From slow to ultraslow: A previously undetected event at the Southwest Indian Ridge at ca. 24 Ma

Abstract

Changes in plate motion are thought to be recorded in the trend of fracture zones, even though fracture zones provide no information about the spreading rate. Using newly compiled published and unpublished magnetic data from the Southwest Indian Ridge, we calculated fi nite rotation poles for A13, A8, and A6, from which we determined a 50% decrease in spreading rate from slow to ultraslow at ca. 24 Ma not accompanied by a signifi cant change in spreading direction. This spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries. Finally, we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca. 24 Ma at the global scale.