Continents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas.

It is generally accepted that oceanic plate subduction has occurred along the eastern margin of Asia since about 500 Ma ago. Therefore, the Japanese Islands have a >500 Ma history of oceanic plate subduction in their geological records. In this paper, the accretionary history of the Japanese Islands is divided into six main stages based on the mode and nature of tectonic events and the temporal gaps in the development of accretionary processes. In the first stage, oceanic plate subduction and accretion started along the margin of Gondwana. After detachment of the North and South China blocks in Devonian time, accretionary complexes developed along island arcs offshore of the South and North China blocks. After the formation of back arc basins such as the Japan Sea, accretionary processes occurred only along the limited convergent margin, e.g. Nankai Trough. Detrital zircons of sandstones revealed the accretionary history of Japan. An evaluation of a comprehensive dataset on detrital zircon populations shows that the observed temporal gaps in the development of the Japanese accretionary complexes were closely related to the intensity of igneous activity in their provenance regions. Age distributions of detrital zircons in the accretionary complexes of Japan change before and after the Middle Triassic period, when the collision of the South and North China blocks occurred.

The eastern Nankai accretionary prism toe was surveyed to evaluate the nature and deformation of its frontal thrust. According to the determined porosities and yield strengths, turbidites were successively buried down to depths of 250–300 m before accretion, and were then exposed at the prism toe by uplift along the Tenryu frontal thrust during 3.4–1.98 Ma. Consolidation tests provided reasonable estimates of burial depth and, when combined with exposed sediment dates, yield prism toe uplift rates of 0.74–2.27 m ka–1. The displacement along the frontal thrust is estimated to be 500–900 m and the slip rates are 1.47–4.55 m ka–1, corresponding to the highest class of active faults on land in Japan. During the surveys of the Tenryu frontal thrust zone, we discovered a new active fault scarp that was several tens of centimetres high, interpreted to be a protothrust located c. 100 m south of the frontal thrust. This scarp is associated with chemosynthetic biocommunities. The thrust might potentially be the result of displacement during the East Nankai (To-Nankai) earthquake (Mw 8.1) in 1944. These lines of evidence indicate that the Tenryu frontal thrust is still active and that displacement along the thrust might induce a tsunami during future Tokai or To-Nankai earthquakes.

Eighty-six new acoustic survey lines along and across the Japan Trench revealed active sediment creep deformation on a deep-sea terrace at water depths of 400–1200 m in an area of arcuate-shaped depressions that are probably associated with tectonic erosion. The most active region of creep is located on the top at the surface of the depression south of 38° N. The area of creep deformation is characterized by arcuate-shaped topographic lineaments with active folds and active normal faults stepping down trenchward. In contrast to the southern region, normal faults at the top of the depression north of 38° N cut a sedimentary sequence (Unit 1) that is acoustically transparent with continuous weak reflectors, and this is covered by the undeformed layered sediment sequence of Unit 2. Unit 2 corresponds to the period of rising sea level that extended from the latest Pleistocene to the early Holocene (14–6 ka). Thus, creep is ongoing at the top of the depression south of 38° N in the surface layer, whereas it stopped north of the depression between 14 and 6 ka. These observations might indicate that the active region jumped from north to south due to probably retrogressive sliding.

We present a new dataset of zircon U–Pb ages that document igneous activity in the SW Japan arc during middle Miocene time and discuss its relationship with the opening of the Japan Sea, Philippine Sea plate migration, and subduction of the young hot lithosphere of the Shikoku Basin. Precursory magmatism, characterized by dike and stock intrusions, started c. 15.6 Ma in both Kyushu and the Kii Peninsula. Most plutonism occurred between 15.5 and 13.5 Ma in an area 600 km long and 150 km wide. No along-arc trend was recognized in the U–Pb ages of igneous activity near the trench. Our data indicate that all near-trench middle Miocene igneous activity occurred immediately after the opening of the Japan Sea ceased, i.e. after 16 Ma, implying that melt extraction and the emplacement of granites in the near-trench region had some influence on the back-arc opening. Our data also imply that the trench–trench–trench-type triple junction between the Japan arc and the Izu–Bonin–Mariana arc must have reached the east side of the Kii Peninsula by 15.6 Ma. The wide distribution of contemporaneous magmatic activity along the arc requires a trench-parallel heat source, such as the subduction of a trench-parallel ridge or a young and highly segmented ridge–fracture zone system in addition to the hot wedge mantle condition related to the opening of Japan Sea.

The Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume–ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (>80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.

Many orogens on the planet result from plate convergence involving subduction of a continental margin. The lithosphere is strongly deformed during mountain building involving subduction of a plate composed generally of accreted continental margin units and some fragments of downgoing oceanic crust and mantle. A complex deformation involving strong partitioning of deformation modes and kinematics produces crustal shortening, accompanied by crustal thickening. Partitioning depends on three main factors: (1) rheologic layering of the lithosphere; (2) interaction between tectonics and surface processes; (3) subduction kinematics and 3D geometry of continental margins (oblique convergence, shape of indenters). Here we present an original view and discussion on the impact of deformation partitioning on the structure and evolution of orogens by examining the Taiwan mountain belt as a case study. Major unsolved questions are addressed through geological observations from the Taiwan orogen and insights from analogue models integrating surface processes. Some of these questions include: What is the role played by décollements or weak zones in crustal deformation and what is the impact of structural heterogeneities inherited from the early extensional history of a rifted passive continental margin? What is the relationship between deep underplating, induced uplift and flow of crustal material during erosion (finite strain evolution during wedge growth)? Are syn-convergent normal faults an effect of deformation partitioning and erosion? What is the role of strain partitioning on the location of major seismogenic faults in active mountain belts? What can be learned about the long-term and the present-day evolution of Taiwan?

The submerged forearcs of Pacific subduction zones of North and South America are underlain by a coastally exposed basement of late Palaeozoic to early Tertiary age. Basement is either an igneous massif of an accreted intra-oceanic arc or oceanic plateau (e.g. Cascadia(?), Colombia), an in situ formed arc massif (e.g. Aleutian Arc) or an exhumed accretionary complex of low and high P/T metamorphic facies of late Palaeozoic (e.g. southern Chile, Patagonia) and Mesozoic age (e.g. Alaska). Seismic studies at Pacific forearcs image frontal prisms of trench sediment accreted to the seaward edge of forearc basement. Frontal prisms tend to be narrow (10–40 km), weakly consolidated and volumetrically small (∼35–40 km3/km of trench). In contrast, deep seismic imaging of submerged forearcs commonly reveals large volumes (∼2000 km3/km of trench) of underplated material accreted at subsurface depths of ∼10–30 km to the base of forearc basement. Underplates have been imaged below the southern Chile, Ecuador–Colombia, north Cascade, Alaska, and possibly the eastern Aleutian forearcs. Deep underplates have also been observed below the Japan and New Zealand forearcs. Seismic imaging of northern and eastern Pacific forearcs supports the conclusion drawn from field and laboratory studies that exposed low and high P/T accretionary complexes accumulated in the subsurface at depths of 10–30 km. It seems significant that imaged underplated bodies are characteristic of modern well-sedimented subduction zones. It also seems likely that large Pacific-rim underplates store a significant fraction of sediment subducted in Cenozoic time.

Our field-based geochemical studies of the Triassic, Jurassic and Cretaceous granitoids on Hainan Island indicate that their magmas had different geochemical affinities, changing from alkaline in the Triassic through ocean island basalt (OIB) in the Jurassic, to calc-alkaline in the Cretaceous. We show that these changes in the geochemical affinities of the Mesozoic granitoids on Hainan and in SE China reflect different melt sources and melt evolution patterns through time. Our new geodynamic model suggests that: (1) Triassic geology was controlled by flat-slab subduction of the palaeo-Pacific plate beneath SE China. This slab dynamics resulted in strong coupling between the lower and upper plates, causing push-over tectonics and contractional deformation in SE China. Flat subduction-induced edge flow and aesthenospheric uprising led to the production of high-K granites, syenites and mafic rocks. (2) Slab foundering, accelerated subduction rates and subduction hinge retreat in the Early Jurassic caused rapid rollback of the downgoing slab. Strong decoupling of the upper and lower plates resulted in pull-away tectonics, producing extensional deformation in SE China. Decompression melting of the upwelling aesthenosphere produced OIB-type melts, which interacted with the subcontinental lithospheric mantle (SCLM) to form A- and I-type granitoids. (3) Segmentation of the palaeo-Pacific plate in the Early Cretaceous resulted in steeply dipping slabs and their faster rollback, facilitating lithospheric-scale extension and oceanward migration of calc-alkaline magmatism. This extensional deformation played a significant role in the formation of metamorphic core complexes, widespread crustal melting and development of a Basin and Range-type tectonics and landscape evolution in SE China.

The Zhangguangcai Range in the Xing’an Mongolian Orogenic Belt, NE China, contains Early Jurassic (c. 188 Ma) Dabaizigou (DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have (87Sr/86Sr)i ratios of 0.7033–0.7044, relatively uniform positive ɛNd(t) values of 2.3–3.2 and positive ɛHf(t) values of 8.5–17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under garnet- to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W–NW beneath East Asia.

We report new U–Pb zircon age data, zircon in situ oxygen isotope, mineral chemistry, whole-rock geochemistry and Sr–Nd isotopic compositions from the Early Devonian ultrapotassic Gucheng pluton in the North China Craton, and discuss its petrogenesis. The Gucheng pluton is exposed in the northern part of the North China Craton and forms a composite intrusion, consisting of K-feldspar-bearing clinopyroxenite, clinopyroxene-bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43–48En19–35Fs18–38), sanidine (An0Ab3–11Or89–97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in situ oxygen isotopes (δ18O = 5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the North China Craton. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the North China Craton facing the Palaeo-Asian Ocean.

The interaction of the Palaeo-Pacific and Palaeo-Asian Oceans is an enigmatic issue as their temporal and spatial features are controversial. To address this issue, we present a systematic study of large volumes of early Permian volcanic rocks and intrusions developed in the East Tianshan. The represented samples of basaltic andesites and rhyolites yield zircon crystallization ages of 285.1 ± 5.9 Ma and 275.3 ± 1.8 Ma, respectively. The basalts have normal mid-ocean ridge basalt (N-MORB) and arc-related geochemical signatures with high TiO2 contents, negative Rb, Th, U, Nb and Ta anomalies and positive Eu anomalies. Basaltic andesites and andesites have arc-related geochemical characteristics with moderate TiO2 contents and relatively negative Nb, Ta and Ti anomalies, together with slightly negative to positive Eu anomalies. The rhyolites show an affinity with A2-type granite with high SiO2, K2O + Na2O, Fe/Mg, Ga, Zr, Nb, Y, HFSE, REE and Y/Nb levels (>1.2). These geochemical data suggest that the rocks formed in a supra-subduction zone. The presence of high ϵNd(t) values of +4.6 to +8.2 and low (87Sr/86Sr)i (0.70342–0.70591) values indicates that the volcanic rocks originated from a depleted mantle. We propose that oblique subduction with slabs breaking off gave rise to transtension and to the emplacement of large volumes of mantle-derived melts in the early Permian in the East Tianshan, serving as an important record of the subduction zone of the Palaeo-Pacific Ocean.