Strain Accommodation by Fracturing During Folding of Sedimentary Rock

PI建议研究褶皱过程中的破裂机制，因为现场数据表明这是脆性地壳的一种重要的应变调节机制。裂缝的形核和扩展是应力集中的结果，褶皱内的应力场可以通过了解相关的裂缝在空间和时间上的演化来约束。以前的工作提出了简单、对称和均匀的应力折叠关系，并提出了在这种理想化应力场中形成裂缝的各种模型。这些方法的共同点是它们忽略了诸如关节之类的预折叠结构，如果它们被重新激活，这些结构可能会扰乱褶皱内的应力场。然后，新的裂缝将在与简单模型预测的应力场不同的应力场中形成。在位于怀俄明州卡斯珀附近的移民缺口背斜，一个拉莱姆褶皱，以前模型的简化关系不能解释大多数观察到的裂缝。PI的初步野外观测表明，褶皱前存在两套构造节理，褶皱上观察到的大部分裂缝与这两套构造节理处于次平行状态。一些较老的关节显示出剪切偏移量，证明它们在折叠过程中重新激活。与重新激活相关的是形成新的裂缝，其走向平行于剪切关节的走向，而不是褶皱轴线。褶皱上两个裂缝组的实测裂缝强度大约是相同地层中未褶皱地层中各自节理组强度的四倍。PI将决定褶皱前节理、层理界面和地层的对比本构性质对褶皱过程中裂缝发育和应变调节的影响。他们计划将详细的现场观察与这些结构的机械建模结合起来。他们的具体目标是：(L)绘制和记录移民缺口背斜各砂岩单元的裂缝和褶皱几何图形，包括三维裂缝几何图形、褶皱地层的三维形状以及页岩单元内层状-平行剪切的证据。2)利用ABAQUS有限元程序设计并进行了大应变数值实验，以了解在现场观测的基础上，褶皱过程中破裂应变调节的基本力学过程。3)结合现场观测和数值分析所获得的见解，构建一个概念性模型，以预测相似地质和构造环境中的裂缝。该模型将对现有的基于对称褶皱-破裂关系的模型进行修改，并在移民缺口背斜等情况下取代现有模型。

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.