This proposal will address the motion between East and West Antarctica. The estimates of motion in the Late Cretaceous and Cenozoic come from summing the plate circuit linking East Antarctica to Australia to the Lord Howe Rise to the Pacific plate to West Antarctica. However, major uncertainties remain in the history of the plate motions in this region, leading to widely different implications for the global plate circuit, without a clear indication of which model is correct. The PIs propose to analyze this motion in two parts: motion since 42 Ma and motion before 42 Ma. For the younger time interval, the PIs will improve accuracy of rotations by taking advantage of an unusual plate geometry that enables them to solve a five-boundary, four-plate configuration. For the older time interval, the only way to calculate motion between East and West Antarctica is via the long Aus-Pac plate circuit. The difficulty in this time interval is that there are three sets of rotations with distinctly different results for the Aus-East Ant boundary. The disagreement over the fundamental motion between Aus and Ant before 50 Ma leads to large differences in the predicted motion in the Western Ross Sea and near Ellsworth Land. The PIs propose to re-examine the key elements in the Aus-Pac plate circuit with the objective of reducing uncertainties, using quantitative methods where possible, and resolving some of the critical issues involving alternative interpretations.
Broader impacts: The scientific impacts of this work are an improved understanding of Antarctic plate tectonic history to incorporate into global models. This project will support one graduate student at each institution. The results of the project will be made available in the global plate motion and plate reconstruction databases. In addition, results will be used for outreach work to middle school groups. Cande has made an animated movie of the plate motions around the Aus-Pac plate circuit that he uses as part of his undergraduate and graduate teaching. He plans to update and expand this animation using the results of this project.
We analyzed marine geophysical data from the Ross Sea and Southern Ocean, near Antarctica, in order to better constrain the history of tectonic plate motions and formation of the seafloor. This region now contains the Antarctica plate as a single tectonic plate, with the Australia and Pacific plates spreading northward away from it on the side near New Zealand. The spreading boundaries are known as the Southeast Indian Ridge (south of Australia) and the Pacific-Antarctic ridge (southeast of New Zealand). Before 26 million years ago, these spreading ridges were much closer to Antarctica, and Antarctica itself consisted of two separate plates: East Antarctica and West Antarctica. East Antarctica (south of Australia) and West Antarctica (south of the Pacific plate) were spreading away from each other along a mid-ocean ridge in the Ross Sea. This mid-ocean ridge created the seafloor in the Adare Basin, and left behind a fossil spreading center, called the Adare Trough. This was connected to a region of continental extensional basins and strike-slip faults along a plate boundary that continued towards the Antarctic Peninsula, but which cannot be easily studied since it is now completely covered by ice. Before 55 million years ago, there was also another plate in this region (the Lord Howe plate which is now part of the Australia plate in the eastern Tasman Sea). To see how these plates moved relative to one another, through time, we reconstruct their past positions. To do this, we identify features on the seafloor that should match back together. These features include linear magnetic anomalies, gravity anomalies, and bathymetric features (fracture zones) that are perpendicular to the magnetic anomalies. The magnetic anomalies and fracture zones match back across the boundaries between each pair of plates: Pacific-West Antarctica, West Antarctica-East Antarctica, East Antarctica-Australia, and Australia-Pacific. We analyzed all of the available data, including existing marine magnetic anomaly data and new gravity field and aeromagnetic data, relevant to matching back these boundaries. We developed a more exact method of matching the magnetic anomaly shapes back to the magnetic reversal time scale, by correcting for skewness (which is most useful for the data from E-W oriented ridges, such as the Southeast Indian Ridge, or for magnetic anomalies formed by seafloor spreading farther away from the Polar Regions, such as the Pacific-Antarctic ridge). We submitted previously unreported data points, and relative plate rotations derived from them, to a new online, public data repository for seafloor magnetic anomaly identifications. A major question about the tectonic history of this region is the spatial and kinematic connection between the seafloor spreading in the Ross Sea and the inferred continental extension farther south (in the Northern Basin) along the plate boundary. The marine magnetic anomalies and fracture zones only form in new seafloor, not in regions of continental extension. We used previously collected active source seismic data and magnetic anomaly data to constrain the limit of the new seafloor and the characteristics of the crust found at the transition between the former spreading ridge and the zone of continental extension. The seismic wave speeds we detected show that the oceanic crust in the southern Adare basin is quite thin, similar to other regions in the oceans where low-angle normal faults bring the crust-mantle boundary close (within 4 km) of the seafloor. Because we did not detect a major structural change at the southern end of the Adare Basin, it is possible that this thinned oceanic crust continues southward into the Northern Basin. We used the plate rotations derived for East Antarctica-West Antarctica motion to estimate the amount and direction of opening of the continental rift system that is buried under the ice. We created a public access web site with the measurements and interpretations of the active source seismic data (sonobuoy data) that show the seismic velocity structure from place to place in the study area, so that these could be used by any other interested educators or researchers. This project supported the Ph.D. thesis of a female graduate student, Michelle Selvans, and produced 3 papers published so far in peer-reviewed journals as well as 2 more papers that are in revision for peer-reviewed journals. In addition, data from this project were used for teaching classes at Caltech and at Northern Virginia Community College.
