Abstract:

In the central Qinghai-Tibet Plateau, there is a NE trending and S-shaped graben called Erdaogou basin .The graben, about 25 km long and about 2 km wide, cuts the east-west trending Fenghuo Shan thrust-folds belt which had been formed during Tertiary or from Late Cretaceous to Palaeogene. The basin floor is covered by Late Quaternary deposits.The graben is apparently asymmetric with the main boundary fault along its northern margin. Many phenomena indicate the active normal faulting with the component of left lateral strike-slip along the fault zones striking NE60°-90° and dipping 50°-80° to the south Along the footwall of fault, there is a prominent triangular facet, which is 200-300 m above the surface of piedmont plain.At the base of triangular facets, faulting is pressed by south-east facing fault scarps cutting fluvial fans and travertine platform. The well-preserved fault scarps offset vertically the fluvial fans about 1,5 m and 4,5±1 m formed during Holocene, and Late Pleistocene respectively and maybe offset left lateral the fluvial fans 8.10 m formed during Late Pleistocene,Age results of U-series and thermo luminescence (TL) indicate that offsets of the Holocene and Late Pleistocene fluvial fans represent the fault throws since 5-6 ka B.P. and 34-16 ka B.P. respectively,These suggest that the average vertical slip rate is 0.28±0.06 mm/a and the left lateral strike slip rate is 0.5-0.8 mm/a along the faults at north side of Erdaogou graben. This corresponds to a “spreading” rate of 0.1-0.2 mm/a across Erdaogou graben. The relief across the normal fault indicate that the minimum cumulative displacement on fault maybe is about 500 m. Based on the minimum cumulative throws and average slip rate of fault, the initiation age of the normal faulting may be no younger than 2.3 Ma.The new data from Erdaogou graben are similar to those from Shuang Hu graben and Wen Quan graben in central Qiangtang block and demonstrate that the rate of Late Quaternary extension in central part of the Qinghai-Tibet Plateau is roughly an order of magnitude lower than in southern Tibet.The lower slip rate of normal faults is also coincide with less prominent morphologic expressions of grabens or rifts in central Tibet, less continuous and shorter than those in southern Tibet. Based on the slip magnitude of fault in central Tibet, the cumulative extensional rate perhaps did not exceed about 3 mm/a across the Qiangtang block during Late Quaternary.

Key words: Qinghai-Tibet Plateau, Fenghuo Shan, Quaternary, extensional deformation, active fault