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

项目概述：该项目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，作为晚白垩纪-早古近纪低角度滑脱断裂对塞拉岩浆岩的快速构造剥蚀的测试。这一阶段的剥蚀使内华达山脉最南端的一个斜地壳剖面暴露出来，其古深度约为35公里。他在北部的大内华达山脉地区的研究表明，在同一时间间隔内，一个长波长高起伏的河流主导地形的高立造山高原持续存在。与此形成鲜明对比的是，在始新世早期，与构造剥蚀作用相结合，南部最深处的基底暴露已经下沉到海洋条件。Sierran岩浆弧古地貌的这些沿走向的深刻变化，可以与Sierran岩浆弧最南端-莫哈韦沙漠北部地区的亚基石器时代地幔岩石圈被俯冲海洋板块的浅平板段构造移走以及相关的Franciscan沟缘变质组合的构造底沉积联系起来。PI已经在塞拉利昂最南端和莫哈韦沙漠北部地区确定了一系列已知的和明显的高级底石器分离板，它们似乎代表了被分离系统取代的上部地壳岩石的一部分。他们还在南sierra地区的基底岩中发现了现代地形异常和异常的构造-结构叠层，这些叠层与基底位压的陡峭梯度的开始和其隐含的剥蚀深度相对应。他们解释说，这些地区代表了靠近分裂带的下盘复合体的残余，在那里，最北端的分离片从造山带边缘分离出来。垂直磷灰石和锆石热年代学剖面被提出，作为快速冷却和构造剥蚀的检验。这些剖面将沿着等高剖面向南延伸至最深处暴露的基层，以便进一步测试区域快速冷却，并测试随着时间的推移逐渐向南迁移的更深层次剥蚀。更广泛的目标是将南塞拉地区的古地貌演化与岩石圈尺度的构造-砂动力学联系起来，并将这些发现与深部结晶基底向上迁移和地表暴露的全球重要问题联系起来，这些问题与隆起、侵蚀、构造剥蚀以及下地壳和上地幔的演化有关。拟议的研究计划为期三年，包括样品收集、矿物分离、地球化学分析和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，并构成加州理工学院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.