Crustal structure and tectonics of northeast Tibet from controlled source and broadband seismology [electronic resource]
Keywords: Geophysics

In this dissertation I present my contributions to our understanding of three-dimensional crustal structure and plateau growth in the north Tibetan Plateau from the Qiangtang terrane to the Qaidam Basin. Tibet is an ideal location to study plateau development processes in an active continent-continent collision zone. Recent seismic projects have collected an abundance of seismic data in north Tibet, and I use a range of active- and passive-source seismic data to investigate tectonic processes in that area. In particular, I look at 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. I further probe the causes of crustal thickness changes across the plateau and whether the plateau is actively growing. I also ask whether there is present-day subduction of Eurasia beneath north Tibet and what role the Kunlun strike-slip fault plays in the regional tectonics. I derive a velocity model from wide-angle reflection and refraction data using a high-resolution, 270-km active-source seismic profile crossing the Kunlun Mountains and south Qaidam Basin. 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. Using that method along with P-wave receiver function imaging, I describe details of the crustal thickness change from 70 km beneath the Kunlun Mountains to 50 km beneath central Qaidam. In contrast to previous work, I relocate the change in crustal thickness 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. I further discuss differences in Vp/Vs across the terranes and calculate the Vp/Vs of the lower crustal material between the deep and shallow Mohos. Crustal velocities and impedance contrasts suggest that instead weak Tibetan lower crust is injected northward beneath stronger Qaidam crust without southward subduction of Eurasian lithosphere. Finally, at the broadest scale, I 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), I observe striking velocity changes at the major east-west Bangong-Nujiang and Jinsha suture zones as well as the Kunlun-Qaidam boundary. I see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, then higher velocities in the Songpan-Ganzi, and finally low velocities beneath the Qaidam Basin. Mid-to-upper crustal velocities also change laterally within the Songpan-Ganzi terrane, perhaps indicating subterrane boundaries, as hypothesized previously from geologic data. No velocity change is observed at the active North Kunlun Fault for periods 8-12 s, but for periods 18-24 s (~20-35 km depth) the wedge of Songpan-Ganzi crust between the South and North Kunlun Faults appears lower velocity than the surrounding Songpan-Ganzi terrane and Kunlun Mountains. Velocities west of ~93° E appear higher velocity than the Songpan-Ganzi terrane to the east. Finally, much of north Tibet (~ 32°-36° N) is underlain by a low velocity zone (LVL) at ~20-30 km depth.