SUBDUCTION INITIATION AND EARLY EVOLUTION OF THE ALEUTIAN ARC: COMBINED ON-LAND AND OFF-SHORE STUDIES

The origin and initiation of the Aleutian Subduction Zone, forming the boundary between the Pacific Ocean and Bering Sea, remains elusive. A continuous belt of arc volcanism extended from mainland Alaska to the Siberian margin with the youngest volcanism along the Beringian margin ranging from 54-50Ma (Davies et al., 1989, CanJEarthSci26). Magnetic anomalies indicate that plate motion changed from a northerly to a westerly direction at ~56-53Ma (chron 25-24). Thereafter volcanism shifted to the Aleutian Arc, presumably trapping a fragment of the Kula plate that formed the basement of the Bering Sea. Interestingly the oldest rocks from the forearcs of the Izu-Bonin-Mariana (IBM) and the Tonga subduction systems yield ages of 50-52Ma (e.g. Reagan et al. 2013, EPSL380; Meffre et al., 2013, G3_13), suggesting subduction initiation as a consequence of a Pacific-wide tectonic reorganization. Did the Aleutian Arc also form during this plate-wide event? The oldest Aleutian ages come from the submarine Murray Canyon in the central Aleutian Arc (46Ma; Jicha et al., 2006, Geology34) and from Medny, the smaller Komandorsky Island, in the westernmost Aleutians (47Ma). The earliest volcanism on Medny (47-21Ma) is tholeiitic with clear subduction-related incompatible element signatures, e.g. relative enrichments in mobile elements such as U, Sr and Pb and depletions in Nb and Ta. Isotopic compositions for Sr-Nd-Pb indicate contributions from subducted sediment and ocean crust, as well as Komandorsky mantle wedge. In the IBM and Tonga Arc systems, boninites have been found among the earliest lavas. In contrast, no boninites have been found in the Aleutians thus far, which could suggest that the oldest lavas have not been found and that subduction may also have initiated earlier, possibly also between 52-50Ma. In order to further constrain the age of arc initiation and the compositions of lavas formed during arc initiation, we have just completed detailed sampling of the lowermost portions of the Adak, Amchitka, Murray and Attu Canyons in the Aleutian forearc and at Kresta Ridge in the rear-arc on R/V SONNE Cruise 249/1. On SO249/2, we will sample forearc canyons in the westernmost Aleutians southeast of Medney Island. Results from the Komandorsky Islands and preliminary results from the cruises will be presented.