Imbricate structure of the Luobusa Ophiolite and surrounding rock units, southern Tibet Hiroshi Yamamoto a, * , Shinji Yamamoto b , Yoshiyuki Kaneko c , Masaru Terabayashi d , Tsuyoshi Komiya b , Ikuo Katayama b,1 , Tsuyoshi Iizuka b a Department of Earth and Environmental Sciences, Kagoshima University, Kagoshima 890-0065, Japan b Department of Earth and Planetary Science, Tokyo Institute of Technology, Meguro, Tokyo 152-8851, Japan c Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan d Department of Safety Systems Construction Engineering, Kagawa University, Takamatsu 761-0396, Japan Received 4 January 2005; accepted 5 April 2006 Abstract The Cretaceous Luobusa Ophiolite is a tectonic slice less than 1.2 km thick. The structurally underlying unit is the Tertiary Luobusa Formation and the overlying unit is composed of feebly metamorphosed Triassic sedimentary rocks. The top and bottom contacts of the ophiolite dip gently to the south. The major part of the Luobusa Ophiolite is mantle peridotite, including podiform chromitite. This chro- mitite has received much attention because it contains an ‘unusual mineral assemblage’ which includes diamond and moissanite. A ser- pentinite me ´lange zone, including clasts of basaltic and sedimentary rocks, occurs underneath the mantle peridotite. Mesoscopic– microscopic structures were observed in the sheared rocks. Shear-indicating structures (C 0 -type shear bands and r-type porphyroclasts) in the rocks near the top and bottom boundaries of the Luobusa Ophiolite show consistent top-to-the-north (or northeast) reverse dis- placement. The results reveal that the Luobusa Ophiolite was overturned and intercalated into an imbricate structure. The thrust faults on the top and bottom of the ophiolite can be correlated with north-vergent back-thrusting which was associated with crustal shortening along the Main Central Thrust due to the continued northward movement of India, after the welding of India to Asia. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ophiolite; Emplacement; Structure; Shear zone; Tibet 1. Introduction The Indus-Yarlung Zangbo suture zone (IYSZ) sepa- rates the Indian subcontinent from the Lhasa Terrane (e.g. Alle `gre et al., 1984). Ophiolite sequences of Creta- ceous age lie along the IYSZ, and radiolarian chert gives an age of 120 ± 10 Ma (Ziabrev et al., 2003). A large ophiolite sequence, called the Luobusa Ophiolite is locat- ed in southern Tibet, about 200 km east-southeast of Lhasa. The ophiolite extends 43 km east-west, and its total extent is about 70 km 2 (Zhou et al., 1996). The Luo- busa Ophiolite is composed mainly of mantle peridotite (harzburgite), a me ´lange zone and an intercalated transi- tion zone. Podiform bodies of chromitite are sporadically distributed in the harzburgite (Zhou et al., 1996). The podiform chromitite has received much attention because it contains an ‘unusual mineral assemblage’ including ultra-high pressure (UHP) minerals such as diamond and moissanite, as well as a number of reduced phases such as graphite, chromium, gold, nickel, copper and sil- icon (Bai et al., 1993, 2002; Yamamoto et al., 2003). Pre- vious studies of the Luobusa Ophiolite have concentrated on the petrology and mineralogy of the mantle peridotite. However, the mechanism by which the mantle peridotite was emplaced has not been made clear. The difficulty in tectonic interpretation arises from the limited amount of structural data. In this study we analyzed deformation 1367-9120/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2006.04.004 * Corresponding author. Tel.: +81 99 285 8130; fax: +81 99 259 4720. E-mail address: hyam@sci.kagoshima-u.ac.jp (H. Yamamoto). 1 Present address: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511, USA. www.elsevier.com/locate/jaes Journal of Asian Earth Sciences 29 (2007) 296–304