Reconstructing the exhumation history of the Lesser Himalaya, NW India, from a multitechnique provenance study of the foreland basin Siwalik Group

Abstract

This research presents the first multitechnique provenance study of the Siwalik Group in the Himalayan foreland basin in India, using the Jawalamukhi section, magnetostratigraphically dated at 13–5 Ma. Combined with provenance data from a Dharamsala Formation sedimentary section (21–13 Ma) located close by, it forms the longest temporally continuous record of Himalayan erosion in the Indian foreland basin. Sandstone petrography and heavy mineral analysis, conglomerate clast composition, Ar‐Ar dating of detrital white micas, and Sm‐Nd analyses on siltstones, conglomerate matrix and conglomerate clasts was undertaken to determine (1) shifts in source region through time and (2) changes in detrital lag times related to exhumation rates in the hinterland, together interpreted in the light of thrusting events. We interpret the data to show a slow down in exhumation rate of the Higher Himalaya by 16–17 Ma, after which time the locus of thrusting propagated south of the Main Central Thrust, and erosion of the low grade Haimanta Formation to the south became significant. The nonmetamorphosed Inner Lesser Himalaya breached its Haimanta cover by 9 Ma with the metamorphosed Inner Lesser Himalaya (Lesser Himalayan Crystalline Series) exhuming to surface by 6 Ma. This event caused sufficient disruption to established drainage patterns that all Higher Himalayan material was diverted from this location at this time.

Neogene marine isotopic evolution and the erosion of Lesser Himalayan strata: Implications for Cenozoic tectonic history

“…Cooling of leucogranite bodies at ∼16 Ma (Horton and Leech, 2013) along the MCT was linked to cessation of movement and southward propagation of the thrust belt (Jessup et al, 2006). Finally, foreland basin deposits show a dramatic decrease in Himalayan exhumation rates at ∼17 Ma, as shown by a shift in lag times between the depositional ages of samples and the ArAr ages of the youngest detrital white micas (White et al, 2002;Najman et al, 2009).…”

“…Recent studies in the Himachal Himalaya (White et al, 2002;Najman et al, 2009) also posit unroofing of a younger succession starting around 17 Ma, prior to erosional exposure of the older rocks of the lower Lesser Himalayan succession at 11 Ma. In particular, they demonstrate that a fundamental shift in petrologic and thermochronological character of foreland basin deposits takes place at ∼17 Ma, where strata record a shift from sediment sourced from higher-grade GH rocks to low-grade sedimentary sources (mostly carbonate and shale).…”

“…Exhumation rates in the Himalaya varied both spatially and temporally over the Cenozoic, but most estimates are less than 2 mm/yr (White et al, 2002;Najman et al, 2009;Deeken et al, 2011). An average rate of 1.9 mm/yr would be required to unroof the estimated 9300 m of younger upper Lesser Himalayan strata over the interval of ∼5 million years (from 16 to 11 Ma) required to expose enough of the older lower Lesser Himalayan succession to provide its detrital signal to the foreland basin.…”

Detrital zircon U–Pb geochronology of the Siwalik Group of the Nepal Himalaya: implications for provenance analysis

“…Along the Nepal Himalaya, geochronological studies have established depositional age from 13.2 Ma spanning a period till 1 Ma (DeCelles et al 1998a, b;Ojha et al 2000;DeCelles et al 2001DeCelles et al , 2004Szulc et al 2006;Chirouze et al 2012), which corresponds to the age ranges yielded by paleomagnetic studies (Appel et al 1991;Harrison et al 1993;Gautam and Rösler 1999;Gautam et al 2012). Similarly, there are number of milestone works in the northwestern Himalaya that interlinked the erosional records of the foreland basin with tectonic history (Burbank et al 1996;Najman et al 2004;Jain et al 2009;Najman et al 2009). These studies revealed that sediments in the foreland basin have been transported from the Lhasa terrane, Gangdese batholiths, Indus-Tsangpo suture zone, ultra-high-pressure gneiss terrane and Tethys Himalaya until Early Miocene time.…”

“…However, a similar study in the Nepal Himalaya has shown that Neogene foreland basin sediments were mainly derived from the Lesser Himalaya, Higher Himalaya and Tethys Himalaya (DeCelles et al 1998a). Recently, a number of studies have been carried out in the Nepalese foreland basin to constrain the exhumation history of the source region using zircon and apatite fission track dating ) and Ar-Ar dating Najman et al 2009;Chirouze et al 2012).…”