The Golmud Step: New details of the 15 km Moho offset between the Tibet Plateau and Qaidam Basin from INDEPTH IV Seismic Results

Zhu and Helmberger (1998) reported a 15-20 km Moho offset over a narrow 5 km range under the northern margin of the Tibetan Plateau near Golmud from anomalous double-pulse teleseismic P-wave arrivals. They proposed a hotter, more ductile, less dense lower crust under the Tibetan Plateau in the south abutting a cold, rigid Qaidam Basin crust to the north, the latter serving as a buttress to lower crustal flow beneath the plateau. A subsequent teleseismic receiver function profile in the same vicinity by Vergne et al (2002) was interpreted to indicate a similar Moho offset of about 20 km located about 100 km north of Kunlun fault. Another teleseismic receiver function study by Shi et al (2009) also reported a 15 km Moho offset at the southern margin of the Qaidam Basin along a transect located about 270 km to the east of Golmud. In the summer of 2007, a N-S seismic reflection profile was collected as part of INDEPTH IV to extend from the central Qaidam Basin in the north to the Songpan-Ganzi terrane of the Tibetan plateau in the south (Zhao et al., 2008). The wide-angle reflection recordings from the larger shots (1-2 tons), especially KS5, provides a fresh perspective on Moho morphology across the Tibet- Qaidam boundary. Figure 1 shows the shot gather for KS5 displayed after applying an NMO correction using an average crustal Vp of 6.2 km/s. There is a clear reflection from a depth of about 51 km beneath the southern Qaidam basin, to a distinctive, albeit a weaker and lower frequency reflection at about 65 km depth beneath the Tibetan plateau to the south. Identifying both events with the Moho implies a 15-km offset about 20 km north of the North Kunlun Thrust along a line about 30 km to the east of Golmud. Close examination suggests that this increase in depth occurs over a zone that may be as narrow as 20 km, corresponding to a 35° dip of any linking segment. The magnitude of the offset implied by this correlation of reflectors, 15 km, is comparable with that estimated by the previous studies. The location of this offset lies about 20 km north of the Moho offset calculated by Zhu and Helmberger (1998) and Vergne et al (2002). We suggest that the abrupt nature of this crustal thickening implies significant strength in the lower crust at this margin of the plateau, and thus represents an important constraint on rheological models used to explain crustal thickening and uplift of the plateau. Figure 1. Shot gather as depth section for KS5 (blow up)