名古屋大学博物館報告
Bull. Nagoya Univ. Museum
No. 28, 39–43, 2012

Description of a NW-trending brittle shear zone,Ulaanbaatar, Mongolia
Bakhat Nuramkhaan1), Yuki Nakane2), Shoko Yamasaki2),
Jamiyandagva Otgonbaatar1), Manchuk Nuramkhaan1),
Makoto Takeuchi2), Kazuhiro Tsukada3), Yusuke Katsurada3),
Sersmaa Gonchigdorj1) and Khishigsuren Sodnom1)
1) School of Geology and Petroleum Engineering, Mongolian University of Science and Technology, Ulaanbaatar,Mongolia2)	Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan
3) Nagoya University Museum, Nagoya 464-8601, Japan

Abstract
A fault trending NW-SE from central Ulaanbaatar to Nalaikh, Mongolia, has been inferred from a topographical viewpoint, however, its lithological and kinematical characteristics remain uncertain. This paper describes
the lithology and kinematics of this fault. The rocks of the Gorkhi Formation are intensely fractured to form abrittle shear zone sub-parallel to the topographical lineament from central Ulaanbaatar to Nalaikh. The fractured
rocks are partly foliated with vertical planes striking N 15˚ to 30˚W. The shear zone extends for ca. 40 km northfrom Nalaikh. Dragging fold and P−Y fabric of the fractured rocks show top-to-the south sense of shear.

Introduction
Although many studies on Paleozoic basement rocks in and around Ulaanbaatar, Mongolia, have been
carried out to discuss the tectonic development of the Central Asian Orogenic belt (e.g., Kashiwagi etal., 2004; Minjin et al., 2006; Kurihara et al., 2009), stratigraphy and geological structure of this area
have far only been published in a geological map sheet (Magic Project, 1998). In addition, a fault trending NW-SE from central Ulaanbaatar to Nalaikh has been inferred from a topographical viewpoint,
however, its lithology and kinematics remain uncertain and it has never been drawn in the previous geological map (e.g. Magic Project, 1998). A geological mapping project between Mongolian University of
Science and Technology and Nagoya University, Japan, has been carried out to reveal the detail stratigraphy and structure of the Gorkhi, Altan-Ovoo and Orgioch Formations in the Ulaanbaatar area since2009, and a brittle shear zone sub-parallel to the topographical lineament, which might be related to thetopographically-inferred fault, was discovered in the course of this project (Figs. 1, 2 and 3). The lithology and kinematics of the shear zone are described in this paper.
Geological outline of the Ulaanbaatar area
Paleozoic sandstone and mudstone with minor amounts of felsic tuff, chert, basalts lime stone andconglomerate are exposed in this area (Takeuchi et al., 2013). The Paleozoic rocks of this area is divided
into the Gorkhi, Altan-Ovoo and Orgioch Formations of the Devonian to Carboniferous (?) accretionary
complexes (e.g. Minjin et al., 2006; Kurihara et al., 2009; Takeuchi et al., 2013). The Paleozoic rocks are
intruded by Mesozoic Bogd-Khaan mountain granite.
In the area between the central Ulaanbaatar to Nalaikh, thick sandstone with minor chert lenses of theGorkhi Formation are exposed (Nakane et al., 2013; Suzuki et al., 2013; Takeuchi et al., 2013). The rocksof this area are cut by NW-SE trending and NE-SW trending vertical faults to form fault-bound blocks(Takeuchi et al., 2013).

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BayanzurkhFig. 1. Index map of the study area. The localities of the brecciated rocks are also shown with white stars. Dotted lines
denote NW-trending fault. The topographic images were from Google (2012). UB: central Ulaanbaatar.
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Fig. 2. Route map along the trail at Bayanzurkh.

Description of the brittle shear zone
A dissected straight valley of ca. 1 km wide is developed along the road from central Ulaanbaatar toNalaikh to form topographical lineament and clear terminal facets (Figs. 1 and 3a). The tops of the facets
are several tens meters high from the bottom of the valley. The foliated / non-foliated clastic rocks witha chert intercalation of the Gorkhi Formation are intensely fractured to form a brittle shear zone subparallel to this lineament (Figs. 1 and 3a). The fractured rocks, i.e. fault gouge and brecciated rocks, arewell-exposed along the trail at Bayanzurkh (Figs. 2 and 3). Sandstone and mudstone of the Gorkhi
Formation are brecciated and crushed for ca. 700 m wide (Figs. 2 and 3). The fractured rocks are unconsolidated. Open cracks are generally developed in the rocks, and some rocks include many clasts of
2 to 5 cm diameter in a finer matrix with random fabric to form fault gouge (Figs. 3b and c). The gouge
is foliated with a spacing of several mm to 10 cm in some places and the foliation strikes N 15˚ to 30˚ W
and vertical. Some minor vertical faults, 1 to 5 m wide, striking N 15˚ to 30˚ W are exposed along thetrail (Figs. 2 and 3d). The shear zone extends at least for ca. 40 km north from Nalaikh (Fig. 1). Intensely
fractured rocks of the chert and sandstone with foliation striking N 65˚ to 80˚ W and dipping 50˚ N to90˚ are exposed at ca. 20 km northwest of the trail (Fig. 1). The planes are folded and show composite
planer structures. The dragging fold and P−Y fabric of the fractured rock show top-to-the south sense ofshear (Figs. 1 and 3e). A fault gouge including many well-rounded red chert clasts in a finer siliseous
matrix around the Hoh torgoi is estimated to be a northern extension of this shear zone (Fig. 1).
Summary
1. The rocks of the Gorkhi Formation are intensely fractured to form a brittle shear zone sub-parallel tothe topographical lineament from central Ulaanbaatar to Nalaikh.
2. The shear zone extends at least for ca. 40 km north from Nalaikh.
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Fig. 3. Photographs showing field evidence of the brecciated rocks. a: Terminal facets along the road from central
Ulaanbaatar to Nalaikh. b and c: The fractured rocks at the brittle shear zone sub-parallel to the lineament. d: A
minor vertical fault along the trail. e: Dragging fold showing the top-to-the south sense of shear along a minor
fault at the Hoh torgoi. Perpendicular view.

3. A part of the fractured rocks are foliated with a spacing of several mm to 10 cm in some places andthe foliation strikes N 15 ˚ to 30˚ W and vertical.
4. Dragging fold and P−Y fabric of the fractured rocks show top-to-the south sense of shear.
Acknowledgments
We would like to thank Prof. Mamoru Adachi of Nagoya University for his help on this work. We express
our special thanks to Dr. Nobuhiko Sugito of the Graduate School of Environmental Studies, Nagoya
University, Japan and Dr. Enkhtaivan Dangaa of the Geography Institute of Mongolian Academy of Sciencefor their critical reading of the manuscript. This work was supported by Grant in Aid for scientific researchby the Ito Kagaku Shinko-kai and the Japan Society for the Promotion of Science (No. 20403013), Grant— 42 —

in Aid for JSPS Fellowship (No. 02087), and the Socho-Sairyo-Keihi of the Nagoya University, Japan.
References
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（2012 年 10 月 15 日受付，2013 年 1 月 28 日受理）

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