Initiation of the Pacific-North America Plate Boundary in NW Mexico

点击翻译按钮获取中文摘要

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).This project is studying the early history of the geological boundary zone between the Pacific and North America plates in northwestern Mexico. The peninsula of Baja California lies on the Pacific plate, and the rest of Mexico belongs to the North America plate. Twelve million years ago, before the San Andreas Fault and the Gulf of California formed, the Baja California peninsula and the islands in the Gulf of California were all connected together as a single land mass. A large volcanic eruption in this region produced pyroclastic flows that deposited volcanic rocks up to at least 150 kilometers from the vent in different directions. These rocks have now been faulted and separated into different mountain ranges on the two plates, due to plate boundary faulting. Students and faculty from the California Institute of Technology will work in collaboration with Mexican scientists to map these rocks in a transect from the Central Baja California peninsula to the Coast of the Gulf of California, and from the Sonoran coast a distance 150 kilometers eastward into Mexico, as well as targeted locations on Isla Tiburon and Isla Angel de la Guarda. The objective is to verify the correlation of these 12-million-year-old volcanic rocks, to check whether they originated in one mega-eruption or in several eruptions, and to study the rocks underneath them in order to understand the landscape upon which they were deposited. From these data, the original extent and depositional pattern of these volcanic rocks can be reconstructed. This allows measurement of the amount of offset by faulting and determination if they are only displaced 300 km (the amount of displacement on the San Andreas fault) or as much as 600 km (which is the amount of displacement implied by some models of fault slip in the southern Gulf of California). The techniques to be used include geological mapping and sampling in the field, and magnetic studies (paleomagnetics and anisotropy of magnetic susceptibility) of the rocks in the laboratory.This project will give a better understanding of the initiation of a major plate boundary that affects the western side of North America - the boundary between the Pacific and North America plates. The history of motion along this boundary between twelve million years ago and six million years ago and solve a controversy regarding how much plate boundary faulting took place in this region. The results of this study will help understand how new plate boundaries form in continents, and how long it can take for a broad zone of faulting to turn into a mid-ocean ridge system, such as the one that is now present beneath the basins of the Gulf of California.