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

大多数山带在不同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，但很少有例子表明，从地质证据中充分量化了造成造山曲率的变形历史和过程。这些历史被嵌入到区域构造走向、古地磁方向、介观构造和变形岩石的粒度结构中。塞维尔造山带的犹他州-爱达荷州-怀俄明州凸起是研究在一定范围内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南北向弯曲90度，在交叉走向转移带附近有额外的局部弯曲。凸起中的岩石也携带着多种古地磁成分，并显示出系统的细观和微观结构（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旋转和内部应变的记录。在怀俄明凸起的初步研究揭示了初级预冲断层和次级顺折叠古地磁成分，这些成分记录了与冲断层曲率相关的复杂旋转历史。初步研究还揭示了广泛一致的区域应变模式，与逆冲趋势成大角度的层平行缩短，与逆冲趋势平行的小切向延伸，以及小的扳手剪切。方解石双应变，在早期层平行缩短期间积累，在大规模推力之前，提供了另一个记录局部显著旋转的参考框架。目前正在进行的工作主要集中在古地磁和结构研究上，这些研究分别位于凸起的南部、中部和北部前缘，以及两个地层水平（侏罗纪双溪灰岩和三叠纪Ankareh组的红层），这些地层暴露良好，具有多重磁化，并包含多种应力标志。对主要逆冲断层的滑线化数据分析和二维剖面构建表明，逆冲输运方向总体上由北部的ENE向南部的ESE转变，且向北、南两端的断层滑动幅度减小。在大量地震数据的约束下，结合本研究的新应变和旋转数据，正在构建三维截面，以开发凸起的鲁棒运动学模型。该模型将用于确定山带曲率的控制机制，包括地层厚度变化、断层强度变化、压痕运动和前陆扶壁效应。这项研究解决了有关山带演化的几个基本问题。(1)旋转、应变、逆冲断层滑动变化与凸起曲率之间的关系是什么？(2)将应变和旋转数据纳入造山带三维模型的结果是什么？(3)对弯曲山带发育最重要的因素是什么？如何识别这些因素？最终，怀俄明州隆起的发现将与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，将应变和旋转数据纳入三维可恢复变形模型，可以提高我们对褶皱-冲断带油气成藏的总体认识。

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.