Paleomagnetism of the Upper Triassic rocks from south of the Ailaoshan Suture and the timing of the amalgamation between the South China and the Indochina Blocks

• Two HTC directions were observed from the Upper Triassic Yiwanshui Fm.
• One is deemed inaccurate due to uncertain determination of paleohorizontal.
• The other is interpreted as primary based on positive foldtest and dual polarity.
• By late Triassic the Simao and the Baoshan terranes may have coalesced.
• Collision of the Indochina Block with the SCB may have continued into end-Triassic.

A high temperature remanence with thermally-discrete unblocking temperature spectra has been isolated from the redbed samples of the Upper Triassic Yiwanshui Formation collected from south of the Ailaoshan Suture in southern Yunnan Province of China. Two groups of directions were observed: the tilt-corrected mean is D/I = 141.4°/31.2° with α95 = 13.0° based on 12 sites for the first group, and D/I = 207.5°/4.9° with α95 = 16.0° based on 5 sites for the second group. The second group of directions was derived from massively bedded rocks with large uncertainties in determination of the paleo-horizontal and is thus deemed inaccurate. The first group of directions passes the foldtest with dual polarity, and is consistent with results previously reported from the same type section, and probably represents primary magnetization of the rock formation. Comparison with the Triassic paleomagnetic poles for the Baoshan Terrane and the South China Block indicates that the Simao and the Baoshan terranes were probably already amalgamated by late Triassic times but the coalescence between them and the South China Block may have not been completed until near the end of the Triassic.