Structure and composition of early arc crust: observations and samples from Shinkai 6500 Dives in the southeastern Mariana Trench

The forearc of the Izu-Bonin-Mariana Trench comprises crust formed during initiation of subduction and earliest arc volcanism. Five dives with Shinkai 6500 during Cruise YK0612 in 2006 recovered forearc crustal rocks from exposures at a major N-S scarp south of Guam and along complex strike-slip and normal faults in the trench slope southeast of Guam. These exposures are little affected by post-Eocene back-arc extension and volcanism. We present here a first attempt to constrain the stratigraphic relationships of lower crustal units in the region. Dive 973 southwest of Guam recovered partially serpentinized dunites and harzburgites (6500 to 5900 m). Most samples have prominent high-temperature deformation fabrics. An earlier Shinkai Dive recovered only volcanic and hypabyssal rocks shallower than 5500 m to the north. Dives 974, 975 and 976 provide a transect of the landward slope near 145o20'E (6500 m to 3000 m). OL basalt, HB-andesite, and hypabyssal silicics are exposed at the shallowest levels, with abundant volcanic silt-and clay-stones, and some coral debris. Exposures of crust in two different structural blocks at 6500 m to 5800 m yielded medium- grained gabbro (more abundant in the lower section), abundant diabase, boninitic volcanics, and diabases intruded into gabbro. A dredge deeper (7500m) on the transect collected ultramafic exposures, with minor tonalite and gabbro. Dive 977 at 145o50'E found mostly diabases and some dikes (6380 to 5500 m). These results document a coherent crustal section floored by CPX-poor ultramafic rocks; overlain by a relatively thin gabbroic layer, then a thick zone of hypabyssal rocks from basaltic to dacitic in composition, intruded into and mixed with volcanic and fine-grained gabbro screens; topped by a volcanic and sedimentary section possibly correlative with Eocene and lower Oligocene sections on Guam. The Moho appears to deepen to the east, perhaps reflecting subsidence of the outer parts of the forearc due to tectonic erosion. The gabbroic section is consistently thinner than typical of most ophiolites. Two dives ended near the summits of seamounts in shallow intrusive rocks capped by sediments, consistent with subaerial erosion before down-dropping to their present 3-6 km. The correlation of geochemical, petrologic, and age characteristics of the dive samples to subaerial exposures on Guam provides a rare opportunity to examine the structure and composition of in situ early island arc lower crust.