川东—湘鄂西褶皱-冲断带构造指向反转机制与变形演化的物理模拟研究

The most typical and clear phenomenon of structural vergence reverse we have hitherto seen in intracontinental fold-and-thrust belts (FTBs) on Earth occurred in the FTBs of Eastern Sichuan–Western Hunan-Hubei of South China, where there is a close correlation between oil and gas accumulation and structural deformation evolution. Structural vergence reverse in FTBs (or thrust wedges) is an uncommon phenomenon in nature that is quite difficult to interpret by the classic Coulomb wedge theory, the most typical cases of which occurred in the ocean-continental subduction zones of Cascadia and Sumatra. The formation mechanism of such non-Coulomb wedges has confused geologists for many years and still remains unclear so far. In contrast, the phenomena of structural vergence reverse occurred in the intracontinental FTBs of Eastern Sichuan–Western Hunan-Hubei are more typical than those occurring in the ocean-continental subduction zones. The formation mechanism of them as well as the deformation evolution of the FTBs, however, is still far from being fully understood. In this project, we plan to conduct analyses of the structural geometries of the FTBs based on the ground geological and deep geophysical data, and then design a series of 2D and 3D models for analogue as well as numerical modeling. In the light of results from systematic analogue and numerical modeling, we shall reveal the mechanism of structural vergence reverse and the processes of deformation evolution in the FTBs, and discuss further the formation mechanism of non-Coulomb wedges, a basic problem in structural geology that has remained controversial for many years. This study may significantly improve and perfect the formation theory of FTBs and help to reveal the relationship between the structural evolution and oil or gas accumulation in the FTBs.

川东—湘鄂西褶皱-冲断带存在迄今陆内褶皱-冲断带中所见最为独特清晰的构造指向反转现象，且该带油气成藏与构造变形演化关系密切。褶皱-冲断带（或冲断楔）构造指向反转是自然界罕见、且难以用经典的库仑冲断楔理论加以解释的构造现象，最典型实例见于卡斯卡底和苏门答腊洋-陆俯冲带部位。这种非库仑冲断楔的形成机制问题已困扰学界多年、至今仍无定论。与卡斯卡底和苏门答腊典型实例相比，川东—湘鄂西陆内褶皱-冲断带的构造指向反转现象则更具特色，目前对其形成机制、以及该构造带变形演化规律尚无全面深入了解。本项目拟在地面地质与深部地球物理资料详细分析基础上，设计系列二维和三维模型，通过系统物理模拟、并结合数学模拟，专门研究该褶皱-冲断带构造指向反转机制与变形演化规律，深入探讨非库仑冲断楔形成机制这一困扰学界多年的构造基本问题。本项目研究对于完善褶皱-冲断带成因理论、揭示该区构造演化与油气成藏关系具有重要意义。

川东—湘鄂西褶皱-冲断带存在迄今陆内褶皱-冲断带中所见最为独特清晰的构造指向反转现象，并且该区的油气成藏与构造变形演化关系密切。褶皱-冲断带（或冲断楔）构造指向反转是自然界罕见、且难以用经典的二维库仑冲断楔理论加以解释的构造现象，这种现象已困扰学界多年、至今仍无定论。同时，有关川东—湘鄂西褶皱-冲断带的构造指向反转现象的形成机制与变形演化规律尚无全面深入了解。本项目基于系统的二维和三维模型的物理模拟及数值模拟结果，并结合研究区地质与地球物理资料，对该褶皱-冲断带构造指向反转的机制与主控因素、以及变形演化规律进行了深入探讨。研究结果表明，褶皱-冲断带的构造指向主要受侧向剪切强度与基底剪切强度比值（τs/τd比值）的控制，并非通常所认为的那样，受多种复杂因素的控制。褶皱-冲断带的成因机制解释无法根据经典的二维库仑冲断楔理论得到完善解释，将来只能从三维角度才有可能得到妥善解释。具有构造指向截然相反的双层冲断系统的川东—湘鄂西褶皱-冲断带的是中生代北西向挤压作用的产物、而非两期变形叠加的结果，它的形成可能与其上、下冲断变形系统具有明显不同的τs/τd比值有关。由于研究区软弱滑脱层性质明显，该区构造变形的传递方式以无序为主要特征。当滑脱层性质变化较大的情况下，褶皱-冲断带构造变形的传递可能以无序方式为主。川东南马尾状褶皱带的形成与边界断裂形态、地层和基底流变学特性有关。本项目成果可能会引起学界对褶皱-冲断带成因机制问题的深入思考，推动褶皱-冲断带成因机制理论的发展。同时可为深入了解研究区的构造演化、形成机制以及天然气藏的有利区带分析提供实验依据和新的启示。