Lg coda Q variations across South America and their relation to crustal evolution

Nine broadband seismograph stations in South America have provided 389 reco rdings of Lg coda with paths that cover most of the continent. Lg coda Q (Q (0)) and frequency dependence (eta) values at 1 Hz, obtained from these rec ords, were inverted using back-projection tomography to obtain regionalized maps of Q(0) and eta. The entire western margin of the continent (the acti ve Andean mountain belt) is typified by low Q(0) (250-450), whereas broad r egions of high Q(0) (700-1100) span the central Brazilian shield and contig uous regions to the north and south. Intermediate Q(0), (450-700) character izes the northern Patagonia platform and most of the Atlantic shield. Reduc ed Q(0) in the Atlantic shield may be related to tectonic or igneous activi ty that occurred during the breakup of Gondwanaland during the Jurassic per iod. This Q distribution is generally consistent with earlier studies where Q(0) was found to be directly proportional to the time that has elapsed si nce the most recent episode of major tectonic or orogenic activity in any r egion. Reduced Q(0) in the Patagonian platform may, however, be due to youn g sediments there. Q(0) is slightly higher in two portions of the Andean belt (between latitud es 2.0 degrees N and 10.0 degrees S, and between latitudes 24.0 degrees S a nd 34.0 degrees S) than in other portions of the belt. These variations are consistent with results of earlier studies of body-wave attenuation and he at flow in the Andean mountain belt. Spatial variations of eta generally vary inversely with Q(0), being low (0. 0-0.2) throughout a broad region centered in the central Brazil shield and extending to the northeastern coast. All surrounding regions except that to the northeast exhibit intermediate to high (0.4-0.8 and possibly higher) e ta values. Possible biasing of Lg coda Q measurements by proximity to the t ransition between the South American and Pacific plates was examined using records from a station near that boundary and was found to be small.