GEOLOGY OF YOUNG SUBMARINE VOLCANOS WEST OF EASTER-ISLAND, SOUTHEAST PACIFIC

Submarine volcanoes, probably related to the Easter hotspot, were foun d between the Easter Microplate and Easter Island near 26-degrees-15'S , 111-degrees-W. Hydrosweep swath mapping accompanied by deep-towed ca mera observations and sampling revealed the presence of fresh lava flo ws confined to two distinct fields, the previously-known Ahu and the n ewly discovered Umu Volcanic Fields, covering a combined area of more than 4300 km2. These lava fields form constructional ridges with volca nic cones as well as isolated seamounts. Another large volcanic area, probably also belonging to the Easter hotspot, was found 100 km south of the Ahu/Umu region. Thus the young hotspot region has a diameter of about 200 km and its centre probably lies underneath the Umu Volcanic Field rather than under Ahu Volcanic Field. The fresh volcanics recov ered from these areas are olivine and plagioclase phyric enriched and, more rarely, depleted tholeiites. The enriched lavas are comparable t o E-MORB from the Easter Microplate spreading axes and volcanic ridges and seamounts west of the microplate.