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

点击翻译按钮获取中文摘要

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.