Timing, Rates, Episodicity, and Sediment Provenance of Subduction Accretion: Establishing a Geochronologic Framework for Long-term Accretion in the Franciscan Subduction Complex

加利福尼亚的方济各俯冲复合体是世界上研究得最好的古代增生棱镜。来自Franciscan的广泛而多样的数据集是非常广泛的俯冲相关过程模型的关键输入，补充了现代俯冲带（几乎完全是海洋）的地球物理和地质研究以及其他古代陆地俯冲复合体的研究。然而，关键的是，方济各和大多数其他俯冲复合体已被证明是非常困难的，这严重阻碍了对其发展的许多方面的解释。近十年来，一些强大的地质年代学技术取得了重大进展，这些技术非常适用于俯冲复杂岩石，包括砂岩碎屑锆石颗粒的U-Pb测年、碎屑锆石的U-Th-He测年、云母的40Ar/39Ar测年以及石榴石和lawsonite的Lu-Hf测年。该项目包括一项综合活动，将这些方法应用于方济各杂岩全宽度范围内已经很好表征的目标单元，以及从侏罗纪到中新世逐渐增加的单元。这个新的地质年代学框架将有效地补充现有的大量方济各会工作，这些工作因年龄控制不佳而受到损害。新的锆石资料与Great Valley前弧盆地、Klamath省、Cordilleran前陆盆地等其它大型碎屑锆石资料具有很强的互补性。综合起来，这些数据集将解决以下问题：(1)在这个经典的俯冲系统中，增生的年表是什么？吸积速率是多少？它们是如何随时间变化的？(2)在1.65亿年的地质时间里，大方济各-大峡谷-内华达山脉俯冲体系的构造和古地理是如何演化的？（30）吸积的速率和事件与内华达山脉岩浆弧、大峡谷前弧盆地、科迪勒兰前陆以及太平洋盆地古板块运动的关系如何？(4)方济会变质沉积岩的沉积来源是什么？它们是如何随时间演变的？潜没是地球上的一个主要构造板块在相邻板块下移动并最终下降到地幔的地质过程，经过数千万年的时间。它是创造和改变地球表面的最重要的全球过程之一。它产生了火山链、沉积盆地、矿藏，并改变了地壳和地幔的结构和组成。俯冲产生的火山、地震和海啸是地球上最严重的自然灾害。对俯冲过程的研究使用两种广泛互补的方法。现代的活动俯冲带是地球物理、钻探、地质、地球化学和建模研究的目标，但几乎全部位于海洋表面以下，这严重限制了直接观测。古代灭绝的俯冲带是广泛的地表测绘、采样和地球物理研究的目标。一些古老的俯冲带暴露出一个随时间变化的俯冲的岩石记录，这种俯冲持续了很长时间，有时超过1亿年。这个项目的目标是提供一个大大改进的框架，在加利福尼亚得到充分研究的古方济会俯冲带内各种岩石亚基的年龄。这种改进的地质年代学框架将允许对来自该俯冲带的许多其他现有数据集进行更可靠的解释。

The Franciscan subduction complex of California is the world's best studied ancient accretionary prism. The extensive, diverse data sets from the Franciscan are critical inputs for models for a very broad range of subduction-related processes, complimenting geophysical and geologic studies of modern subduction zones (which are almost entirely marine) and studies of other ancient on-land subduction complexes. Critically, however, the Franciscan and most other subduction complexes have proven very difficult to date and this severely impedes interpretation of many aspects of their development. The last ten years have seen major advances in several powerful geochronologic techniques that are highly applicable to subduction complex rocks, including U-Pb dating of detrital zircon grains from sandstones, U-Th-He dating of detrital zircons, 40Ar/39Ar dating of micas, and Lu-Hf dating of garnet and lawsonite. This project involves an integrated campaign to apply these methods to already well characterized target units across the full width of the Franciscan complex, to units accreted progressively from Jurassic through Miocene time. This new geochronologic framework will effectively complement a very large body of existing Franciscan work that is compromised by poor age control. The new zircon data will also be highly complementary with other large detrital zircon data set being generated from the Great Valley forearc basin, Klamath Province, Cordilleran foreland basin, etc. Synthesized, these data sets will address such issues as: (1) What was the chronology of accretion in this classic subduction system? What were rates of accretion and how did they vary over time? (2) How did the tectonics and paleogeography of the greater Franciscan-Great Valley-Sierra Nevada subduction system evolve over 165 million years of geologic time? (30 How do rates and episodes of accretion correlate with events in the Sierra Nevada magmatic arc, Great Valley forearc basin, Cordilleran foreland, as well as motions of Pacific basin paleoplates? (4) What were the sediment sources for Franciscan metasedimentary rocks and how did they evolve over time?Subduction is the geologic process by which one of the Earth's major tectonic plates moves beneath an adjacent plate and eventually descends into the Earth's mantle, over time periods of tens of millions of years. It is one of the most important global processes creating and modifying the Earth's surface. It generates chains of volcanoes, sedimentary basins, mineral deposits, and modifies the structure and composition of the crust and mantle. Subduction-generated volcanoes, earthquakes, and tsunamis are among the most severe natural hazards on Earth. Investigations of subduction processes use two broadly complementary approaches. Modern, active subduction zones are targets for geophysical, drilling, geologic, geochemical, and modeling studies, but lay almost entirely beneath the ocean surface, which severely limits direct observation. Ancient, extinct subduction zones are targets for extensive surface mapping, sampling, and geophysical study. Some ancient subduction zones expose a rock record of time-varying subduction over protracted periods sometimes exceeding 100 million years. The goal of this project is to provide a greatly improved framework of the ages of various rock subunits within the well-studied ancient Franciscan subduction zone in California. This improved geochronologic framework will allow much more robust interpretations of many other existing data sets from this subduction zone.