Cenozoic clockwise rotation of the Tengchong block, southeastern Tibetan Plateau: A paleomagnetic and geochronologic study

Abstract

Paleomagnetic data from ~50–35Ma (likely ~40Ma) mafic dykes in Yunnan, southeastern Tibetan Plateau, cutting ~115Ma granitoids record the rotation of the Tengchong (Lhasa) block around the East Himalayan Syntaxis (EHS). Ti-rich titanomagnetite and magnetite carry a primary magnetic component (Group1); a magnetic overprint resides in magnetite (Group2), likely induced by low-grade metamorphism between ~30 and 10Ma. The tilt-corrected overall mean directions are D/I=89.8°/35.1° for Group1 and D/I=33.3°/41.4° for Group2. These data imply a clockwise rotation of ~87° (87.3±12.5°) of the Tengchong block since remanence acquisition at ~40Ma with respect to stable Eurasia. The average rotation rate of 2.18±0.31°/Myr is at the upper limit of the present-day rotation rates around the EHS obtained from GPS velocities and Quaternary strain rates. The remagnetized Group2 indicates a—poorly defined—rotation of ~31° (31.2±32.9°). Our key resultsthe detection of ~87° clockwise rotation, with high rotation rates following the India–Asia collision, and a decrease in rotation rates during the Miocenesuggest that first the Tengchong block rotated rapidly around the EHS, synchronous with eastward tectonic escape of lithospheric blocks. The later, slower, clockwise rotation—similar to those recorded geodetically—may be related to viscous flow of Tibetan crust.