Ulban Terrane (Zone) as Part of the Jurassic Accretionary Complex of the Sikhote-Alin Orogenic Belt

Abstract

The paper presents new data on the age of Triassic–Jurassic cherts replaced up section by siliceous mudstones, then by mudstones, which together with basalts form thin (up to hundreds of meters) plates and lenses that wedge out along strike at different levels of the multikilometer terrigenous section of the Ulban terrane (zone). These deposits in the Nimelen subzone (Albazino gold deposit area) formed in the Norian–Early Toarcian; host terrigenous deposits formed in the Pliensbachian–Bathonian. The age of cherts and siliceous–clayey rocks in the Mewachan subzone is Middle Triassic–Callovian, and that of host terrigenous rocks is Middle–Late Jurassic (including the Tithonian). Comparison of deposits of the Ulban terrane with well-studied deposits of the Samarka terrane similar in age and composition allows us to consider the Ulban terrane (zone) as part of the Jurassic accretionary complex of the Sikhote-Alin orogenic belt. There are also sufficient signs to consider that its structural features are consistent with the subduction model. This idea is based on the fact that Triassic–Jurassic cherts and siliceous–clayey rocks, which accumulated over about 45 Ma at a rate of about 1.8 m/Ma, are considered the sedimentary cover of the oceanic plate. At the same time, a gradual transition from cherts to mudstones is evidence of movement of the oceanic plate into the subduction zone. The occurrence of its fragments in the terrigenous section is the result of their detachment in the trench and under the overhanging slab. Tectonostratigraphic columns of the subzones of the Ulban terrane and a zonal scheme of the Jurassic accretionary wedge, including four structural-age levels, have been compiled.