HR: 1340h
AN: T23C-0563    [Abstracts]
TI: Drainage Pattern, Along-Strike Topography and Three-Dimensional Construction of the Himalayan orogen
AU: * Yin, A
EM: yin@ess.ucla.edu
AF: An Yin, Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095-1567 United States
AB: Past studies on the Himalayan orogen have mostly emphasized its 2-D evolution in cross-section view. As a result, how the orogen has grown in 3-D remains poorly understood. For example, it is not clear if the >1500-km long Main Central Thrust (MCT) and Main Boundary Thrust (MBT) were initiated synchronously along the whole orogen or started at one segment and then propagated laterally during subsequent fault motion. To clarify this issue, I examined the Himalayan drainage pattern, along-strike topography, and geometry and kinematics of growing contractional structures across both the Himalayan front and the Shillong Plateau. The main observations may be summarized as follows. (1) The five rivers that cut across the Himalaya are arranged approximately symmetric with respect to the Himalayan-arc axis (~85° E): the Arun River (87° E) is in the middle with the Sutlej (77° E) and Indus (72° E) Rivers in the west and the Subansiri (93° E) and Yalu-Brahmaputra (96° E) Rivers in the east. (2) Between the eastern and western syntaxes, south-flowing drainages east of 85.5° E are consistently deflected to the east by east-growing anticlines and thrusts, whereas drainages west of 85.5°E are deflected systematically to the west by west-growing anticlines and thrusts along the Himalayan front. The only region where no drainage deflection is observed is the Bhutan Himalaya. There all rivers flow straight across the Himalayan front. (3) The deflected drainage pattern indicates that the Shillong Plateau south of the eastern Himalaya has been growing westward. (4) The along-crest Himalayan topographic profile concaves downward, starting from ~5200 m just inside the two syntaxes and reaching 8848 m at Mt. Everest (~87° E). (5) There are a total of 17 major growing contractional structures in the Main Frontal Thrust Zone (MFTZ), with 10 in the west and 7 in the east. Each structure has a length between ~20 km and >150 km, but the west-growing structures in the west are significantly longer (with four > 150 km) than the east-growing structures (all of them < 70 km). The above observations have the following implications. (A) The Main Frontal Thrust Zone consists of a series of en echelon folds and thrusts instead of being a single thrust. The en echelon arrangement of contractional structures indicates that a significant component of right-slip and left-slip motion occurs along the western and eastern limbs of the Himalayan arc. The sense of shearing indicates that the Himalayan arc has been progressively tightened. (B) The MFTZ was initiated in the middle segment of the Himalayan arc and has subsequently propagated laterally to the east and west. (C) The eastward increase in convergence velocity between India and Asia has played a minor role in controlling the late Cenozoic development of the Himalayan arc. That is, the divide for both the easterly and westerly deflected Himalayan drainages and east- and west-growing structures only departs from the central Himalayan-arc axis for ~1° to the east. Similarly, the highest point in the along-crest topographic profile only deviates ~2° east of the arc axis. I suggest that the eastward increase in convergence rate between India and Asia has been accommodated partially by the west-growing Shillong Plateau along the north-dipping Dauki thrust. That is, the thrust has absorbed extra shortening in the eastern Himalaya resulting from this asymmetric boundary condition. (D) In light of the late Cenozoic development of the MFTZ and the Dauki thrust, the Himalayan orogen may have been constructed by alternation or coeval development of structures initiating either in its center or at one of its two ends.
DE: 1814 Energy budgets
DE: 8100 TECTONOPHYSICS
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8104 Continental margins: convergent
DE: 8108 Continental tectonics: compressional
SC: Tectonophysics [T]
MN: Fall Meeting 2005