Bainang Terrane, Yarlung-Tsangpo suture, southern Tibet (Xizang, China): a record of intra-Neotethyan subduction-accretion processes preserved on the roof of the world

Journal of the Geological Society, May 2004 by Ziabrev, Sergey V, Aitchison, Jonathan C, Abrajevitch, Alexandra V, Et al

The Yalongmai unit is characterized by repetitions of intensely sheared and foliated tectonic slices composed of different lithologies. Bedding and tectonic layering dip in a manner similar to that seen in other units. Tectonic layering is subparallel to bedding. Shearing patterns arc lithologically controlled. Some chert is tectonically dissociated into lenses, assemblages of which are mappable over several kilometres. Cherts exhibit S-C or phacoidal fabrics and are rarely foliated whereas foliation in calcareous shales is typically penetrative. Foliation becomes more pervasive to the SE. Stretching lineation, crcnulation and kink bands are also associated with foliation. Lineation is manifested by fine penetrative mineral fibres. Sporadically developed crenulation occurs on some foliation surfaces as small patches of closely spaced wrinkles. Kink bands arc more abundant in the SE part of the unit and arc locally arranged into swarms of conjugate bands. Stretching lineations are deformed by both crenulation and kink bands. In each subunit they plunge gently to moderately to the north and are strongly grouped around mean values of 352731� and 352738� (Fig. 3). Crenulation crests are almost horizontal and their orientations vary from west to SSW with mean values of 244705� and 250704� for each subunit. Kink band axes arc oriented WSW and arc nearly horizontal with close mean values 255705� and 253704� in both subunits. Orientations of all linear structural elements indicate NNW-SSE transport. Rare secondary fine-scale asymmetric cleavage associated with crenulation indicates south-directed thrusting. Small-scale S and Z intrafolial folds (more common in the Chiangdui subunit) deform both bedding and tectonic layering. Stretching lineation and crenulation are warped in one of these folds. Morphologies and hinge orientations of the folds indicate south-directed thrusting with components of either sinistral or dextral strike slip.

Renchingang unit. This southernmost and structurally lowermost, 1750 m wide unit is oriented ENE-WSW, parallel to the Baigang subunit, and is thrust southwards over the Indian terrane. Yellowish grey calcareous shale characterizes the unit. In the north, it contains lenticular fragments of normally graded yellowish grey calcarenites. The unit also incorporates lithologies characteristic of the Yalongmai unit, such as varicoloured calcareous shale and red radiolarian chert, which occur in metre-thick tectonic lenses and slices disseminated within calcareous shale. Chert is thinbedded and includes layers (1-3 cm) of pink to red micritic limestone as well as finely laminated chert and limestone, amounting to c. 25% of the succession. The presence of thin pelagic limestones intercalated with chert indicates sedimentation above or near the CCD.

The southern part of the unit consists of structurally homogeneous shales. To the north, thin slices of different lithologies are tectonically repeated. Penetrative foliation accompanied by stretching lineation characterizes the unit. Abundant kink bands deforming foliation and stretching lineation occur in swarms. Crenulation is rare. Foliation generally dips north (mean 359�/ 40�) with local overturning. Stretching lineations generally dip north (mean 002746�). Kink band axes are nearly horizontal (mean 262704�). Linear structural elements indicate north-south transport. Small intrafolial S-shaped folds deform foliation and stretching lineation and indicate sinistral along-strike displacement.