关键词: 白马雪山垭口, 泥石流堆积, 古气候, 构造抬升

Abstract: There are many Quaternary debris-flow accumulations in southeastern marginal areas of the Tibetan Plateau （TP），especially in valleys of the upper reaches of the Jinsha River. They provide rich geologic and geomorphologic information for river evolution，tectonic movement and climate change. A large fan-shaped diamicton accumulation of debris flow intercalated with reddish weathering crust occurs on the summit plane of the Baima Mountains，Yunnan Province，Southwest China，about 2 km away from the modern Jinsha River. The region is characterized by semiarid climate and the precipitation is typically controlled by the southwest monsoon. The annual average temperature is 12.2 ℃. The precipitation is concentrated between June and September，accounting for 80% of the annual average precipitation of about 300 mm. The elevation of the summit plane （erosion surface） is 2 600-2 700 m. The large debris-flow accumulation located at the Linigongka Pass is 700 meters wide and averages 100 meters in thickness. The debris-flow accumulation is composed of debris-flow deposits and buried reddish weathering crust. The matrix-supported gravels mainly consists of slate and limestone belonging to the Middle Devonian，and generally 3-10 cm sized; the largest exceeding 20 cm. The debris-flow accumulation is generally exposed along highway cut slopes and gullies. Two profiles were chosen to observe，describe，sample and analyze，with an analysis of particle size，major elements，pollen and electronic spin resonance （ESR） dating of samples from the debris-flow accumulation. In the debris-flow units，gravel ranges from 63.1% to 78.9%，sand from 10.3% to 23.0%，silt from 4.4% to 7.1%，and clay from 0.5% to 1.2%. In the reddish gravel soil units，gravel ranges from 40.0% to 71.8%，sand from 17.9% to 45.3%，silt from 9.0% to 14.6% and clay from 1.4% to 2.1%. The chemical element analysis suggests that the intercalated reddish weathering crust originated from the similar parent material （debris-flow sediments） and have apparent differences in element composition from the debris-flow sediments，indicating that debris-flow sediments experienced weak weathering processes. The ESR dating indicates that the debris-flow accumulation formed around 2.48～1.54 Ma BP，which is consistent with the morphological structure. The pollen analysis shows that climate of the Jinsha River valley during the Early Pleistocene was characterized by a remarkable wet-dry alternation and would have been warmer than today. The modern reddish weathering crust usually occur in regions where the annual average temperature is 22～25 ℃. Combined with the morphological and sedimental feature of the buried ancient weathering crust，it suggests that the paleo-debris-flow accumulation occurred on the summit plane would have experienced strong tectonic uplifting. This provides important morphological evidence that the Baima Mountains had surface uplift amplitude by 1 300 m since the Early Pleistocene.

Key words: the Baima Mountains, debris-flow accumulation, paleoclimate, tectonic uplift