Thermochronological evidence for the movement of the Ailao Shan–Red River shear zone: A perspective from Vietnam

Abstract

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Onset timing of left-lateral movement along the Ailao Shan–Red River Shear Zone: 40Ar/39Ar dating constraint from the Nam Dinh Area, northeastern Vietnam

“…Excluding the less reliable estimate of the cooling path for RR58C K-feldspar, the general cooling path of the metamorphic rocks in this outcrop can be summarized as starting fast cooling at around 27 to 25 Ma at a rate greater than 1008C my À1 . This cooling path is very similar to those obtained by previous studies for samples from areas further northwest of the Day Nui Con Voi metamorphic massif (Nam et al, 1998;Wang et al, 1998), except that the age of the initiation of fast cooling appears to be slightly older (Fig. 7).…”

“…This observation has been con®rmed further by thermochronological results, that indicate a rapid change of cooling rates during the late Oligocene± Early Miocene (27±17 Ma) for the shear zone in southern Yunnan and northern Vietnam (Harrison et al, 1996;Wang et al, 1998). This would imply that the initiation of movement along the ASRR Shear Zone actually postdated the initiation of sea¯oor spreading of the South China Sea which started from magnetic anomaly 11 (Taylor and Hayes, 1983;Briais et al, 1993), corresponding to 30 Ma, according to the new geomagnetic polarity timescale of Cande and Kent (1995).…”

“…4(a). extrusion model may need further evaluation, as suggested by Chung et al (1997) and Wang et al (1998).…”

“…Nevertheless, the good correlation of the age of fast cooling, the sample location, and the northwesterly progression of the age for fast cooling remain the Harrison et al (1985) a Fe=Fe/(Fe+Mg): Since the argon diusivity of biotite is controlled by its chemical composition, the diusion data are calculated with using a regression of diusion data for the biotites published in Giletti (1974) and Harrison et al (1985). same as those found by previous studies (Harrison et al, 1996;Wang et al, 1998).…”

“…In order to further tackle this problem, the age of the onset of the activity along the ASRR Shear Zone needs to be addressed in greater detail. Systematic thermochronological analyses of the metamorphic massifs within the ASRR Shear Zone indicate that the exhumation and/or deformation front was propagating from the southeast to the northwest at a rate of 4 to 5 centimeters per year (Harrison et al, 1996;Wang et al, 1998). Accordingly, the earliest record of left-lateral motion along the shear zone should be observed at the southernmost part of the Day Nui Con Voi metamorphic massif in northeastern Vietnam.…”