龙门山南段前陆区古冲积扇晚第四纪构造变形

The Longmen Shan thrust belt has a distinctly north-south segmented structural pattern. The late Quaternary tectonic shortening rate in the southern segmental thrust zone is approximately 1 mm/yr, which is less than the 2-3 mm/yr of north segment. However, there developed a fold belt in the foreland of southern Longmen Shan, which is the thin-skinned structure most developed area along the eastern edge of the Tibetan Plateau. How the crustal shortening in the southern section of Longmen Shan is distributed to the foreland fold belt is one of the important scientific issues in the region. Qingyijiang River runs through the southern section of Longmen Shan and its foreland area. In the foreland area, Mingqiong and Dansi Quaternary ancient alluvial fans were developed. The alluvial fans have been abandoned on account of Qingyijiang River's migration, showing a pattern of distribution around multiple anticlinal structures in the foreland area. Therefore, it is an ideal material for studying the late Quaternary tectonic activity in the foreland area. In this study, the digital elevation model analysis and high-accuracy network RTK measurements will be used to restore the late Quaternary deformation of the ancient alluvial fans. In addition, the 10Be and 26Al cosmogenic nuclide dating result will be applied to quantitatively analyze the activity of the foreland anticlines. Eventually, the late Quaternary tectonic activity pattern in the foreland area of the southern Longmen Shan will be obtained. In this study, compared with the band-distributing river terrace, the employment of alluvial fans as the deformation's base-surface is an improvement. Moreover, this study provides a new method for tectonic geomorphology.

龙门山逆冲推覆带存在明显南北分段特征，南段的山区冲断带晚第四纪地壳缩短速率约1mm/yr，远小于北段的2-3mm/yr。但龙门山南段发育了前陆褶皱带，是青藏高原东缘薄皮构造最发育的区域。龙门山南段的第四纪地壳缩短怎样分配到前陆褶皱带，是该区域的重要科学问题之一。青衣江贯通龙门山南段及其前陆区，在前陆区发育了名邛与丹思第四纪古冲积扇。冲积扇因青衣江改道现今已废弃，呈现出围绕前陆区多个背斜构造分布的形态，是研究前陆区晚第四纪构造活动的理想素材。本项研究拟通过数字高程模型分析与高精度网络RTK测量，恢复古冲积扇的晚第四纪变形量，结合10Be、26Al宇宙成因核素测年工作，定量的分析前陆区背斜的构造活动，最终得到龙门山南段前陆区晚第四纪的构造活动模式。本项研究利用面状分布的古冲积扇作为变形研究的基准面，弥补了河流阶地带状分布的局限性，为构造地貌研究提供了新的方法。

在2008年汶川Ms8.0级地震与2013年芦山Ms7.0级地震发生后，龙门山地区的活动构造问题备受关注。龙门山南段山区冲断带第四纪逆冲活动量有限，地壳缩短速率不超过1 mm/yr，远小于龙门山中段的2-3 mm/yr，龙门山南段第四纪构造变形怎样分配到前陆地区，是该区域的一个重要科学问题。开展前陆区的构造活动研究，对于理解龙门山南段逆冲活动特征、高原向东扩展模式具有重要意义，也是该地区地震危险性评价的需要。在龙门山南段前陆区具有独特的河流地貌形态，废弃的名邛古冲积扇与丹思古冲积扇分布于青衣江北侧，形成了宽阔的洪积台地，横跨了前陆地区的熊坡背斜、三苏场背斜、大塘断裂等活动构造，这两套古冲积扇是研究前陆区晚第四纪构造变形的理想素材。本项目基于实地调查、网络RTK测绘技术、第四纪年代学技术等开展了一系列研究，包括古冲积扇的沉积与演化过程、古冲积扇年代学、古冲积扇第四纪构造变形等，最终分析得到了龙门山南段前陆区晚第四纪构造变形样式，建立了区域晚第四纪构造变形模型。主要结论包括以下几点：一是发现了名山地区的古青衣江河道，重新建立了青衣江自晚更新世以来的演化过程与古冲积扇的发育关系，明确了名邛冲积扇是由名山地区的古河道连通；二是厘定了名邛古冲积扇与丹思古冲积扇的年代学，宇宙成因核素测年结果表明古冲积扇的废弃时代比前人估测结果更年轻；三是通过多条实测地质地形剖面分析得到多个构造的变形特征与缩短速率，如大塘构造晚第四纪缩短速率为0.3mm/yr；四是建立了区域构造模型，龙门山南段前陆区同时受到了龙门山推覆与川滇块体挤出的作用，两种动力有各自相应的构造与之匹配，因此该地区既存在垂直于龙门山的构造缩短，又存在近东西向的构造缩短。