Structural and kinematic analysis of the Cuonadong dome, southern Tibet, China: Implications for middle-crust deformation

Interpreted deformation temperature versus structural thickness for the Cuonadong dome (blue bars and dots; this study), the Lhagoi Kangri dome (LKD, short dots; Diedesch et al., 2016), the Kangmar dome (KD, dark gray; after Wagner et al., 2010), the Mabja-Sakya dome (MD, light gray; after Langille et al., 2010), and the Dzakaa Chu transect (DC, dots; after Cottle et al., 2011). Dotted lines correspond to calculated deformation temperature gradients. [Display omitted] •High-strain D2 kinematic fabrics with a downward progression in the Cuonadong Dome.•Deformation temperatures associated with D2 kinematic fabrics increase with structural depth from ∼450 °C to > 630 °C.•Interpreted deformation temperatures yielded an average thermal field gradient of ∼ 180 °C/km over 1.05 km of structural thickness. The Cuonadong Dome is located at the eastern part of the North Himalayan gneiss domes. Structural and kinematic analysis in the middle unit of the Cuonadong Dome provide significant new insights into the structural deformation of the middle crust in the Northern Himalaya. Field observations, microscopic kinematic and EBSD analyses, and deformation temperature estimates from the mid-crustal rocks of the Cuonadong Dome show high-strain D2 (top-to-N) kinematic fabrics with a downward-to-north progression from dominantly top-to-north shear in the garnet zone, to solely top-to-north shear in the kyanite/sillimanite zone. A combination of mineral assemblages, microstructures, and quartz crystal preferred orientation patterns indicates deformation temperatures associated with D2 kinematic fabrics increase with structural depth from ∼450 °C in garnet zone to > 630 °C in the kyanite/sillimanite zone. These values broadly overlap temperature range deduced from metamorphic petrography (450–630 °C). Microtextural relations indicate that peak metamorphism occurred post-D1 and pre-D2 deformation. Combined with the ∼1000-m-thick ductile deformation zone, the interpreted deformation temperatures yielded an average thermal field gradient of ∼ 180 °C/km over 1.05 km of structural thickness. Based on the geochronologic data, midcrustal D2 extensional deformation in the Cuonadong Dome initiated as early as early Oligocene (32 Ma) and ended during the middle Miocene (∼14 Ma). Comparative studies of D2 ductile deformation in midcrustal rocks from other North Himalayan gneiss domes and the South Tibetan Detachment System indicate that D2 shear zone in the Cuonadong Dome represent the n