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

ABSTRACT

Trans-Himalayan geodetic data show that, between both syntaxes, India/Asia convergence is steadily oriented ≈ N20°E. However, surface faulting near both syntaxes, along the 2005 and 1950 earthquake ruptures, imply long-term thrusting directed ≈ 130° apart, and post-LGM (last Glacial Maximum) shortening rates of ≈ 5 to 6 mm/y, ≈ 2 to 3 times slower than in Nepal (≈ 15 to 20 mm/y). Syntaxial earthquakes' return-time are also ≈ 3 times longer (>2,000 y) than in Nepal (≈ 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 ≈ 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 ≈ 65° 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 (≈ 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 ≈ 9 mm/y, N100 to 110°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.