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

许多工作人员将北美西部边缘的构造和岩浆事件的显着变化和时间与基本的板块边界过程联系起来。这些过程包括板块运动的变化或沿边缘的扩张中心的迁移。从晚白垩世到古近纪(距今约8000万年-4000万年)，海底磁异常记录了在大洋中部扩张中心分离的两个板块的运动，表明了几个重要的板块运动变化，但关于扩张中心相对于大陆边缘位置的一些基本问题仍然存在。后者形成三个板块汇聚在一起的三重结合部，这些结合部的位置深刻地影响着边缘的构造和岩浆作用。将北科迪勒构造与板块边界过程联系起来的一个长期而重大的问题是晚白垩世至始新世(约7000万-5500万年前)库拉-法拉隆-北美三重结合部的位置。相互竞争的假设提供了几种选择，这些选择对科迪勒边缘的板块边界相互作用产生了深远的影响。板块模型通常认为库拉-法拉隆-北美交界处在距今约6000万年前，大约在北纬40度(今天的坐标)，但所有的作者都认识到，这个位置的约束非常差。地质证据支持这一位置的海沟-海脊-海沟三重结点，但同样强烈或更强烈的理由是，在同一时间(距今6200-5000万年前)沿阿拉斯加南部边缘(今天北纬约57-61度)通过一个海沟-海脊-海沟三重结点。这一矛盾的一个解决方案是在三重结点通过期间和之后将阿拉斯加边缘向北平移，来自阿拉斯加南部的两项古地磁研究支持这一假说。最近，一些工作人员提出，地质证据要求在库拉和法拉隆之间有两个独立的TRT三叉路口，因此需要在库拉和法拉隆之间增加一个板块，称为复活板块。该项目通过对科迪亚克群岛早古新世(大约6200万-6500万年前)的火山和沉积单元的详细古地磁和构造研究来检验这些相互竞争的模型。古地磁研究是从基岩悬崖规模的露头钻出的样品中进行的，根据它们在空间中的当前位置进行定位，然后在实验室的磁强计中进行测量，以确定岩石冷却或沉积时的磁场方向。这些岩石记录的磁场可以用来确定它们喷发或沉积的纬度。这项研究的结果对特定的古地磁领域和广泛的板块构造领域都有意义。这项研究正在重新审视25年前调查过的一个问题，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纬度，但没有明确确定这些岩石沿着科迪勒边缘形成的地点。自第一次研究以来，古地磁设备和研究方法有了很大的改进，通过使用更广泛的岩石类型和分析方法，结果将表明第一次研究的结果是否正确，表明这些岩石来自南方。汇聚边缘或俯冲带过程的广泛领域从这项研究中受益，因为许多科学家想要将火成岩的类型、相关的贵金属矿床以及古代边缘的构造过程与俯冲到它们下面的海洋板块的特定特征联系起来。如果一个人不知道该板块相对于边缘的年龄、速度或方向，那么形成大陆的一级过程就很难确定。通过获取距今约6200万年前科迪亚克岛三重结合部的纬度，可以更准确地了解整个板块边缘过程。因此，这项研究涉及来自西华盛顿大学和加州大学戴维斯分校的合作者。这项研究涉及两名硕士学生，指导他们的是具有非常不同的科学背景的顾问，一名研究古地磁学，一名研究构造地质学。

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.