AN INBOARD PERSPECTIVE OF THE CENOZOIC TECTONO-MAGMATIC HISTORY OF ALASKA'S SOUTHERN CONVERGENT MARGIN
The Paleocene–Eocene active ridge subduction event beneath southern Alaska is associated with spatially variable basin subsidence and inversion, near-trench slab-window magmatism sourced by asthenospheric mantle, an increase in exhumation rate, and an elevated geothermal gradient lasting up to ~10 My after the ridge subduction event. The complex late-Eocene southern Alaska tectonic history included re-establishment of arc magmatism (with inherited slab window mantle signatures) and several lesser known events including forearc and accretionary prism magmatism (~45-36 Ma), a likely minor ridge-subduction event, rock cooling associated with a depressed geothermal gradient, punctuated pluton emplacement along the Denali Fault system at ~40 Ma, and potential flattening of the young and hot northward subducting Pacific slab. Based on rock cooling rates and basin analysis, the period between ~35 and ~25 Ma, was likely one of limited contractional deformation for much of southern Alaska away from the margin.
Late Oligocene (~25 Ma) to present tectonics of southern Alaska have been dominated by the flat-slab subduction and accretion of the Yakutat microplate with rock-deformation focused along pre-existing structures and an absence of arc magmatism. The presence of major Oligocene-Miocene unconformities in the Cook Inlet, Susitna, and Nenana basins support this interpretation. The ~6 Ma southern Alaska tectonic event is the only documented region-wide Neogene event. The southern Alaska deformation response was likely driven by the thicker part of the Yakutat microplate converging along the now transpressive Fairweather Fault, a result of increased obliquity due to a plate motion change at ~6 Ma. This indentor event was magnified by the geometric complexity of the corner syntax of NE Pacific subduction zone leading to an inferred change in both the velocity of rotation and shape of the southern Alaska block.