The persistence of rapid exhumation in the eastern Himalayan syntaxis

Thesis (Ph.D.)--University of Washington, 2014At the eastern margin of the Indian-Eurasian plate collision, margin-normal plate convergence transitions to dextral strike-slip motion, warping tectonic units southward to form the eastern Himalayan syntaxis. Large southeast Asian rivers (e.g., the Yangtze, Salween and Mekong rivers) closely following the periphery of this syntaxis reflect the accumulation of crustal strain from the ongoing collision. In contrast, the Yarlung-Siang-Brahmaputra River bisects the syntaxis, abruptly dropping over two kilometers from the Tibetan Plateau through the steep Tsangpo Gorge. Along this reach, surface erosion rates increase sharply, contributing nearly half of the erosional efflux to the foreland from only ~3% of the river's drainage area. Curiously, this intense erosion directly coincides with the Namche Barwa metamorphic massif, a crustal-scale antiform that has been rapidly exhumed in the Late Pliocene and Pleistocene. Thermo-mechanical modeling indicates that if erosion has remained focused in this area over a multi-million year timescale, the continuous removal of upper crustal material may have resulted in the local exhumation of a crustal-scale structure. This consequential rock uplift would have maintained a steep topographic gradient at the southeastern margin of the Tibetan Plateau, such that the high relief and elevated erosional fluxes observed today may in fact have persisted for many millions of years in the eastern Himalayan landscape. This idea presents an intreseting concept in landscape evolution -that the legacy of processes shaping the surface of the Earth may extend much deeper to direcly influence the dynamics of the Earth's crust. This thesis specifically evaluates the circumstances by which rapid exhumation of the eastern syntaxis initiated and has been sustained in the Late Cenozoic. To do this, I extend the existing Late Pliocene and Pleistocene exhumation history by interpreting detrital evidence from syntaxis-proximal foreland basin deposits. First, I reconstruct continental-scale river drainage patterns from detrital analyses of foreland basin units, demonstrating a connection of the Yarlung-Siang-Brahmaputra River prior to the Middle or Early Miocene. Second, I document the potential of glacial ice and debris dam-burst flood events to exacerbate Quaternary erosion rates within this connected river system. Finally, I interpret the cooling histories of detrital minerals in the same foreland basin units to constraining the onset of rapid exhumation rates in the Late Miocene. I conclude from this work that an antecedent river has locally sustained rapid exhumation of the eastern syntaxis since the Late Miocene. In the Late Miocene, localized tectonic uplift accelerated exhumation where an antecedent river crossed the southeastern Tibetan Plateau margin. By the Early Pliocene, thermo-mechanical feedbacks between surface erosion and rock uplift may have developed to sustain rapid exhumation rates at the plateau margin to the present day