Paper No. 8
Presentation Time: 3:45 PM
PALEOGENE IGNIMBRITE FLARE-UP AND FAULT REACTIVATION DRIVEN BY RAPID THERMAL MATURATION OF THE LOWER CRUST IN THE NORTH-CENTRAL SIERRA MADRE DEL SUR, SOUTHERN MEXICO
MORÁN ZENTENO, Dante J.1, GONZÁLEZ-TORRES, Enrique A.
1, MARTINY, Barbara M.
2 and MORI, Laura
1, (1)Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria Delegación Coyoacán, Mexico D.F, 04510, Mexico, (2)Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F, 04510, Mexico, dantez@servidor.unam.mx
The Sierra Madre del Sur (SMS), in southern Mexico, is made up of an extensive record of Paleocene calc-alkaline volcanic successions and plutonic rocks related to the subduction of the Farallon plate. These rocks represent an extinct arc that was active previous to the episodes of continental margin truncation that occurred between 28 and 20 Ma. Subsequent arc magmatism reinitiated farther north, in the present-day active Trans-Mexican volcanic belt. In the north-central Sierra Madre del Sur, late Eocene-early Oligocene volcanism was marked by an ignimbrite flare-up episode with a major pulse between 36.2 and 31.5 Ma. The flare-up in this sector is represented at by least 9 silicic volcanic centers, including large collapse calderas that have since been exhumed and exposed. This condition allows examination of the volcanic infrastructure and establishing a relationship between the volcanic stratigraphy and coeval fault structures. The preserved volume of ignimbrites and related units is ~4000 km
3 and the volume removed by erosion could be just as large. The largest ignimbrite eruptions occurred in the Nanchititla, Goleta, Taxco, Tilzapotla and Huautla volcanic centers at 36.2, 36, 38-32, 34.5 and 31.5 Ma, respectively.
The (SMS) silicic volcanic centers are mainly distributed in a relatively narrow WNW-ESE 230 km-long strip characterized by WNW-ESE and N-S fracture zones with left-lateral and dip-slip components active during and after the flare-up episode. The volcanic stratigraphy in most silicic centers indicates that the ignimbrite outburst initiated without, or with very scarce, less evolved precedent magma injections, which is suggestive of a relatively rapid thermal maturation of the lower crust and also high rates of magma flux toward the upper crust. Relative concentration of fracture zones along the strip of silicic volcanism with respect to the surrounding regions as well as chronologic relationships with main volcanic episodes, indicate that the rapid thermal evolution of the crust along the strip led to rheological weakening and deformation of the crust that in turn facilitated the magma ascent into the upper crust.
