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Geologic framework of the northern Indo-Burma Ranges and lateral correlation of Himalayan-Tibetan lithologic units across the eastern Himalayan syntaxis
AuthorHaproff, Peter J.
Harrison, T. Mark
Manning, Craig E.
Dubey, Chandra S.
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The Cenozoic India-Asia collision generated both the east-trending Himalayan orogen and the north-trending Eastern and Western Flanking Belts located along the margins of the Indian subcontinent. Although the tectonic development of both flanking belts is key to understanding mechanisms of continental deformation during indenter-induced collision, few field-based studies coupled with geochronological and geochemical methods have been applied to these tectonic domains. In this study, we investigate the lateral correlation of lithologic units between the northern Indo-Burma Ranges, the northernmost segment of the Eastern Flanking Belt, and the eastern Himalayan-Tibetan orogen by integrating field observations, U-Pb zircon geochronology, and whole-rock geochemistry. Our findings provide new quantitative constraints to interpretations that the northern Indo-Burma Ranges expose the eastward continuation of several lithologic units of the Himalayan orogen and Lhasa terrane. Our field work documents a stack of thrust-bounded lithologic units present in the study area. The northernmost and structurally highest Lohit Plutonic Complex consists of Mesoproterozoic basement rocks (ca. 1286 Ma) and Late Jurassic-Cretaceous granitoids (ca. 156-69 Ma) with positive epsilon(Nd) values and initial Sr-87/Sr-86 ratios of similar to 0.705, which are correlative to the Bomi-Chayu complex and the northern Gangdese batholith, respectively. The structurally lower Tidding-Mayodia melange complex, composed of basalt, gabbro, ultramafic rocks, and mafic schist of a dismembered ophiolite sequence, is interpreted in this study as the eastward extension of the Indus-Yarlung suture zone. Structurally below the suture zone are the Mayodia gneiss and Lalpani schist, which are interpreted to correlate with the Lesser Himalayan Sequence based on comparable metamorphic lithologies, negative epsilon(Nd) values, and similar Mesoproterozoic-Cambrian detrital zircon age spectra. In contrast to the above metamorphic units, the structurally lowest Tezu unit consists of siliciclastic strata that may be correlated with the Miocene-Pliocene Siwalik Group of the Himalayan orogen. Despite the above correlations, notable Himalayan-Tibetan lithologic units are absent in the northern Indo-Burma Ranges, including the Mesozoic-Cenozoic southern Gangdese batholith belt and its cover sequence of the Linzizong volcanic rocks, Xigaze forearc basin, Tethyan Himalayan Sequence, and Greater Himalayan Crystalline Complex of south-central Tibet and the central Himalaya. We interpret the absence of these lithologic units to be a result of a greater magnitude of crustal shortening and/or underthrusting of the Indian cratonal rocks than that across the Himalayan orogen to the west. This interpretation is supported by a southward decrease in the map-view distance between the active range-bounding thrust and the Indus-Yarlung suture zone in the northern Indo-Burma Ranges, from similar to 200 km in the north near the eastern Himalayan syntaxis to similar to 5 km in the south across a distance of similar to 200-300 km.
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