Geologic and Tectonic Evolution of the Yinkiong Group and Abor Volcanic Rocks in the Eastern Himalaya: An Overview of Geologic Data.

The Siang valley in the eastern part of Arunachal Himalaya contains Paleocene–Eocene sediments of Yinkiong Group along with the Abor volcanic exposed in a wide lateral extent in the middle and lower Siang valleys. Based on available geologic data and detail geologic mapping present study proposes a new evolution model of the basin in the context of India–Asia collision tectonics. Integration of the field data on the disposition of different litho units from Pasighat (south) to Tuting (north) along the Siang valley and in the Mishmi block, helped us to constrain several major thrust fault and strike-slip faults in the area showing dextral and sinistral senses. Some of these faults are conjugate. It is postulated that during the early Paleocene, the Indian Plate collided with the Eurasian Plate and this collision resulted in the breakage of an edge of the Indian Plate along its northeastern extremity. This broken edge rotated towards the Burmese Plate and eventually formed a rift basin that led to the ingress of the Tethys Sea where sedimentation along with syn-sedimentary volcanism took place. The sedimentation in the basin continued throughout the Paleocene–Eocene time. The absence of rocks younger than Yinkiong Group indicates that this basin was closed at the end of the Eocene. The development of a highly folded sequence along the Siang valley in response to north south-directed compression can be constrained due to subsequent India‒Asia collision. The broken edge of the Indian Plate and western part of the Burmese Plate amalgamated through Tuting–Tidding suture. This Indo-Burmese amalgamated part of the Mishmi block thrusted over the Paleocene–Eocene sequence from the east. The continued NNW ward movement of the Mishmi block resulted in the swinging of the MCT towards the northeastern direction. On the other hand, the overriding of the Himalayan metamorphic belt along the Tidding Thrust was intense, leaving only a narrow exposure of garnetiferous graphite-schist and garnetiferous amphibolites as slivers of the ophiolite package. The Lesser Himalayan sequences of the Mishmi block overrode the frontal molasses basin in many places and are in direct contact with the Brahmaputra alluvium in the southeastern part along the Lohit and Dibang valleys. The Lesser Himalayan sequence also overrode the Paleocene–Eocene sequence from the western direction showing significant occurrences in the Boleng and Bomdo area of the Siang valley. [ABSTRACT FROM AUTHOR]