晚中新世龙门山南段及前缘地区盆山体系形成的动力学机制

Dynamic Mechanism of Formation of Basin-Mountain System in Southern Segment of Longmenshan and Frontal Area in Late Miocene

新生代龙门山的隆升机制存在上地壳缩短和下地壳流两种端元模式,两种模式会造成四川盆地不同的构造变形响应,形成不同的盆山系统.为了厘定龙门山南段及前缘地区盆山系统的构造变形特征及其动力学指示意义,利用低温热年代学技术(AFT和AHe)对龙门山南段前缘地区乐地1井7个砂岩样品进行了分析,约束该地区的剥露特征.结果表明,龙门山南段前缘地区中新世(约21 Ma)以来浅部地层经历了约10~11 Ma的快速剥露阶段(约500~700 m/Ma),与前人龙门山南段揭露的快速剥露时间一致.结合区域地质资料,认为约10~11 Ma龙门山南段向南东的逆冲推覆作用,导致构造应力通过四川盆地西南部多层滑脱层向盆地传递,造成龙门山前缘地区大范围构造变形及快速剥露,该认识支持龙门山上地壳缩短的隆升机制.

The uplift mechanism of the Cenozoic Longmenshan has two endmember modes:upper crustal shortening and lower crustal flow.The two modes will cause different tectonic deformation responses in the Sichuan basin and form different basinmountain systems.In order to determine the tectonic deformation characteristics of the basin-mountain system in the southern Longmenshan and the frontal area and its dynamic indicative significance,seven sandstone samples from Well Ledi 1 in the southern Longmenshan frontal area were analyzed by low-temperature thermochronology(AFT and AHe)to constrain the exhumation characteristics of the front area.The results show that the shallow strata in the frontal area of the southern Longmenshan have experienced a rapid exhumation stage(~500 to 700 m/Ma)of~10 to 11 Ma since the Miocene(~21 Ma).This rapid exhumation stage is synchronized with the rapid exhumation time revealed by the predecessors in the southern Longmenshan.Combined with regional geological data,it is believed that the thrust and nappe from the southern Longmenshan to the southeast at~10 to 11 Ma,caused the tectonic stress to be transferred to the basin through the multi-layered detachment layers in the southwestern Sichuan basin,resulting in large-scale tectonic deformation and rapid exhumation in the front area.This knowledge indicates the uplift mechanism of the southern Longmenshan is the upper crustal shortening model.