The Continuing Structural Evolution of the Southern Edge of the Tibetan Plateau

The physiographic transition from the steep and rugged topography of the southern flanks of the Himalayan ranges to the much more subdued topography of the Tibetan Plateau is among the most dramatic on Earth. This transition, which can be defined as the southern edge of the plateau, is remarkably abrupt, always occurring over a distance of less than 50-75 km, and typically over a much shorter distance. It corresponds to a profound discontinuity in modern surface strain between the Tibetan Plateau and the Himalaya. To the north, the surface strain field of southern Tibet is one of essentially E-W extension expressed as N-S-striking normal fault systems and NW-SE-striking and NE-SW-striking transcurrent fault systems. To the south, the modern surface strain of the Himalaya is characterized by N-S contraction by deformation on active, N-S-striking, S-vergent thrust fault systems. This discontinuity persists at least as far back in time as the initiation of N-S graben systems of southern Tibet in Middle-Late Miocene time. Bedrock geologic studies have shown that this transition zone also sometimes includes segments of the Miocene South Tibetan fault system (STFS), an important family of structures dominated by north-dipping, low-angle detachments with normal-sense kinematics. One popular model of plateau evolution holds that the STFS trace was the Miocene southern plateau margin and marked the tip of a decoupling horizon between brittle upper crust of the plateau and a fluid middle and/or lower crust of tectonically reworked Indian Plate lithologies that flowed southward from beneath Tibet and toward the Himalayan foreland under the influence of gravity. Is it possible that the southern edge of the plateau is defined by Neotectonic structures that are similar to, and perhaps played a similar geodynamic role as, Miocene structures of the STFS? Our structural, geomorphologic, and thermochronologic studies along the plateau margin from Bhutan to NW India suggest that this may be the case. We find that the plateau edge is coincident with knick zones on large trans-Himalayan rivers as they flow off the plateau, and many of these knick zones are coincident with mappable faults and mineral cooling age discontinuities suggestive of significant post-Pliocene displacement. Since these discontinuous features are not precisely coincident with older STFS strands, we regard them as an entirely younger family of structures that we refer to collectively as the Tibet-Himalaya Accommodation Zone. In most cases, this zone lies south of the trace of the Miocene STFS, implying a southward migration of the southern plateau margin toward the Indian foreland, as well as a southward expansion of the Tibetan strain field.