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1. The India-Asia collision results from two possible pre-collisional crustal configurations of northern Greater India

   https://doi.org/10.1016/j.epsl.2023.118098  

   Li, Yipeng; Robinson, Delores M. (May 2023, Earth and Planetary Science Letters)

   Interpretations of pre-collisional configurations of Greater India are highly controversial and predict distinct processes during the India-Asia collision. To better determine the possible pre-collisional configuration(s) of Greater India, we conduct a mass-balance analysis combined with previously published geologic, paleomagnetic, and geodynamic evidence. The mass-balance analysis determines the magnitude of northern Greater India (NGI) width needed to provide sufficient crustal accretion to form the Tethyan-Greater Himalaya orogenic wedge in Cenozoic time. Applying endmember crustal thicknesses of 10–40 km to a mass-balance equation yields a broad range of plausible pre-collisional NGI widths of ∼3016±1000 km and ∼956±283 km, respectively, which we further assess considering contrasting models/evidence. The integrated evidence requires a thin NGI continental crust to form 1) continuous Tethyan-Greater Himalayan crustal thickening, 2) a narrow foredeep width of Himalayan foreland basin, 3) continuous Gangdese arc magmatism with oceanic-subduction-style mantle wedge, and 4) low-magnitude exhumation in the North Himalaya and Gangdese arc-forearc from ∼60-30 Ma. Adding the structurally restored ∼740 km wide southern Greater India, the synthesized analyses yield two possible configurations: 1) an ∼1350±440 km wide and ∼23-30 km thick NGI indicating an ∼2080±450 km wide Greater India with ∼500-1000 km wide oceanic basin systems in both Asia and NGI; and 2) a ≥1815±630 km wide and ∼10-23 km thick Zealandia-type NGI indicating a ≥2550±640 km wide pre-collisional Greater India without or with limited ∼500-1000 km Xigaze back-arc oceanic basin. The former is conditionally consistent with the integrated evidence by assuming no Cenozoic oceanic subduction initiation within NGI and predicts multi-stage collision since ∼60 Ma. The latter is consistent with the integrated evidence and predicts an approximate-single-stage collision at ∼60 Ma. Both configurations predict significant post-collisional NGI crustal shortening that may have been accommodated by the Eocene-Oligocene Greater Himalayan structural discontinuities. 

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2. Expedition 355 Scientific Prospectus: Arabian Sea Monsoon

   https://doi.org/10.14379/iodp.sp.355.2014  

   Pandey, D.K.; Clift, P.D.; Kulhanek, D.K. (June 2014, Scientific prospectus)

   null (Ed.)

   Interactions between the solid Earth and climate system represent a frontier area for geoscientific research that is strongly emphasized in the International Ocean Discovery Program (IODP) Science Plan. The continental margin of India adjoining the Arabian Sea offers a unique opportunity to understand tectonic-climatic interactions and the net impact of these on weathering and erosion of the Himalaya. Scientific drilling in the Arabian Sea is designed to understand the coevolution of mountain building, weathering, erosion, and climate over a range of timescales. The southwest monsoon is one of the most intense climatic phenomena on Earth. Its long-term development has been linked to the growth of high topography in South and Central Asia. Conversely, the tectonic evolution of the Himalaya, especially the exhumation of the Greater Himalaya, has been linked to intensification of the summer monsoon rains, as well as to plate tectonic forces. Weathering of the Himalaya has also been linked to long-term drawdown of atmospheric CO2 during the Cenozoic, culminating in the onset of Northern Hemisphere glaciation. No other part of the world has such intense links between tectonic and climatic processes. Unfortunately, these hypotheses remain largely untested because of limited information on the history of erosion and weathering recorded in the resultant sedimentary prisms. This type of data cannot be found on shore because the proximal foreland basin records are disrupted by major unconformities, and depositional ages are difficult to determine with high precision. We therefore propose to recover longer records of erosion and weathering from the Indus Fan that will allow us to understand links between paleoceanographic processes and the climatic history of the region. The latter was partially addressed by Ocean Drilling Program (ODP) Leg 117 on the Oman margin, and further studies are proposed during IODP Expedition 353 (Indian Monsoon Rainfall) that will core several sites in the Bay of Bengal. Such records can be correlated to structural geological and thermochronology data in the Himalaya and Tibetan Plateau to estimate how sediment fluxes and exhumation rates change through time. The drilling will be accomplished within a regional seismic stratigraphic framework and will for the first time permit an estimation of sediment budgets together with quantitative estimates of weathering fluxes and their variation through time. Specific goals of this expedition include 1. Testing whether the timing of the exhumation of Greater Himalaya correlates with an enhanced erosional flux and stronger chemical weathering after ~23 Ma, 2. Determining the amplitude and direction of the environmental change at 8 Ma, and 3. Dating the age of the base of the fan and the underlying basement to constrain the timing of India-Asia collision. Drilling through the fan base and into the underlying basement in the proposed area will permit additional constraints to be placed on the nature of the crust in the Laxmi Basin (Eastern Arabian Sea), which has a significant bearing on paleogeographic reconstructions along conjugate margins in the Arabian Sea and models of continental breakup on rifted volcanic margins. 

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3. Three-dimensional strain accumulation and partitioning in an arcuate orogenic wedge: An example from the Himalaya

   https://doi.org/10.1130/B35528.1  

   Fan, Suoya; Murphy, Michael A. (April 2020, GSA Bulletin)

   In this study, we use published geologic maps and cross-sections to construct a three-dimensional geologic model of major shear zones that make up the Himalayan orogenic wedge. The model incorporates microseismicity, megathrust coupling, and various derivatives of the topography to address several questions regarding observed crustal strain patterns and how they are expressed in the landscape. These questions include: (1) How does vertical thickening vary along strike of the orogen? (2) What is the role of oblique convergence in contributing to along-strike thickness variations and the style of deformation? (3) How do variations in the coupling along the megathrust affect the overlying structural style? (4) Do lateral ramps exist along the megathrust? (5) What structural styles underlie and are possibly responsible for the generation of high-elevation, low-relief landscapes? Our model shows that the orogenic core of the western and central Himalaya displays significant along-strike variation in its thickness, from ∼25−26 km in the western Himalaya to ∼34−42 km in the central Himalaya. The thickness of the orogenic core changes abruptly across the western bounding shear zone of the Gurla Mandhata metamorphic core complex, demonstrating a change in the style of strain there. Pressure-temperature-time results indicate that the thickness of the orogenic core at 37 Ma is 17 km. Assuming this is constant along strike from 81°E to 85°E indicates that, the western and central Nepal Himalaya have been thickened by 0.5 and 1−1.5 times, respectively. West of Gurla Mandhata the orogenic core is significantly thinner and underlies a large 11,000 km2 Neogene basin (Zhada). A broad, thick orogenic core associated with thrust duplexing is collocated with an 8500 km2 high-elevation, low-relief surface in the Mugu-Dolpa region of west Nepal. We propose that these results can be explained by oblique convergence along a megathrust with an along-strike and down-dip heterogeneous coupling pattern influenced by frontal and oblique ramps along the megathrust. 

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4. Expedition 354 Scientific Prospectus: Bengal Fan

   https://doi.org/10.14379/iodp.sp.354.2014  

   France-Lanord, C.; Schwenk, T.; Klaus, A. (February 2014, Scientific prospectus)

   null (Ed.)

   Expedition 354 will drill a transect of holes in the Bay of Bengal to address interactions among the growth of the Himalaya and Tibet, the development of the Asian monsoon, and processes affecting the carbon cycle and global climate. Because sedimentation in the Bengal Fan responds to both climate and tectonic processes, its terrigenous sediment records the past evolution of both the Himalaya and regional climate. The histories of the Himalayan/Tibetan system and the Asian monsoon require sampling different periods of time with different levels of precision. Accordingly, we propose a transect of six holes in the fan at 8°N with two complementary objectives. (1) We will study the early stages of Himalayan erosion, which will bear on the India-Eurasia collision and the development of the Himalaya and Tibet as topographic features. We will drill a deep site (MBF-3A to ~1500 m) in the west flank of the Ninetyeast Ridge where a reflector interpreted as a Paleocene-Eocene unconformity could be reached at a reasonable depth. (2) We will study the Neogene development of the Asian monsoon and its impact on sediment supply and flux. Our east–west transect of drill sites at 8°N will include Site MBF-3A and two other 900 m penetration sites (MBF-1A and MBF-2A) to reach sediment at least as old as 10–12 m.y. Records from the Arabian Sea and the Indian subcontinent suggest that at ~7–8 Ma the intensity of the monsoon increased and C4 plants expanded. Moreover, these changes appear to be linked to changes in the erosional regime as recorded by Ocean Drilling Program Leg 116 and possibly to the tectonic evolution of southeast Asia. This transect will allow study of the extent to which a strengthening of the monsoon encompassed the Bay of Bengal, where increased rainfall, not strengthened wind, characterizes the monsoon, and will allow quantitative studies of the interrelations of climate change and sediment accumulation. In addition, three sites (MBF-4A, MBF-5A, and MBF-6A) will document how the depocenter migrated across this transect during the Pleistocene and will provide the most complete record of channel-derived terrigenous material through this time interval. 

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5. Constraints on the timing of the India‐Asia collision and unroofing history of the Himalayan orogen using detrital zircon U‐Pb‐Hf and whole‐rock Sr‐Nd isotopes in Cretaceous‐Miocene Lesser Himalayan sedimentary rocks

   https://doi.org/10.1111/bre.12742  

   Feng, Wei; Meng, Qingquan; Song, Chunhui; Fang, Xiaomin; Zhuang, Guangsheng; He, Pengju; Yang, Shufen; Zhang, Jing; Chen, Yongfa; Zhang, Yihu (December 2022, Basin Research)

   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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