EARLY EVOLUTION OF THE MARIANA FORE-ARC NEAR GUAM

We have prepared a preliminary geological map of the southern Mariana fore-arc south and east of Guam based on the spatial distribution of lithologies encountered during Shinkai 6500 submersible diving as well as dredging and drilling. The sequence of rock types encountered from the trench westward to Guam and east of the West Santa Rosa Boundary Fault (WSRBF) is similar to the typical stratigraphic sequence found in ophiolites: peridotite, gabbro to tonalite, diabase, MORB-like lavas, transitional lavas, low- to high-Ca boninites, and tholeiitic to calcalkaline series arc lavas. The most abundant igneous lithologies in this area are the MORB-like pillow lavas and associated intrusive rocks termed “fore-arc basalts” (FAB) in Reagan et al. (2010, G-cubed). The fore-arc west of the WSRBF has similar lithologies but lacks FAB. The compositions of FAB, their broad distribution along the Izu-Bonin-Mariana system (cf. Ishizuka et al. 2009, EOS), and their likely eruption ages of 51-52 Ma (Tani et al., 2009, EOS) suggest that they were the first lavas to erupt after subduction began. The best geochronologic constraints suggest that boninites erupted between 48 and 44 Ma. The oldest arc lavas erupted between 45 and about 34 Ma, before the opening of the Parece Vela Basin. High-Mg andesites erupted at about 31 Ma and were synchronous with rifting. The youngest arc sequence is Miocene and erupted before the opening of the Mariana Trough. The preservation of the lithologies associated with subduction initiation (SI) in the fore-arc suggests that basal subduction erosion of the Philippine Plate has not been robust over the history of the arc. In contrast, the presence of FAB-related volcanic and intrusive rocks near the trench suggests that low-angle normal faulting removed the upper fore-arc crust. Our best age estimate of SI in the Mariana of 51-52 Ma is the age of many changes in plate motion globally including the collision of India and Asia. We speculate that stress from the collision was transferred to the boundary that became the Mariana subduction system and might have triggered overthrusting that nucleated subduction. The presence of Cretaceous fossils and andesitic glasses at DSDP sites 460 and 461 (Hussong and Uyeda et al., 1982) suggests that a sliver of the overthrusted plate might be exposed in the trench-slope.