Partially melted, mica-bearing crust in Central Tibet

Abstract

Surface wave tomography shows that the central Tibetan Plateau (the Qiangtang block) is characterized by S wave speeds as slow as 3.3 km/s at depths from 20–25 km to 45–50 km and S wave radial anisotropy of at least 4% (VSH > VSV) that is stronger in the west than the east. The depth of the Curie temperature for magnetite inferred from satellite magnetic measurements, the depth of the α-β quartz transition inferred from VP/VS ratios, and the equilibration pressures and temperatures of xenoliths erupted from the middle to deep crust indicate that the Qiangtang crust is hot, reaching 1000°C at the Moho. This inferred thermal gradient crosses the dehydration melting solidi for crustal rocks at 20–30 km depth, implying the presence or former presence of melt in the Tibetan middle to deep crust. These temperatures do not require the wholesale breakdown of mica at these depths, because F and Ti can stabilize mica to at least 1300°C. Petrology suggests, then, that the Qiangtang middle to deep crust consists of a mica-bearing residue from which melt has been extracted or is being extracted. Wave speeds calculated for mica-bearing rocks with a subhorizontal to gently dipping foliation and 2% silicate melt are a good match to the wave speeds and anisotropy observed by seismology.