Article 79 by Inna Safonova1,2*, Shigenori Maruyama1,3, Nikolai Kruk1,4, Olga Obut1,4, Pavel Kotler1,2, Olga Gavryushkina1,2, Sergey Khromykh1,2, Maxim Kuibida1,2, and Sergey Krivonogov1,2 Pacific-type orogenic belts: linking evolution of oceans, active margins and intra-plate magmatism 1 Novosibirsk State University, Pirogova St. 2, Novosibirsk, 630090, Russia; *Corresponding author, E-mail: [email protected] 2 Sobolev Institute of Geology and Mineralogy SB RAS, Koptyuga ave. 3, Novosibirsk, 630090, Russia 3 Earth-Life Science Institute, Tokyo Institute of Technology, E-16, 2-12-1, Ookayama, Meguro, Tokyo 152-8550, Japan 4 Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Koptyuga ave. 3, Novosibirsk, 630090, Russia (Received: November 20, 2016; Revised accepted: January 15, 2018) http://dx.doi.org/10.18814/epiiugs/2018/018008 Pacific-type orogens (fold belts) hosting accretionary rated from real geology and therefore cannot be considered trust- complexes are places keeping records of the evolution of worthy. We propose a new conceptual model based on both real and paleo-oceans, and formation and transformation of con- “model” geologies. We suggest that indicative minerals, rocks and tinental crust at their active convergent margins. Pacific-type rock complexes, such as garnet and other minerals of deep-seated orogeny induces destruction of crustal materials, their xenoliths, alkaline basalt, carbonatite, etc. which are present on the subduction to the deep mantle, generation of hydrous- Earth’s surface and accessible for our study, are not the only source of carbonated plumes in the mantle transition zone (MTZ) information about deep mantle processes. The absence of certain rocks or complexes can also reflect past geological processes on the and its related intra-plate magmatism. We propose a new surface and their effect on the deep interior of our planet. approach for linking paleo-oceans, active margins and The popular model of mantle plumes and superplumes implies their intra-plate magmatism in central and eastern Asia. The generation at the core-mantle boundary (CMB), i.e., at a depth of approach “stands” on three “whales”: the model of Ocean 2900 km; those plumes are also often termed “dry” plumes (Maruyama, Plate Stratigraphy (OPS), the parameters of Pacific-type 1994; Maruyama et al., 2007). The superplumes result in voluminous, convergent margins and a model of hydrous-carbonated but typically, rather rapid basaltic eruptions (Reichow et al., 2009) plumes. The OPS model evolved from extensive studies of and in the formation of large igneous provinces, for example, Sibe- accretionary complexes in the western Pacific, in particular, rian traps or Ontong-Java. According to different evaluations, super- in Japan; it allows differentiating oceanic plates of one paleo- plume cycles can be from several tens to hundred million years ocean and evaluating their sizes and ages. An important issue (Maruyama et al., 2007; Li and Zhong, 2009; Nance and Murphy, for reconstructing the history of paleo-oceans is to estimate 2013). Therefore, the model of “dry” plumes cannot explain the numer- major parameters of the Pacific-type convergent margins: ous and almost ceaseless manifestations of intra-plate continental accreting vs. eroding, geometrical length, and life time. For the magmatism in central and eastern Asia since the amalgamation of the eroding margins we must define major periods of tectonic Laurasia supercontinent at ca. 300 Ma and up to the present time erosion and transportation of oceanic and continental (Kuzmin et al., 2010; Safonova and Maruyama, 2014; Yarmolyuk et materials to the deep mantle and evaluate a possibility of al., 2014). Recently, an alternative/complementary hypothesis of their accumulation in the MTZ. All this would allow us to hydrous-carbonated plumes generated in the MTZ, i.e., at 410–660 develop a holistic model linking the evolution of paleo- km, has been proposed in relation to the processes of subduction and accretion at Pacific-type convergent margins (Kawai et al., 2013; oceans, the accretion and erosion of oceanic and continen- Safonova et al., 2015). During Pacific-type orogeny, new continental crust tal crust materials at Pacific-type convergent margins, forms at magmatic arcs and the rocks of oceanic crust are incorpo- mantle metasomatism and intra-plate magmatism. rated into accretionary complexes; consequently, both continental and oceanic crust materials can be destructed and subducted to the deep mantle. In a wide sense this problem is a part of the problem of global Introduction material circulation from the mantle to the crust at divergent margins and from the crust back to the mantle at convergent margins. How- In recent decades, there has been done a big progress in experimen- ever, to turn that hypothesis into a model more evidence from geology, tal and numerical modeling of global material circulation and deep geochronology and petrology of Pacific-type convergent margins, geodynamics. Unfortunately, many of those models appeared sepa- their formation and evolution, mantle metasomatism and mantle Episodes Vol. 41, No. 2 80 plumes, is necessary. In this paper, we concisely propose a new During subduction OPS units get accreted to active margins (Fig. 3a) approach for developing a holistic model linking paleo-oceans, active to be later incorporated into accretionary complexes or subducted to margins and plume magmatism recorded in the Pacific-type orogenic the deep mantle. The OPS model can be used as a basis for the recon- belts of Central and East Asia. Such an approach has logically struction of paleo-oceans and processes of accretion and subduction at evolved from the results of our IGCP Project #592 “Continental con- their active margins, or, for example, it can provide solid evidence struction in Central Asia” (2012–2016; Safonova et al., 2011, 2013, about the direction of subduction by studying the geometry of duplex 2016a; Xiao et al., 2013). structures (Maruyama et al., 2010; Fig. 3b). In addition, the OPS model allows evaluating the size and age of a paleo-oceanic plate (Safonova et al., 2016a), which was subducted million years ago but its Pacific-type Convergent Margins and Subduction small pieces remained on the surface and are now accessible for obser- Zones: Definitions and Importance vation and study (for more details on the methodology see section 3 below). Pacific-type convergent margins (ocean-continent) and their related orogenic belts exist/form over subduction zones, which are the only Accretionary Complexes and Ocean Plate Stratigraphy ways to deliver surface materials to the deep mantle (Fig. 1). The parameters of paleo-oceans and old subduction zones are typically The fragments of oceanic crust or OPS units are important constitu- reconstructed by methods of paleomagnetism and paleontology (e.g., ents of Pacific-type orogenic belts (Isozaki et al., 1990; Maruyama et Scotese et al., 1994; Stampfli and Borel, 2002; Metcalfe, 2011), al., 2010; Kusky et al., 2013; Safonova et al., 2016b; Safonova, 2017). which results remain disputable though. A geology-based model of The subdivision and mapping of accretionary complexes have been Ocean Plate Stratigraphy (OPS) was proposed by Japanese scientists always very problematic due to their extremely complicated structure (Isozaki et al., 1990; Maruyama et al., 2010) and later developed by other (Fig. 3). Accretionary complexes consist of OPS units scrapped off researchers (e.g., Kusky et al., 2013; Safonova and Santosh, 2014; Safon- the subducting oceanic plate and of the material eroded from adjacent ova et al., 2016b). OPS is a regular succession of igneous (MORB, arcs. The OPS rocks are of special importance as they record the full mid-ocean ridge basalt; OIB, oceanic island basalt; gabbro-peridot- history of the oceanic plate from its “birth” at mid ocean ridges to its ite) and sedimentary (pelagic chert, hemipelagic siliceous sediments, “death” at subduction zones. Main OPS types are (1) sedimentary and trench turbidite, seamount carbonate “cap”, slope and foothill facies) igneous rocks of the oceanic floor: pelagic chert, MORB, gabbro, per- rocks of the oceanic lithosphere, that were, respectively, erupted and idotite; (2) hemipelagic oceanic slope and shelf sediments: siliceous deposited on the sea floor as the underlying oceanic basement trav- shale, mudstone, and siltstone, which accumulate closer to a trench by eled from the mid-oceanic ridge to the subduction zone (Fig. 2). transportation of fine-grained material eroded from accretionary Figure 1. A scheme of a Pacific-type convergent margin showing a typical intra-oceanic arc system with adjacent back-arc basin, major crustal and upper mantle components and major sites of melt generation. ① – at mid-oceanic ridges, ② – above the subducting slabs (modi- fied from Stern, 2010). June 2018 81 Figure 2. A principal scheme of Ocean Plate Stratigraphy (OPS) with seamounts (modified from Maruyama et al., 2010). Figure 3. A schematic model for the accretion of deep-sea pelagic sediments and formation of accretionary wedge/complex (a) and for the formation of duplex structures (b) based on the study of the Inuyama area, the OPS type locality, of the Mino terrane in SW Japan (b; Mat- suda and lsozaki, 1991). Episodes Vol. 41, No. 2 82 wedge and adjacent arcs; (3) trench fill turbiditic sandstone and con- glomerate (Kusky et al., 2013; Fig. 2). A special case is seamount OPS (oceanic islands, seamounts, plateaus), which includes OIB-type basalts, which may be capped by shallow-water carbonates (massive/ micritic/reefal limestone), carbonate-volcanogenic epiclastic slope facies and basal siliceous shale and chert (Safonova and Santosh, 2014; Yang et al., 2015). The accreted seamount OPS is of special importance as it can be preserved much more often during subduction, even at erod- ing active margins, unlike other types of OPS. Although most of the oceanic crust is subducted into the mantle, fragments of OPS may remain on the surface and appear accessible to our study. Convergent Margins Pacific-type convergent margins are places of major continental growth by island-arc juvenile magmatism and accretion (Fig.