Strain Accommodation by Fracturing During Folding of Sedimentary Rock

由于现场数据表明，这是脆性地壳的一种重要应变调节机制，因此PI建议研究折叠过程中的压裂机制。应力集中导致裂缝成核和扩展，通过了解相关裂缝在空间和时间上的演化，可以限制褶皱内的应力场。以往的研究主张简单、对称和均匀的应力-褶皱关系，并提出了在这种理想应力场中裂缝形成的各种模型。这些方法的共同点是，它们忽略了折叠前的结构，如关节，如果它们被重新激活，可能会扰乱褶皱内的应力场。新的裂缝将在一个不同于简单模型预测的应力场中形成。在怀俄明州Casper附近的Laramide褶皱Emigrant Gap背斜，先前模型的简化关系无法解释大多数观察到的裂缝。PI的初步野外观测表明，褶皱之前存在两个构造节理组，褶皱上观察到的大多数裂缝与这两个节理组近平行。一些较老的节理表现出剪切偏移，这表明它们在折叠过程中被重新激活。与再激活相关的是新裂缝的形成，其走向与剪切关节平行，而不是与褶皱轴平行。褶皱上两个裂缝组的测量断裂强度大约是同一地层未展开地层中各自节理组强度的四倍。PI将确定褶皱前节理、层理界面和地层对比本构特性对褶皱过程中裂缝发育和应变调节的影响。他们计划将详细的实地观察与这些结构的力学建模结合起来。他们的具体目标是：1)绘制和记录移民峡背斜各种砂岩单元的裂缝和褶皱几何形状，包括三维裂缝几何形状、褶皱地层的三维形状以及页岩单元内层理平行剪切的证据。2)利用ABAQUS有限元代码设计并执行大应变数值实验，以了解在现场观测的激励下，折叠过程中破裂应变调节的潜在力学过程。3)结合现场观测和数值分析的见解，构建一个概念模型，以预测类似地质和构造环境下的裂缝。该模型将修改，并在移民缺口背斜等情况下取代现有的基于对称褶皱-断裂关系的模型。

The PI's propose to investigate the mechanics of fracturing during folding because field data indicate this is a significant strain-accommodating mechanism of the brittle crust. Fractures nucleate and propagate as a result of stress concentrations, and the stress fields within folds can be constrained by understanding the associated fracture evolution in space and time. Previous efforts have argued for simple, symmetric and homogeneous stress-fold relationships, and various models of fracture formation in such idealized stress fields have been proposed. These methods have in common that they ignore pre-folding structures such as joints, which can perturb the stress field within the fold, if they are reactivated. New fractures then would form in a stress field different from that predicted by the simple models. At the Emigrant Gap anticline, a Laramide fold located near Casper, Wyoming, the simplified relationships of previous models are not capable of explaining the majority of the observed fractures. The PI's preliminary field observations show that two tectonic joint sets existed prior to folding, and that most fractures observed on the fold are sub-parallel to these two sets. Some of the older joints show shear offsets, arguing for their reactivation during folding. Associated with the reactivation is the formation of new fractures with strikes that parallel those of sheared joints rather than the fold axis. Measured fracture intensities for both fracture sets on the fold are approximately four times the intensities of the respective joint sets in unfolded strata of the same formation.The PI's will determine the influence of pre-folding joints, bedding interfaces, and contrasting constitutive properties of the strata on fracture development and strain accommodation during folding. They plan to integrate detailed field observations with mechanical modeling of these structures. Their specific objectives are: l) Map and document fractures and fold geometry of various sandstone units at the Emigrant Gap anticline, including fracture geometries in 3-D, the 3-D shape of the folded strata, and evidence for bedding-parallel shearing within the shale units. 2) Design and execute large-strain numerical experiments using the FEM code ABAQUS to understand the underlying mechanical process of strain accommodation by fracturing during folding as motivated by the field observations. 3) Integrate the insights gained from the field observations and numerical analyses to construct a conceptual model to predict fractures in similar geologic and tectonic settings. This model will modify and, in cases such as the Emigrant Gap anticline, replace existing models based on symmetric fold-fracture relationships.