Cenozoic exhumation of the Danba antiform, eastern Tibet: Evidence from low-temperature thermochronology

Abstract

The Danba antiform (DA) exposes the highest grade metamorphic rocks in eastern Tibet. The metamorphic grades characterizing the DA evolve from sillimanite-migmatite grade to greenschist grade over a relatively short distance of ∼20 km from core to limb. This metamorphic event indicates an important Mesozoic to Cenozoic doming and exhumation history. However, the Cenozoic history of the antiform is poorly constrained due to a lack of data. Consequently, we used fission track dating on zircon and apatite from 22 samples collected throughout the DA. The zircon fission track (ZFT) data show a transition from Cenozoic non-reset (202 Ma), to partially reset (53–37 Ma), to totally reset (24–8 Ma) ages from the periphery to the core of the DA. The oldest totally reset ZFT ages are ca. 25 Ma and likely indicate the onset of Cenozoic folding of the DA. Compared to the apatite fission track (AFT) ages of ca. 10 Ma in the peripheral region, the youngest AFT ages are younger than 3 Ma in the core of the DA, suggesting that folding could be ongoing. Based on these multithermochronometer data, the cooling rate increases from ∼8 °C/m.y. on the periphery to ∼12–56 °C/m.y. in the core of the DA since ca. 12 Ma. The DA shares a similar cooling history with the Longmen Shan (LMS) fault belt, implying that the detachment fault of the LMS may extend to the DA, resulting in linked uplift histories. The differential exhumation among the samples in the core of the DA and the surrounding area indicates that both upper crustal deformation and crustal channel flow may have developed simultaneously (mainly since ca. 12 Ma) in the DA.