NSF Award Abstract - #0336961| NSF Award Abstract - #0336961 |
| NSF Org | EAR |
| Intial Amendment Date | December 18, 2003 |
| Latest Amendment Date | December 18, 2003 |
| Award Number | 0336961 |
| Award Instrument | Standard Grant |
| Program Manager |
David Fountain EAR Division of Earth Sciences GEO Directorate for Geosciences |
| Start Date | January 1, 2004 |
| Expires | December 31, 2005 (Estimated) |
| Awarded Amount to Date | $218796 |
| Investigator(s) | Joann Stock jstock@gps.caltech.edu (Principal Investigator) |
| Sponsor |
California Institute of Technology 1200 E California Blvd Pasadena, CA 91125 626/395-6073 |
| NSF Program(s) | TECTONICS |
| Field Application(s) | 0000099 Other Applications NEC |
| Program Reference Code(s) |
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| Program Element Code(s) |
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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.