ACTIVATION OF THE NYAINQENTANGHLA SHEAR ZONE - IMPLICATIONS FOR UPLIFT OF THE SOUTHERN TIBETAN PLATEAU

Neogene extension of the tibetan plateau is manifested as a series of north-south trending graben, the most prominent of which is the Yadong -Gulu rift. The Nyainqentanghla Shan, a NE-SW trending mountain range similar to 100 km NW of Lhasa, bounds the western margin of the Yangba jian graben, the central segment of the Yadong-Gulu rift. The eastern edge of the Nyainqentanghla massif is marked by a low angle (similar t o 25 degrees) detachment fault shear zone of amphibolite grade mylonit es. The Ar-40/Ar-39 thermal history results from samples collected alo ng two deeply incised valleys within the massif reveal that a rapid co oling event propagated from similar to 8 Ma in the core of the range t o similar to 4 Ma within the high strain zone at the eastern boundary. Assuming that faulting initiated at high angle (similar to 60 degrees ), thermal histories were fit to a numerical simulation of slip on a n ormal fault to yield estimates of both the age of fault initiation and the slip rate history. The form of the isotopically derived thermal h istories are similar to general form predicted by the thermal model an d suggests that significant movement began at 8+/-1 Ma in the southern valley (Goring-la) and proceeded at an average slip rate of similar t o 3 mm/yr between similar to 8 and 3 Ma. A more complex history is req uired to fit the data from the northern valley (Balum Chun), but the t iming of initiation and average slip rate are similar to the Goring-la result. Numerical simulations in which the fault angle is varied indi cate that the isotopically derived temperature histories are inconsist ent with slip occurring at low angle (<40 degrees). Because the extens ion direction of the Yangbajian graben is representative of most rifts on the southern Tibetan plateau, our data suggest that crustal thickn ess and elevation reached close to their present values by 8+/-1 Ma. A carbon isotopic shift in pedogenic carbonates from the Siwalik Format ion at about 7.5 Ma appears to reflect intensification of the Asian mo nsoon and, by inference, that the plateau had attained an important th reshold elevation by that time. Formation of a diffuse plate boundary in the Indian oceanic lithosphere beginning at 75-8.0 Ma is also consi stent with this history. We suggest that the plateau had attained a th reshold area and elevation by 8+/-1 Ma sufficient to trigger these thr ee independent manifestations.