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TitleOrogen-parallel deformation of the Himalayan midcrust: Insights from structural and magnetic fabric analyses of the Greater Himalayan Sequence, Annapurna-Dhaulagiri Himalaya, central Nepal
AuthorParsons, A J; Ferré, E C; Law, R D; Lloyd, G E; Phillips, R J; Searle, M P
SourceTectonics vol. 35, 2016 p. 1-23, https://doi.org/10.1002/2016TC004244
Year2016
Alt SeriesEarth Sciences Sector, Contribution Series 20160286
PublisherAGU Publications
Documentserial
Lang.English
Mediaon-line; digital
File formatpdf
AreaAfghanistan
Lat/Long WENS -83.7500 -83.5000 28.7500 28.2500
Subjectsstructural geology; Himalayan orogen; midcrustal deformation; structural analyses; magnetic stretching; mineral stretching
Illustrationslocation maps; tables; graphs; plots; stereograms
ProgramWestern Cordillera, Redefinition of crustal blocks, GEM2: Geo-mapping for Energy and Minerals
AbstractThe metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal, recorded orogen-parallel stretching during midcrustal evolution. Anisotropy of magnetic susceptibility and field-based structural analyses suggest that midcrustal deformation of the amphibolite facies core of the GHS occurred under an oblate/suboblate strain regime with associated formation of low-angle northward dipping foliation. Magnetic and mineral stretching lineations lying within this foliation from the top of the GHS record right-lateral orogen-parallel stretching. We propose that oblate strain within a midcrustal flow accommodated oblique convergence between India and the arcuate orogenic front without the need for strain partitioning in the upper crust. Oblate flattening may have also promoted orogen-parallel melt migration and development of melt-depleted regions between km3 scale leucogranite culminations at ~50-100 km intervals along orogen strike. Following the cessation of flow, continued oblique convergence led to upper crustal strain partitioning between orogen-perpendicular convergence on thrust faults and orogen-parallel extension on normal and strike-slip faults. In the Annapurna-Dhaulagiri Himalaya, orogen-parallel stretching lineations are interpreted as a record of transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel stretching. Our findings suggest that midcrustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose an orogen-wide transition from midcrustal orogen-perpendicular extrusion to upper crustal orogen-parallel extension during the mid-Miocene. The 3-D nature of oblate strain and orogen-parallel stretching cannot be replicated by 2-D numerical simulations of the Himalayan orogen.
Summary(Plain Language Summary, not published)
The metamorphic core of the Himalaya (Greater Himalayan Sequence, GHS), in the Annapurna-Dhaulagiri region, central Nepal, recorded stretching deformation parallel to the mountain belt during midcrustal evolution. Magnetic fabric analysis and field observations suggest that midcrustal deformation of the GHS occurred through flattening. We propose that midcrustal flattening accommodated oblique collision between India and the curved Himalayan mountain belt without needing to partition deformation into different parts of the crust. Flattening may have also promoted migration of magma and development of magma-depleted regions. After flattening, continued oblique collision led to upper crustal extensional deformation. Our findings suggest that midcrustal flattening and upper crustal extension could not be maintained simultaneously and support other studies from across the Himalaya, which propose a transition in deformation mechanisms during the mid-Miocene.
GEOSCAN ID299472