Constraining erosion during arc-arc collision from turbidite sands in the Nankai

沉积杂岩中的沉积岩可能记录了很长一段地质时期俯冲带的演化。增生活动大陆边缘既是陆壳返回地幔的主要下沉，也是陆壳形成的场所。因此，了解构成这些边缘的过程对于限制大陆地壳的建造和维护至关重要。洋弧向大陆边缘的吸积是大陆地壳形成的关键阶段，但这些事件的证据往往很模糊。在这个项目中，我们将确定什么过程控制沉积物流向海沟系统，从而确定哪些过程可能从较老的复合体中的类似沉积物中解卷出来。特别是，我们将研究弧弧碰撞对活动边缘系统的侵蚀所产生的影响。我们之所以选择研究日本西南部的南开海槽，是因为它是典型的增生大陆边缘，其沉积物不仅来自本州的中生代增生杂岩，而且来自伊豆-博宁弧与日本大陆碰撞形成的山脉。进一步验证了这种碰撞或中更新世-0.9 Ma过渡时期的气候变化是否是日本侵蚀的主要控制因素。我们将把矿物学和热年代学方法应用于IODP探险316号回收的沉积物，尼科尔森曾作为船上科学家在这艘船上航行。样品年代为上新世-新近世(2.0-0.5 Ma)，为南开边缘的增生砂体。我们还建议对直接从钻探地点陆上排出伊豆碰撞带和岛曼托增生杂岩的现代河流沉积物进行表征。利用岛曼托杂岩与伊豆碰撞带对比的构造/岩浆史，我们将限制它们对沉积物产生和流向海沟的相对影响。目前的模型预测，伊豆-博宁弧在1 Ma左右与日本相撞。我们将通过分析横跨最后2.0-0.5 Ma的15个砂体来检验这一点，以确定该事件的物质诊断何时首次到达南开海槽。我们将使用已证实的重矿物分析，这些分析可以识别已知对碰撞带有诊断作用的特定颗粒。辉石和角闪石颗粒的单颗粒主量元素分析将限制物源随时间的变化，使我们能够确定侵蚀模式在100Ky内的变化。此外，由于伊祖-博宁碰撞带提供了年龄小于40 Ma的锆石，与来自石门托带的中生代年龄形成对比，因此将利用锆石颗粒的LA-ICPMS U-Pb定年进一步约束源区的变化。来自磷灰石和锆石裂变径迹分析(AFT和ZFT)的热年代学数据，结合生物地层学数据，将提供对沉积物源区折返时间和速度的限制，从而能够重建弧体与日本大陆碰撞期间的地貌演变。通过这样做，我们将更好地了解弧吸积如何进行，从而更好地了解亚洲大陆地壳是如何在很长一段地质时期内建造的。

Sedimentary rocks in accretionary complexes potentially record the evolution of subduction zones over long periods of geological time. Accretionary active continental margins are both major sinks of continental crust back into the mantle and locations for continental crustal formation. As a result understanding the processes that construct these margins is essential to constraining how the continental crust is built and maintained. Accretion of oceanic arcs to continental margins is a key stage in the building of continental crust, yet evidence for these events is often obscure. In this project we will determine what processes control the flux of sediment to a trench system and thus what might be deconvolved from similar deposits in older complexes. In particular, we will examine the impact that arc-arc collision has had on the erosion of an active margin system. We choose to study the Nankai Trough of SW Japan because it is the type-section accretionary continental margin and derives sediment, not only from Mesozoic accretionary complexes in Honshu, but also from the mountains formed by the collision of the Izu-Bonin Arc with the Japanese mainland. We further test whether this collision or the climate changes during the Mid-Pleistocene transition ~0.9 Ma are the primary controls on erosion of Japan. We will apply mineralogy and thermochronology methods to sediments recovered by IODP Expedition 316, on which Nicholson sailed as a shipboard scientist. The samples are dated as Pliocene-Recent (2.0-0.5 Ma) and are accretionary sands of the Nankai margin. We also propose to characterize modern river sediments draining both the Izu collision zone and the Shimanto Accretionary Complex, immediately onshore from the drill sites. Using the contrasting tectonic/magmatic histories of the Shimanto Complex compared to the Izu collision zone we will constrain their relative influences on sediment production and flux to the trench. Present models predict that the Izu-Bonin Arc collided with Japan around 1 Ma. We shall test this by analyzing 15 sands spanning the last 2.0-0.5 Ma in order to determine when material diagnostic of that event first reaches the Nankai Trough. We will use proven heavy mineral analyses that can identify specific grains known to be diagnostic of the collision zone. Single grain major element analysis of pyroxene and amphibole grains will constrain changes in provenance through time, allowing us to date changes in erosion patterns within 100 ky. In addition, LA-ICP-MS U-Pb dating of zircon grains will be used to further constrain changes in source area, because the Izu-Bonin collision zone supplies zircons younger than 40 Ma, contrasting with Mesozoic ages in those derived from the Shimanto Belt. Thermochronological data derived from apatite and zircon fission track analysis (AFT and ZFT), combined with biostratigraphic data, will provide constraints on the timing and rate of exhumation of the sediment source areas, allowing reconstructions of the evolution of the landscape during the collision of the arc with the Japanese mainland. In doing so we will better understand how the arc accretion progresses and thus how the continental crust of Asia has been constructed over long periods of geological time.