Tectonic framework of the easternmost Himalayan orogen based on U-Pb zircon geochronology and detailed geologic mapping, NE India
Abstract
While the configuration of major lithologic units and structures in the central Himalayas has been a topic of intense study for over a century, correlation of rocks and structures across the Eastern Himalayan Syntaxis, however, has not been adequately examined. To test this, we conducted systematic geologic mapping along two southwest-northeast traverses across the northernmost north-trending Indo-Burma Range, directly east of the Eastern Himalayan Syntaxis. The northern Dibang River Valley traverse is ~70 km long and transects predominantly northeast-dipping thrusts that bound major lithologic units. From southwest to northeast, units consist (1) Quaternary Indo-Gangetic river plain deposits, (2) low-grade metasediments, (3) ultramafics and ophiolite-sequence rocks, (4) low- to medium-grade phyllite, biotite schist, and paragneiss, (5) mélange of suture material containing metavolcanics, mafics, and ophiolites, (6) medium- to high-grade micaceous schist and gneiss, (7) deformed ortho- and paragneiss, and (8) mylonitized to undeformed diorite and granite of the Lohit Plutonic Complex. Each lithologic unit is structurally above the previous unit. The southern Lohit River Valley traverse is ~50 km long and consists the following units and structures. At the range front, low- to medium-grade quartzite, phyllite, and schist is thrust over modern river plain deposits. Structurally above is a thrust sheet of medium-to high-grade schist, paragneiss, and orthogneiss. Further northeast is the thrust-bounded Tidding suture zone containing metavolcanics, ophiolites, and ultramafic rocks. Structurally above the suture are diorite and granite of the Lohit Plutonic Complex. First order structures of the study area are km-wide ductile thrust shear zones with mylonitic fabrics indicating top-southwest dip-slip motion. Preliminary metamorphic isograd mapping suggests a northward decrease in metamorphic grade within major thrust sheets. The range front is bounded by an active right-slip transpressional system, which is expressed by right-laterally offset of stream channels. U-Pb geochronology of detrital zircon from units (2), (4), and (7) in the Dibang River Valley yielded minimum ages of 27 ×1 Ma, 646 ×13 Ma, and 85 ×2 Ma. Metasediments containing 27 Ma detrital zircon may have been deposited in the Himalayan foreland and later incorporated into the northern Indo-Burma transpressional zone. Biotite schist with 646 Ma detrital zircon is interpreted to be Neoproterozoic Lesser Himalayan-equivalent metasediments. Lastly, paragneiss containing 85 Ma zircon may be sourced from the Lohit Plutonic Complex. Our field observations are consistent with a model of Tibetan crustal material flowing around the eastern syntaxis as India indents into Asia.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T11A2422H
- Keywords:
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- 8038 STRUCTURAL GEOLOGY Regional crustal structure