Tectonic Evolution of the Early Mesozoic Arc in California and Arizona

PI将测试亚利桑那州和加利福尼亚州晚三叠世至中侏罗世早期弧的构造环境的推测模型。这个大陆弧占据了一个深的、有断层的凹陷，类似于今天的中美洲弧。现代的模拟提供了弧地堑洼地表面过程的“快照”，而亚利桑那州和加利福尼亚州中生代早期的弧则提供了一个伸展弧的三维视图，因为它演化了5000万年。这项工作要么记录推测模型，要么对其进行修正，但需要记录的关系包括：(1)确定沿弧沉降的模式和速率，利用U-Pb锆石年代学对大陆地层剖面进行测年；(2)相对于中生界深部岩体的高阶流纹岩侵入体及其伴生破火山口丰度特征；(3)利用绿片岩级变质过程中相对不动的主微量元素记录弧内碱性火山活动模式；(4)通过沉积岩和火山岩相分析识别同沉积正断层；以及断层与侵入体之间的交叉关系的锆石定年。这一推测模型部分基于我们新的U-Pb锆石年龄数据，其中包括克拉通衍生的石英砂在整个下侏罗世和中侏罗世可能沿着弧地堑凹陷的长度漏斗状分布，并在其宽度上分布，并向弧前地区扩散。因此，这些石英砂将被用作“示踪剂”，以识别从美国科迪勒拉山脉西南部侏罗纪弧或弧前地区的起源地点迁移的外来地体，特别是如果这些砂可以通过碎屑锆石的U-Pb定年来识别指纹。由厚大陆弧序列组成的连续带可能主要起源于伸展环境。PI将把变质火山和变质沉积岩的相分析和岩石学研究与U-Pb锆石结合起来，以记录长寿命弧中的断裂、沉降和火山作用模式。这些结果可以应用于解释所有年龄和地点的更多断裂的大陆弧序列。

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.