HR: 0830h
AN: S41D-0125    [PDF]
TI: Crustal Thickness Beneath the South-Central Andes and Sierras Pampeanas at $30^o$S Inferred From Pn Apparent Phase Velocities
AU: * Fromm, R
EM: rfr@geo.arizona.edu
AF: University of Arizona, 1040E Fourth Street, Tucson, AZ 85719 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: University of Arizona, 1040E Fourth Street, Tucson, AZ 85719 United States
AU: Beck, S L
EM: beck@geo.arizona.edu
AF: University of Arizona, 1040E Fourth Street, Tucson, AZ 85719 United States
AU: CHARGE Working Group, .
EM: rfr@geo.arizona.edu
AF: University of Arizona, 1040E Fourth Street, Tucson, AZ 85719 United States
AB: The south-central Andes and Sierras Pampeanas at $30^o$S represent a unique environment where the major tectonic units evolved predominantly since the Miocene as a result of the flat subduction of the Nazca plate and its associated processes. In this context, we developed a methodology to obtain a crustal thickness model in this area to constrain some aspects of the evolution of this orogenic system. We used Pn arrivals of 9 crustal earthquakes to obtain apparent inter station Pn phase velocities along an E-W trending transect consisting of 8 broadband stations deployed as part of the Chile Argentina Geophysical Experiment (CHARGE) project. The apparent velocities are defined as the ratio of the distance between two consecutive stations and the differential travel time. A two stage inversion scheme was used to match the observed and synthetic apparent phase velocities for a crustal model consisting of 8 Moho depths along the profile assuming homogeneous P-wave velocities in the crust and upper mantle. In the first stage, an initial crustal model was found by trial and error. The second stage consisted of a refinement of the optimal solution by a random search in model space. Our preferred model shows a crustal thickness of 65km beneath the High Cordillera and Precordillera and 45-60km beneath the western Sierras Pampeanas. A progressive shallowing is observed towards the east, where a 36km thick crust is observed 700km east of the trench. These crustal thicknesses are consistent with earlier results based in part on gravity modeling and deep reflection surveys. However, isostatically compensated Moho depths computed along the profile for a range of reasonable density models appear to be systematically shallower than our preferred model. This discrepancy is particularly acute (up to 15-20km) beneath the western Sierras Pampeanas, where low topographic elevations and only minor surface crustal shortening apparently contradict such deep Moho depths. This deep rooted crust might be explained by a combination of an unusually dense lower crust or upper mantle, or by lower crustal flow induced by dynamic forces caused by to the flat slab.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting