Regional Pliocene exhumation of the Lesser Himalaya in the Indus drainage

Abstract

<p><strong>Abstract.</strong> New bulk sediment Sr and Nd isotope data, coupled with U–Pb dating of detrital zircon grains from sediment cored by the International Ocean Discovery Program in the Arabian Sea, allow the reconstruction of erosion in the Indus catchment since <span class="inline-formula">∼17</span>&amp;thinsp;Ma. Increasing <span class="inline-formula"><i>ε</i>_Nd</span> values from 17 to 9.5&amp;thinsp;Ma imply relatively more erosion from the Karakoram and Kohistan, likely linked to slip on the Karakoram Fault and compression in the southern and eastern Karakoram. After a period of relative stability from 9.5 to 5.7&amp;thinsp;Ma, there is a long-term decrease in <span class="inline-formula"><i>ε</i>_Nd</span> values that corresponds with increasing relative abundance of <span class="inline-formula">&amp;gt;300</span>&amp;thinsp;Ma zircon grains that are most common in Himalayan bedrocks. The continuous presence of abundant Himalayan zircons precludes large-scale drainage capture as the cause of decreasing <span class="inline-formula"><i>ε</i>_Nd</span> values in the submarine fan. Although the initial increase in Lesser Himalaya-derived 1500–2300&amp;thinsp;Ma zircons after 8.3&amp;thinsp;Ma is consistent with earlier records from the foreland basin, the much greater rise after 1.9&amp;thinsp;Ma has not previously been recognized and suggests that widespread unroofing of the Crystalline Lesser Himalaya and to a lesser extent Nanga Parbat did not occur until after 1.9&amp;thinsp;Ma. Because regional erosion increased in the Pleistocene compared to the Pliocene, the relative increase in erosion from the Lesser Himalaya does not reflect slowing erosion in the Karakoram and Greater Himalaya. No simple links can be made between erosion and the development of the South Asian Monsoon, implying a largely tectonic control on Lesser Himalayan unroofing.</p>