Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya

Abstract

Provenance analysis of the Sub-Himalayan Late Miocene-Pleistocene foreland basin deposits (Siwaliks) from the Dehradun reentrant area provides a 10-Myr long record of the denudation history and tectonic evolution of the northwestern Indian Himalaya. We studied Siwalik sediments exposed along the Mohand-Rao and Haripur-Khol sections, using detrital zircon U-Pb geochronology, major and trace elements, and Sr-Nd isotope geochemistry. Results suggest that the erosion pattern has been relatively stable since the Late Miocene with sediments derived from the Tethyan Himalayan (THS), Greater Himalayan (GHS), and outer-(oLHS) and inner-Lesser Himalayan (iLHS) sequences. Provenance data indicate that erosional unroofing of the Lesser Himalayan Crystalline sequences (LHCS) initiated around 6 Ma, possibly related to out-of-sequence movement of the Ramgarh-Munsiari Thrust. Our data also suggest erosional recycling of older foreland basin deposits into younger Siwaliks since~5.5 Ma, which may indicate the time of thrust propagation from the Lesser Himalaya into the foreland basin. While the iLHS has been exposed to erosion since at least~10 Ma, the Siwaliks were dominated by materials derived from the GHS and THS sources. We interpret these results as an indication that tectonic uplift and erosion of the orogenic wedge occurred in response to duplexing of the iLHS and concomitant high topography and rock uplift rate in the Greater and Tethyan Himalaya. Comparing the provenance of the Siwalik sediments with that of the modern Ganga and Yamuna river sediments further indicates that deposition during the Late Cenozoic was most likely accomplished by southward flowing transverse Himalayan rivers, analogous to the modern ones.

A 6 Ma record of palaeodenudation in the central Himalayas from in situ cosmogenic 10Be in the Surai section

“…To reconstruct palaeodenudation rates from in situ cosmogenic 10 Be analyses of foreland sediments, it is, therefore, critical to trace potentially recycled sediments. In the Himalayas, although Sr-Nd isotopes have been widely used to study sediment provenance, our results and those of Mandal et al (2019) suggest that these geochemical proxies are insensitive to sediment recycling, which is better identified by simpler major and trace element analyses, notably Al/Si and Na/Si, which are linked to weathering and sorting processes, respectively Lupker et al, 2011).…”

Neogene Kinematic Evolution and Exhumation of the NW India Himalaya: Zircon Geo‐ and Thermochronometric Insights From the Fold‐Thrust Belt and Foreland Basin

“…No significant increase in the relative proportion of 1,700–1,900‐Ma grains is observed throughout these lower to middle Miocene sections, indicating that the iLH had yet to significantly breach the surface at this time. However, compilation of new and recently published detrital zircon U‐Pb ages from the middle to upper Siwalik Group within the Dehradun reentrant reveal a significant increase in abundance of 1,700–1,900‐Ma grains when compared to lower to middle Miocene foreland basin results (Figure ), providing compelling evidence that iLH unroofing initiated by at least ~7–10 Ma in NW India (Mandal et al, ). This is further supported by a similar shift in provenance documented within the Siwalik Group in Nepal (Bernet et al, ; DeCelles et al, ; Najman et al, ).…”

“…Detrital zircon U‐Pb ages results from all samples collected within the ~3‐km‐thick package of lower to middle Miocene foreland basin deposits of NW India showed minimal variation (Figure S5), and Figure shows a compilation of these data for simplicity. New detrital zircon U‐Pb ages from one sample of the upper Siwalik Group (15HP80) were compiled with published ages from middle‐upper Siwalik group of the Dehradun reeantrant (Mandal et al, ), and these data revealed a significant increase in the abundance of ~1,700–1,900‐Ma grains with an increase in the abundance of grains younger than ~400 Ma when compared to the age distribution of lower to middle Miocene deposits (Figure ). For detailed explanations of sample localities and individual sample results, we refer the reader to the supporting information.…”

“…Najman et al () suggest that the iLH began contributing its erosional products to the foreland by 9 Ma based on εNd values, with higher‐grade iLH rocks exposed by 6 Ma. Evidence from the Siwalik Group suggests the timing of Paleoproterozoic LH unroofing by ~11 Ma in Nepal and by ~7–10 Ma in NW India, when a significant increase in 1,700–1,900‐Ma detrital zircon grains, likely sourced from the LH, is observed in these deposits (DeCelles et al, ; Mandal et al, ; Figure ). While foreland provenance data presented here cannot directly constrain the initial timing of iLH unroofing in NW India, average bedrock iLH ZHe cooling ages of ~10 Ma agree with the initiation of iLH unroofing between 9 and 11 Ma.…”

“…Compilation of zircon U‐Pb ages from the middle to upper Siwalik Group reveal a significant influx of ~1,700–1,900‐Ma grains when compared to pre‐Siwalik Group deposits, indicating that initiation of iLH unroofing began by at least ~7–10 Ma in NW India. Siwalik U‐Pb ages from the Dehradun reentrant were compiled from Mandal et al (), and this compilation includes the results from one upper Siwalik Group sample analyzed in this study (15HP80; n = 93). Pre‐Siwalik U‐Pb ages are compiled from this study, and bedrock U‐Pb ages are compiled from this study and previously published data (Colleps et al, ; Mandal et al, , ; McKenzie et al, ; Myrow et al, , ; Spencer et al, ; Webb, Yin, et al, ).…”