3.a. Crustal architecture and geology of Hainan

Hainan Island is located in the continental shelf of southern China and adjacent to the Indochina Block in the west (Fig. 1). It is also tectonically situated within the NE–SW-trending South China Marginal Basin Rift System, which extends into the Taiwan Strait in the NE (Fig. 3). The basement rocks on Hainan are covered peripherally by Quaternary alluvium, and by Quaternary intra-plate basaltic lava flows in the northern part of the island (Fig. 4; Tian et al. Reference Tian, Yan, Huang, Dilek, Yu, Liu, Zhang and Zhang2020). Similar lava fields continue to the north on the Leizhou (Liuchow) Peninsula in mainland China, making up the Leiqiong volcanic field. The structural fabric of Hainan Island is defined mainly by NE–SW-oriented, oblique normal fault systems, forming locally well-developed horst blocks (Fig. 4). The inverted Baisha basin, which is composed of Cretaceous sedimentary and volcanic rock sequences in the west-central part of the island with a highest peak of 1417 m a.s.l., represents one of these fault blocks.

Fig 4. Geological map of Hainan Island, depicting the distribution of main fault systems, granitoid groups and sample locations for this study. Data are from XH Li et al. (Reference Li, Zhou, Chung, Ding, Liu, Lee, Ge, Zhang and Zhang2002), Ge, (Reference Ge2003), Li et al. (Reference Li, Li, Li and Wang2006), Pedoja et al. (Reference Pedoja, Shen, Kershaw and Tang2008), Y Hu et al. (Reference Hu, Hao, Jia and Song2016), D Xu et al. (Reference Xu, Wang, Wu, Zhou, Shan, Hou, Fu and Zhang2017), Yao et al. (Reference Yao, Li, Li and Li2017), Tian et al. (Reference Tian, Yan, Huang, Dilek, Yu, Liu, Zhang and Zhang2020) and this study.

The crystalline basement in Hainan consists of metamorphosed Mesoproterozoic magmatic, volcanic and clastic rocks that are exposed mainly in the W-SW and E parts of the island (Fig. 4). Amphibolite-facies gneissic granitoids, gneiss and metaclastic rocks of the 1430 Ma Baoban Complex represent a Mesoproterozoic continental rift sequence (Yao et al. Reference Yao, Li, Li and Li2017). Greenschist-facies metasedimentary and metavolcanic rocks of the 1439 ± 9 Ma Shilu Group constitute the upper crustal volcanic–sedimentary units of this rift sequence (Yao et al. Reference Yao, Li, Li and Li2017). The unconformably overlying 1200–1000 Ma metamorphosed quartz–sandstones of the Shihuiding Formation make up foreland deposits. Metamorphosed Lower Palaeozoic sedimentary rocks are locally exposed both in the western and eastern parts of the island. These Mesoproterozoic crystalline and Lower Palaeozoic metasedimentary rocks are intruded by Permian (Wuzhishan), Triassic (Indosinian) and Jurassic–Cretaceous (Yanshanian) granitoids and are overlain by Lower and Upper Palaeozoic sedimentary–volcanic rock assemblages, and Upper Triassic clastic rocks (Fig. 4).

Permo-Carboniferous (333 Ma) metasedimentary and metavolcanic rocks with mid-ocean-ridge basalt (MORB) affinities in the west-central part of the island represent a major episode of continental rifting in its Palaeozoic tectonic history (Li et al. Reference Li, Zhou, Chung, Ding, Liu, Lee, Ge, Zhang and Zhang2002). Li et al. (Reference Li, Li, Li and Wang2006) have demonstrated the occurrence of 267–262 Ma (Guadalupian), calc-alkaline metagranites (Wuzhishan orthogneiss) in east-central Hainan, which they have interpreted as the first major product of continental arc magmatism in SE China, developed above a NW-dipping subduction zone. These authors have also noted that this Permian, Andean-type arc magmatism was coeval with widespread crustal uplift along the coast of SE China that provided clastic sediments into terrestrial basins further inland. The timing of the Permian arc plutonism on Hainan is consistent with the findings of Zhang and co-workers in Anhui Province in SE China (FQ Zhang et al. Reference Zhang, Dilek, Zhang, Chen, Zhu and Hao2020) where Guadalupian (270–264 Ma) tuff deposits within the Gufeng Formation (Permian carbonate platform of SE China) represent the initial products of subaerial volcanism and magmatic arc construction in SE China during the Late Palaeozoic.

3.b. Mesozoic granitoids and associated intrusive rocks on Hainan

In this study, we have distinguished two major (early–Middle Triassic and Cretaceous) igneous events in the geological record of Hainan. Ge (Reference Ge2003) also reported the existence of Jurassic granitic intrusions (186 Ma) on Hainan Island. Triassic magmatic rocks include bimodal, high-K granites and hypabyssal dolerite, hornblende gabbro and syenite that are all alkaline to subalkaline in nature (Fig. 5). The distribution of these Triassic intrusions and their internal structural fabric are predominantly NE–SW oriented (Fig. 4). Contact relationships observed in the field suggest that the Xinglong granite intruded into the dolerite via magma stoping (Fig. 6a–c) and along extensional shear zones (Fig. 6d). These spatial relationships are best exposed below the river-bed dam in Dazhou Village, where irregular and anastomosing granite veins intrude the fine-grained dolerite and the larger outcrops of granite contain enclaves of dolerite. The dolerite rock is dark grey and displays an ophitic texture; it is composed mainly of plagioclase (70–60%), clinopyroxene (25–20%), amphibole (15–10%) and biotite (10–5%) (Fig. 7a). The Xinglong granite is red in both outcrop and hand sample and is coarse-grained. It is composed of plagioclase with microcline twinning (70–60%), quartz (25–20%), hornblende (10–5%) and biotite (10–5%) (Fig. 7b).

Fig 5. Total alkali vs SiO₂ diagram for the Triassic and Cretaceous intrusive rock suites on Hainan Island (this study). FJZ = Fenjiezhou syenite. Alkaline–subalkaline boundary is from Le Bas et al. (Reference Le Bas, Le Maitre and Streckelsen1986).

Fig 6. Outcrop images of the primary intrusive relationships between the Triassic Xinglong granite and dolerite. (a–c) Xinglong granite is intrusive into the dolerite, forming jigsaw puzzle rafts of angular mafic rock pieces floating in the felsic rock, typical of magma stoping. (d) Syn-kinematic intrusion of the Xinglong granite along extensional shear zones in the dolerite.

Fig 7. Microphotographs of representative samples from: (a) Xinglong dolerite; (b) Xinglong granite; (c) Wanning gabbro (coarse-grained) with euhedral to subhedral and prismatic plagioclase crystals with Carlsbad–albite twins; (d) Ledong diorite; (e) Wenshi dykes; (f) Sanya dykes. Key for acronyms: Cpx – clinopyroxene; Hrb – hornblende; Qz – quartz; Pl – plagioclase. See the text for descriptions of the mineralogy and textures of these rock samples.

The Wanning gabbro is exposed on the north side of the Wanning reservoir near Wanning City (Fig. 4) and is intrusive into ~251–234 Ma mylonitic gneissic granites. It is coarse-grained and composed predominantly of plagioclase (65–60%), hornblende (30–25%) and pyroxene (4–3%) (Fig. 7c). Accessory minerals include apatite, zircon and magnetite (2–1%). The Fenjiezhou syenite crops out on the small Fenjiezhou Island (or Boundary Island) in the Riyue Bay immediately to the SE of Hainan and is intrusive into Permo-Carboniferous metasedimentary rocks. It is composed of 75–70% feldspar, 10–5% albite, ~10% biotite and ~3% quartz.

Middle to Late Jurassic (180–142 Ma) granitoids (coexisting with minor dolerite, diorite and gabbro to syenite) and volcanic rock assemblages occur widely in NE–SW-trending belts in Hainan and SE China (Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010; Zhou et al. Reference Zhou, Sun, Shen, Shu and Niu2006; Li & Li, Reference Li and Li2007). Jurassic granitoids have geochemical characteristics of I- and A-type granites, whereas the coeval syenites show incompatible trace-element patterns of ocean island basalt (OIB)-type melts (Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010; Zhou et al. Reference Zhou, Sun, Shen, Shu and Niu2006). These Jurassic granite and syenite intrusions are spatially and temporally associated with alkaline basalts and bimodal (basalt–rhyolite) volcanic rocks (180–170 Ma).

Cretaceous magmatic rocks on Hainan occur as mainly NE–SW-striking doleritic dyke swarms in the Proterozoic metamorphic basement units and in the Triassic granitoids (Chahe dykes), in Lower Cretaceous sandstone-conglomerate units (Wenshi dykes) and in the Jurassic–Cretaceous granite suites (Sanya dykes) (Fig. 8). These dykes were intruded along NE–SW-oriented fault systems. Also part of the Cretaceous magmatic series is the Ledong diorite–granodiorite suite in the SW part of Hainan. Diorite occurs as enclaves within the granodiorite. The Chahe dykes occur in the west-central part of Hainan near the city of Changjiang, the Wenshi dykes occur in the east-central part near Wenshi Town in Qionghai City, and the Sanya dykes crop out along the Sanya City Highway in southern Hainan, west of the city of Sanya (Fig. 4). All dyke rocks are composed of dolerite with a diabasic texture and consist of plagioclase, pyroxene and hornblende with accessory minerals of apatite, zircon and magnetite (Fig. 7e–f).

Fig 8. Outcrop images of the Cretaceous Sanya dykes with their granitic host rock (Sanya granite) and the Cretaceous Wenshi dykes with their Cretaceous red sandstone host. Sanya dykes were emplaced along NE–SW-striking extensional fractures and fault systems that show consistently top-to-the west displacement and shearing.

3.c. Geochronology of Mesozoic intrusive rocks on Hainan

The hypabyssal dolerite and the Xinglong granite intrusions have U–Pb sensitive high-resolution microprobe (SHRIMP) zircon weighted mean ²⁰⁶Pb/²³⁸U concordia ages of 238 ± 2 Ma (MSDW = 0.69) and 234 ± 2 Ma (MSDW = 0.14), respectively (see Data Repository File #1 (in the Supplementary Material available online at https://doi.org/10.1017/S0016756820001211); Tang et al. Reference Tang, Chen, Dong, Yang, Shen, Cheng and Fu2013). SHRIMP zircon dating of the Wanning gabbro and Fenjiezhou syenite have weighted mean ²⁰⁶Pb/²³⁸U concordia ages of 240 ± 2 Ma and 231 ± 3.4 Ma, respectively (Tang et al. Reference Tang, Chen, Dong, Yang, Shen, Cheng and Fu2013). We interpret these ages as the timing of crystallization of the Wanning gabbro and hypabyssal dolerite, followed by the emplacement of the granite and syenite in the Middle to Late Triassic.

The dolerite dyke swarms display magmatic zircon ages between 101 ± 4 Ma and 93 ± 2 Ma (U–Pb SHRIMP dating), and inherited zircon ages of 238 ± 4 Ma and 237 ± 4 Ma (see Data Repository File #1 (in the Supplementary Material available online at https://doi.org/10.1017/S0016756820001211); Tang et al. Reference Tang, Chen, Dong, Yang, Shen, Cheng and Fu2013). Magmatic zircons represent the timing of a widespread NE–SW-dyking event in a regional, NW–SE-oriented extensional stress regime. Inherited zircon ages are identical to the ages of the Triassic dolerite and granite intrusions on Hainan, indicating that the Cretaceous magmas came up through these earlier-formed intrusive series. The Ledong granodiorite zircons have weighted mean ²⁰⁶Pb/²³⁸U concordia ages of 101 ± 0.9 Ma (see Data Repository File #1 (in the Supplementary Material available online at https://doi.org/10.1017/S0016756820001211); Tang et al. Reference Tang, Chen, Dong, Yang, Shen, Cheng and Fu2013), indicating contemporaneous emplacement of dolerite dykes and the Ledong granodiorite–diorite.

5.a. Triassic geology and magmatism

Lower Triassic shallow marine rocks rest unconformably on the Permo-Carboniferous sedimentary and volcanic units. In the Zhejiang region, for example, the Lower Triassic Changxing Formation includes black bioclastic limestones with graded-bedding and cross-bedding features, characteristic of high-energy, shallow marine deposits, and the Lower Triassic Zhengtang Formation further south consists of clastic and carbonaceous turbiditic sequences; collectively, these two formations represent shelf-break and continental slope deposits, and are deformed by NW-vergent folds and thrust faults (Xiao & He, Reference Xiao and He2005; Chu et al. Reference Chu, Faure, Lin, Wang and Ji2012). These deformed Lower Triassic rocks are unconformably overlain by Upper Triassic lacustrine and terrestrial rocks that are interpreted as foreland basin (Li & Li, Reference Li and Li2007) or molasse deposits (Xiao & He, Reference Xiao and He2005).

Triassic sedimentary strata and some of the NW-vergent thrust faults are cross-cut by high-K, I-type granitoid, gabbro, syenite and hypabyssal dolerite intrusions (Fig. 12a). The negative ε _Nd(T) values (−5.90 to −3.86) of the granite and dolerite rocks suggest partial melting of an enriched mantle source (EMII), accompanied by high-T crustal melting for the origin of their magmas (Fig. 11). Granitoid plutons and thrust faults become younger to the NW (249 to 217 Ma; Fig. 3), in the direction of thrusting, as documented by U–Pb zircon SHRIMP and ⁴⁰Ar/³⁹Ar mica ages of representative rocks (for a comprehensive age data table, see the data repository Files in Li & Li, Reference Li and Li2007). This early Mesozoic regional structure is known as the SE China Fold and Thrust Belt (Fig. 3; Xiao & He, Reference Xiao and He2005).

5.b. Jurassic geology and magmatism

Jurassic volcanic and sedimentary rock sequences in SE China rest unconformably on the folded–faulted Triassic foreland basin strata and deformed granitoids (Fig. 12b) or on the deformed Devonian to Carboniferous siliciclastic rock assemblages. The Jurassic magmatism produced A- and I-type granitoids, alkaline granites and syenites with minor gabbro, diorite, dolerite and lamprophyre dyke intrusions, which were emplaced along NE–SW-trending extensional fault systems (YH Jiang et al. Reference Jiang, Jiang, Zhao and Ling2006; XM Zhou et al. Reference Zhou, Sun, Shen, Shu and Niu2006; C Guo et al. Reference Guo, Chen, Zeng and Lou2012; Quelhas et al. Reference Quelhas, Mata and Dias2020). These plutons locally intruded into thick sequences of basaltic, dacitic, rhyolitic volcanic rocks and alkaline basalts, which were also erupted within NE–SW-oriented grabens (Fig. 12b; Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010; C Guo et al. Reference Guo, Chen, Zeng and Lou2012; Wei et al. Reference Wei, Song, Hou, Chen, Faure and Yan2018). The Jurassic magmatism was hence associated with extensional deformation in a ~400 km wide belt, characterized by a Basin and Range-type crustal structure, which included lacustrine–playa lake deposits, evaporites and alluvial fan systems interspersed with bimodal volcanic sequences (see Wang & Shu (Reference Wang and Shu2012), and references therein, for an overview of the distribution of Mesozoic metamorphic core complexes in SE China, and Ni et al. (Reference Ni, Liu, Tang, Yang, Xia and Guo2013) for the occurrence of a metamorphic core complex in E China).

Jurassic granites have a relatively wide range of ε _Nd(T) values (−6.55 to −0.78) and are geochemically similar to A-type granites (Fig. 11). However, geochemical compositions and isotopic fingerprints of these A-type granites display major differences, which appear to have resulted from isotopically distinct crustal domains within the South China Block, particularly in Cathaysia (Quelhas et al. Reference Quelhas, Mata and Dias2020). Syenites have high K contents (shoshonitic) and a large range of Nd–Sr isotopic compositions (ε _Nd(T) = −3.54 to +3.44). Gabbros are geochemically analogous to intraplate transitional basaltic rocks and display a narrow range of ε _Nd(T) values (−0.8 to +0.55). These geochemical and isotopic features suggest that magmas of the gabbros and syenites were derived from partial melting of a continental lithospheric mantle, which was modified by OIB-type (aesthenospheric) melts (Jiang et al. Reference Jiang, Jiang, Zhao and Ling2006), whereas the granites were produced by magmas that formed from partial melting of hornblende-bearing crustal rocks (Li et al. Reference Li, Li, Wartho, Clark, Li, Zhang and Bao2010; Yao et al. Reference Yao, Li, Li and Li2017; Quelhas et al. Reference Quelhas, Mata and Dias2020).

5.c. Cretaceous geology and magmatism

Cretaceous plutonic and volcanic rocks occur in a NE–SW-trending belt (Figs 2, 3). Granites and rhyolites within a 300 km wide zone adjacent to the modern coastline form coeval, felsic intrusive and extrusive assemblages with a minor occurrence of gabbros and basalts. Dominantly composed of high-K, I-type and A-type granites, the Cretaceous granitoids in SE China were emplaced in multiple episodes within discrete time windows (Li, Reference Li2000; XY Li et al. Reference Li, Li, Huang, Wang, Yu, Cao and Xie2019; X Zhao et al. Reference Zhao, Yu, Jiang, Mao, Yu, Chen and Xing2019). The 146–136 Ma and 132–123 Ma granites were intruded in southern Anhui, Jiangsu, Zhejiang and northern Jiangxi provinces (Gao et al. Reference Gao, Wang, Tan and Li2019b), whereas the 109–101 Ma high-K, calc-alkaline I-type granites were emplaced near the modern coastline of SE China (Z Li et al. Reference Li, Qiu and Xu2012). The younger, 97–87 Ma calc-alkaline I-type granites and some A-type granites were emplaced in coastal Fujian and eastern Taiwan (Li, Reference Li2000; X Zhao et al. Reference Zhao, Yu, Jiang, Mao, Yu, Chen and Xing2019). Cretaceous granites in Fujian, Guangdong and Zhejiang provinces that are closer to the present coastline are spatially associated with monzogranite, biotite granite, hornblende gabbro and syenite, and have I-type geochemical characteristics with enrichment in LREE and LILE, and relative depletion in HFSE (Fig. 9; Lapierre et al. Reference Lapierre, Jahn, Charvet and Yu1997; Guo et al. Reference Guo, Chen, Zeng and Lou2012a, b; Z Li et al. Reference Li, Li, Chung, Lo, Xu and Li2012; Tao et al. Reference Tao, Pan, Liu, Jin, Jia and He2020). These rocks have a relatively narrow and homogeneous Sr–Nd isotopic compositional range, showing small negative ε _Nd(T) values (−4.1 to −2.2) (Fig. 12c).

Cretaceous granitoids (132–123 Ma and 117 Ma) further inland in Hunan Province are high-K, calc-alkaline and weakly to strongly peraluminous in character, represented by two-mica or biotite monzogranites and minor granitoids. They display LREE enrichments and negative Ba, Sr, N, P and Ti anomalies, indicating strong subduction influence in their melt evolution (Ji et al. Reference Ji, Lin, Faure, Chen, Chu and Xue2017). Their negative zircon ε _Hf(t) values (−12.5 to −3.6) and crustal Hf model (T_DM) ages (1.4–2.0 Ga), combined with other geochemical features are interpreted to indicate partial melting of Neoproterozoic metapelitic rocks of the South China Block as their magma source (Ji et al. Reference Ji, Lin, Faure, Chen, Chu and Xue2017). Further to the E-NE in Jiangxi and Anhui provinces the Early Cretaceous granitoid plutons (i.e. Lingshan pluton in the Gan-Hang belt and Huayuan-gong granitoids in the Lower Yangtze River Belt) display A-type geochemical affinities with metaluminous to weakly peraluminous compositions, enriched Hf isotope ratios of zircon and a large range of high Y/Nb ratios (Yang et al. Reference Yang, Wang, Ye, Li, He, Siebel and Chen2017; Wang et al. Reference Wang, Zhao, Huang, Xing and Wang2018). Magmas of these A-type plutons were derived from mixing of aesthenospheric melts with magmas originated from partial melting of sedimentary rocks of a Mesoproterozoic crystalline basement (Yang et al. Reference Yang, Wang, Ye, Li, He, Siebel and Chen2017; Wang et al. Reference Wang, Zhao, Huang, Xing and Wang2018).

Cretaceous volcanic rocks near the coastal regions in SE China fall into two broad time intervals. The 143–124 Ma, acidic- to intermediate-composition volcanic rocks show variable enrichment in K, Rb, Ba, Th and Ce, and negative Nb, Ta, Zr, Hf and Yb anomalies. Geochemically, these volcanic rocks are similar to the age-equivalent, first and second episode, I-type granitic plutons in the region. This phase of volcanism in SE China also coincided with extensive ignimbrite flare-up episodes and with the formations of nested caldera complexes above large, overlapping silicic magma chambers near the modern shorelines in SE China (for example, in Fujian (Qiu et al. Reference Qiu, Wang and Zhou1999); in Western Guangdong (Geng et al. Reference Geng, Xu, O’Reilly, Zhao and Sun2006); in Jiangxi (Yang et al. Reference Yang, Jiang, Jiang, Zhao and Fan2011); and in Hong Kong (Sewell et al. Reference Sewell, Tang and Campbell2012)). Thus, it appears that the Early Yanshanian magmatic phase in SE China culminated in a ‘surge of pulsed supereruptions’ during the latest Jurassic – Early Cretaceous (Sewell et al. Reference Sewell, Tang and Campbell2012, p. 15), reminiscent in scale of the Yellowstone Caldera collapses in the Western USA during the Quaternary (Vazquez & Reid, Reference Vazquez and Reid2002). The 97–88 Ma bimodal volcanic rock suites exhibit significant negative Nb anomalies and negative ε _Nd(T) values, suggesting that their magmas were derived from partial melting of an enriched lithospheric mantle source (Li, Reference Li2000; XY Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018).

Cretaceous volcanic rocks occurring further inland from the coastal regions are composed of tholeiitic and OIB-type basalts and minor alkaline rhyolites (YM Wu et al. Reference Wu, Guo, Wang, Zhang, Zhang, Alemayehu and Wang2020), interlayered with red sandstone, siltstone and mudstone, all deposited in NE–SW-oriented normal fault-bounded basins (Lapierre et al. Reference Lapierre, Jahn, Charvet and Yu1997; Y Wang et al. Reference Wang, Wang, Lia, Seagrenc, Zhang, Zhang and Qian2020). The geochemical and isotopic nature of these tholeiitic basalts suggests that their magmas were derived from a combination of aesthenospheric decompression melting and low-pressure, lower crustal melting in a continental setting (Fig. 12c). The 90 Ma Ji’an basalts in central Jiangxi Province have OIB-like geochemical signatures and display Sr–Nd–Pb–Hf isotopic compositions characteristic of a depleted aesthenospheric mantle, which was enriched by melts derived from dehydration of a subducted oceanic crust (Wu et al. Reference Wu, Guo, Wang, Zhang, Zhang, Alemayehu and Wang2020).

Syn-volcanic Cretaceous sedimentary rocks represent fluvial to lacustrine deposits, formed in a broad intracontinental rift system. This regional extensional structure represents the East China Rift System (Fig. 3; Tian et al. Reference Tian, Han and Xu1992). Cretaceous plutonic rocks and their volcanic counterparts within the East China Rift System represent the products of magmas, which were derived from partial melting of a subduction-metasomatized SCLM and which were variously affected by crustal contamination. NE–SW-running 101–93 Ma doleritic dyke swarms on Hainan reflect a discrete pulse of hypabyssal mafic magmatism during the Late Cretaceous, consistent with the geometry and kinematic evolution of the East China Rift System. Cretaceous mafic dyke intrusions with a predominant NE–SW orientation and age clusters of ~140 Ma, ~105 Ma and ~90 Ma also occur in Guangdong and Fujian, and display tholeiitic geochemical affinities (Li & McCulloch, 1998). These extensive NE–SW-striking doleritic dyke swarms near the coastal areas and the NE–SW-running East China Rift System point to a NW–SE-oriented tensile stress regime and the associated extensional deformation phase in SE China during the Cretaceous.

In summary, rifting, crustal subsidence, extensional doming and exhumation were widespread in the South China Block, particularly within Cathaysia during the Cretaceous (Figs 2, 3). The timing of the NW–SE-directed extension in Cathaysia has been constrained as 136–80 Ma based on thermochronological studies (Li, Reference Li2000; Zhou & Li, Reference Zhou and Li2000; Li et al. Reference Li, Zhang, Dong and Johnston2014). The results of these studies also indicate that the Cretaceous extensional tectonics and its attendant magmatism in Cathaysia migrated to the SE (oceanward in the present coordinate system) through time (Li, Reference Li2000; Li J et al. Reference Li, Zhang, Dong and Johnston2014, Reference Li, Dong, Zhang, Zhao, Johnston, Cui and Xin2016; Yang et al. Reference Yang, Wang, Ye, Li, He, Siebel and Chen2017; Wang et al. Reference Wang, Zhao, Huang, Xing and Wang2018, Reference Wang, Wang, Lia, Seagrenc, Zhang, Zhang and Qian2020).

7.a. Onset of active margin tectonics, flat-slab subduction and upper plate deformation

The Late Palaeozoic (Carboniferous to the earliest Permian) tectonics of E-SE China and its continental margin was largely dominated by extensive carbonate platform construction and passive margin development (FQ Zhang et al. Reference Zhang, Dilek, Zhang, Chen, Zhu and Hao2020, and references therein). This geological process was abruptly halted as the palaeo-Pacific oceanic plate foundered and began to subduct beneath continental Asia towards the end of the Palaeozoic (Fig. 13A). Passive margin transference into an active margin implies spontaneous subduction, but what causes this sudden passive margin collapse is not well understood, and the concept of subduction initiation is the subject of extensive interdisciplinary research (see Stern & Gerya, Reference Stern and Gerya2018, for an overview). Far-field stress influence (e.g. collision between the Indochina and Shibumasu blocks and the terminal closure of Palaeotethys), sudden acceleration of the palaeo-Pacific oceanic plate velocity against the Asian continent, and/or a collapse of an old fracture zone system near the Asian continental margin might have played a major role in spontaneous subduction initiation beneath the SE Asian continental margin at this time. Our study does not allow us to determine unequivocally the cause(s) of subduction initiation beneath East Asia in the latest Palaeozoic – early Mesozoic.

The research done by other geoscientists has shown, however, that active margin tectonics along the SE periphery of continental Asia persisted throughout the Mesozoic Era with changing slab geometry and dip angles (Zhou & Li, Reference Zhou and Li2000; XH Liet al. 2006, 2012; Zhou et al. Reference Zhou, Sun, Shen, Shu and Niu2006; Li & Li, Reference Li and Li2007; ZX Li et al. Reference Li, Li, Chung, Lo, Xu and Li2012; Zhu et al. Reference Zhu, Li, Xu and Wilde2013, Reference Zhu, Li, Xu and Wilde2014, Reference Zhu, Li, Xia, Xu, Wilde and Chen2017; JH Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018). This convergent margin tectonics produced an intermittently active magmatic arc (ZX Li et al. Reference Li, Li, Chung, Lo, Xu and Li2012; Duan et al. Reference Duan, Meng, Christie-Blick and Wu2018; JH Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018) and a retroarc foreland basin system with a protracted, complex tectonic history of both extensional and contractional deformation history (Li & Li, Reference Li and Li2007; XH Li et al. Reference Li, Li, Chung, Lo, Xu and Li2012; ZX Li et al. Reference Li, Li, Chung, Lo, Xu and Li2012, Reference Li, Zhang, Dong and Johnston2014; Pang et al. Reference Pang, Krapež, Li, Xu, Liu and Cao2014). Permian magmatic arc products are not exposed extensively in SE China, except on Hainan Island where Guadalupian calc-alkaline granites (Wuzhishan granites; et al. 2006) exist, and in Anhui Province where Guadalupian volcanic tuff deposits with calc-alkaline geochemistry are well preserved in the Permian sedimentary record (FQ Zhang et al. Reference Zhang, Dilek, Zhang, Chen, Zhu and Hao2020). However, the widespread occurrence of Permian detrital zircons in the Upper Permian through Jurassic sedimentary rock sequences in SE China has been interpreted to have sourced from a magmatic arc in the region (XH Li et al. Reference Li, Li, Chung, Lo, Xu and Li2012), including the proto-Japan arc (Duan et al. Reference Duan, Meng, Christie-Blick and Wu2018; Hara et al. Reference Hara, Hirano, Kurihara, Takahashi and Ueda2018; X Zhang et al. Reference Zhang, Wu, Zhu, Zhang, Xing, Chen, Chen and Zhang2018; Wakita et al. Reference Wakita, Nakagawa, Sakata, Tanaka and Oyama2020).

We infer that an initially large width of the subducting palaeo-Pacific plate against the entire coast of East Asia resulted in flat subduction and resisted slab retreat (Fig. 14a). A global synthesis and three-dimensional modelling of 17 modern subduction zones and their kinematics suggest that the trench-parallel extent of a subducted slab (its width) may vary between 300 and 7000 km, and that it affects the slab dip angle and subduction partioning along a trench more than the age of the downgoing oceanic lithosphere (Schellart et al. Reference Schellart, Stegman, Farrington, Freeman and Moresi2010; Schellart, Reference Schellart2020). Very large widths of downgoing oceanic plates (8000 to >10 000 km) may resist slab retreat, have fast trench-normal subduction plate velocity and be pushed over by the continental upper plate, causing flat subduction. Conversely, small widths (~1500 km or less) result in slab-buoyancy-driven trench retreat, slow trench-normal subduction plate velocity (and small subduction partitioning – <0.5), and slab steepening with rollback (Schellart et al. Reference Schellart, Freeman, Stegman, Moresi and May2007; Schellart, Reference Schellart2020). The former configuration with flat slab leads into significant crustal shortening in the upper continental plate, as was the case in western North America during the Late Cretaceous – Early Eocene Sevier–Laramide orogeny due to the rapid Farallon plate motion caused by its enormous slab width (>10 000 km; Schellart et al. Reference Schellart, Stegman, Farrington, Freeman and Moresi2010). Subsequent segmentation of the Farallon plate into a series of smaller plates, such as the Juan de Fuca plate, with significantly reduced widths created slab rollback and upper plate extension during the Neogene (Mooney & Kaban, Reference Mooney and Kaban2010; YJ Wang et al. Reference Wang, Fan, Zhang and Zhang2013; Schmandt & Lin, Reference Schmandt and Lin2014).

Fig 14. Geodynamic interpretation of subduction-related magmatism and its migration through time and space as a result of the slab geometry of the palaeo-Pacific (Izanagi) oceanic plate and active margin tectonics in East Asia. Broad time windows, keyed into those in Figure 13, include Early to Middle Triassic (250–240 Ma), Jurassic (180–150 Ma) and Albian–Cenomanian (110–90 Ma) in these inferred reconstructions. Outlines of the Japanese archipelago, Sakhalin Island and the Kamchatka Peninsula are provided as reference in these reconstructions. The colour difference on land: buff colour = the Asian continent bounded by the modern shorelines; yellow colour = inferred continental crust along the active continental margin of Asia during Mesozoic times. Lines I and II refer to the tectonic profiles (shown below) across SE China and NE China – SE Russia, respectively. Distributions of major magmatic provinces with their characteristic geochemical affinities are depicted schematically in each reconstruction. QDS – Qinling–Dabie Suture; SZ – Sulu Suture Zone. Major sinistral fault systems that developed inboard of the magmatic arc system during the Late Cretaceous are shown tentatively: KTSF – Korea – Taiwan Strait Fault; TLF – TanLu Fault Zone; YKF – Yongdong–Kwangju Fault. Large grey arrows show the convergence direction of the palaeo–Pacific (Izanagi) oceanic plate with respect to Eurasia. Data are from Faure & Natal’in (Reference Faure and Natal’in1992), Maruyama et al. (Reference Maruyama, Isozaki, Kimura and Terabayashi1997), Lee (Reference Lee1999), WD Sun et al. (Reference Sun, Ding, Hu and Li2007), Hall (Reference Hall2012), ZX Li et al. (Reference Li, Li, Chung, Lo, Xu and Li2012), Wang & Shu (Reference Wang and Shu2012), Egawa (Reference Egawa and Itoh2013), Zahirovic et al. (Reference Zahirovic, Seton and Müller2014), Li & Li (Reference Li and Li2015), YQ Liu et al. (Reference Liu, Kuang, Peng, Xu, Zhang, Wang and An2015), SF Liu et al. (Reference Liu, Gurnis, Ma and Zhang2017), Tang et al. (Reference Tang, Xu, Wang and Ge2018), Zhao et al. (Reference Zhao, Yu, Jiang, Mao, Yu, Chen and Xing2019), Y Zheng et al. (Reference Zheng, Xu, Zhao and Dai2018) and Wakita (2020). See text for discussion of the model.

Flat subduction of the palaeo-Pacific plate beneath SE China resulted in push-over tectonics (v₄ < v₃ < v₂ in Fig. 13a), characterized by slow retreat of the subduction hinge and rapid advancement of the continent over the trench. This plate interaction dynamics resulted in strong coupling between the two plates (Fig. 13a). The push-over geodynamic scenario caused landward-migrating shortening in the overlying continent and major crustal shortening (>300–km wide in the N–S direction) that manifested in the formation of the SE China Fold and Thrust Belt (Fig. 3; JH Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018). As this flat subduction continued through time, thrust faulting and folding migrated inland (NW) until ~195 Ma. Flat subduction-induced edge flow and aesthenospheric uprising (Ballmer et al. Reference Ballmer, Conrad, Smith and Johnsen2015) led to enriched melts (EMII-type) and the necessary heat to cause partial melting of the lherzolitic subcontinental lithospheric mantle (Fig. 10) and the lower crust to produce high-K granites, syenites and mafic rocks (Fig. 12).

7.b. Slab foundering, trench retreat and upper plate extension

The flat geometry of the subducting palaeo-Pacific oceanic lithosphere for ~50 Ma during the Triassic led to its foundering and delamination, which in turn facilitated the initiation of slab-buoyancy-driven trench retreat and slab steepening in the Early Jurassic (Figs 13b, 14b). Accelerated subduction rate and subduction hinge retreat caused steepening of the downgoing slab, its rapid rollback, and slab segmentation (Fig. 13b). Segmentation of the subducting palaeo-Pacific slab resulted in a major decrease in slab width, which in turn led to pull-away subduction dynamics, causing strong decoupling of the upper and lower plates, and then lithospheric-scale extension and magmatism in the upper plate during the Cretaceous (Figs 13c, 14c). Aesthenospheric upwelling, replacing the delaminated lower plate, caused metasomatism of the continental mantle lithosphere by OIB-type melts. Partial melting of this modified mantle and the continental crust produced A- and I-type granitoids, syenites with OIB-like geochemical features, and both alkaline and bimodal volcanic rocks (Fig. 11; Yang et al. Reference Yang, Dilek, Weinburg and Liu2018; W Zhang et al. Reference Zhang, Wu, Zhu, Zhang, Xing, Chen, Chen and Zhang2018; X Zhao et al. Reference Zhao, Yu, Jiang, Mao, Yu, Chen and Xing2019; this study). Slab-driven trench retreat and slab rollback resulted in upper plate extension, which produced NE–SW-trending, fault-bounded depocentres (grabens), fault-block ranges and metamorphic core complexes (Figs 12, 13b; Lin et al. Reference Lin, Faure, Monié, Schärer and Panis2008; Wang & Shu, Reference Wang and Shu2012; Li & Li, Reference Li and Li2015; JH Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018; Wang et al. Reference Wang, Wang, Lia, Seagrenc, Zhang, Zhang and Qian2020). Late Jurassic – Early Cretaceous SE China may have hence resembled the Cenozoic Basin and Range Province in Western North America (Dilek & Moores, Reference Dilek and Moores1999; XH Li, Reference Li2000; D Wang & Shu, Reference Wang and Shu2012) (see Section 8 for brief discussion of the Basin and Range tectonics and its analogy for Late Jurassic – Early Cretaceous SE China).

7.c. Slab segmentation and steepening, plate decoupling and continental back-arc extension

It has been well demonstrated that changes in rollback velocities along the strike of a subduction zone commonly lead to tearing and segmentation of the subducting lithospheric slab (Govers & Wortel, Reference Govers and Wortel2005; Schellart et al. Reference Schellart, Freeman, Stegman, Moresi and May2007; Rosenbaum et al. Reference Rosenbaum, Gasparon, Lucente, Peccerillo and Miller2008; Dilek & Altunkaynak, Reference Dilek, Altunkaynak, van Hinsbergen, Edwards and Govers2009). We think that the velocity of subduction rollback along the length of the palaeo-Pacific subduction system beneath the Asian continent became varied in time during the Jurassic, resulting in the development of sharp, hairpin-type cusps along its length (Maruyama et al. Reference Maruyama, Isozaki, Kimura and Terabayashi1997; Li & van der Hilst, Reference Li and van der Hilst2010; Hall, Reference Hall2012; Müller et al. Reference Müller, Seton, Zahirovic, Williams, Matthews, Wright, Shephard, Maloney, Barnett-Moore, Hosseinpour, Bower and Cannon2016; SF Liu et al. Reference Liu, Gurnis, Ma and Zhang2017; Zhou & Li, Reference Zhou and Li2017). Furthermore, the arrival of a large oceanic plateau at the palaeo-Pacific subduction trench, such as the Permian Akasaka–Kuzuu Seamount Province (Maruyama et al. Reference Maruyama, Isozaki, Kimura and Terabayashi1997), likely resulted in significant changes in along-strike slab subduction velocities and in slab tearing (Fig. 14c). Accreted fragments of this seamount province occur in the Kuzu area of Tochigi Prefecture in the central part of Honshu Island in Japan (Kobayashi, Reference Kobayashi2006). These developments in the palaeo-Pacific subduction dynamics caused slab segmentation and hence decreased slab width. The combination of decreased slab width, continued slab rollback and a more steeply dipping subduction zone in the Cretaceous produced an extended arc-trench and back-arc system in the upper plate (Figs 13c, 14c; Malinovsky et al. Reference Malinovsky, Golozoubov, Simanenko and Simanenko2008; J Deng et al. Reference Deng, Liu, Wang, Dilek and Liang2017; JH Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018; XY Li et al. Reference Li, Li, Suo, Dai, Guo, Ge and Lin2018, Reference Li, Li, Huang, Wang, Yu, Cao and Xie2019).

The southeastward-younging ages of the Cretaceous calc-alkaline rocks in E-SE China are consistent with this slab rollback interpretation. Slab dehydration triggered hydrous melting, which generated arc-like rocks in a magmatic belt close to the coastline (Figs 13c, 14c; Zhou et al. Reference Zhou, Liang, Kröner, Cai, Shao, Wen, Jiang, Fu, Wang and Dong2015). Aesthenospheric decompression melting and low-pressure lower crustal melting beneath the trench-distal back-arc setting formed crustally contaminated MORB-like tholeiitic basalts in an intracontinental extensional system (Fig. 12c; Sun et al. Reference Sun, Chen, Milan, Wilde, Jourdan and Xu2018; Tang et al. Reference Tang, Xu, Wang and Ge2018).

8.a. Basin and Range tectonics of the Western US Cordillera

The Basin and Range geological province in western North America extends for ~2500 km from Canada in the north, through the Great Basin in the USA, to central Mexico in the south, and consists of NNW–SSE-oriented, fault-controlled basins (grabens and half-grabens) and mountain ranges (mostly horst blocks). The modern Great Basin is bounded to the west by the Sierra Nevada batholith and mountains in California and the Wasatch mountain range in Utah to the east, and by the Snake River Plain and the Mojave Block in the north and south, respectively. It is a unique physiographic province with a >600 km E–W width, mean elevation of 1500 m a.s.l. , average crustal thickness of 30 km and a closed, internal drainage system (Dilek & Moores, Reference Dilek and Moores1999). The temperature within its crust and upper mantle is higher than normal, as indicated by the high heat flow and the low seismic velocities in the underlying mantle.

The Basin and Range province behind (or to the east of) the Sierra Nevada magmatic arc was a high plateau with a thick crust (~60 km), supporting high topography (~3 km a.s.l.) in the Late Cretaceous – Palaeogene, after the Cordilleran orogeny (Dilek & Moores, Reference Dilek and Moores1999; Liu, Reference Liu2001; Best et al. Reference Best, Barr, Christiansen, Gromme, Deino and Tingey2009). Saline lakes were developed on this altiplano in the Eocene, reminiscent of today’s Tibetan plateau north of the Himalaya Mountains (Dilek & Moores, Reference Dilek and Moores1999). The Late Cretaceous – Palaeogene Laramide magmatism, which was associated with the flat Farallon slab, migrated eastward across the Great Basin (Dickinson, Reference Dickinson2006). However, by the Late Eocene – Oligocene the unusually thick crust and high topography of the Great Basin became unstable as the excess gravitational potential energy, thermal relaxation and enhanced radiogenic heating caused partial melting in the middle to lower crust that in turn resulted in the onset of orogenic collapse and extensional deformation (Liu, Reference Liu2001). This orogenic collapse also overlapped in time and space with re-migration of magmatism westwards across the Great Basin and towards the continental margin, as the segmented Farallon plate started dipping more steeply below the North American continent (Atwater & Stock, Reference Atwater and Stock1998; Dickinson, Reference Dickinson2006; Schellart et al. Reference Schellart, Stegman, Farrington, Freeman and Moresi2010; Wang et al. Reference Wang, Wu, Zhang, Fan, Zhang, Zhang, Peng and Yin2012). Aesthenospheric upwelling associated with the steeply dipping and fast-retreating, narrow Farallon slabs provided the necessary heat source for extensive crustal melting and induced thermal weakening, which collectively led to significant extensional deformation, crustal exhumation, and denudation of crystalline basement rocks forming metamorphic core complexes in the Great Basin during the Oligo-Miocene (Dilek & Moores, Reference Dilek and Moores1999; Dickinson, Reference Dickinson2006).

The Late Tertiary metamorphic core complexes in western North America are situated in the hinterland of the Late Jurassic – Early Eocene Sevier fold-and-thrust belt (Taylor et al. Reference Taylor, Bartley, Martin, Geissman, Walker, Armstrong and Fryxell2000), and stretch from the British Columbia (Canada) latitude in the north, through the Great Basin in the USA, to the Sonora Desert in northern Mexico. They are characterized by dome-shaped, highly deformed metamorphic and magmatic rocks in the footwalls of low-angle detachment faults, overlain by undeformed, terrestrial clastic rocks deposited in supradetachment basins and structural grabens. The exhumation of these metamorphic core complexes was synchronous with widespread outburst of commonly caldera-centred ignimbrite eruptions (Dilek & Moores, Reference Dilek and Moores1999). The ‘ignimbrite flare-up’ episode was coeval with the emplacement of metaluminous to mildly peraluminious granitoid plutons at depth and voluminous eruptions of rhyolitic, rhyodacitic ash-flow tuffs and high-K dacitic to andesitic lava flows (Best et al. Reference Best, Barr, Christiansen, Gromme, Deino and Tingey2009; Henry & John, Reference Henry and John2013). This volcanic phase was temporally associated with thin-skinned extensional deformation across the Great Basin that produced closely spaced, high-angle and shallow listric normal faults, and fault-bounded and strongly tilted stratigraphic–volcanic sequences (Proffett, Reference Proffett1977; Lipman, Reference Lipman1984; Johnson et al. Reference Johnson, Shannon, Fridrich, Chapin and Zidek1989; Bachl et al. Reference Bachl, Miller, Miller and Faulds2001). All these tectonic and magmatic events were largely a result of back-arc extension in the western North American Cordillera during the Mid-Cenozoic.

A second phase of the Basin and Range extension started around 18–16 Ma and developed block-faulting, regional subsidence and bimodal basaltic–rhyolitic volcanism (Dilek & Moores, Reference Dilek and Moores1999; Best et al. Reference Best, Barr, Christiansen, Gromme, Deino and Tingey2009). Eruption of alkaline basaltic lavas during this phase indicates the involvement of SCLM in the melt evolution. Strike-slip faulting in the western end of the Great Basin later in the Miocene shows that the mode of extensional deformation was largely controlled by transtension, rather than back-arc extension, as the San Andreas transform fault system replaced the active margin tectonics along the Pacific – North American plates (Atwater & Stock, Reference Atwater and Stock1998). Strain partitioning along and across the San Andreas transform fault system produced a series of NNW–SSE-striking, dextral transtensional fault systems (i.e. the Walker Lane Shear Zone), which have been accommodating ~25–15% of the Pacific – North American plate motion (Faulds et al. Reference Faulds, Henry and Hinz2005) and facilitating the N-NW motion of the Sierran – Great Valley block with respect to the Colorado Plateau (fixed North American plate) at an average rate of 15 mm a⁻¹ since 10–8 Ma (Wernicke & Snow, Reference Wernicke and Snow1998).

8.b. Basin and Range analogue for Late Jurassic – Cretaceous SE China

There are many similarities between the Cenozoic geology and tectonics of the Western US Cordillera and the Late Jurassic – Cretaceous geology and tectonics of SE China, suggesting that similar geodynamic processes may have controlled their continental dynamics, specifically their tectonomagmatic and landscape evolution. Here, we draw some first-order parallels between these two continental regions, regarding the nature of major geological events and their consequences in a relative time frame.

1. (1) Both the Western US Cordillera and SE China experienced flat-slab subduction of oceanic plates against their continental margins that resulted in contractional deformation, which migrated inland through time.

2. (2) The onset of the following extensional deformation and attendant magmatism in both continental regions was associated with more steeply dipping and retreating oceanic slabs. It is inferred that this slab dynamics was a result of slab segmentation and hence major reduction in the widths of subducting slabs. Coeval magmatism migrated across the continental upper plates, towards the active margins, and produced bimodal volcanic rocks and granitoid intrusions, some of which resulted from partial melting of continental lower crust. This magmatism caused thermal weakening of the continental crust.

3. (3) Thermally weakened continental crust in the back-arc tectonic setting in both the Western US Cordillera and SE China subsequently underwent crustal exhumation and denudation of crystalline basement rocks. This process led to the development of metamorphic core complexes in the footwalls of low-angle detachment faults, which were onlapped by syn-extensional and terrestrial, supradetachment depocentres. This tectonic episode was also synchronous with the development of playa lakes within basins, separated by ranges, representing uplifted fault blocks. The landscape evolution in both regions was a manifestation of thin-skinned tectonics, with extensive alluvial fan system and internal drainage formation, and deposition of evaporites and lacustrine sediments.

4. (4) Spatially and temporally overlapped with metamorphic core complex formation was widespread eruption of rhyolitic, dacitic and andesitic tuffs and production of ignimbrites and ashfall deposits. In both SE China and in the Great Basin of the Western US Cordillera this unique magmatic event is known as ‘ignimbrite flare-up’, which was associated with the emplacement of shallow-level granitic plutons.

5. (5) Later-stage extensional deformation involved block-faulting, regional subsidence and bimodal (basaltic–rhyolitic) volcanism in both SE China and the Great Basin. Basaltic magmatism was a result of aesthenospheric decompression melting beneath significantly thinned continental lithosphere, and was locally fed by mafic dyke swarms, which were emplaced perpendicular to the regional tensile stress regimes.

6. (6) With the convergence direction of the subducting plates becoming more oblique with respect to the active margins of E-SE China and the Western US Cordillera through time, oblique subduction and the associated strain partitioning in the upper continental plates resulted in transtensional deformation. Major strike-slip fault systems developed, such as the Tanlu Fault in East China (Y Zhang et al. Reference Zhang, Dilek, Zhang, Chen, Zhu and Hao2020) and the Walker Lane shear zone in western Nevada and eastern California (Faulds et al. Reference Faulds, Henry and Hinz2005), which strongly affected the crustal architecture of the continental margins in both regions.

This comparative summary shows that the time-progressive tectonic, magmatic and landscape evolution of Mesozoic SE China and the Cenozoic Western US Cordillera followed similar tectonic paths through time while undergoing similar geodynamic events. Although these two regions had significantly different geodynamic histories prior to the onset of flat-slab subduction along their continental margins, their SCLM and continental crust responded to the subducting slab dynamics and changing slab geometries in similar ways.