HR: 11:50h
AN: S41F-07    [PDF]
TI: Regional Tomographic Inversion of P-wave Travel Times in Central Chile and Argentina, between $30\deg$ and $36\deg$ south.
AU: * Wagner, L S
EM: lwagner@geo.arizona.edu
AF: Department of Geosciences University of Arizona, 1040 E. 4th St, Tucson, AZ 85721 United States
AU: Beck, S
EM: beck@geo.arizona.edu
AF: Department of Geosciences University of Arizona, 1040 E. 4th St, Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences University of Arizona, 1040 E. 4th St, Tucson, AZ 85721 United States
AU: Araujo, M
EM: maraujo@inpres.gov.ar
AF: INPRES, 47 Norte Roger Balet, San Juan, n/a Argentina
AU: Campos, J
EM: jaime@dgf.uchile.cl
AF: Departamento Geofisica Universidad de Chile, 2085 Avda. Blanco Encalada, Santiago, n/a Chile
AU: CHARGE Working Group, .
EM: charge@geo.arizona.edu
AF: Department of Geosciences University of Arizona, 1040 E. 4th St, Tucson, AZ 85721 United States
AB: We use regional P-wave travel times to model upper mantle structure beneath Chile and Argentina between $30\deg$ and $36\deg$ S. This area is of particular interest, because north of $33\deg$S, the subducted Nazca plate descends normally to a depth of 100 km, but then flattens out, continuing along a subhorizontal path for some 400 km before resuming a normal subduction angle and descending into the mantle. South of $33\deg$, the Nazca plate subducts with a constant dip. This difference in subduction style from the north to the south is correlated with changes in volcanism, seismicity, and orogenic structure. Data for this study comes from the CHARGE project, a 22 station broadband portable seismic deployment, and first arrival P-picks from the ISC catalog recorded at an additional 30 stations operated by INPRES and the Seismological Service of the Universidad de Chile. We use the tomographic inversion method of Zhao et al. [1994] to invert over 3100 P-wave picks from over 170 events recorded at 54 stations. This tomography method allows for the introduction of discrete discontinuities, such as the Moho and the top of the subducting slab, which allows for clearer vertical resolution because smearing across these boundaries is reduced. Preliminary results show evidence of low velocity zones possibly associated with the dehydration of the subducting oceanic plate. To the south where the Nazca plate descends at a dip of $30\deg$, the low velocity zone appears to be confined to a narrow area above where the subducted plate reaches ~100 - 125 km depth and corresponds to the region beneath the active volcanic arc. To the north, the low velocity zone persists, extending east beneath the Sierras Pampeanas. This correlates well with the flat slab area, indicating that the Nazca plate may dewater continuously as it subducts east until it resumes a normal descent angle into the mantle. However, arc volcanism north of $33\deg$ S has been dormant for 8 Ma, suggesting that it is the cold thermal regime of the flat slab, rather than a lack of water, that is primarily responsible for shutting down the arc volcanism. Zhao, D., A. Hasegawa, and H. Kanamori, Deep structure of Japan subduction zone as derived from local, regional, and teleseismic events, \emph{Journal of Geophysical Research, 99}, 22313-22329, 1994.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8180 Tomography
SC: Seismology [S]
MN: 2003 Fall Meeting