SLAB-WINDOW SEGMENTATION AND THE EVOLUTION OF THE SANAK-BARANOF BELT: AN EXAMPLE FROM KODIAK ISLAND, AK

Study of the Paleocene forearc plutons on Kodiak Island has allowed us to construct a model of slab-window evolution. This model can be extrapolated to the rest of the 2100-km long Sanak-Baranof Belt based on the statistical distribution and thermal characteristics of the slab-window plutons. Emplacement mechanisms for the Kodiak batholith indicate that its magma ascended vertically through the accretionary prism for a minimum distance of 5-10 km with little regard to pre-existing structures. The plutons are direct markers of the underlying slab-window. Therefore, U/Pb geochronology, geometry, composition and thermal characteristics of the overlying plutons can be used to constrain the evolution of the slab-window. Preliminary U/Pb zircon dates allow for a 1 Ma age progression from west (59 Ma) to east (58 Ma) along the axis of the batholith. Also, the spatial distribution of the plutons also changes from west to east. On the southwest side of Kodiak Island the slab-window plutons occur over a trench perpendicular distance of 10-15 km, whereas on the northeast side of the Island the plutons are distributed distance of 60-70 km. These observations are interpreted to be the result of the widening of the slab-window as it migrated eastward at approximately 19cm / year. The high migration rate is due to oblique subduction of the spreading-ridge. A similar spatial pattern of forearc plutons is observed in the Kenai Peninsula / Prince-William Sound region and in the Chugach metamorphic complex. Each of these regions is bounded by a gap in slab-window magmatism. Within each segment, the plutons have a Poisson spatial distribution, whereas for the entire Sanak-Baranof belt the plutons are spatially clustered. Together these data suggest that in each of the three locations a distinct spreading-ridge segment was subducted. In each segment the resulting slab-window first formed with a linear geometry similar to that of a spreading ridge, but as it was progressively subducted the slab-window grew in size. A larger-scale tectonic effect of slab-windows and the subduction of very young oceanic crust appears to be a high volume magmatic pulse in the volcanic arc. Inland of the Sanak-Baranof belt, both the Alaska-Aleutian range batholith and the Coast Plutonic Complex exhibit periods of significantly increased plutonism shortly before the passage of the slab-window.