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The metamorphic core of the Himalayan orogen, or Greater Himalayan sequence, is a northward tapering prism bound at the bottom by a N dipping family of thrust faults (the Main Central thrust system) and at the top by a N dipping family of normal faults (the South Tibetan detachment system). Research in the central Annapurna Range of Nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the Early Miocene. The Main Central thrust system is represented here by a 2‐ to 3‐km‐thick zone of high strain that developed during two or more episodes of movement. Most of its displacement was concentrated along the Chomrong thrust, a sharp, late‐metamorphic discontinuity that places middle amphibolite facies rocks of the Greater Himalayan sequence on top of lower amphibolite facies rocks of the Lesser Himalayan sequence. The earliest demonstrable movement on this thrust system occurred ∼22.5 Ma; the most recent movement may be as young as Pliocene. The oldest element of the South Tibetan detachment system in this area is the Deorali detachment, which appears to have been active at the same time as the earliest shortening structures of the Main Central thrust system. Fabrics related to the Deorali detachment are disrupted by a previously unrecognized, SW vergent, thrust structure, the Modi Khola shear zone. The effect of this structure, which is constrained to be between 22.5 and 18.5 Ma, was to shorten rock packages that had been extended previously during movement on the Deorali detachment. Transition back to a local extensional regime after 18.5 Ma was marked by development of the Machhupuchhare detachment and related splays. Geologic evidence for rapid, two‐way transitions between contraction and extension in the Annapurna Range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle; it also appears to be an important factor in mountain range development.

The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5‐km‐thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low‐grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobarometric and thermochronological data demonstrate that two major tectonometamorphic events characterize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite‐grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma 40Ar/39Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite‐grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T =880 K (610 °C) and P =940 MPa (=35 km), probably close to peak of metamorphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T =810 K (540 °C) and P=650 MPa (=24 km), indicating re‐equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late‐ to post‐metamorphic, out‐of‐sequence thrusting. The entire GHS cooled homogeneously below 600–700 K (330–430 °C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.

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The South Tibetan detachment system separates the high-grade metamorphic core of the Himalayan orogen from its weakly metamorphosed suprastructure. It is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front. Geochronologic data from the Rongbuk Valley, north of Qomolangma (Mount Everest) in southern Tibet, demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago, an interval characterized by large-scale crustal thickening at lower structural levels. These data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of Himalayan tectonics in Miocene time.