Syn-Compression Normal-Sense Low-Angle Detachment in the Himalayan Foreland, Western Nepal: Implications for Orogenic Models
Syn-Compression Normal-Sense Low-Angle Detachment in the Himalayan Foreland, Western Nepal: Implications for Orogenic Models
Previously Published Material: Results were partially presented in: Soucy La Roche, R., Godin, L., and Braden, Z., 2014, Recognition of the South Tibetan detachment in the Karnali klippe, western Nepal: Implications for emplacement of Himalayan external crystalline nappes, in Montomoli C. et al., eds., proceedings for the 29th Himalaya-Karakoram-Tibet Workshop, Lucca, Italy, p. 155.
Abstract ID#: 33713
English Abstract:
The Himalayan metamorphic core, exposed between two opposite sense shear zones, is locally preserved in a series of foreland klippen. The upper shear zone, the South Tibetan Detachment (STD), is a key element in many competing tectonic models. One of these, the tectonic wedging model, requires that the STD merges with the reverse-sense basal shear zone towards the foreland. The Karnali klippe in western Nepal is a doubly-plunging synform underlain by a ca. 2 km thick folded reverse-sense shear zone. It comprises amphibolite metamorphic facies rocks overlain by greenschist to subgreenschist facies sedimentary rocks. Based on quartz recrystallization textures and crystallographic preferred orientations, the metamorphic units record both coaxial and reverse-sense non-coaxial strain at temperature increasing structurally upward from 550-600°C just above the basal thrust to ca. 750 °C in the middle and upper levels. In contrast, a ca. 1 km thick low-angle normal-sense shear zone, correlated with the STD, separates the metamorphic rocks in the footwall from low grade sedimentary rocks in the hanging wall on both flanks of the klippe. Temperature of deformation decreases abruptly to 580 °C and to 475 °C in the lower and upper part of the STD, respectively. Calcite twin types suggest that sedimentary units of the hanging wall were deformed at temperatures of 150-200 °C. In situ U-Th/Pb geochronology on monazite, combined with chemical characterization, suggests that prograde metamorphism occurred between 36 and 30 Ma in the immediate footwall of STD, followed by tectonic exhumation from 28 to <24 Ma, possibly starting as early as 30 Ma. Transport-parallel exposure of the STD in this area implies a minimum slip of 145 km. The presence of the STD on both flanks of the Karnali klippe contradicts previous interpretations that it merges at depth with the basal shear zone, thus weakening the argument for tectonic wedging in western Nepal.