EVIDENCE OF FOSSIL FLAT SUBDUCTION IN TOMOGRAPHIC IMAGES

The mid-continent region of the United States lies within the North American craton and has been relatively stable since the Reelfoot Rift formed around 500Ma. Significant deviations in S-velocities from that of the surrounding uppermost mantle are thus not expected. Yet, in our recent tomographic S-velocity study for this region, we image an anomalous region of slow S-velocities in the mid-lithosphere. This anomalously slow region exists from the base of the crust to depths of ~100 km, and is slower than a cratonic average by about 200 m/s. The slow uppermost mantle beneath the mid-continent is underlain by a faster lithosphere typical of the North American craton to depths of ~200 km. We tend to attribute this anomalous feature to a remnant fossil subduction zone from the Mesoproterozoic, as the southern edge of Laurentia was accreting terranes.

To better understand the nature of this fossil flat subduction zone in the mid-continent, US, we have been updating the North American S-velocity model, NA04, with events from 2000 through 2005. This updated S-velocity model allows better resolution of the structure of the upper mantle beneath the mid-continent and the western US, where the Farallon Plate subducted at flat dip angles around 60 Ma. We will compare our updated model for North America with tomographic S-velocity models for South America, such as SA99. Model SA99 includes the regions of current flat subduction beneath Peru and Chile, but its resolution for the subducting Nazca Plate beneath South America is limited because of the existing event-station distribution. We hope to learn how flat subduction has affected the formation of the continental lithosphere throughout recent and more distal times.