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

- Neoproterozoic/Cambrian rocks blanketed Indian craton and all lithotectonic zones.
- Uplift of the Lesser Himalaya occurred at ∼16 Ma, ∼5 m.y. earlier than assumed.
- Requires an average exhumation rate of 1.9 mm/yr, similar to Cenozoic erosion rates.
- Uplift caused major changes in 187Os/188Os and 87Sr/86Sr Neogene record of seawater.
- Work supports Himalayan tectonic model of break-forward thrusting.

An extensive, northward deepening blanket of Neoproterozoic and Cambrian sedimentary rocks once extended from the Himalayan margin far onto the Indian craton. Cambrian deposits of this “upper Lesser Himalayan” succession, which include deposits of the “outer” Lesser Himalaya tectonic unit, are enriched in radiogenic 187Os. They make up part of a proximal marine facies belt that extends onto the craton and along strike from India to Pakistan. By contrast, age-equivalent facies in the Tethyan Himalaya are more distal in nature. Neoproterozoic to Cambrian strata of the upper Lesser Himalayan succession are now missing in much of the Lesser Himalaya, with their erosion exposing older Precambrian Lesser Himalayan strata. We suggest that exhumation and weathering of the upper Lesser Himalaya and related strata caused dramatic changes in the 187Os/188Os and 87Sr/86Sr Neogene record of seawater starting at ∼16 Ma. First-order estimates for the volume of upper Himalayan strata, as well as the volume of all LH rock eroded since this time, and geochemical box modeling, support this idea. Exhumation at 16 Ma is a fundamental event in the evolution of the Himalayan orogeny and the geochemical evolution of the oceans, and will be a critical part of the construction of future models of Himalayan thrust belt evolution.