Role of the Aleutian Arc and NW Pacific seafloor in Pacific-wide plate reorganization in the Paleogene

The Paleogene marked a Pacific-wide tectonic reorganization with cessation of subduction in the western Pacific at 60-55 Ma, due to arc-parallel subduction of the Izanagi-Pacific spreading center (Whittaker et al., 2007). The Izu-Bonin-Mariana (IBM) and Tonga subduction zones initiated at 52-51Ma (Ishizuka et al., 2011; Meffre et al., 2013) and the Hawaiian-Emperor Bend (HEB) formed at 49-47Ma (O'Connor et al., 2013; Jicha et al., 2018). Here we report new 40Ar/39Ar ages for the initiation of the Aleutian Arc and for secondary (deformation-style) volcanism on the NW Pacific seafloor. The oldest Aleutian ages (55-52Ma) are from Medny Island (westernmost Aleutians) and south of Aggatu Island (western Aleutians). Preliminary ages older than 55Ma have not been replicated thus far. Both relative (between West Antarctica and the Pacific Plate; Wright et al., 2015) and absolute plate motions (Muller et al., 2019) show a clockwise swing of the Pacific/Kula Plates resulting in nearly northward absolute plate motion at 56-55Ma, consistent with initiation of Aleutian subduction at that time. At 53-52Ma, plate motion shifted back to a NW direction, coincident with initiation of the IBM and Tonga arcs. At 48-47Ma plate motion shifted from NW to a more westerly direction, consistent with formation of the HEB.

MORB-like basalts produced by rejuvenated volcanism on the Mesozoic NW Pacific Plate yield ages close in time to IBM and Tonga initiation and HEB events. A ridge on the Krusenstern Fracture Zone west of the northern Emperor Seamounts (Smts) of Tenji (71Ma) and Suizei (73Ma) yielded an age of 52Ma, as did a sample from the top of a guyot smt east of these Emperor Smts. A sample from a ridge on the western margin of the Emperor Trough, formed in Cretaceous seafloor, yielded an age of 49Ma. A tuff drilled at DSDP Hole 192A on Meiji Seamount (oldest Emperor Smt on the Pacific seafloor, ~85Ma) yielded an age of 48Ma. These volcanic rocks fall into the same two age groups (53-52 and 48-47 Ma) as rejuvenated volcanism from the Musician Ridges (O'Connor et al., 2015), interpreted to result from plate deformation related to the IBM and Tonga subduction initiation and HEB events. Rejuvenated volcanism associated with these large-scale tectonic events is consistent with plate cracking as a result of changes in plate motions at these times.