HR: 11:20h
AN: S41F-05    [PDF]
TI: Crustal structure of central Chile and Argentina
AU: * Gilbert, H J
EM: hgilbert@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Strees, Tucson, AZ 85721 United States
AU: Beck, S
EM: beck@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Strees, Tucson, AZ 85721 United States
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: University of Arizona, Department of Geosciences 1040 E. 4th Strees, Tucson, AZ 85721 United States
AU: CHARGE Working Group, .
EM:
AF: University of Arizona, Department of Geosciences 1040 E. 4th Strees, Tucson, AZ 85721 United States
AB: Utilizing data collected during the recent Chile-Argentina Geophysics Experiment (CHARGE) we study the crustal structure of central Chile and Argentina. The CHARGE PASSCAL array consisted of 22 broadband seismometers that were configured in two transects at 30$\deg$S and 36$\deg$S. Records from teleseismic P and PP arrivals were used to calculate receiver functions to image crustal and upper mantle structure. We study geographic variations in structure by stacking receiver functions into common conversion point bins. A strong Moho signal on the northern transect can be clearly seen at a depth near 40 km below the eastern Sierras Pampeanas. However, to the west of a major suture zone at 67$\deg$W, between the eastern and western Sierras Pampeanas, we no longer detect a simple Moho signal as receiver function arrivals are much more elusive. Instead, we find low-amplitude broad arrivals around 50-60 km depth under the western Sierras Pampeanas, Precordillera, and Andes. These arrivals indicate that the crust thickens to the west and correlate well with crustal thickness variations based on Pn arrivals. Mid-crustal arrivals are also present in this region of thicker crust. By comparing arrival times of multiple phases on receiver functions we determine that the crustal Vp/Vs ratio varies with lower values near 1.75 to the east of 67$\deg$W, indicative of felsic crust, and higher values over 1.8 to the west, that can result from a more mafic crust. On the eastern portion of the southern transect we detect a strong Moho signal near 40 km depth that appears to be deepening to the west. When investigating how receiver function characteristics vary azimuthally, we find a positive Moho signal for the western portion of the northern transect when stacking only data sampling the Moho from the southwest backazimuth. When the area is sampled by data from northwestern backazimuths a negative arrival is present at the same depth. The azimuthal dependence we observe for the Moho signal can explain why stacking data from multiple azimuths results in low amplitude signal from the Moho. We do not detect any significant azimuthal dependence on Moho characteristics in the southern transect, or the eastern portion of the northern transect. Results of receiver function forward modeling indicates that an anisotropic layer at the base of the crust between depths of 30 and 50 km with a fast axis oriented towards the northwest and an axis of symmetry plunging near 50 degrees can produce arrivals similar to those observed on the western portion of the northern transect. Although investigating the properties of anisotropic structures in this manner suffers from non-uniqueness, these models provide a possible explanation to the observed azimuthally dependent signal. If anisotropy in this depth range results from crustal flow, our observations could be produced by flow in the northwest-southeast direction that may be associated with crustal thickening.
DE: 7205 Continental crust (1242)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting