Identifying the Himalayan Hinterland-Foreland Transition in Central Nepal

Recumbent isoclinal folds and orogen transport direction-parallel stretching lineations indicative of extending flow commonly characterize orogenic hinterlands. Strain preserved in orogenic forelands, in contrast, is typified by thrust faulting and related folding characteristic of compressing flow. Exhumed mid-crustal rocks exposed in central Nepal, which are mapped as part of the Greater Himalayan sequence (GHS), record a progressive, multi-stage metamorphic and deformation history. The GHS comprises material in the hanging wall of the Main Central thrust (MCT), here mapped at the base of Tertiary pervasive deformation and metamorphism. In the study area the GHS is separated into two distinct tectonometamorphic domains. Metamorphism and deformation of the migmatitic upper domain is interpreted to have occurred synchronously at ca. 20 Ma. Metamorphic depth estimates define an apparent field gradient of 62 MPa/km, twice that expected for pelitic gneiss density. The distorted pressure field gradient of the upper domain is interpreted to reflect post-metamorphic vertical thinning. Assuming plane strain conditions, as indicated by petrofabric data, vertical thinning of the GHS would have been paired with horizontal stretching. In contrast, metamorphism and strain in the lower domain is diachronous, younging away from the migmatitic core toward the MCT. The lower domain is interpreted to comprise thrust slices added to the hanging wall of the MCT while the fault migrated downward structurally beneath the base of the migmatitic rocks after ca. 20 Ma. The net effect of the migration of the MCT was to vertically thicken and horizontally shorten the GHS. The vertical thinning and horizontal stretching of the upper domain of the GHS indicates hinterland-style extending flow. In contrast, the vertical thickening and horizontal shortening of the lower domain of the GHS is more typical of compressing flow commonly observed in foreland regions. The transition between the upper and lower domains, therefore, represents the change from hinterland-style deformation to foreland-style deformation as the mid-crustal material was extruded laterally from the back of the Himalayan orogenic wedge. The identification of the Himalayan hinterland-foreland transition may serve to reconcile some of the current contrasting interpretations of the GHS.