Mantle shear structure beneath the Americas and surrounding oceans

Abstract

Maps of lateral variation in shear velocity within the mantle beneath North and South America, their surrounding oceans, and parts of Africa and Eurasia are produced from inversion of travel times of horizontally polarized shear body waves. The data consist of S and ScS waves as well as multibounce phases SS, SSS and SSSS. Waves that bottom within the upper mantle are modeled using synthetic seismograms in order to estimate travel times for each of the multiple arrivals caused by velocity discontinuities near 400 and 660 km depth. The model consists of blocks with uniform slowness anomalies relative to a one‐dimensional starting model and extends from the surface to the core‐mantle boundary. The blocks have horizontal dimensions of roughly 275 by 275 km and vary in the vertical dimension from 75 to 150 km. The data are inverted using a simultaneous iterative reconstruction technique algorithm. The upper 400 km of the model is dominated by lateral variations that correspond to surface tectonic environments. Three shields on three separate continents have higher than average velocities down to between 320 and 400 km depth. Young tectonically active regions are very slow in the upper 250 km. The transition zone from 400 to 660 km depth is the most poorly resolved region. High velocity beneath western South America in the transition zone is probably associated with subducting slab. The transition zone velocity beneath the western and central part of North America also appears to be slightly faster than average. The lower mantle is dominated by large‐scale sheets of higher than average velocity and more equidimensional regions of slow velocity. From South America to Siberia, sheet‐like high‐velocity anomalies are observed from 750 km depth to the core‐mantle boundary. Another lower mantle high‐velocity anomaly is seen beneath southern Eurasia. The high‐velocity lower mantle anomalies seem to be associated with subduction during the last 150 Ma. Comparing the location of past subduction with the location of lower mantle anomalies, the identification of lower mantle anomalies with old subducted slabs suggests slow sinking of slabs in the lower mantle (about 1 to 2 cm/yr). If this interpretation is correct then high velocity in the deepest mantle off the west coast of South America through the western United States requires significant subduction from 120 to 150 Ma a few thousand kilometers off the coast of the Americas. The slowest deep region found in this study is at the base of the mantle beneath the eastern Atlantic Ocean and may be associated with hotspots in that region. Other hotspots do not appear to be associated with slow lower mantle velocity.