Pliocene Kinematic History of the Northern Gulf of California

我们正在研究一个非常大的地壳块体（Isla Angel de la Guarda地壳块体）是如何在过去的300万至600万年里与加州湾北方的相邻陆地分离并成为一个岛屿的。 天使岛地壳块最初是下加利福尼亚半岛的一部分。 大约600万年前，当沿着太平洋-北美板块边界的北方加州湾开始开放时，它从蒂伯龙岛（索诺拉）分裂出来。 大约2-3百万年前，板块边界的位置发生了变化，IAG和半岛的其他部分之间形成了断层，将IAG作为一个岛屿转移回北美板块。 从那时起，天使岛地壳块应该已经相对于下加利福尼亚半岛移动了大约100公里，这是由于Ballenas海峡（天使岛地壳块以西）断层的滑动，该海峡是太平洋-北美板块边界的一部分，向北连接到圣安德烈亚斯断层。 天使岛拉瓜尔达地壳块体和半岛之间的总位移对于限制太平洋-北美板块边界的演化、圣安德烈亚斯断层系统的历史以及对于了解北方加州湾、下加利福尼亚和岛屿之间、下德尔芬盆地中有多少下沉的大陆地壳存在于水下是重要的。 解决这一问题的关键是对过去不同时期喷发的火山岩进行对比，以及对更古老的基底岩石进行对比。 当天使岛地壳块体与下加利福尼亚半岛相连时，基底岩石是连续的。 该地区的火山喷发在岛上和邻近的半岛上沉积了熔岩和凝灰岩（火山碎屑流沉积物）。 对下加利福尼亚半岛上的火山岩研究相对较好，因此其地层和分布是已知的。 然而，天使岛是无人居住的，其火山岩尚未被详细研究。 我们正在研究这些岩石首先使用卫星图像和航空照片，以确定我们需要访问的露头。 随后在岛上的实地工作将使我们能够在最重要的地点对岩石进行取样，并绘制出它们的分布图。 我们正在使用火山地层学、古地磁学、地球化学和岩石样品的地质年代学（年龄测定）技术，以确定它们的特征，并确定哪些与周围地区暴露的岩石相匹配。 不同火山单元的厚度和数量因地而异。 我们将能够利用这些变化来确定该岛脱离下加利福尼亚时可能的原始位置，并知道它在多久以前成为一个与半岛其他部分分离的孤岛。 我们还将比较天使岛的拉瓜尔达地壳块体岩石Tiburon岛上的岩石，看看我们是否可以确定这两个是如何最初的相关性，在裂谷开始之前，这项研究补充了现有的墨西哥政府研究项目，该项目正在评估整个加州湾火山岩的相关性。 墨西哥项目将提供地球化学分析和氩年代测定，从而减少该项目的费用。 来自加州理工学院和墨西哥的科学家和学生正在岛上进行实地工作。 加州理工学院的实验室正在进行古地磁和其他实验室研究。 年龄和地球化学分析的结果正在北美火山岩社区数据库中存档。 研究结果正在科学出版物中传播，并在美国和墨西哥的专业会议上介绍。 其结果还被用于以英语和西班牙语对学生或公众进行的外联讲座。 最后，我们的研究结果有助于生物学家研究岛上的动物和植物，因为它们限制了岛上的生物群落与墨西哥邻近地区的生物群落隔离的时间。这项研究得到了地球科学部构造计划和国际科学与工程办公室美洲计划的资助。

We are studying how a very large crustal block (the Isla Angel de la Guarda crustal block) separated from the adjacent land in the northern Gulf of California in the past 3 to 6 million years and became an island. The Isla Angel de la Guarda crustal block was originally part of the Baja California Peninsula. It split away from Tiburon Island (Sonora) when the northern Gulf of California started to open about 6 million years ago along the Pacific-North America plate boundary. About 2-3 million years ago, the position of the plate boundary changed, and a fault developed between IAG and the rest of the peninsula, transferring IAG back onto the North America plate as an island. Since then, the Isla Angel de la Guarda crustal block should have moved approximately 100 kilometers relative to the Baja California Peninsula due to slip on the faults in the Ballenas channel (west of the Isla Angel de la Guarda crustal block), which are part of the Pacific-North America plate boundary and connect northwards to the San Andreas fault. The total displacement between the Isla Angel de la Guarda crustal block and the peninsula is important for constraining the evolution of the Pacific-North America plate boundary, the history of the San Andreas fault system, and for understanding how much sunken continental crust is present under water in the northern Gulf of California, between Baja California and the island, in the Lower Delfin Basin. The key to solving this problem is the correlation of volcanic rocks that were erupted at various times in the past, as well as correlation of the older basement rocks. When the Isla Angel de la Guarda crustal block was attached to the Baja California peninsula, the basement rocks were continuous. Volcanic eruptions in the region deposited lava and tuffs (pyroclastic flow deposits) on both the island and on the adjacent peninsula. The volcanic rocks on the Baja California Peninsula are relatively well studied, so their stratigraphy and distribution is known. However, Isla Angel de la Guarda is uninhabited and its volcanic rocks have not been studied in detail. We are studying these rocks first using satellite imagery and air photos to identify the outcrops we need to visit. Subsequent field work on the island will allow us to sample rocks in the most important locations and to map their distribution. We are using the techniques of volcanic stratigraphy, paleomagnetics, geochemistry, and geochronology (age dating) of rock samples, in order to characterize them and determine which ones match the rocks exposed in the surrounding regions. The thickness and number of the different volcanic units vary from place to place. We will be able to use these variations to determine the likely original position of the island when it broke away from Baja California, and to know how far back in time it became an isolated island separated from the rest of the peninsula. We will also compare Isla Angel de la Guarda crustal block rocks to the rocks on Tiburon Island to see if we can determine how the two were related initially, before the rifting started.This study complements an existing Mexican government research project that is evaluating correlations of volcanic rocks across the Gulf of California. The Mexican project will provide geochemical analyses and argon dates, reducing the costs to this project. Field work on the island is being conducted by scientists and students from Caltech and Mexico. The paleomagnetic and other laboratory studies at are being done using laboratories at Caltech. The results of the ages and geochemical analyses are being archived in the community data base on volcanic rocks of North America (NAVDAT). The results are being disseminated in scientific publications and presented at professional meetings in the USA and Mexico. The results are also being used for outreach talks to students or the general public, in both English and Spanish. Finally, our results are helping biologists studying the animals and plants of the island, because they constrain how long the biological communities on the island have been isolated from those of the adjacent regions of Mexico. The research is being supported by funds from the Tectonics Program, Division of Earth Sciences, and the Americas Program of the Office of International Science and Engineering.