Intraplate volcanism with complex age-distance patterns: A case for small-scale sublithospheric convection

Many volcano chains in the Pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions. One possible explanation for non-hot spot intraplate volcanism is small-scale sublithospheric convection (SSC), and we explore this concept using 3-D numerical models that simulate melting with rheology laws that account for the effects of dehydration. SSC spontaneously self-organizes beneath relatively mature oceanic lithosphere. Whenever this lithosphere is sufficiently young and thin, SSC replaces the shallow layer of harzburgite, which was formed by partial melting at the mid-ocean ridge, with fresh peridotite. This mechanism enables magma generation without any preexisting thermochemical anomalies. However, the additional effect of melting-induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous SSC, overturn of the compositional stratification, and related magmatism. The intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of SSC into very shallow and cooler levels. On the one hand, such a restriction precludes high degrees of melting, but on the other hand, it slows asthenospheric cooling and thus prolongs the duration of melting (to similar to 25 Ma). Volcanism over such an elongated melting anomaly continues for at least 10-20 Ma and occurs on seafloor ages of similar to 20 to similar to 60 Ma. These seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid-ocean ridge melting. The long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains. SSC thus gives an explanation for previously enigmatic volcano ages along the Line Islands and the Gilbert and Pukapuka ridges, as well as along the individual subchains of the Wakes, Marshalls, and Cook-Australs.