2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 6
Presentation Time: 9:15 AM


OWNBY, Steven, Dept. of Geological Sciences, Univ of Michigan, 2534 C.C. Little Bldg, 425 E. University Ave, Ann Arbor, MI 48109-1063, DELGADO-GRANADOS, Hugo, Instituto de Geofisica, UNAM, Coyoacan, DF 04510 and LANGE, Rebecca A., Department of Earth and Environmental Sciences, Univ Michigan, 425 E University, Ann Arbor, MI 48109, sownby@umich.edu

Volcán Tancítaro is one of the few, large stratovolcanoes (~57 km3 and 3840m above sea level) in the Michoacán-Guanajuato Volcanic field located in the Trans Mexican Volcanic Arc. Our newly acquired 40Ar/39Ar date on an andesite lava from V. Tancítaro yielded an age of 422±56 ka (error is 2σ), which corresponds well with the K-Ar date of 530±60 ka from Ban et al. (1992). The majority of the exposed lavas from V. Tancítaro are andesites, with phenocrysts of hornblende, two pyroxenes, and abundant plagioclase. The edifice is located at the intersection of two planes of crustal weakness delineated on the surface by alignment of cinder cones and fault plane trends. V. Tancítaro has a history of collapsing catastrophically by large volume debris avalanches, one of which has left a horseshoe shaped scar on the eastern flank. The minimum volume of material removed from the flank of V. Tancítaro by the debris avalanche is ~0.9 km3, estimated from high-resolution digital elevation models (DEMs) using geographic information systems (GIS) software. The debris avalanche deposit (DAD) resulting from the formation of the horseshoe scar has an estimated area of 176 km2 and volume of 3.52 km3 (Capra et al., 2002).

A second debris avalanche deposit is located on the western side of V. Tancítaro, and has not been previously described. This western debris avalanche deposit has a vertical drop of 2800-3000 m, maximum run out distance of 27.8-29.5 km, for an H/L of ~0.1. The deposit has not been mapped in detail but has an area >90 km2 and an estimated average thickness of 20m, which yields a volume ~1.8 km3. The deposit contains features typical of DADs: jigsaw fracturing, horizontal grading, large boulders, and a few elongated hummocks. V. Tancítaro presents a serious geologic hazard owing to three factors: (1) weakening of the edifice by weathering, (2) active seismicity in the region, and (3) V. Tancítaro’s history of collapsing to form large volume debris avalanches. These factors motivate use of the LAHARZ program and historical deposit dimensions to predict the potential hazards imposed by V. Tancítaro on the surrounding region.