The shape of the Himalayan "Arc": An ellipse pinned by syntaxial strike- slip fault tips

Trans-Himalayan geodetic data show that, between both syntaxes, India/Asia convergence is steadily oriented approximate to N20 degrees E. However, surface faulting near both syntaxes, along the 2005 and 1950 earthquake ruptures, imply long-term thrusting directed approximate to 130 degrees apart, and post-LGM (last Glacial Maximum) shortening rates of approximate to 5 to 6 mm/y, approximate to 2 to 3 times slower than in Nepal (approximate to 15 to 20 mm/y). Syntaxial earthquakes' return-time are also approximate to 3 times longer (>2,000 y) than in Nepal (approximate to 700 y). In a structural frame centered halfway between the syntaxial cusps, the tectonic features of the range show remarkable symmetry. In map view, the overall shapes of the Main Front Thrust (MFT) and the Main Central Thrust (MCT) closely fit ellipses, with major-to-minor axis ratios of approximate to 2.5 to 3. This suggests that the range growth atop subducting India is "pinned" by the strike-slip faults that bound it to the east and west. Discrete Element Modeling corroborates a late-Tertiary elliptical range growth. This accounts for the approximate to 65 degrees angles and twofold to threefold decrease in active thrusting between Nepal and the syntaxes, for the maximum Himalayan heights (>= 8,000 m), larger magnitudes (>= 8), and shorter return-time (approximate to 700 y) of great earthquakes in Nepal, for the existence of two 500- to 600 km-long, south-concave mountain ranges north of both syntaxes and for the approximate to 9 mm/y, N100 to 110 degrees E extension across southern Tibet. It also suggests that predictions of impending or frequent great earthquakes in the eastern- and westernmost Himalayas may be overstated.