Collaborative Research: Three-Dimensional Kinematic History of the Wyoming Salient: Implications for Development of Curved Orogens

大多数山脉带在一系列尺度上表现出不同程度的曲率，记录了复杂的三维变形历史。然而，尽管它们无处不在，但很少有实例表明，造山曲率的变形历史和过程已经从地质证据中得到了充分的量化。这些历史埋藏在区域构造趋势、古地磁方向、介观结构和变形岩石的粒度组构中。塞维尔造山带的犹他州-爱达荷州-怀俄明州突出点是研究在一系列尺度上产生曲率的过程的理想地点。主要逆冲断层和褶皱的区域走向从凸起的北端到南端呈90度曲线，在交叉走向转移带附近有附加的局部曲率。凸起中的岩石还携带多种古地磁成分，并显示出系统的中观和微观结构套件(包括解理、小褶皱、小断层网络和变形化石)，提供了单个逆冲片内的旋转和内部应变的记录。怀俄明州凸起的初步研究揭示了主要的前冲断和次要的同步褶皱古地磁成分，它们记录了与推力曲率有关的复杂旋转历史。初步研究还揭示了大致一致的区域应变模式，普遍存在与逆冲趋势成大角度的层状平行缩短，平行于逆冲趋势的较小的切向伸展，以及较小的扭动剪切。方解石-孪生应变在大规模逆冲作用之前的早期层平行缩短过程中积累，提供了另一个记录局部显著旋转的参考框架。正在进行的工作重点是对位于凸起南部、中部和北部前缘部分的三个“狭长地带”和两个地层水平(侏罗纪双溪石灰岩和三叠纪安卡雷组红层)进行的古地磁和构造联合研究，这些地层暴露良好，具有多个磁化作用，并包含多个应变标志。主要逆冲断裂的滑动线理资料分析和二维剖面构造表明，逆冲断层运移方向总体上由凸起北段的ENE向向南的ESE方向变化，断层滑动强度向凸起的南北两端减小。在丰富的地震数据的约束下，结合这项研究的新的应变和旋转数据，正在构建三维横截面，以开发一个稳健的凸起运动学模型。该模型将被用来确定导致山脉带弯曲的控制机制，包括地层厚度的变化、沿断层的强度变化、凹陷的运动和前陆支撑效应。这项调查解决了与山脉带演化有关的几个基本问题。(1)旋转、应变、逆冲断层滑动的变化和显著的曲率之间有什么关系？(2)将应变和旋转数据纳入造山带的三维模型中会产生什么后果？(3)在形成弯曲山带的过程中，最重要的因素是什么？如何最好地识别这些因素？最终，来自怀俄明州凸起的发现将与正在进行的其他弯曲山带的类似研究进行比较，为弯曲造山带的发展提供一系列可行的模型。最后，将应变和旋转数据包含到三维可恢复变形模型中，应该会提高我们对褶皱冲断带油气聚集的总体理解。

Most mountain belts display varying degrees of curvature over a range of scales, recording complex, three-dimensional deformation histories. However, despite their ubiquitous occurrence, few examples exist where deformation histories and processes responsible for orogenic curvature have been adequately quantified from geologic evidence. These histories are embedded in regional structural trends, paleomagnetic directions, mesoscopic structures, and grain-scale fabrics of deformed rocks. The Utah-Idaho-Wyoming salient of the Sevier orogenic belt is an ideal location to study processes that produce curvature over a range of scales. Regional trends of major thrust faults and folds curve through 90 degrees from the north to south end of the salient, with additional local curvature near cross-strike transfer zones. Rocks in the salient also carry multiple paleomagnetic components and display systematic suites of mesoscopic and microscopic structures (including cleavage, minor folds, minor fault networks, and deformed fossils), providing a record of rotations and internal strain within individual thrust sheets. Initial studies in the Wyoming salient reveal both primary pre-thrusting and secondary syn-folding paleomagnetic components that record complex rotation histories related to thrust curvature. Initial studies also reveal broadly consistent regional strain patterns, with widespread layer-parallel shortening at high angles to thrust trends, minor tangential extension parallel to thrust trends, and minor wrench shear. Calcite-twin strains, which accumulated during early layer-parallel shortening prior to large-scale thrusting, provide another reference frame that records locally significant rotations. Ongoing work is focused on combined paleomagnetic and structural studies of three 'swaths' located in the southern, central, and northern frontal parts of the salient, and on two stratigraphic levels (Jurassic Twin Creek Limestone and redbeds of the Triassic Ankareh Formation) that are well exposed, carry multiple magnetizations, and contain multiple strain markers. Analysis of slip lineation data from major thrust faults and construction of two-dimensional cross sections indicate that thrust transport directions change overall from ENE in the northern part to ESE in the southern part of the salient, and fault slip magnitude decreases toward the north and south ends of the salient. Three-dimensional cross sections, constrained by abundant seismic data, and incorporating new strain and rotation data from this study, are being constructed to develop a robust kinematic model of the salient. The model will be used to identify controlling mechanisms responsible for mountain belt curvature, including variations in stratigraphic thicknesses, strength changes along faults, motion of indentors, and foreland buttress effects. This investigation addresses several fundamental questions concerning the evolution of mountain belts. (1) What are the relations between rotations, strain, changes in thrust fault slip, and salient curvature? (2) What are the consequences of incorporating strain and rotation data into three-dimensional models of orogenic belts? (3) What factors are most important in developing curved mountain belts and how can these factors be best identified? Ultimately, findings from the Wyoming salient will be compared with similar ongoing studies of other curved mountain belts, providing an array of viable models for the development of curved orogens. Lastly, inclusion of strain and rotation data into three-dimensional restorable deformation models should improve our general understanding of hydrocarbon accumulations in fold-thrust belts.