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（海拔和地势）是地球主要山脉（如喜马拉雅山脉、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重要组成部分。尽管对地表隆起的估计很常见，但我们缺乏对过去造山带地形的精确记录：海拔、折返模式和排水系统演化的量化变化。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高原，受到挤压边界力的支持，在新近纪伸展塌陷之前平均海拔为3-4公里。然而，古近纪向伸展构造的转变，以及推测的地壳对平板移除的动态响应仍然知之甚少，特别是在新近纪伸展所覆盖的腹地部分。内华达州东部的始新世河流和湖泊沉积岩（靠近拟议的古分水岭）与火山单元互层，跨越了这一构造转变的时间。初步数据表明，这些岩石为了解从平板俯冲和收缩到伸展构造过渡期间的构造和地表过程提供了重要的见解。这项多学科研究将重建该地区的地形和形态，限制约 50 至 3000 万年前初始伸展的时间和幅度，并建立向伸展过渡的壳幔动力学构造模型。这包括以下内容：(1) 河流和湖盆沉积学和地层学，以重建水系和盆地形态；(2) 互层凝灰岩的氩年代学，以揭示沉积历史、沉积物积累速率以及沉积类型随时间的变化； (3) 水合火山玻璃和湖相碳酸盐的稳定同位素分析，以限制古海拔和湖水化学随时间的变化，(4) 碎屑锆石 U-Pb 年代学，以重建河流排水网络并确定沉积物来源模式，(5) (U-Th)/He 碎屑锆石颗粒双重测年，以查明相似结晶年龄的来源并量化折返 费率。整合多学科来量化地球动力学是构造学研究的前沿。拟议的研究将区分所提出的盆地形成机制，以便通过表面记录跟踪深层地幔过程，创建科迪勒拉腹地塌陷的可重复构造模型。我们的研究结果将有助于量化冲断带腹地地区地壳对加热、失稳和分层的响应，并有可能记录造山高原围墙盆地形成和盆地水文的新机制。我们关于造山高原塌陷表面表现的最终模型将提高我们对驱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，地球第二高的山脉。从那时起，这个高地区域塌陷并延伸成一系列由低海拔盆地分隔的较小范围。在塌陷的最初阶段，在现在非常干燥的沙漠中形成了一个大湖（或一系列较小的湖泊），类似于玻利维亚和秘鲁的的喀喀湖。这个湖和流入它的河流的沉积物包含了有关该地区演变的重要线索。该项目是一项跨学科研究，旨在研究这座古老山脉如何以及为何被破坏，以及这如何影响该地区的气候和环境。了解过去的地形和地理对于理解以下内容至关重要：(1) 板块构造运动导致山脉的构建、演化和崩塌；(2) 地形变化对气候、降水和地表/地下水输送的影响；(3) 地球表面的风化、侵蚀和塑造；(4) 扩张与超级火山发生之间的关系；(5) 具有重要经济意义的火山的形成和发展。 石油、天然气和金矿。我们将使用多种尖端物理和化学技术研究内华达州在山体崩塌初始阶段积累的层状沉积物（地层）。在野外，我们将测量湖泊地层的厚度和成分，这可以告诉我们湖泊是深还是浅，是咸还是淡水。我们还将收集湖中积累的多个火山灰床，并使用这些床内的晶体，通过测量晶体内钾的放射性衰变来确定高精度年龄。利用这些火山灰床中玻璃碎片中的水，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。最后，我们将收集古代河流沉积的砂岩，并从中分离出锆石晶体以进行多种分析。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何时从形成它们的岩石中被侵蚀。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多少？ (2) 30-5000万年前，地表有哪些类型的岩石暴露并受到侵蚀，它们的侵蚀速度有多快？ (3) 内华达州过去的地形和地貌是怎样的？ (4) 内华达州何时以及以多快的速度扩展到当今存在的孤立的沙漠盆地和山脉？除了该项目的研究目标外，该奖项还有助于支持两名早期职业研究人员；扩大代表性不足的群体对 STEM 学科的参与；研究生和本科生参与研究； 对研究基础设施的贡献；对过去延伸的地质测绘和量化的贡献，这对于了解整个地区的矿化趋势、金矿的形成以及地震和火山等地质灾害的发生至关重要。