侧缘前陆构造带:一种前陆构造带的新类型——以龙门山为例

前陆构造带是造山带研究中一个非常重要的概念,指与造山带相邻的带状构造区域,由前陆冲断带和前陆盆地组成,可以分为周缘前陆构造带和弧后前陆构造带。前人在研究中国中—西部盆山系统时又提出了再生前陆构造带的概念,由再生造山带的前陆冲断带与再生前陆盆地组成。然而,晚三叠世由龙门山前陆冲断带与川西前陆盆地组成的龙门山前陆构造带在构造位置、沉积特征、地球动力学机制等方面与这3类前陆构造带有很大不同。在综合前人研究成果的基础上,研究提出龙门山前陆构造带在晚三叠世为一种新的前陆构造带类型——侧缘前陆构造带。由于松潘—甘孜褶皱带楔形的几何学特征,叠加其深部滑脱层的解耦作用,使得滑脱层之上的造山带表壳物质与下部"解耦",向SE方向发生大规模侧向挤出,导致造山带侧缘龙门山地区的地壳发生缩短增厚和挠曲沉降,形成龙门山侧缘前陆冲断带和川西侧缘前陆盆地。小金弧形构造带内显示区域应力状态由松潘—甘孜褶皱带内部NE—SW向挤压,向龙门山前陆冲断带NW—SE向挤压过渡,是对侧缘前陆构造带特殊应力环境的应变响应。侧缘前陆构造带在形成和演化上依赖于造山带特殊的楔形几何样式和深部软弱沉积层构成的滑脱层。前者为强烈缩短变形的造山带表壳物质提供一个"出口",后者使得滑脱层以上的表壳物质与下部的基底"解耦",随着造山带的持续挤压缩短,受空间限制的表壳物质无法消减,从而向容易挤出的方向发生大规模位移。

Lateral foreland belt—a new type of foreland belt:a case study of Longmenshan

The foreland belt is a very important concept in research of orogenic belt. It refers to the belt-shaped structural area adjacent to the orogenic belt. It consists of foreland thrust belt and foreland basin, and can be divided into peripheral foreland belt and retro-arc foreland belt. Previous researchers put forward the concept of rejuvenated foreland belt when studying the basin-orogen system in central-western China; it is composed of the foreland thrust belt of the rejuvenated orogenic belt and the rejuvenated foreland basin. However, in the Late Triassic, the Longmenshan foreland belt composed of the Longmenshan foreland thrust belt and the western Sichuan foreland basin was very different from these three types of foreland belts in terms of tectonic location, sedimentary characteristics, and geodynamic mechanisms. Based on the previous research results, the study proposes that the Longmenshan foreland belt was a new type of foreland belt in the Late Triassic, i.e., lateral foreland belt. Due to the wedge-shaped geometric characteristics of the Songpan-Garze fold belt, superimposed with the decoupling effect of the deep detachment layer, the orogenic supracrustal material above the detachment layer is decoupled from the lower part, and large-scale lateral extrusion occurs to the southeast, thus leading to lithospheric shortening, thickening and flexural subsidence in Longmenshan area on the lateral margin of the orogenic belt, and forming the Longmenshan foreland lateral thrust belt and the western Sichuan lateral foreland basin. The regional stress state of the Xiaojin arcuate tectonic zone shows a transition from NE-SW compression in the Songpan-Garze fold belt to the NW-SE compression in the Longmenshan foreland thrust belt, which is the strain response to the special stress environment of the lateral foreland belt. The formation and evolution of the lateral foreland belt relies on the special wedge-shaped geometry of the orogenic belt and the detachment layer formed by deep weak sedimentary layers. The former provides an "exit" for the orogenic supracrustal material that is strongly shortened and deformed, and the latter decouples the supracrustal material above the detachment layer from the lower basement. As the orogenic belt continues to squeeze and shorten, the supracrustal material restricted by space cannot be subtracted, and a large-scale displacement occurs in the direction where extrusion may easily occur.