Collaborative Research: Comparison of Numerical, Physical and Natural Extensional Fault Systems

有两种基本的断层模式可以在脆弱的上地壳中进行伸展：地垒-地堑几何和书架几何。为地堑地垒构造，相邻正断层向相反方向倾斜；相邻断层呈书架状，向同一方向倾斜。干砂的数值模型和比例物理模型通常产生地堑-地堑几何。书架几何只在特定的、不对称的边界条件下发展。然而，湿粘土的比例物理模型会产生具有对称或不对称边界条件的书架几何形状。这些观察结果提出了以下问题。自然界中书架的几何形状通常是不对称应力模式的结果吗？砂土和粘土的力学性能有什么显著差异？数值模型能重现更像粘土的行为吗？自然界中许多岩石的力学性质是不是更像粘土而不是沙子？数值模型、砂模型和粘土模型，以及一系列的边界条件，正在解决这些问题。这些模型的分析有几个目标：(1)确定哪些数值参数最能再现砂和粘土模型中观察到的断层模式；(2)考察砂土模型中变形随深度的传递；(3)数值模拟低黏性、各向异性和断层附近孔隙压力变化，更好地理解粘土模型的结果。这项合作的数值/实验研究的最终目标是量化和理解地质条件和物质行为的范围，这些条件和物质行为可以产生在大陆伸展的自然带中观察到的断层几何形状。该项目具有经济影响，因为伸展断层系统通常与产油区有关。此外，约束断层和岩石圈的性质可以提高对地震动力学的理解。

Two fundamental fault patterns accommodate extension in the brittle, upper crust: horst-and-graben geometry and book-shelf geometry. With a horst-and-graben geomety, neighboring normal faults are inclined in opposite directions; with a book-shelf geometry, neighboring faults are inclined in the same direction. Numerical models and scaled physical models with dry sand commonly produce a horst-and-graben geometry. The book-shelf geometry only develops under specific, asymmetric boundary conditions. Scaled physical models with wet clay, however, produce a book-shelf geometry with either symmetric or asymmetric boundary conditions. These observations raise the following questions. Is the book-shelf geometry in nature generally the result of fundamentally asymmetric stress patterns? What are the significant differences in the mechanical behavior of sand and clay? Can numerical models reproduce more clay-like behavior? Is the mechanical behavior of many rocks in nature more clay-like than sand-like?Numerical, sand, and clay models, with a range of boundary conditions, are underway to address these questions. The analyses of these models has several objectives: (1) to define which numerical parameters best reproduce the fault patterns observed in sand and clay models; (2) to examine the transfer of deformation with depth in the sand and clay models; and (3) to numerically simulate low-viscosity behavior, anisotropic behavior, and pore-pressure changes near faults to better understand the results of the clay models. The ultimate goal of this collaborative numerical/experimental research is to quantify and understand the ranges of geologic conditions and material behaviors that can produce the geometries of faults observed in natural zones of continental extension. The project has economic impact because extensional fault systems are commonly associated with hydrocarbon-producing regions. Also, constraining fault and lithospheric properties may improve the understanding of earthquake dynamics.