TESTING TECTONIC MODELS USING TIME-VARYING CRUSTAL THICKNESS VARIATIONS ACROSS SOUTHERN TIBET

More than 30 years after the classic tectonic models for the Tibetan-Himalayan orogen were formulated, no clear test has been devised to select between a pure-shear thickening history and one involving pre-collisional topography and crustal extrusion. That these models imply vastly different crustal thickness histories underscores how little we know of this parameter. From isotopic data, we previously estimated a crustal thickness profile at the initiation of collision (~50 Ma) ranging from 20 km at the ITS to 45 km thick 130 km to the N. Since S. Tibet has a uniform ~85 km thickness today (inc. ~30 km of Indian crust) and experienced little post-50 Ma upper crustal deformation, we inferred that ~20 km was added by tectonic accretion and ~15 km from juvenile magma since collision. To assess pre-collisional variations, we undertook a traverse across S. Tibet at 92^oE. Zircons from 82 granitoids (28 to 230 Ma) were analyzed for U-Pb, δ¹⁸O, ε_Hf, and trace elements; results show the high degree of variability expected from RAFC processes. Zircon ε_Hf data show a similar distribution to whole rock ε_Nd permitting its use as a crustal thickness proxy. Zircons from 195 to 50 Ma granitoids near the ITS all show +ve ε_Hf whereas the northernmost samples (100 to 230 Ma) range from -7 to -22. Although RAFC modeling is often limited by its multivariate nature relative to available geochemical controls, zircon T^xlln data permit new insights into these processes. We developed an RAFC model featuring a parameterized T-crystallization relationship coupled to a zircon growth model that predicts zircon ε_Hf as a function of T^xlln. The model shows that recharge yields a zircon population that records the full spectrum of ε_Hf in the system whereas no recharge yields a much narrower distribution. Thus zircons may have the potential to preserve a record of the magnitude of recharge. Insights gained from modelling reinforce our view that the relationship between assimilation and geothermal structure can be used to estimate past crustal thickness of convergent margins. Interpreted through this model, our data show that the southern margin of Asia remained thin (<25 km) until collision began whereas the northern terrane was continuously thick from 200 Ma. This observation reinforces the growing view that Tibet hosted significant topography prior to collision.