Collaborative Research: Plate Boundary Reconstructions in the Northern Cordillera: Revisiting the Paleomagnetism of a Paleocene Trench-ridge-trench Triple Junction

合作研究：北科迪勒拉板块边界重建：重新审视古新世海沟-山脊-海沟三重交界处的古地磁学

• 批准号: 0609957
• 负责人: Bernard Housen
• 金额: $ 17.9万
• 依托单位: Western Washington University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-06-01 至 2009-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0609957&HistoricalAwards=false
• 关键词: Collaborative; Research; Plate; Boundary; Reconstructions

Many workers have related significant variations and timing of tectonic and magmatic events along the western North American margin to fundamental plate boundary processes. These processes include changes in plate motion or migration of a spreading center along the margin. From Late Cretaceous through Paleogene (approximately 80-40 million years before present) the constraints from seafloor magnetic anomalies, which record the motions of two plates separating at mid-ocean spreading centers indicate several important plate motion changes, but some fundamental questions regarding the locations of spreading centers with respect to the continental margin remain. The latter sites form triple-junctions, where three plates come together, and the location of these profoundly affects the tectonic and magmatic processes along the margin. A persistent and significant problem for relating northern Cordilleran tectonics to plate boundary processes is the location of the Kula-Farallon-North America triple junction during the Late Cretaceous to Eocene (approximately 70-55 million years ago). Competing hypotheses offer several options which have far-reaching consequences for plate boundary interactions along the Cordilleran margin. Plate models commonly place the Kula-Farallon-North American junction at about 60 million years before present at approximately 40 degrees North latitude (present-day coordinates), but all authors recognize this location is very poorly constrained. Geologic evidence supports this location of a trench-ridge-trench triple junction, but an equally strong or stronger case can be made for passage of a trench-ridge-trech triple junction along the southern Alaska margin (about 57-61 degrees North in present-day) at the same time (62-50 million years before present). One resolution of this contradiction is to translate the Alaskan margin northward during and after passage of the triple junction, and two paleomagnetic studies from southern Alaska support this hypothesis. Recently some workers have suggested the geologic evidence calls for two separate TRT triple junctions, thus requiring an additional plate between the Kula and the Farallon, named the Resurrection plate. This project tests these competing models with a detailed paleomagnetic and structural study of volcanic and sedimentary units of early Paleocene age (about 62-65 million years before present) on the Kodiak Islands. The paleomagnetic research is done from samples drilled out of cliff-scale outcrops of bedrock, oriented with respect to their current position in space, then measured in a magnetometer in a lab to determine the magnetic field orientation when the rocks cooled or were deposited. The magnetic field recorded by these rocks can be used to determine the latitude at which they were erupted or deposited. The results from this research have implications both for the specific field of paleomagnetism and for the broad field of plate tectonics. This study is re-examining a problem investigated over 25 years ago, which discovered these anomalous paleolatitudes on volcanic rocks preserved in southern Alaska but did not unequivocally determine where these rocks formed along the Cordilleran margin. Paleomagnetic equipment and research methods have improved considerably since the first study was conducted and by using a wider variety of rock types and analytical methods the results will show whether the first study's results, indicating a southerly origin for these rocks, were correct or not. The broad field of convergent-margin, or subduction zone, processes benefits from this research because many scientists want to relate the type of igneous rocks, and associated precious metals deposits, and tectonic processes along ancient margins to specific characteristics of the ocean plate being subducted beneath them. If one does not know what the age, velocity, or orientation of that plate was with respect to the margin, then the first-order processes that shape the continents are difficult to determine. By obtaining the latitude of the Kodiak Island triple junction at about 62 million years before present, the overall plate margin processes can be much more exactly known. The research thus involves collaborators from Western Washington University and the University of California, Davis. The research involves two Master's students, guided by advisors with very different scientific backgrounds, one in paleomagnetism and one in structural geology.

许多工人在北美西部边缘沿着构造和岩浆事件的显着变化和时机与基本的板块边界过程相关。这些过程包括板块运动的变化或沿边缘扩散中心的迁移。从白垩纪晚期到古近纪（在现场之前约80-4亿年），海底磁异常的限制记录了两个在海洋中部扩散中心分离的板的运动，这表明了几个重要的板块运动变化，但有关扩展中心相对于大陆边缘的位置的一些基本问题。后一个位置形成了三个结合，其中三个板聚集在一起，这些位置深刻影响了边缘沿着边缘的构造和岩浆过程。将北部山脉构造与板边界过程联系起来的一个持续且重大的问题是库拉 - 弗拉隆 - 北美三连接期间在白垩纪晚期到始新世（大约70-55亿年前）的位置。竞争假设提供了几种选择，这些选择对沿山脉边缘的板边界相互作用产生了深远的影响。板模型通常将Kula-Farallon-North American交界处放置在大约6000万年之前，大约是北纬40度（当今的坐标），但所有作者都认识到这个位置受到了限制的约束。地质证据支持沟槽式 - 沟槽三连接点的位置，但是可以通过同一时间沿着阿拉斯加南部的边缘（当今约57-61度）（在当前的62-50百万年前）沿着阿拉斯加南部的边缘（北约57-61度）来通过沟槽 - 遥远的三重连接处。这一矛盾的一种解决方案是在三连接通过期间和之后将阿拉斯加边缘向北转换，而来自阿拉斯加南部的两项古磁研究支持了这一假设。最近，一些工人提出地质证据要求需要两个单独的TRT三重连接，因此需要Kula和Farallon之间的额外板，称为复活板。该项目通过在科迪亚克群岛（Kodiak Islands）上对古新世早期（早期约62-65亿年）的火山和沉积单元的详细古磁和结构研究测试这些竞争模型。古磁研究是从从基岩悬崖尺度露头钻出的样品进行的，该样品相对于它们在太空中的当前位置而定向，然后在实验室中的磁力计中测量，以确定当岩石冷却或沉积岩石时的磁场方向。这些岩石记录的磁场可用于确定它们爆发或沉积的纬度。这项研究的结果对古磁性的特定领域和板块构造的广泛领域都有意义。这项研究正在重新审查25年前研究的问题，该问题在阿拉斯加南部保存的火山岩上发现了这些异常的古介质，但并未明确地确定这些岩石沿山脉边缘形成的位置。自从进行了第一项研究以来，古磁设备和研究方法已经大大改善，并且通过使用多种岩石类型和分析方法，结果将表明第一个研究的结果表明这些岩石的南部起源是否正确。收敛 - 利润或俯冲带的广泛领域从这项研究中受益，因为许多科学家希望将火成岩的类型和相关的贵金属沉积物以及沿古代边缘的构造过程与海洋板的特定特征相关联。如果一个人不知道该板的年龄，速度或方向是什么相对于边缘，那么塑造大陆的一阶过程很难确定。通过在现场约6200万年获得Kodiak Island Triple Junction的纬度，可以更加清楚地知道整体板缘过程。因此，这项研究涉及西部华盛顿大学和加利福尼亚大学戴维斯分校的合作者。这项研究涉及两名大师的学生，由具有非常不同的科学背景的顾问指导，一项是古磁性，一个在结构地质学中。