The Tuff of San Felipe: A Widespread Ignimbritic Marker Unit for Gulf of California Extension

圣费利佩凝灰岩是一种大体积的火山灰流凝灰岩，它是1260万年前加利福尼亚湾打开之前从现在加利福尼亚湾附近的墨西哥火山喷发出来的。在这个时候，下加利福尼亚州毗邻墨西哥的索诺拉。这种凝灰岩最初是作为一个连续的薄片沉积在下加利福尼亚州和索诺拉州，但后来的断层作用将该地区分成了不同的山脉，在这些山脉中，圣费利佩凝灰岩的暴露是倾斜和旋转的。在一些地区，凝灰岩已经被侵蚀掉了，但在许多地方，凝灰岩仍然被保存下来，包括圣费利佩的西部和西南部山脉、下加利福尼亚州、加利福尼亚湾的蒂布龙岛，以及索诺拉州的基诺湾到埃莫西约。在这个项目中，对圣费利佩的凝灰岩进行了非常详细的研究，以便确定所有不同山脉的原始相对位置。正在研究的凝灰岩的方面包括：总体厚度、从下到上的成分变化（包括凝灰岩中发现的整体化学成分和单个晶体或斑晶的化学成分）、从下到上的碎屑含量、从下到上的古磁剩余物方向、凝灰岩离开喷口时的流动方向（由古磁芯测量的磁化率的各向异性决定），以及由同位素定年确定的年龄。凝灰岩在火山口附近最厚，离火山口越远越薄。此外，在填满原有河道或河谷的地方，水会更厚。从底部到顶部的变化以及与喷口的距离的变化（相变化）可以指导凝灰岩在喷发和沉积后的初始形态。通过更好地了解凝灰岩沉积后的样子，以及它随后在不同山脉中断裂、倾斜和旋转的方式，可以了解自1260万年前以来该地区所有山脉是如何变形的。其中一个主要的问题是，在1260万到600万年前（在圣费利佩凝灰岩喷发之后，但在地质学家已经绘制出的一些更年轻、更小的火山灰流凝灰岩喷发之前），这个地区的板块边界发生了多少变形。其他技术预测，在这段时间里，可能发生了数百公里的走滑和伸展运动。圣费利佩的凝灰岩覆盖了一个非常大的区域，因此为评估这里是否发生了大量的运动提供了很好的标志水平。这将有助于了解圣安德烈亚斯断层系统是如何形成的。

The Tuff of San Felipe is a large-volume ash-flow tuff that erupted from a Mexican volcano near the present Gulf of California, 12.6 million years ago, before the Gulf of California opened. At this time, Baja California was adjacent to Sonora, Mexico. This tuff was initially deposited as a continuous sheet in both Baja California and Sonora, but later faulting has broken the region into different mountain ranges in which exposures of the Tuff of San Felipe are tilted and rotated. In some of the blocks the Tuff has been eroded away, but it is still preserved in many places, including the mountain ranges W and SW of San Felipe, Baja California, on Tiburon Island in the Gulf of California, and from Kino Bay to Hermosillo in Sonora. In this project, the Tuff of San Felipe is being studied in great detail, so that the original relative positions of all the different mountain ranges can be determined. The aspects of the tuff that are being studied include: overall thickness, compositional variations from bottom to top (including bulk chemical composition and chemical compositions of the individual crystals, or phenocrysts, found in the tuff), clast content from bottom to top, direction of paleomagnetic remanence from bottom to top, direction of flow of the tuff when it left the vent (which is determined from the anisotropy of the magnetic susceptibility, measured on paleomagnetic cores), and age determined by isotopic dating. The tuff is thickest near the vent, the source of the eruption, and thinner with distance away from the vent. In addition, it is thicker where it filled pre-existing channels or river valleys. The changes from bottom to top and with distance from the vent (facies variations) are a guide to the initial morphology of the tuff just after it was erupted and deposited. By understanding better what the tuff looked like just after it was deposited, and which way it was subsequently faulted, tilted and rotated in different mountain ranges, allows understanding of how all of the mountain ranges have deformed in this area since 12.6 million years ago. One of the major questions being answered is how much plate boundary deformation occurred in this region between 12.6 and 6 million years ago (after the time of eruption of the Tuff of San Felipe, but before the eruption of some younger, smaller ash-flow tuff sheets that have already been mapped by geologists). Other techniques predict that possibly several hundreds of kilometers of strike-slip and extensional motion occurred here during this time period. The Tuff of San Felipe covered a very large area and therefore provides good marker horizon for evaluating whether this amount of motion occurred here or not. This will help understand how the San Andreas fault system initiated.