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

加利福尼亚的内华达山脉被大量的花岗岩（一种岩基）所覆盖，这些花岗岩代表了2.5亿至8000万年前中生代北美洲西部边缘俯冲形成的古代火山的根部。内华达山脉（Sierra Nevada）在加州北部的苏珊维尔（Susanville）附近结束，但它的花岗岩一直延伸到东部的大盆地（Great Basin）。这里没有对它们进行详细的研究。该项目的第一部分将详细描述这些岩石与内华达山脉北部之间的精确匹配，并将确定基岩的特定部分穿过大盆地北部的位置。这个项目还将解决一系列非常有争议的问题，这些问题是关于加利福尼亚和内华达州从白垩纪（8000万年前）到现在的景观。有人提出，我们今天所知道的内华达山脉自白垩纪以来就存在了，而内华达山脉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发生，高原塌陷，形成了大盆地。然而，来自内华达州西北部的数据表明，这部分岩基下的地壳从未厚到足以形成这样一个高原，河流似乎将古塞拉山脉侵蚀的碎片带入了北部的海洋（现在该地区被广泛的年轻火山熔岩流覆盖）。这些不同的想法将通过两种方法进行验证：1)对盆地和山脉断块中倾斜的火山-花岗岩不整合下方的岩基花岗岩样品阵列进行Ar/Ar、裂变径迹和(U-Th)/He热年代学分析，将详细描述侵入后的冷却和岩基的侵蚀历史；2)加利福尼亚和俄勒冈州晚白垩纪和早第三纪海相和非海相沉积岩中的锆石年龄群将有助于确定大量的底石器物质被侵蚀和运输到哪里。该项目填补了我们对北美科迪勒拉地区白垩纪至新生代地球动力学演化认识的重大空白。内华达山脉是一个被广泛研究的大陆弧岩浆活动的例子，然而基基的北部，以及它最近被承认的向内华达西北部延伸的情况，却很少受到关注。该地区的构造和岩浆演化是北美西部地质演化中尚未解决的关键问题。该项目将吸引研究生和本科生参与实地和实验室阶段的研究。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使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.