Subduction of a segmented ridge along a curved continental margin: Variations between the western and eastern Sanak–Baranof belt, southern Alaska

Abstract

Between the western and the eastern parts of the southern Alaskan Sanak–Baranof belt there exist variations in pluton spacing, geochronology, geochemistry and metamorphism. All such characteristics can be explained by increase in heat flux towards the eastern parts of the belt. Despite such differences, the initiation of magmatism propagated eastward at a constant rate of 19 cm/year throughout the entire belt. The average spacing of magmatic centers changes from 165 ± 88 km in the west to 67 ± 41 km in the east. The duration of magmatism as measured by U/Pb zircon and monazite ages increases from 1–2 Ma in the west to 4–5 Ma in the east. The duration of Ar–Ar biotite cooling ages also increases to the east. In agreement with the extended cooling times, large regions of greenschist and amphibolite facies metamorphism are present in the eastern segments, whereas outside of narrow pluton aureoles high-temperature metamorphism is not present in the western Sanak–Baranof belt. Finally, intrusions with adakitic characteristics such as high Sr/Y ratios are present in the east, but not in the west suggesting that some degree of slab melting occurred in the eastern Sanak–Baranof belt. We suggest that the above east–west thermal and magmatic variations can be explained by a decreasing rate of subduction and a persistent slab window in the eastern Sanak–Baranof belt caused by the oblique subduction of a segmented spreading ridge along a curved continental margin, and that this model is compatible with the Resurrection plate hypothesis.