Mesozoic terranes of the northwest Pacific continental margin (Russia): Radiolarian ages and sedimentary environments

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Abstract
Geological mapping using detailed tectonic and complex radiolarian analysis revealed significant northward displacement of a number of Russian Far and Northeast Asia terranes. It was recorded that some terranes possibly crossed the equator. Terranes of north‐east Russia were composed of different allochthonous formations, ranging in age from Middle Triassic to Maestrichtian‐Paleocene and accumulated from the margin to oceanic basins. The Middle to Upper Triassic interval included two formations: (i) volcanogenic, consisting of typical volcanic rocks of the island arcs (up to 800 m thick); and (ii) a chert‐limestone‐terrigenous one composed of marginal sandstone, siltstone, limestone and tuffic chert (about 400 m). Lower Jurassic allochthonous formations are represented by chert‐terrigenous (about 300 m) and jasper‐alkaline‐basaltic (WPB‐type) seamount deposits (about 100 m). Middle Jurassic to Hauterivian allochthonous terranes from the northern part of the Koryak‐Kamchatka region include five formations: jasper (bedding jaspers with condensed limestone lenses with Buchias, 80 m), jasper‐basalt (with MORB, 100‐150 m), ferrotitanic basalt (WPB with lenses of jasper mainly composed of genus Parvicingula, about 75%, 150 m), terrigenous‐volcanic (with MORB, IAT, CA basalts and olistostrome, 600 m), tuffic‐jasper‐basalt (MORB and deposits of arc‐trench system, about 500 m) with the same age according to radiolarian data. Aptian? Albian‐Maestrichtian ones are predominantly terrigenous‐tuffaceous‐siliceous. Moreover, the Early and Middle Jurassic faunas of the northwest Pacific margin contain many boreal elements similar to those of New Zealand (Southern Hemisphere), Japan, ODP Site 801. The Late Jurassic faunas of the Koryak and Kamchatka region are mainly North Tethyan and seldom Central Tethyan and are very closely related to those of the Americas. The Tithonian to Early Cretaceous radiolarian are predominantly Central Tethyan and Equatorial in contrast to Boreal Late Cretaceous. The combining in the same region at 60°N Pacific margin of the formations accumulated in different tectonic paleoenvironments and paleoclimatic provinces, is good evidence for the possible significant northward displacement of some terranes in the northwestern Pacific.