Island Arc Development: Some Observations, Experiments, and Speculations

Abstract

Island arcs have narrow, segmented volcanic fronts which may reflect a similar segmentation in the downgoing plate. Secondary volcanic centers appearing some 3-4 m.y. later than the front and at some distance behind the front are common to most island arcs. Secondary arc volcanism in the Aleutian, Kamchatka, Kurile, Japan, Indonesia, and Scotia arcs is examined to expose correlations between lava chemistry and volcano position relative to the segmented front. Potash is universally higher, as expected, but often so are soda and alumina. The regular spacing of volcanic centers of the front is generally systematically greater than the separation or spacing between the fronts. Often this transverse spacing (d') (i.e. distance to the secondary centers) is proportional to the spacing (d) along the front and inversely so to the dip ($$\theta$$) of the local Benioff zone. These centers roughly obey the empirical relation $$d' = d cos \theta$$. It is speculated that this relation results from the gravitational instability of a ribbon-like region of magma inclined at the angle of the Benioff zone. A set of fluid instability experiments, scaled to the earth, are performed and examined analytically to show that a scatter similar to that observed in the spacing histograms also occurs experimentally under the most deterministic instability conditions and to prove the theoretical result of Selig (1965) relating spacing to source thickness and viscosity. These results are then applied to island arc magmatism to expose the strengths and weaknesses of the overall model. The discovery of a fundamental relationship (equation (17)) linking the size and viscosity of a diapir to its source thickness and mantle viscosity provides a new and critical link between pluton size and source characteristics. A set of equations is found which determines, from the observed spacing of volcanic centers and the volume of magma, the dimensions and viscosity of the source and its diapirs. At the time of instability the source is thin ($$\leq 1 km$$), wide (~ 30 km, on average) and only, perhaps, a thousand times less viscous than the overlying asthenosphere-found here to have a viscosity of about $$4 \times 10^{20} p$$; the ascending partially molten diapirs have an initial radius of about 3-5 km. Increased partial melting and magma extraction during ascent eventually produces a smaller diapir of magma. Finally, the general theory of gravitational instability is critically examined in reference to its application to a magma source which is probably segmented and of an irregular thickness. Even under these conditions the theory of gravitational instability is found to be applicable to the earth.