ABSTRACT
The Ailao Shan-Red River metamorphic belt in southern Yunnan province, China, was one of the largest left-lateral, ductil strike-slip shear zone developed during the mid-Tertiary in Asia. To better understand the nature and timing of this feature, 40Ar/39Ar age spectrum and fission track analyses were undertaken on 18 K-feldspars and 4 apatites from seven sampling sites (including three sections of cross metamorphic belt) from Yuanyang to Zhelong for about 225 km along this belt. Using the MDD model (Multiple-Diffusion Domain model) these results yield 18 curves of cooling history in the temperature range from 400 – 150 °C. The common characteristics of these curves were to have one (or two) rapid cooling (>100 °C/Ma), rapid uplift event, in the age range from 25.7 – 17.8 Ma. The fission track ages range from 10.6 to 5.61 Ma. Both results indicate dicreasing from SE to NW along the shear zone.
When ploted as a function of distance along the range, the starting time of rapid uplift and the fission track ages are getting younger linearly from Yuanyang (SE) to Zhelong (NW) at the slope of 34mm/yr and 30 mm/yr respectively. The surprising linear correlation can be interpreted by two probable tectonic models in different time ranges: (1) the slope of 34 mm/yr was probably just the speed of left-lateral strike-slip movement with uplift (slope angle of about 11°) and the starting time of the rapid uplift decrease linearly during 25.7 to 17.8 Ma from SE to NW; (2) The slope of 30 mm/yr was considered a homogeneous extended speed of tectonic uplift from SE to NW during 10 Ma (or 15.5 Ma) to 5.5 Ma, propably due to a resistance of unfree interface at the SE edge of the shear zone. These results also support that a 15.5 Ma unconformity separates a younger left-lateral transpressional regime from an older left-lateral transtensional regime in the Red River shear zone in the Tonkin Gulf, Vietnam.
