Tectonic Evolution of the Early Mesozoic Arc in California and Arizona

PI将在亚利桑那州和加利福尼亚州测试一个关于晚三叠世到中侏罗世早期弧形构造背景的推测模型。这个大陆弧占据了一个深的断层式凹陷，类似于今天的中美洲弧形。虽然现代的模拟提供了弧地陷中的表面过程的“快照”，但亚利桑那州和加利福尼亚州的早期中生代弧提供了一个伸展弧的三维图像，因为它在5000万年的演化过程中。这项工作要么记录下推测的模型，要么修改它，但要记录的关系包括：(1)确定沿弧线的下沉模式和速率，使用U-Pb锆石年代学来确定大陆地层的年代学；(2)相对于中层深成岩体，表征高水平流纹岩侵入体和相关火山口的丰度；(3)利用绿片岩级变质过程中相对不活动的常量元素和微量元素记录弧内碱性火山作用的模式；(4)通过沉积和火山岩的相分析以及断层与侵入体之间的横切关系的锆石测年来识别同沉积正断层。这一推测模型部分基于我们新的锆石U-Pb年龄数据，其含义包括克拉通来源的石英砂可能在整个下侏罗世和中侏罗世期间沿着弧地凹陷的长度被漏斗状分布，并分布在弧前地区的宽度上。因此，这些石英砂将被用作“示踪剂”，以识别在美国西南部科迪勒拉山脉的侏罗纪弧区或弧前地区从原产地移位的外来地体，特别是如果可以通过碎屑锆石的U-Pb测年对这些砂岩进行指纹识别的话。由厚重的陆弧序列组成的连续带可能主要起源于伸展环境。PI将结合变质火山岩和变质沉积岩的相分析和岩石学研究，并结合锆石U-Pb定年，记录长寿命弧内的断裂、沉陷和火山活动的模式。这些结果可以用来解释所有时代和地区更多的陆弧破裂序列。

The PI's will test a speculative model for the tectonic setting of the late Triassic to early middle Jurassic arc in Arizona and California. This continental arc has occupied a deep, fault- bounded depression, similar to the present-day arc of Central America. While the modern analogue provides a "snapshot" of surficial processes in an arc graben-depression, the early Mesozoic arc of Arizona and California provides a three- dimensional view of what was an extensional arc, as it evolved over 50 million years. The work will either document the speculative model or modify it, but the relationships to be documented include: (1) determination of patterns and rates of subsidence along the arc, using U-Pb zircon geochronology to date sections of continental strata; (2) characterization of the abundance of high-level rhyolitic intrusions and associated calderas, relative to mesozonal plutons; (3) documentation of patterns of alkalic volcanism within the arc, using major and trace elements that are relatively immobile during greenschist grade metamorphism, and (4) recognition of syndepositional normal faults through facies analysis of sedimentary and volcanic rocks, as well as by zircon dating of cross-cutting relations between faults and intrusions. Implications of this speculative model, which is based in part on our new U-Pb zircon age data, include the possibility that craton-derived quartz sands were funneled along the length of the arc graben-depression and distributed across its width, and beyond to forearc regions, throughout lower and middle Jurassic time. These quartz sands would thus be used as "tracers" to recognize exotic terranes that have been displaced from their site of origin in the Jurassic arc or forearc region of the southwest Cordilleran U.S., particularly if the sands can be fingerprinted through U-Pb dating of detrital zircon. Continuous belts composed of thick continental arc successions may largely originate in extensional settings. The PI's will integrate facies analysis and petrologic study of metavolcanic and metasedimentary rocks with U-Pb zircon dating to document patterns of faulting, subsidence and volcanism in a long- lived arc. These results can then be applied to interpretation of more disrupted continental arc successions of all ages and localities.