Abstract Our understanding of subduction initiation remains limited by the difficulty of making direct observations of geologically recent examples. In the geologic record, supra‐subduction zone ophiolites are often interpreted to record syn‐initiation slab rollback and upper plate extension. Unfortunately, many ophiolites are decoupled from records of subsequent arc maturation and/or lack robust geochronological constraints. In this contribution, we investigate the initiation of the Late Triassic‐Jurassic Talkeetna Arc of southern Alaska. The Talkeetna Arc is one of the world's preeminent examples of an oceanic magmatic arc, yet the timing and tectonic record of its initiation remain unconstrained. Our study integrates geologic mapping with a chemostratigraphic analysis of ∼3.5 km of pillow basalts that comprise the Lower Shuyak Formation (Kodiak Archipelago, Alaska). The Lower Shuyak Formation hosts Late Triassic calc‐alkaline plutons and is overlain by ∼2 km of volcaniclastic strata. Initial mafic volcanism was characterized by high‐Ti, E‐MORB‐like tholeiites that are enriched in high field strength elements (HFSEs) and light rare earth elements. Compositions transition up section to HFSE/LREE‐depleted island arc tholeiites that become enriched in Th. These trends are consistent with petrologic processes during subduction initiation. High‐precision U‐Pb zircon dates (CA‐ID‐TIMS) constrain initiation to ∼232 Ma or slightly earlier. Our results demonstrate that initiation of the Talkeetna Arc occurred earlier than previous estimates, slightly preceding the eruption of regionally extensive flood basalts interpreted by previous studies as plume‐related. This relationship may suggest that subduction initiation was plume‐induced, or that rifting of the upper plate was the primary driver of basaltic volcanism regionally.
Keough et al. (Wed,) studied this question.