Telescoping of isotherms beneath the South Tibetan Detachment System, Mount Everest Massif

• Published in: Journal of structural geology 2011-11, Vol.33 (11), p.1569-1594
• Main Authors: Law, R.D., Jessup, M.J., Searle, M.P., Francsis, M.K., Waters, D.J., Cottle, J.M.
• Format: Article
• Language: English
• Subjects: Deformation temperatures; Detachment; Mount Everest; Quartz fabrics; Strain; Strain rate
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2011.09.004

Petrologic and microstructural/crystal fabric data indicate that isotherms recorded in Greater Himalayan Series (GHS) schists and gneisses in the footwall to the South Tibetan Detachment System (STDS) have undergone extreme telescoping during penetrative flow associated with southward extrusion of the GHS. In the Rongbuk Valley, to the north of Mount Everest, we have made three vertical sampling traverses from the STDS down into the GHS and estimated temperatures associated with penetrative deformation using the opening angles of quartz c-axis fabrics measured on dynamically recrystallized grains. From north to south, the deformation temperature data indicate apparent thermal field gradients of 369, 385 and 420 °C per km for our three traverses, traced over a maximum vertical sampling distance of 0.5 km. Adopting a differential flow path model, simple geometric analysis using sections drawn parallel to the local transport direction indicates that detachment-parallel transport magnitudes of 25–170 km are needed to explain the extreme telescoping of isotherms in the immediate footwall to the STDS, depending on assumed original geothermal gradient, dip of detachment, etc. These particle transport estimates are similar to those previously calculated from barometry data of GHS rocks in the Everest region and are compatible with channel flow models for extrusion and exhumation of the GHS. ► Deformation temperatures in the STDS footwall are estimated using quartz fabric opening angles. ► Results are described from three sampling transects on the north side of Mount Everest. ► Linear apparent thermal gradients of 370–420 °C per km are recorded at 0–500 m beneath the STDS. ► Models for this extreme telescoping of isotherms are discussed. ► Telescoping increases towards the south and may indicate dip-slip of 25–170 km on the STDS.