Quaternary extension in southern Tibet: Field observations and tectonic implications

We summarize evidence for Quaternary and active faulting collected in the field during three Sino‐French expeditions to southeastern Tibet (1980–1982). Detailed mapping of Quaternary and active faults as well as microtectonic measurements indicate that normal faulting has been the dominant tectonic regime north of the Himalayas in the last 2 ± 0.5 m.y. The maximum horizontal principal stress in south Tibet appears to be only the intermediate principal stress σ 2 , σ 1 being vertical. South of the “chord” joining the eastern and western syntaxes of the Himalayan arc, extensional strains are principally localized within seven regularly spaced rift zones, three of which have been studied in some detail. The extension direction is determined to be N96° E ± 7° mainly from statistical averaging of strikes of newly formed normal faults. Throw rates on normal faults are evaluated for different time spans (2 ± 0.5 m.y., 60 ± 40 kyr, and 10 ± 2 kyr B.P.), using structural and topographic reliefs, as well as synglacial and postglacial vertical offsets. The rate of Quaternary extension is about 1% m.y. −1 along an 1100‐km‐long ESE traverse across south Tibet. This corresponds to a “spreading” rate of 1 ± 0.6 cm/yr. This rate and the divergent horizontal projections of slip vectors of earthquakes along the Himalayan front constrain the rate at which rigid India underthrusts southern Tibet to be 2 ± 1 cm/yr. Although most of the normal faults appear to be independent of, and nearly orthogonal to, the ≃E‐W Mesozoic‐Tertiary tectonic fabric, the Yadong‐Gulu rift appears to be guided for over 130 km by the older, oblique (≃NE‐SW) Nyainqentanglha range and fault zones along it. This reactivated zone is the most prominent left‐lateral strike‐slip fault system in SE Tibet. Excepting this zone, and the vicinity of the SE extremity of the Karakorum fault, Quaternary strike‐slip faulting is rare in south Tibet, i.e., south of the chord between the syntaxes of the Himalayan arc. North of the chord, the tectonic style is different. There minor conjugate strike‐slip faulting is widespread and appears to control Quaternary normal faulting, which is more diffuse and subdued than in south Tibet. Along the chord, the presence of a major zone of active right‐lateral, en échelon strike‐slip faults (Karakorum‐Jiali fault zone) probably reflects the greater facility of eastward extrusion in north central Tibet, in response to the northward push of India. The eastern Himalayan syntaxis may be an obstacle to such extrusion movements south of the chord.