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

两种基本的断层模式适应了脆弱的上地壳的伸展：地垒和书架几何结构。对于地垒地块，相邻的正断层朝相反的方向倾斜；对于书架几何，相邻的断层朝同一方向倾斜。对于干燥的沙子，数值模型和比例物理模型通常会产生地垒和地块的几何形状。书架几何仅在特定的、不对称的边界条件下展开。然而，湿粘土的比例物理模型产生了具有对称或非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。书架的几何形状本质上是不是基本上不对称的应力模式的结果？砂土和粘土的力学行为有什么显著区别？数值模型能再现更多类似粘土的行为吗？自然界中许多岩石的力学行为是不是更像粘土，而不是类似沙子？为了解决这些问题，人们正在用带有一系列边界条件的数值模型、砂子模型和粘土模型来解决这些问题。这些模型的分析有几个目标：(1)定义哪些数值参数最好地再现了砂土和粘土模型中观察到的断层模式；(2)检查砂土和粘土模型中变形随深度的传递；以及(3)数值模拟断层附近的低粘度行为、各向异性行为和孔隙压力变化，以更好地理解粘土模型的结果。这种合作的数值/实验研究的最终目标是量化和了解能够产生在大陆伸展自然带中观察到的断层几何形状的地质条件和物质行为的范围。该项目具有经济影响，因为伸展断层系统通常与油气产区有关。此外，约束断层和岩石圈的性质可能会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.