On-going uplift rate of the Saint Peter Saint Paul Peridotite Ridge, Equatorial Atlantic Ocean, base on geomorphologic analyses of wave-cut bench and14C dating for coral fossils

The Saint Peter Saint Paul Islets is situated at the Equatorial Atlantic Ocean on the top of the morphologic elevation of 90km in length, 25km in width, and 3800m in height constituted by abyssal mantle peridotite. The submarine morphology around the islets is highly accidental characterised by steep slopes and subvertical cliffs. The morphological features and the absence of large coral reef suggest that the islets are formed by recent tectonism that may continue up to the present. It is considered that the peridotite ridge top was present at the ocean bottom and it has just reached to the present sea level. The summit level map shows a wave-cut bench of 7~9 m above sea level. Based on the correlation of this bench to the Flandrian Transgression, the uplift rate in recent 6000 years is calculated as 1.5 mm/year. The C datings for the coral fossils also indicate the same uplift rate. If the peridotite ridge corresponds to a tectonically deformed megamullion, total uplift is 1500 to 3000m and the tectonism started at 1 to 2Ma. On the other hand, if the ridge is originated directly from subcrustal mantle, the total uplift becomes about 9000m and tectonism is active in recent 6 million years.