The exhumed Himalayan midcrustal core exposed in the Likhu Khola region of east-central Nepal includes upper-greenschist-to upperamphibolite-grade metamorphic rocks that record pervasive, top-to-the-south sense deformation. Metamorphic temperature estimates are within error across the mapped area ranging from 772 ± 37 °C in the structurally lower, southern part of the study area to 853 ± 58 °C in the structurally higher, northern area. Estimated metamorphic pressures are relatively constant at lower structural levels, but they decrease from 11.8 ± 1.4 kbar to a minimum of 6.5 ± 1.3 kbar up structural section. The decrease in pressures coincides with an abrupt change in pressure estimates up structural section that is interpreted to mark a tectonometamorphic discontinuity that separates two domains with distinct structural, thermal, and metamorphic histories. In situ laser-ablation split-stream monazite U-Th/Pb and rare earth element (REE) petrochronology outlines dates ranging from ca. 27.8 Ma to 15.1 Ma in the hanging wall of the interpreted discontinuity; monazite REE data indicate the spread in ages is the result of a protracted metamorphic history and late-stage anatexis. Metamorphic and petrochronologic data from the Likhu Khola are consistent with a kinematic model in which material structurally above the discontinuity was metamorphosed in the deep hinterland of the orogen and was subsequently incorporated into the foreland of the orogen. The transition from hinterland-to foreland-style processes was marked by a shift to discrete deformation and the development of the discontinuity. Movement across the discontinuity is interpreted to have driven metamorphism and deformation of the rocks structurally below at or after 15 Ma. Discontinuities similar to that identified in this study are being identified and described across the orogen, indicating they are important, orogenwide features.

Detailed fi eld study of the Likhu Khola region in east-central Nepal has provided the basis for new lithological, metamorphic, and structural interpretations of the region. Metamorphic mineral assemblages defi ne an inverted metamorphic fi eld gradient from middle greenschist facies rocks (micaceous phyllitic schist) in the structurally lowest levels observed to upper amphibolite facies migmatitic rocks (sillimanite migmatite) in the structurally highest levels observed in this study. Quartz textures and c-axis orientations indicate that deformational temperatures also generally increase upward in the structural section, ranging from ~490 °C to >650 °C. These temperatures are compatible with observed metamorphic mineral assemblages at lower structural levels, indicating that deformation and metamorphism may have been contemporaneous. All rocks in the mapped area are pervasively deformed and typically record a top-to-the-south sense of shear; no discrete large-scale thrust or normal-sense structures were observed. The deformational temperatures recorded, however, are similar to those observed in the immediate hanging wall of the Main Central thrust, the base of the exhumed metamorphic core. The new observations from the Likhu Khola region are compatible with nearby studies that highlight structural, metamorphic, and temporal discontinuities within the exhumed Himalayan mid-crust. † Volume, mineralogy, and form estimated by outcrop documentation in the fi eld.

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