Sedimentary facies and micro-fossil analyses, and AMS14C dating were performed in order to reveal the water-level fall events and draining process of the lake (Paleo-Kathmandu Lake) that existed in the past in the Central Nepal Himalaya. The sedimentary facies change from the lacustrine Kalimati Formation to the deltaic Sunakothi Formation in the southern and central Kathmandu basin, and the abrupt and prominent increase of phytoliths Bambusoideae and Pediastrum, and contemporaneous decrease of sponge spicule and charcoal grains around 48 and 38 ka support the lowering of water level at these times. According to the pollen analysis, both events occurred under rather warm and wet climate, thus supporting that they were triggered by tectonic cause and not by climate change. The first event might be linked to a possible occurrence of a large earthquake with an epicenter in the vicinity of the Paleo-Kathmandu Lake. The occurrence of a mega landslide in Langtang area close to the north of the Kathmandu Valley producing pseudotachylite dated at 51 ± 13 ka could be linked to this earthquake. Finally, the water was completely drained out from the remnant lake at the central part of the Kathmandu basin by ca.12 ka.
Precise stratigraphic analysis of Middle-Upper Permian and Lower Triassic sequence at Chaotian in northern Sichuan, China, identified two remarkable mass extinction horizons, one at the Middle-Upper Permian (Guadalupian-Lopingian; G-L) boundary and the other at the Upper Permian-Lower Triassic (P-T) boundary. Across each of the boundaries, biodiversity declined sharply in fusulinid, rugose coral, brachiopod, ammonite, conodont, and radiolaria. Both boundaries are characterized by two biohorizons, i.e., one marked by major extinction of pre-existing fauna and the other by the first appearance of younger fauna. It is noteworthy that a peculiar rhyo-dacitic tuff bed occurs at each of the extinction horizons. Thus the Late Permian biosphere was strongly affected twice by highly explosive, severe volcanism. Regional correlation of the G-L and P-T boundary tuff beds throughout South China, and partly to Japan, positively suggests a cause-effect link between large-scale explosive volcanism and mass extinction. (Communicated by Tatsuro MATSUMOTO, M.J.A., Jan. 13, 2004)
Abstract. A continuous lacustrine sediment core obtained from the Kathmandu Valley in the Central Himalayas revealed that cyclical changes in C3/C4 vegetation corresponded to global glacial-interglacial cycles from marine isotope stages (MIS) 15 to MIS 4. The C3/C4 vegetation shifts were reconstructed from significant changes in the δ13C values of bulk organic carbon. Glacial ages were characterized by significant 13C enrichment, due to the expansion of C4 plants, attributed to an intensification of aridity. Thus, the southwest (SW) summer monsoon, which brings the majority of rainfall to the Central Himalayan southern slopes, would have been weaker. Marine sediment cores from the Indian Ocean and Arabian Sea have demonstrated a weaker SW monsoon during glacial periods, and our results confirm that arid conditions and a weak SW monsoon prevailed in the continental interior of the Central Himalayas during glacial ages. This study provides the first continuous record for the continental interior of paleoenvironmental changes directly influenced by the Indian monsoon.
The Himalayan range was formed and uplifted in association with the southward migration of plate boundary thrusts that separate the Himalaya into four belts.Initially, during 50-35 Ma, the Tibetan marginal mountain range was uplifted after slab break-off of the Tethyan oceanic plate, which was subducted under the Asian continent to depths of up to 100 km.During the second stage at ~35-25 Ma, the Mesoproterozoic sediments deposited on the northern passive margin of the Greater India were subducted and underwent moderate-pressure metamorphism at depths of ~40 km.Subsequent to metamorphism, metamorphosed continental crust was separated from the underlying mantle by delamination, and its rapid exhumation and associated amphibolite facies metamorphism occurred during 22-16 Ma.Partially melted metamorphic rocks generated granitic melt that intruded both metamorphic rocks and Tibetan Tethys sediments during the Miocene.Exhumation of the metamorphic belt continued after its exposure at ~15 Ma, forming an extensive metamorphic nappe covering the Lesser Himalayan sediments.After ceasing movement at 11-10 Ma, the Indian plate started to subduct along the Main Boundary Thrust (MBT), which was newly formed in front of the southern margin of the metamorphic nappe.At the same time, the nappe and weakly metamorphosed underlying Lesser Himalayan sediments started to cool laterally from the southern front to the root zone at a rate of ~10 km/Myr.At 3-2.5 Ma, the plate boundary fault shifted to the Main Frontal Thrust (MFT) to the south of the MBT, causing rapid uplift of the marginal range of the Lesser Himalaya and the Siwalik Hills.Today, the Indian Plate is converging with the Asian Continent at the rate of 58 mm/yr, and half of this convergence is consumed by uplift of the Siwalik Hills along the MFT.
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