T 11 A1853 NET SLIP ACROSS THE BALLENAS

T 11 A-1853 NET SLIP ACROSS THE BALLENAS TRANSFORM FAULT MEASURED FROM OFFSET IGNIMBRITE DEPOSITS J Stock 1, A Martin Barajas 2, M Martinez-Lopez 2, A Chapman 1 1 - California Institute of Technology, Pasadena CA USA 2 - CICESE, Ensenada, Baja California, Mexico jstock@gps. caaltech. edu, amartin@cicese. mx, marlopez@cicese. mx, alan@gps. caltech. edu Abstract The Ballenas Transform Fault in the Gulf of California separates the Baja California Peninsula (to the west, on the Pacific Plate) from Isla Angel de la Guarda (to the east, on the North America plate). This active right-lateral fault is thought to have originated in Pliocene time when a local plate boundary reorganization caused part of the rift margin to jump westward into the Baja California peninsula, transferring Isla Angel de la Guarda from the Pacific plate onto the North America plate. The net slip along the Ballenas transform fault system is therefore expected to be less than the 250 -300 km of opening of the northern Gulf of California basins since late Miocene time. Here we constrain the amount of net slip using geological similarities between the west coast of the island the Baja California peninsula. Reconnaissance geological mapping on Isla Angel de la Guarda reveals the presence of a middle Miocene high-silica rhyolite ignimbrite with an Ar/Ar age of 11. 8 +/- 0. 2 Ma on sanidine (Martin-Barajas et al. , 2008). This deposit is preserved in a paleo-low in Cretaceous (? ) granitic rocks in the central part of the island, near the western coast. The major and trace element compositions are similar to those of the Tuff of San Felipe, identified in Sonora and Baja California, but not previously recognized on Isla Angel de la Guarda. The deposit from the island exhibits a paleomagnetic remanence vector toward the SW and nearly horizontal, similar to that known for the Tuff of San Felipe in its outcrop area in other locations, and far outside of the expected paleomagnetic remanence direction. The ignimbrite deposits on Isla Angel de la Guarda are not densely welded, and the age is younger than that previously reported for the Tuff of San Felipe, but is within the range of ages determined for other outcrops of the Tuff of San Felipe in Sonora (Vidal-Solano et al. , 2008). We correlate this ignimbrite to a similar unit preserved in isolated locations northwest of Cataviña on the Baja California peninsula, for which paleomagnetic work shows a very similar remanence direction. Using the SE limit of these outcrops against the granitic rocks in both locations yields an estimate of ~130 km right-lateral strike-slip offset along the Ballenas channel. We expect to refine this estimate further with additional field work. Baja California Peninsula, Cataviña region C Paleomagnetic Methodology Densely welded tuff Brown glassy base (vitrophyre not well developed) conglomerate Sample Preparation: v Oriented hand samples were obtained from Isla Angel de la Guarda and the Baja California Peninsula in the region near Catavina. v. Three to five cores were drilled from each rock; these were re-oriented back to their original field positions and their orientations measured. They were then sliced into standard lengths. Demagnetization and analysis at the Caltech Paleomagnetics Laboratory: v. Natural Remanent Magnetization (NRM) was measured. v. Low temperature step: chips were cooled to 77 K (liquid nitrogen treatment) then allowed to warm up back to room temperature. Magnetization of sample was then measured. v. Alternating Field (AF) steps from 25 m. T to 75 m. T with increments of 25 m. T. v. High AF demagnetization from 100 m. T to 800 m. T in steps of 50 m. T. v. High AF steps are used to obtain the best fit for a linear vector of magnetic remanence for each core; these are combined for each locality to obtain Fisher and Bingham statistics POR-08 -02 POR-08 -03 Marker unit: the Tuff of San Felipe This processed ASTER satellite image (short-wave infrared) shows outcrops of this ignimbrite in light blue v. Peralkaline rhyolitic pyroclastic flow deposit crops out in scattered mesas in coastal and central Sonora and Baja California, México. Correlated in Baja California by Stock et al. (1999) and on Tiburon Island coastal Sonora by Oskin (e. g. , Oskin, 2002; Oskin & Stock 2003). v 40 Ar /39 Ar geochronology on sanidine and U-Pb geochronology on zircon crystal found within the tuff date the unit at around 12 -12. 5 Ma (Vidal-Solano et al. , 2005; Vidal-Solano et al. , 2007 a; Vidal-Solano et al. , 2007 b). v. Unit exposed over a distance of at least 430 km; it is a key horizon in the reconstruction of the Gulf Extensional Province because it was erupted at the time of the onset of extension in part of the Gulf of California (Stock et al. , 1999). NORTH AMERICA C IAG PACIFIC Figure 1: Map of the Sonora and Baja California region of Mexico, separated by the Pacific-North America plate boundary in the Gulf of California. Black dots: outcrops of the Tuff of San Felipe previously reported. Red dots: newly correlated locations. C= Cataviña; IAG= Isla Angel de la Guarda. Ballenas Transform Fault Cataviña Ignimbrites form flat-topped mesas overlying conglomerates above the granitic rocks Figure 2: Tie points on opposite sides of the Gulf of California (Oskin & Stock, 2003). v. Crystal-rich, strongly indurated, lithic-lapilli pyroclastic flow deposit with black basal vitrophyre, grading down into a narrow zone of brown, moderately welded tuff, and grading up into a red spherulitic horizon followed by a zone of densely welded magenta colored cliff forming ignimbrite (Stock et al. , 1999). v It has 5 -15% alkali feldspars and can be up to 180 m thick in some locations near the vent (Stock et al. , 1999). v. Distinctive mineralogical association with fayalite, Fe-rich augite, alkali feldspar phenocrysts and zircon as a common trace mineral (Vidal-Solano et al. , 2008). v. Unit has unique, low-inclination, reverse magnetization vital for correlation of tuff along the region (Stock et al, 1999). Lower hemisphere projection of POR-08 demagnetization vectors between 300 m. T and the origin, for 8 cores. Because a strong and anomalous NRM was removed by the earlier demagnetization steps, more samples need to be studied to get reliable uncertainties for this location. IAG Tilt-corrected vectors of paleomagnetic remanence, for sample IAG-07 -44, from Isla Angel de la Guarda, Gulf of California, México. N=9 Decl = -242. 2; Incl=-9. 5 a 95=3. 6 Isla Angel de la Guarda Conclusions Trachyte inclusion Matrix Sum of the probabilities of the ages of Tuff of San Felipe, coastal Baja California The paleomagnetic remanence direction of the ignimbrite discovered on Isla Angel de la Guarda, and that exposed in mesas near Cataviña, Baja California, is nearly horizontal towards the SSW. This is similar to the direction found in the Tuff of San Felipe in Baja California and coastal Sonora. Geochemical analyses from Isla Angel de la Guarda, geochronology from the same location, and field observations from both regions suggest that these two localities belong to a previously unrecognized lobe of pyroclastic density current deposits (ignimbrite) from the ca. 12. 5 Ma eruption of the Tuff of San Felipe. This lobe of the eruption appears to have filled a depression with thin conglomerates overlying basement granitic rocks. The southern edge of the exposures in both locations are offset ca. 130 km parallel to the strike of the active Ballenas Transform Fault, giving a maximum of 130 km for the net slip on this fault. This is less than the ca. 300 km of net post-6. 3 Ma slip in the Northern Gulf of California because the Ballenas Transform Fault is a relatively young structure; it formed in Pliocene time, due to a plate boundary reorganization and cessation of activity on the Tiburon Fracture Zone. Acknowledgments: This research was supported by the U. S. National Science Foundation under Grant No. EAR-0610011 and by Consejo Nacional de Ciencias y Tecnología, Mexico, under Project CONACy. T P 46600 - F. Gianna Hernandez-Mendez assisted with paleomagnetic measurements. References • Oskin, M. E. , 2002. Tectonic evolution of the northern Gulf of California, México, deduced from conjugate rifted margins of the upper Delfin Basin. : Ph. D thesis, California Institute of Technology, Pasadena, 487 pp. • Oskin, M. E. , and Stock, J. M. , 2003. Cenozoic volcanism and tectonics of the continental margins of the Upper Delfin basin, northeastern Baja California and western Sonora. In: S. E. Johnson, S. R. Paterson, J. M. Fletcher, G. H. Girty, D. I Kimbrough & A. Martin-Barajas. Eds. , Tectonic evolution of northwestern México and southwestern USA: Geol. Soc. Amer. Sp. Paper, v. 374, 421 -438 • Stock, J. M. , Lewis, C. J. , Nagy, E. A. , 1999. The Tuff of San Felipe: an extensive middle Miocene pyroclastic flow deposit in Baja California, México: Journal of Volcanology and Geothermal Research, v. 93, p. 53 -74 • Vidal-Solano, J. , Paz-Moreno, F. A. , Iriondo, A. , Demant, A. , Cochemé, J. J. , 2005. Middle Miocene peralkaline ignimbrites in the Hermosillo region (Sonora, México). Geodynamic implications: C. R. Geoscience, 337, 1421 -1430. • Vidal-Solano J. R. , Paz-Moreno, F. A. , Demant, A. , Lopez-Martinez, M. , 2007 a. Ignimbritas hiperalcalinas del Mioceno medio en Sonora Central: revaluación de la estratigrafía y significado del volcanismo terciario: Revista Mexicana de Ciencias Geológicas, v. 24, p 47 -67 • Vidal-Solano, J. , Paz-Moreno, F. A. , Demant, A. , Iriondo, A. , Wooden, J. L. , 2007 b. 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