Development features of the Early Pleistocene debris-flow deposits at the Baima Mountain Pass, Yunnan Province and their paleoclimatic and tectonic significance
第四纪泥石流蕴含了关于河流地貌演化、新构造运动和气候变化的丰富信息。对发育于云南省德钦白马里你共卡雪山垭口海拔2 700 m,年代为2.48 ~ 1.54 Ma BP的古泥石流堆积物进行了系统研究。研究表明:泥石流堆积体呈扇状分布,其沉积构造有叠瓦构造、砾石支撑-叠置构造和石线构造,以稀性泥石流为主。古泥石流夹含的红色古风化壳由古泥石流堆积物风化形成,属于弱风化的红化土类型,反映了温和较湿的气候特征。主量化学元素和孢粉分析表明,古泥石流形成的早更新世时期金沙江河谷地区气候温和湿润,远比现在气候温暖。青藏高原东南缘白马雪山剥蚀面上发育的巨厚层古泥石流扇形堆积体的发现,表明青藏高原东南缘在早更新世早期已经有稀性泥石流出现,青藏高原快速隆升、夏季风加强和暴雨形式降水出现是该区早更新世泥石流发育的重要动力学因素。依据现代红化土的发育条件,估算自~1.54Ma以来青藏高原东南缘白马雪山一带的地表隆升幅度达1 300 m。
英文摘要:
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 °C. 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 analys is 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 °C. Combined with the morphological and sedimental feature of the buried ancient weathering crust, it suggests that the paleodebris-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.