Rotation of the Philippine Sea Plate (IODP Expedition 351)

构造（或岩石圈）板块是由世界各大洋中洋脊的海底扩张形成的，并在俯冲带循环回到地球内部。板块的形成和破坏产生了地球上最重要的表面特征，是地球物理和化学进化的主要驱动力。虽然在类地行星中，俯冲带是地球所特有的，但到目前为止，我们对它们是如何形成的还没有很好的了解。我们知道，俯冲与致密、古老的海洋岩石圈相对于底层地幔的引力不稳定有关。然而，目前尚不清楚俯冲是由作用在洋中脊等现有结构上的水平力引发的，还是由垂直重力自发引发的。推进我们对这一基本过程的理解的关键是检查沉积岩序列随时间的变化，这些变化是由沉积在经历俯冲起始的板块上积累的，以便检查板块条件的变化。这样的序列将由国际海洋探索计划第351远征队前往菲律宾海板块进行钻探和取样。在5200万年前开始俯冲之后，这个板块现在位于伊豆-博宁-马里亚纳（IBM）俯冲带系统之上。将对1300 m厚的层序进行采样，其中包括在IBM系统启动之前和之后形成的岩石。对这些岩石在沉积过程中获得的磁化强度进行分析，将使取样剖面的年代测定更加准确，因为在过去的地质时期，地球磁场几何形状变化的时间（极性反转）是众所周知的。这将为与菲律宾海板块构造历史相关的地质事件的时间提供必要的控制。然而，要充分了解该系统的演化，需要了解菲律宾海板块过去运动的边界条件。这一信息可以再次通过覆盖在板块火成岩基底上的岩石序列的磁化来获得。就像地球磁场的倾角是纬度的一个简单函数一样，岩石样本的磁化强度与水平面（其倾斜度）的夹角提供了它形成时的纬度的直接度量。先前对菲律宾海板块岛屿上暴露的岩石的研究表明，它在演变过程中向北漂移了25度。然而，这些解释中使用的古地磁数据来自构造复杂的板块边缘，并且在时间上分布广泛。相比之下，这个项目将产生近连续的随时间的倾斜记录，将以高分辨率解决板块运动。除了向北移动外，现有的古地磁数据还表明，自板块形成以来，它的垂直轴向旋转了90度。要验证这一点，需要了解磁化方位角（或磁偏角）随时间的变化。不幸的是，科学的海洋钻探样本并没有指向地理上的北方，因此不能直接从351考察期间回收的岩芯材料中获得赤纬。然而，这些信息可以通过将无定向岩心中的地质特征与井壁内部定向图像中的相同特征进行匹配来检索。另外，磁化方向的记录也可以通过模拟古代井下岩石磁化变化对当前地磁场的扰动来间接获得。这两种方法都将用于该项目，以获得菲律宾海板块构造旋转的高分辨率记录，提供了解俯冲起始过程所需的区域构造演化的关键信息。

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.