HR: 08:30h
AN: T41F-03    [Abstracts]
TI: New Map of Subducted Slab Geometry in the Chile-Argentina Flat Slab Subduction Zone, South America: Implications for Ridge Buoyancy
AU: * Anderson, M
EM: anderson@geo.arizona.edu
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85721 United States
AU: Alvarado, P
EM: alvarado@geo.arizona.edu
AF: Department of Geosciences Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85721 United States
AU: Beck, S
EM: beck@geo.arizona.edu
AF: Department of Geosciences Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85721 United States
AU: Rodi, W
EM: rodi@erl.mit.edu
AF: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology 79 Massachusetts Ave., Cambridge, MA 02139 United States
AB: Earthquake hypocenter locations and focal mechanisms for intermediate-depth earthquakes between 30° and 36°S in South America reveal subducting slab geometry and deformation that is slightly different from past studies. The CHARGE broadband seismic network was arranged in two transects across the Andes in this region between 2000-2002. During this period, we recorded 1098 intermediate-depth earthquakes with enough P- and S-wave arrivals for obtaining reliable locations. We use the multiple-event location algorithm GMEL to refine the hypocenter locations of these events which yields tighter clustering of events within the slab compared to hypocenters produced by the ISC and single-event hypocenters calculated with our data. In addition, station corrections produced by this method are consistent with independently-determined crustal thickness variations across the network and local changes in uppermost mantle velocity. The Wadati-Benioff zone in the region between 30° and 33°S has been shown by previous earthquake location studies to dip eastward into the mantle to a depth of 100 km and then extend another 300 km with a flat geometry before dipping again into the mantle. To the south of 33°S the slab dips into the mantle at a consistent 30° angle. Using our new hypocenter locations, we produce contours of the top of the Wadati-Benioff zone which exhibits the shallowest depths coinciding with the projected extent of the Juan Fernandez aseismic ridge (JFR) within the subducting slab. All other parts of the slab to the north, east and south dip away from this zone, though more steeply to the east and south than to the north. We estimate a conservative confidence interval on the depths of our events to be between 7-12 km, which means the small variation in depth (15-20 km) between the JFR and the slab to the north near 30°S is close to the edge of our resolution. However, previous studies have also noted deeper events north of the JFR, which supports our depth determinations. We also determine focal mechanism solutions from first motions for many of the located intermediate-depth events, the results of which show normal faulting dominating brittle slab deformation. The orientation of the sub-horizontal T-axes is perpendicular to local slab strike almost everywhere in the slab, given our new slab contours, which is consistent with slab-pull acting throughout the slab. There is an exception to this trend within a small portion of the slab near the slab-dip transition (near 33°S) at 150 km depth, where T-axes are rotated away from the slab dip direction. The implication is that there may be a gap in the slab at this depth. The absence of a double seismic zone below 90 km depth along the JFR and tomography results that show a dry mantle wedge immediately above the JFR suggests that dehydration embrittlement may not be the root cause of these events. We suggest an alternative hypothesis that the JFR, because it is the shallowest portion of the flat slab, may be a locus of bending of the slab, hence increased seismic activity compared to other portions of the slab.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8123 Dynamics: seismotectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 9360 South America
SC: Tectonophysics [T]
MN: Fall Meeting 2005