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

许多工作者将沿北美西部边缘的构造和岩浆事件的显著变化和时间与基本板块边界过程联系起来。这些过程包括板块运动的变化或扩张中心沿边缘的迁移。从晚白垩世到古近纪（距今约8000万至4000万年），海底磁异常记录了两个板块在洋中扩张中心分离的运动，这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的位置的一些基本问题仍然存在。后一种位置形成了三联结，在那里三个板块聚集在一起，它们的位置深刻地影响着边缘的构造和岩浆过程。将北科迪勒构造与板块边界过程联系起来的一个持久而重要的问题是晚白垩世至始新世（大约7000万至5500万年前）的库拉-法拉伦-北美三重交界处的位置。相互竞争的假设提供了几种选择，这些选择对科迪勒拉边缘的板块边界相互作用具有深远的影响。板块模型通常将库拉-法拉龙-北美交界处定位在距今6000万年前的大约北纬40度（现在的坐标），但所有作者都认识到这个位置是非常有限的。地质证据支持海沟-山脊-海沟三重连接点的位置，但在同一时期（距今6200万至5000万年前），沿阿拉斯加南部边缘（约北纬57-61度）也存在海沟-山脊-海沟三重连接点，这一点同样有力或更有力。解决这一矛盾的一种方法是将阿拉斯加边界在三联点通过期间和之后向北平移，阿拉斯加南部的两项古地磁研究支持这一假设。最近，一些工作人员提出，地质证据表明存在两个独立的TRT三重交叉点，因此在库拉和法拉隆之间需要一个额外的板块，命名为复活板块。该项目通过对科迪亚克群岛早古新世（距今约6200万至6500万年前）的火山和沉积单元进行详细的古地磁和结构研究，来检验这些相互竞争的模型。古地磁研究是从基岩的悬崖尺度露头中钻出的样本，根据它们在空间中的当前位置进行定向，然后在实验室中用磁力计测量，以确定岩石冷却或沉积时的磁场方向。这些岩石所记录的磁场可以用来确定它们喷发或沉积的纬度。这一研究结果对古地磁的特定领域和板块构造的广泛领域都具有启示意义。这项研究重新审视了25年前调查过的一个问题，即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，但并没有明确确定这些岩石沿着科迪勒拉山脉边缘形成的位置。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，表明这些岩石的起源是南方的。汇聚边缘或俯冲带过程的广泛领域受益于这项研究，因为许多科学家希望将火成岩的类型、相关的贵金属矿床以及沿着古代边缘的构造过程与它们下面俯冲的海洋板块的特定特征联系起来。如果一个人不知道那个板块相对于边缘的年龄、速度或方向，那么塑造大陆的一级过程就很难确定。通过获得大约6200万年前科迪亚克岛三重交界处的纬度，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研究生，他们的导师有着非常不同的科学背景，一个是古地磁，一个是构造地质学。

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.