Reconciling Different Deformation Mechanisms in Adjacent Sedimentary Lithologies at Raplee and Comb Folds, Monument Upwarp, UT

本项目对Laramide造山运动期间纪念碑上曲段千米尺度褶皱期间相邻地层露头尺度上发生的脆性变形（粉砂岩和砂岩的节理）和韧性变形（石灰岩中有脉状和溶缝的剪切带发育）进行了比较和评价。研究的目的是：1)描述和绘制犹他州东南部Raplee背斜和Comb单斜的节理和共轭剪切带；2)使用有限元方法构建和评估连续体力学模型，以研究与这些结构相关的脆性弹性和韧性塑性变形。我们试图调和这些不同的机制是如何以及为什么在近距离和相似的载荷条件下在折叠过程中被激活的，并推断出宿主地层的应力-应变行为。假设是不同的本构性质和强度导致了不同机制的激活。为了验证这一假设，我们将结构的系统几何特征（例如开口分布和静脉形状）与数值模型的结果进行了比较。我们预计，某些范围的本构性质和应力状态将导致相似。这将为砂岩和石灰石的力学行为提供重要的见解，在没有这种结构的情况下，无法从分米尺度的实验室测试中获得。褶皱是最常见的地质结构之一，发生在从手样到山脉的所有层状岩石类型中。沉积岩中的褶皱作为碳氢化合物的圈闭具有额外的重要性，因此是油气勘探的主要目标。由于熟悉用作铺路石或台面的沉积岩板的硬度和强度，一般公众一定想知道这些地层是如何扭曲成像Comb和Raplee背斜这样壮观的褶皱的。拟议的项目解决了这个问题，这也是构造地质学的一个基本问题：在千米尺度的褶皱中，沉积地层内的变形机制是什么？其中一个机制很明显：砂岩层是由脆性裂缝（节理）破坏的。第二种机制相当微妙：石灰石层缺乏节理，但包含剪切带、方解石被溶解的表面排列（溶缝）和充满方解石的正交开口裂缝（脉）。相邻露头中的脆性（节理）和韧性（剪切带）机制并置，为推断各自地层在褶皱时的力学特性变化提供了机会。详细测绘和力学建模的相互作用有望提供关于岩石褶皱变形的新见解和原始概念，并为石油地质学、岩石力学和地球动力学等密切相关学科的科学家提供新的见解。除了该项目的研究目标外，该奖项还支持研究生的培训；包括与Dine学院的教师合作，为参与收集数据的美洲原住民学生组织实地考察；并正在为扩大代表性不足的群体参与地球科学作出贡献。

This project compares and evaluates the brittle deformation (jointing in siltstone and sandstone) and the ductile deformation (shear zone development with veins and solution seams in limestone) that occurred at the outcrop scale in adjacent strata during kilometer-scale folding on the Monument Upwarp during the Laramide orogeny. The goals of the research are: 1) to describe and map the joints and conjugate shear zones at Raplee anticline and Comb monocline in southeastern Utah; and 2) to construct and evaluate continuum mechanical models using finite element methods to investigate the brittle elastic and ductile plastic deformation associated with these structures. We seek to reconcile how and why these different mechanisms were activated in close proximity and under similar loading conditions during folding, and to deduce the stress-strain behavior of the host strata. The hypothesis is that different constitutive properties and strengths led to the activation of the different mechanisms. To test this hypothesis we compare the systematic geometric features of the structures (e.g. opening distributions and shapes of veins), to the results of the numerical models. We anticipate that certain ranges of constitutive properties and states of stress will result in similarity. This will provide important insights about the mechanical behavior of sandstone and limestone, not obtainable from laboratory testing at the decimeter scale in the absence of such structures. Folds are one of the most common geologic structures, occurring in all layered rock types at scales from hand samples to mountain ranges. Folds in sedimentary rock take on the added importance of being traps for hydrocarbons, and therefore a primary target for oil and gas exploration. Being familiar with the stiffness and strength of sedimentary rock slabs used as paving stones or counter tops, the general public must wonder how such strata is contorted into spectacular folds such as those Comb and Raplee anticlines. The proposed project addresses this question, which also is a fundamental issue for structural geology: what deformation mechanisms operate within sedimentary strata during kilometer-scale folding? One mechanism is obvious: sandstone layers are broken by brittle fractures (joints). A second mechanism is quite subtle: limestone layers lack joints, but contain shear zones, alignments of surfaces (solution seams) along which calcite has been dissolved, and orthogonal opening cracks (veins) filled with calcite. The juxtaposition of brittle (jointing) and ductile (shear zone) mechanisms in adjacent outcrops offers the opportunity to deduce how the mechanical properties of the respective formations varied at the time of folding. The interplay of detailed mapping and mechanical modeling promises to offer new insights and original concepts about rock deformation during folding and to contribute new insights for scientists working in the closely related disciplines of petroleum geology, rock mechanics, and geodynamics. In addition to the research objectives of this project, the award is supporting the training of a graduate student; includes a partnership with faculty at Dine College to organize field trips for Native American students who will participate in collecting data; and is contributing to the broadening of participation of underrepresented groups in the Earth sciences.