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Geological Sciences Seminar: Simon Klemperer, Stanford University

When:
Tuesday, Sep 29, 2020 12:00 PM
Where:
Zoom
More Info:

Free

Audience:
Faculty/Staff, Students, Alumni/Friends
Sponsor:
Department of Geological Science

Continental subduction re-imagined: Mapping the mantle suture beneath Tibet using regional 3He/ 4He analysis of geothermal springs

An enduring question about continent-continent collision is the extent to which continental crust is subducted into the mantle, versus underplated beneath the overriding plate. For the Himalaya-Tibet archetype we use helium-isotopic analyses of geothermal springs to map a boundary that parallels the Yarlung-Zangbo suture in southern Tibet for >1000 km from 80–90°E, separating a Himalayan domain of no detectable mantle helium from a Tibetan domain with significant mantle helium. We interpret the helium boundary zone to overlie the ‘mantle suture’ where cold Indian lithospheric mantle is juxtaposed at the Moho against, and subducts beneath, a sub-Tibetan partially molten asthenospheric mantle wedge. Across several of the south-Tibetan rifts that parallel plate convergence, the helium boundary steps southward to the east, likely controlled by tears in underthrusting Indian lithosphere, as imaged by our seismic data. This segmented Indian slab likely subducts at low-angle beneath a thin layer of hot therefore weak mantle. Continental subduction as recorded beneath Tibet may resemble the modern-day ‘flat-slab’ subduction beneath the Andes’ Altiplano. Continental collision may closely resemble oceanic subduction, rather than being a distinct process. 

Simon joined the Stanford faculty in 1990, and began fieldwork in Tibet in 1992, collecting controlled-source seismic data with Chinese colleagues in the 'INDEPTH' (International Deep Profiing of Tibet and the Himalaya) transect that would eventually span Tibet from the High Himalaya to the northern Kunlun margin of Tibet. Klemperer’s group has also participated in a wide variety of active- and passive-seismic imaging programs spanning the Himalayan arc from west to east.  But the geologic problem is more interesting than the technique, and for the last decade (in addition to his seismic studies) Simon has sought to characterize the mantle beneath Tibet by sampling 200 geothermal springs over multiple field seasons, yielding the results shared today.

contact Kelly Wells, kcwells7@stanford.edu, for the Zoom link

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