VOLCANOS OF THE SOUTHWESTERN EXTENSION OF THE ACTIVE MARIANA ISLAND-ARC - NEW SWATH-MAPPING AND GEOCHEMICAL STUDIES

Until recently it was thought that the volcanoes of the Mariana island arc of the western Pacific terminated at Tracey Seamount at similar t o 14 degrees N immediately west of Guam. Sea floor mapping in 1995 sho ws a series of large volcanic seamounts stretching westward for nearly 300 km beyond that point. The morphology, spacing, and composition of those sampled are consistent with their having formed as a consequenc e of eruption of suprasubduction zone arc magmas. The relationships of the volcanoes to the tectonic processes of subduction of the Pacific plate beneath the southern portion of the Mariana convergent plate mar gin are becoming increasing-ly clear as new bathymetry and geochemical data are amassed. The volcanoes along this trend that lie closest to Guam are forming where the center of active extension in the back-arc basin intersects the line of are volcanoes. They develop well-defined rifts that are parallel to rift structures along the extension center, whereas volcanoes of the spreading axis to the north are smaller than the frontal are volcanoes and tend to form along lineaments. Composit ions of lavas from these intersection volcanoes bear some similarities to back-arc basin basalt, but are on the whole well within the range of compositions for Mariana island arc lavas. The Pacific plate subduc ts near ly orthogonal to the strike of the trench along the southern p art of the Mariana system and the distance to the are line ii om the t rench axis is only similar to 150 Km. Several deep fault-controlled ca nyons on the inner slope of the southern Mariana trench indicate an en hanced tectonic extension of this plate margin. The presence of these active are volcanoes and the existence of the orthogonal normal faulti ng along the southern. Mariana forearc supports a model of radial exte nsion for formation of the Mariana Trough, a model previously dismisse d because of the lack of evidence of these two major geological featur es.