Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins Gary S. Chulick a , Shane Detweiler b, * , Walter D. Mooney b a Mt. Aloysius College, 7373 Admiral Peary Hwy, Cresson, PA 16630, USA b United States Geological Survey, MS 977, 345 Middlefield Road, Menlo Park, CA 94025, USA article info Article history: Received 19 July 2011 Accepted 5 June 2012 Keywords: Crustal structure Seismic velocity South America abstract We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (P n and S n ). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. 8.7 km), which is w1 km thinner than the global average of 39.2 km (s.d. 8.5 km) for continental crust. (2) Histograms of whole-crustal P-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 (6.9e7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (P cc ) is 6.47 km/s (s.d. 0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average P n 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. (5) A region across northern Chile and northeast Argentina has 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. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) 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. Published by Elsevier Ltd. 1. Introduction The construction of continent-scale maps of geophysical prop- erties provides a broad picture of the structure of the Earth. For example, a map of crustal thickness indicates the lateral extent of tectonic provinces such as highly extended regions and orogenic zones. Likewise, maps of crustal seismic velocities can help to delineate platforms, shields, sedimentary basins, and exotic accreted terrains (Christensen and Mooney, 1995). Geophysical maps provide a means of identifying crustal properties that delin- eate geologic provinces (e.g., Prodehl, 1984; Meissner,1986; Collins, 1988; Pakiser and Mooney, 1989; Blundell et al., 1992; Pavlenkova, 1996; Yuan, 1996; Chulick and Mooney, 2002; Fuck et al., 2008; Cordani et al., 2009; Cordani et al., 2010). We present the first set of contour maps based on seismic- refraction work combined with other forms of seismic data (i.e., seismic reflection, sonobuoy, receiver function and earthquake models) for the entire continent of South America and the surrounding ocean basins. There are several reasons why new maps are warranted at this time. First, the quantity and quality of data on South American crustal structure has grown substantially in the past decade or so, with new seismic surveys (e.g. Wigger et al.,1994; Flueh et al.,1998; Patzwahl et al.,1999; Bohm et al., 2002; ANCORP Working Group, 2003; Berrocal et al., 2004; Schmitz et al., 2005; Rodger et al., 2006; Scherwath et al., 2006; Soares et al., 2006; Greenroyd et al., 2007) conducted that cover hitherto unexplored regions (e.g. the Chilean Andes and Amazonia) as well as provide better resolution in previously studied areas (e.g., the Peruvian Andes, and eastern Brazil). However, this work in South America has generally not been done at the continental scale and frequently depends on passive source data (e.g. Schmitz et al., 1999; Assumpção et al., 2002, 2004; An and Assumpção, 2005; Tavera et al., 2006; Lange et al., 2007; Heit et al., 2008; Wölbern et al., 2009). This study consolidates the * Corresponding author. Tel.: þ1 650 329 5192; fax: þ1 650 329 5163. E-mail address: shane@usgs.gov (S. Detweiler). Contents lists available at SciVerse ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Journal of South American Earth Sciences 42 (2013) 260e276 0895-9811/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jsames.2012.06.002