An Apatite/Zircon (U-th)/He Thermochronometric Study of the Rapid Tectonic Denudation of an Oblique Crustal Section-Southern Sierra Nevada, California

项目总结派建议在内华达山脉最南端地区制作一系列(U-the)/He磷灰石和锆石热年代学剖面，作为对白垩纪末期-古近纪早期西尔兰石器时代岩石被低角度拆离断层非常快速的构造剥蚀的测试。这一阶段的剥蚀暴露出一个穿过内华达山脉最南端的倾斜地壳部分，其古深度约为35公里。他在北部大内华达山脉地区的磷灰石研究揭示了在同一时间间隔内持续存在的具有长波长-高起伏河流主导的地形的高站立造山高原。与之形成鲜明对比的是，在始新世早期，随着构造剥蚀，岩基向南的最深水平暴露已经下沉到海洋条件。Sierran岩浆弧古地貌沿走向的这种深刻变化可以与最南端的塞拉利昂-北部莫哈韦沙漠地区下的石器时代以下的地幔岩石圈被俯冲板块的浅平板段构造剥离，以及与之相关的弗朗西斯海沟-亲缘变质组合的构造底侵作用有关。PI已经确定了沿最南端的塞拉利昂和最北端的MojaveDesert地区的一系列已知和明显的高级别旧石器时代滑脱岩层，它们似乎代表了被滑脱系统置换的部分地壳上部岩石。他们还在锡拉尔南部地区的基岩中发现了现代地形异常和异常的构造-结构叠加，这与岩基侵位气压测量中陡峭梯度的开始及其隐含的剥蚀深度相对应。他们解释说，这些区域代表了分离带附近的下盘杂岩的残留物，在那里，最北端的滑脱岩片从造山高原的边缘崩解出来。垂直磷灰石和锆石的热年代学剖面被提出，作为非常快速的冷却和构造剥蚀的测试。这些剖面将沿着恒定的海拔剖面向南延伸至岩基最深的裸露水平，以便进一步测试区域快速冷却，并测试可能的向南迁移，随着时间的推移，剥蚀程度逐渐加深。更广泛的目标是将塞拉利昂南部地区的古地貌演化与岩石圈尺度的构造和动力学联系起来，并将这些发现与全球重要的问题联系起来，即深部结晶基岩的向上迁移和地表暴露，以及相关的抬升、侵蚀、构造剥蚀和下地壳和上地幔的演化。拟议的研究计划为期三年，包括样本收集、矿物分离、地球化学分析以及新数据与现有地质和地球化学数据集的综合。该项目将需要联合PI及其技术人员的部分努力，并构成加州理工大学博士论文的主要组成部分。

PROJECT SUMMARYThe PI's propose to produce a series of (U-The)/He apatite and zircon thermochronologic profiles in the southernmost Sierra Nevada region as a test of the very rapid tectonic denudation of the Sierran batholithic rocks by low-angle detachment faulting in latest Cretaceous-early Paleogene time. This phase of denudation exposed an oblique crustal section through the southernmost Sierra Nevada batholith to a paleodepth of ~35 km. Apatite He studies in the greater Sierra Nevada region to the north have revealed the persistence of a high standing orogenic plateau with long wavelength-high relief fluvial dominated topography during the same time interval. In marked contrast the deepest level exposures of the batholith to the south had subsided to marine conditions by early Eocene time in conjunction with tectonic denudation. These profound along strike variations in the paleogeomorphology of the Sierran magmatic arc can be linked to the tectonic removal of the sub-batholithic mantle lithosphere beneath the southernmost Sierra-northern Mojave Desert region by a shallow flat-slab segment of the subductingoceanic plate, and the related tectonic underplating of Franciscan trench-affinitymetamorphic assemblages. The PI's have identified an array of known and apparent high-level batholithic detachment sheets along the southernmost Sierra and northern MojaveDesert region that appear to represent part of the upper crustal rocks that were displacedby the detachment system. They have also identified modern topographic anomalies andanomalous structural-textural overprints in the basement rocks of the southern Sierraregion which correspond to the onset of steep gradients in batholith emplacementbarometry and its implicit depth of denudation. They interpret these areas to represent theremnants of footwall complexes proximal to the breakaway zones where the northernmost detachment sheets calved off the edge of the orogenic plateau. Verticalapatite and zircon thermochronologic profiles are proposed across these zones as a test ofvery rapid cooling and tectonic denudation. These profiles will be extended southwardalong constant elevation profiles to the deepest exposed levels of the batholith in order tofurther test for regional rapid cooling, and to test for a possible southward migration inprogressively deeper levels of denudation with time. The broader goals are to relate thepaleogeomorphic evolution of the southern Sierra region to lithosphere-scale tectonicsand dynamics, and to relate these findings to the globally important problem of theupward migration and surface exposure of deep crystalline basement rock as related touplift, erosion, tectonic denudation, and the evolution of lower crust and upper mantle.The proposed research program runs for a period of three years and includes a vigorousprogram of sample collecting, mineral separation, geochemical analysis, and thesynthesis of the new data with existing geological and geochemical data sets. The projectwill entail partial efforts of the joint PI's and their technical staff, and constitute a majorcomponent of a Caltech Ph.D. Dissertation.