FOREARC INTRUSIONS IN SOUTHERN ALASKA: EXAMPLE OF LONG-TERM INFLUENCE OF A SLAB WINDOW ON CONVERGENT MARGIN MAGMATISM
The Matanuska forearc intrusions signify emplacement of depleted MORB mantle (DMM) through a slab window that formed by spreading ridge subduction beneath southern Alaska. A slab window first opened beneath southern Alaska during Paleocene to early Eocene ridge-trench encounters, triggering near-trench igneous and related events in and outboard of the Matanuska forearc region. We newly propose that during mid-Eocene time there was renewed asthenospheric upwelling after the subduction of one (or more) additional small ridge segment(s) that were offset from earlier ridge segments. In this model, a narrow portion of the southern Alaska margin experienced repeated ridge subduction with prolonged mantle upwelling and heating, driving upper plate extension and ongoing mafic magmatism between ~60-40 Ma.
We envision this to be a process by which anhydrous depleted mantle is added beneath a convergent margin (i.e., depleted mantle growth). Any subsequent magmatism, including re-established arc systems, would inherit this mantle which may explain the relatively depleted and juvenile nature of the eastern Aleutian arc system compared to pre-slab window arc rocks of southern Alaska. With evidence for ridge subduction and near-trench slab-window magmatism along much of western North America this process has likely been an important control on the composition of past and present Cordilleran arc magmas and highlights the long-term role that slab windows can have on mantle evolution beneath convergent margins.