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Title: Geologic framework of the northern Indo-Burma Ranges and lateral correlation of Himalayan-Tibetan lithologic units across the eastern Himalayan syntaxis
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National Science Foundation
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  1. Abstract

    Cretaceous‐Miocene sedimentary rocks in the Nepalese Lesser Himalaya provide an opportunity to decipher the timing of India‐Asia collision and unroofing history of the Himalayan orogen, which are significant for understanding the growth processes of the Himalayan‐Tibetan orogen. Our new data indicate that detrital zircon ages and whole‐rock Sr‐Nd isotopes in Cretaceous‐Miocene Lesser Himalayan sedimentary rocks underwent two significant changes. First, from the Upper Cretaceous‐Palaeocene Amile Formation to the Eocene Bhainskati Formation, the proportion of late Proterozoic‐early Palaeozoic zircons (quantified by an index of 500–1200 Ma/1600–2800 Ma) increased from nearly 0 to 0.7–1.4, and the percentage of Mesozoic zircons decreased from ca. 14% to 5–12%. The whole‐rock87Sr/86Sr and εNd(t = 0) values changed markedly from 0.732139 and −17.2 for the Amile Formation to 0.718106 and −11.4 for the Bhainskati Formation. Second, from the Bhainskati Formation to the lower‐middle Miocene Dumri Formation, the index of 500–1200 Ma/1600–2800 Ma increased to 2.2–3.7 and the percentage of Mesozoic zircons abruptly decreased to nearly 0. The whole‐rock87Sr/86Sr and εNd(t = 0) values changed significantly to 0.750124 and −15.8 for the Dumri Formation. The εHf(t) values of Early Cretaceous zircons in the Taltung Formation and Amile Formation plot in the U‐Pb‐εHf(t) field of Indian derivation, whereas εHf(t) values of Triassic‐Palaeocene zircons in the Bhainskati Formation demonstrate the arrival of Asian‐derived detritus in the Himalayan foreland basin in the Eocene based on available datasets. Our data indicate that (1) the timing of terminal India‐Asia collision was no later than the early‐middle Eocene in the central Himalaya, and (2) the Greater Himalaya served as a source for the Himalayan foreland basin by the early Miocene. When coupled with previous Palaeocene‐early Eocene provenance records of the Tethyan Himalaya, our new data challenge dual‐stage India‐Asia collision models, such as the Greater India Basin hypothesis and its variants and the arc–continent collision model.

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  2. Abstract

    Pressure‐temperature (P‐T) conditions and high‐resolution paths from 11 garnet‐bearing rocks collected across Himalayan fault systems exposed along the Bhagirathi River (Uttarakhand, NW India) reveal the tectonic conditions responsible for their growth. A garnet from the Tethyan metasedimentary unit has a 50.3 ± 0.6 Ma (Th‐Pb, ±1σ) monazite inclusion, suggesting that ductile mid‐crustal metamorphism occurred synchronously or soon after (<10 Myr) India‐Asia collision, depending on timing. High‐resolution garnet P‐T paths from the same rock show ∼1 kbar fluctuations in P as T increases over a ∼20°C interval, consistent with a period of erosion. We report garnets from the Main Central Thrust (MCT) hanging wall that have Eocene to Miocene monazite ages, and one garnet yields paths consistent with motion along the Main Himalayan Thrust (MHT) décollement. Most high‐resolution MCT footwall P‐T paths fluctuate in P (±1 kbar) as T increases, consistent with imbrication and paths from equivalent structural assemblages in central Nepal. Like those rocks, MCT footwall (Lesser Himalayan Formation, LHF) monazite ages are Early Miocene (9.3 ± 0.6 Ma) to Pliocene (3.0 ± 0.2 Ma). The results demonstrate the consistency in timing and conditions across the MCT at locations ∼650 km apart. If the present‐day Himalayan tectonic framework has not significantly changed since the Pliocene, the LHF duplex can be considered when attributing seismic events to particular faults. The MHT is undisputedly the significant factor in accommodating Himalayan seismic activity, but MCT footwall faults may explain some shallower hypocenters, without the need for unusual MHT geometries.

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