Collaborative Research: Testing Theories of Synconvergent Extension in the Hinterland of the Sevier Orogen

越来越多的人认识到，高海拔山脉和大陆高原的高海拔是由于地壳缩短造成的，它们在板块汇聚过程中通常会经历伸展。尽管这一过程得到了广泛的认识，但同辐合伸展的原因目前还不完全清楚，这仍然是大陆构造学中的一个突出问题。本研究评价了美国西部白垩系塞维尔造山带的同辐合伸展过程，并将其应用于其他山带。本文评价了三种常被引用的共收敛伸展机制：(1)构造增厚地壳的热松弛，导致地壳与地幔的部分熔融和解耦；(2)双断层作用导致的局部地壳增厚，形成了超临界造山锥；(3)岩石圈地幔剥离。这些过程中的每一个都有可预测的影响，通过对塞维尔造山带腹地的两个地方的研究进行了测试：犹他州西北部和爱达荷州南部的Raft river - albiion - grouse Creek山脉和加利福尼亚州东南部的Funeral Mountains。这些是评估与同步收敛扩展相关的过程的理想领域。前人的研究表明，中生代造山活动的长期变质和变形记录被很好地保存了下来，这是由于在明确的新生代伸展剪切带和滑脱断裂之间和下面存在少量或不存在热覆印，以及中生代构造的保存。作为该研究项目的一部分，我们正在进行详细的野外测绘、结构和运动学分析、石榴石的定量变质热压测量和PT路径测定、独居石的原位离子探针钍-铀-铅定年、镥-铪和钐-钕石榴石的地质年代学以及氩-氩热年代学。这种综合方法对于(1)确定压力-温度-时间路径是必要的，(2)评估早期逆冲埋藏和随后的同收敛伸展的运动学，(3)通过将压力-温度-时间路径与数值模拟预测的路径进行比较来验证同收敛伸展的假设，以及(4)关联腹地和前陆演化。本研究采用的多种地质年代学方法为完善石榴石定年方法中镥铪法和钐钕法的温度依赖性校准提供了基础。这项综合研究代表了三所大学和主要研究人员在构造地质学、变质岩石学、数值热模拟、同位素地球化学和地质年代学方面的专业知识的研究成果。拟议的工作在所有各级都是充分协作和综合的。在这项研究中发展的信息和技术正在向构造学、岩石学和地球化学界传播。该项目为四名研究生和四名本科生提供研究培训，其中包括代表性不足的群体成员。学生参与本研究的重要组成部分，作为研究生和本科生的论文，并获得实地观察，数据采集和数据分析的经验。他们通过在大学研究展览和地质会议上的演讲，为传播这项研究的发现做出了贡献，并在评审期刊上发表了科学论文。学生通过接触本研究的调查方法，以及理解和整合合作者的数据和分析的需要，从项目的合作性质中受益。这项研究补充了内华达大学拉斯维加斯分校在松鸡溪和阿尔比恩山脉的大理石峰和米尔溪地区的两个地质夏季野外营地，并为葬礼山所在的死亡谷国家公园的科学知识库做出了贡献。

It is increasingly well recognized that high altitude mountains and continental plateaus, which owe their high elevations to crustal shortening, commonly undergo extension during plate convergence. Despite widespread recognition of this process, the causes of synconvergent extension are currently incompletely understood and remain an outstanding problem in continental tectonics. This study evaluates the processes that led to synconvergent extension in the Cretaceous Sevier orogen in the western United States, with application to other mountain belts. Three frequently cited mechanisms for synconvergent extension are being evaluated: (1) thermal relaxation of tectonically thickened crust, leading to partial melting and decoupling of crust from mantle; (2) localized crustal thickening resulting from duplex faulting leading to supercritical orogenic taper, and (3) the delamination of lithospheric mantle. Each of these processes has predictable effects that are being tested through studies of two localities within the hinterland of the Sevier orogen: the Raft River-Albion-Grouse Creek ranges in northwestern Utah and southern Idaho and the Funeral Mountains in southeastern California. These are ideal areas to evaluate processes related to synconvergent extension. As prior studies indicate, protracted metamorphic and deformational records of Mesozoic orogenesis are well preserved due to minimal or absent thermal overprints and preservation of Mesozoic fabrics between and beneath well-defined Cenozoic extensional shear zones and detachment faults. As part of this research project we are conducting detailed field mapping, structural and kinematic analysis, quantitative metamorphic thermobarometry and PT path determination on garnet, in situ ion microprobe Thorium-Uranium-Lead dating of monazite, Luticium-Hafnium and Samarium-Neodymium garnet geochronology, and Argon-Argon thermochronology. This integrated approach is necessary to (1) determine pressure-temperature-time paths, (2) evaluate the kinematics of early thrust burial and subsequent synconvergent extension, (3) test hypotheses for synconvergent extension by comparing the pressure-tempearture-time paths to those predicted by numerical simulations, and (4) correlate hinterland and foreland evolutions. The multiple geochronologic methods employed in this study provide a basis for refining the calibrations of temperature dependence of the Luticium-Hafnium and Samarium-Neodymium methods of dating garnet. This integrated study represents a research effort between three universities and principal investigors with expertise in structural geology, metamorphic petrology, numerical thermal modeling, isotope geochemistry and geochronology. The proposed work is fully collaborative and integrated at all levels. Information and techniques developed in this research is being disseminated to tectonic, petrologic, and geochemical communities. This project provides research training for four graduate students and four undergraduates, including members of underrepsented groups. The student participants conduct important components of this research as graduate and undergraduate theses, and gain experience in making field observations, data acquisition, and data analysis. They contribute to the dissemination of the findings of this study through presentations at university research expositions and geologic conferences, and contribute to scientific papers in refereed journals. Students benefit from the collaborative nature of the project through exposure to the investigative methods of this study, and the need to understand and integrate data and analysis from collaborators. The research complements two University of Nevada-Las Vegas geology summer field camps in the Marble Peak and Mill Creek areas of the Grouse Creek and Albion Mountains, and contributes to the scientific knowledge base for Death Valley National Park, where the Funeral Mountains are located.