Pliocene Kinematic History of the Northern Gulf of California

我们正在研究一个非常大的地壳区块(Isla Angel de la Guarda地壳区块)是如何在过去的3到600万年里从加利福尼亚州海湾北部的相邻陆地上分离出来并成为一个岛屿的。瓜尔达岛(Isla Angel De La Guarda)地壳区块最初是下加利福尼亚半岛的一部分。大约600万年前，当加利福尼亚州北部的海湾沿着太平洋-北美板块边界开始开放时，它从提伯龙岛(索诺拉)分裂出来。大约200-300万年前，板块边界的位置发生了变化，在IAG和半岛其他部分之间发展了断层，将IAG作为一个岛屿重新转移到北美板块上。从那时起，由于巴列纳斯海峡(瓜尔达岛地壳地块以西)的断层滑动，瓜尔达岛地壳块体应该相对于下加利福尼亚半岛移动了大约100公里，巴列纳斯海峡是太平洋-北美板块边界的一部分，向北连接到圣安德烈亚斯断层。Isla Angel de la Guarda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、圣安德烈亚斯断裂系统的历史以及了解下加利福尼亚州海湾北部、下加利福尼亚州和下德尔芬盆地该岛之间存在多少沉没的陆壳具有重要意义。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，以及对较老的基岩进行对比。当Isla Angel de la Guarda地壳区块连接到下加利福尼亚半岛时，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都沉积了熔岩和凝灰岩(火山碎屑流沉积)。下加利福尼亚半岛上的火山岩研究相对较好，因此它们的地层和分布是已知的。然而，拉瓜尔达岛无人居住，其火山岩还没有得到详细的研究。我们正在首先利用卫星图像和航空照片研究这些岩石，以确定我们需要探访的露头。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，并绘制其分布图。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年代学(年龄测定)技术，以确定它们的特征，并确定哪些岩石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从下加利福尼亚州分离出来时可能的原始位置，并知道它在多长时间前成为一个孤立的岛屿，与半岛的其他部分分开。我们还将比较瓜尔达岛的地壳块状岩石和蒂伯龙岛上的岩石，看看能否确定两者在裂谷开始之前最初是如何联系的。这项研究是对墨西哥政府现有研究项目的补充，该项目正在评估加利福尼亚州海湾地区火山岩的相关性。墨西哥项目将提供地化分析和Ar数据，从而降低该项目的成本。来自加州理工学院和墨西哥的科学家和学生正在进行岛上的实地工作。古地磁和其他实验室研究正在利用加州理工大学的实验室进行。年龄和地球化学分析的结果保存在北美火山岩社区数据库(NAVDAT)中。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公布。结果还被用来用英语和西班牙语向学生或普通公众进行外展演讲。最后，我们的结果有助于生物学家研究岛上的动植物，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.