Effect of the cold Nazca Slab on the depth of the 660 km discontinuity in South America

Abstract

The Nazca plate subducting beneath South America makes the 660 km discontinuity deeper and the mantle transition zone (MTZ) thicker under the continent. MTZ variations are often associated with mantle temperature and, therefore, can help confirm the slab position at greater depths. Recent P- and S-wave tomography results show the Nazca plate, near 20° S, being held below the MTZ for longitudes between 70° W and 55° W. We used 63,809 P-wave receiver functions from 1216 stations (using the LQT components of the incident ray system) to image the MTZ in South America. The receiver functions were corrected for move-out, stacked in cells of 3° x 3° degrees every 1° x 1°. We obtained 54,389 RF traces imaging the 410 km and the 660 km discontinuities. The discontinuity times were corrected using the SL2013 global tomography model to obtain depths. A thickened MTZ follows the trend of the Nazca plate beneath the sub-Andes. To the north of 18oS the thickened MTZ is only about 250 km wide; to the south, the thickened zone reaches up to a 1100 km width. This observation clearly indicates that the Nazca slab flattens close to the 660 km discontinuity lowering the mantle temperature and thickening the MTZ. The 660 km discontinuity is more affected than the 410 km, which is consistent with the Nazca slab being held just below the MTZ and not inside the MTZ in most of the region.