Collaborative Research: Paleogeographic Record of Contractional to Extensional Tectonics in the Cordilleran Hinterland, Nevada

合作研究：内华达州科迪勒拉腹地收缩到伸展构造的古地理记录

• 批准号: 1322015
• 负责人: Michael Smith
• 金额: $ 8.6万
• 依托单位: Sonoma State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-09-01 至 2015-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1322015&HistoricalAwards=false
• 关键词: Collaborative; Research; Paleogeographic; Record; Contractional

Orogen topography - elevation and relief - is a critical component in geodynamic models of the growth, evolution, and collapse of Earth's major mountain belts such as the Himalayas, Andes, and North American Cordillera. Although estimates of surface uplift are common, we lack precise records of past orogen topography: quantified changes in elevations, exhumation patterns, and drainage system evolution. The Cordilleran hinterland from Nevada to western Utah is interpreted as a Paleogene orogenic plateau, supported by compressional boundary forces, with mean elevations of 3-4 kilometers prior to Neogene extensional collapse. The Paleogene transition to extensional tectonics, however, and the presumed dynamic crustal response to flat slab removal remain poorly understood, particularly in portions of the hinterland overprinted by Neogene extension. Eocene fluvial and lacustrine sedimentary rocks in eastern Nevada - near the proposed paleo-divide - interbedded with volcanic units, span the time of this tectonic transition. Preliminary data show that these rocks provide crucial insights into the tectonic and surface processes during the transition from flat-slab subduction and contraction to extensional tectonics. This multi-disciplinary study will reconstruct the topography and morphology of the region, constrain the timing and magnitude of initial extension from ~50 to 30 million years ago, and build a tectonic model for the crust-mantle dynamics of the transition to extension. This includes the following: (1) fluvial and lacustrine basin sedimentology and stratigraphy to reconstruct drainages and basin morphology, (2) Argon geochronology of interbedded tuffs to reveal depositional history, sediment accumulation rates, and changes in deposition style over time; (3) stable isotope analyses of hydrated volcanic glasses and lacustrine carbonates to constrain paleoelevations and lake water chemistry over time, (4) detrital zircon U-Pb geochronology to reconstruct the fluvial drainage network and identify sediment provenance patterns, and (5) (U-Th)/He double dating of detrital zircon grains to pinpoint sources of similar crystallization age and quantify exhumation rates. The integration of multiple disciplines to quantify geodynamics is at the forefront of tectonics research. The proposed research will differentiate between proposed mechanisms for basin formation, so as to create a reproducible tectonic model of the collapse of the Cordilleran hinterland by tracking deep mantle processes through the surface record,. Our findings will help quantify the crustal response to heating, destabilization, and delamination in thrust belt hinterland regions, and have the potential to document a new mechanism for walled basin formation and basin hydrology on orogenic plateaus. Our final model of the surface expression of orogenic plateau collapse will improve our understanding of the crustal and mantle dynamics that drive the evolution and eventual degradation of regions of high elevation worldwide.Approximately 45 million years ago, the state of Nevada resembled the Andes of western South America, the Earth's second highest mountain range. Since then, this high area has collapsed and extended into a series of smaller ranges separated by low elevation basins. During the initial phases of this collapse, a large lake (or series of smaller lakes) formed in what is now a very dry desert, similar to Lake Titicaca in Bolivia and Peru. The deposits of this lake and the rivers that flowed into it contain important clues about the evolution of the area. This project is an interdisciplinary study of how and why this ancient mountain range was destroyed, and how this affected the climate and environments of the region. Knowing past topography and geography is critical to understanding: (1) the construction, evolution, and collapse of mountains through plate tectonic movements, (2) the effect of changing topography on climate, precipitation, and surface/ground water transport, (3) the weathering, erosion, and shaping of Earth's surface, (4) the relationship between extension and the occurrence of super-volcanoes, and (5) the formation and development of economically-important oil, gas, and gold deposits. We will study layered sedimentary deposits (strata) that accumulated in Nevada during the initial phase of mountain collapse using several cutting-edge physical and chemical techniques. In the field, we will measure the thickness and composition of lake strata, which can tell us whether the lake was deep or shallow and salty or freshwater. We will also collect multiple volcanic ash beds that accumulated in the lake, and use crystals within those beds to determine high-precision age determines by measuring the radioactive decay of potassium within the crystal. Using water trapped within glass shards in these volcanic ash beds, we will determine the isotopic composition of ancient precipitation in order to estimate past elevations of the region. Finally, we will collect sandstone that was deposited by ancient rivers, and separate zircon crystals from them for a number of analyses. By measuring the respective amounts of Uranium, Thorium, and Lead from zircon mineral grains, we will determine the age of individual grain and when it was eroded from the rock they formed in. Combining all of the techniques outlined above, we will be able to answer the following questions: (1) What was the size and extent of this ancient lake and the corresponding drainage system? (2) What types of rocks were exposed and eroding at the surface 30-50 million years ago, and how quickly did they erode? (3) What was the past topography and relief of Nevada? and (4) When and how quickly did Nevada extend into the isolated desert basins and ranges that exist today? In addition to the research objectives of this project, the award is contributing to support of two early career researchers; broadening of participation of underrepresented groups in an STEM discipline; involvement of graduate and undergraduates in research; contributions to research infrastructure; and contributions to geologic mapping and quantification of past extension, which is critical to understanding mineralization trends across the region, the formation of gold deposits, and the occurrence of geohazards such as earthquakes and volcanoes.

造山带地形——海拔和起伏——是地球主要山脉带（如喜马拉雅山、安第斯山脉和北美科迪勒拉山脉）生长、演化和崩塌的地球动力学模型的关键组成部分。虽然对地表隆起的估计是常见的，但我们缺乏过去造山带地形的精确记录：量化的海拔变化、挖掘模式和排水系统演化。从内华达州到犹他州西部的科迪勒拉腹地被解释为古近纪造山带，受挤压边界力支撑，在新近纪伸展崩塌之前平均海拔为3-4公里。然而，古近纪向伸展构造的过渡以及假定的地壳对平板移动的动态响应仍然知之甚少，特别是在被新近纪伸展覆盖的腹地部分。内华达东部的始新世河流和湖相沉积岩——靠近提议的古分水岭——与火山单元互层，跨越了这个构造转变的时间。初步数据表明，这些岩石为研究从平板俯冲和收缩向伸展构造过渡期间的构造和地表过程提供了重要的见解。该多学科研究将重建该地区的地形和形态，限制约5000万至3000万年前初始伸展的时间和幅度，并建立过渡到伸展的壳幔动力学的构造模型。这包括：(1)河流和湖泊盆地沉积学和地层学，以重建流域和盆地形态；(2)互层凝灰岩的氩年代学，以揭示沉积历史、沉积堆积速率和沉积样式随时间的变化；(3)水合火山玻璃和湖泊碳酸盐的稳定同位素分析，以限制古海拔和湖泊水化学随时间的变化；(4)碎屑锆石U-Pb年代学，以重建河流流域网络，确定沉积物物源模式；(5)碎屑锆石颗粒的(U-Th)/He双定年，以确定相似结晶年龄的来源，并量化挖掘速率。整合多学科来量化地球动力学是构造学研究的前沿。拟开展的研究将区分不同的盆地形成机制，从而通过地表记录追踪深部地幔过程，创建可复制的科迪勒拉盆地腹地崩塌构造模型。我们的发现将有助于量化逆冲带腹地地壳对加热、不稳定和剥离的响应，并有可能记录造山带高原壁式盆地形成和盆地水文的新机制。我们的造山高原崩塌的最终表面表达模型将提高我们对推动全球高海拔地区演化和最终退化的地壳和地幔动力学的理解。大约4500万年前，内华达州类似于南美洲西部的安第斯山脉——地球上第二高的山脉。从那时起，这个高海拔地区崩塌并扩展成一系列由低海拔盆地分隔的较小山脉。在这次崩塌的最初阶段，一个大湖（或一系列较小的湖泊）在现在非常干燥的沙漠中形成，类似于玻利维亚和秘鲁的的的喀喀湖。这个湖的沉积物和流入它的河流包含了该地区演化的重要线索。这个项目是一个跨学科的研究，研究这个古老的山脉是如何以及为什么被破坏的，以及它是如何影响该地区的气候和环境的。了解过去的地形和地理对于理解：(1)板块构造运动中山脉的构造、演化和崩塌；(2)地形变化对气候、降水和地表水/地下水输送的影响；(3)地球表面的风化、侵蚀和塑造；(4)伸展与超级火山的发生之间的关系；(5)具有重要经济意义的石油、天然气和金矿床的形成和发展至关重要。我们将使用几种尖端的物理和化学技术研究在山体崩塌初期在内华达州积累的层状沉积沉积物（地层）。在野外，我们将测量湖泊地层的厚度和组成，这可以告诉我们湖泊是深还是浅，是咸水还是淡水。我们还将收集湖中积累的多个火山灰床，并利用这些床中的晶体通过测量晶体中钾的放射性衰变来确定高精度的年龄。利用这些火山灰床玻璃碎片中的水分，我们将确定古代降水的同位素组成，以估计该地区过去的海拔高度。最后，我们将收集由古河流沉积的砂岩，并从中分离出锆石晶体进行一系列分析。通过测量锆石矿物颗粒中铀、钍和铅的含量，我们将确定单个颗粒的年龄以及它们从形成的岩石中被侵蚀的时间。结合上述所有技术，我们将能够回答以下问题：(1)这个古老湖泊的大小和范围以及相应的排水系统是什么？(2)在3000万至5000万年前，地表暴露并受到侵蚀的岩石类型是什么？它们的侵蚀速度有多快？(3)内华达州过去的地形和地形是怎样的？(4)内华达州是何时以多快的速度扩展到今天存在的孤立的沙漠盆地和山脉的？除了该项目的研究目标外，该奖项还为两位早期职业研究人员提供支持；扩大代表性不足的群体参与STEM学科；研究生和本科生参与研究；对研究基础设施的贡献；并对过去伸展的地质填图和量化做出贡献，这对于了解整个地区的成矿趋势、金矿床的形成以及地震和火山等地质灾害的发生至关重要。