An inverted metamorphic field gradient associated with a crustal-scale south-vergent thrust fault, the Main Central Thrust, has been recognized along the Himalaya for over 100 years. A major problem in Himalayan structural geology is that recent workers have mapped the Main Central Thrust within the Greater Himalayan Sequence high-grade metamorphic sequence along several different structural levels. Some workers map the Main Central Thrust as coinciding with a lithological contact, others as coincident with the kyanite isograd, up to 1-3 km structurally up-section into the Tertiary metamorphic sequence, without supporting structural data. Some workers recognize a Main Central Thrust zone of high ductile strain up to 2-3 km thick, bounded by an upper thrust, MCT-2 (¼ Vaikrita thrust), and a lower thrust, MCT-1 (¼ Munsiari thrust). Some workers define an 'upper Lesser Himalaya' thrust sheet that shows similar P-T conditions to the Greater Himalayan Sequence. Others define the Main Central Thrust either on isotopic (Nd, Sr) differences, differences in detrital zircon ages, or as being coincident with a zone of young (,5 Ma) Th-Pb monazite ages. Very few papers incorporate any structural data in justifying the position of the Main Central Thrust. These studies, combined with recent quantitative strain analyses from the Everest and Annapurna Greater Himalayan Sequence, show that a wide region of high strain characterizes most of the Greater Himalayan Sequence with a concentration along the bounding margins of the South Tibetan Detachment along the top, and the Main Central Thrust along the base. We suggest that the Main Central Thrust has to be defined and mapped on strain criteria, not on stratigraphic, lithological, isotopic or geochronological criteria. The most logical place to map the Main Central Thrust is along the high-strain zone that commonly occurs along the base of the ductile shear zone and inverted metamorphic sequence. Above that horizon, all rocks show some degree of Tertiary Himalayan metamorphism, and most of the Greater Himalayan Sequence metamorphic or migmatitic rocks show some degree of pure shear and simple shear ductile strain that occurs throughout the mid-crustal Greater Himalayan Sequence channel. The Main Central Thrust evolved both in time (earlymiddle Miocene) and space from a deep-level ductile shear zone to a shallow brittle thrust fault.

Greater Himalayan sequence rocks exposed in the Manaslu-Himal Chuli Himalaya can be separated into distinct upper and lower parts. Deformation recorded in both parts occurred at temperatures ranging between ~450 °C and ~640 °C and is characterized by almost equal coaxial and noncoaxial components. Across the upper Greater Himalayan sequence, peak metamorphic temperatures are essentially isothermal, whereas correspond ing metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars/km. In the lower Greater Hima layan sequence, however, both metamorphic pressure and temperature decrease with structural depth. The abnormal pressure gradient in the upper Greater Himalayan sequence is attributed to ~50% vertical thinning during southward displacement, while the inverted gradient in the lower portion is interpreted to be the result of coeval exhumation and downward expansion of the Main Central thrust shear zone and the progressive incorporation of more rock into the Greater Himalayan sequence. Deformation in the upper portion of the Greater Himalayan sequence was characterized by extending fl ow, i.e., extension in the direction of fl ow, whereas deformation in its lower portion was characterized by compressing fl ow, i.e., compression in the direction of fl ow. Extending fl ow is a distinctive feature of displacement and distortion in deep orogenic hinterlands, while compressing fl ow is emblematic of displacement and distortion in orogenic foreland regions. The transition between the upper and lower parts of the Greater Himalayan sequence therefore represents a fundamental transition be-tween hinterland-style deformation, involving processes such as lateral midcrustal fl ow, and foreland-style deformation, involving critical-taper thrust-fold wedge development.

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In the Dhaulagiri region of central Nepal quartz-rich specimens sampled from the Greater Himalayan sequence yield well-defined quartz c -axis fabrics with a dominant top-to-the-SW sense of shear. These fabrics reflect pervasive crystal-plastic deformation extending more than 8 km structurally below previously mapped locations of the Main Central thrust fault. Quartz c -axis fabric opening angles suggest deformation temperatures of c . 500 ± 50 °C within the lower portion of the Greater Himalayan sequence and up to c . 670 ± 50 °C within the migmatitic upper portion. These temperatures generally plot within error of garnet–biotite temperature estimates for metamorphic assemblages and are interpreted to reflect Tertiary deformation during extrusion of the mid-crust. The quartz c -axis data, and a new, detailed tectonostratigraphic map, constrain the position of the Ramgarh thrust in the Dhaulagiri region to be coincident with the Main Central thrust at the base of the pervasively deformed exhumed mid-crustal core. Mean kinematic vorticity numbers ( W m ) measured in specimens sampled from the lower portion of the Greater Himalayan sequence range between 0.49 and 0.80 ( c . 66–41% pure shear) with an average value of 0.67 ( c . 53% pure shear). These data indicate that extrusion of the mid-crust was facilitated by a significant component of pure shear strain.