[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-P10] Deformation structure of ocean plate stratigraphy in the Paleozoic Gorkhi Formation, northern Mongolia: Constraints on subduction tectonics in the Mongol-Okhotsk Ocean

*Tomohiko Sato¹, Kazumasa Aoki¹, Jargalsaikhan Batsukh², Khishigjav Tsogtbaatar² (1.Center for Fundamental Education, Okayama University of Science, 2.Institute of Paleontology, Mongolian Academy of Sciences)

Keywords:Central Asian Orogenic Belt, accretionary complex, oceanic plate stratigraphy, deformation structure

The Central Asian Orogenic Belt (CAOB) has grown through repeated collisions of island arcs, seamounts, and small continents in paleo-oceanic realm surrounded by the Siberia, North China, and Tarim cratons from the Paleozoic to the Mesozoic time, resulting in accretionary complexes (ACs) of different ages. The Silurian–Carboniferous AC, called the Gorkhi Formation, is distributed in the Ulaanbaatar belt of northern Mongolia in the central part of the CAOB. This formation consists mainly of sandstones, mudstones, cherts and basalts, and comprises tectonic thrust units that internally retain ocean plate stratigraphy (OPS) (e.g., Kurihara et al., 2009; Savinskiy et al., 2022). Five NE-SW trending units have been recognized, i.e. Unit 1 to 5 in ascending order from NW to SE. The direction of plate subduction at the time of the accretion of the Gorkhi Formation was estimated to be northwest, based on the stratigraphic top of the OPS (Savinskiy et al., 2022); however, data on local structural features were insufficient, and the tectonic development of this formation remains controversial.
In this study, we investigated 25 outcrops of the Gorkhi Formation in the Sergelen area of the Ulaanbaatar belt, and examined the stratigraphy and deformation features. As a result, a complete set of OPS, i.e. basalt, chert, mudstone, and sandstone from bottom to top, was recognized from 11 outcrops. Their layers are nearly vertical with the stratigraphic top pointing north with exceptions in Unit 3 showing a synclinal structure. In addition, it was found that there is no need to distinguish Unit 1 and 2, and Unit 4 and 5, based on the dip-strike pattern, degree of deformation and lithology (hereafter referred to as Unit 1-2 and Unit 4-5). Duplex structures of chert and sandstone with thrust planes dipping to north to west were identified in 3 outcrops. Folding structures of chert with the axis planes dipping slightly to north to west were also identified from 3 outcrops in the Unit 1-2. These observations suggest that the direction of subduction during the accretion of the Gorkhi Formation was basically north to northwest; this is consistent with the preliminary interpretations by previous studies. Observation of the most strongly deformed bedded cherts in the Unit 1-2 suggests that this unit was located closer to the front of the subduction zone at that time.
For the future study, we are planning to conduct U-Pb dating of sandstones in each geological unit in order to clarify the accretionary process with detailed temporal resolution.

[Reference]
Kurihara, et al., 2009, Journal of Asian Earth Sciences, 34, 209–225.
Savinskiy, et al., 2022, International Journal of Earth Sciences, 111, 2469-2492.