Collaborative Research: The Cretaceous Sierra Nevada Batholith in the Northwestern Basin and Range: Its Intrusive History and Erosional Exhumation

加利福尼亚州的内华达山脉被大量的花岗岩(岩基)覆盖，这些花岗岩代表了2.5亿到8000万年前的中生代在北美西部边缘之下俯冲形成的古老火山的根源。内华达山脉(山脉)在加利福尼亚州北部的苏桑维尔附近结束，但它的花岗岩继续延伸到东部的大盆地。在这里，它们还没有得到详细的研究。该项目的第一部分将详细说明这些岩石与内华达山脉北部之间的精确匹配，并将解决基岩的特定部分穿过大盆地北部的问题。这个项目还将解决从白垩纪(8000万年前)到现在加州和内华达州的一系列非常有争议的问题。有人提出，我们今天所知的内华达山脉自白垩纪以来就存在，塞拉利昂以东的地区构成了一个巨大的高原，就像安第斯山脉的高原一样。随着盆地和山脉断裂的开始，高原坍塌，导致了大盆地的形成。然而，来自内华达州西北部的数据表明，这部分岩基下面的地壳从未厚到足以形成这样一个高原，河流似乎将古代塞拉山脉侵蚀的碎片带到了北部的海洋中(该地区现在被广泛的年轻火山熔岩流覆盖)。这些不同的想法将用两种方法来检验：1)Ar/Ar，裂变径迹和(U-Th)/He热年代学，从盆地和山脉断块倾斜的火山-花岗岩不整合下面的深度收集的岩基花岗岩样品的(U-Th)/He年代学，将详细描述岩基的侵入后冷却和侵蚀历史；2)加利福尼亚和俄勒冈州晚白垩世和早第三纪海相和非海相沉积岩中的锆石年龄组将有助于确定这些大量的石器时代物质被侵蚀和运往哪里。该项目填补了我们对北美科迪勒拉大片白垩纪到新生代地球动力学演化的认识的一个重大空白。内华达山脉是一个被广泛研究的大陆弧岩浆活动的例子，但岩基的北部，以及最近发现的延伸到内华达州西北部的部分，几乎没有受到关注。该地区的构造和岩浆演化是北美西部地质演化中一个悬而未决的关键问题。该项目将使研究生和本科生参与研究的实地和实验室阶段。一个项目网站、在当地学校和社区中心的演讲以及报纸文章将继续让当地社区参与研究的进展和成果。

The Sierra Nevada, California, is underlain by a large volume of granite (a batholith) that represents the roots of ancient volcanoes formed by subduction beneath the western margin of North America during the Mesozoic, 250 to 80 million years ago. The Sierra Nevada (as a mountain range) ends near Susanville, northern California, but its granites continue into the Great Basin to the east. Here they have not been studied in detail. The first part of this project will detail the exact match between these rocks and the northern Sierra Nevada and will address where specific parts of the batholith cross the northern Great Basin. This project will also address a very controversial set of questions about the landscape of California and Nevada from the Cretaceous (80 million years ago) to present. It has been proposed that the Sierra Nevada as we know it today existed since the Cretaceous and that the region to the east of the Sierra constituted a vast high plateau like the Altiplano of the Andes. The plateau collapsed with the onset of Basin and Range faulting that led to formation of the Great Basin. Data from northwestern Nevada, however, suggests that the crust beneath this part of the batholith was never thick enough to produce such a plateau and that rivers appear to have carried debris eroded from the ancient Sierra into the ocean to the north (a region now covered by extensive young volcanic lava flows). These different ideas will be tested using two methods: 1) Ar/Ar, fission track, and (U-Th)/He thermochronology for arrays of samples from the granites of the batholith, collected with depth beneath the tilted volcanic-granite unconformity in Basin and Range fault blocks, will detail the post-intrusive cooling and the erosional history of the batholith; and 2) age populations of zircons in Late Cretaceous and early Tertiary marine and non-marine sedimentary rocks in California and Oregon will allow determination of where this large volume of batholithic material was eroded and transported to.This project fills a major gap in our knowledge of Cretaceous to Cenozoic geodynamic evolution large area of North American Cordillera. The Sierra Nevada is a widely studied example of continental arc magmatism yet the northern portion of the batholith, and its recently recognized continuation into northwestern Nevada, has received little attention. The tectonic and magmatic evolution of this area is a key unresolved problem in western North American geologic evolution. The project will engage graduate and undergraduate students in the field and laboratory phases of the research. A project web site, presentations at local schools and community centers, and newspaper articles will continue to engage local communities in the progress and outcomes of the research.