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TitlePreservation of the early evolution of the Himalayan middle crust in foreland klippen: insights from the Karnali Klippe, west Nepal
AuthorSoucy La Roche, RORCID logo; Godin, L; Cottle, J M; Kellett, D AORCID logo
SourceTectonics 2018 p. 1-33, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20170309
Mediapaper; on-line; digital
File formatpdf; xlsx
AreaHimalayas; Nepal; India; Bhutan
Lat/Long WENS 79.0000 92.0000 31.0000 27.0000
Subjectsstructural geology; tectonics; geochronology; mineralogy; geochemistry; crustal studies; crustal evolution; tectonic evolution; klippen; orogenies; metamorphism; deformation; foliation; lineations; thermal history; pressure-temperature conditions; crystallization; strain; crustal shortening; sutures; bedrock geology; structural features; faults; faults, thrust; faults, normal; faults, strike-slip; folds; homoclines; anticlines; synclines; shear zones; thermobarometry; radiometric dating; argon argon dating; uranium thorium ratios; uranium lead ratios; fabric analyses; petrographic analyses; crystallography; modelling; Karnali Klippe; Asian Plate; Indian Plate; Himalayan Orogen; Greater Himalayan Sequence; South Tibetan Detachment; Lesser Himalayan Sequence; Indus-Yarlung Zangbo Suture; Phanerozoic; Tertiary
Illustrationsgeoscientific sketch maps; cross-sections; photomicrographs; equal-area stereonet projections; plots; photographs; profiles; phase diagrams; spectra; schematic representations; bar graphs
ProgramScience laboratory network
Released2018 05 05
AbstractAlthough the India-Asia collision has been ongoing since the Eocene, the exposed hinterland of the Himalayan orogen was pervasively deformed and metamorphosed at high temperature during the Miocene and hence reveals little information about the Eocene-Oligocene period of collision. New pressure-temperature-time-deformation data from the Karnali klippe in west Nepal foreland demonstrate that Greater Himalayan sequence (GHS) rocks there escaped the Miocene overprint and consequently unveil the early tectonometamorphic evolution of the middle crust. Prograde metamorphism in the GHS occurred at 40 Ma and peak suprasolidus conditions in the kyanite stability field (i.e., >650-700°C and >0.7-1.0 GPa) were attained between 35 and 30 Ma. Peak metamorphism was followed by cooling, decompression, and melt crystallization at circa 30 Ma during tectonic exhumation below the South Tibetan detachment. As the middle crust was exhumed, strain propagated up section within the South Tibetan detachment high-strain zone, which remained active through circa 16 Ma. The GHS cooled below ~450-475°C at 20-17 Ma on the southwest flank and 17-14 Ma on the northeast flank of the Karnali klippe. In marked contrast, GHS rocks now exposed in the hinterland were still buried, hot and actively deforming, while the foreland was cooled and exhumed. Oligocene cooling of the frontal tip of the GHS is compatible with the southward extrusion of partially molten midcrustal rocks followed by renewed shortening along out-of-sequence shear zones in the hinterland.
Summary(Plain Language Summary, not published)
This manuscript presents new data on the timing of metamorphism and deformation in high-metamorphic grade rocks of the Himalayan orogen. The rocks under investigation are preserved in a 'klippe' or isolated packaged of metamorphic rocks that sits south of the main slab of metamorphic rocks. The new data presented here show that the rocks in the klippe have an earlier (Eocene to Oligocene) metamorphic history than the rocks in the main slab (Miocene), and thus inform us about the early metamorphic history of the India-Asia collision, a period which has hitherto been poorly studied and is thus poorly constrained.

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