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

加州的方济各俯冲复合体是世界上研究得最好的古代增生棱柱体。方济各会提供的广泛而多样的数据集是一系列与俯冲相关过程的模型的关键输入，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上俯冲复合体的研究。然而，至关重要的是，方济各会和大多数其他俯冲复合体已被证明非常难以确定日期，这严重阻碍了对其发展的许多方面的解释。近10年来，在几种非常适用于俯冲杂岩的年代学方法上取得了重大进展，包括砂岩碎屑锆石U-Pb定年、碎屑锆石U-Th-He定年、云母40 Ar/39 Ar定年以及石榴石和硬铝石Lu-Hf定年。该项目涉及一个综合的运动，将这些方法应用于已经很好地表征了整个方济各复杂的宽度的目标单位，从侏罗纪到中新世时间逐渐增加的单位。这个新的地质年代学框架将有效地补充现有的方济各会工作的一个非常大的机构，是由穷人的年龄控制妥协。新的锆石数据也将是高度互补的其他大型碎屑锆石数据集产生的大峡谷弧前盆地，克拉马斯省，科迪勒什前陆盆地等综合，这些数据集将解决这样的问题：（1）什么是在这个经典的俯冲系统的增生年表？什么是吸积率？它们如何随时间变化？(2)在1.65亿年的地质时间里，大方济各-大峡谷-塞拉内华达州俯冲系统的构造和古地理是如何演变的？(30加积的速率和阶段如何与内华达州岩浆弧、大峡谷弧前盆地、科迪勒拉前陆以及太平洋盆地古板块运动的事件相关？(4)方济各会变质沉积岩的沉积物来源是什么？它们是如何随时间演变的？俯冲是一种地质过程，其中地球的一个主要构造板块移动到相邻板块之下，并最终下降到地球的地幔，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。它产生了一连串的火山、沉积盆地、矿藏，并改变了地壳和地幔的结构和组成。俯冲引起的火山、地震和海啸是地球上最严重的自然灾害之一。对俯冲过程的研究使用两种广泛互补的方法。现代活跃的俯冲带是地球物理、钻探、地质、地球化学和建模研究的目标，但几乎完全位于海洋表面之下，这严重限制了直接观测。古老的灭绝俯冲带是广泛的地表测绘、取样和地球物理研究的目标。一些古老的俯冲带暴露出岩石记录的时间变化的俯冲在漫长的时期，有时超过1亿年。该项目的目标是提供一个大大改进的框架内的各种岩石亚基的年龄研究的古方济各会在加州俯冲带。 这种改进的地质年代框架将允许更强大的解释许多其他现有的数据集从这个俯冲带。

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.