龙门山新生代构造格架的横向差异：来自三维热-动力模拟的制约

Cenozoic deformation in the Longmenshan, the eastern margin of the Tibetan Plateau, has been regarded as one line of evidence for the outward expansion of the plateau. Several geodynamic models, such as lower crustal channel flow, upper crustal shortening, and extrusion of the whole crust, have been proposed for explaining the eastward expansion. These models have different predictions for deformation nature, structural architecture and thus rock uplift and thermochronological age distribution in the Longmenshan. The proposed project aims to use a modified 3D thermo-kinematic modelling tool to interpret the thermochronological data of different portions of the plateau margin, so as to quantify the Cenozoic structural architecture and fault slip-rates in the Longmenshan. A pilot 3D thermo-kinematic modelling study in the southern segment of the Longmenshan suggests that the thermochronological age distribution and late Cenozoic erosion is controlled by a set of listric reverse faults, consistent with the upper crustal shortening model. However, it is noted that southern, central and northern segments of the Longmenshan are characterized by significantly different features in landscape, surface geology, and active tectonics, implying variable deformation styles and mechanisms. Through applying the 3D thermo-kinematic modelling method to other segments of the Longmenshan, this project focuses on quantifying the lateral structural variations in the proposed study area, so as to provide a tectono-thermochronolgical perspective into the strain distribution and the underpinning geodynamic mechanism in the region.

龙门山新生代构造变形被认为是青藏高原向四周扩展的证据之一，对于其动力机制分歧很大，现今流行的观点包括：下地壳管流、上地壳缩短和全地壳挤出等模型。在龙门山地区，这些模型分别预测了不同的构造变形样式和不同的岩石抬升、剥露与热年代学年龄空间变化特征。本申请拟利用热年代学数据三维热-动力模拟方法，定量反演龙门山的深部结构和主控断层的滑移速率。使用该方法在龙门山南段的预研究发现：热年代学年龄和晚新生代剥蚀空间分布受控于上地壳铲形逆断层系统，支持上地壳缩短模式。然而，龙门山南、中、北三段的地貌、地质和活动构造等特征差异显著，很可能指示了这些区段构造变形样式的迥异。本研究计划将三维热-动力模拟方法应用到龙门山其他区段，进而量化龙门山新生代构造格架的横向差异，为进一步认识该地区的应变分配和构造变形机制提供构造-热年代学方面的科学约束。

龙门山新生代构造变形被认为是青藏高原向四周扩展的证据之一。本项目通过低温热年代学方法，结合一维和三维热-动力模拟、地震剖面及露头构造分析，揭示了龙门山南、中、北三段及四川盆地的构造演化。取得的成果可概括为：（1）厘清了龙门山-四川盆地新生代以来构造变形时空序列，提出青藏高原东缘（龙门山-四川盆地）新生代构造变形受控于后展式的褶皱逆冲：早新生代构造变形被龙门山和四川盆地共同吸收，而晚新生代构造变形后撤并主要集中在龙门山。另外，在龙门山内部，晚新生代以来（～10Ma）后山断裂（汶川-茂县断裂和虎牙断裂）吸收了更大量的变形，表现为乱序（out-of-sequence）变形的特征，导致后山断裂上下盘约0.6 -1.0 mm/yr的差异剥蚀。此外，沿着龙门山走向，构造变形由南段的挤压转变为北段的右旋走滑，本研究提出龙门山北段走滑构造的发育始于始新世（～40Ma）：始新世前后，北段腹地剥蚀由快变慢，而前陆地区则由沉降与埋藏转换为剥蚀，这一差异的浅表演化与龙门山北段在始新世由挤压转变为走滑的构造体制相一致。进而推测青藏高原东部的变形经历了始新世和晚中新世两期重要的构造调整，龙门山的几何形态可能发生了显著的变化，反映了深部递进式的构造变形与浅表过程之间的相互作用。（2）开发了基于温度历史的剥蚀过程反演计算方法，该方法考虑了岩石剥露对地温场的影响，输入热历史模拟软件HeFTy和QTQt所限定的热历史，输出具有阶段性变化的剥蚀速率演变历史，为剥蚀历史的量化提供更加科学的研究工具。研究成果和方法对于进一步深入理解青藏高原的东向扩展及褶皱冲断带-前陆盆地系统的演化具有借鉴意义。