Pliocene Kinematic History of the Northern Gulf of California

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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.