Seismic Structure of the Crust and Uppermost Mantle of South America and Surrounding Ocean Basins

We present a new set of contour maps showing the seismic structure of South America and the surrounding ocean basins. These maps include crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle, i.e., Pn and Sn. These maps include much new seismic reflection/refraction data from the continent. By summarizing all available data in this way, we are able to compare the crust of South America to other continents and the global crust in general. Our conclusions show: (1) The average weighted thickness of the crust under South America is 37.4 km (standard deviation, s.d. +/- 9.3 km), which is 1.8 km thinner than the global average of 39.2 km (s.d. +/- 8.5 km) for continental crust. (2) Histograms of whole-crustal P- and S-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9-7.3 km/s) lower crustal layer. (3) A region across northern Chile and east into Argentina has an anomalously low P- and S-wave crustal velocity structure. This likely corresponds to the shallowly-subducted portion of the Nazca plate ("flat slab" of Isacks et al., 1968) which is also a region of extension. Thus, it may be tectonically analogous to the Basin and Range province of the southwestern United States. (4) The average Pn velocity beneath South America is 8.06 km/s (s.d. +/- 0.16 km/s). (5) The well-known thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (6) The crust may be thinned by extension in the Amazon basin and along the western edge of the Brazilian craton. The western edge of the continental crust correlates with the Chile-Peru trench, while the eastern edge generally correlates with the Atlantic passive margin. (7) The average Pcc velocity (P-wave velocity of the crystalline crust) is 6.43 km/s (s.d. +/- 0.25 km/s). This is only slightly lower than the global average of 6.45 km/s. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor due to the thicker sediment layer on the older Atlantic seafloor, as noted in previous work (Chulick and Mooney, 2002).