Collaborative Research: Evaluating controls on orogenic structural style by constraining the spatio-temporal evolution of a retroarc thrust belt

尽管所有的大陆山带都是由于大陆地壳在板块汇聚区缩短而形成的，但它们在宽度、海拔和所涉及的岩石类型上都有很大的差异。鉴于这些山脉中的许多与地球上一些最严重的地质灾害有关，并拥有丰富的石油和矿藏，确定造成这些差异的主要因素具有相当大的社会和科学兴趣。这项研究将检验一种假设，即这些山带的主要差异是由于造山开始之前存在的岩石类型和强度的差异造成的。这一贡献意义重大，因为预计它将重新定义这些变化的相对重要性，这些变化不仅影响山脉带的几何形状，而且影响它们形成的机制。除了该项目的科学贡献外，重要的社会成果还包括通过对一个重要的科学、技术、工程和数学(STEM)学科的研究生和本科生进行教育来发展劳动力。它通过针对K-12学生的外联努力培养公众的科学素养，这将导致培训学生并吸引他们投身STEM职业。该项目支持扩大STEM中任职人数不足的群体。该项目通过实施落基山脉北部高中科学教师实地课程，为落基山脉北部地区高中教师的STEM教育者发展做出了贡献。研究结果将通过在专业学会会议上的陈述和同行评议的科学文献来传播。主要研究人员将对爱达荷州中东部和蒙大拿州西南部的构造风格--主要是沉积单元的“薄皮”变形或基底单元的“厚皮”变形--的时空转换进行构造、沉积和热年代学记录，并将其与怀俄明州“拉莱姆”平面俯冲的时间进行比较。需要检验的中心假设是，沿着提议的横断面的构造样式的转变是由于上板块的造山前地层结构而发展的，该横断面位于经典的拉莱米德地区之外。这项工作将对上板块的内在性质，而不是板块边界地球动力学的变化，如平板俯冲，是否可以成为控制造山构造样式的主导机制提供一个关键的检验。该团队将通过1)爱达荷州中东部和蒙大拿州西南部的构造和地层调查来验证这一中心假设，以限制构造样式和同时代沉积中心的空间格局，以及2)逆冲板块的热力学计时，以限制与逆冲和同时代前陆沉积有关的折返时间。研究设计包括锆石和磷灰石铀铅和(铀-钍)/氦测年，以及在野外构造和地层调查背景下的磷灰石裂变径迹热年代学；这些结果将被用来建立区域运动学模型，以评估假想的非平板控制的构造样式的时空转换。因此，这项工作的结果有可能改变我们对大陆褶皱冲断带薄皮和厚皮之间因果关系的认识，并可能重新评估造山前上地壳结构在影响现代和古代大陆山带一级几何形状方面的重要性。

Though all continental mountain belts form as continental crust is shortened in regions of plate convergence, they exhibit major variations in width, elevation, and types of rocks involved. Given that many of these mountain belts are associated with some of Earth's most significant geological hazards and host prolific petroleum and mineral deposits, there is considerable societal and scientific interest in identifying the main factors responsible for their differences. This research will test a hypothesis that major differences in these mountain belts result from variations in the types and strengths of rocks that are present prior to the beginning of mountain building. This contribution is significant because it is expected to redefine the relative importance of these variations in influencing not only the geometry of mountain belts, but also the mechanisms through which they form. In addition to the scientific contributions of the project, important societal outcomes include workforce development through the education of graduate and undergraduate students in an important science, technology, engineering and mathematics (STEM) discipline. It fosters scientific literacy of the public through outreach efforts aimed at K-12 students that will result in the training and attraction of students to STEM careers. The project supports broadening of underrepresented groups in STEM. The project contributes to STEM educator development for high school teachers in the northern Rocky Mountain region through the implementation of a field-based course for high school science teachers in the northern Rockies. Results of the research will be disseminated through presentations at professional society meetings and peer-reviewed scientific literature. The principal investigators will construct a structural, sedimentary, and thermochronologic record of the spatial and temporal transition in structural style--"thin-skinned" deformation of primarily sedimentary units or "thick-skinned" deformation of basement units--in east-central Idaho and southwestern Montana and compare it to the timing of "Laramide" flat subduction beneath Wyoming. The central hypothesis to be tested is that the transition in structural style along the proposed transect, which is outside the classic Laramide region, developed as a result of the pre-orogenic stratigraphic architecture of the upper plate. This work will provide a critical test of whether an intrinsic property of the upper plate, rather than a change in plate boundary geodynamics such as flat-slab subduction, can be a dominant mechanism for controlling orogenic structural style. The team will test the central hypothesis using 1) structural and stratigraphic investigations in east-central Idaho and southwestern Montana to constrain the spatial pattern of structural styles and coeval depocenters and 2) thermochronometry of thrust sheets to constrain the timing of exhumation related to thrusting and coeval foreland sedimentation. The research design includes zircon and apatite uranium-lead and (uranium-thorium)/helium dating, as well as apatite fission track thermochronometry in the context of field-based structural and stratigraphic investigations; these results will be used to build a regional kinematic model to evaluate the hypothesized non-flat-slab control on the spatio-temporal transition in structural style. Results from this work thus have the potential to transform our knowledge of the causal relationship between thin- and thick-skinned portions of continental fold-thrust belts and could warrant reevaluation of the importance of the pre-orogenic upper crustal architecture in affecting the first order geometries of modern and ancient continental mountain belts.