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

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。这个项目正在研究墨西哥西北部太平洋板块和北美板块之间的地质边界带的早期历史。下加利福尼亚半岛位于太平洋板块上，墨西哥的其余部分属于北美板块。1200万年前，在圣安德烈亚斯断层和加利福尼亚湾形成之前，下加利福尼亚半岛和加利福尼亚湾的岛屿都连接在一起，成为一个单一的陆地块。该地区的一次大型火山喷发产生了火山碎屑流，这些碎屑流将火山岩从火山口向不同方向沉积了至少150公里。由于板块边界的断裂作用，这些岩石现在已经断裂，并在两个板块上分离成不同的山脉。加州理工学院的学生和教师将与墨西哥科学家合作，绘制这些岩石的分布图，从中下加利福尼亚州半岛到加利福尼亚湾海岸，从索诺兰海岸向东150公里进入墨西哥，以及蒂布隆岛和瓜尔达天使岛的目标位置。目的是验证这些1200万年前的火山岩的相关性，检查它们是起源于一次大型喷发还是几次喷发，并研究它们下面的岩石，以了解它们沉积的景观。根据这些资料，可以重建这些火山岩的原始范围和沉积模式。这样就可以测量断层的偏移量，并确定它们是只偏移了300公里（圣安德烈亚斯断层的偏移量）还是多达600公里（加利福尼亚湾南部断层滑动的一些模型所暗示的偏移量）。所使用的技术包括野外的地质测绘和取样，以及实验室岩石的磁学研究（古磁学和磁化率各向异性）。这个项目将更好地理解影响北美西部的主要板块边界的起源——太平洋板块和北美板块之间的边界。在1200万年前到600万年前之间沿着这个边界的运动历史，并解决关于这个地区发生了多少板块边界断裂的争议。这项研究的结果将有助于理解新的板块边界是如何在大陆上形成的，以及一个广泛的断层带需要多长时间才能变成一个洋中脊系统，就像现在在加利福尼亚湾盆地下面出现的那样。

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.