Tectonic evolution of the Himalaya constrained by detrital ⁴⁰Ar–³⁹Ar, Sm–Nd and petrographic data from the Siwalik foreland basin succession, SW Nepal

Abstract

40Ar–39Ar dating of detrital white micas, petrography and heavy mineral analysis and whole‐rock geochemistry has been applied to three time‐equivalent sections through the Siwalik Group molasse in SW Nepal [Tinau Khola section (12–6 Ma), Surai Khola section (12–1 Ma) and Karnali section (16–5 Ma)]. 40Ar–39Ar ages from 1415 single detrital white micas show a peak of ages between 20 and 15 Ma for all the three sections, corresponding to the period of most extensive exhumation of the Greater Himalaya. Lag times of less than 5 Myr persist until 10 Ma, indicating Greater Himalayan exhumation rates of up to 2.6 mm year−1, using one‐dimensional thermal modelling. There are few micas younger than 12 Ma, no lag times of less than 6 Myr after 10 Ma and whole‐rock geochemistry and petrography show a significant provenance change at 12 Ma indicating erosion from the Lesser Himalaya at this time. These changes suggest a switch in the dynamics of the orogen that took place during the 12–10 Ma period whereby most strain began to be accommodated by structures within the Lesser Himalaya as opposed to the Greater Himalaya. Consistent data from all three Siwalik sections suggest a lateral continuity in tectonic evolution for the central Himalayas.

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“…The timing of the final unroofing of the Greater Himalaya is also poorly defined Bernet et al, 2006;Najman, 2006;Szulc et al, 2006) and should be indicated by influxes of high-grade metamorphic minerals into the Indus Fan. Seeing whether the arrival of this material from the Greater Himalaya coincides with, or likely follows, intensification of the summer monsoon rains would be a critical test of the channel flow and competing orogenic wedge models.…”

Lithology, Monsoon and Sea-Surface Current Control on Provenance, Dispersal and Deposition of Sediments over the Andaman Continental Shelf

“…The Irrawaddy River flows, in its upper reaches, through the Gangdese batholith, metamorphic rocks and ophiolites of FIGURE 3 | The lithology map of the drainages of the Irrawaddy, the Salween and the Tanintharyi River basins falling in the Tibet, China, and the Myanmar, modified after (Awasthi et al, 2014;Chapman et al, 2015). possible eastern continuation of the Indus Tsangpo Suture Zone, the volcanics from a Cretaceous arc and sediments produced during the collision (Stephenson and Marshall, 1984;Maury et al, 2004;Najman et al, 2004;Szulc et al, 2006;Allen et al, 2008), associated post-collision intrusive igneous rocks (Darbyshire and Swainbank, 1988) and basic/ultrabasic rocks of the eastern syntaxis of the Himalaya. In the middle reaches, the Irrawaddy drains the Mogok Metamorphic Belt containing schists, gneisses, marble, migmatites, and calc-alkaline plutonics (Figure 3; Chapman et al, 2015).…”

Laser 40Ar/39Ar age constraints on Miocene sequences from the Bengal basin: Implications for middle Miocene denudation of the eastern Himalayas

“…Distal eastern equivalents of the molassic Siwalik sequences have been studied in the Bengal fan, in seismic surveys [Curray, 1991] and on two drilling legs (Deep Sea Drilling Project Leg 22 and Ocean Drilling Program Leg 116) [Ingersoll and Suczek, 1979;Amano and Taira, 1992]. Drilling has recovered strata as old as about 18 Ma, and detrital geochronology and isotopic studies on these strata indicate that orogenesis had begun prior to this time [e.g., Copeland and Harrison, 1990;Galy et al, 1996;White et al, 2002;Hodges et al, 2005;Szulc et al, 2006].…”