| Abstract |
Tectonic (or lithospheric) plates are formed by seafloor spreading at mid-ocean ridges in the world's oceans and are recycled back into the Earth's interior at subduction zones. The formation and destruction of plates produces the most important surface features on Earth, and are the major drivers of the physical and chemical evolution of our planet. Although subduction zones are unique to Earth among the terrestrial planets, as yet we do not have a good understanding of how they form. We know that subduction relates to gravitational instability of dense, old ocean lithosphere with respect to the underlying mantle. However, it remains unclear whether subduction is triggered by horizontal forces acting on existing structures like mid-ocean ridges or initiates spontaneously from vertical gravitational forces. The key to advancing our understanding of this fundamental process is to examine changes in sedimentary rock sequences through time that have accumulated by deposition on a plate that has experienced subduction initiation, in order to examine changes in plate conditions. Such a sequence will be drilled and sampled by International Ocean Discovery Program Expedition 351 to the Philippine Sea Plate. Following subduction initiation 52 million years ago, this plate now sits above the major Izu-Bonin-Mariana (IBM) subduction zone system. A 1300 m thick sequence will be sampled that includes rocks formed prior to and after initiation of the IBM system. Analysis of magnetizations acquired by these rocks during their deposition will allow accurate dating of the sampled section, as the timing of changes in the geometry of the Earth's magnetic field (polarity reversals) during the geological past is well known. This will provide essential control on the timing of geological events associated with the tectonic history of the Philippines Sea Plate. Full understanding of the evolution of this system, however, requires the boundary conditions of the past motions of the Philippine Sea Plate to be known. This information can again be obtained using the magnetization of the sequence of rocks that overlie the igneous basement of the plate. The angle the magnetization of a rock sample makes with the horizontal (its inclination) provides a direct measure of the latitude at which it formed, as the inclination of the Earth's magnetic field is a simple function of latitude. Previous studies of rocks exposed on islands on the Philippine Sea Plate have suggested that it has drifted northwards by 25 degrees during its evolution. The palaeomagnetic data used in these interpretations, however, come from the tectonically complex margins of the plate and are widely distributed in time. In contrast, this project will yield near-continuous records of inclination through time that will resolve plate motion at a high resolution. In addition to northwards translation, existing palaeomagnetic data also suggest a 90 degree vertical axis rotation of the plate since it formed. To test this requires knowledge of changes in the azimuth (or declination) of magnetizations through time. Unfortunately, scientific ocean drilling samples are not oriented with respect to geographic north, so declinations cannot be acquired directly from the core materials recovered during Expedition 351. However, this information can be retrieved by matching geological features seen in unoriented cores with the same features seen on oriented images of the inside of the borehole wall. Alternatively, records of magnetization direction can be obtained indirectly by modeling of the disturbances to the present day geomagnetic field that result from variations in the ancient magnetizations of rocks downhole. Both of these approaches will be used in this project to obtain high-resolution records of the tectonic rotation of the Philippine Sea Plate, providing key information on the regional tectonic evolution that are required to understand subduction initiation processes. |
