IMPORTANT changes in South Asian climate occurred in the Late Miocene epoch (∼8 Myr ago)1,2, and these have been attributed by some researchers to uplift of the Tibetan plateau at about the same time 3-5. Unfortunately, this link has been difficult to test because the timing of plateau uplift remains poorly constrained by independent evidence. One way to determine the minimum age of uplift is to establish the initiation age of the north-striking normal fault systems in southern Tibet that are widely regarded6-10 as being related to gravitational collapse of the Tibetan plateau. Here we report an40Ar/39Ar age of ∼14 Myr for hydrothermal mica from an extensional fracture belonging to such a fault system in north-central Nepal. This age implies that east-west extension began before ∼14 Myr ago in at least some parts of the Tibetan plateau, suggesting that the plateau attained its high mean elevation well before Late Miocene time.
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