Evolution of the Himalayan Foreland and Contemporary Vegetation During the Late Miocene-Pliocene: A Case Study from the Siwalik Group of Rocks, NW Himalaya, India

Ghosh, Sambit (2019) Evolution of the Himalayan Foreland and Contemporary Vegetation During the Late Miocene-Pliocene: A Case Study from the Siwalik Group of Rocks, NW Himalaya, India. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The India-Asia collision at ca. 55 Ma resulted in the formation of Himalayan mountain chain. It is believed that the rise in Himalayan mountain imparted a significant impact on the late Cenozoic climate in Asia by intensifying the monsoon. However, understanding the influence of tectonics (uplift of the Himalaya) on climate (monsoonal rainfall) or vice versa in mountain building processes remain challenging for a large orogen like the Himalaya. The late Cenozoic climate change was thought to be an important factor to trigger the evolution and expansion of the C₄ plants during late Miocene. However, the causes of C₄ plant expansion are still debated as the expansion was asynchronous globally as well as regionally. The foreland basin in the southern fringe of the Himalaya which continuously received orogenic sediments can provides clues to understand the role of allocyclic (tectonics and climate) process in mountain building and the late Miocene vegetation history in the foreland area. In the present study, the late Miocene-Pliocene Siwalik Group of rocks which represent the late overfill stage of foreland basin have been used to understand the factors which led to the evolution and expansion of the C₄ plants in the Himalayan foreland area and the relative influence of climate and tectonics in the mountain building processes. The study has been carried out in the Naladkhad (ca. 12–8 Ma), Ranital (ca. 11.5–7 Ma) and Jabbarkhad (ca. 5–2.5 Ma) regions of the Kangra sub-basin, and the Ghaggar (last 2.5 Myr), Moginanda (last 2 Myr) and Haripur Khol (last 6 Myr) regions of the Subathu sub-basin of the NW Himalaya, India. In addition, a high resolution record of monsoonal rainfall variation and its impact on the vegetation has been reconstructed from the Holocene relict Benital lake sedimentary archive (last 10 kyr). The δ¹³C values of long-chain n-alkanes from paleosol samples (n = 74) of Naladkhad, Ranital and Jabbarkhad sections of the Kangra sub-basin and Haripur Khol section of the Subathu sub-basin have been used to reconstruct the relative abundance of C₃-C₄ plants, after isotopic characterization of modern plants from the Ganges floodplain, considered equivalent to the past Siwalik floodplain. The values from the Naladkhad (11.6 to 8.9 Ma) and Ranital (11.3 to 7.4 Ma) sections of the Kangra sub-basin indicate presence of ca. 20% C₄ plants at ca. 11 Ma. The existence of C₄ plants at ca.11 Ma indicate an early appearance of C₄ plants, compared with the published data from the Siwalik regions. The Ranital section shows a patchy occurrence of C4 plants between 11.3 Ma and 7.4 Ma and the Jabbarkhad section shows (5.2 Ma to 3.0 Ma) a gradual increase in C4 plant abundance. The C4 plant abundance fluctuated between 83% and 0% (5.8 Ma to 1.3 Ma) in the Haripur Khol section of the Subathu sub-basin. Comparison of C₄ plants abundance from the four sections shows that each section was characterized by a distinct evolutionary pattern. The δD values of the same n-alkanes from the Kangra and Subathu sub-basins indicate two episodes of Indian summer monsoon intensification at ca. 9 Ma and ca. 4 Ma. In lieu of vegetation and climate data from the Upper Siwalik of the Kangra sub-basin, the Ghaggar and Moginanda sections from the adjacent Subathu sub-basin have been considered. The δ¹³C and δ¹⁸OSC values of soil carbonate (SC) from the Ghaggar (n=7) and Moginanda (n=3) regions and δ¹³C values of fossil tooth enamel from the Haripur Khol (n=10) region of the Subathu sub-basin have been measured. The δ¹³C values indicate the abundance of C₄ plant was ca. 70% in the Ghaggar (2.5 to 1 Ma), 82% in the Moginanda (2 to 1 Ma) and ca. 85% in the Haripur Khol (5 to 1 Ma) regions. The δ¹³Osc values indicate a decreasing trend in monsoonal rainfall during ca. 2.5 to 1 Ma. Data from different parts of the globe including the Himalayan foreland suggests that the expansion of C₄ plants was not synchronous and hence invoke influences of regional factors in controlling the evolution of C₄ plants. The cross plots between climate (δDc₂₉ and δ¹⁸Osc) and vegetation (δ¹³Cc₂₉ and δ¹³Csc) proxy from the studied regions of the Kangra and Subathu sub-basin and other Siwalik regions suggest that that climate was not the only main driver to control the C4 plants abundance. It was also observed that the C4 plant abundance variation could be linked to the variable channel/overbank ratio of the studied sections. The comparison of the sedimentary architecture among the different Siwalik regions indicate that the abundance of overbank fine sediment is higher at the Pakistan Siwalik (68%) and in contrast, the NW India and Nepal Siwalik regions are characterized by 47% and 26% fine overbank sediments, respectively. The cross-plots between abundance of C4 plants and overbank fine sediment from the different Siwalik regions show a positive correlation and suggest a significant role of sedimentary architecture in controlling the abundance of C₄ plant in the Himalayan foreland. The variation in the sedimentary architecture which is controlled by the accommodation space and sediments supply to the foreland area varied significantly along the foreland. The dominance of gravel in the foreland basin has traditionally been related to either syn- or post- tectonic activity in the hinterland area. However, apart from the tectonic connection, gravel progradation towards the foreland is also dependent on the balance between the rate of creation of accommodation space and sediment supply, which vary temporally as well as spatially. Therefore, past geomorphic condition in the basin is essential to understand the syn- versus post- tectonic gravel progradation history in the foreland basins. The extensive conglomerate of the Middle and Upper Siwalik Group has routinely been linked with the thrusting activity in the Himalaya. So far, little attention has been paid to the actual geomorphic context of the Middle and Upper Siwalik conglomerates. Towards this, sedimentological and paleohydrological analysis of the Siwalik Group of sediments were carried out at the Naladkhad and Ranital sections of the Kangra sub-basin. The facies architecture and textural parameters suggest that the Naladkhad section was situated close to the source area, whereas the Ranital section was situated away from the source area. When compared with the modern fans forming at the mountain front, sedimentological and paleohydrological analysis suggest that the Siwalik Group of sediments in the Kangra sub-basin were deposited in a piedmont alluvial fan setting, dominated by braided rivers. This inference is in stark contrast with the earlier proposed mega-fan origin of the Siwalik Group of sediments exposed in the Kangra sub-basin. Interestingly, our proposed depositional settings lend support for a syn-tectonic gravel progradation for the Middle and Upper Siwalik conglomerates which has been strongly advocated recently. The geomorphic setting of the Siwalik Group of rocks at the Kangra sub-basin allow us to understand the role of tectonic and climate in depositing the foreland sediments. Understanding the mutual interaction among climate, tectonics and exhumations in an active orogen like Himalaya have been a perennial challenge for the last few decades. While, the core of the Himalayan orogen received significant attention, the persistently growing southern fringe of the Himalaya i.e., the foreland basin remain less explored and therefore, our understanding of the lateral growth of the orogeny is limited. In the present study, along with the tectonic and climate proxy data, the sedimentological and sequence stratigraphic analysis of the >4000 m logged section in the foreland indicate that, both spatially and temporally, the basin filling style alternates between high and low accommodation system tract and record a lateral growth of the orogen in response to tectonics only. It is therefore assumed that the evolution of the Himalayan orogen is compartmentalized into two segments, while core of orogen responds to the both tectonics and climate changes, the fringe responds only to tectonics. These findings will help our understanding of the mountain building process in other orogens and numerical modeling attempts. It has been observed that rainfall amount plays a major role in controlling the ¹³C/¹²C ratio of the plants. Therefore, ¹³C/¹²C ratio of the organic matter from the paleo-archive has been used to understand the past climate variation. The long-term climate record reconstructed from the Siwalik archive is not only regulated by the orbital forcing but tectonic forcing (uplift of the Himalaya and Tibetan Plateau) also played crucial role in controlling the Indian summer monsoon (ISM) rainfall variations. In the present study, an attempt has been made to comprehend the role of ISM rainfall variations in determining the ¹³C/¹²C ratio of the vegetation from the Holocene lake sedimentary archive, where orbital forcing mainly controlled the climate signatures. The early Holocene was characterized by strong monsoonal rainfall, however none of the paleoclimatic records show the magnitude required to shape the observed landform in the Ganges plain and sediment discharge in Bay of Bengal. The Tropical Rainfall Measurement Mission data suggests that the Central Himalaya (ca. 2 km altitude) is sensitive to rainfall changes and hence paleoclimate proxies from this region would provide an excellent opportunity to reconstruct the Holocene monsoon variation. Here we present, for the first time, n-alkane δDc₂₉ values to reconstruct the Holocene monsoon from the relict lake sediments of Benital area in the Central Himalaya which receives ca. 80% of the mean annual rainfall during summer monsoon. The δDc₂₉ values indicate that the early Holocene was characterized by a wet phase with a 70% increase in monsoonal rainfall at ca. 9 ka. Subsequently the middle-late Holocene was characterized by less monsoonal rainfall. However, the large change in monsoonal rainfall during the early Holocene as inferred in the present study is the largest of the existing records. The comparison of δDc₂₉ values and the solar insolation data at 30°N suggests that migration of the Intertropical Convergence Zone primarily controlled the variations in monsoonal rainfall. Based on the distribution and δ¹³Cc₂₉ values of modern plants, the δ¹³Cc₂₉ values of the lake sediments indicate that during pre and post 7 ka the lake catchment was dominated by woody and non-woody plants, respectively. The cross plot between δDc₂₉ and δ¹³Cc₂₉ indicates that at higher rainfall, the δ¹³Cc₂₉ values of catchment vegetation were less-responsive.

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