Late Miocene dismemberment of the Himalyan arc: Deformation of the Shillong Plateau, NE India

A common feature of convergent plate boundaries is the self-organization of strain, exhumation and topography along discrete, arcuate boundaries. Deviations from this geometry can represent first-order changes in the static stress applied to the plate boundary that must affect how strain is partitioned within the interior of the orogen. The simplicity of the Himalyan fold-and-thrust belt seen along its central portions breaks down along the extremities of the arc, particularly in the east, where a 400 km long antiformal fold (Shillong Plateau) has developed in the Indian cratonic basement. It is arguably the largest active basement fold structure in the world, and is 5 to 10 times larger than its commonly-sited analogs found in the Laramide orogeny of the western US or the Sierras Pampeanas of the Andean orogen. New apatite (U-Th-Sm)/He data suggest deformation initiated in mid to late Miocene time, significantly earlier than was previously estimated from the sedimentary record alone. Helium ages collected within 150 meters depth from the plateau surface vary systematically with depth between 116 and 14 Ma and are suggestive of slow cooling during this time interval. Samples collected on a vertical transect along the southern limb of the anticline indicate rapid cooling between 14 and 8 Ma, which we associate with the initiation of fold growth. Geomorphic and fluvial analyses are used to assess the fault geometry that underlies the anticline. Fault geometry, combined with our helium data, allow us to estimate a horizontal shortening rate of 0.28 to 0.63 mm/yr. The timing of fold initiation suggests that deformation of the Shillong Plateau may be temporally linked to a number of kinematic changes within the Himalaya/Tibetan orogen, as well as along the eastern India/Burma- Sundaland plate boundary. These events include the onset of E-W extension in central Tibet, eastward expansion of high topography of the Tibetan Plateau, onset of rotation of crustal fragments in southeastern Tibet, and re-establishment of eastward subduction beneath the Indo-Burman ranges. We suggest that the coincidence of these tectonic events is related to the "dismemberment" of the eastern Himalayan arc, signifying a significant change in regional stress along the India-Eurasian-(Burma)-Sundaland plate boundaries that affects the deformation field more than 1000 km into to the orogen.