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灰流凝灰岩，是 1260 万年前，在加利福尼亚湾打开之前，从现今加利福尼亚湾附近的一座墨西哥火山喷发出来的。 此时，下加利福尼亚州与墨西哥索诺拉州相邻。 这种凝灰岩最初在下加利福尼亚州和索诺拉州以连续的片状沉积，但后来的断层作用将该地区分成不同的山脉，其中圣费利佩凝灰岩的暴露是倾斜和旋转的。 在一些区块中，凝灰岩已经被侵蚀殆尽，但在许多地方仍然保留着凝灰岩，包括下加利福尼亚州圣费利佩的西部和西南山脉、加利福尼亚湾的蒂伯龙岛以及从基诺湾到索诺拉州埃莫西约的山脉。 在这个项目中，对圣费利佩凝灰岩进行了详细研究，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各个方面包括：总体厚度、从下到上的成分变化（包括在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、从下到上的古地磁剩磁方向、凝灰岩离开喷口时的流动方向（由磁化率的各向异性确定，在 古磁芯），以及通过同位素测年确定的年龄。凝灰岩在火山口（喷发源）附近最厚，随着远离火山口而变薄。 此外，它填充预先存在的渠道或河谷的地方更厚。从底部到顶部以及距喷口距离的变化（相变化）可以指导凝灰岩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以何种方式断层、倾斜和旋转，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 12.6 至 600 万年前（在圣费利佩凝灰岩喷发之后，但在一些较年轻、较小的灰流凝灰岩片喷发之前，地质学家已经绘制了地图），该地区发生了多少板块边界变形。 其他技术预测，在此期间，这里可能发生了数百公里的走滑和伸展运动。 圣费利佩凝灰岩覆盖了非常大的区域，因此为评估这里是否发生了如此大的运动量提供了良好的标记地平线。这将有助于了解圣安地列斯断层系统是如何启动的。