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TitleDirect shear fabric dating constrains early Oligocene onset of the South Tibetan detachment in the western Nepal Himalaya
AuthorSoucy La Roche, RORCID logo; Godin, L; Cottle, J M; Kellett, D AORCID logo
SourceGeology vol. 44, no. 6, 2016 p. 403-406,
Alt SeriesEarth Sciences Sector, Contribution Series 20160008
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf
AreaHimalaya; Nepal
Lat/Long WENS 81.6667 82.1667 29.3333 28.9167
Subjectsstructural geology; geochronology; tectonics; structural analyses; structural features; shear stress; shearing; shear zones; Oligocene; metamorphism; analytical methods; uranium lead dates; uranium thorium dates; tectonic setting; tectonic environments; tectonic interpretations; South Tibetan detachment; Karnali klippe
Illustrationslocation maps; stereonets; cross-sections; photographs; photomicrographs
ProgramGEM: Geo-mapping for Energy and Minerals GSC Science Laboratory Network
Released2016 04 19
AbstractA newly identified and dated segment of the South Tibetan detachment in the Karnali klippe, western Nepal Himalaya, constrains initiation of mid-crustal tectonically driven exhumation to the early Oligocene. The folded top-to-the-northeast high-temperature (~600 °C) shear zone separates amphibolite-facies rocks with a ca. 36-30 Ma prograde metamorphic history in the footwall from weakly to non-metamorphosed upper crustal rocks in the hanging wall. In situ dating of syn-kinematic-post-metamorphic peak monazite indicates that the base of the shear zone was active from ca. 30-29 to <24 Ma, and a post-deformation muscovite cooling age implies that ductile shearing had ceased by ca. 19 Ma. Deformation along the South Tibetan detachment in western Nepal was thus synchronous with thrustsense shearing along the lower boundary of a zone of migmatitic rocks, compatible with tectonic models involving mid-crustal channelized flow during the Oligocene. Along with other published data from the Himalayan range, this suggests that the South Tibetan detachment actively exhumed the middle crust for almost 20 m.y.
Summary(Plain Language Summary, not published)
The Himalayan mountain belt is forming by the collision of two tectonic plates, India and Asia. By studying the major fault lines in the collision zone, we can understand the structure of big continental collisions that have occurred throughout Earth's history. The South Tibetan Detachment is a major fault in the Himalaya. In this work, we use geochronological data (ages of rocks and minerals) to show that the South Tibetan Detachment formed by the Early Oligocene, about thirty million years ago. This is earlier than previous studies have found, and coincides with the known timing of another major fault, the Main Central Thrust. This result allows us to compare feasible models for the formation of the Himalayan mountain belt.

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