Himalayan source-to-sink dynamics and the detrital zircon Hf isotope record in Bengal Fan Turbidites (IODP Exp. 354)

Ocean drill cores recovered by IODP Expedition 354 Bengal Fan (BF) shed light on the world’s largest source-to-sink (S2S) sedimentary system, providing insight into its geodynamic history during the Late Cenozoic. Here, we investigate Hf isotopic compositions of detrital zircon (DZ) grains (N = 1300) by laser ablation multi-collector ICP-MS in Miocene through Pleistocene age turbidites from the middle BF. Prominent DZ age populations in BF turbidites at ca. 25 Ma, 50 Ma and 120 Ma are a close match for modern DZ ages obtained from Brahmaputra River sands, suggesting strong ties to Tibetan sources within the paleo-Brahmaputra drainage area. Previous studies documented increased proportions of young U-Pb DZ age populations (< 300 Ma) in BF turbidites, showing they nearly double in abundance starting in the Late Pliocene (ca. 2.7 Ma). We obtained Hf isotope compositions on individual U-Pb dated DZ grains, with an emphasis on placing further constraints on the provenance of the < 300 Ma DZ age group. BF turbidite sands all contain DZ showing a wide range of hafnium isotopic compositions (ε_Hf   = +20 to -30); however, Pleistocene turbidites contain a higher proportion (by 2:1) of more negative ε_Hf DZ signatures compared to Miocene/Pliocene-age turbidites. This is reflected in the ε_Hf signatures of the ca. 25, 50 and 120 Ma DZ U-Pb age groups, which all trend more negative in Pleistocene turbidites compared with Miocene/Pliocene -age turbidites. The effect is most pronounced for the ca. 50 Ma DZ age group; comparison with published zircon U-Pb and zircon Hf isotope data from bedrock sources indicate that the ca. 50 Ma DZ age group is derived from the Gangdese or Bomi Chayu batholiths of the Lhasa Terrane, whilst the ca. 120 Ma DZ age group is primarily sourced from the Bomi Chayu batholith; the ≤25 Ma DZ age group carries the signature of sources in the Namche Barwa massif (eastern Himalayan syntaxis portion of the Brahmaputra drainage). We hypothesize that greater integration of the Lhasa Terrane into the Yarlung-Brahmaputra river drainage system changed the dynamics of paleo-sediment transport to the Bengal Fan starting in the Late Pliocene. A growing U-Pb age and Hf isotope database from modern Himalayan river sands will help place further constraints on sediment routing, tectonic and climatic influences in the Himalaya-Bengal Fan S2S system since the mid-Miocene when delivery of turbidite sands to the middle Bengal Fan rapidly intensified.