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

圣费利佩凝灰岩：加利福尼亚湾延伸区广泛分布的凝灰岩标记单元

• 批准号: 0336961
• 负责人: Joann Stock
• 金额: --
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-01-01 至 2008-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0336961&HistoricalAwards=false
• 关键词: Tuff; San; Felipe; Widespread; Ignimbritic

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.

圣费利佩凝灰岩是一种大量的火山灰流凝灰岩，1260万年前，在加利福尼亚州海湾开放之前，从现在的加利福尼亚州海湾附近的墨西哥火山喷发。当时，下加利福尼亚州毗邻墨西哥的索诺拉。凝灰岩最初在下加利福尼亚州和索诺拉都是连续沉积的，但后来断层作用将该地区划分为不同的山脉，圣菲利佩凝灰岩的暴露在这些山脉中倾斜和旋转。在一些街区，凝灰岩已经被侵蚀，但在许多地方仍然保存下来，包括下加利福尼亚州圣费利佩的W山脉和西南山脉，加利福尼亚州海湾的蒂伯龙岛，以及从基诺湾到索诺拉的赫尔莫西洛。在这个项目中，圣费利佩的凝灰岩正在进行非常详细的研究，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、自下而上的成分变化(包括凝灰岩中发现的单个晶体或斑晶的整体化学成分和化学成分)、碎屑含量、从底部到顶部的古地磁剩磁方向、凝灰岩离开喷口时的流动方向(这是由磁化率的各向异性确定的，由古地磁岩心测量)，以及由同位素测年确定的年龄。凝灰岩在喷口附近最厚，距离喷口越远，凝灰岩越薄。此外，在填满现有河道或河谷的地方，它的厚度会更大。凝灰岩从下到上的变化和与喷口的距离(相变化)是凝灰岩喷发和沉积后的初始形态的指南。通过更好地了解凝灰岩沉积后的样子，以及随后它在不同山脉中被断层、倾斜和旋转的方向，可以了解自1260万年前以来该地区所有山脉是如何变形的。正在回答的主要问题之一是，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的一些更年轻、更小的火山灰流凝灰岩板块喷发之前)，该地区发生了多少板块边界变形。其他技术预测，在这段时间内，这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很大的面积，因此为评估这种运动量是否发生在这里提供了很好的标志层。这将有助于理解圣安德烈亚斯断层系统是如何启动的。