Abstract Joining geological mapping, structural analysis, petrology and geochronology allowed the internal architecture of the Greater Himalayan Sequence (GHS) to be unraveled. Several top-to-the-south/SW tectonic–metamorphic discontinuities developed at the regional scale, dividing it into three main units exhumed progressively from the upper to the lower one, starting from c. 40 Ma and lasting for several million years. The activity of shear zones has been constrained and linked to the pressure–temperature–time–deformation (P – T – t – D) evolution of the deformed rocks by the use of petrochronology. Hanging wall and footwall rocks of the shear zones recorded maximum P – T conditions at different times. Above the Main Central Thrust, a cryptic tectonometamorphic discontinuity (the High Himalayan Discontinuity (HHD)) has been recognized in Central-Eastern Himalaya. The older shear zone, that was active at c. 41–28 Ma, triggered the earlier exhumation of the uppermost GHS and allowed the migration of melt, which was produced at peak metamorphic conditions and subsequently produced in abundance at the time of the activation of the HHD. Production of melt continued at low pressure, with nearly isobaric heating leading to the genesis and emplacement of andalusite- and cordierite-bearing granites. The timing of the activation of the shear zones from deeper to upper structural levels fits with an in-sequence shearing tectonic model for the exhumation of the GHS, further affected by out-of-sequence thrusts.

ABSTRACT 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-of-sequence reactivation fits with the evolution of a brittle thrust wedge affected by sur-ficial 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.

ABSTRACT The Sub-Himalayan Zone constitutes a tectonic wedge of synorogenic sediments along the southern edge of the Himalayan Belt. Sediments are incorporated into the prism from the foreland Indo-Gangetic plain, undergo a tectonic cycle within it, and eventually are eroded. The structural sketch map exhibits westward-plunging arcuate structures on the foremost location of the Outer Belt. Investigations from spatial imagery and digital elevation modeling (DEM), together with kinematic data, allow us to calculate velocities for the geomorphologic development. Four velocities rule the general evolution of the wedge. The foremost geomorphic structure (ridge) is the assemblage of elementary structures. The lateral ridge propagation velocity is estimated to be 40 cm/yr, which supports a general cylindrical development of the Outer Belt, in spite of the asymmetrical development of each independent elbow-shaped structure. The sediment’s burial history can be quantified from geometric and kinematic data. We emphasize that because of the cylindrical behavior of the prism, extrapolation of the sediment transfer to the entire western Nepal Siwalik is valid. Burial in the foreland basin takes two times longer than the entire tectonic cycle, which only lasts for about 6.5 m.y. Sediments reaching 6 x 10– 5 km 3 per year and per linear kilometer accrete along the Siwalik range. About 21% of the flowing material within the wedge is captured and withdrawn from it as subducted duplexes. Assuming a steady-state development of the wedge, and according to the Coulomb-wedge theory for the Siwalik, mean erosion rates are estimated to be about 1.9 mm/yr, which is in accord with previous estimates. We emphasize that this consistency supports the overall estimates for the general development of the wedge.