中科大地球与行星物理学术报告通知-焦利青

Trans-Himalayan GPS data confirms that, between both syntaxes, India/Asia convergence is steadily oriented ≈ N20°E, perpendicular to the range trend. However, measurements of co-seismic and Holocene surface faulting near both syntaxes, along the 2005 and 1950 earthquake ruptures, imply long-term thrusting directed ≈ 130° apart, and post-LGM shortening rates of only ≈ 5-6 mm/yr, ≈ 2 to 3 times slower than in Nepal (≈ 15-20 mm/yr). Syntaxial earthquakes' return-times are also ≈ 3 times longer (> 2000 yrs) than those in Nepal (≈ 700 yrs). 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 Frontal 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 was “pinned” by the strike-slip faults that bound it to the east and west. 3D Discrete Element Modelling corroborates a late-Tertiary elliptical range growth. This accounts for the ≈ 65° angles and 2-3-fold decrease in active thrusting between Nepal and the syntaxis, for the maximum Himalayan heights (≥ 8000 m), larger magnitudes (≥ 8), and shorter return times (≈ 700 years) 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/yr, N100-110°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. In this talk, Dr Jiao will analyze geometry/kinematics of the entire range and of its two main megathrusts (MFT and MCT), in keeping with broad-scale, along-strike geodetic measurements, earthquake’s focal mechanisms and field-studies of thrust faulting. She will show by using a simple, planar, laterally bounded Discrete Element Modelling (DEM) simulation, that, as predicted by solid mechanics, the range’s shape and growth best fit those of ellipses pinned at both syntaxes by ≈ NS, nearly orthogonal, conjugate strike-slip fault tips. Dr. Jiao will also discuss the mechanisms behind the formation of the Himalayan Arc.