The Hawaii-Emperor Bend: A Seafloor Spreading Perspective

At the beginning of all plate tectonic reconstructions stands the thorough identification of seafloor spreading anomalies wherever this is possible. While this seems to be a relatively simple thing to do and a largely finished task for large parts of the world's oceans, there have been recent advances in the identification of seafloor spreading anomalies in the tropical East Pacific. These lead to significant revisions of the Paleogene isochrons compared to widely used global age models in certain places of the East Pacific and allow for a better determination of spreading rate changes over time including a large increase in East Pacific Rise spreading rates within Chron 21, which coincides with the HEB. A factor that receives less attention than it may deserve in this context is the question of the reliability of reversal timescales, especially for the time around Chron 21, because this is directly related to the calculated spreading rates. Any major change in the spreading regime of the East Pacific Rise has an impact on the plate tectonic evolution of the other large ocean basins and especially the Atlantic Ocean, simply because the Pacific plate is so big that it has common boundaries with all other big plates of the Earth except for the African plate. This is well documented for a number of tectonic events like the break-up of the Farallon plate at Chrons 6C-6B. However, with respect to Chron 21 the currently used reversal timescales are based on the assumption that during the Eocene the South Atlantic has seen no sudden changes in seafloor spreading rates. If this assumption is wrong and a spreading rate acceleration near Chron 21 was missed then this would minimize the increase we calculate for the Pacific as well. We present our updated seafloor spreading identifications for the Paleogene Northeast Pacific crust and the evidence we find for a significant increase in spreading rates near Chron 21. Rate increases are larger for a modest younger revision of the Chron 21 age. The implications which even relatively small changes in the age dating of this crust have on plate tectonic reconstructions involving the HEB are obvious and highlight the benefit of investing some effort in answering the remaining open questions about spreading rates and reversal ages near Chron 21.