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 Yangbajian graben, the central segment of the Yadong-Gulu rift. The eastern edge of the Nyainqentanghla massif is marked by a low angle (similar to 25 degrees) detachment fault shear zone of amphibolite grade mylonites. The 40Ar/39Ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from similar to 8 Ma in the core of the range to 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 normal fault to yield estimates of both the age of fault initiation and the slip rate history. The form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8+/-1 Ma in the southern valley (Goring-la) and proceeded at an average slip rate of similar to 3 mm/yr between similar to 8 and 3 Ma. A more complex history is required to fit the data from the northern valley (Balum Chun), but the timing of initiation and average slip rate are similar to the Goring-la result. Numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle (<40 degrees). Because the extension direction of the Yangbajian graben is representative of most rifts on the southern Tibetan plateau, our data suggest that crustal thickness and elevation reached close to their present values by 8+/-1 Ma. A carbon isotopic shift in pedogenic carbonates from the Siwalik Formation at about 7.5 Ma appears to reflect intensification of the Asian monsoon and, by inference, that the plateau had attained an important threshold elevation by that time. Formation of a diffuse plate boundary in the Indian oceanic lithosphere beginning at 75-8.0 Ma is also consistent with this history. We suggest that the plateau had attained a threshold area and elevation by 8+/-1 Ma sufficient to trigger these three independent manifestations.