西藏南部吉隆盆地中新世-早更新世孢粉组合带及其地质意义

吉隆盆地为高喜马拉雅中新世晚期约10 Ma时期形成的一个南北向断陷盆地, 其东侧为同沉积正断层, 沃马剖面位于盆地沉降中心的东南部.在该剖面下部新发现一套中新世巨厚砾岩层(旦增竹康组).通过锆石和磷灰石裂变径迹年代学研究得出吉隆盆地控盆断裂早期活动时间为13.4±1.9 Ma, 源区12~11 Ma发生构造热事件, 据此推算出吉隆盆地初始裂陷后开始沉积的底界年龄约为10 Ma.综合前人在吉隆盆地得出的7.20~1.67 Ma古地磁测年值, 可得出吉隆盆地旦增竹康组年龄为10.0~7.4 Ma, 沃马组年龄为7.40~1.67 Ma.根据孢粉组合带和孢粉组合反映的植物类型和古环境变化, 沿剖面自下而上划分为3个孢粉组合带和9个孢粉组合及其对应的植被类型.吉隆地区古气候变化可划分为3个阶段: (1)组合带Ⅰ和孢粉组合1~2, 为温暖偏干环境的常绿与落叶针阔叶混交林, 地层对比时代为晚中新世(10.0~7.0 Ma); (2)组合带Ⅱ和孢粉组合3~7, 为寒冷干旱环境的落叶针叶林, 期间存在一次暖湿气候的波动, 地层对比时代为晚中新世晚期-早上新世(7.0~3.3 Ma); (3)组合带Ⅲ和孢粉组合8~9, 为温凉偏干的气候下生长暗针叶林和落叶阔叶林构成的针阔叶混交林, 地层对比时代为晚上新世(3.30~1.67 Ma). 

Geological Significance of Miocene-Early Pleistocene Palynological Zones in the Gyirong Basin, Southern Tibet

The Gyirong basin is one of east-west extensional basins distributed among the high Himalayas. The Late Cenozoic sequence is mainly Woma Formation, consisting of lacustrine and fluvial deposits, from which a Hipparion fauna in 7 Ma was found. However, Danzengzhukang Formation is newly found at the base of the section attributed to alluvial sequence. The zircon and apatite fission track thermochronology shows that eroded source experienced tectonic thermal event at 12-11 Ma, and early activity of the east boundary normal fault of the basin occurred during 13.4±1.9 Ma. Based on the previous paleomagnetostratigraphic researches of the basin, the age interval of the section is most reasonably determined as between 10 and 1.67 Ma. Through abundant palynological analyses, 3 palynological zones and 9 palynological assemblages and vegetable types are recognized, and 3 phases of paleoclimatic changes are revealed: (1) 10.0-6.7 Ma, correlating with palynological zone Ⅰ and assemblages 1-2, it was warm and damp-dry coniferous-leaved and broad-leaved mixed forests, but became cool and humid during 9.5-7.0 Ma. (2) 6.7-3.3 Ma, correlating with palynological zone Ⅱ and assemblages 3-7, it turned cold and arid deciduous coniferous-leaved forests, marked by the increase in the cold-tolerant tree taxa and drought-tolerant herb taxa, but the relative increase of broad-leaved taxa suggests climate underwent warm and humid fluctuations during 5.0-4.3 Ma. (3) 3.30-1.67 Ma, correlating with palynological zone Ⅲ and assemblages 8-9, it got cool and damp-dry deciduous coniferous-leaved and broad-leaved mixed forests.