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

人们越来越清楚地认识到，由于地壳缩短而导致的高海拔山脉和大陆高原，通常在板块会聚过程中经历伸展。尽管这一过程得到了广泛的认识，但目前对共聚伸展的原因尚不完全清楚，仍然是大陆构造中的一个突出问题。这项研究评估了导致美国西部白垩纪Sevier造山带同向会聚伸展的过程，并将其应用于其他山脉带。评价了三种经常被提及的同聚伸展机制：(1)构造增厚地壳的热弛豫，导致壳幔部分熔融和解耦；(2)双重断裂导致的局部地壳增厚，导致超临界造山锥体；(3)岩石圈地幔的拆离。这些过程中的每一个都有可预测的影响，正在通过对塞维尔造山带腹地两个地点的研究进行测试：犹他州西北部和爱达荷州南部的福特河-阿尔比翁-格鲁兹溪山脉，以及加利福尼亚州东南部的殡仪山。这些都是评估与突触扩展相关的过程的理想领域。前人的研究表明，由于在明确的新生代伸展剪切带和滑脱断层之间和之下保留了中生代组构，因此中生代造山作用的长期变质和变形记录得到了很好的保存。作为这一研究项目的一部分，我们正在进行详细的野外填图、构造和运动学分析、石榴石的定量变质温压测定和PT路径测定、独居石的原位离子微探针Th-U-Pad定年、Luticum-Hafnium和Sm-Nd石榴石年代学以及Ar-Ar热年代学。这种综合方法对于(1)确定压力-温度-时间路径，(2)评价早期逆冲埋藏和随后的同向会聚伸展的运动学，(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检验同向会聚伸展假说，以及(4)关联腹地和前陆演化是必要的。本研究所采用的多种年代学方法为进一步完善Luticum-Hafnium和Sm-Ne法测定石榴石的温度依赖关系提供了依据。这项综合研究代表了三所大学和具有构造地质学、变质岩石学、数值热模拟、同位素地球化学和地质年代学专业知识的主要研究人员之间的研究努力。拟议的工作在各级都是充分协作和综合的。在这项研究中开发的信息和技术正在传播到构造、岩石学和地球化学界。该项目为四名研究生和四名本科生提供研究培训，其中包括贫困群体成员。作为研究生和本科生论文，学生参与了这项研究的重要组成部分，并在实地观察、数据获取和数据分析方面积累了经验。他们通过在大学研究博览会和地质学会议上发表演讲来传播这项研究的结果，并在被引用的期刊上发表科学论文。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，从项目的协作性质中受益。这项研究补充了内华达大学-拉斯维加斯大学在格罗斯克里克和阿尔比昂山脉的大理石峰和磨坊溪地区的两个地质夏令营，并为葬礼山脉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.