These contour maps depict the seismic structure of the crust and uppermost mantle of South America and the surrounding oceanic basins. Notable geographic features include a region across northern Chile and northeast Argentina with anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.
Figure 7b: Cross section A goes across the Brazilian Shield (vertical exaggeration is ~110x). B crosses the continent at latitude 20 degrees south (vertical exaggeration is ~110x). C crosses the continent at latitude 33.5 degrees south (vertical exaggeration is ~130x). D crosses the continent at latitude 55 degrees south (vertical exaggeration is ~130x). E crosses the continent near the Atlantic coast (vertical exaggeration is ~200x).
- The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. +/- 8.7 km), which is ~1 km thinner than the global average of 39.2 km (s.d. +/- 8.5 km) for continental crust.
- The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. +/- 0.25 km/s). This is essentially identical to the global average of 6.45 km/s.
- The average Pn velocity beneath South America is 8.00 km/s (s.d. +/- 0.23 km/s), slightly lower than the global average of 8.07 km/s.