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TitleDiachronous deformation along the base of the Himalayan metamorphic core, west-central Nepal
AuthorGibson, R; Godin, L; Kellet, D AORCID logo; Cottle, J MORCID logo; Archibald, D
Source 2016, 19 pages,
Alt SeriesEarth Sciences Sector, Contribution Series 20150160
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf
AreaHimalayas; Nepal
Lat/Long WENS 74.0000 88.0000 36.0000 26.0000
Subjectsigneous and metamorphic petrology; structural geology; tectonics; orogenesis; orogenic regions; orogenies; faults, thrust; faults, strike; plate tectonics; metamorphic environment; metamorphic rocks; metamorphic zones; metamorphism, prograde; metamorphism, retrograde; Jurassic
Illustrationslocation maps; sketch maps; graphs; geochronological charts; geochemical charts; structural diagrams; p-t tables; p-t diagrams; photomicrographs; cross-sections, structural; cross-sections, stratigraphic
ProgramScience laboratory network
Released2016 02 01
AbstractGeologic mapping accompanied by microstructural and geochronologic analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style despite variability in the timing of metamorphism and deformation. The Main Central thrust zone at the base of the Himalayan metamorphic core varies in thickness, tectonostratigraphy, and metamorphic gradient along the 250 km of strike length studied. In situ U-Th/Pb petrochronology of monazite sampled from an along-strike transect at the top of the high-strain zone records Eocene-Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The timing of prograde and retrograde metamorphism and the muscovite 40Ar/39Ar dates gradually decrease along strike from northwest to southeast. This age trend is punctuated by an abrupt ~3-8 Myr decrease in the age of prograde and retrograde metamorphism and muscovite 40Ar/39Ar dates near the Marsyandi River in central Nepal. Quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high-strain zone document similar flow style at ~440°C in central Nepal. Muscovite
40Ar/39Ar ages, interpreted to approximate the age of deformation at this structural level, decrease from ca. 7 to 4 Ma along strike from northwest to southeast. Along-strike tectonometamorphic variability in west-central Nepal spatially corresponds to faults in the Indian basement bounding the subsurface Faizabad ridge, highlighting the probable influence of inherited basement faults on the geometry of the basal Himalayan detachment, the Main Himalayan thrust, as well as the tectonic evolution of the structurally-overlying Himalayan metamorphic core.
Summary(Plain Language Summary, not published)
The record of deformation in the metamorphic core of the Himalayan mountain belt is similar along a 250 km length of the belt in central Nepal. However, the time at which deformation occurred, determined by radiometric age dating of the minerals monazite and muscovite, is shown to be diachronous across this length. The diachroneity may be reflective of the effects of inherited basement features of the Indian continental plate which underlies the mountain belt. This study demonstrates that linking radiometric age information to rock deformation is critical for revealing complexities in the evolution of mountain belts. It also demonstrates that basement structures influence the resulting formation of overlying mountain belts.

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