Relationships between displacement and distortion in orogens: Linking the Himalayan foreland and hinterland in central Nepal

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 similar to 450 degrees C and similar to 640 degrees C and is characterized by almost equal coaxial and noncoaxial components. Across the upper Greater Himalayan sequence, peak metamorphic temperatures are essentially isothermal, whereas corresponding metamorphic pressure estimates across the same section decrease downward with an apparent gradient of 620 bars/km. In the lower Greater Himalayan sequence, however, both metamorphic pressure and temperature decrease with structural depth. The abnormal pressure gradient in the upper Greater Himalayan sequence is attributed to similar 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 flow, i.e., extension in the direction of flow, whereas deformation in its lower portion was characterized by compressing flow, i.e., compression in the direction of flow. Extending flow is a distinctive feature of displacement and distortion in deep orogenic hinterlands, while compressing flow 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 between hinterland-style deformation, involving processes such as lateral midcrustal flow, and foreland-style deformation, involving critical-taper thrust-fold wedge development.