JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B14, PAGES 13,803-13,872, DECEMBER 10, 1986 QUATERNARY EXTENSION IN SOUTHERN TIBET: FIELD OBSERVATIONS AND TECTONIC IMPLICATIONS Rolando Armijo and Paul Tapponnier Institut de Physique du Globe, Paris J. L. Mercier Universit• de Paris-Sud, Orsay, France Han Tong-Li n Ministry of Geology, Beijing, People's Republic of China Abstract. We summarize evidence for Quater- nary and active faulting collected in the field during three Sino-French expeditions to south- eastern Tibet (1980-1982). Detailed mapping of Quaternary and active faults as well as micro- tectonic 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 prin- cipal stress 02, o1 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 stu- died in some detail. The extension direction is determined to be N9•E _+ •o mainly from statis- tical averaging of strikes of newly formed nor- mal 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 struct,•ral and topographic reliefs, as well as synglacial and postglacial vertical offsets. Th• rate of Quaternary extension is about 1% m.y. along a 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 hori- zontal projections of slip vectors of earth- quakes 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) Nyainqentadglha 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 vici- nity 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 wide- spread and appears to control Quaternary normal faulting, which is more diffuse and subdued than in south Tibet. Along the chord, the presence of Copyright 1986 by the American Geophysical Union. Paper number 5B5855. 0148-0227/86/005B-5855 $05. O0 a major zone of active right-lateral, en •chelon strike-slip faults (Karakorum-Jiali fault zone) probably reflects the greater facility of east- ward 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. 1. Introduction Landsat image interpretation and earthquake fault plane solutions imply that normal faulting characterizes the present tectonics of the Tibe- tan plateau [e.g., Molnar and Tapponnier,1978; Ni and York, 1978; Romanowicz, 1982]. Such an extensional regime contrasts with active thrust faulting in areas of lower elevation to the north and south. Besides, it seems to have star- ted only recently, after important episodes of crustal shortening and thickening. Indeed, much of the overall structure, high elevation, and thickened crust of Tibet appears to be the result of three subduction-collision cycles which, since the Palaeozoic, have accreted Gond- wanJan or Cathaysian continental blocks to Laurasia [Chang and Cheng,1973; StSckiln, 1980; Seng•r, 1981a, 1984; Tapponnier et al., 1981a; All•gre et al., 1984; Hirn et al., 1984]. A bet- ter comprehension of the development of post- orogenic extension in Tibet is important to understanding the way in which high plateaus evolve during continental collision but has so far been hampered by the scarcity of direct evi- dence. As a step toward such a comprehension, we present a field study of active and Quaternary tectonics performed during the 1980-1982 Sino- French expeditions to Tibet. Much of the work described in this paper was done in areas near Kung Co and Dinggye and between Yadong and Nagqu (Plate 1). It involved a combination of approa- ches. Structures deforming Quaternary sediments, or exhibiting youthful morphology in the field and on air photographs were mapped at a scale of 1/100,000. Morphological observations were gathered to constrain offsets of different sizes on faults. Measurements of small-scale features reflecting tectonic strains (slicken- sides, tension gashes, etc.) were also made, usually near the faults in Quaternary sediments or in fractured bedrock and cataclasites. Final- ly, we tried to relate recent surface breaks to historical and instrumental seismic records [Institute of Geophysics, 1976]. 13,803