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The latest Cretaceous(?)–Paleocene Barra Honda Formation represents one of the largest carbonate shoals (>900 km2, 350 m thick) of the convergent margin of Costa Rica. Although the mode of formation of the carbonate shoal is well understood, how environmental and tectonic factors interacted to cause its demise near the Paleocene‐Eocene boundary remains poorly constrained. Stable isotopic, biostratigraphic, mineralogical, and geochronological analyses from the Barra Honda Formation and overlying siliceous Buenavista Formation provide new constraints on the timing and causes of the demise of the carbonate shoal. We report one new U–Pb zircon chemical abrasion, isotope dilution, and thermal ionization mass spectrometry date (56.30 ± 0.13 Ma, 2σ) obtained from an ash‐rich layer at the boundary between the two formations. The sharp transition from Barra Honda massive limestones to Buenavista marl‐chert alternations coincides with a negative shift in carbon isotope (δ13Ccarb) values of about 3–5 ‰ and a 50% decrease in carbonate contents. The timing of the combined lithological‐mineralogical‐isotopic change is coeval with the Paleocene‐Eocene Thermal Maximum (PETM, 56 Ma). The onset of clay‐rich sedimentation is consistent with a PETM‐related increase in the terrestrial influx of nutrients and detrital particles, which promoted eutrophication and decreased light availability in the photic zone. Combined with seawater acidification and warming, these environmental parameters were fatal to the carbonate‐producing benthic communities of Barra Honda. High subsidence rates of the forearc basin and renewed arc volcanic activity must have closely followed the cessation of shallow carbonate production, preventing further formation of the carbonate shoal. Plain Language Summary The Barra Honda Formation in Costa Rica is a large area of carbonate rocks formed during the latest Cretaceous(?)–Paleocene interval. We wanted to understand why this carbonate shoal, which was formed by a favorable combination of environmental and tectonic factors, disappeared around the Paleocene–Eocene boundary. We studied different aspects, that is, stable isotopes, microfossils, minerals, and ages of rocks from the Barra Honda Formation and the overlying Buenavista Formation. We present a new age date from zircons recovered from an ash layer between these formations, showing that the lithological change happened around 56 million years ago. At that time, there was a sudden shift in the types of sedimentary rocks deposited and in their carbon isotope values. This change coincides with a global event called the Paleocene–Eocene Thermal Maximum (PETM), known for causing environmental upheavals such as ocean warming and increased ocean water acidity. These factors caused more nutrients to flow in from land and reduced light in the seawater, and likely led to the decline of benthic communities in the Barra Honda carbonate shoal. Additionally, the basin where these carbonate rocks formed started sinking rapidly after the PETM event, preventing the shoal from recovering. Key Points U‐Pb age, biostratigraphic, and stable isotope compositions show that the Barra Honda carbonate shoal ended at the Paleocene‐Eocene boundary PETM‐related ocean acidification and increased detrital/nutrient influx may be the primary causes of the demise High subsidence rates of the forearc basin during the early Eocene terminated the shallow carbonate sedimentation

The causes for forearc basin and megathrust rupture zone segmentation are controversial. The Nankai forearc, Japan, is separated into five domains based on topography: Enshu, Kumano, Muroto, Tosa, and Hyuga. The boundaries of these domains correspond to the rupture limits of large earthquakes. We examined the geologic structure of the boundary region between the Kumano and Muroto domains off the Kii Peninsula using multichannel seismic reflection data to evaluate the role of upper plate composition in controlling segmentation. The results suggest that thick cover sediments and underlying accretionary prism are obliquely thrust landward over the igneous basement complex rock in the region of offshore of Cape Shionomisaki and separate the forearc basin. The igneous basement complex rocks directly overlying the plate interface in the hypocentral regions of 1944 Tonankai and 1946 Nankai earthquakes. The 1944 earthquake originated at the base of the complex, and the rupture extent slipped past its basement boundary, whereas the 1946 event nucleated at the updip boundary of the basement complex. The dense igneous rocks might have worked as a heavily loaded barrier on the seismogenic megathrust and separated the rupture area of both the earthquakes. Upper plate geology may be an important factor in controlling seismogenesis in the Nankai Trough and may serve as an example for understanding the controls on megathrust slip in other subduction zones. Plain Language Summary The Nankai Trough, Japan is the place where quakes nucleate in the margin and tsunamis repeatedly devastated circum‐Pacific societies. When, where, why, and how the quakes were started in the trench is a central scientific question but unsolved. The historical records of earthquakes suggest that the rupture started around Cape Shionomisaki of the Kii Peninsula, where there is also a topographic segment boundary of the ∼2,000 m deep basin in the middle continental slope. Our research of the geologic structure beneath the earthquake epicenters indicates a dense and hard rock mass situated upon the hypocenters of 1944, and 1946 megaquakes off Cape Shionomisaki. The rock mass might have worked as a heavy load and barrier on the low angle dipping plate boundary fault when the fault slipped the large rupture was propagated in the Nankai Trough. Key Points The Nankai forearc basin is separated off the Kii Peninsula owing to the existence of middle Miocene igneous basement in the upper plate The rupture areas of the 1944 Tonankai, 1946 Nankai, and 2016 off‐Mie earthquakes appear to be controlled by the igneous basement The shallow distribution of very low‐frequency earthquakes and tremors are related to the geological heterogeneity of the upper plate

The long‐term state of stress in the subduction forearc depends on the balance between margin‐normal compression due to the plate‐coupling force and the margin‐normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin‐normal compression due to the dominance of the plate‐coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin‐normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin‐normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin‐normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner‐forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi. Plain Language Summary The state of stress in the overriding plate between the trench and the volcanic arc of subduction zones depends on frictional coupling between the overriding and subducting plates and gravitational force, which causes lateral compression and tension, respectively. The trench‐ward portion of this so‐called “forearc” region is generally in compression due to the dominant effect of plate coupling, but for the arc‐ward portion, the relative importance of the two forces varies spatially. We constrain the state of stress in the forearc using earthquake data for five subduction zones and inferring results from previous studies for four other subduction zones. The results indicate that the forearc stress state seems to correlate with the downdip depth of plate coupling and the width of the forearc. A relatively shallow downdip extent of coupling in a wide forearc tends to have the arc‐ward portion in tension or neutral stress state as observed in parts of Nankai, Hikurangi, Mexico, and Cascadia although this tendency is impacted by the local tectonic settings. Key Points Focal mechanism inversion results indicate the correlation of inner forearc stress state with the downdip depth of plate coupling Margin‐normal horizontal deviatoric tension in the inner forearc tends to occur where plate coupling is shallow and the forearc is wide The variation in the inner forearc stress state does not require a variation in the subduction fault strength

The island of Donousa consists of a variegated metasedimentary sequence cut by arc-type granitoid dykes. Detrital zircon U–Pb ages and the presence of volcanic and ultramafic detritus suggest that the protoliths of the metasedimentary rocks were deposited in a Campanian forearc basin later than 76 Ma. D1 stacking of the forearc deposits occurred at upper crustal levels (P max  = 0.4 GPa) with staurolite present as a stable phase. The subsequent isobaric heating led to growth of andalusite followed by D2 extension in the stability field of sillimanite, K-feldspar and high-quartz ( P  = 0.3 GPa, T  = 660 °C). Based on U–Pb ages of garnet (74.6 + 2.3/ − 2.5 Ma) and zircon (74.0 ± 0.8 Ma), separated from an early monzogranite dyke, the timing of forearc deposition, Buchan metamorphism, D2 extension and dyke emplacement is constrained to a maximum period of 2.6 m.y. in the late Campanian. Cooling below T = ca. 600 °C occurred during the Maastrichtian (71.3 + 0.6/ − 1.3 Ma, U–Pb on titanite) and was followed by D3 folding under greenschist facies conditions. A younger generation of Danian granitoid dykes (64.1 + 0.9/ − 1.4 Ma, U–Pb on titanite; 60 ± 4 Ma, Rb–Sr on muscovite) intruded at higher structural levels and was associated with a pervasive fluid-assisted overprint. The sequence of forearc deposition, burying, Buchan metamorphism, extension, and igneous dyking within < 2.6 m.y. is strong evidence for fast (ca. 4 cm a −1 ) trenchward arc retreat that was probably related to late Campanian subduction accretion and magmatic flare-up. Graphical abstract

Active serpentinite mud volcanoes in the forearc region of the Izu‐Bonin‐Mariana system represent an excellent natural laboratory for studying the geochemical processes along convergent plate margins and the associated forearc. During IODP Expedition 366, serpentinite mud with lithic clasts from the underlying forearc crust and mantle as well as from the subducting Pacific Plate was recovered. Ultramafic clasts from Fantangisña Seamount reveal very high degrees of serpentinization with mesh and bastite textures as well as development of late lizardite and chrysotile veins, which suggests serpentinization temperatures below 200°C. On the other hand, recovered harzburgites and, on occasion, dunites from Asùt Tesoru Seamount show a well‐preserved primary assemblage with low degrees of serpentinization and forearc peridotite characteristics. Fine‐grained antigorite associating with lizardite has been identified throughout the serpentine mud matrix, suggesting an alteration temperature of c. 340°C. Furthermore, alteration conditions during rodingitization point to temperatures of at least 228°C, estimated via chlorite geothermometry. Additionally, a rare ophicarbonate clast containing andraditic as well as Cr‐rich hydrogarnets from Asút Tesoru Seamount indicates crystallization temperatures of at least 230°C. Hence, a trend of lower temperature of serpentinization and higher degree of alteration closer to the trench. The detailed characterization of the fluid‐rock alteration conditions as well as fluids composition and transport permits a better constraining of the fluid–rock interactions and related mass transfers within subduction zones and during ascent of serpentinite fault gouge within mud volcano conduits and in mudflows after their emplacement on the flanks of the edifices. The fluid migration and circulation in subduction zones play a crucial role in the geochemical cycling as well as the physical and mechanical processes taking place there. Characterizing their nature, source and pathways would contribute to a better understanding not only of the rheology and fluid recycling but also of the tectonic and metamorphic processes operating deep within the Earth's lithosphere as a whole. During an International Ocean Discovery Program (IODP) Expedition, 366 serpentinite mud volcanoes located on the fractured forearc of the Mariana subduction system were drilled. The recovered material consists of highly hydrated rocks that experienced varying degrees of metamorphism and alteration. Mineralogical and chemical composition study of these rocks showed that they are former mantle rocks formed by the infiltration of fluids within the subduction zone. Some of them experienced additional transformation during their ascent to the seafloor by acquiring substantial amount of CO 2 ‐rich minerals. Furthermore, the results show a trend in which alteration temperature is decreasing but transformation degree is increasing with proximity to the subduction channel. Serpentinization and rodingitization in the subduction channel as well as carbonation in mud volcano conduits were identified Overall evolution of the slab‐derived fluid was narrowed down to temperatures from c. 350° to c. 100°C and below Serpentinization degree increases with proximity to the trench

The East Kunlun Orogen on the northern margin of the Tethyan orogenic system records a history of Gondwana dispersal and Laurasian accretion. Uncertainties remain regarding the detailed histories of northern branches of the Paleo-Tethys Ocean in East Kunlun Orogen (Buqingshan Ocean). Based on a synthesis of sedimentary, structural, lithological, geochemical, and geochronological data from the East Kunlun Orogen and adjacent regions, this paper discusses the spreading and northward consumption of the Paleo-Tethys Ocean during Late Paleozoic–Early Mesozoic times. The main evolutionary stages are: (1) during Carboniferous to Middle Permian, the Paleo-Tethys Ocean (Buqingshan Ocean) was in an ocean spreading stage, as suggested by the occurrence of Carboniferous MORB-, and OIB-type oceanic units and Carboniferous to Middle Permian Passive continental margin deposits; (2) the Buqingshan Ocean subducted northward beneath the East Kunlun Terrane, leading to the development of a large continental magmatic arc (Burhan Budai arc) and forearc basin between ~270–240 Ma; (3) during the late Middle Triassic to early Late Triassic (ca. 240–230 Ma), the Qiangtang terrane collided with the East Kunlun–Qaidam terranes, leading to the final closure of the Buqingshan Ocean and occurrences of minor collision-type magmatism and potentially inception of the Bayan Har foreland basin; (4) finally, the East Kunlun Orogen evolved into a post-collisional stage and produced major magmatic flare-ups and polymetallic mineral deposits between Late Triassic to Early Jurassic (ca. 230–200 Ma), which is possibly related to asthenospheric mantle upwelling induced by delamination of thickened continental lithosphere and partial melting of the lower crust. In this paper, we propose that the Wilson cycle-like processes controlled the Late Paleozoic–Early Triassic tectonic evolution of East Kunlun, which provides significant implications for the evolution of the Paleo-Tethys Ocean.

Peridotites exposed in the southern Mariana Trench provide a rare opportunity to investigate mantle processes operating at the interface between arc and back-arc tectonic domains. This study presents petrographic observations and major element mineral chemistry of 41 depleted mantle harzburgites collected from three sites (Sites A–C) in the southern Mariana Trench. Site A is located on the east-facing slope of the West Santa Rosa Bank Fault, whereas Sites B and C are situated on the southern slope of the South Mariana Forearc Ridge along the eastern side of the Challenger Deep. The harzburgites exhibit variable microstructures ranging from coarse-grained (>1 mm) to medium-grained (<1 mm) to small-grained (>0.1 mm) textures, with or without porphyroclasts, and commonly contain amphibole associated with orthopyroxene and spinel. Olivine Mg# (Mg/[Mg + Fe]) (0.902–0.925) and spinel Cr# (Cr/[Cr + Al]) (0.304–0.720) indicate a wide range of mantle depletion across the three sites. Based on the integrated chemical characteristics of olivine, spinel, and amphibole, the harzburgites are classified into three distinct compositional trends (Trends 1–3). Trend 1 is characterized by high olivine Mg# (~0.925), high spinel Cr# (>0.6), low TiO2 contents (<0.1 wt%), and K2O-enriched but TiO2-poor amphibole (TiO2/K2O < ~0.5), consistent with strongly depleted forearc mantle modified by slab-derived hydrous melts or fluids. In contrast, Trend 2 is defined by relatively high olivine Mg# (>~0.91), lower spinel Cr# (<0.6), slightly higher TiO2 contents (up to ~0.2 wt%), and amphibole moderately enriched in both K2O and TiO2 (TiO2/K2O = 1–4), recording an intermediate geochemical signature that cannot be uniquely attributed to a purely forearc origin. Trend 3 is characterized by lower olivine Mg# (~0.90), lower spinel Cr# (<0.6), distinctly higher TiO2 contents (up to ~0.8 wt%), and TiO2-rich but K2O-poor amphibole (TiO2/K2O > 4), indicating a back-arc mantle origin related to decompression melting. Trends 1 and 2 occur in harzburgites from Sites B and C of the South Mariana Forearc Ridge, whereas Trend 3 is exclusively identified in harzburgites from Site A of the West Santa Rosa Bank Fault, highlighting the juxtaposition of forearc-type, transitional, and back-arc-type mantle domains within a single forearc region.

Recordings of earthquakes by a temporary deployment of 84 short period seismometers in northern Chile were used to derive regional 3D seismic velocity models for the Taltal segment. We used the Regressive ESTimator (REST) package for event detection and automatic onset estimation of P‐ and S‐wave arrival times to create an earthquake catalog with 23,985 hypocenters. We followed standard acceptability criteria (i.e., azimuthal gap and residual cutoff) to create a high‐quality data set and inverted for 3D Vp, Vs and Vp/Vs models using local earthquake tomography. Plots of hypocenters from the catalog, comprising 16,349 earthquakes, reveal active structures in the upper crust, dip changes along the slab and fracturing within the oceanic crust. Moreover, the wavespeed models illuminate anomalies in both the Nazca and South American plates that correlate with the observed seismicity distribution, including variations from low (1.75) to high (>1.80) Vp/Vs near the Atacama fault system on the coastline and the Domeyko Fault System in the forearc. The seismic velocity models also provide evidence for fluid circulation caused by the subducting Taltal ridge on the coast and partial melting feeding a volcanic complex close to the Andes. Finally, the observed low Vp/Vs ratios (∼1.75) are associated with copper mining operations in the area, suggesting that this kind of imaging can be used to characterize the distribution of potential ore deposits in the area. Plain Language Summary We recorded earthquakes in northern Chile with a network of 84 seismometers and used the arrival times of P and S waves to generate 3D wavespeed models of the region. These models reveal several structures in the area, including changes in the angle of the subducting Nazca plate and fractures in the oceanic crust. Among features observed in both the Nazca and South American plates are the Atacama and Domeyko fault systems. We also infer fluid circulation caused by the subducting Taltal ridge and partial melting that feeds a volcanic complex near the Andes. Low values of the Vp/Vs ratio are associated with copper mining operations in the area and could be used to identify new ore deposits. Key Points Seismic catalog reveals forearc activity and slab dip variations. Vp anomalies in the oceanic plate are related to mid‐depth seismic events Velocity models uncover anomalies in Salar de Atacama and Taltal ridge that might influence seismicity distribution and hydration changes Shallow low Vp/Vs (<1.75) correlate with ore deposits; deep high Vp/Vs (>1.80) suggest fluids and melting for the Lastarria volcanic complex

Persistent arc magmatism archives fluid transport and mantle partial melting in subduction zones. However, arc magmatism often exhibits different magmatic records along the strike, as seen in the Tethyan orogenic belt. During Neo‐Tethys subduction under Iran, there was pulsed arc magmatism with Middle Jurassic and Eocene magmatic flare‐ups. There are few Late Cretaceous magmatic records preserved in Iran. The Zagros fold‐and‐thrust belt of Iran and Iraq includes the deformed passive continental margin of Arabia, but sparse records of pre‐collisional, forearc rocks along the suture zone and southwest of the arc are preserved on the Eurasian side of the collision zone. Through provenance analysis of the western Makran accretionary wedge in southeastern Iran, we identify a Late Cretaceous continental arc and forearc that once existed in southwest Iran but disappeared and/or displaced to the northwest in Turkey by later subduction zone tectonics. We suggest that subduction erosion and large‐scale strike‐slip displacement by oblique subduction are responsible for destroying the continuous Tethyan continental arc.

At the southernmost part of the Ryukyu subduction zone, six long-offset multi-channel seismic profiles were collected across three forearc basins and the southern Ryukyu accretionary prism during the TAIGER experiment in 2009 and the TAICRUST project in 1995. These profiles were reprocessed to generate pre-stack depth migration (PSDM) sections. In addition, two velocity-interface models were obtained by reanalyzing active source data recorded from 28 ocean-bottom seismometers during the same TAIGER experiment, in consideration of the PSDM sections and previous tomography models. Due to the northwest convergence of the Philippine Sea Plate (PSP), it is suggested that the Gagua Ridge may have been obliquely subducting northwestward beneath the Ryukyu prism and below the Nanao Basin. The PSDM sections and the velocity-interface models indicate the subducted Gagua Ridge causing the uplift of the sedimentary basement and the lower crustal structure below the Nanao Basin. The sedimentary and crustal structures near ~ 122.5° E beneath the Nanao Basin were also uplifted where shallow earthquakes had occurred by the oblique subduction of the Gagua Ridge depicted in the isopach map of the crust. Furthermore, a recent earthquake (Mw ~ 6.0) occurred near the northeast coast of Taiwan in 2018 at a depth of approximately 12 km below the Hsincheng Ridge. Our study suggests that this earthquake was caused by a thrust fault near the décollement, which might have been formed by the subduction of the PSP.