Wednesday, May 30th, 2018

Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)

“Crustal Flow & Melting in the Tibetan Plateau”

      – by Dr. Bradley Hacker, UCSB

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 alpha-beta 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 degrees 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 degrees 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.

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