High Resolution Imaging of Mantle Discontinuities across South America at 20 Degrees South

PIs: Kelly Liu and Stephen Gao
06/2000 - 05/2002 by National Science Foundation; EAR#: 0001000

We have conducted a high resolution study of the major discontinuities beneath a 3000 km long profile along the 20 degree latitude line across central South America. The study utilized a unique data set, which is a combination of broadband data sets from three portable seismic experiments conducted during 1992--1995. We have re-processed the data sets and combined them into a uniform format.

Preliminary results by non-linear stacking of 752 high quality radial receiver functions suggest clear spatial variations of both the 410 and the 660 across the profile. Some interesting features include:

1). The 660 appears to be depressed by at least 35 km, or perhaps as large as 65 km by the subducting slab;

2). West of the Andes, the depth to the Moho is about 40-45 km. It increases to 65-80 km over a distance of less than 50 km, and then decreases to about 45-50 km beneath the eastern boundary of the Andes, over a distance of about 120 km;

3). The transition zone thickness is about 250 km on the ocean side, and 260 km on the continent side, indicating that the former is hotter than the latter.

4). There exists a group of strong negative-polarity arrivals above 300 km depth that exist only above the slab on the continental side. If this preliminary observation holds up after further testing, it would reflect first-order differences between oceanic and continental upper mantle.