Episodicity and rates of thrust sheet motion in Himalaya (Western Nepal)
Résumé
The kinematics and rates of displacement along single faults of the Outer Belt and through the entire thrust belt of the Himalayas of western Nepal have been estimated by structural field work, balanced cross sections, fluviatile terrace-deposit studies, and geodesy. GPS geodetic studies indicate that the shortening is 15 ± 2mm/yr and is perpendicular to the trend of the western Nepal Himalayas and compatible with elastic strain accumulation. This surface shortening would be induced by a 19 mm/yr slip rate, at depth, on the basal detachment beneath the Great Himalayas, whereas the detachment is locked beneath the Lesser Himalayas. In the outer zone, most of the displacement presumably occurs during major seismic events, and the 2.5- to 5-m thrust movements observed locally for imbricate fans branching off the major thrusts of the Sub-Himalayas could reflect surficial rupture during these events. During the Holocene, the Main FrontalThrust (MFT) has been active, but portions of the piggyback thrusts of the Outer Belt also show episodes of activity that vary from a few meters to several tens of meters of displacement. The ratio of MFT shortening to total Himalayan shortening varies laterally, from 1 to less than 0.5, and out-of-sequence thrusting occurs in the Sub-Himalayas and possibly in the Lesser Himalayas. This pattern of episodic out-ofsequence reactivation fits with the evolution of a brittle thrust wedge affected by surficial mass transport and/or fluid pressure variation due to fault-valve behavior. The total shortening rate through the Himalayas of western Nepal following the Miocene is 19 mm/yr, with present-day, Holocene, and long-term shortening-rate uncertainties of 2, 6, and 5 mm/yr, respectivelyThe kinematics and rates of displacement along single faults of the Outer Belt and through the entire thrust belt of the Himalayas of western Nepal have been estimated by structural field work, balanced cross sections, fluviatile terrace-deposit studies, and geodesy. GPS geodetic studies indicate that the shortening is 15 ± 2mm/yr and is perpendicular to the trend of the western Nepal Himalayas and compatible with elastic strain accumulation. This surface shortening would be induced by a 19 mm/yr slip rate, at depth, on the basal detachment beneath the Great Himalayas, whereas the detachment is locked beneath the Lesser Himalayas. In the outer zone, most of the displacement presumably occurs during major seismic events, and the 2.5- to 5-m thrust movements observed locally for imbricate fans branching off the major thrusts of the Sub-Himalayas could reflect surficial rupture during these events. During the Holocene, the Main FrontalThrust (MFT) has been active, but portions of the piggyback thrusts of the Outer Belt also show episodes of activity that vary from a few meters to several tens of meters of displacement. The ratio of MFT shortening to total Himalayan shortening varies laterally, from 1 to less than 0.5, and out-of-sequence thrusting occurs in the Sub-Himalayas and possibly in the Lesser Himalayas. This pattern of episodic out-ofsequence reactivation fits with the evolution of a brittle thrust wedge affected by surficial mass transport and/or fluid pressure variation due to fault-valve behavior. The total shortening rate through the Himalayas of western Nepal following the Miocene is 19 mm/yr, with present-day, Holocene, and long-term shortening-rate uncertainties of 2, 6, and 5 mm/yr, respectively