GEOLOGIC RECORD OF SUBDUCTION INITIATION PROCESSES FOR THE LATE TRIASSIC-JURASSIC TALKEETNA ARC, ALASKA: PRE-INITIATION DEFORMATION, SYN-INITIATION VOLCANISM, AND ESTABLISHMENT OF THE ARC FRONT

The Late Triassic-Jurassic Talkeetna Arc is an archetypal example of an oceanic arc crustal section. Yet, the processes associated with the initiation of subduction for this system remain unstudied. This study investigates pre-, syn-, and post-initiation records on the Kodiak Archipelago and Alaska Peninsula. The pre-initiation stage is recorded by unconformity development consistent with regional tectonic shortening. On the Alaska Peninsula, Permian limestone, volcaniclastic conglomerate, and chert-pebble conglomerate comprise a compositionally diverse basement overlain by the Norian Kamishak Formation. On the Kodiak Archipelago, a previously unmapped exposure of the unconformity is characterized by distal volcaniclastic turbidites that were isoclinally folded, erosionally beveled, and subsequently overlain by mafic volcanic rocks. Unconformity development was followed by a significant subsidence event prior to deposition of the first Norian deepwater volcaniclastic strata representing Talkeetna Arc activity.

On the Kodiak Archipelago, syn-initiation processes are represented by greenschist grade pillow basalt of the Lower Shuyak Formation. Geochemical data show that most of the section consists of tholeiitic basalts that become progressively more depleted up section. An airfall tuff interbedded with pillowed basalt flows yielded a U-Pb zircon age of ~233 Ma. We tentatively interpret the Lower Shuyak Formation as syn-initiation, upper plate crust that formed near the position of the future arc axis. The early arc front is represented by the Afognak batholith which intrudes the Carnian pillow basalts in the study area. Sparse published ages for the batholith range from ~217-213 Ma. A new U-Pb zircon age of ~207 Ma was obtained in this study. Norian volcaniclastic strata of the Upper Shuyak Formation (Kodiak Archipelago) and Kamishak Formation (Puale Bay) are interpreted as the extrusive products of the batholith following initiation. These strata were deposited as sediment aprons along the flanks of edifices. As an integrated dataset, this record of pre-, syn-, and post-subduction initiation processes for the Talkeetna Arc is an unusually complete example of tectonic processes resulting from the development of a nascent convergent margin, subduction zone, and oceanic arc system.