Symmetric Himalayan Topography and Drainage Pattern Induced by Asymmetric Strain Partitioning and Exhumation

A key question about the Himalayan evolution is how its topography is expressed as a result of interaction between strain distribution and climate-induced exhumation in space and time. To quantify this problem we investigate the modern Himalayan system by examining its GPS velocity field, kinematics of active structures, along-strike crest-line topography, climate-induced exhumation and drainage patterns across the Himalayan arc. GPS data show an eastward increase in arc-perpendicular shortening from 14 mm/yr to 29 mm/yr, 5-10 mm/yr right-slip arc-parallel shear in the western Himalaya, and 2-6 mm/yr left-slip arc-parallel shear in the eastern Himalaya. The transpressional tectonics across the Himalayan arc is expressed in two ways. Within the Himalaya, the strike-slip component is accommodated by motion on the discrete right-slip Karakorum fault in the west and distributed left-slip Dinggye-Chigu fault zone in the east. Across the Himalayan front, the transpressional tectonics is expressed by the development of en echelon folds indicating right-slip and left-slip shear in the west and east across the Main Frontal Thrust Zone. The lateral growing folds deflect south-flowing Himalayan drainages to the west in the western Himalaya and to the east in the eastern Himalaya. The along-strike Himalayan crest-line topography is quite symmetric, with the highest elevation in the central arc (8848 m at Mt. Everest) that decreases gradually to ~5000 m at the two ends of the range inside the two syntaxes. Due to the effect of the Asian monsoon system, the magnitude of Quaternary exhumation increases eastward along Himalayan strike. Our results show that despite the eastward increase in the magnitude of shortening, the Himalayan drainage pattern and crest-line topography are remarkably symmetric across the arc, suggesting that tectonics and climate have worked in concert in maintaining the dynamic equilibrium of the Himalayan morphology.