8, average –6.1) prior to 15 Ma indicate that water masses in the abyssal SCS resulted from the mixing of more radiogenic Upper Circumpolar Deep Water (UCDW, average –4.5) and less radiogenic Lower Circumpolar Deep Water (LCDW, average –6.4). The general decrease in εNd values was attributed to an increasing influence of the unradiogenic LCDW at the studied site, consistent with the subsidence and the associated deepening of the SCS plain. After 15 Ma, seawater εNd dropped significantly to a range of –8.9 to –6.1 (average –7.5), indicating a slowdown in the hydrological connection between the deep-water masses in the SCS and the western Pacific Ocean. We argue that the formation of the Luzon Strait due to the uplift of the Luzon arc in the late Miocene led to the shallowing and narrowing of the SCS-Pacific channels. Consequently, penetration of LCDW was reduced and water masses in the abyssal SCS would have been less ventilated and strongly influenced by lithogenic input from the unradiogenic sediments of large Asian rivers draining the peri-Himalayan region.

Recycled materials and secondary processes controlled the chemical and isotopic compositions of bubbling gases discharged from two adjacent geothermal springs in the Northern Luzon Arc

Gas emissions from hydrothermal systems can serve as indicators of subsurface activity. In addition to gas sources, hydrothermal gas geochemistry is strongly influenced by secondary processes that occur during/after hydrothermal circulation. Here, we observed statistically significant differences in the geochemical characteristics (except for helium isotopes) of bubbling gases discharged from two adjacent vents in the Northern Luzon Arc. Helium (3He/4He = 4.25–7.09 Ra) in both vents was controlled by mixing between mantle and crustal components, where about 74% of helium was contributed by the mantle. Differences in N2/Ar ratios (∼ 300–330) of the two neighboring springs are attributed to subducted materials and seawater mixing (contributing ∼2.5% N2 and Ar), rather than phase separation in the reaction zone. Specifically, Ar was mainly supplied by atmospheric components that dissolved in the percolated seawater with only 8%–9% contributed by the excess radiogenic 40Ar. Excess N2 relative to Ar was mainly supplied by the decomposition of subducted materials (83%–92%) of the South China Sea plate beneath the Philippine Sea Plate. The Lutao gases showed low CO2 concentrations (0.07–22.2 mmol/mol), despite the high 3He/4He ratios indicating a significant contribution of magmatic components. Magmatic CO2 may have been largely consumed by the high Ca Lutao vent fluids via carbonate precipitation in the reaction zone. Alternatively, stable carbon isotope compositions (δ13C) indicate that Lutao CO2 may be supplied by microbial oxidation of alkanes (e.g., CH4 with concentrations of 14.6–173 mmol/mol in the samples), with fractionation factor ΔCO2–CH4 ranging from −15‰ to −25‰ and conversion rates of <10%. Up to 65% of the CO2 in the 2016 samples experienced secondary calcite precipitation in the discharge zone. Our results indicate that recycled subducted materials could potentially affect the geochemical characteristics of gases discharged from arc-volcanic systems. In addition, the influence of secondary processes needs to be considered before tracing the sources of hydrothermal fluids and/or gases, especially in shallow-water hydrothermal systems.

Sub-arc Mantle Heterogeneity of the Northern Luzon Volcanic Arc: Mineral and Whole Rock Compositional Variability in Mantle Xenoliths from Lutao Island

Abstract Mantle xenoliths hosted in volcanic rocks from the island of Lutao offer a glimpse into the nature of the mantle beneath the northern Luzon volcanic arc. The xenoliths are spinel-bearing and composed mostly of harzburgite with one lherzolite and one olivine orthopyroxenite. The olivine (Fo92.5–88.9), orthopyroxene (Mg# = 94.6–89.2), and clinopyroxene (Wo49.1–38.1En57.0–45.4Fs3.0–11.0) compositions are similar to those of abyssal peridotites. The spinel compositions are variable and can be principally divided into high-Al (Cr# &amp;lt; 45) and low-Al (Cr# &amp;gt; 45) groupings. The whole rock compositions are similar to abyssal peridotite (Al2O3 = 0.95–2.07 wt %; Mg# = 88.5–90.9) and have U-shaped chondrite normalized rare earth element patterns. The Sr-Nd isotopes of the xenoliths are broadly chondritic (87Sr/86Sri = 0.704400–0.707908; εNd(t) = 0.0 − +1.5). The two-pyroxene equilibrium temperatures range from 900 to 1200 °C with the majority of temperature estimates &amp;gt;1000 °C. The olivine-orthopyroxene-spinel oxygen barometry estimates yielded ΔFMQ values from 0 to +2 and correspond to moderately oxidizing to oxidizing conditions. The xenoliths are likely derived from the Philippine Sea Plate lithospheric mantle that was modified by melt extraction and/or fluid enrichment processes. Trace element and isotopic mixing modeling indicate that 1–2% contamination by subducted South China Sea sediment can explain the Sr-Nd isotopic enrichment and Th and U elemental variability within the xenoliths assuming an initial composition similar to enriched depleted mid-ocean ridge mantle (E-DMM). The anomalously high two-pyroxene equilibrium temperatures of the Lutao xenoliths relative to other regions of the northern Luzon volcanic arc (Iraya &amp;lt;1000 °C) indicate that they were affected by a high-temperature event that was likely a consequence of recent intra-arc rifting that occurred after collision (&amp;lt;6 Ma) between the Luzon arc and the Eurasian margin.

Metasomatic enrichment of the sub-arc mantle: Evidence from the petrography and geochemistry of the xenolith-bearing andesites from Dinem Island, Batanes, Philippines

The Babuyan Segment of the Luzon Arc in the Philippines is divided into an older west volcanic chain (WVC) and a younger east volcanic chain (EVC) based on the geochemical and geochronological characteristics of their volcanic rocks. Mantle and crustal xenoliths have been previously reported in the volcanic rocks of young Batan (EVC) and Sabtang (WVC). In this work, we present new petrographic and geochemical data on volcanic rocks from Dinem Island (EVC), which also host mantle and crustal xenoliths. These volcanic rock data are compared to the data of other volcanic centers in the region to better understand magmatic-metasomatic processes that occur beneath island arcs.The Dinem volcanic rocks are generally porphyritic andesites, with plagioclase and hornblende phenocrysts, minor clinopyroxene and Fe-Ti oxides. Correlation between major oxides (i.e., K2O, Na2O, TiO2, CaO and Al2O3) and trace elements (i.e., Ni and Cr) with FeO*/MgO suggests that the Dinem volcanic rocks are related by magmatic differentiation. These rocks also record enrichment in large ion lithophile elements (e.g. Rb, Ba, Th) and depletion in high field strength elements (e.g. Nb, Ta, Ti), which are typical of arc volcanic rocks. The volcanic rocks from Dinem also have elevated (La/Sm)N and Th/Yb and low Ba/Th values relative to WVC volcanic rocks. These trace element ratios and enriched LREE signature suggest that the Dinem and younger Batan volcanic rocks of the EVC were derived from a mantle wedge that is highly metasomatized by slab-derived fluids/melts relative to the source region of the WVC volcanic rocks.

The volatile record of volcanic apatite and its implications for the formation of porphyry copper deposits

Abstract Volatile saturation influences the physicochemical behavior of magmas and is essential for the sequestration of metals in porphyry copper deposits. Tracking the evolution of volatile components (F, Cl, H2O, S) in arc systems is complicated by their mobility and tendency to rapidly re-equilibrate with late-stage melts. We demonstrate that accurate measurements of volatile concentrations in apatite offer a reliable method for identifying the occurrence of volatile saturation. Fluorine, Cl, S, and calculated OH concentrations in apatite obtained by scanning electron microscope–energy-dispersive X-ray spectroscopy and electron microprobe analysis were used to compare two end-member volcanic systems in the West Luzon Arc (Philippines): Pinatubo (a fluid-saturated analogue for porphyry copper deposits) and Taal (a barren and fluid-undersaturated comparator). Apatites from Pinatubo are S-rich (0.04–0.64 wt%) and show a progressive decrease in XCl/XOH (0.6–0.25) and an increase in XF/XCl (1.5–8) and XF/XOH (0.75–1.2) during crystallization. Modeling indicates that these changes result from efficient partitioning of Cl into a continuously saturated H2O-rich fluid, while high regions of S in apatite reflect episodic flushing by a separate S-rich flux. Little S is evident in apatites from Taal (&amp;lt;300 ppm), which show increasing XCl/XOH and XF/XOH together with constant XF/XCl during crystallization. This cannot be explained using an H2O-saturated model, and instead reflects fluid-undersaturated crystallization and cooling in a reduced and/or S-depleted system. Measured volatiles in apatite therefore effectively discriminate volatile-saturated and undersaturated magmatic systems, providing an important ‘fertility’ filter for porphyry exploration.

Retro‐Foredeep Basin Evolution in Taiwan: Zircon U‐Pb and Hf Isotope Constraints From the Coastal Range

AbstractThe evolution of the Taiwan orogen, an active arc–continent collision zone between the North Luzon Arc (NLA) and the Eurasian continent, is recorded in arc volcanic and sedimentary rocks from the Taiyuan retro‐foredeep basin of the Coastal Range, eastern Taiwan. Arc volcanism in the northern NLA experienced a magmatic culmination at 9–7 Ma and a cessation at 5.3–4.3 Ma, based on new U‐Pb zircon ages. Young zircon U‐Pb ages (1.74, 1.58, and &lt;1.2 Ma) and low Hf isotope signatures also indicate that three biotite‐bearing lapilli tuff layers deposited in the retro‐foredeep basin are associated with eruptions of the Lutao volcano in the southern NLA. NLA volcanism terminated progressively from ∼5.3 to &lt;1.2 Ma in response to a southward‐propagating collision zone that followed the uplift of the orogenic wedge by &lt;3 myr. The detrital zircon U‐Pb age spectra suggest no significant shift in provenance during the Plio‐Pleistocene period and indicate that the synorogenic sediments in the Taiyuan Basin were mainly derived from the Yuli Belt, a Miocene accretionary complex in the eastern Taiwan orogen. The accumulation rate of synorogenic deposits varies from 0.53 to 5.14 km/myr, with an abrupt increase at ∼2 Ma. This new rate is higher than the previously proposed rates and that of the pro‐foreland basin to the west, suggesting asymmetric and rapid uplift of the Taiwan orogen.

Heavy Mo isotope composition of northern Bataan adakites, Philippines: Evidence for fore-arc subduction erosion?

Abstract The identification of an eroded fore-arc crust component in arc magmas is challenging due to the combined effects of mantle metasomatism and crustal assimilation–fractional crystallization. In this study, molybdenum (Mo) isotope compositions are used in conjunction with Sr-Nd-Hf isotopic and elemental data to identify eroded fore-arc crust components in adakites from the Cuyapo and Balungao volcanoes of the northern Bataan segment of the Luzon arc (Philippines). The Mo isotopic ratios (δ98/95Mo, relative to the NIST SRM 3134 standard) of these adakites increase with increasing εNd (+4.3 to +5.6) and Ba/Nb (206–286). The low δ98/95Mo (−0.36‰ to −0.26‰) in the Cuyapo adakites coupled with low Sr-Nd-Hf isotopic ratios suggests contributions from the residual slab, which lost isotopically heavy Mo during dehydration. Interestingly, the high δ98/95Mo (−0.18‰ to 0.00‰) Balungao adakites have Mo-Sr-Nd-Hf isotopic ratios similar to those of the Luzon basement. Fractionated Nb/Ta (16–18) and high Sr/Y indicate the coexistence of melt with residual rutile and garnet ± amphibole assemblages, corresponding to a source region (&amp;gt;~45 km) below the present Luzon crust (~33 km). This thus suggests an origin of heavy Mo from partial melting of eroded crust in the mantle wedge rather than in the upper-plate crust. Our work not only demonstrates that Mo isotopes may be a potential tracer of eroded crust but also highlights that lavas with combined high δ98/95Mo, εNd, and Ba/Nb emplaced at subduction zones with juvenile arc crust may be a result of subduction erosion.

Crustal structure of the northern Manila subduction zone: Is thinned continental crust or oceanic crust subducting beneath the Luzon arc and forearc?

Forward and inverse P-wave velocity models along an E-W 280-km-long wide-angle reflection/refraction profile at 21°N across the northeastern South China Sea margin and Manila subduction zone reveal a 10–12 km thick thinned continental crust (TCC) located west of the Manila transcurrent fault (MTF) and Manila slab. The TCC is divided in two parts separated by the intra-continental velocity difference boundary (VDB). West of the VDB, the TCC is located above a high velocity layer (HVL). East of the VDB, the TCC consists of three layers: an undefined body with a velocity inversion at its base above the upper and lower crusts. The VDB and MTF merge east of the Hengchun peninsula showing that the TCC located east of the VDB decreases in size and disappears to the north. The distribution of earthquakes (EQs) shows that ∼80 km of crust have subducted east of the MTF since the onset of Luzon arc collision with Eurasia at ∼7 Ma. All published tomographies suggest the subduction of oceanic crust (OC). However, due to crustal corrections, up to a maximum width of 40 km of TCC might eventually exist in the upper part of the Manila slab instead of OC. Two end-members with 40 km of TCC or not in the upper part of the Manila slab were restored at ∼7 Ma. Since that time, the north Luzon island moved ∼600 km in the northwest while part of this motion was absorbed near the Hengchun peninsula by shrinking of Eurasia TCC, resulting in a ∼ 20° clockwise rotation of the Manila slab and MTF. Small Luzon arc blocks started to rotate 10°-30° clockwise at ∼7 Ma below northern Taiwan, 3–4 Ma in the Coastal range and 1 Ma in Lutao and Lanyu islands and not south of the Hengchun peninsula. Thus, both the clockwise rotation of the individual Luzon arc blocks and of the Manila slab and MTF might explain the collision process of the Luzon arc with the Eurasia continent.

Magma Chamber Process of Post‐collisional Magmatism: Insight from Textural and Elemental Characteristics of Plagioclase from the Tatun Volcanic Group, Northern Taiwan Volcanic Zone

AbstractThe Taiwan mountain belt, one of the youngest orogenies in the world, is caused by the collision of the Luzon arc with the Eurasian margin, which leads to post‐collisional extension and magmatism in the Northern Taiwan Volcanic Zone (NTVZ). The magma chamber process in this region has not previously been elucidated in detail. In this paper, the textural and compositional features of plagioclase phenocrysts in basalt from the Tatun Volcanic Group (TTVG) were studied to restrict the dynamics of magma system. Results show that the magma melts in TTVG are mainly sourced from the underlying MORB‐like mantle wedge but influenced by incorporation of subduction components, causing the elevated Sr/Y and Ba/Y ratios in magma melts. The subduction components are mainly transported in the form of sediment melt. The plagioclase phenocrysts in the TTVG volcanic rocks are generally coarsely core‐sieved with a clear rim. The An contents in the rims of plagioclase are much lower than those of cores, and elevated FeO concentrations are detected in the plagioclase rims. We propose there exists a double‐layer magma chamber in this region. The core of the plagioclase was crystalized in the deeper quiescent magma chamber (∼21 km), which was subsequently partially dissolved during the ascent of magma melt under H2O‐undersaturated condition, forming the typical coarsely sieved texture and synneusis. When this crystal‐rich melt migrates into the shallower chamber, water saturation is reached and more sodic plagioclase formed as the rim of phenocryst. Due to the considerably higher fO2 in the shallow chamber than in the deeper one, and the distribution of Fe between plagioclase and melt positively correlates with fO2, the FeO content in the plagioclase rim elevates in conjunction with increasing fO2.

Applying unsupervised machine-learning algorithms and MUSIC back-projection to characterize 2018–2022 Hualien earthquake sequence

The city of Hualien in eastern Taiwan is located at the junction of the Luzon arc–Eurasian continent collision and the oblique subduction of the Philippine Sea plate along the Ryukyu Trench. This complex tectonic setting creates one of the most intense seismic hazards in the world. Since the mid-twentieth century, three earthquake sequences (1951, 1986–1992, and 2018–2022) have caused severe damages and revealed a diversity of focal mechanisms, demonstrating a complex seismogenic tectonic interactions of collisions and subductions. In this study, we used the two-dimensional back-projection method to examine rupture characteristics of three strong earthquakes during the latest sequence (2018–2022). We also studied the same earthquake sequence using unsupervised machine-learning via density-based spatial clustering of applications with noise (DBSCAN) and principal component analysis (PCA). Our results revealed two westward dipping seismogenic structures. The deeper one lies beneath the eastern flank of the Central Range and the shallower one rotates clockwise along the coastline from south to north. Overall, focal mechanisms within these two structures are consistent with planar geometries obtained using PCA. The deeper structure beneath the Central Range is likely an imbricated fault system. The coastline structure is characterized by a transition of strike and dip angles from south to north. We suggest that the subhorizontal nodal planes in earthquakes offshore Hualien are related to the shallow northward dipping patch in the northern part of the coastal structure. Our Coulomb stress change results revealed strong linkages between these two structures.

Synorogenic extension and extrusion in southern Taiwan

Paleostress inversion of 141 outcrop-scale faults across the eastern flank of the southern Central Range of Taiwan where leveling and GPS data suggest a steep gradient in rock uplift rates yields two main kinematic phases of deformation. Phase 1 consists of 93 normal faults that generally dip moderately northeast whereas phase 2 consists of 48 strike-slip faults that generally dip steeply west-northwest. Both phases record NE-trending sub-horizontal extension but different orientations of principal shortening; in phase 1, the principal shortening axis is nearly vertical whereas in phase 2 it plunges gently to moderately southeast. The northeast extension is consistent with extension directions obtained from GPS and earthquake focal mechanisms in the central part of the southern Central Range. However, these indicators of contemporary deformation also reveal more complicated states of stress along the eastern and western flanks of the range and in the deep crust southwest of the range. We interpret these more complicated stress states as reflecting “forceful extrusion” of the southern Central Range where the lower crust is being pinched between more rigid crustal blocks represented by the Peikang High and the Luzon Arc. In this context, the temporal progress from strike-slip to normal faulting observed in outcrops may reflect the advection of the rocks from lower to higher structural levels. The northeast extension normal faults can be interpreted that accommodating the lateral and vertical movement of the crust in the southern Central Range. Based on thermochronological data and the onset of extrusion in southwest Taiwan in the late Pleistocene, we infer that this SW extrusion process may be younger the 0.5 Ma.

Three‐Dimensional Stress Model of the Collision‐Subduction Junction East of Taiwan: Implications for the Decoupling of the Luzon Arc During Subduction

AbstractCarried by the Philippine Sea Plate converging with the Eurasian Plate, the Luzon Arc is obliquely colliding with and accreting to the Eurasian continental margin to build the Taiwan orogen. Northward along the collision suture, the Luzon Arc extends offshore and is being subducted under the Ryukyu Trench. How the northern segment of the Luzon Arc responds to both collision and subduction remains poorly understood. At present, seismic tomography and conventional stress analyses cannot fully capture images of this arc or discern how it deforms following subduction. We employed a three‐dimensional spatial clustering approach in eastern Taiwan to reveal how stress varies along the collision suture. Our results show that the maximum horizontal compression orientation, SH, is sub‐parallel to the plate motion at all depths across the lithosphere, except at shallow depths for the northern segment, where SH exhibits a clockwise rotation from convergence‐parallel by 10°–30°. Accompanying the SH rotation in the northern crustal segment is a tilt of the maximum compressive P‐axes by approximately 20°. We interpret this systematic variation along the suture and with depth as resulting from the rotation of the stress regime in the crust of the Luzon Arc forced by the oblique subduction, and likely augmented by the bending of the Central Range crustal block. The location of rotation/tilting and its disparity with depth may signify a decoupling of the volcanic arc from the mantle lithosphere during early subduction.

Introduction to the special issue on the Hualien earthquake swarms

The Hualien city is located near the pole of rotation induced by the along-strike transition from Ryukyu subduction to the collision between the Luzon arc and the Chinese continental margin in northeastern Taiwan (Rau et al., J Geophy Res 113: B09404, 2008). Such a tectonic regime creates varied seismogenic structures that are responsible for the ruptures of the 1951 M L 7.3 Hualien-Taitung earthquake sequence (Chen et al., J Geophy Res 113: B02304, 2008), the 2018 M w 6.4 Hualien earthquake sequence (e.g., Rau and Tseng, Terr Atmos Ocean Sci 30: 281–283, 2019), the 2021 M L 6.2 Shoufeng earthquake sequence, and numerous and persistent moderate-to-small size earthquake swarms in the Hualien area. This special issue was introduced by the several month-long 2021 episode of moderate-to-small size earthquake swarms occurred in the Hualien area (Figs. 1 and 2, Central Weather Bureau, 2022). The 2021 Hualien earthquake episode includes small-to-moderate-size earthquakes that release minute strain energy on the small asperities and larger events (M > 5.5) that ruptured nearby asperities. Figure 3 shows the spatiotemporal distribution of the Hualien earthquake sequence between April 1 and December 31 of 2021. The 2021 Hualien earthquake sequence provides us a great opportunity to examine the earthquake physics, characteristics, seismogenic processes, and the early warning algorithm of an earthquake swarm, which are the key elements for the understanding of generation of earthquakes. In this special issue, we have collected six papers that detailed various aspects of the 2021 Hualien earthquake sequence, which provide us a better understanding of the nature of the Hualien earthquake swarms.Fig. 1The crustal (≤ 40 km) Hualien earthquake swarm activities observed from April to December of 2021. Star symbols show the epicenters of the largest three earthquakes in this time period. Rainbow color represents the focal depth of each event. The radius of each circle is proportional to the observed MLFig. 2The temporal distribution of seismicity in the upper 40 km beneath this area observed from April to December of 2021. Several swarm activities can be identified, in which intense seismicity happened within a short time interval. The main earthquake swarms occurred from April to July of 2021. Numbers indicate the days after April 1st, 2021Fig. 3The spatiotemporal distribution of the Hualien earthquake swarm activities from April to December of 2021. The symbol color represents the time in days after April 1st, 2021

Taiwan river muds from source to sink: Provenance control, inherited weathering, and offshore dispersal pathways

Clay mineralogy and geochemistry of river muds around Taiwan Island reveal dominant provenance control. Abundant illite with subordinate chlorite in the western, northern, and southern parts of the island are derived from Cenozoic mudrocks of the axial Slate Belt and Western Foothills. In eastern Taiwan, chlorite is most common in muds generated from the Tananao Complex, whereas smectite is produced by erosion of arc andesites of the Coastal Range. Kaolinite occurs in minor amounts in western Taiwan, where it is mostly recycled from mudrocks originally generated in mainland China and accumulated along the Chinese passive margin before being tectonically accreted in the Western Foothills of the Taiwan thrust belt. Only locally in more humid regions of northern Taiwan may moderate kaolinite content associated with relatively high illite chemistry index hint at present kaolinite formation. Mineralogical and geochemical indications of weathering are thus considered to be mostly inherited rather than indicative of present climatic and geomorphological conditions.A comparison of clay-mineral assemblages in Taiwan river muds and offshore deep-sea clays allows us to trace sediment transport from the land to the deep-sea. Mostly illite is supplied to the Taiwan Strait and the South China Sea, whereas subequal amounts of illite and chlorite are delivered to the Ryukyu forearc basin from eastern Taiwan rivers draining the Tananao Complex. Smectite from Coastal Range andesites is transferred via the Xiuguluan River and Chimei Canyon directly to the Ryukyu trench. Additional amounts of smectite produced in the Ryukyu and Luzon arcs are transported southward to the Ryukyu forearc basin and northward by the Kuroshio Current, northwestward to the northern South China Sea and northeastward to the Huatung Basin and Okinawa Trough.

Deformation of the colliding Luzon volcanic arc: Strain analysis using magnetic fabrics of syn-orogenic mudstones in the Coastal Range, eastern Taiwan

Exposed in the Coastal Range of eastern Taiwan is a portion of the Luzon volcanic arc that has collided with the South China continent since 6 Ma, as well as syn-orogenic sedimentary rocks deposited on the arc and subsequently intensely folded and thrusted. This study sampled syn-orogenic mudstones for the measurement of anisotropy of magnetic susceptibility (AMS) at four sites on the sides of the range. The AMS fabrics reveal a contribution from burial compaction and, less significantly, tectonic shortening which varies slightly across the range. This lateral variation is considered as, for simplicity, a result of two-phase deformation because these rocks were involved in an eastward propagating back thrust system on the colliding arc. A new strain analysis method is developed to estimate the overall tectonic strain by unstraining the pseudo-strains with respect to the AMS measurements at each site with the knowledge about the distribution of the susceptibility ellipsoids in the unstrained state. Strain results reveal that shortening phases occurred before and after the folding, with a shortening ratio of 0.177–0.195 and 0.181–0.248, respectively. The post-folding shortening on the western side is sub-parallel to the range, and the pre-folding shortening on both sides is layer-parallel and nearly perpendicular to the range. Therefore, the mudstones underwent layer-parallel shortening, fold-related tilting and eventually orogen-parallel shortening after deposition. This sheds light on the complexity in deformation of the colliding Luzon arc.

Late Miocene–Quaternary seismic stratigraphic responses to tectonic and climatic changes at the northeastern margin of the South China Sea

Abstract Tectonic and climate evolution could be well archived in deep-water stratigraphy. Based on newly acquired high-resolution two-dimensional (2-D) multichannel seismic profiles and multibeam bathymetry, together with Ocean Drilling Program/International Ocean Discovery Program (ODP/IODP) data, this study investigated the late Miocene–Quaternary deep-water seismic stratigraphy, sedimentary evolution, and responses to regional tectonic and climatic variations at the northeastern margin of the South China Sea. The late Miocene–Quaternary stratigraphy consists of three units (i.e., SU-1, SU-2, and SU-3 from bottom to top) that are dated to 10.5–6.5 Ma (stage 1), 6.5–0.9 Ma (stage 2), and 0.9 Ma–present (stage 3), respectively. SU-1 is dominated by sheeted drifts with slight thickness variation, but SU-2 and SU-3 mainly consist of mounded, lenticular contourite drifts and channel-like moats. This suggests that the bottom currents intensified from stage 1 to stage 2, which was probably caused by the sill uplifting at the Luzon Strait under the influence of tectonic collision between the Luzon arc and Eurasia since ca. 6.5 Ma. SU-2 and SU-3 are separated by a basinwide high-amplitude seismic reflection, across which the average sedimentation rate shows a dramatic increase from ~28 m/m.y. in SU-2 to ~144 m/m.y. in SU-3. The high sediment supply since 0.9 Ma could be linked to the mid-Pleistocene climate transition, which resulted in abundant rainfall that promoted the Taiwan orogen to contribute more sediments to the South China Sea. This study indicates that the deep-water stratigraphy recorded the tectonic collision around the sole deep-water gateway (i.e., Luzon Strait) and mid-Pleistocene climate transition event at the northeastern South China Sea margin, which has important implications for understanding the relationships among the tectonic, paleoclimatic, paleoceanographic, and deepwater sedimentary processes in the largest marginal basin of the western Pacific Ocean.

Magnetic field characterization of Macolod Corridor (Luzon, Philippines): New perspectives on rifting in a volcanic arc setting

The Macolod Corridor (MC) is a NE-SW trending zone of Quaternary volcanism which perpendicularly bisects the Luzon Arc. This peculiar zone of volcanism is in a junction of different tectonic elements. This led to several models of formation associated with subduction or crustal extension-related processes. New information about its subsurface structure can provide constraints in resolving outstanding questions on its tectonic development. However, elucidating the origin and deep structure of the MC has been problematic due to the extensive young volcanic deposits that blanket the area. In this study, ground magnetic and aeromagnetic datasets are merged to characterize the magnetic signature in the MC. Short wavelength and high amplitude anomalies suggest complex interactions of magnetic bodies within the region. Edge detection techniques reveal linear magnetic anomalies with NE-SW, NW-SE, and E-W trends in the Corridor. These may indicate rift structures at depth. Analytic signals show maximum amplitudes over volcanic centers suggesting maximum magnetic property contrasts due to deep structural controls. Euler solutions cluster in the NE-SW, NW-SE, and E-W trends. These may correspond to conjugate structures at depth suggesting the primary role of the NW-SE shearing between the bounding Philippine Fault and the Sibuyan Verde Passage Fault. Clustering of solutions in a graben-like pattern is also observed beneath volcanic complexes indicating an extensional regime. The MC is also characterized by a shallow magnetic basement as reflected in the Curie point depths. This may be attributed to high heat flow associated with active volcanism and probably crustal thinning. These results skew our interpretation to a rift-related origin of the MC. Thus, it is interpreted to be the manifestation of the continuum of tectonic processes including near-field influences (shearing between the two bounding faults) and far-field influences (arc-continent collision and resulting northwestward translation of the Philippine Mobile Belt).