Along-strike comparison of strain and timing of deformation in the lower Himalayan metamorphic core, west-central Nepal

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Gibson, Rohanna
Structural geology , Geochronology , Himalayan tectonics
Orogens are typically analysed and modelled as two-dimensional cross-sections, downplaying their three-dimensionality and potential variability along strike. Geologic mapping accompanied by microstructural and geochronological analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style, despite variability in the timing of both metamorphism and deformation. The Main Central thrust high-strain zone at the base of the Himalayan metamorphic core varies in thickness, lithotectonic stratigraphy, and field metamorphic gradient along the mapped 250 km strike length. Sampling from two along-strike transects targeted specific lithotectonic units near the base and top of the high-strain zone. In situ monazite U-Th/Pb geochronology and trace-element analysis of samples from the top of the high-strain zone record protracted Eocene-Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The metamorphic ages, as well as muscovite 40Ar/39Ar age, gradually decrease from northwest to southeast along strike, punctuated by an abrupt ca. 3-8 Myr age decrease in the east central Annapurna foothills. Geochronology results are not fully consistent with predictions from either channel flow or wedge extrusion models, and may exemplify the continuum between these end-member models. Quartz crystallographic preferred orientation fabrics from the base of the high-strain zone document similar flow style and plane-strain conditions at ca. 440°C along 70 km strike length in central Nepal. Muscovite 40Ar/39Ar ages, which are interpreted as the approximate ages of these deformation fabrics, decrease from ca. 7 to 4 Ma from northwest to southeast along strike at this structural level. Differences in the crustal architecture and timing of metamorphism and deformation indicate there is structural variation and diachronous evolution along strike of the lower metamorphic core in west-central Nepal. This variability spatially corresponds with the subsurface Faizabad ridge, a feature on the Indian plate that projects beneath the Himalaya in this region, and highlights the possible impact of subsurface ridges on the geometry of the basal Himalayan décollement and the tectonic evolution of the metamorphic core.
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