2012-09-26 – Seminar: Dr. Marianne Karplus
26 September 2012 – Seminar
When: 11am on Wednesday, 26 September 2012
Where: DIAS, Geophysics Section, 5 Merrion Square, Dublin 2, (library)
Speaker: Dr. Marianne Karplus
Title: Crustal structure and tectonics of north Tibet from controlled-source and broadband seismology
I will present on the three-dimensional crustal structure and plateau growth in the north Tibetan Plateau from the Qiangtang terrane to the Qaidam Basin. The INDEPTH IV project seismology group recently collected an abundance of data in north Tibet, and we use those active- and passive-source seismic datasets to investigate how north Tibet is deforming in response to the India-Asia collision including whether there may be mid-or lower-crustal flow within the plateau.
Our wide-angle reflection and refraction velocity model shows that the central Qaidam Basin resembles average continental crust, whereas the Songpan-Ganzi terrane and East Kunlun Mountains exhibit thickened, lower-velocity crust also characteristic of southern Tibet. Combining those results with P-wave receiver functions, we relocate the 20-km crustal thickness change to ~45 km north of the Kunlun Mountains topographic front, in a region of overlapping bright Moho reflectors at ~70 km and ~50 km. The crustal thickness change is unrelated to the strike-slip North Kunlun Fault. Crustal velocities and impedance contrasts suggest that weak Tibetan lower crust is injected northward beneath stronger Qaidam crust without southward subduction of Eurasian lithosphere.
Finally, at the broadest scale, we use ambient noise tomography to map regional differences in crustal Rayleigh wave group velocities across a larger area of north Tibet including the Qiangtang terrane. For periods of 8-24 s (sampling from ~10-35 km depth within the crust), we observe striking velocity changes at the major east-west Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary in the mid- to upper-crust. Our data also supports the hypothesis that north Tibet (~ 32°-36° N) is underlain by a low velocity zone at ~20-30 km depth.