Evolution of the Aleutian arc
Monday, 15 December 2014
Recently published and ongoing research at several of the circum-Pacific oceanic arcs (Izu-Bonin-Marianas, Tonga) has led to the development of tectonic models for how these subduction zones have formed and evolved since the Eocene. An outstanding question that has yet to be answered is how the timing of initiation and evolution of the Aleutian arc is linked to these current tectonic models. More than 70 new 40Ar/39Ar ages and over 100 major and trace element analyses of lavas and plutons exposed on numerous central and western Aleutian islands have been determined as part of a reconnaissance effort to address this question. Exposures of > 42 Ma rocks remain elusive, and likely require a concerted sampling campaign in the submarine forearc. The new geochemical and geochronologic data support the suggestion that initial growth of the Aleutian arc was rapid and subsequent growth occurred in three distinct pulses (37-29, 17-11, 6-0 Ma) (e.g., Jicha et al., 2006), a trend also observed in other circum-Pacific arcs. Eocene-Oligocene lavas and gabbroic plutons of the western Aleutians are tholeiitic with island arc-like trace element patterns that can be light rare earth element (LREE) depleted. These compositions are distinctly different from the calc-alkaline, light rare earth element (REE) enriched and variably heavy REE depleted, Pleistocene to Holocene primitive andesites and dacites found atop the thinner, faulted crust of the western Aleutians. Widespread calc-alkaline magmatism does not appear to commence until the Miocene, although some Oligocene calc-alkaline lavas and plutons have been found throughout the arc and along the Bowers Ridge (Wanke et al., 2012). A major transition in post-Pliocene arc chemistry occurs as a sediment melt component, which was not available early in the development of the Aleutian Arc becomes important in the Pleistocene (e.g., Kay and Kay 1994; Schaen, 2014).